njta dm section 5 geotechnical design

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NJTA Design Manual Geotechnical Engineering

SECTION 5

GEOTECHNICAL ENGINEERING

Table of Contents

Page No

5.1 INTRODUCTION.........................................................................................................15.1.1 GENERAL ..........................................................................................................1

5.2 GEOTECHNICAL STUDIES.......................................................................................15.2.1 GENERAL ..........................................................................................................15.2.2 SUBSURFACE SOIL PROFILE ..............................................................................25.2.3 SOIL PROPERTIES .............................................................................................25.2.4 DESIGN CONSIDERATIONS .................................................................................4

5.2.4.1 General............................................................................................................ 45.2.4.2 Design Bearing Pressure ................................................................................ 45.2.4.3 Settlement Analysis ......................................................................................... 45.2.4.4 Time-Dependent Consolidation....................................................................... 55.2.4.5 Stability ............................................................................................................ 65.2.4.6 Lateral Earth Pressure..................................................................................... 65.2.4.7 Deep Foundations-Piles ..................................................................................75.2.4.8 Deep Foundations-Drilled Shafts .................................................................... 9

5.2.5 FOUNDATION RECOMMENDATIONS.....................................................................95.2.5.1 Roadways........................................................................................................ 95.2.5.2 Structures ......................................................................................................135.2.5.3 Geotechnical Engineering Reports................................................................14

5.3 CONSTRUCTION CONTROL...................................................................................155.3.1 SETTLEMENT PLATFORMS ...............................................................................165.3.2 PIEZOMETERS ................................................................................................. 165.3.3 SLOPE INDICATORS .........................................................................................165.3.4 OBSERVATION WELLS ..................................................................................... 17

REFERENCES......................................................................................................................17List of Exhibits

Page No

Exhibit 5 - 1 Pile Design Loads Guide..................................................................................18Exhibit 5 - 2 Structure Foundation Recommendation ..........................................................19Exhibit 5 - 3 Typical Excavation Section ..............................................................................20

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SECTION 5

GEOTECHNICAL ENGINEERING

5.1 INTRODUCTION

5.1.1 General

The performance of engineering works (buildings, bridges, embankments,dams, etc.) is affected, to a major degree, by the performance of theirfoundations. It therefore becomes very important that the behavior of thematerials on which the foundation is to be placed be investigated verythoroughly. Adequate knowledge of the behavior of the foundation conditionsleads to a greater degree of confidence in design with consequent savings incost.

This Section is intended to provide all Engineers with an outline of the workand methods expected that relate to geotechnical investigations, analyses,preparation of plans and specifications, and construction monitoring.

The use of this Section is intended to provide a uniform approach to thegeotechnical aspects of Authority projects and the presentation in contractplans and specifications. It also provides an outline of the requireddocumentation of foundation design and related decisions.

5.2 GEOTECHNICAL STUDIES

5.2.1 General

The purpose of the boring and testing programs is to obtain adequateinformation relating to the subsurface conditions and the behavior of the soils,in order to facilitate the design of satisfactory and economic foundations. Theadequacy of the foundation will, in large measure, be reflected by thebehavior of the structure.

The boring and testing programs shall be completed in sufficient time to allowthe Engineer to complete the soil studies and make the foundationrecommendations prior to the Phase B construction plan submittal. Thesesoil studies and foundation recommendations shall be incorporated into aGeotechnical Engineering Report, with all relevant back-up information, andshall be submitted to the Authoritys Engineering Department with the PhaseB submittal for review.

The boring program is designed to provide information from which the soilprofile can be constructed, and the testing program is to provide the soilproperties from which the behavior can be predicted. The analysis of theresults of both of these programs requires a great deal of thought and

judgment on the part of the soils engineer.

In spite of extensive research in the field, soils analysis is still as much an artas it is a science, and there is no substitute for experience and sound

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judgment. In view of this, all soils exploration and analysis is to be performedunder the supervision of, and reviewed by, an Engineer, licensed by the Stateof New Jersey, who has had at least five (5) years experience in the field ofgeotechnical engineering.

5.2.2 Subsurface Soil Profile

A majority of the subsurface soil profiles encountered in nature areconsiderably varied and complex. The simple, uniform soil profile is more theexception than the rule. The borings provide the geotechnical engineer withthe general characteristics of the subsurface materials and, hopefully, thelocation of potential sources of trouble. With this information, the soilsengineer is faced with the task of constructing idealized soil profiles outliningthe boundaries of potential trouble zones. An estimate of performance basedon these profiles can furnish the Engineer with knowledge which would aid inavoiding the undesirable consequences of these potential trouble zones bythe use of appropriate design methods.

Subsurface soil profiles shall be constructed for the mainline roadways andramps under embankments. Profiles shall also be constructed at eachfoundation unit for each structure. The purpose of these subsurface soilprofiles is to ascertain variations in the soil boundaries, which shall be takeninto consideration in design. The soil profiles shall contain StandardPenetration Test (SPT) data, generalized soil descriptions with boundaries,and ground water levels. The soil profile shall be prepared in such a mannerthat it can be included in a Soils Report. The preliminary investigations shallevaluate stability and settlements, their effects on the proposed constructionschedule, the effects of the proposed construction on adjacent structures andembankments, and any other considerations that could affect theperformance of existing or proposed structures. This step in the designprocedure is very important and can lead to the avoidance of problems duringand after construction.

5.2.3 Soil Properties

The laboratory testing program provides the data from which the properties ofthe soils are computed. These properties may be classified as index orclassification properties and behavioral or quantitative properties. The indexproperties are used essentially in classifying the soils and, on minor projects,may be used to predict behavior by comparing them with the behavior ofsimilar soils. Minor projects include sign structures, simple span bridges andnoise barriers. On projects not covered by minor projects or as directed bythe Authoritys Engineering Department, the soil is tested under conditions

similar to those expected during and after construction so that the behavioralproperties under similar conditions may be determined. These properties arethen utilized to predict the behavior of the soils which, in turn, will control theperformance of the foundation.

The soil properties to be used for design are the same as those derived fromthe Subsurface Soil Profile laboratory reports. Therefore, it is important thatthe laboratory testing program be based on conditions similar to thoseexpected in design.

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Soils with high acidic properties are not suitable for the growing of grassesand shrubs. Areas containing these soils shall be identified and delineatedas part of the soils testing program, so that remedial measures may beincorporated into the grading plans.

The purpose of the test is to determine the acidity level of all the varioustypes of soils that will be located within the top three feet of the final ground.The location of the tests will vary with the proposed grading. If acidic soilsare encountered within the top three feet of the final ground, these soils shalleither be removed and replaced with non-acidic soils or shall be treated toneutralize the acidity for the top three feet. The above treatment shall takeplace for soils with pH values of 3.0 or less within the construction limits only.

Areas containing soils, within the top three feet of the final grading, with pHvalues between 3.0 and 6.5, within the construction limits, shall be tabulatedby stations, and this tabulation sent to the Authoritys EngineeringDepartment as early as possible after the tests are completed. A complete

list of all pH tests, listed by Station and Offset, shall be included in the SoilsTesting Report.

As a general guide, the following is suggested:

1. In proposed cut sections, test the jar samples obtained from the boringswithin the three feet below the final ground elevation.

2. In proposed fill sections, the borrow material to be used should be tested.If the material is to be obtained from cut sections, it may be necessary totest all the jar samples within the cut area to ascertain the suitability of thesoils as fill material and if any acidic soils are present. If the borrowmaterial is to be obtained from sources outside the construction limits,

this borrow material should be tested.

It is recommended that any electro-chemical device, such as theChemtrix unit, be used since the readout is obtained directly from ameter, which eliminates a common operator error. The colormetric kits,such as the Sudbury unit, are acceptable; but they are subject tooperator error in color comparison to determine the pH value. The resultsobtained from either device are only as good as the sample used,therefore it is important that the operator choose a good representativesample for all types of material tested.

The hand-held, direct contact instrument is not recommended but may be

used if it is properly calibrated and used. It has the disadvantage in that itcan only obtain spot readings and would need to be checked periodically,probably using prepared standard solutions. A few samples should bechecked using either the electro-chemical or the colormetric unit to verifythe accuracy of the results.

Proposed remedial measures shall be coordinated with the AuthoritysEngineering Department.

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5.2.4 Design Considerations

5.2.4.1 General

This section is intended as a general guide to the proceduresemployed by the Authority in design. Technical design methods shallfollow the current AASHTO LRFD (Load and Resistance Factor

Design) Guide with modifications noted herein. If other methodsobtained from standard text books and technical papers onGeotechnical Engineering are used, those text books and papersshould be referenced.

5.2.4.2 Design Bearing Pressure

The following shall supplement section 10.6.3.1 of the AASHTO LRFDInterim 2006.

Bearing capacity is a function of the shear strength of the soil. Theallowable bearing pressure of granular materials may also beobtained from curves of allowable bearing pressure vs. footing width

for given values of standard penetration. These curves are predicatedon certain settlement criteria which should be compatible with thestructure usage.

It is especially important when using these curves that the soil profilebe determined for a minimum depth equal to twice the width of thefooting. The soils within this depth must have the same or higherpenetration resistance as is used to determine the allowable bearingpressure or detrimental settlements will occur. In addition to thiscriterion, it should also be established that no compressible layers arewithin the zone of influence of the stresses which would consequentlycause settlement of the footing.

The unconfined compression strength used in the bearing capacityequations can be obtained: (a) for minor structures by using indextests and comparing with similar soils and; (b) for other structures byusing unconfined compression laboratory testing of undisturbedsamples.

5.2.4.3 Settlement Analysis

The following shall supplement section 10.6.2.4 of the AASHTO LRFDInterim 2006.

Knowledge of the magnitude of settlement that may occur is essential

in the determination of the performance of the structure. It is,therefore, important that the geotechnical engineer make a settlementanalysis of the foundation soils. When the foundation soil is a loosesand or clay type soil, this analysis can be very critical.

Prior to making the settlement analysis, the overburden pressure andthe intensity and distribution of the applied vertical stresses arecomputed. The applied vertical stresses are computed using theBoussinesq equations which assume that the soil is a perfectly

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homogeneous, elastic and isotropic medium. In addition to theforegoing assumption, the structure is also considered to be perfectlyflexible.

1. Settlement of Clay Soil

The compression index of a clay soil is determined from the logpressure vs. void ratio or log pressure vs. strain consolidationcurves. At least one unload reload cycle shall be performedduring each consolidation test, to determine the preconsolidationpressure and the recompression index. The unload reload cycleshall preferably be performed beyond the preconsolidationpressure, or in the straight portion of the curve. In the calculationof settlement, the log pressure vs. strain or log pressure vs. voidratio curve is to be used directly in the computations, using theunload reload cycle adjusted to the preconsolidation pressure andthe present overburden pressure. This method takes intoconsideration the stress history of the soil and gives settlementvalues that are more realistic.

2. Settlement of Sands

The settlement of sandy soils may be calculated by empiricalmethods based on observations. The Schmartmann Procedurewhich is based on the results of displacement measurementswithin sand masses loaded by model footings, as well as finiteelement analyses and deformations of materials with non-linearstress-strain behavior, may also be used. These methods areonly approximate but may be used to predict the relativemagnitude of settlement under a foundation.

5.2.4.4 Time-Dependent Consolidation

In clays and silts with low permeabilities, the time rate of consolidationbecomes important. The coefficient of vertical consolidation isdetermined from the time rate data obtained during the consolidationtest. The data is plotted using the Taylor Method of square root oftime vs. strain or log of time vs. strain to determine the coefficient orvertical consolidation.

Vertical permeability tests can be performed in conjunction with theconsolidation test at specified load increments. By turning the samplehorizontally and trimming it to fit the consolidometer, the horizontalpermeability may also be obtained in the same manner. In addition to

these, vertical and horizontal permeability tests can also be performedusing the triaxial apparatus with specified chamber pressures. Theseresults are utilized in computing vertical and horizontal coefficients ofconsolidation.

The time rate calculation is based on Terzaghis one dimensionaltheory of consolidation. It is used to predict the time required forsettlement to occur and the necessity of special treatment foraccelerating the consolidation. It can also be used to predict the

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excess pore pressure within the foundation soils. The excess porepressure measured during construction is compared with thepredicted, and may be used to control the rate of placement of theembankments.

5.2.4.5 Stability

Whenever poor foundation conditions are encountered, the stability oftie embankments becomes critical and the safety factor against failuremust be determined. A minimum safety factor of 1.2 is desired duringconstruction. Whenever the safety of existing facilities may beendangered by a failure, this safety factor shall be increased to aminimum of 1.5 or higher. The safety factor may be achieved eitherby using toe berms or relying on shear strength gain fromconsolidation within the foundation soils or a combination of both.This judgment is left to the geotechnical engineer who will consider,among other factors, time scheduling, rate of shear strength gain andeconomy.

The Authority recommends the use of computers for performing thestability analyses. A large number of failure circles can be analyzedby this method in a short period of time, giving the geotechnicalengineer a certain degree of flexibility in his analysis.

5.2.4.6 Lateral Earth Pressure

In the design of earth retaining structures, lateral earth pressures arecomputed either by the Rankine Method, which uses the PlasticEquilibrium Theory, or by the Coulomb Method, which uses theWedge Theory. In both cases, the soil is considered to be at the pointof incipient failure. The Coulomb method shall be used if the soil ispredominantly granular, otherwise the Rankine method shall be used.

A retaining structure must yield at the top, a minimum of 0.001H inorder for the soil to develop the full active pressure condition. Beforespecifying the lateral pressures to be used in design, the type ofstructure that will be used and whether it can yield enough to developthe active pressure condition must be determined. If not, some valueintermediate between the active and at rest condition shall bespecified.

The strain required to develop the full passive pressure is greater thanthat required to develop the active pressure condition. Whenever thepassive pressure is used in design, consideration shall be given to thepossible movement of the structure, and a suitable safety factorapplied. The passive pressure shall be ignored in the calculations ifthe possibility of scour exists.

In designing sheet pile cofferdams, consideration shall be given toseepage pressure that may apply. Leakage between the interlocksusually is not large enough to completely relieve these pressures.

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When designing temporary cofferdams, the above theories shall beused in the sheeting design. However, the struts shall be designedusing the earth pressure distributions given by Terzaghi and Peck inSoil Mechanics in Engineering Practice, Second Edition, Article 48or other methods approved by the Authoritys EngineeringDepartment.

The stability of the cofferdam shall be checked for uneven loading onopposite sides, as for example when the cofferdam is placed at theedge of a river with the embankment higher on one side than on theother side. Consideration shall also be given to constructionconditions that may have an adverse effect on the stability of thecofferdam. An example of this may be excavated material placed onone side of the outside of the cofferdam against the sheeting or liveloading from construction equipment. If the cofferdam is not designedfor this type of loading, it shall be so stated on the plans.

The bottom of an excavation in soft clays shall be checked against

failure by heaving. This condition is particularly critical in deepexcavations that have not been dewatered below the bottom, prior toexcavation. The maximum stability number shall be 4.

5.2.4.7 Deep Foundations-Piles

The following shall supplement sections 10.5.3.3; 10.5.5.2.3; 10.7.1;10.7.2; 10.7.3; 10.7.4; 10.7.5; 10.7.6 of the AASHTO LRFD Interim2006.

A structure is founded on piles if it is determined that the soil belowthe footing does not have adequate bearing capacity, if there are softor loose layers within the zone of influence of stresses from the

footing that may produce excessive settlement, or if a cost estimateindicates that it is more economical to use piles.

Exhibit 5 - 1 provides a guide of alternate pile types and LRFDService I design loads that is based upon past experience on theAuthoritys roadways and other sources. It should be noted thatExhibit 5 - 1 is for reference only and shall not be used for actual piledesign loads. There may be cases where other types of piles ordesign loads would be more appropriate.

Where the preliminary investigations indicate the need for pile or deepfoundations, the recommended type of pile or deep foundation and

AASHTO Group I load should be noted on the Structure FoundationRecommendation form (Exhibit 5 - 2).

Other types of deep foundations that may warrant consideration aredrilled shafts and caissons. The applicability and potential economyof these alternatives will be a function of the structure configurationand subsurface conditions.

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If relatively high pile loads are proposed then load tests should bemade. The cost of these load tests should be compared to thepotential saving by using the proposed higher allowable loads. Steelpile allowable design loads shall be computed on the net crosssectional area after deduction for corrosion losses.

Where conditions of excessively corrosive materials are expected,such as meadow mat, peat, unburned carbon, cinders, or otherchemically active materials which cannot be economically removed,additional protection of the piles may be necessary. Considerationshall also be given to additional protection in the event of nearbyinstallations producing stray currents causing corrosion losses.

Timber piles shall be specified for temporary structures or light loadingconditions where the required length of pile is not too great. Thenatural taper of these piles makes them ideal to be used as shortfriction piles. Where corrosive conditions or marine animals mayattack the wood, the piles shall be pressure treated for protection.

Consideration shall also be given to the use of cast-in-place concretepiles and pipe piles, filled or unfilled. If an economic comparisonindicates that these piles are more advantageous than timber or steelpiles, they shall be used.

Where the rock surface is not fully defined or where friction piles arespecified, a minimum of two test length piles shall be used at eachstructure location. The purpose of these piles is to verify theestimated pile lengths prior to ordering piles. These piles may belocated within the footing area or, nearby, where the subsurfaceconditions are similar. The overburden above the bottom of the

footing elevation shall be removed prior to driving the test length piles.The use of a cleaned out shell through the overburden is not a desiredprocedure and shall only be used under exceptional circumstances,and then only with prior approval of the Authoritys EngineeringDepartment.

When it is desired to verify the design capacity of the piles, a pile loadtest shall be performed either on a test length pile or on another piledriven solely for this purpose.

The pile design may allow for some bending to resist lateral loads inthe foundation. The maximum bending allowed in the piles is a

function of the type of pile (its material properties) and the coefficientof horizontal subgrade reaction of the soils. In no event shall thecombined stresses cause an overstress in the pile after allowing for asuitable safety factor. Batter piles shall be used where the lateralloads exceed the capacity of the piles in bending.

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5.2.4.8 Deep Foundations-Drilled Shafts

The following shall supplement sections 10.5.3.4; 10.5.5.2.4; 10.8 ofthe AASHTO LRFD Interim 2006.

Drilled shafts shall be considered for foundation support when spreadfootings cannot be founded on suitable soil or rock strata within areasonable depth and when piles are not economically feasible due tohigh loads or obstructions to driving. Drilled shafts shall also beconsidered when high lateral loads or uplift loads must be resistedand deformation tolerances are small, or as a direct support elementfor columns used as pier bents. In addition, drilled shafts shall beconsidered for sign structures to be constructed on existing roadwayembankments and cuts.

The minimum diameter of a drilled shaft shall be 36 inches and theminimum center to center spacing of any two drilled shafts shall bethree times the diameter of the shaft.

In addition to the methods outlined in the AASHTO LRFD Interim2006, the procedures in the Federal Highway publication FHWA-IF-99-025 shall be followed.

5.2.5 Foundation Recommendations

Designs for roadways, bridges and buildings are based on the soils andfoundation recommendations of the geotechnical engineer.

The importance of these recommendations cannot be overemphasized, sincethe performance, as well as the economics of the structures, will in largemeasure depend on them.

The geotechnical analysis and foundation recommendations shall becompleted in sufficient time to be incorporated into the Phase B plansubmission. These geotechnical studies and foundation recommendationsshall be incorporated in a Geotechnical Engineering Report, with all relevantback up information, and shall be submitted to the Authoritys EngineeringDepartment for review with the Phase B construction plan submittal.

5.2.5.1 Roadways

When the geotechnical engineer has determined, from anexamination of the subsurface profile, that the foundation soils arecompetent to support the proposed embankment loadings, and that

no compressible layers exist within the zone of influence of thestresses, his task is essentially complete. If there should be a deeplayer of loose sand or clay type soils, and the proposed embankmentstress is greater than the preconsolidation stress, a settlementanalysis shall be made. The Engineer shall determine whether theactual embankment quantity placed shall be measured for payment orwhether the contractor shall be advised to allow for the settlement inhis bid prices. Whatever decision is made should be reflected in theSpecifications.

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Significant sections of the existing New Jersey Turnpike Authorityroadways are underlain by soft, weak, compressible soils whichinclude peats, organic silty clays and varved clays. These areas haverequired special foundation treatment to maintain a stableembankment and minimize roadway settlements. In some areas the

Turnpike roadways are in cuts that extend into clay soils which haverequired underdrains and/or undercutting to maintain stability and agood pavement surface for high speed traffic. These potentialproblems should be investigated and evaluated as part of thepreliminary investigation of embankment foundation and cut areas.

If potential problems of stability or excessive settlement exist for aproposed embankment then methods to overcome these problemsshall be investigated and evaluated. Several foundation treatmentsthat should be considered in such cases are:

1. Excavation and backfill.

2. Toe berms.3. Preload or surcharge.4. Sand drains or wick drains.5. Controlled rate of construction.6. Structure.7. Other method deemed appropriate by the Engineer.

In evaluating these alternate treatments, consideration should begiven to relative stability, expected post construction settlement andtreatment cost.

A number of the above listed foundation treatments require close

control during construction. Where any such methods are proposedfor construction the necessary instrumentation should be included inthe construction contract. Also, instrumentation control criteria shouldbe presented in the Geotechnical Engineering Report. Somecommonly found unsatisfactory foundation areas are outlined below:

1. Surface Marsh and Swamps

The materials within this type of deposit can vary fromundecomposed meadow mat to organic silt. Also the thickness ofthese deposits can vary considerably.

These materials possess very high moisture content, very low

shear strength and extremely high compressibility. In addition to alarge hydrodynamic consolidation, they also possess largesecondary consolidation characteristics due to creep anddecomposition. In general, they are undesirable as foundationmaterials.

Recommended practice is normally to excavate these materials tofirm bottom and backfill the excavation with a select granular

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backfill. Special Subgrade Material, Grade B, Backfill with lessthan 10% passing the #200 sieve is used for underwater backfill.No Compaction below water level is required. Typical excavationsections are shown in Exhibit 5 - 3.

2. Special Man-Made Foundation Problems

Problems for roadway cuts and embankments can be caused by anumber of manmade features, such as uncompacted fills, dumps,sanitary landfills, and old buried foundations. Usually theseconditions cause roadway settlement problems. Often, sanitarylandfills can cause a problem involving methane gas.

If these conditions are encountered, or suspected, then test pitsshould be utilized so that the contents can be disclosed. The testpit logs should give a complete verbal description and includephotographs.

A controlled sanitary landfill is one in which the refuse is placed in

spread lifts not exceeding five feet, with a one or two foot layer ofeither sand or silt placed over each lift and rolled. If it can bedetermined that the sanitary landfill was built under conditionssimilar to those outlined above, it is sometimes possible to placelow embankments, in the magnitude of five feet, on this material.An overload shall be placed above the profile grade and left inplace until most of the settlement has occurred, to precompressthe material. Before constructing the pavement, the overload isremoved and the embankment is proof-rolled using a 50-ton rollerwith minimum tire pressures of 100 pounds per square inch. Softor weaving areas shall be removed and replaced with a selectgranular backfill.

Uncontrolled sanitary landfill is not desirable as a foundationmaterial even for low embankments. These materials shall beexcavated for their entire thickness and replaced with selectgranular materials. Typical excavation sections are shown inExhibit 5 - 3.

In view of the recent emphasis on toxicity, excavation methodsmay be frowned upon because of the disposal problem. Ifsatisfactory methods of disposal cannot be found, the Engineermay be forced to examine alternative methods of design. Onesuch method is Dynamic Deep Compaction where a heavy weight

is dropped, repeatedly, three to eight times at the same spot froma 30- to 120-foot height. In this event, the post-constructionsettlement shall be estimated and the degree of futuremaintenance brought to the Authoritys Engineering Departmentsattention.

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3. Soft Clays and Silts

Clays and silts with low shearing strength make poor foundationmaterial. However, with proper treatment, their shearing strengthcan be increased, thus improving their capacity as a foundationmaterial. Methods commonly used for increasing the shearingstrength of soft clays and silts include overloading or overloadingin combination with wick or sand drains.

These materials have very low permeability and may require asubstantial time period for consolidation to take place. Thecoefficient of consolidation shall be investigated in both thevertical and horizontal directions, and the soil samples shall becarefully inspected for possible horizontal sand or silt layers. All ofthese can be used as aids in reducing the required consolidationtime.

With low embankments, where stability using in-situ undrainedshear strength is not critical, only overloading may be necessary

to consolidate the soft clays and silts. In this case theconstruction schedule must allow enough treatment time after theembankment is placed for complete consolidation to occur.However, the in-situ undrained shear strength may not besufficient for the stability of higher embankments, and shearstrength gain will have to be accelerated during embankmentconstruction. In this event, wick or sand drains are to be used,with or without toe berms and in some cases lightweight fill maybe considered. Overload may be required for completeconsolidation to occur within a reasonable time period.

Embankments placed on stabilized foundation soils may settle

after construction. This post construction settlement may consistof primary and secondary consolidation or secondaryconsolidation only, depending on the available treatment time.The estimated magnitude of this settlement will be computed inadvance and later verified using actual field settlement data.

4. Roadway Cuts

Most problems encountered in roadway cuts involve either, orboth, high ground water conditions and soft subgrade. Two otherproblems less frequently encountered are unstable cut slopes andsloughing of cut slopes. These latter two problems are usually

encountered when clay or silt soils are encountered in highwaycuts. The potential for these problems shall be investigated in allproposed cuts of significant size.

Ground water problems can usually be handled by underdrains. Asoft subgrade can usually be handled by underdrains and/orundercutting and backfilling with granular soil.

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Cut slope stability may require one or more of several potentialtreatments: flattening the slope, control of surface or subsurfacedrainage or benching. Cut slope sloughing is normally caused bysubsurface drainage and is best controlled by controlling thesubsurface drainage and/or benching.

The evaluation of the potential for these roadway cut problemsshould be addressed in the Geotechnical Engineering Report.

5.2.5.2 Structures

A subsurface profile should be drawn for each foundation unit of eachstructure. The foundation recommendations shall be based on thisprofile, unless the geotechnical engineer has sufficient reason to varyfrom it. In this event, the reasons shall be stated with therecommendation.

Major structures, such as bridges and retaining walls shall beinvestigated for stability and settlement. Usually the latter will control.

For proposed normal size structures and conditions utilizing soilbearing foundations the settlement evaluation may be based oncharts correlating settlement with the Standard Penetration Test(SPT) blow per foot on the split spoon sampler. For structuresfounded on bedrock presumptive bearing values from recognizedapplicable building codes may be utilized.

For large span structures or unusual conditions a more detailedevaluation of proposed soil bearing foundations will be required.

Proposed large size box culverts will require at least a preliminaryinvestigation and evaluation. Pipe culverts that are proposed for

embankments that are expected to settle shall be evaluated for theeffects of any differential settlements.

The evaluation of these conditions shall be included in the StructureFoundation Geotechnical Engineering Report along with the StructureFoundation Recommendation form (Exhibit 5 - 2) giving therecommended AASHTO LRFD using Strength Limit States loads.

Allowable bearing capacity is computed by methods previouslyoutlined in this Section. Soil bearing foundations may be consideredunyielding if the allowable bearing pressure is 3.0 tons per square footor greater, provided that there are no compressible layers below the

footing level that will cause unacceptable settlement. The settlementshall be computed so that the structural designer can take it intoconsideration if continuous spans are being considered.

When embankments are placed on stabilized foundation soils, post-construction settlement may occur. This settlement may consist ofprimary and secondary consolidation or secondary consolidation only,depending on the treatment time available. Since the magnitude of

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this post-construction settlement may affect the type of structure to bechosen, it shall be computed and if necessary, later verified usingactual settlement data.

The embankment post-construction settlement will cause an abutment(soil bearing or pile supported) to rotate backwards. (A rule of thumb

used to estimate the amount of lateral movement into theembankment is one-third to one-quarter of the post-constructionembankment vertical settlement.) The abutment shall be designed sothat the bearing shoes can be adjusted for this lateral shift.

Piles for pile supported footings shall be designed according to theprocedures outlined in this Section. The foundation recommendationshall state the type of pile, the design load in tons and the minimumand/or estimated pile tip elevation. Any special treatment required forthe piles (e.g. coating for corrosion protection, creosoting wood piles,pile shoes, special pile tips or points, etc.), shall be stated in thefoundation recommendations in the remarks section.

Piles driven through stabilized foundation soils that are expected tocontinue to settle after construction will experience drag forces due tothe consolidation of these soils. These drag forces can be severe insome cases and may overload piles that have not been designed forthis additional load. The foundation recommendations shall alsostate, in the remarks section, the estimated magnitude of the dragforces, so that the Engineer can take them into consideration.

5.2.5.3 Geotechnical Engineering Reports

Detailed Geotechnical Engineering Report submission guidelines areprovided in Section 5 of the Procedures Manual. The following is a

brief outline of the report requirements.

The Engineer is to prepare a Geotechnical Engineering Report on allthe proposed construction that will be designed. This report is tofacilitate the review of contract plans and specifications and to providea documentation of all potential problems for future reference.

The Geotechnical Engineering Report can consist of one or moreletter reports or formal reports that cover all the proposed constructionfor bridges, sign structures, retaining walls, major culverts, roadwayembankments and roadway cuts, and other significant structures.

Three (3) copies of the Geotechnical Engineering Report are to besubmitted to the Authoritys Engineering Department with the PhaseB plan submittal.

This report should contain the following items:

1. A brief description of the structure or structures.

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2. A brief description of the soils and foundation conditions.

3. A soils profile showing SPT data, ground water elevations, soilstrata and a generalized description of the soils and bedrockencountered.

4. A table of undisturbed soil properties.

5. A brief description of any potential foundation problems and theproposed treatments.

6. Any required criteria necessary for the use of constructioninstrumentation.

7. For all bridges, retaining walls and major culverts documentationof recommended AASHTO LRFD Strength Limit States allowableloads (Exhibit 5 - 1) using the Structure FoundationRecommendation form (Exhibit 5 - 2).

5.3 CONSTRUCTION CONTROL

Control of normal methods of construction is covered by the New Jersey TurnpikeStandard Specifications, the construction contract specifications and is discussed inthe New Jersey Turnpike Construction Manual. The earthwork and structure itemsnoted in these references include compaction, select material gradations, pile loadtests and pile driving.

Construction of embankments on soft foundations often requires treatments thatneed special construction control methods with which most Contractors and FieldEngineers are not familiar. Consequently, the methods of construction andconstruction control have to be clearly stated in the plans, specifications anddirectives to the Field Engineers and Inspectors. The directives necessary for anyspecial control of earthwork construction not included in the contract plans andspecification will be included in the Geotechnical Engineering Report.

Typical methods of embankment foundation treatment that require constructioncontrol instrumentation are preload, surcharge, toe berms, controlled rate ofconstruction and sand drain or wick drain treatments.

The planning of instrumentation for construction control should consider duplicationand location of the instruments because of potential damage due to constructionoperations. The instrumentation equipment and installation should be included in theconstruction plans and specifications. The reading of these instruments andreduction of the data will be the responsibility of the Engineer.

Instrumentation commonly used on Authority Projects includes A) SettlementPlatforms, B) Piezometers, C) Slope Indicators and D) Observation Wells. The needfor other instruments, (e.g., strain gauges and load cells), may only occasionallyarise in special circumstances and will not be discussed here.

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5.3.1 Settlement Platforms

These devices serve two functions. First, in the stabilization of foundationsoils, they provide information on the time rate of consolidation of the soils.The shape of the settlement time curve is used to determine the end ofprimary consolidation, and to predict the rate and magnitude of post

construction settlement. Secondly, the magnitude of settlement at the timethe final survey is made is used to determine the additional quantity ofmaterial placed to compensate for settlement of the subsoils.

The design of the settlement platform shall be as simple as possible tofacilitate installation, maintenance and observation. An initial reading shall betaken as soon as the settlement platform is installed and before anyembankment is placed, and at close enough intervals thereafter to be able todraw a smooth curve through the points. Readings shall be made to thenearest one hundredth of a foot by any satisfactory means.

5.3.2 Piezometers

Piezometers are used to measure pore pressures in the foundation soilsduring consolidation and stabilization. They are used essentially to controlconstruction operations during embankment placement. In order to facilitatethe construction control, the geotechnical engineer shall prepare a chartshowing allowable pore pressure vs. embankment height. This chart shall bebased on stability calculations of the embankment.

There are several types of piezometers available from various manufacturers.The Authority has found the hydraulic heavy liquid piezometer, manufacturedby the Piezometer Research and Development Corporation, to be satisfactoryin response to pressures and for ease of maintenance.

5.3.3 Slope IndicatorsThese devices are essentially early warning devices for measuring horizontaldisplacements in the weak subsoil strata. They are especially useful inmonitoring unstable conditions in cuts or fills.

Essentially, these devices consist of a specially designed aluminum or plasticcasing installed into the ground with wash boring equipment. The bottom ofthe casing is anchored in rock or in a very dense soil stratum. If largesettlements are anticipated, the sections shall be of the telescoping type toprevent buckling.

A specially designed torpedo is inserted into the casing and the angle of

inclination at selected depth is measured. The inclination angles aretranslated into displacement measurements which, by integrating from thebottom to the top produces the displacement profile. The magnitude and rateof displacement are monitored and provide the basis for determining apossible failure condition.

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5.3.4 Observation Wells

These are open perforated pipes placed in the ground to monitor the groundwater level. They shall be used wherever the ground water level is critical toconstruction progress.

REFERENCES

1. American Association of State Highway and Transportation Officials, LRFD, 4th Edition,Interim 2006.

2. American Association of State Highway and Transportation Officials, StandardSpecifications for Highway Bridges, 17th Edition, 2002.

3. Burmister, D.M., Principals and Techniques of Soil Identification, Proceedings of theHighway Research Board, December, 1949.

4. Federal Highway Administration, Drilled Shafts: Construction Procedures and DesignMethods, Publication No. FHWA-IF-99-025, 1999.

5. New Jersey Turnpike Construction Manual.

6. New Jersey Turnpike Standard Specifications.

7. Rutgers University College of Engineering, Engineering Soil Survey of New Jersey,Rutgers University College of Engineering Research Bulletins - Nos. 15 through 36.

8. Terzaghi, K. and Peck, R.B., Soil Mechanics in Engineering Practice, 2nd Edition, 1967.

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EXHIBIT 5 - 1PILE DESIGN LOADS GUIDE

For AASHTO Service 1 Limit State

(Final Pile Designs shall be in accordance with AASHTO LRFD Specifications)

A. Piles driven to bedrock:HP 8x36 45-55 tons (1)

HP 10x42 55-65 tons (1)

HP 12x53 65-80 tons (1)

HP 10x57 80-100 tons (1)

HP 12x74 105-130 tons (1)

HP 12x84 120-150 tons (1)

HP14x102 150-180 tons (1)

HP 14x117 175-205 tons (1)

(1) with-points on pile tips.

10-3/4 pipe w/0.375 wall 50(2) - 70(3) tons

12-3/4 pipe w/0.375 wall 75(2) - 100(3 ) tons

(2) for fc = 3,000 psi

(3) for fc = 4,000 psi

B. Friction Piles

1. Timber Piles (treated or untreated) 20-40 tons

2. Cast-in-Place Piles (3)

12 35-55 tons14 50-75 tons16 60-100 tons

3. Prestressed Concrete Piles (5)

10 Square 30-70 tons12 Square 50-105 tons14 Square 70-140 tons16 Square 90-185 tons18 Square 115-235 tons

(5) for fc= 5,000 psi

4. Steel H PilesHP 10x42 35-55 tonsHP 12x53 45-70 tonsHP 14x73 60-95 tons

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EXHIBIT 5 - 2STRUCTURE FOUNDATION RECOMMENDATION

NEW JERSEY TURNPIKE AUTHORITYJOHN DOE ASSOCIATES

Consulting Engineers

SECTION NO. CONTRACT NO.

STRUCTURE FOUNDATION RECOMMENDATION

Structure No. Job No. By Date

Location

Roadway Over-Under

Lower Rdway. in-on ft. Cut-Fill PG to PG =

Recommended Foundations

Soil Bearing Piles/Drilled Shaft

FootingElev.

DesignLoad

Tip Elev.DesignLoad

FoundationUnit

Ref. BoringNo.

BoringElev.

GroundWaterElev. *

NormalFooting

Elev.

ft. TSF Est. Min.

Type

Tons

Remarks:

* Water Elev. - Indicates Water Table except for borings located in water it indicates Mean Low Water for area.

Design Load - for AASHO LRFD Strength Limit States. Form N23-A - Rev. 2007

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EXHIBIT 5 - 3TYPICAL EXCAVATION SECTION

njta dm section 5 geotechnical design

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