airfield pavement design

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AIRFIELD PAVEMENT DESIGN CHAPTER

This chapter provides informat ion to help select design, and con-

struct airfield structures and landing facilites. Close battle support,

and rear area TO air-fields are designed according to specific aircraft

characteristics and requirements governing thickness, strength, and

quality of materials. Airfield location and soil strength determine

the dif ferent minimum pavement thicknesses and design procedures.

The proper placement of the base, subbase, and subgrade determine

the ef fect iveness of the airf ield under al l cl imat ic and seasonalconditions.

AIRFIELD STRUCTURE TYPE

Airfield structures fall into three categories:expedient-surfaced, aggregate-surfaced, andf l ex ib le-pavement . Expedient -surfaced andaggregate-surfaced airfields are used primar-i ly in the close bat t le and support areas.Flexible-pavement ai rf ields are primari ly

const ructed in the rear area.

PRELIMINARY INFORMATION

Field condition, soil strength, and soil behaviorare the three most important pieces of informa-tion used to determine the feasibil i ty of con-structing an airfield at a particular location.

Fie ld Condi t ion

Knowledge of the current field condition isi m por t an t w hen des i gn i ng and cons t ruc t i ngan airfield. A proper description of the field

condi t ion at a proposed const ruct ion si te in-cludes the following elements:

Ground cover (vegetation).

Natural slopes.

Soil density.

Mois ture content .

Soil consistency (soft or hard).

Existing drainage.

Natural soil strength (in terms of California Bearing Ratio (CBR)).

Informat ion about the kind and dist r ibut ionof ground cover, slopes, moisture content,

and na tura l s t rength i s used to es t imate theconstruction effort required for a specific typeof airfield. The surface condition and thesoil type must be known to predict potentialdust problems at the si te . Moisture contentdata is required to determine the effect oftraffic on soil strength and to estimate waterneeds during const ruct ion. Soi l s t rengthdata i s needed to determine the surfacing re-quirements as wel l as the thickness design.

So i l S t r eng t h

From an engineering viewpoint, shearing re-

sistance (or shear strength) is one of themost important propert ies that a soi l pos-sesses. A soils shearing resistance undergiven conditions is related to its ability towithstand a load. The shearing resistanceis especially important in i ts relation to thesupporting strength or bearing capacity of asoi l used as a base or subgrade beneath a

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road, runway, or other st ructure. For mostmili tary pavement applications, the CBRvalue of a soil is used as an empirical meas-ure of shear strength. The CBR is deter-mined by a standardized penetrat ion shear

test and is used with empirical curves fordesigning and evaluat ing unsurfaced aggre-gate-surfaced, and flexible pave ments formilitary airfields. The CBR test is usuallyperformed on laboratory compacted testspecimens when used in pavement design.When used in pavement evaluat ions, de-st ruct ive test pi ts are usual ly dug to deter-mine pavement layer thicknesses, and in-place field CBR tests are conducted on thebase course , subbase , and subgrade mater i -als . In-place CBR tests are t ime-consumingto run and are usual ly impract ical for use

in the TO.

For expedient-surfaced airfields in the closebat t le and support areas, the laboratoryCBR test (which usual ly takes about fourdays to complete) is inappropriate due tot ime and equipment const raints . Therefore,several field-expedient methods of determin-ing CBR are available in the TO.

The Unified Soil Classification System(USCS) correlation is the quickest meansavailable for estimating CBR. For each soil

classification, empirical studies have deter-mined a range of CBR values. Theseranges can be found in FM 5-410 (Table 5-3, page 5-11). Since the CBR ranges areonly estimates, use the lowest CBR value inthe range. The soil type usually variesacross the entire airfield.

A better method for determining the CBRfor in-place soils is with a penertrometer.There are currently three types of pene-trometers available for airfields: the airfieldcone penetrometer, the trafficabili ty pene-

t rometer , and the dual -mass dynamic conepenetrometer (DCP).

The airfield cone penetrometer described inAppendix I is used to determine an index ofsoil strengths (Fenwick 1965) for variousmilitary load applications. The airfield pene-trometer consists of a 30-degree cone witha 0.2-square-inch base area. The force re-

quired to penetrate to various depths in thesoi l i s measured by a spring, and the ai r-field index (AI) is read directly from thepenetrometer. The airfield cone penetrome-ter has a range of 0 to 15 (CBR value of 0

to approximately 18). (The AI-CBR correla-tion is shown in Figure 12-1.) The airfieldcone penetrometer i s compact and sturdy.Its operat ion is simple enou gh tha t inexperi-enced mili tary personnel can use i t to deter-mine soil strength. A major drawback tothe airfield cone penetrometer is that i t willno t penet ra t e many crus t s , t h in basecourse, or gravel layers that may lie oversoft layers. Relying only on the surface AItest results could cause the loss of vehiclesor aircraft .

The ai rf ield cone penetrometer must not beconfused with the trafficabili ty penetrome-ter , a standard mil i tary i tem in the soi l testset . The trafficabili ty penetrometer has adial-type load indicator (0 to 300 range)an d is equipped with two cones: one is 1/ 2inch in diameter wi th a cross-sect ional areaof 0.2 square inch, and the other i s 0.8

Figure 12-1. Correlation of CBR and Al

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inch in diameter with a cross-sectional areaof 0.5 square inch. It also will not pene-trate gravelly soils or aggregate layers, butit may be useful for subgrades. If the traffi-cabiIity penetrometer is used to measure AI,the readings obtained with the 0 .2-square-

inch cone must be divided by 20; the read-ing with the 0 ,5-square-inch cone must bedivided by 50. Use th e same test ing proce-dures as discussed in Appendix I for the air-field cone penetrometer.

The dual-mass DCP described in AppendixJ will overcome some of the shortfalls asso-ciated with traffic ability and airfield conepenetrometers. The DCP was originally de-signed and used for determining thestrength profile of flexible-pavement struc-tures. It will penetrate soil layers having

CBR strengths in excess of 100 and willa lso measure soil s trengths less than 1CBR. The DCP is a powerful, relatively com-pact , s turdy device that can be used by in-

experienced military personnel to determinesoil strength. The DCP relation to CBR isshown in Figure 12-2. Presently, the DCPis not in the Army inventory. It was re-cently modified and studied by the UnitedStates Army Engineer (USAE) Waterways Experiment Station (WES). Information onprocurement and use of the DCP should bedirected to USAE WES, Pavement SystemsDivision, Geotechnical Laboratory, 3909Halls Ferry Road, Vicksburg, MS 39180-6199.

Soil Behavior

Soil is compacted to improve its load-carry-ing capacity and to prevent differential set-tlement (rutting) under aircraft traffic loadsHigh soil strength is usually associatedwith a high degree of compaction. However

at ta ining and maintaining a desiredstrength in soils is contingent upon thewater content a t the t ime of constructionand throughout the period of use . Some

Figure 12-2. Correlation plot of CBR versus DCP index

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basic moisture content-density relations forcohesive and cohesionless soils are dis-cussed in FM 5-410. Generally, it is desir-able for the soil to be compacted to Ameri-can Society of Test Materials (ASTM) 1557or to compactive effort, 55 blows per layer(CE 55), while it is within the desired mois-ture content range. The 4 percent moisturerange and the 5 percent densi ty range arederived from initial soil tests, and theymake up the specification block. Soils aretreated to improve their s trength or to re-duce the effects of plasticity and high liquidlimits. Stabilizing a soil can also providedust control and waterproofing. During con-struction, the type of soil treatment is deter-mined by the soil characteristics and avail-ability of stabilizing materials. (See FM 5-

410 for additional information.)

DESIGN CONSIDERATIONS

The design of airfield structures is basedon

Airfield location/ m ission.

Using aircraft and associated grossweight.

Strength of subgrade and available con-struction mater ia ls .

Susceptibility of geographic area andconstruction mater ia ls to frost act ion.

Traffic areas.

Expected number of passes of aircraft.

Air f ie ld Loca t ion / Miss ion

The location of the airfield within the TO isbroken down into three major areas , as de-scribed in Chap te rs 10 a n d 11 :

Close battle area.

Suppor t a rea .

Rear area.

These areas areof the airfield.

Design Aircraf t

Weight

designated by the mission

and Assoc ia ted Gross

In TO airfield design, the design aircraft isbased solely on the airfield location, asshown in Table 12-1 . The gross weight is

the maximum allowable weight during take-off (worst case) and is the basis for thethickness design. Of the aircraft listed inTables 11-1 a n d 11-2, pages 11-2 and 11-3,that can possibly use the airfield, the de-sign aircraf t is the one that presets the mostextreme load distribution characteristics.

Ex p e c t e d N u m b e r o f P a s s e s

For a runway, passes are determined by thenumber of a ircraf t movements across animaginary traverse line placed within 500feet of the end of the runway. More simply,

a pass on a runway is equivalent to a take-off and landing of an aircraft similar inweight to the design aircraft. For taxiwaysand aprons, passes are determined by thenumber of aircraft cycles across a line onthe primary taxiway that connects the

Table 12-1. Design aircraft

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runway and parking apron. At s ingle run-way airfields, the pass level of the runway,taxiway, and apron should be the same.

For expedient-surfaced airfields, the in-

place soi l s t rength determines the numberof passes. If the mission requires a longerservice life, the designer must adjust the de-s ign so measures are taken to improve thein-place soil. When designing aggregateand flexible-pavement surfaces, there is a di-rect correla t ion between the number of passes and the thickness of the design.

Traf f ic Areas

On expedient-surfaced airfields in the closebat t le and support areas , t raff ic areas are

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Type A. The airfield is capable of support-ing missions as soon as the runway is con-s tructed. The layout of the runway andhammerhead tu rnaround a reas a re shownin Figure 12-3. Specific dimensions for the

ent i re runway are shown in Chap te r 11 .The 63-foot turnaround area is required forthe design aircraft (C-130). When C-17sare ant ic ipated, the turnaround area doesnot increase the airfield width, which is 90feet. Ensure that an extra 90-foot sectionis added to both ends of the runway be-cause tu rnaround p rocedures can be de t r i -mental to an airfield surface. Taxiways anaprons should then be cont inual ly devel-oped to support cont inuous t raff ic .

Figure 12-3. Typical layout for expedient-surfaced airfield in close battle and support areas

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On aggregate-surfaced airfields in the sup-port area, t raffic areas are designated asshown in Figure 12-4. Type A areas in-clude primary taxiways, parking aprons,washrack areas, power check pads, and

1,000 feet on both ends of the runway.The interior port ion of the runway and theladder taxiway are considered Type C areas.Since the l ift on the wings accounts forsome of the aircraft load, Type C areas aredesigned for only 75 percent of the totalload.

On pavement ai rf ields in the rear area, pave-ments can be grouped into four t raffic ar-eas designated as Types A, B, C, and D.They are defined below and shown in Fig-ure 12-5 .

Type A traffic areas include all primary taxi-ways, including st raight sect ions, turns,and intersections. The ends (1,000 feet) arealso considered Type A since the aircraftload is st i l l fully transferred to the pave-ment. Although traffic tends to channelize

Figure 12-4. Typical layout of aggregate-surfaced airfields in the support area



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in the center lane on long, st raight taxiwaysections, i t is not practical in the TO to con-struct pavement sections of varying thick-nesses. Type B areas include all apronsand hards tands . Type C areas inc lude thecenter, 75-foot width of runway interior be-tween the 1,000-foot runway ends and atthe runway edges adjacent to intersect ionswith ladder taxiways. Washrack pavementsare also included in Type C areas. Type Dareas include those areas where traffic vol-u me is ext remely low, an d/ or the a ppl ied

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weight of the operating aircraft is muchlower than the design weight. Type D areasinclude the edges of the entire runway ex-cept for the approach and exi t areas at taxi -way intersections.

In designing flexible-pavement structures,the area type determines the actual load onthe pavement . Type A and B areas supportthe entire design weight, while Types C andD should be designed for only 75 percent ofthe design weight.

Figure 12-5. Typical layout of flexible-pavement airfields in the rear area



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So i l S t r eng t h

The st rength of const ruct ion materials canbe determined in terms of CBR by usingthe laboratory CBR test , airfield cone pene-trometer, trafficabili ty penetrometer, or

DCP, as discussed earl ier in this chapter.The st rength of both the subgrade and avai l -able const ruct ion materials can be deter-mined in terms of CBR based on proce-dures out l ined in Chapter 5, FM 5-430-00-1/ AFPAM 32-801 3, Vol 1. Stren gth of thein-place soil or subgrade will determine thetype of surface and the number of passesfor expedient-surfaced airfields. It also willdetermine the total thickness design in ag-gregate and flexible-pavement surfaces.

Fros t Act ion

In regions subject to frost action, the de-sign of aggregate-surfaced and flexible-pave-

ment airfields must give consideration tomeasures that wi l l prevent serious damagefrom frost action. Three conditions must ex-ist simultaneously for detrimental frost ac-tion to occur: (1) soil must be frost-suscep-

t ible, (2) temperature must remain belowfreezing for a considerable period t ime, and(3) ample supply of groundwater must beavailable. Precise methods for estimatingthe depth of freeze and thaw in soils arecontained in AFR 88-19, Vol 1. In addition,Chapter 4, Air Force Manual (AFM) 88-6(TM 5-818-2), contains the cri teria and pro-cedures for design and const ruct ion of pave-ments subject to seasonal frost act ion.

Specific design procedures for frost are dis-cussed in detai l in aggregate-surfaced and

flexible-pavement airfield design sections,pages 12-22 an d 12-35, respectively.

EXPEDIENT-SURFACED AIRFIELDS

Unsurfaced deserts , dry lake beds, and flatvalley floors serve as possible airfield sites.Normally, expedient-surfaced airfields areused for very short periods of t ime (zero tosix months) and support C-130s, C-17s,and Army aircraft operations. Although ex-pedient-surfaced airfields require very littleinit ial construction, they may require exten-sive daily maintenance.

Expedient-surfaced airfields are primarilyused for the movement of t roops and sup-pl ies in the close bat t le and support areas.Only those Army and Air Force aircraft con-figured for expedient surfaces will be al-lowed to use the airfields. The C-130 hasbeen the primary aircraft for missions inthe close bat t le area because i t can land onunpaved or semiprepared surfaces. The C-17, which is used primarily for strategic mo-

bili ty, can also land on austere airfields.Therefore, expedient-surfaced airfields aredesigned for the C-130 or the C-17.

Since the close batt le area is expected tochange quickly, minimal resources shouldbe committed to airfields in this area. Al-though the design l ife of expedient surfaces

ranges from zero to six months (init ial con-struction), the airfield is usually only re-quired from zero to two weeks, unless i t isupgraded to a support area. If a soil willsupport an unsurfaced ai rf ield for the de-sign aircraft , do not surface the airfieldwith matting unless the service l ife becomessignificant. Use the following design proce-dure to determine the expedient surfacetype and its expected service life:

DESIGN STEPS

1. Determine the airfield location.

2. Determine the design ai rcraft and asso-ciated gross weight.

3. Determine the in-place soil strength.

4. Determine the required number ofpasses (service life).

5. Determine the al lowable number ofpasses and surface type .

6. Outline corrective actions to increaseservice l ife as necessary.



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STEP 1. DETERMINE THE AIRFIELD cise determina tion of gradients sh ould b e

LOCATION surveyed by Army engineering teams usintheodolites, auto levels, and Philadelphia

The general area (close batt le , support area) rods .will be given in the mission statement. Inthis case, expedient-surfaced airfields only After a potential airfield site has been se-

occur in the c lose ba t t le and suppor t a reas . lected, i t must be tested to ensure i ts suDetermining the best location should be abil i ty. The reconnaissance leader must

based on a thorough reconnaissance of the first determine the alignment of the airfie

area, if possible. and the loca t ion of the runway, hammer-head turnaround, tax iway, and park ing

S i t e R e c o n n a i s s a n c e apron (if any). Airfield approach zones a

Potential LZ areas fall into three basic cate-must be evaluated for sat isfactory gl ide a

gories:gles. (See Chapter 2, FM 5-430-00-1/ AFPAM 32-80 13, Vol 1.) The criteria m

Existing. Roads, highways, and other dictate the airfield alignment even befor

paved sur faces tha t can be used for soil testing begins.

cargo aircraft (beyond the scope of thischapter) .

STEP 2. DETERMINE THE DESIGNUnsurfaced. Natura l a reas such as de- AIRCRAFTserts, dry lake beds, and flat valleyfloors that may or may not include amembrane (geosynthetic covering thatdoes not contr ibute strength) .

Surfaced. Unsurfaced airfields requir-ing a mat t ing or membrane sur face be-cause of the in-place soil or to increasethe service life of the airfield.

USAF combat control teams (CCTs) aretrained to perform airf ield surveys in sup-port of C-130 an d C-17 aircraft operat ions .CCTs gather al l available data on the air-field and perform site visits to evaluate ap-proach-zone obstruction clearances andweight bearing. CCTs are equipped withhand-held pocket t ransi ts , cl inometers , andlevels to check approach-zone clearance.Airfield and DCPs are used to check weightbearing of unsurfaced LZs. CCTs are notqualified to evaluate deteriorating existingpavements for traffic cycles and weight bear-ing.

Design aircraft , as discussed previously, merely a function of the area, which is dtermined by the mission. The design aircraft for expedient surfaces is the C-130 C-17. When a C-17 is expected, it will bcome the design aircraft . The C-17 has greater load capacity in the close batt le support areas. The gross weights for bothaircraft are shown in Table 12-1 , page 1

STEP 3. DETERMINE THE IN-PLACSOIL STRENGTH

This design step is significant in determing the thickness design and the serviclife. Therefore, it is important that accrate readings are taken from one of theped ien t CBR me thods . Use t hese p rocedures for determining soil s trength for uniform soil , which has equivalent CBRreadings and so i l charac ter i s t ics (At terlimits, gradation) to a depth of 24 inchafter organics and loose granular soi l h

CCTs gather data from an on-si te survey, been removed. Specia l cases of vary ingpresent an LZ survey package, and recom- subgrades are d iscussed la ter in th is c

men d app roval/ disapproval for use of a pro- t e r .

posed airfield. Airlift force commanders atth e Nu m bered Air Force/ Airlift Cont rol Cen - Potential ly soft or dangerous areas shou

ter/ Air Force Special Operat ions Base m ake be tested first. Areas with poor drainage

the final decision. Airfields that require pre- with moist or discolored soil, or where vtation is growing well may indicate a

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problem. Additionally, animal burrow holes,areas prone to flash flooding, previously for-ested areas, and dry lake beds may al l posepotential problems. The airfield may need tobe real igned, taking into considerat ion an

area that will not lend itself to traffic.

Once the initial alignment of the airfield hasbeen decided, determine critical CBR for thesi tes . To do this , test each aspect of the air-f ield to ensure accurate coverage in locatin gpotential problem area. (Appendices I a n d Jdetai l the recomm ended test ing intervals forthe airf ield cone penetrometer and DCP, re-spectively.) Many soils may not be a uniformclassif icat ion throughout the depth concerned(usually 24 inches.) Cases where specific lay-ers have different CBRs pose special concerns

in determining the critical CBR These casesare discussed in detail in following sections.

STEP 4. DETERMINE THE REQUIRED

NUMBER OF PASSES

From the miss ion s ta tement or an es t imateof the si tuat ion, determine the minimumnumber of design aircraft passes that wil laccomplish the mission. Remember, a passis considered one takeoff and one landing.Given the design aircraft, the in-place soilCBR, and the number of required aircraft

passes, you can determine the airf ield sur-face type needed. While unsurfaced air-fields are favorable in minimizing resourcesinvolved in construction, some soils in theirna tura l s ta te cannot suppor t t ra f f ic wi thouta surface. For more information about spe-c i f ic mats and membranes , see Appendix N.

STEP 5. DETERMINE THE ALLOWABLE

NUMBER OF PASSES AND SURFACE

TYPE

The service life is a function of taking the

design aircraf t and the in-place soil CBR en-tering into Figure 12-6 and de termining thenumber of al lowable passes. The surfacetype (unsurfaced, l ight-duty mat, or me-dium-duty mat) is also a variable in deter-mining the al lowable number of passes. I tdoes not direct ly increase the strength of the soil, but a surface does increase a soilsservice life. Determine the surface type bychecking the least resource-intensive

method first. For example, if the intersec-t ion of the soil CBR and the unsurfacedcurve does not meet the requi red number of passes, use the l ight-duty mat curve. Usea medium-duty mat i f the number of re -

quired passes is still not met. If the soilCBR cannot suppor t the requi red numberof passes for any surface type, go to Step 6.

STEP 6. OUTLINE CORRECTIVE

ACTIONS TO INCREASE SERVICE LIFE

After determining the al lowable number of passes , compare i t to the number of passesrequired by the mission or construction direc-tive. There are certain courses of action avail-able to increase the al lowable number of passes. Each course of act ion involves in-

creasing the strength of the in-place soil: (1)compact the in-place soil, (2) stabilize the in-place soil using mechanical, chemical, or geo-synthetic stabilization, or (3) add a basecourse. Each of these methods is discussedin more detai l later in this chapter .

DESIGN EXAMPLES

E x a m p l e 1

Design an airfield for 200 passes in the closebattle area given an in-place soil with AI=13.No C-17s are expected to use the airfield.

N O TE: M ul t ip ly the num ber o f r equ i r edp a s s e s b y t w o t o a c c o u n t f o r t h e a i r c r a f tt a x i i n g d o w n t h e r u n w a y t o t a k e o f f o ru n l o a d .

So lu t ion 1

Step 1. The airfield location is the closebat t le a rea .

Step 2. From Table 12-1 , page 12-4, the de-sign aircraft is a C-130, which has a grossweight of 130 kips.

Step 3. The soil strength is given as an AI.I t can be converted to a CBR value throughFigure 12-1, page 12-2. AI = 13 is equiva-lent to CBR = 14.1.

Step 4. The required number of passes is 200.

Step 5. Determin e the al lowable passesfrom Figure 12-6. Enter the char t wi thCBR = 14.1. Read the number of passes onthe horizontal axis where the CBR inter-sects the C-130 curve for the appropriate

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surface. In this case , the unsurfaced curveexceeds the required number of passes fora CBR = 14.1 soil; therefore, the soil willcarry the 200 required passes .

E x a m p l e 2

Design an airfield for logistics missions of aC-17 in the support area. The in-place soilhas a DCP index of 60. The division AirForce liaison estimates the need for 600passes .

S o l u t i o n 2

Step 1. The airfield is located in the sup-port area (given).

Step 2. Design aircraft is a C-17, whichhas a gross weight of 430 kips in the sup-

port area (Table 12-1 , page 12-4).S t ep 3 . D CP in d e x is 6 0 . F ro m Figure 12-2 , page 2-3, CBR = 3.

Step 4. The required number of passes is600.

Step 5. From Figure 12-6, page 12-11, theintersect ion of the unsurfaced curve andthe soil CBR yeilds zero passes. The onlysurface available for this low CBR is a me-d iu m - d u t y m a t . Th e a llo wa b le n u m b e r of passes for a C-17 weighing 430 kips on amedium-duty treat is 540. Since the allow-able number of passes exceeds the requirednumber , proceed to Step 6 .

S tep 6 . Out line cor rect ive ac t ions to in -crease service life. Since the DCP index re-f lects the soi l s t rength in an undis turbedstate, first determine the DCP index afterseveral passes with a roller suitable to thesoil type. If you improve the index onlyslightly, then you can meet the service lifewith compaction only. Consider stabiliza-t ion or adding a base course as other meth-ods if compaction alone is not enough.

EXPEDIENT AIRFIELD

DESIGNSPECIAL CASES

The previous discussion of soil-strength de-terminat ion was adequate for a soi l thathas a uniform CBR and soil characteristics(Atterberg criteria, gradation) to a depth of24 inches after organics and any loose,granular soil is moved aside. It is possible

to determine a critical CBR for soils withvarying strengths by evaluating each cases e pa r a t ely. Th is c h a n g es t h e m e t h od o f d e -termining the in-place CBR but not the ac-tual design procedure.

S o i l - S t r e n g t h P r o f i l e - I n c r e a s i n g w i t h

D e p t h

If a soil-strength profile increases withdepth, the critical CBR is the average CBRfor the upper 12 inches . Soi l s t rength usu-ally increases with depth so the weakest 12inches are considered critical, and they con-trol the evaluation. If the average CBR ofthe top 12-inch layer yields a CBR thatdoes not meet any surfacing requirement inFigure 12-6, consider stabilizing the sub-

grade. (See Chapter 9, FM 5-410, for de-tails on the type and depth of stabilization.)

E x a m p l e 3

Determine the number of allowable trafficpasses for a C-130 aircraft in the close bat-tle area. Gross weight is 130 kips. Thesoil-strength profile is shown below.



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Solu t ion 3

Step 1. Airfield location is the close battle area.

Step 2. Design aircraft is the C-130, grosswe igh t = 1 3 0 k ip s .

Step 3. From the soil profile, the CBR in-creases with depth . The cr it ica l CBR is an av-erage of the top 12 inches; therefore, CBR =5.5 .

Step 4. The required number of passes isnot given in the mission statement, so go toStep 5.

Step 5. While th e required n umber of passesis not given, use Figure 12-6, page 12-11, todetermine the surface type and al lowablen u m b er of p a s s es . F or a CB R = 5 .5 , a n u n -

surfaced airfield allows only 42 passes. If alight-duty mat is used, however, the servicel ife increases to 5,000 passes (use the largestnumber if i t runs off the scale) . Either sur-face would be correct, depending on the tacti-cal s i tuat ion; but i f t ime and resources exist ,us e the light -duty mat .

Soi l -St rength Prof i le-Very Sof t Layer on

a H ard Layer

Determining the critical CBR on a soil witha very soft layer over a hard layer can besubjective, depending on the AI or CBRvalue and design aircraft weight. A softlayer can be a thin layer in which there isan extreme contrast between the upper fewinches and the lower level.

If a very soft layer is 4 inches thick or less,discard the CBR values from the soft layer,and determine the cri t ical CBR from the 12-inch layer below the soft layer . For unsur-faced airfields, the allowable number of traf-f ic passes may be reduced due to rut t ing inthe top 6 inches, which causes excessivedrag on aircraft during takeoff . This must

be carefully monitored by airfield personnel.The maximum rutt ing depths (Table 12-2)are based on the orientat ion of the ruts aswell as the soil strength.

If the very soft layer is more than 4 inchesthick, the soft layer should be reduced bygrading to at least 4 inches. If you cannotred uce the depth of the soft layer because of t ime or equipment constraints , determine the

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Table 12-2. Maximum rutting depth

critical CBR as an average of the top 12inc h es . The resul t ing low CBR will pre-

scribe matting, which reduces the effects orutting. Generally, the area will not be suable as an airf ield without placing matt ingon the traff ic area or blading the soft material off and waste it, if the equipment isavailable.

Exam ple 4

Determine the surface type needed to sup-port 2,000 passes of a C-130 aircraft in thsuppor t a r ea . Gros s we igh t is 130 k ips ;soil-strength profile is indicated below.



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Solu t ion 4

Step 1. Airfield location = support area.

Step 2. Design aircraft = C-130; grosswe igh t = 1 3 0 k ip s .

Step 3. The soil profile shows a soft layerthat is roughly 4 inches deep. It is fol-lowed by a hard layer. Discard the datafrom the soft layer since the critical AI isan average of the 12-inch layer below thesoft layer. The average AI from the 6.2-inchdepth to the 16.7-inch depth is 7. Theequivalent CBR from Figure 12-1, page12-2, is 5.4.

Step 4. The required number of passes is2 ,000 .

Step 5 . The surface type is a light-duty

mat , capable of supporting 5,000 passes(Figure 12-6, page 12-11).

NOTE: If the CBR was high enough t o

jus t i fy an unsur faced a i r f ie ld , check Ta -

b le 12-2 , p a g e 1 2 - 1 3 , f o r m a x i m u m r u t -

t ing dep th . Ai r f ie ld personne l should

c a r e f u l l y m o n i t o r t h e r u n w a y t o e n s u r e

r u t s d o n o t e x c e e d t h e m a x i m u m .

Hard Layer Over a Sof te r Layer

Some soils may yield a profile that shows ahard layer over a soft layer. This type pro-

file is generally exhibited by a soil that hasa gravel surface over a natural or fill soil,o r by a na tu ra l so i l tha t has a ha rd c rus tin the u pper layer . If the top layer o f so ilis adequate to support the desired aircraftpasses , then the s t rength of the weaker soi llayers beneath the top layer is used tocheck for the critical CBR.

The airfield cone penetrometer cannot beused to determine soil strength in a grav-elly soil, but the DCP can be used. If theDCP is not available, dig a test hole or test

pi t to determine the thickness of the hardlayer.

If the hard layer is less than 4 inchesthick, the hard layer is d iscarded, and thecritical CBR is determined by the averageCBR of the 12-inch layer profile below thehard layer. The number of traffic passes isdetermined as before (Figure 12-6).

If the hard layer is greater than 4 inchesthick, the critical layer is the 12 inches di-rectly beneath the hard layer. If the hardlayer is greater than 12 inches, simply aver-age the CBR values of the 12- and 24-inchlayers.

E x a m p l e 5

Determine the surface type and the numberof allowable traffic passes for a C-130 air-c ra ft in the c lose ba t t le a rea . Gross weigh tis 130 kips. The soil-strength profile yields5 inches of gravel, and the 12-inch soil pro-file below the gravel layer has an averageCBR = 6. The commander indicated thathe needed 60 passes to accomplish the mis-sion.

Solu t ion 5

Steps 1 -3 . Close ba t t le a rea ; C-130 (130kips); CBR = 6 (given).

Step 4. The required number of passes is60 (given).

Step 5. Allowable passes = 70 (Figure 12-6)for an unsurfaced airfield.

E x a m p l e 6

Design an airfield in the support area foruse by both C-130s (130 kips) and C-17s(430 kips) for 1,000 passes. The soil ana-

lysts used a DCP to determine the soilstrength profile below.



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S o l u t i o n 6

Step 1 . Air fie ld loca t ion = c lose ba t t le a rea(given).

Step 2. Although both aircraft will use theairfield, the C-17 is the design aircraft

when both are present .Step 3. The soil profile above shows that asoft layer exists under a hard layer. TheAIs are consistently above 17 until the 18-inch depth, when they drop significantly to10. Since the hard layer is greater than 12inches and the soil is only evaluated to 24inches, calculate the average of the bottom12 inches or the 12.4- inch to the 24.3- inchlayer:

The critical AI is 12.4, which yields a CBR= 13 (Figure 12-1 , page 12-2).

Step 4 . The required number of passes is1,000 (given).

Step 5. From Figure 12-6, page 12-11, anunsurfaced airfield allows 130 passes for asoil CBR = 13. Since this does not meetthe commanders guidance, check other sur-faces. A light- or a medium-duty mat canbe used in this s i tuat ion.

A hard soil layer over a soft layer can usu-ally be found in dry lake beds having ahigh evaporat ion ra te and a high water ta-ble. The upper crust is often 2 to 6 inchesthick, and the soil beneath it generally can-not support an a ircraf t .

Soi l -S t reng th Prof i le Decreas ing wi th

D e p t h

This type profile is similar to a hard layerover a soft layer. Generally, the soil exhibitsa weakening with depth without a verystrong surface layer. This type profile canreadily be seen in areas of dry lake beds orwhere the groundwater can be found close tothe surface. Areas such as these a lso maybe subjected to seasonal fluctuation if thewater table causes the soil profile to change.

Determine the critical CBR for this type pro-file by evaluating various layers to a depthof 24 inches. Determine the profiles criti-

5-430-00-2/AFJPAM 32-8013, Vol

cal CBR by choosing the lowest averageCBR from the following layers: 6-18inches , 8-20 inches , 10-22 inches , and 12-24 inches .

E x a m p l e 7Determine the number of allowable C-130traffic passes on an airfield in the close bat le area , gross weight 130 kip-pounds, anda soil strength profile shown below:

S o l u t i o n 7

Step 1 . Airfield location = close battle are(given).

Step 2. Design aircraft = C-130; grossweight = 130 kips.

Step 3. Determine the in-place soilstrength by calculating averages for the following layers:



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Since the lowest average CBR for the differ-ent layers is 7.2, it is the critical CBR.

Step 4. The required number of passes isnot specified.

Step 5. From Figure 12-6, page 12-11, theallowable number of passes = 180 (unsur-faced) and 5,000 (light-duty mat).

SMOOTHNESS REQUIREMENTS FOR

UNSURFACED AIRFIELDS

While unsurfaced airfields require littlepreparat ion, both the C-130 and the C-17require relatively smooth surfaces for take-off and landing. The overall grades, gradechanges , and s lopes mus t be within the lim-its indicated in Table 11-3, page 11-4. Therandom surface deviations and obstacles al-lowed depend on the s t rength, hardness ,and s ize of i tems that cause roughness .They should not exceed the following limits:

Rocks in traffic areas must be removed,embedded, or interlocked in a mannerthat will preclude displacement whentraversed by aircraf t . Tree s tumps mustbe cut to within 2 inches of the ground.

Dried, cohesive dirt clods (clay ex-cluded) and soil balls (as much as 6

inches in diameter) that will burst upontire impact are allowed. Because hard-ened clay clods may have charac-teristics similar to those of rocks, theymust be pulverized or removed from traf-fic areas.

Contours of dirt patterns are allowed when

they resu lt from p lowing to redu ce erosion,

aid water d rain off, an d prep are th e soil for

planting. Thes e contours contain a s oftcore that does not require removal.

Limitations on rutting are a function ofthe orientation, depth of ruts, and soilbear ing s t rength. Maximum ruts thatcan be traversed safely are shown in Ta-ble 12-2, page 12-13.

Potholes must be filled if they exceed 15inches across their widest point and 6inches deep . Potho les a re c ircu la r o roval and are distinguished from depres-sions by their smaller size and sharp-an-


gled corners. Distance between repairsshould be at least 20 feet apart.

Ditches more than 6 inches deep mustbe eliminated from traffic areas. Whenditches are filled, the bearing strength

must approximate that of the surround-ing s oil.

If it is decided (after final analysis of thesubgrade s t rength) to change the a l ignmentof the airfields, test the new area as re-quired.

Remember , these evaluat ions do not guaran-tee risk-free operation; and evaluations areaffected by airfield condition, weather, anda ir cr a ft u s e. Th e co m m a n d er m u s t k e epthese risks in mind when making decisions

on airfield use.

DESIGN REQUIREMENTS FOR

MEMBRANE- AND MAT-SURFACED

AIRFIELDS

Many high-performance Air Force aircraftcannot operate on the degree of surfaceroughness permit ted by unsurfaced cr i ter ia .Heavy cargo aircraft will rarely operate onunsurfaced airfields because of their sensi-tivity to foreign object damage and soils t rength requirements . Matt ing and mem-branes can alleviate some of these prob-lems. A thorough discussion on mem-bra n e s a n d m a tt in g placement is containedin Appendices L, M, a n d N.

S m o o t h n e s s R e q u i r e m e n t f o r M a t s a n d

Membrane Ai r f ie lds

Membrane-surfaced airfields. Membrane cov-erings are impermeable nylon fabrics thatprotect an airfield from harmful drainage ef-fects and act as a rustproofing agent. Mem-branes are used on both types of expedientsurfaces (unsurfaced and surfaced) with alight- or medium-duty mat. Although themembranes do not increase the s t rength of in-place soil, they may increase the servicelife in many geographic areas. The sur-face smoothness requirements for this air-field category apply to the subgrade sur-face before placement of membrane or to
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an existing unsurfaced airfield where sus-tained operations of the C-130 are ex-pected. Smooth grade the runway and taxi-way to a crown or transverse slope thatmeets design standards. The overall gradesand slopes will not exceed that required forthe unsurfaced airfield either longitudinallyor transversely a t any location on the sur-face of the runway, taxiway. or apron.

Mat-surfaced airfields. Mat surfacing pro-vides a very smooth, well-drained, fine-graded surface free of local depressions orpotholes. Surface smoothness requirementsfor this air-field category apply to the sur-face of subgrade before placement of mem-brane and landing mat. For satisfactoryperformance, the landing mat must be sup-

5-430-00-2/AFJPAM 32-8013, Vo

ported by the subgrade and must not bequired to bridge over depressions or potholes. Prepare a satisfactory surface forthe landing mat by compacting and finegrading to a predetermined grade. Table

12-3 and 12-4, page 12-18, list some mcharac te r is t ic s .

Grade runways and the taxiway to provia crown section or transverse slope thatmeets the design standards. Overall graand slopes must be within the limits givin Table 11-3, page 11-4. Random surfdeviations in grade will not exceed 1 inceither longitudinally or transversely from12-foot straightedge or string line placedany location on the surface of the taxiwrunways, or aprons.

Table 12-3. Characteristics of M19 and ancillary items

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Table 12-4. Mat dimensions

DESIGN IMPROVEMENTS FOR

EXPEDIENT-SURFACED AIRFIELDS

When suitable in-place soils cannot befound to support expedient-surfaced ai r-fields, improve the in-place soil of the de-

sired location as a last resort . The extratime and resources involved in improving in-place soil is minimal when compared to re-configuring missions based on finding asui table subgrade.

The easiest way to increase the allowablenumber of passes i s by compact ing the in-place soil or subgrade. Through compac-tion, soil particles orient themselves in adenser formation, which increases the soilCBR. Compaction will only be effective ifdone for the entire cri t ical layer. For uni-

formly distributed soil profiles, that meansthe top 12 inches. Since most rollers onlycompact to a depth of 6 to 8 inches, scarifyand windrow the top 6 inches to the side inorder to compact the bot tom 6-inch layer.Specific guidelines for the type of roller touse can be found in FM 5-410. After youincrease the soil CBR, go back through thedesign steps to determine the new allowable

number of passes . Depending on the un-compacted CBR and the amount tha t i tchanged by compact ion, the surface type orthe need for a surface al together also mayhave changed.

Normally, compaction wiIl improve thestrength of soils. However, there are somespecial cases where working a soil may actu-ally decrease its strength. Specifically, thefine-grained soils, types CH and OH, canhave high st rengths in an undisturbed con-dition; but scarifying, grading, and compact-ing may reduce thei r shearing resistance.For more information on these soils, seeChapter 8, FM 5-410.

Another way to increase the strength of thesubgrade is through soil stabil ization.

There are many methods of stabil izationavailable to increase soil CBR. The threemajor types of stabilization are stabilizationexpedients (or geosynthetics), mechanicalstabil ization, and chemical stabil ization.Choosing the best one depends on the soi lcharacterist ics as well as available re-sources. Specific information on each typeof stabil izer can be found in Chapter 9,



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FM 5-410. Stabilizing an in-place soil ismost commonly done to increase the soilsCBR, but i t can also be used to negate theharmful effects of dust and water. Table12-5 summarizes the possible functions of

stabilizers in traffic and nontraffic areas onexpedient surfaces.

Dust Cont ro l and Waterproof ing

Much information needs to be developed toform comprehensive criteria for selectingand using dust-control agents and soi l wa-terproofers on expedient airfields. Therea re many poss ib le cho ices . Unt il one o rtwo vastly superior dust-control agents orsoil waterproofers are developed, the engi-neer should be aware of the potentially ac-ceptable systems and some of their charac-

teristics.

Dust Cont ro l . The presence of dust-sizedparticles in a soil surface may not indicatea dust problem. An external force imposedon a ground surface will generate dust.Dust may be generated as a resul t of ero-sion by an aircrafts propeller wash, engineexhaust blas t , je t -blas t impingement , or thedraft of moving aircraft. The kneading andabrading action of tires can loosen particlesfrom the ground surface. These particlesmay become airborne as dust .

On unsurfaced airfields, the source of dusmay be the runway, taxiways, shoulders ,overruns, or parking areas . In areas of open terrain and prevailing winds, soil pacles may be blown in from distant location

and deposited on an airfield. This can cotr ibute to dust potent ia l despi te adequate itial control measures of the soil within thcons t ruc t ion a rea . Where b lowing dus t isproblem, it may be necessary to apply addional dust-control agents to an airfield.

The primary objective of a dust-controlagent is to prevent soil particles from be-c om in g a ir b or n e . Th e s e a ge n t s m a y b eneeded on traffic and nontraffic areas. Ifprefabr icated landing mat , membrane, orconventional pavement surfacing is used the traffic areas of an airfield, dust-controagents are needed only on nontraffic areaThe substance used in these areas must rsist the maximum intensity of air blast impingement of aircraft.

Dust-control agents used for traffic areasmust withstand the abrasion of wheels anb las t impingement . Although du s t -con t roagents may provide resistance against airimpingement , they may be unsui table as wearing surface. An important factor

Table 12-5. Stabilization functions



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limiting the applicability of a dust-controlagent in traffic areas is the extent of sur-face rutting that occurs under traffic. Un-der these conditions, the effectiveness of ashal low dust-control t reatment could be de-

s troyed rapidly by breakup and subsequentstripping from the ground surface. Somedust-control agents will tolerate deforma-t io n s b et t er t h a n o t h er s . No rm a lly, r u t s inexcess of 1/ 2 inch will resu lt in the destr u c-tion of any thin layer or shallow dust-con-trol t reatment .

Waterproofing. Water may enter a soil bythe (1) leaching of precipitation or pondedsurface water, (2) capillary action of underly-ing groundwater, (3) a rise in the water ta-ble, or (4) condensation of water vapor and

the accumulat ion of mois ture under a vapor-impermeable surface.

As a general rule, areas with an existingshallow water table will have a low soil bear-ing s t rength and should be avoided when-ever possible.

The objective of a soil surface waterprooferis to protect soil against water and preserveits strength during wet-weather operations.The use of soil waterproofers is limited totraffic areas except where excessive soften-

ing of nontraffic or limited traffic areassuch as shoulders or overruns must be pre-vented.

Soil waterproofer may prevent soil erosionresulting from surface-water runoff. Likedust-control agents, a thin or shallow soilwaterproofing treatment loses its effective-ness when damaged by excessive rutting.These treatments can be used efficientlyonly in areas that are initially firm.

Many soil waterproofers also function well

as dust-control agents. A single materialmay be used as a t reatment in areas withboth wet- and dry-soil surface conditions.

Materials. Many materials for dust controland soil waterproofing are available. Noone choice, however, can be singled out asacceptable for all problem situations. Tosimplify the discussion, materials are

grouped into five general classifications:Group I, bituminous materials: Group II, ce-menting materials: Group III, resinous andlatex systems; Group IV, salts; and GroupV, miscellaneous materials.

A summary of the var ious mater ia ls and aguide to their applications as a dust-controlagent or soil waterproofer are given in Table12-6. This summary is the best es t imate of the applicability of the materials based onexis t ing in fo rmat ion . Two mate r ia ls in thetable (asphalt penetrative soil binder (APSB)(Peneprime) and polyvinyl acetate dust-con-trol agent (DCA 1295) warrant special men-tion.

APSB (Peneprime) is a special cutback

asphalt having good penetration capabil-ity and rapid curing characteristics.This material is effective in sand, gravel,s i lt , an d lean clay. It is not effect ive inheavy clay or clay with excessive shrink-age or swelling characteristics. Surfaceapplication assures good penetration ingranu la r so ils . In clay , s ilt , and granu -lar soils that are highly compacted, thesurface should be scarified to a shallowdepth before the material is applied.

Compaction should be initiated whenpenetration is complete. In traffic ar-eas , compact ion can be accomplishedby normal traffic. These mater ia ls areeffective in traffic and nontraffic.When the mater ia l is appl ied on un-scarified areas of well-compacted soil,reapplication may be necessary if thetraffic is moderate to heavy.

Polyvinyl acetate (DCA 1295) is an emul-s ion that is appl ied to the surface usinga fiberglass scrim (screen) fabric to rein-force it . This material can be used onall types of soil, and it cures in four

hours or less. This system is applicableto shoulders and overruns and is effec-tive as a waterproofing agent. It will notsupport heavy fixed-wing aircraft traffic.

The following information is provided in Ta -ble 12-6:



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Table 12-6. Dust-control and waterproofing applications

Airfield Pavement Desig


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Column 1 identifies the material.

Column 2 indicates the usual form inwhich the material is supplied.

Column 3 indicates the most acceptablemethod of application. Where a mate-ria l may be applied e ither as an admix-ture or as a surface penetration treat-ment, the preferred and most generallyused method is indicated first.

Column 4 shows applicable soil ranges.The range of soils indicated will nor-mally result in reasonably satisfactoryresults with the particular materia l .Sometimes the materia ls may be usedoutside this range with decreased effec-

tiveness. In general, granular soils(gravel to coarse sand) may or may notrequire treatment for dust control or wa-terproofing, depending on the amounto f fin e s p rese n t . F in e sa n d s (su c h a sdune or windblown sands) will probablyrequire a dust-control treatment butwill not need to be waterproofed. Soilsranging from silty sand to highly plasticclay may require a dust-control agent ora soil waterproofer.

Columns 5, 6 , and 7 show the primary

function of the materials as either a

dust-control agent or soil water-proofer, and where known, therelative degree of effectivenessthat can be expected. Rarely willnontraffic areas require water-

proofing because there is usuallyno need to maintain soil strengthin nontraffic areas. If such a re-quirement exists, materia ls suit-able for traffic areas can be con-sidered acceptable for use in non-traffic areas.

Columns 8 and 9 reflect thequantity requirements applicableto the soil range indicated in col-umn 4. The lower quantity of therange generally is suitable for

coarse soils, and the greaterquantity is needed for fine soils.These quantity requirements aregiven only as a general guide,and in some cases, effective re-sults may be achieved with lesseror g rea te r amounts than thosegiven in the table. (Detailed infor-mation on dust control is in TM5-830-3.)

Column 10 indicates the mini-mum cur ing t ime requ irements .

AGGREGATE-SURFACED AIRFIELDS

While time and resources are limited inthe close battle area, it may be possible tocommit resources in the support area to ag-gregate-surfaced airfie lds. Most a irfie ldsin the support area initially have expedientsurfaces, which may be upgraded to aggre-gate surfaces for susta ined operations.Sometimes a former close battle area is re-designa ted as a suppor t a rea and up-graded to an aggregate surface for ensuingopera t ions .

The design of aggregate-surfaced airfields issimilar to the design of expedient-surfaceda ir fie lds . In aggrega te -surfaced a ir fie lds ,however, a layer of high-quality material isplaced on the compacted subgrade to

improve its strength. The thicknessdesign is a function of the CBR ofthe in-place soil and the design air-craft. Instead of determining thenumber of allowable passes based onthe CBR, use the required number ofpasses to determine the tota l thick-ness design. For a given CBR, thethickness design increases with in-creased number of passes. Normally,aggregate-surfaced airfields are usedfrom one to six months and supportC-17 and C- 130 sorties.

Design the layout of aggregate-sur-faced airfie lds l ike expedient-sur-faced airfie lds. The runway witht u r n a r o u n d s s h o u l d b e c o n s t r u c t e dfirst as shown in Figure 12-3, page



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12-5. As time permits, complete the airfieldlayout according to Figure 12-4, page 12-6.

MATERIALS

Materials used in aggregate airf ields must

meet the requi rements s ta ted in Chapter 5 ,FM 5-430-00 -1/ AFPAM 32-801 3, Vol l, an din the following paragraphs. The materialsshould have grea ter s t rength than the sub-grade and should be placed so the higherquality material is on top of the lower qual-ity material. All layers in an airfield designrequire a minimum layer thickness of 6inches and should conform to the CBR andcompaction cri ter ia shown in Table 12-7.

Table 12-7. Compaction criteria and CBR re-quirements for aggregate-surfaced airfields

(MIL STD 621 method 100 CE 55)

5-430-00-2/AFJPAM 32-8013, Vol I

Subgrade

The in-place soil or subgrade requires moreattention in aggregate-surfaced airfields t ruc tu re s . Befo re deve loping the th i cknes sdesign, determine the compacted CBR ofthe subgrade . Since laboratory CBR testsare impractical for ini t ial construction, usethe penetrometers discussed earl ier .

Determine the soils CBR profile as dis-cussed previously for expedient surfaces.Like road design, the CBR of the subgradedetermines the thickness of the whole de-sign. If you can improve the CBR throughcompac t ion , the th i cknes s of t heaggregate airfield structure will decrease.The depth to which an in-place soil shouldbe compacted is normally 6 inches, but the

depth is determined in the design procedur

Se lec t and Subbase M ate r i a l s

Select and subbase materials used in aggregate airfields provide granular fill to meetthe th ickness des ign based on the subgradCBR. Select materials and subbase coursemust meet the Atterberg l imits and grada-t ion requirements of Table 12-8, which arethe same cri ter ia used for roads.

Base Course

Only good quali ty materials should be usedin base courses of aggregate airfields.Since the base course is also the surfacecourse, it must meet specifications for boths t rength ant i gradat ion . The minimum CB

Table 12-8. Maximum permissible values for subbases and select materials

Airfield Pavement Design 12-


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for an airfield base course is 80 (Table 12-7 , page 12-23). Since CBR tests requiret ime, use one of the base-course materialsshown in Table 12-9, if possible. They arematerials of known strength. If a material

not listed is more easily obtained, use atest s tr ip to determine i ts compacted CBRwith a DCP.

Gradation requirements for aggregate-sur-faced layers are given in Table 12-10, wherethe specif icat ions depend on the maximumsize aggregate (MSA). These are the same gra-dation requirements as given in Chapter 5,FM 5-430 -00-1/ AFPAM 32-80 13, Vol 1, forbase courses for aggregate-surfaced roads.

FROST CONSIDERATIONSAggregate-surfaced airfields, unlike roads orexpedient surfaces, require much more re-str ict ive tolerances in construction and gen-era l main tenance . For th is reason and thepotential for catastrophic accidents in thecase of s tructural fai lure, frost must be con-sidered in the design of aggregate airfields.The specif ic areas where frost has an im-pact on the design are discussed in the fol-lowing paragraphs:

Table 12-9. AssignedCBR ratings for base-

course materials

Table 12-10.rock or slag,

aggregates

Desirable gradation for crushed

and uncrushed sand and gravelfor nonmacadam base courses

As discussed earl ier , three condit ions mustexist for detrimental frost action to occur:(1) the subgrade must be frost susceptible,(2) the temperature must remain belowfreezing for a considerable amount of time,

and (3) an ample supply of groundwatermust be available. Since aggregate-sur-faced airfields have a design life up to sixmonths, the effects of frost may not be rele-vant because of the t ime of year . In anycase, evaluate the frost effects during thedesign process in the event the airfield isneeded for sustained operat ions.

In general, frost-susceptible soils are thosewith considerable amount of fines or withat least 6 percent of materials f iner than0.02-millimeter grain size by weight. You

do not have to relocate or f ind another soilwhen faced with one of these si tuat ions;however, you need to adjust the thicknessdesign to account for the frost act ion.When water in a subgrade freezes, addi-tional water travels by capillary rise and in-creases the ice lense. The ice lenses candisturb the compacted layers enough to cre-ate large voids during the next thaw cycle.



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THICKNESS DESIGN PROCEDURE

The design procedure for aggregate-surfacedairfields in the support area is very similarto expedient-surfaced airfields. The majordifference is that the outcome is the thick-

ness of the aggregate structure, which is afunction of the subgrade CBR, the designCBR, and the number of passes .

S t e p 1 . D e te rm ine the A i r f i e ld Loca t ion

The airfield location is always the supportarea. While aggregate-surfaced airfields aretoo resource intensive for the close battlearea (unless they are existing airfields),they do meet the surface requirements forthe rear a rea .

Step 2 . D e te rm ine the D es ign A i rc ra f t

and G ross W eigh t

The C-17 and C-130 are the only possibili-t ies for design aircraft in the support area.

Aggregate surfaces are considered a semi-prepared surface where only the C-17 andC -1 3 0 ca n la n d . S in c e th e s u p p or t a re a isprimari ly a connector of the rear and closebatt le areas, i t is logical that the design air-craft be able to land in al l three areas. Theaircraft also have the same design weightsfor the suppor t and close batt le areas asshown in Table 12-1 , page 12-4.

5-430-00-2/AFJPAM 32-8013, Vol

Step 3 . C heck So i l s and C ons t ruc t ion A

g r e g a t e s

This design step has three parts: (1) checkthe local area for possible borrow sites tobe used as se lec t mater ia l s and subbases ,

(2) check the strength and gradation of apossible base course, and (3) check thefrost susceptibility of all materials, if necesary.

Check construction aggregates for uas select and subbase materials . This s imilar to road design discussed inChap ter 9, FM 5-430-00-1/ AFPAM 38013, Vol 1. Conduct soi l tests on aborrow sites to determine the soilsCBR, gradation, and l iquid and plaslimits . Compare these values to Tab

12-8 , page 12-23, to determine if theborrow material can be used in a layof the design.

Check the s t rength and gradat ion of thbase course . The base course must haa CBR = 80 or higher and meet the grdation criteria ofTable 12-10.

Check frost susceptibility of materialsIf detrimental frost action (as definedearlier) is a concern, evaluate each lasoil below the base course for frost suceptibility. A soil is frost susceptible

it ha s 6 percent fines a nd/ or 6 perce(by weight) 0.02 millimeter grain size.

For frost design purposes, soils have beedivided into seven groups (Table 12-11 ,page 12-26). Only the nonfrost-suscepti(NFS) group is suitable for base course.Soils are listed in approximate order of dcreasing bearing capability during periodof thaw.

The percentage of fines should be restrictein all the layers to facilitate drainage and

duce the loss of s tabil i ty and strength during thaw periods.

Do not use a soil above the compacted sugrade if it is frost susceptible. For exampa borrow material that meets the cri ter ia a subbase should not be used in the des iif i t has more than 6 percent f iner than0.02 millimeter by weight.



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Table 12-11. Soil classification for frost design

If a subgrade is frost suscept ible, determine S t e p 4 . D e t e r m i n e t h e N u m b e r o f P a s s e si ts frost group from Table 12-11, and findthe frost area soi l support index from Table12-12. This value is needed to adjust thethickness design in Step 8.

Table 12-12. Frost area soil support index

R e q u i r e d

Unlike design for expedient surfaces, youcan control the thickness design by thenumber of passes requi red . As the numberof passes increases, so does the thicknessdesign and, consequent ly, the const ruct ioneffort . You may be given the required num-ber of passes in a mission statement or youcan ad jus t i t based on the th i ckness des ign .

S t ep 5 . De t e rm i n e t h e T ot a l S u rfa c e

T h i c k n e s s a n d C o v e r R e q u i r e m e n t s

The total thickness of the aggregate st ruc-ture i s a funct ion of the subgrade CBR, thedesign ai rcraft , and the number of passes.Since the thickness design is usual ly



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greater than 6 inches ( the minimum layerthickness), multiple soil layers are used.For example, if the thickness required overa subgrade was 18 inches, i t would be ex-pensive and wasteful to fi l l the entire 18inches wi th a high-qual i ty base course. In-stead, use borrow materials to fi l l al l butthe top 6 inches. The CBR of each soilused in the design determines the requiredcover.

5-430-00-2/AFJPAM 32-8013, Vol II

After evaluating the available soils and de-termining the number of passes, enter Fig-ure 12-7 or 12-8 , page 12-28, wi th the sub-grade CBR unt i l i t intersects the grossweight of the design aircraft . Trace a l inehorizontally until you intersect the desired

number of passes and de termine theminimum cover required over the subgradefrom the horizontal axis. Determine theminimum cover required over each soi l thatcould be used in the design.

Figure 12-7. C-130 design curves for gravel-surfaced airfields

Airfield Pavement Design 12-2


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S t e p 6 . C o m p l e t e t h e T e m p e r a t e T h i c k -

ness Design

After finding the cover requirements, youcan complete the thickness design withoutconsidering the effects of frost. The method

is s imilar to road design in that you deter-mine the layer thicknesses that sat isfy theminimum cover requirements for each layer .Remember tha t each layer must be a mini -mum of 6 inches thick. Also, do not usesoil layers in the design if they are not nec-essary to satisfy the cover requirements.For example, if a subgrade only requires 5inches of granular f i l l , then the base courseis the only aggregate layer required. Eventhough you may have de termined tha t sub-bases and select materials are readily avail-able nearby, they are not necessary for the

a ir fie ld des ign . Figure 12-9 illus t ra tes therelat ionship between minimum cover andlayer thickness.

Step 7 . A d jus t Th ickness D es ign fo r

Fros t Suscep t ib i l i t y

If you are not designing in a frost area or ifthe subgrade in a frost area is not frost sus-ceptible (see Step 1), go to Step 9.

Since the freeze-thaw cycles associated withfrost areas weaken soils, you now have toconsider frost and how it will affect thethickness design. Since the subgrade is thonly frost-susceptible material at this point

retrieve the subgrade information fromStep 1.

Determine the fros t-area soil-support indexfrom Table 12-12, page 12-26. Use the in-dex to enter Figure 12-7, page 12-27, or Fure 12-8 instead of the compacted CBR.

For example, for a C-130 airfield, if the copacted subgrade CBR was found to haveCBR = 8 and the subgrade was found to ban F2 type soil, enter Figure 12-7 with CB= 6.5 instead of 8. Since the lower value icreases the thickness design for the samenumber of passes , choose the thicker of thtwo designs.

The frost design will not always increasethe thickness design. For instance, if Step7 indicates a total thickness design of 14inches over a subgrade with a CBR = 3 anthe soil is an F3 soil, use Table 12-12, pag12-26, to determine the soil support indexof 3.5. Since 3.5 is greater than 3, it re-quires thinner design (determined by Figu12-7).

After choosing the thicker of the two de-

signs, you must add a frost f i l ter to the design and adjust the layer thicknesses. Afrost filter is sand or a uniformly graded, hesionless material that al lows the lateralmovement of water . P lace a 4-inch layer drectly on the compacted subgrade and compact it to the specifications outlined inStep 8.

Geotextiles may be used over F3 and F4subgrade materials in seasonal frost areasto help prevent intrusion of fines into baslayers during periods of thaw. The geotex-

t i le should provide at least 110 pounds at10 percent s train when the material istested by the Grab Strength Test (ASTM 5034 and D-5035). If the material exhibits different strengths in perpendicular drect ions , the lowes t va lue is u sed . If lontudina l seams are requi red , they mustmeet the requirements in ASTM D-1683.End overlap at t ransverse joints should

Figure 12-9. Thickness design procedure



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a minimum of 2 feet . The fabric will beplaced di rect ly on the subgrade and mustextend laterally to within 1 foot of the toeof slope on each side. A frost-filter layer isnot required when a geotextile is placed di-rect ly on the compacted subgrade.

S t e p 8 . D e t e r m i n e S u b g r a d e D e p t h a n d

C o m p a c t i o n R e q u i r e m e n t s

The layer thickness of an in-place soil isthe depth to which you must ensure ade-quate compact ion. Determine the depth byentering Table 12-13 wi th the appropriatetraffic area and soil information.

The actual depth of subgrade compact ion isthe difference between the total thickness

above the subgrade and the value from Ta-ble 12-13 or 6 inches, whichever is greater.For example, if the thickness above the co-hesive subgrade (Type B traffic area) is 16inches, then the depth of subgrade compac-tion is 21 inches (Table 12- 13) - 16 inches= 5 inches. However, since 5 inches is lessthan 6 inches, compact to a depth of 6inches. Since the equipment effort for com-pact ion is about the same for depths of 1 to6 inches , t he minimum depth of subgradecompact ion is 6 inches.

Because most road const ruct ion missions re-quire cut -and-fi l l operat ions, the subgradedepth requirement is only significant in cutsections since the soil in fill sections isplaced and compacted in l ifts (usually 6inches). In cut sect ions, however, the sub-grade must be scari f ied and compacted in

Table 12-13. Depth of compaction required

for subgrades

place to the depth required after the cut i sm a d e .

C o m p a c t i o n r e q u i r e m e n t s f o r s u b g r a d eand g ranu l a r l aye r s a r e exp re s sed a s apercent of maximum CE 55 densi ty as de-t e rmined by us ing Mi l i t a ry S tandard (MIL-STD) 621 Test Method 100 (ASTM 1557).The speci f i ca t ions for each l ayer in thedesign are l i s ted in Table 12-7 , page 12-23 .

Remember, there are special cases for sub-grades that lose st rength when being re-molded. These are generally soil types CHand OH. See Chapter 8, FM 5-410, formore informat ion on these soi ls .

Step 9 . Draw the F ina l Des ign Prof i l e

This step is a culminat ion of the pre-vious eight steps into a picture that thebu i l de r can unde r s t and . I t show s t helayer th i cknesses , so i l CBRs, and compac-t i on r equ i r em en t s . I t a l so show s t hecompact ive effort of any fi l l sect ions,which is the same soi l as the subgrade.F igure 12-10 shows the speci f i c de ta i l i n -cluded in the profi le .

1 These are optional layers depending on the materials

available and the thickness design.

2

3

Scarifiy and compact in place.

Total depth of fill.

Figure 12-10. Final design profile



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E x a m p l e 8

Design the taxiways and ends of the run-way (Type B area) for an aggregate-surfacedairfield in the support area (Honduras) for1,000 passes of a C-130. The in-place soil

is a well-graded, sandy clay with a PI = 6,and has 7 percent f iner than 0 .02 mil l ime-ter by weight. Your soils analyst reports auniform CBR = 5. After he set up a tests t r ip , he found that the CBR increased to 7w it h c om p a c t io n . F r om t h e r ec on n a i s s a n c eteams, you have one potential borrow sitewith the following soil characteristics:

Borrow A: GP-GC; CBR = 35, PI = 8, LL= 28; 60 percent passes Number 10 s ieve;15 percent passes Number 200 s ieve.

Base course : Nearby c ivilian ba tch p lan thas been leased by the US; well-graded,crushed limestone available with the fol-lowing gradation specifications:

S o l u t i o n 8

S t ep 1 . Air fie ld l oc a t io n = s u p p o r t a r e a(given).

Step 2 . Design aircraft = C- 130/ 130 k ips(given).

Step 3 . Check construct ion aggregates .

a . Check mater ia ls for use as s elect / su b-

base. Since there is only one potentialsource, check it according to Table 12-9,page 1 2-24. Since PI = 6, the soil does notmeet the Atterberg criteria for a subbase.Therefore, determine whether it meets selectmaterial criteria. Since its LL < 25 and thePI < 12, i t can be used as a select mater ia lCBR = 20.

5-430-00-2/AFJPAM 32-8013, Vol

b. Determine the base course CBR. FromTable 12-10, page 12-24, s ince the basecourse material is a well-graded, crushed agregate (limestone), the CBR = 100.

c. Check materials for frost susceptibility

Since the location of the airfield is Hondu-ras , f rost is not a concern.

Step 4 . Determine the number of passes rquired. Passes required = 1,000 (given).

Step 5 . Determine the tota l surface thick-ness and cover requirements . Using CBRsfor each soil layer that requires cover, enteFigure 12-7, page 12-27, to determine thecover requirements.

Step 6 . Complete the temperate th icknessdesign. Draw a figure to determine thelayer thicknesses based on the cover re-qu i rements .

Calculate the layer th icknesses f rom thesu r face down. F ir s t , look a t the cover re -quired above the select layer. It requires aminimum of 4 inches above it . The basecourse has a layer th ickness of 6 inches b

cause the min imum layer th ickness in anairfield is 6 inches. Next, look at the coverequired above the CBR = 7 subgrade.While 10 inches are required, you alreadyhave 6 inches in the base. Therefore, thesubgrade requires an addi t ional 4 inches ocover. Again, since the minimum layer.th ickness is 6 inches , round the selectlayer th ickness up to 6 inches .

Airfield Pavement Design 12-3


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Step 7. Not applicable since the airfield islocated in a nonfrost area.

Step 8. Determine the subgrade depth an dc om p a ct ion r eq u ir em e n ts . (S ee Table 12-

13 , page 12-30, to f ind the minimum depthof compaction below the surface.) Becausethe subgrade soil has a PI = 6, i t is a cohe-sive soil. For a cohesive soil in a Type Carea, the required depth of compact ion is17 inches below the surface. Since the to-tal thickness design is 12 inches, the actualdepth of subgrade compact ion is 17 - 12 =5 ( rounded up to 6 inches.) The compactionrequirements (from Table 12-7 , page 12-23)for the three layers is shown below:

Step 9 . Draw the fina l des ign profile .

E xam pl e 9

Design a Type B area for an aggregate-sur-faced ai rf ield in northeastern Turkey thatcan wi thstand 10,000 passes of a C-17(gross weight = 430 kips). The area is sub-

jected to seasonal frost conditions (assume

that seasonal frost wi l l occur during the ai r-field service life). Below is a summary ofsoi l and const ruct ion aggregate data.

Sub g rade : C L: P I = 14 : na t u ra l CB R =3: compacted CBR = 5: 7 percent finerthan 0.02 mill imeter by weight.

Borrow A: GP: CBR = 35: PI = 8; LL =28: 10 percent pass Number 80 sieve; 5percent pass Number 200 sieve; NFS.

Borrow B: GW-GC; CBR = 45; PI = 5;

LL = 23; 65 percent pass Number 10sieve; 12 percent pass Number 200sieve; NFS.

B as e c ou r s e : Lim e s ton e ; meet s grada-tion limits for 2-inch MAS (Table 12-10,page 12-24).

So l u t i on 9

S te ps 1 a n d 2 . S u p p or t a re a is t h e on lychoice for aggregate-surfaced airfields; C-17(430 kips) is the design aircraft .

Step 3. Check soi ls and const ruct ion aggre-gates.

a. Check possible subbases and select ma-terials.

Borrow A: Fails as a subbase due to At-terberg cri teria, but meets select mate-rial cri teria. Therefore, i t can be usedas a select material CBR = 20.

Borrow B: Meets cri teria for a subbaseCBR = 50; therefore, use i t as a subbaseCBR = 45.

b. Check st rength and gradat ion of thebase course. Since th e base cou rse is lime-stone, the CBR = 80 (Table 12-9, page 12-24). The soils analyst already checked thegradat ion informat ion and said i t met thespecificat ions.

c. Check for frost susceptibil i ty. No mate-rials above the compacted subgrade arefros t su scep tib le . S ince t he subg rade hasgreater than 6 percent f iner than 0.02 mil l i -meter by weight, i t is frost susceptible.

From Table 12-11 , page 12-26, the soil fallsinto frost group F3. The soil support indexfrom Table 12-12, page 12-26, is 3.5.

Step 4. The number of passes required is10,000 (given).



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Step 5. Determin e the cover requirementsfrom Figure 12-8, page 12-28.

Step 6. Complete the temperate thicknessdesign.

The required cover above the select materialCBR=20 is only 6 inches. Since the basecourse already has a layer thickness of 6

inches, the selects cover requirement is sat-isfied. Therefore, there is no need for thesu bbase layer . The cover requi red over thesubgrade is 23 inches; consequently, the se-lect material must be 23 - 6 = 17 inches.This is the most cost-effective design undernormal condit ions because there are fewerrestr ict ions on select materials than on sub-bases . Keeping a subbase layer would beacceptable if the material is readily avail-able and usable in i ts borrowed or quarrieds ta te .

Step 7. Adjust thickness design for frost.Since the subgrade is a frost-susceptiblesoil (frost group F3) and the area is sub-

jected to frost, the total thickness designmust be derived from the soil-suppor t in-dex, which is 3.5 (Table 12-12, page 12-26).Entering Figure 12-8 with 3.5 yields a mini-mum required cover of 29.2 inches(rounded up to 30 inches.) Since this thick-

5-430-00-2/AFJPAM

ness i s grea ter than the 23

32-8013, Vol

inches requiredfor the temperate design, use this designfor the airfield. Also, you must add a 4-inch frost filter. This changes the thicknesdesign shown below.

Step 8. Determine subgrade dep t h a n dcompaction requirements. From Table 12-13 , page 12-30, the required depth of com-paction below the surface is 21 inches.Since the actual thickness design is greatethan 21 inches , use the minimum depth osubgrade compaction = 6 inches. To findthe compaction requirements for the soil laers , see Table 12-7, page 12-23.

Step 9. Draw the final design profile.

SPECIAL DESIGN CONSIDERATIONS

Stabi l ized Soi l Design

The use of stabilized soil layers for aggre-gate-surfaced pavement structures (as de-scribed in Cha pter 5, FM 5-430-00 -l/ AFP



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32-8013, Vol 1, and FM 5-410) provides theopportuni ty to reduce the overal l thicknessrequired to support a given load. Designingan airfield with stabilized soil layers re-quires the application of equivalency factors

to a layer or layers of a conventionally de-signed st ructure.

To qualify for the application of equivalencyfactors, the stabi l ized layer must meet ap-propriate st rength and durabi l i ty require-m en t s . An equivalency factor representsthe number of inches of a convent ionalbase or subbase tha t can be rep laced by 1inch of stabil ized material . Equivalency fac-tors are determined as shown in Table 9-21, p age 9-76, FM 5-430 -00-l/ AFPAM 32-8013, Vol 1, for bituminous-stabil ized mate-

rials and in Figures 9-55 and 9-56, page 9-76, FM 5-43 0-00-1 / AFPAM 32-801 3, Vol 1,for materials stabil ized with cement, l ime,or fly ash mixed with cement or l ime. Se-lecting an equivalency factor from the tabu-lation depends on the classification of thesoil to be stabilized. Selecting an equiva-lency factor from Figures 9-55 and 9-56 re-quires the unconfined compressive st rength(as determined by ASTM D 1633) be known.Figure 9-55 shows equivalency factors forsubbase materials , and Figure 9-56 showsequivalency factors for base materials.

Minimum thickness . The minimum thick-ness requirement for a stabi l ized base orsubbase i s 6 inches .

Application of equivalency factors. The useof equivalency factors requires that a roador airfield be designed to support the de-sign load conditions. If a stabil ized base orsubbase course is desi red, the thickness ofa convent ional base or subbase is dividedby the equivalency factor for the applicablestabilized soil. (See page 9-77, FM 5-430-00-1/ AFPAM 32-80 13, Vol 1, for exam plesof applying equivalency factors to base andsubbase th i cknesses . )

D r a i nage R equ i r em en t s

Adequate surface drainage should be pro-vided in order to minimize moisture dam-age. Expeditiously removing surface waterreduces the potent ial for absorpt ion and en-

sures more cons i s t en t s t rength and reducedm a in t en a n c e. D ra in a ge , h owe ve r, m u s t b eprovided in a manner to preclude damageto the aggregate-surfaced airfield from ero-sion of fines or the entire surface layer.

Also, ensure the change in the overall drain-age regime, as a resul t of const ruct ion, canbe accommodated by the surrounding topog-raphy wi thout damage to the environmentor to the newly constructed airfield.

The surface geometry of an airfield shouldbe designed to provide drainage at allpoints . Depending on surrounding terrain,surface drainage of the roadway can beachieved by a continual cross slope or by aseries of two or more interconnecting crossslopes. Judgment i s required to arrange

the cross slopes in a manner to removewater from the airfield at the nearest possi-ble points while taking advantage of thenatural surface geometry.

It is also essential to provide adequatedrainage outside the airfield area to accom-modate maximum flow from the area to bedrained. One or more such provisions willbe required if they do not already exist . Ad-di t ional ly, adjacent areas and thei r drainageprovisions should be evaluated to determineif rerouting is needed to prevent water from

other areas flowing across the airfield.

Drainage should be considered a cri t ical fac-tor in aggregate-surface airfield design, con-st ruct ion, and maintenance. Therefore,drainage should be considered before con-st ruct ion and, when necessary, serve as abasis for si te selection.

M a i n t e n a n c e R e q u i r e m e n t s

Environment and surface migrat ion of mate-rials as the result of traffic are the primaryreasons that an aggregate surface requires

frequent maintenance. Also, rainfall andwater running over the aggregate surfacetend to reduce cohesiveness by washing thefines from the surface course. Maintenanceshould be performed at least weekly and, ifrequired, more frequently. Experience withaggregate surfaces indicates that the fre-quency of maintenance is init ially high, butit will decrease over t ime to a constant



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value. Although the design life of an aggre-gate-surfaced airfield is only 6 months, thedecreasing maintenance al lows the designlife to be easily increased for sustained op-erat ions in the support area. Most mainte-nance consists of replacing f ines and grad-ing periodically to remove ruts and potholescreated by passing traff ic and the environ-ment. During the lifetime of the airfield, oc-casionally scarifying the surface layer mightbe required to bring f ines back to the sur-face. Additional aggregate must be addedto restore the thickness, and the wearingsur face must be recompacted to thespecified density. Additional maintenanceinformation is provided in Chapter 8, FM 5-430 -00-1/ AFPAM 32-801 3, Vol 1.

D us t C on t ro lThe primary objective of a dust palliative isto prevent soil particles from becoming air-borne as a result of wind or traffic. Wheredust palliative are considered for traffic ar-

5-430-00-2/AFJPAM 32-8013, Vol

eas , they must wi ths tand the abras ion of wheels and tracks. An important factor l iiting the applicability of the dust palliativein traffic areas is the extent of surface rutt ing or abrasion that occurs under traff ic.Some palliative tolerate deformations bet-ter than others, but normally ruts in exceof 1/ 2 inch wil l resu lt in th e vir tual destrtion of any thin layer or shallow penetratidust-pall iat ive treatment. The abrasive action of aircraft landing gear may be too severe for the use of some dust palliative ina traffic area.

A wide selection of materials for dust con-trol is available to the engineer. No onechoice, however, can be singled out as be-ing the most universally acceptable for all

problem s i tua t ions tha t may be encoun-tered. However, several materials havebeen recommended for use and are dis-cussed in TM 5-830-3.

FLEXIBLE-PAVEMENT AIRFIELDS

Bituminous (flexible)-pavement designs per-mit the maximum use of readily avai

airfield pavement design

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