a procedure for the analysis of benkelman beam …

A PROCEDURE FOR THE ANALYSIS OF BENKELMAN BEAM DEFLECTION BOWLS

E. BARAN, B.Tech.(Civil) , Grad.Dip.App.Hydrogeol., Grad.I.E.Aust., Engineer, (Pavement Testing Services), Pavements Branch, Main Roads Department, Queensland

D.J. ANGELL, B.E. , M.Eng.Sc., M.I.E.Aust., Engineer, (Pavement Design), Pavements Branch, Main Roads Department, Queensland

S.W. EARL, B.Sc. , Computer Systems Officer, Information Services Branch, Main Roads Department, Queensland

S.W. WALSH, B.Sc., Computer Systems Officer, Information Services Branch, Main Roads Department, Queensland

ABSTRACT

In the past back analysis of deflection bowl results, to obtain estimates of pavement and subgrade strength parameters, has involved use of trial and error curve fitting techniques comparing measured rebound bowl shapes with predicted bowl shapes from elastic analysis programs. The US Corps of Engineers developed an iterative program utilising CHEVRON to simplify this procedure. However, difficulties have been experienced in obtaining realistic estimates of pavement and subgrade stiffness particularly when analysing bowls from stiff (eg cement treated) pavements. The cause of this has been that the measured bowl is not a true indication of the actual deflection bowl because of the influence of the bowl on the legs of the Benkelman beam. Attempts to correct the measured bowl for the influence of the actual bowl on the beam's legs have not been totally successful because a knowledge of existing pavement strength properties is required to make an accurate correction and it is these strength properties that are sought from the back analysis. This paper presents a procedure for back analysis of deflection bowls to obtain estimates of pavement and subgrade moduli, which utilises the US Corps of Engineers iterative approach but calling in CIRCLY rather than CHEVRON and using the geometry of a Benkelman beam to correct the computer predicted deflection bowl to what would have been measured by a Benkelman beam. Although this procedure

INTRODUCTION

1. The Benkelman beam, since its development in 1953 at the WASHO (1955) Road Test, has become a standard tool, used by road authorities worldwide, for the non destructive testing of pavements. Although many variations of the Benkelman beam test procedure exist, most procedures generally aim at recording the deflection response during the unloading or rebound of the pavement as the standard test wheel load moves away from the tip of the beam. This rebound deflection level gives an indication of the structural condition of an existing pavement and is the major input into overlay design procedures (Asphalt Institute 1983; NAASRA 1987).

2. Researchers found that the deflection bowl shape, which is an influence line of the change in deflection with distance as the load moves during the test, would yield additional information on pavement condition than would just def l ection alone. The steepness of the

bowl near the position of maximum deflection reflects the stiffness of the pavement base with weak bases having steep slopes while stiff bases have flat slopes. From the deflection recorded at approximately 1 m away from the maximum, an indication of subgrade strength could be obtained. Higher deflections in this region indicate weaker subgrade.

3. With the development of computer programs that could represent pavement structures as layered elastic models, deflection bowls could be analysed to quantify the varying pavement and subgrade strength properties that the different bowl shapes indicated. This back analysis of deflection bowl shapes was first done by Main Roads (Queensland) (MR(Q» in 1979 using a multi-layer linear elastic computer program developed by the Chevron Resea rch Company (Chevron 1963) called CHEVRON.

4. This procedure was essentially a trial and error curve fitting exercise. Various combinations of pavement and subgrade moduli

ACKNOWLEDGEMENT: The authors wish to thank Mr E.I'. Finger, Commissioner of Main Roads, Queensland, for permission to present this paper. The views expressed are those of the authors and not necessarily those of Main Roads Queensland.

PROCEEDINGS 14th ARR B CONFERENCE, PART 6 201

BARAN, ANGELL, EARL, WALSH - BENKELMAN BEAM DEFLECTION BOWLS

were input into the program until the predicted deflection response of the pavement model matched, as near as practical, the measured response. This was a time consuming operation and only considered practical when carried out by experienced operators or where unlimited access to computer time was available. In the latter situation a large number of test runs covering a range of pavement and subgrade moduli increments are run and the output scanned to select the best fit solution.

5. In the late seventies, the US Corps of Engineers had' developed an iterative back analysis program, which could handle up to five layers using a multi-layer Chevron elastic analysis program, that greatly reduced the work involved in deflection bowl modelling. A listing of this program, still in a developmental stage was obtained by the Australian Road Research Board (ARRB) in late 1979 from Mr W. Barker of the US Corps of Engineers (Gray 1979; Bush 1980). This program was made operational by MR(Q) staff, modified slightly and metricated by ARRB staff and the current version, titled CHEVDEF has been in use by MR(Q) since 1983.

6. The main drawback of CHEVDEF was that by using CHEVRON, which assumes that pavement layers are isotropic, it could not accurately predict the anisotropic behaviour of granular and cohesive materials. It was therefore decided to modify CHEVDEF to call in an alternative program to CHEVRON that could handle anisotropic behaviour.

7. The C.S.I.R.O's CIRCLY program (Wardle 1977) was selected for this purpose. A copy was obtained in early 1987, the input and output format simplified and a new program

4200

named CIRCDEF, based on CHEVDEF's iterative procedures but calling in CIRCLY rather than CHEVRON, was developed.

8. Even with these developments, problems still existed with the deflection bowl back analysi s procedure. Past MR (Q) experience, particularly in the analysis of bowls from bound pavements, was that although the procedure gave realistic estimates of base modulus, the predicted subgrade modulus was unreasonably high. These subgrade modulus predictions were up to two times the estimate obtained from insitu measurements.

9. It was found fr om analysis of deflection bowls from bound pavements that the influence of the bowl on the feet of the Benkelman beam, during the test, was significant. On heavily bound cement treated pavements this effect contributed to an almost 50% reduc-tion in the measured Benkelman Beam deflection response and was the cause of the high subgrade strength estimates obtained from back analysis of these bowls. On thin granular pavements this effect was greatly reduced .

10. A procedure was developed to correct for the influence of the' deflection bowl on the beam's legs and incorporated into the CIRCDEF program. This overall bowl analysis procedure aims at making the estimate of pavement and subgrade strength from back analysis of deflection bowls as simple and as accurate as possible. This paper presents this developed procedure, outlines the advantages in using this approach, and suggests the direction for further development and verification.

FI XED LEGS CARRYING HANDLE

BALL BEARING FULCRUM

TIP FOLDING PROBE

I BALL BEARING RACE 250

2440 *" 1220* 1-----.. ------=-~----~~

BODY REAR LEG

(ALUMINIUM CONSTRUCTION)

202

Fig. 1 - Benkleman beam

DIMENSIONS IN MILLIMETRES

*CRITICAL DIMENSIONS

PROCEEDINGS 14th ARRB CONFERENCE, PART 6


DESCRIPTION OF BENKELMAN BEAM BOWLS

11. The Benkelman beam consists basically of a 3.66m long beam pivoted at a point 2.44 m from the tip. (Refer Fig 1.) The pivot is held in place by an external frame supported by a pair of fixed legs, near the pivot point and an adjustable leg at the end furtherest from the tip. A dial gauge is mounted on the external frame and makes contact with the beam at a point 1.22 m from the pivot point. A deflection applied to the tip results in, because of the 2:1 lever arm ratio, half that deflection being recorded on the dial gauge. Figure 1 presents the Road Construction Authority (RCA) (formerly Country Road Board) Design (CRB 1967/69) Benkelman beam which is used by MR(Q). It should be noted that, irrespective of design, all Benkelman beams have the same critical dimensions.

Loading Bowl

12. Generally test procedures involve placing the beam tip some distance between the dual tyres of a single axle loaded to 8.2 tonnes an& recording the deflection response as the test axle is driven away from the tip. By fitting a contact displacement transducer to the beam in the vicinity of the dial gauge and using a rotopulse, driven by the test vehicle's wheel to provide a distance measure, the full deflection response can be measured as a function of distance from the test load. Figure 2 shows the deflection bowl test setup and presents typical bowl shapes obtained from the deflection test procedure, a modified WASHO (1955) procedure, used by MR(Q) (1978).

13. Although a loading phase is involved, the MR(Q) procedure is aimed at measuring the unloading or rebound of the pavement as the test load moves away from the tip. Other

Residual Deflection

Unloading Bowl (Rebound Bowll

Lead (mm)

.Travel (mml _____________________ --..,

Fig. 2 - Deflection bowl setup and typical output

PROCEEDINGS 14th ARRB CONFERENCE, PART 6 203

BARAN , ANGELL, EARL, WALSH - BENKELMAN BEAM DEFLECTION BOWLS

Distance (m)

o 2:3 4 o r-----~----~~----~----~. . __ - -r-.

O·I+--- -=...-'-'----c::;;=-""""=-------

0· 2b-:....~---,,£----

LEGEND - 250/50· -.- 400/100 -- 5000/50

Distance (m)

o 2 :3 4 Or------'-----~~----~------J ----------O· I+-----:,,-'----~......,=-------

0 ·2t---r-~----LEGEND ---250/50 -. -400/ 100 ---5000/50

- - - - - 5<X)()/ 1OO 'lit 250/50-2S0MPoiso MPo Deflection

-Povement / &bQrade Modul i. (rTYTl)O.:>+II-+ _ _

- - - - - 5000/100

TYPICAL CHEVRON GENERATED BONLS 06f--f--- PAVEM£NT DEPTH =280 mm 06

TYPICAL CHEVROO GENERATED BOWLS PAVEMENT DEPTH = lBO mm

0 ·8

O· /:HIN! BASE 0·9

1·1

Fig. 3 - Typical rebound deflection bowls (for various extremes of pavement and subgrade modulus)

procedures such as having the test load approach the tip of the beam, thereby producing a loading rather than a rebound deflection bowl are not generally used in Australia although the Lacriox Deflectograph does operate on this principle.

14. It is Main Roads practice to define a bowl by the deflection level at the point of maximum deflection, designated DO' and at some distances from the maximum on the unloading side of the bowl. These distances are 150 mm, 300 mm, 450 mm, 600 mm, 900 mm and 1200 mm. All bowl deflections are measured from a zero datum defined as the tail end of the bowl as indicated in Figure 2. These seven deflections have been found to be sufficient for defining the bowl shape for analysis by the CHEVRON or CIRCLY program. The use of deflections at additional distances increases computation time with negligible improvement in accuracy.

15. Typical computer generated rebound deflection bowls for bound pavements and unbound granular pavements are presented in Figure 3. It can be seen from these bowls that the steepness of the bowl in the vicinity of the maximum deflection reflects the base modulus while the deflection at approximately 1 m from the maximum deflection refects the subgrade strength. Most importantly, it can be seen that significant deflections are still predicted at distances of 2-4 metres from the test load and it these deflections that influence the legs of the Benkelman beam and affect the measured deflection result .

16. The reason for MR(Q) adopting the modified WASHO (1955) method was the concept that by placing the tip of the beam as far forward of the test load as the fixed legs were back from the load at the start of test, the influence of the bowl on the beams legs was minimal . However, this is not the case as the stylised beam sketches in Figure 4 indicate . Even a rebound (CGRA 1959) test with the test load

204

(0)

(b)

{Dial Gouge LOA~II . ~FUlcrum- _

~~~_T_I:f~'--__ yT't-~Z~\.---Rear

Front Leg Legs

(d) [----~ ' L nun n ::""1 x l x'{ ___ _

(e) ~ef:c~l:n-L- --= - -1 Reference 1: line A~orent y

Deflection Reference Line after Legs Move

Diagrams (a) - (c) show the movement of the Benkelman beam during a typical reb ound test on a Bound pavement. x.y and z denote the displacement of the tip. front legs and rear leg respectively.

In Diagram (d) a displacement of "x " mm at the tip is recorded as an u x /2 " mm displacement at the dial guage . Ho wever , if the legs are moved as indicate d in Diagram (e) then the measured deflection is redu ced consid erably . Fo r the exa mple shown, the apparent deflection .. xl .. is a pprox im ate l y 40 % of t he actual deflection x .

1 Tz

Fig. 4 - Effect of beam leg movement on deflection measurement



moving an infinite distance away from the tip will not result in the actual deflection being recorded by the beam.

17. In the past the effect of the bowl on the beam legs had been discounted because it was considered that the deflections at these distances from the load were very small, if they existed at all, but it was conceeded that they may be significant for pavements with very stiff bases. The former conception has to be discarded. The computer analysis programs used to analyse pavement structures predict significant deflections in the region as Figure ] shows. Also obvious frpm these computer generated bowls is that the deflection in the region > 2.0 m is a function of the subgrade strength and independent of pavement strength or thickness.

18. To see if deflections occur at distances of 3-5 m from the load point as predicted, data obtained by MR(Q) from monitoring of full depth deflection gauges on ARRB experimental sites near Brisbane (Sharp, Baran and Potter 1987) were examined. These gauges, an example of which is given in Figure 5, were installed by ARRB pr'imarily for measurement of deflection under moving loads during the 1984 ARRB Axle Load Study (Sharp, Sweatman and Potter 1986). The gauges being installed in the pavement, record the actual deflection (with respect to an anchor fixed 2-3 metres below the surface) free of the influences that effect a Benkelman beam, ' that occurs as a loaded wheel passes over them.

'" Q) ... ~ E

r<> I

N

DCDT Body

DCDT core

Anchor

Fig, 5 - Insitu full depth deflection gauge


19. Typical data from one of these gauges, installed in a pavement with a granular base and a weak cement treated subbase, are presented in Figure 6. At the time of testing, March 1986, the concept of deflections occurring at distances greater than 3-4 metres from the load was not appreciated and the tests were terminated at approximately 5 metres from the gauge. Figure 6, which presents the results of 10 repeat runs over a single gauge, shaw that the majority of results indicate that the deflection had not stabilised even when the load was 5 m from the gauge.

20. If the rate of change of deflection between 2.5 and 5 m is extropolated out to say 10 m and at this point the deflection assumed to be zero (note CHEVRON still predicts a deflection of 0.01 mm at this point for the appropriate site subgrade conditions), then the deflection at 2 metres from the load, measured from this zero datum, would be approximately 0.06 mm. This deflection, which is dependent on subgrade strength alone, lies between 0.05 m and 0.11 mm, predicted by CHEVRON for subgrade strengths of 100 MPa and 50 MPa respectively, (refer Figure 3) ' and would infer a subgrade strength at time of test of CBR 1-10 which is considered appropriate for the insitu site conditions at the time.

21. Accepting that deflections occur at distances of 3-5 metres from the load and are of the magnitude predicted by elastic analysis programs and supported by results of insitu deflection gauges, then it would be logical to reasOh that the effect on a measured deflection bowl, of the movement of legs of a Benkelman beam during a test, would be greatest in very low maximum deflection situations (Le. where the movement of the legs is a significant proportion of the maximum movement of the beam tip) and least in high deflection &ituations (where the magnitude of the deflection at the tip would swamp any relatively minor movements at the legs.

22. From observing the Benkelman beam deflections being recorded on newly constructed, full depth cement treated base (design base modulus of 5000 MPa) it was apparent that the measured deflection response of these stiff pavements was much lower than that predicted by elastic analysis programs. Even failed cement treated base pavements had no trouble meeting the NAASRA (1987) tolerable deflection criteria. This, together with past experience of back analysis of cement treated base deflection bowls, which generally gave realistic values for base modulus and at least twice the expected subgrade strength, confirmed that, at least on stiff pavements, the Benkelman beams were only measuring part of the actual defle ction that wa s oCf urring.

23. With back analysis of deflection bowls from granular pavements using CHEVDEF, although realisti c estimates of base and subgrade moduli were generally obtained, the fit between the measured and the computed bowls was usually poor, espe cially in the 900 mm - 1200 mm region. This poor fit was attributed both to the anisotropi c behaviour of the granular pavem ent material and to a le s ser part the influenc e of the bowl on the legs of the beam.

205


Distance of Travel from Maximum Def lection in metres

Deflection

o 0 ·1

0 ·2

0 ·3 0-4

(mm) 0·5 0·6

0 ·7 0 ·8 0 ·9 1·0

o 2 3 4 5 6 7 8 9

LEGEND 43-Run Number

Fig. 6 - Typical full depth deflection gauge output for rebound bowl

Distance (metres)

00 0 ·2 OA 06 0-8 1·0 1-2 14 1·6 I·S 20

0·2

OA Deflection

(mm) 0 ·6 -f?O 4OOMPo\'tT!2aomm ~ 250MPo ;r9 .,oJ 100 MPo 50 MPo

:t /STRONGJ /WEAK/

/ THICK BASEl

O+-~--~~~--~~--~~~~

0 ·2

OA

1·0

400MPO ::>:~ 180mm-· --250MPo ~ ~

100 MPo 50 MPo /STROVGJ [WEAK}

/THINBASEJ

IA Legend ISO - Isotropic ANISO - Anisotropic

250 MPo/50 MPo-Povement/Subgrode Modul i

Fig. 7 - Granular pavement bowls

24. It was there f ore apparent, that the procedures adopted in the past by Main Roads for the analysis of deflection bowls, needed to be improved to adequately handle the pavement and materials being tested. The steps adopted were to develop an i terative back analysis procedure based on the CIRCLY elastic analysis program and to include in that procedure a method of allowing for the influence of the bowl on the legs of the beam.

206

ADOPTION OF CIRCLY INSTEAD OF CHEVRON

25. Until r ecent years , most pavement analysi s procedures have adopted an is otropic cha rac terisation of all pavement and subgrade materials. In procedures used to back analyse deflection bowls, the same assumptions of isotropy have normally been made.

26. Analysis of an increas ing amount of research test da ta (NAASRA 1987; Anderson 19 83; Youdale 1984) is suggesting t hat unbound granular paving materials and subgrade materials are better characterised using anisotropic as sumptions about their behaviour .

27. Although acceptance of anisotropy for these materials is not universal and the reason for this apparent behaviour is not fully understood, it is now increas~ngly being adopted by pavement designers for analysis. Specifically, cross-anisot ropy is commonly assumed , with both horizonta l moduli being about half of the vertical modulus (NAASRA 1987) .

28. Whether these unbound materials are in fact anisotropic to this degree or if their stress dependency contributes to this appar ent behaviour is not clear but physically it may be attributable to the fact that the materials are normally laid in layers and compacted ver t ica lly.

29. The CHEVRON computer program previously used by MR(Q) for pavement analysis and back analysis of deflection bowls uses only isotropic characterisation of a ll materials. The CIRCLY program has now been adopted both for pavement design and for back analysis, and t his program is able to use either isotropic or cross anisotropic characterisat ion of materials. This capability is a major reas on for the adoption of the CIRCLY program.



30. CIRCLY has other advantages over CHEVRON as a pavement analysis tool, including its ability to handle:-

a greater range of load types including vertical and horizontal loads and moments about three axes . multiple loads and a range of load distributions over the contact areas.

31 . Although the above are significant advantages for pavement analysis, these full capabilities of CIRCLY are not normally used for back analysis of deflection bowls. The loading used for the back analysis is shown in Figure 8.

107-6 mm

330mm

Uniform Vertical Load distribution over circular contact areas with a Tyre Pressure of 550 kPa .

Fig. 8 - Representation of dual tyre loading for back analysis

32. In the MR(Q) procedure for back analysis, all asphalt and cemented layers ("Treated" layers) are assumed to be isotropic while unbound granular and subgrade materials are assumed to be cross anisotropic with vertical moduli twice the horizontal moduli.

OPERATION OF CIRCDEF

33. CIRCDEF, the CIRCLY based iterative back analysis program reads its required input from a data file set up prior to execution. The user is able, for the initial run, to set parameters by specifying a keyword followed by its appropriate value. In subsequent runs only changes to the parameters, need be specified. Parameters which may be set are presented in Tablel.

34. The program also provides, where applicable, defaults for many of these parameters. The defaults are consistent with the standard MR(Q) Benkelman beam test procedure and deflection bowl presentation formats. Hence only a few of the parameters in Table 1 need to be included in the data set.

35. A flow chart of the CIRCDEF iterative procedure is presented in Figure 9 . A typical output from a CIRCDEF run which includes the correction subroutine is given in Figure 10. The correct subroutine, is discussed in detail in the following section. In brief , the CIRCLY predicted bowl is corrected by the subroutine to simulate a Benkelman beam bowl.

36. The first few lines of output echo the input parameters. Details of the loads, layer system and deflection to be analysed are


Keyword Description

ND Number of deflection points to be input. 2~ND90

NS Number of layers in the pavement system. l~NS..1.8

NL Number of variable layers. l~L..1.4 (Number of layers for which a modulus value is to be calculated).

ILV A list of the layer numbers of the variable layers

RR List of the distances from the dual load centre to the measurement positions.

RRD List of the deflections observed at the points specified.

E List of the start moduli for each layer. These values are used as the moduli for each layer in the initial iteration by CIRCDEF and for subsequent iterations if the layer is not variable.

V List of the value of Poissons ratio for each layer.

HH List of the thicknesses of each layer. (0 may be specified for the last layer to indicate semi-infinite).

LT List of flags indicating each layer's type:

T for treated, G for granular, & S for subgrade.

EMIN List of the minimum modulus value for each layer.

EMAX List of the maximum modulus value for

LS

WGT KPA

TOL

MAXIT

CORECT*

START**

each layer. Distance between each of the two circular loads or wheels. The load at each location (wheel). Pressure applied at the load locations. Tyre pressure is used here for calculation of the load radius. Tolerance of the fit. (Maximum absolute sum of the percentage error in an acceptable solution). Maximum number of iterations to be performed. A flag indicating whether correction of the predicted bowl is required. The distance in front of the load where the first measurement will be taken (in mm).

TRAVEL**The travelling distance of the load in Dml.

SEND This keyword is used to separate data sets if required.

* this calls in the deflection bowl correction sub-routine

** only required with CORECT

TABLE 1 CIRCDEF Input Parameters

followed by the deflect ions predicted by CIRCLY with the initial modulus values. Differences and percentage errors between the "measured" and "predicted " are determined when the correction routine is not used and between "measured" and "corrected' when using the correction routine.

207

208

BARAN, ANGELL, EARL, WALSH -- BENKELMAN BEAM DEFLECTION BOWLS

\----oj initialise

Flow Chart of CIRCDEF

Iterative Procedure

Increrent No

Iteration ~--r----------------<: nurrber

Yes

No

Check input ~or

consistancy

Calculate initial deflections (call CIRCLY and correction r outines . )

OJput predic ted, neasured deflections and percentage errors

Store this as the best solut ion.

Calcul?te a new set of rrxxIulus va lues for the variable

layers . (Hethod obtained fran CHEVDEF

program. )

Yes

(A,tput these rrxxIuli as the

predicted values for the next interation.

Predict deflections for the new rrxxIulus values (call CIRCLY and the

Yes

Store as the best solution

OJtput final and best fit rrxxIulus wi th the

terminating condition .

Fig. 9 - Flow chart of CIRCDEF iterative procedure



TYP I CAL CIReDEr RU N ( TlIIH GRA NULAR PAVEHEtlTJ

NUMBER OF VARIABLE LAYERS

HUHBER Of LAYERS rtf SYSTEM

HUHB!;R Of TARGET DEFLECTIONS

TRA Y[I..LIHG DISTANCE or LOAD IN HH :: 6000 . 0

DISTANCE IN YROHT Of THE WAD Of fiRS T HEASUREl-IEHT (HH, lJflO

ll[fLECTIOH READIHGG IN liM , POf.JTI ONNO : 1 2 3 .. ~ 6 7 DErLECT IOI"S: 0 . 9R4(10 n . sonl) O . ~!>9"n O . J~"{l(l 0 . 24700 0 . 14800 0.10100 WEIGHTING rACTOR: 1 . 005 1 . 181 1.189 2 . 625 4 . 0"" 6 . 157 9 . 90J

DETAILS OF VARIABLE LAYERS

LAYER NO f. YSTEli LAYER NO

VALUE 0F 11AKIHIIH VERTICAL HODIILUS

VALUE OF' HI1IIIiUl1 VERTICAL HODULUS

DETAILS 0' LAYERED SYSTEH

20(10 . 0 2000 . 0

10 . 0 10 . 0

LAYER NO VERTICAL POISSONS RATIO HODULUS

THICKNESS LAYER TYP!:

lln . 0 . 350 1 ~O . 00 CROSS-AfHSOTROFIC 10 . 0 . 350 SEHI-INrINITE CROSS-ANISOTROPIC

DETAILS OF LOADS

LOAD TYPE RADIUS REfEREUCE STRESS

(1) VERTICAL FORCE 101.5867 o . f:t!'OOE+OO

LOAD LOCATIONS

LOAD NO.

POSITION I 2 3

o (l(lOOE"OO O . nooor.t {'I{'I U.3300[+03 O. OUOOE~uU

PREOICTED 1 . 0663~B 0 . 621050 0 . 540395 0 . 383608 0 . 293695 0 . 199286 0 . 150651

CORRECTED HEASUREn 0 . 964140 0.994000 0 . 709004 0 . 841000 0 . 423103 0 . 559000 0,2535480.354000 0.183512 0 . 241000 0 . 113472 0.148000 0 . 018101 0 . 101000

ABSOLUTE SUH : ARITIIHET I C SUH :

AVEnAGE GTRESS

0.5500E"00

DlffEREUCE 0 . 029 260 0 . 131996 0 . 135 297 0.100452 0 . 063428 0 . 034528 0 . 022699 0.523858

LOAO/11Ot1ENT PER LOCATION

0 . 2000';'+05

, DIFf. 2 . 9

16 . J 24 . 2 28. ~ 25 . 1 23 . 3 22 . 1

143.496323 143 . "96323

............. "' ...... t:f •••• t1 t ........ , t. t .... t ...... t, .................... f"4" I .... . ••••• $ ••••••••• 1- ............................................................. .

PREDJCT!D HOOULI AT ITERATION J . 135 . 01 84 . 32

POSITION pnEOtCTEO CORRECTED HEASlIRr.(l DIFrERENCE , OJFf . I 1.126057 0 . 9939 81 O . ~ 9 4(lno 0 . 0000 19 0 . 0 2 0 . 814909 0.160476 0.1'41000 0 . O86~24 10 . 2 3 O. 5B~902 0 . 4601:19 0 . 55 9000 O. 098861 17 . 1 ~ 0 . 06694 0 . 2 '16412 O , 3~4000 0.011fl. 28 21.9 5 0 . 318588 0 . 190881 0 . 2.0000 0 . 048119 19.5 6 0 . 21581B 0 . 122559 O. 14fHlOO 0 . 025441 11 , 2 1 0 . 163301 0 . 0 (1 4-\14 0 . 101000 O. a 16586 16 . ..

A EtSOLUTE Slit! : 0 . 353018 102 . 89 5884 ARITHMETIC SlIH : 102 . 895884

AVERAGE : 0 . 050" 14.6994 ..................................... " ................................. . .. ,., ........................ i- ............. ............................. t" ............ . PREDICTED HODULI AT ITERAT.ION 2 ,

311 , 49 fll.29

POSITIOH PREOICTEO CORRECTED HEASURED 01 Ff[RENCE , our , 1 1.151321 0 . 989401 0 , 994000 0 . 004593 0 . 5 2 O. 919~83 0 . 841621 O. f!41000 -0 . 0006 2 1 -l'. 1 3 0 . 1169!.o2 0 . 563241 O . !>59000 -0 . 004241 -0 8 4 0 . ~2"5(' 5 0 . 36159 0 O . .354000 -0 , 001590 - 2 . 1 & O . 39f1S86 0 . 25212 3 0.241000 -0 . 00 (. 123 - 2 . 3 6 0.26421& 0 . 148706 0 . 146000 -0 . 000106 -0 . !J 1 0 . 198699 0 . 101 2 16 0 , 101000 -0 . 00021 B -0 . 3

AB!:i()LUTE SUM: 0 . 023152 S . 501361 ARITlItiETlC SlI~l : -5 [·83159

AVERAGE: 0 . 0034 0 9296 ....... t .............................. t .... ~t ...... t .............................. . ............... * ....... ~ ....................................................... .

PREDICTED HODULI AT ITERATIOn 3 . 348 . 3& 52 . 01

POSITION 1 2 3 4 5 6 1

PREDICTEO 1. 1 !.o411Q o 91640 2 0 . 1 J IJ2C 0 . 5 11023 0,3 92 1 80 o 26068 5 0 . 196453

CORRECTED HEASURED 0 . 994 0 11 0 . 99400 0 O . 8 ·'62~6 0 . 841000 0 .5~ 1393 ' 0 . 559000 0 . 355018 0 . 3 5 4000 0 . 2 4 82400 . 241000 0 . 1461 50 0 . 14 8000 0.100112 0 . tOl000

ABSOLUTE SUH: ARl THHET I C SUI1 :

AVERAGE :

THE FINAL VERTICAL HODULUS VALUES ARE

LAYER HO BEST FIT

DIFFERENCE -0 . 000071 0.000114 0 . 001601

- 0. 001018 -0 .00 12 40

0 . 0 0 1250 0 , 0006 2 8 0 , 006853

0 . 0010

FINAL MODULI (ITERATIOt~ 3)

348 , 35 52 . 01

DEFLECTI ONS ARE IN TOLERAU CE

.. u" EN D Of PROGRAM ... u

348 . 3 5 52 . 01

Fig. 10 - Typical CIRCDEF output

S DIff . 0 . 0 0 . 1 0 . 3

- 0 . 3 -0 . 5 0. 8 0 . 8

2 . 851298 1 . 2 2903& 0 , 4082


37. From these values CIRCDEF calculates a new set of modulus values for the variable layers and outputs these along with a table of the deflections predicted and their corresponding corrected values by CIRCLY for the new values. This output is produced for each iteration performed until either (i) a suitable solution is found, (ii) the maximum number of iterations is reached or (iii) CIRCDEF detects an inconsistency in the system. When one of these conditions applies, final as well as the best fit moduli are output along with the terminating condition.

OPERATION OF BOWL CORRECTION SUBROUTINE

38. The standard Benkelman beam geometry and dimensions are presented in Figure 1. The beam acts as a lever with a single fulcrum twothirds the ·distance along the lever producing a reading a the gauge equal to half the actual deflection at the tip. There are three points along the beam that come into contact with the pavement surface, the tip and the two sets of legs which support the body of the beam. ·

39. As discussed earlier, the influence of the deflection bowl at the start of test extends past both beam legs. The movements of these legs during the test results in an underestimation, by the standard Benkelman beam test procedure, of the actual deflection that occurs.

40. Correction of the measured deflection for the influence on the legs requires the knowledge of the pavement and subgrade moduli so that the deflections at the legs, can be calculated. However it is these pavement and subgrade moduli that are being estimated by the bowl back analysis procedures and hence any corrections to the measured bowl would be based on assumed moduli of the layers. This type of iterative procedure would be cumbersome and time consuming.

41. The MR(Q) approach was to use the much simpler technique of correcting the CIRCLY "predicted" bowl to what would have been measured by a Benkelman beam under the corresponding pavement and subgrade strength conditions and for the deflection test procedure used in the field. In this situation the moduli of the pavement and subgrade, used for the initial computation are the input start values and for subsequent iterations are the CIRCLY predicted values. The "corrected" CIRCLY bowl is then compared with the "measured" bowl and the iterative procedure continues. A flow chart of this procedure is given in Figure 11. A detailed description of the procedure is given in Table 2.

42. To display the benefits of the new bowl analysis procedure and to determine the factors that effect the relationship of the measured bowl to the actual bowl shape, an analysis was undertaken of typical pavement models and

209

210


INPUT: A number of deflection measurements representing half a bowl (PRED) . Let this number be N.

OUTPUT: N deflection measurements representing half the corrected bowl. Note : N may take any values 1 to 10, inclusive, although MR(Q) use 7.

METHOD: (1) Expand the N deflection points to a number spanning the entire bowl at 50 mm intervals by interpolating curves and straight lines between adjacent deflection points. Selection of curves or straight lines is dependent on inbuilt program test conditions, however, curves are generally used between 0-600 mm and beyond 90 0 mm, while stra ight lines are used between 60 0- 900 mm. A Bezier routine is used to calculate the curves while straight line interpolating calculates the lines. Duplicate thi s half bowl to create a representation of a fu l l bowl.

(2) Using this set of deflections, simulate a Benkleman beam test by using the geometry of the beam and bas ic lever principles to derive a set of corrected deflections as a function of the deflec t ions at the fulcrum and at the gauge. In order to simulate t his test: (a) deflections at the fulcrum must be derived as a function of

the deflections at the legs of the beam.

DO = [(D2 - D3 ) x ~~~~J + D3

(b) deflections at the gauge are derived as a function of the deflections at the legs of the beam. ~hese derivations are done by extrapolating and interpolating, respectively.

Dl = ~D2 - D3 ) x l:~~J + D3

Once the delections at the fulcrum and gauge have been established, these measurements along with the deflections at the beam tip are used as input to the simulated Benkelman beam test to produce a reading at the gauge at every 50 mm. The formula to calcula te this corrected bowl is defined:

Dc = [Pf ; Do] + (Dl - DO)

where Df is the deflection at the beam tip where Dc is the resulting corrected deflection where DO is the deflection at the fulcrum where Dl is the deflection at the gauge where D2 is the deflection of the front leg where D3 is the deflection at the rear leg 1 700 mm is the distance from fulcrum to rear leg 1450 mm is the distance between front and rear legs

480 mm is the distance from guage to rear leg

(3) From this array of corrected deflections, interpolate a set of N deflections at the same displacements along the beam as the original predicted set (PRED) by (a) Finding the maximum deflection and making it the deflection

at displacement 0 mm. Read off N-l other deflections at those positions equal to array PRED.

(b) Calculate average of last ten deflections and make the zero deflection line. Normalize N corrected deflections to this line.

(4) Substitute this corrected bowl for the predicted bowl input , and return control back to circdef.

TABLE 2 Description of Correction Procedure



Bound Pavements

Unbound Pavements

cal culate new rroduli

circly creates theoretical bowl fran rrodul i

theoretical bowl

CORRECTION ROUTINE CORRECTS TI1EORErICAL BCML TAKING TIlE INFUJENCE OF TIlE BEAM' S LEGS

INl'O ACCOUNT

corrected bowl

circdef compares measured bowl with corrected bowl

OK

End of Program

if compar ison is favourabl e return cur rent modul i to circdef , otherwise test a di f ferent set of moduli

Fig. 11 - Flowchart of correction routine's role in CIRCDEF

Subgrade Strengths

508100 MPa

50MPa

/THICK/

Subgrode Strengths

508100 MPa

50MPa

/THINJ

Fig. 12 - Pavement models used in analysis

PROCEEDINGS 14th ARRB CONFERENCE, PART 6 211


DIS TANCE Imm) DISTANCElmm l DISTANCE (mm)

o G 100 300 450 600 900 12CO 00 150 300 450 60D 900 1200 o 0 150 300 450 600 900 I 00

0'1

DEF L ECT ION (rrvn)

0,2

0 '3

00

0'1

DEFL ECTION ( mm)

0 ,2

0'3

0,4

0 ,1

02

DEFLECTION l <rm)

0' 3

0 '5

~p7~ ~z:;... 0 '1

h~@} 0 ,2

~I ~ 5000 MPo ,P':'C 280 mm 5000 MPo~iIJ80mm

!.:.CU 100MPo ICOMPo

0 '3 PredicTed Base Modulus 3900 - 5350 MPo Predicted Bose ModukJs 3000 -4000 r.tPo Predicted $ubQrode Modulus 180-230 MPa Predicfed Suborode Modulus [5 0 - 180 ~

0 ,4

/ STRONG SUBGRADE'

150 300 4 600 900 1200 00 150 300 450 600 900 12 0

~~~ 0 ,'

~~ 0 ,2

01

0'2

0

0 '4

0 ,5

0 '9

/GRANULARJ

250 MPo {{::::} 280 mm

5OMPo

Predicted Base MOdulus 150 - 200 MPo Predicted SUbQrode Modulus 75- 80 MPo

/ WEAK SUBGRADE}

o 0 150 300 450 60D 900 1200

;at. . 5000 WFb ~ 1280 MPo

5OMPo

Predicted Bose Modulus 4250 - 7000 MPo Predicted SubgrOde Modulus 75-165 MPo

5OOOMPo:2i}80 mm

5OMPo

0 ,1

0 ' 2 PredicTed Bose Modulus 4OCX)-48(X)MAJ PredicTed Subgrode Modulus 75-115 MPa

0 '3

/WEAK SUBGRADE/ 0 -4

o 100 300 450 600 900 1200 o 0 ,5

<;;>:5:;:1 ~ ,0>..-0-

/<7 0'1 // /,7 ~

I !GRANULARJ 0

;: !GRANULAR/

4 00 MPo Btl 280mm 0 ,3

~ 100MPo

~ MPo ~I 180mm

""'"'"'"' 100MPo

PredicTed Bose Modulus 250 - 270 MPa Predicted Bose Modulus 175-250 MPo

0 '6

0 ,7

Pred icted SubgrOde Modulus 14 0 -160 MPa Predic Ted Subgrode Modulus 135- 150 MPo

/STRONG SUBGRADE) 1,1

/GRANULARJ

2 5 0 MPc :::::::::::' L l80 mm

50MPo

Predicted Bose Modulus 100 -110 MPo Predicted SUbQrooe tvlodulus 70-75 MPo

/ WEAK SUBGRADEJ

_____ Ac tua l Deflection Bowl Shope

Fig. 13 - Actual deflection bowl shape vs predicted Benkelman beam bowl shape (for various test procedures)

"'" '" "",,",,,Predicted Benkelmon 'O...."~ ~ ~ Beom Bowl Shope

212 PROCEEDINGS 14th ARRB CON FERENCE, PART 6


deflection test procedures. To simplify the analysis procedure only full depth single layer pavements were considered. Two pavement types, a full depth cement treated base and a granular pavement, were selected together with two subgrade strengths . This gave four variations of the basic 2 layer pavement and subgrade model. To assess the effect of varying pavement depth, two pavement thickness , 280 mm and 180 mm, were analysed giving a total of 8 different pavement and subgrade models as indicated in Figure 12.

43. Five deflection test procedures were also incorporated giving a total of 40 variations of pavement models and test procedures. The first three test procedures are described below:

Lead = 1350 mm/travel - 6000 mm. The standard MR(Q} test procedure. Lead = 1350 mm/Travel = 4000 mm. This shortened version of the standard MR(Q} procedure was adopted to assess effect of varing travel distance with a fixed lead. Lead = 1650 mm/Travel = 6000 mm. This situation represents the maximum possible distance that the tip can be placed in front of the load at start of test.

44. The above three test methods are modifications of the basic WASHO (1955) test procedure. The following two methods used in the analysis can be classified as Rebound (CGRA 1959) procedures.

Lead 300 mm/Travel 3300 mm. Current RCA (1987) procedure. Lead 100 mm/Travel 6000 mm. A typical rebound test with the same travel as the MR(Q} procedure.

45 . The eight pavement models were analysed with CIRCLY to determine the deflected shape that would be expected under a standard axle load. The deflection bowl correction subroutine was then used, together with the CIRCLY determined deflected shapes, to predict the shape of the Benkelman beam bowls that would have been measured by the various deflection bowl test procedures. The computed Benkelman bowl shapes were then back analysed using the old procedures to display the results that would have been obtained using analysis procedures that did not include the deflection bowl correction subroutine.

46. These results are presented graphically in Figure 13. The shaded band represents the range of Benkelman beam results that would have been obtained, for the various test procedures, from the 8 pavement types analysed. The differen~e between these and the actual bowl shape used to generate them represents the effect of influence of the bowl on the legs of the beam and the resulting measured deflection.

47 . For all pavement types, the procedures that resulted in the shortest deflection bowl "tail" .(i.e. travel minus lead) resulted in the lowest measure of Benkelman beam deflection although this differenc e is not always significant . Generally the shortened MR(Q}


proc edure (1350/4000) and the RCA rebound procedure (300/3300) gave similar results and defined the upper limit of the shaded band. The standard MR(Q} procedure (1350/6000) the long lead test (1650/6000) and the long rebound test (100/6000) produced s i milar results and generally defined the bottom of the shaded band.

48. From examination of these it can be seen that :

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Bowls measured on stiff pavements have the same shape as the actual bowl but the magnitude of the deflections are markedly lower.

On granular pavements the measured bowl, although closer in magnitude to the maximum reading varies in shape from the actual bowl.

For stiff (bound) pavements on strong subgrades there is little difference between the various test methods. Analysis of bowls in this situation would generally indicate the correct base stiffness but overestimate the subgrade strength by 100%. This is because a Benkelman beam would only measure 40-50% of the actual deflection that was occurring.

With stiff (bound) pavements on weak subgrades, the difference between the test procedures becomes significant. Methods with short deflection bowl "tails" produce the lowest deflection readings. A "tail" length of 4000-5000 mm appears to be optimum and no benefit is to be gained from procedures with longer "tails' (e .g. long rebound test Lead-IOO mm/Travel=6000 mm).

Back analysis of bowls from stiff pavements on weak subgrades generally indicate correct base modulus but overestimate subgrade strength by 50% for procedures with long "tails· to 200% for procedures with short "tails".

On granular pavements only minor differences between the test methods exist. However, the difference in bowl shape and the lower measured deflections results in an underestimation of base modulus by 30-35% and an overestimate of subgrade strength by only 50%.

For thick stiff (bound) pavements the subgrade overestimates average (for the various test procedures) 100% and 140% for strong and weak subgrades respectively with the estimate of base modulus being generally correct.

For thin stiff (bound) pavements the subgrade overestimate reduces to 65% and 95% for strong and weak subgrades respectively but at the expense of a slight underestimation of base modulus.

213


Distance (mm) Distance (mm )

o 0 300 600 9CX) 1200 300 600 9CX) 1200

10

20

30 -Full Depth Deflection 30 LEGEND Deflection

(nrnx 10 ) Deflection Gouge (mmxlO) 08 8 Actual Deflection Bowts 40 Results 4 Measured Oller East Go.Jge

-- - - Corrected 0 .8 tlctua I Deflect ion Bowls 50 Benkleman Beam Measured over West Gouge

Deflection Bowl + Actual Deflection Bowl

60 60 Measured at Intermediate Position

70 D Corrected Deflection 70 Gouge Results

Rooge -East a West Gouges

Fig. 14 - Comparison of measured deflection bowls with corrected insitu deflection gauge results

(i) Reduction in granular base thickness does not effect subgrade strength estimates however underestimat es of base stiffness is further reduced to about 50% of the actual stiffness.

49. These results highlight the need to employ correction routines in the back analysis procedures if calculations of pavement and subgrade moduli are required. It is essential i n the ana lysis of cement treated pavements and thin granular pavements on weak subgrades. Lightly trafficked r ural pavements and urban residential s treets would fit into this latter pavement category.

VERIFICATION OF THE BOWL CORRECTION SUBROUTINE

50 . Verification of the bowl correction procedure requires the measurement of actual pavement deflections with insitu def lection gauges and comparing this response with the results obtained from a Benkelman beam test obtained at the same location and time. Limited data of this type was available from the instrument ed ARRB (Sharp et al 1986 and 1987) sites. Thes e full depth deflection gauge r esults were corrected with the cor rection subroutine and the resulting predicted Benkelman bowl shape compared with the ac tual measured shapes as indicated in Figure 14. The pavement used in this example has a 75 mm Asphalt Surfacing , 150 mm Crushed Rock Base and 200 mm weak cement treated subbase .

51 . The predicted corrected Benkelman beam bowl was simi lar to some the measured bowls . The variation in measured Benkelman beam bowl shape at a given location with repeat testing is highlighted in Figure 14. At the time of wri t ing of this paper ARRB and MR(Q) had just completed a more de tailed asse ssment of the deflection gauge instrumented sites and thi s data is cur rent l y being analysed to provide verification for the developed correction procedures.

214

CONCLUSIONS

52. The Benkleman beam has, since its development in 1953 , become the standard tool used by many road authorities for the nondestructive testing of pavements to obtain an indication of their structural condition and hence determine sui table rehabilitation measures.

53. Wi th the development of elastic analysis computer programs , the i nterpretation of the full deflection bowl became pos sible. Since this back analysis procedure has been in use, it has been observed that the procedure appeared to give erroneous re sult s when analysing bowls from s tiff (cement treated ba se ) pavem ents .

54. A computer analysis of predicted bowl shapes and some limited verification from fu ll depth deflection guages, indicat ed that s i gnificant def lections were occuring at di stanc e s of 3.5 metres from the load and that defl ections would r esult in the Benkl eman beam underestimating the actual deflection that occurred.

55. The percentage underestimation of deflections was greatest where maximum deflection level s were low (i.e. on stiff pavements) . It was found also that the deflections in the region that affected the l egs of the Benkleman beam were d ependant on subgrade strength alone and independant on pavement thickness and type.

56 . This investigation highlighted the need to employ correction routines in the back analysis procedures if calculation of pavement and subgrade modulii is r equired. It was also realised that the current procedures based on elastic analysis programs that assumed isotropic behaviour of pavement and subgrade materials were not the most sui table for modelling t he performance of granular and cohesive ma t erials which exhibit anisotropic behaviour .



57. An interative procedure was developed by the authors to:

allow for anisot ropic behaviour of some pavement materials by utilising CSIRO's CIRCLY program. incorporate a routine into the procedure to correc t for the influence of the bowl on the Benkleman beam legs .

58. Thi s overall bowl analysis procedure, still structured on the iterative procedures developed by the U.S. Corps of Engineers, is aimed at making the estimate of pavement and subgrade moduli as simple and as accurate as possible. The use of this new MR(Q) procedure is considered necessary at least for the analysis of bowls from cement treated pavements and thin granular pavements on weak subgrades.

59. With the current trend towards the use of automated deflection measurement devices such as the Lacriox Deflectograph and the Department of Main Roads, NSW Deflectolab, both of which measure very short deflection bowls, bowl correction of these devices is essential if any valid interpretation of the bowl shapes is intended .

60. It is hoped that the findings of this investigation into the factors influencing deflection bowls and procedure developed for the back analysis should be of interest to all engineers and researchers in the pavement evaluation field. The modification of these procedures to suit Deflectograph and Deflectolab deflection bowls would be a simple exercise and will be developed in due course.

REFERENCES

ANDERSON, D.T. (1983) Anisotropy in Pavement Layers. RCA In~ernal Repor~, Oc~ober .

ASPHALT INSTITUTE (1983) Asphalt Overlays for Highway and Street Rehabilitation. (}'(S-17) June 1983.

BUSH, A.J. (1980) Non Destructive Testing for Light Aircraft Pavements - Phase 2 -Development of the Non Destructive Evaluation Procedure. Repor~ N. FAA-R~90-9-II prepared for US FAA November 1980.


CGRA (1959) The CGRA Benkelman Beam Procedure. Canadian Good Roads Assoc.ia~ion, J'echn1'cal Publica~ion No 12.

CHEVRON (1963) Analysis of Stresses and Displacements in an n-Layered Elastic System under a Load Uniformly Distributed on a Circular Area. Chevron Research Company, Sep~ember 1963.

CRB (1967/69) Benkelman Beam Design Plans. Coun~ry Road Board, }.(a~erials and Research Division.

GRAY, W.J. (1979) W. Barker (US Corps of Engineers) to W.J. Gray (Research Scientist, ARRB) . Personal communic8~1·on.

MR(Q) (1978) Benkelman Beam Deflections Test Method No Q701-1978.

NAASRA (1987) Pavement Design - A Guide to the Structural Design of Road Pavements.

RCA (1987) Deflection Testing by Benkelman Beam. RCA J'es~ }.(e~hod 420.01, J'echnical Bulle~1'n No 33.

SHARP , K.G., BARAN, E AND POTTER, D.W. (1987) Field Trials of Pavement Structures : Construction Report - Queensland. AIR 357-5 July 1987.

SHARP, K.G., SWEATMAN, P.F. AND POTTER, D.W. (1986) Comparative Effects of Dual and Wide Single Tyres on Pavement Response. Proc 13 ARRB Conf. (13:4) Adelaide.

WARDLE (1977) Program Circly - A Computer Program for the Analysis of Multiple Complex Circular Loads on Layered Anisotropic Media. Division of App11'ed Geomechanics, CSIRO.

WASHO (1955) The WASHO Road Test (Method of Test for the Determination of the Load Deflection Characteristics of Flexible Pavements Employing the Benkelman Beam) H1'ghway Research Board, Special Repor~ No 22.

YOUDALE, G. (1984) Pavement Materials. April.

Anisotropy of Granular D}'(R In~ernal Repor~,

215


E.Baran

D.J. Angell

S.w. Earl

S.w. Walsh

216

Ed Baran is the Engineer (Pavement Testing Services) with the Pavements Branch of the Queensland Main Roads Department MR (Q). He joined MR (Q) in 1965 as a cadet draftsman working in the road design and transportation studies areas. Tn 1974, he graduated in Civil Engineering from the Queensland 1 nstitute of Technology and was appointed to the Materials Branch of MR (Q) working in the pavements investigation area. Currently, he is involved in the planning, execution and reporting offield testing associated with pavement investigations and with the development of equipment and procedures associated with field testing of pavements.

David J . Angell is the Pavement Design Engineer with Main Roads, Queensland. After graduation from the University of Queensland with a Bachelor of Engineering in 1972, he joined Main Roads and worked on design of roads and construction of roads and bridges in several parts of Queensland until 1982. During this period he completed a Master of Engineering Science degree at the University of New South Wales in 1974. From 1982 he headed the South Western Queensland Materials Unit until appointment to his present position in Pavements Branch in 1986. He has been working on development of pavement design methods, production of the Main Roads Pavement Design Manual, characterisation of paving materials and research into the behaviour of materials and pavements.

Steven W. Earl graduated in Science from the University of Queensland in 1986 majoring in Computer Science. He started work in 1987 with the Main Roads Department of Queensland as a Computer Systems Officer. Since then he has been involved in writing computer programsfor applications of pavement testing, centred on the stress and strain analysis of multilayered pavements, and pavement design, based on MR (Q) design standard. His other areas of programming include work with the analysis of the state's traffic accident statistics.

Sean W. Walsh is a Computer Systems Officer with the Main Roads Department, Queensland. He joined Main Roads in January 1987 having completed a Bachelor of Science at the University of Queensland in 1986. Since joining Main Roads, he has worked in Information Services Branch developing computer systems for pavement analysis, data archival and communication with traffic counting devices.


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