SECTION 5 GEOTECHNICAL ENGINEERING

Transcription

1 SECTION 5 GEOTECHNICAL ENGINEERING Table of Contents Page No 5.1 INTRODUCTION GENERAL GEOTECHNICAL STUDIES GENERAL SUBSURFACE SOIL PROFILE SOIL PROPERTIES DESIGN CONSIDERATIONS General Design Bearing Pressure Settlement Analysis Time-Dependent Consolidation Stability Lateral Earth Pressure Deep Foundations-Piles Deep Foundations-Drilled Shafts FOUNDATION RECOMMENDATIONS Roadways Structures Reports CONSTRUCTION CONTROL SETTLEMENT PLATFORMS PIEZOMETERS SLOPE INDICATORS OBSERVATION WELLS REFERENCES List of Exhibits Page No Exhibit 5-1 Pile Design Loads Guide...18 Exhibit 5-2 Structure Foundation Recommendation...19 Exhibit 5-3 Typical Excavation Section...20 MAY 2007

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3 5.1 INTRODUCTION General SECTION 5 GEOTECHNICAL ENGINEERING The performance of engineering works (buildings, bridges, embankments, dams, etc.) is affected, to a major degree, by the performance of their foundations. It therefore becomes very important that the behavior of the materials on which the foundation is to be placed be investigated very thoroughly. Adequate knowledge of the behavior of the foundation conditions leads to a greater degree of confidence in design with consequent savings in cost. This Section is intended to provide all Engineers with an outline of the work and 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 the geotechnical aspects of Authority projects and the presentation in contract plans and specifications. It also provides an outline of the required documentation of foundation design and related decisions. 5.2 GEOTECHNICAL STUDIES General The purpose of the boring and testing programs is to obtain adequate information relating to the subsurface conditions and the behavior of the soils, in order to facilitate the design of satisfactory and economic foundations. The adequacy of the foundation will, in large measure, be reflected by the behavior of the structure. The boring and testing programs shall be completed in sufficient time to allow the Engineer to complete the soil studies and make the foundation recommendations prior to the Phase B construction plan submittal. These soil studies and foundation recommendations shall be incorporated into a Report, with all relevant back-up information, and shall be submitted to the Authority s Engineering Department with the Phase B submittal for review. The boring program is designed to provide information from which the soil profile can be constructed, and the testing program is to provide the soil properties from which the behavior can be predicted. The analysis of the results 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 art as it is a science, and there is no substitute for experience and sound MAY

4 judgment. In view of this, all soils exploration and analysis is to be performed under the supervision of, and reviewed by, an Engineer, licensed by the State of New Jersey, who has had at least five (5) years experience in the field of geotechnical engineering Subsurface Soil Profile A majority of the subsurface soil profiles encountered in nature are considerably varied and complex. The simple, uniform soil profile is more the exception than the rule. The borings provide the geotechnical engineer with the general characteristics of the subsurface materials and, hopefully, the location of potential sources of trouble. With this information, the soils engineer is faced with the task of constructing idealized soil profiles outlining the boundaries of potential trouble zones. An estimate of performance based on these profiles can furnish the Engineer with knowledge which would aid in avoiding the undesirable consequences of these potential trouble zones by the use of appropriate design methods. Subsurface soil profiles shall be constructed for the mainline roadways and ramps under embankments. Profiles shall also be constructed at each foundation unit for each structure. The purpose of these subsurface soil profiles is to ascertain variations in the soil boundaries, which shall be taken into consideration in design. The soil profiles shall contain Standard Penetration Test (SPT) data, generalized soil descriptions with boundaries, and ground water levels. The soil profile shall be prepared in such a manner that it can be included in a Soils Report. The preliminary investigations shall evaluate stability and settlements, their effects on the proposed construction schedule, the effects of the proposed construction on adjacent structures and embankments, and any other considerations that could affect the performance of existing or proposed structures. This step in the design procedure is very important and can lead to the avoidance of problems during and after construction Soil Properties The laboratory testing program provides the data from which the properties of the soils are computed. These properties may be classified as index or classification properties and behavioral or quantitative properties. The index properties are used essentially in classifying the soils and, on minor projects, may be used to predict behavior by comparing them with the behavior of similar soils. Minor projects include sign structures, simple span bridges and noise barriers. On projects not covered by minor projects or as directed by the Authority s Engineering Department, the soil is tested under conditions similar to those expected during and after construction so that the behavioral properties under similar conditions may be determined. These properties are then utilized to predict the behavior of the soils which, in turn, will control the performance of the foundation. The soil properties to be used for design are the same as those derived from the Subsurface Soil Profile laboratory reports. Therefore, it is important that the laboratory testing program be based on conditions similar to those expected in design. MAY

5 Soils with high acidic properties are not suitable for the growing of grasses and shrubs. Areas containing these soils shall be identified and delineated as part of the soils testing program, so that remedial measures may be incorporated into the grading plans. The purpose of the test is to determine the acidity level of all the various types 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 soils are encountered within the top three feet of the final ground, these soils shall either be removed and replaced with non-acidic soils or shall be treated to neutralize the acidity for the top three feet. The above treatment shall take place 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 ph values between 3.0 and 6.5, within the construction limits, shall be tabulated by stations, and this tabulation sent to the Authority s Engineering Department 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 Soils Testing Report. As a general guide, the following is suggested: 1. In proposed cut sections, test the jar samples obtained from the borings within 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 to test all the jar samples within the cut area to ascertain the suitability of the soils as fill material and if any acidic soils are present. If the borrow material 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 the Chemtrix unit, be used since the readout is obtained directly from a meter, which eliminates a common operator error. The colormetric kits, such as the Sudbury unit, are acceptable; but they are subject to operator error in color comparison to determine the ph value. The results obtained from either device are only as good as the sample used, therefore it is important that the operator choose a good representative sample 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 it can only obtain spot readings and would need to be checked periodically, probably using prepared standard solutions. A few samples should be checked using either the electro-chemical or the colormetric unit to verify the accuracy of the results. Proposed remedial measures shall be coordinated with the Authority s Engineering Department. MAY

6 5.2.4 Design Considerations General This section is intended as a general guide to the procedures employed by the Authority in design. Technical design methods shall follow the current AASHTO LRFD (Load and Resistance Factor Design) Guide with modifications noted herein. If other methods obtained from standard text books and technical papers on are used, those text books and papers should be referenced Design Bearing Pressure The following shall supplement section of the AASHTO LRFD Interim Bearing capacity is a function of the shear strength of the soil. The allowable bearing pressure of granular materials may also be obtained from curves of allowable bearing pressure vs. footing width for given values of standard penetration. These curves are predicated on certain settlement criteria which should be compatible with the structure usage. It is especially important when using these curves that the soil profile be determined for a minimum depth equal to twice the width of the footing. The soils within this depth must have the same or higher penetration resistance as is used to determine the allowable bearing pressure or detrimental settlements will occur. In addition to this criterion, it should also be established that no compressible layers are within the zone of influence of the stresses which would consequently cause settlement of the footing. The unconfined compression strength used in the bearing capacity equations can be obtained: (a) for minor structures by using index tests and comparing with similar soils and; (b) for other structures by using unconfined compression laboratory testing of undisturbed samples Settlement Analysis The following shall supplement section of the AASHTO LRFD Interim 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 settlement analysis of the foundation soils. When the foundation soil is a loose sand or clay type soil, this analysis can be very critical. Prior to making the settlement analysis, the overburden pressure and the intensity and distribution of the applied vertical stresses are computed. The applied vertical stresses are computed using the Boussinesq equations which assume that the soil is a perfectly MAY

7 homogeneous, elastic and isotropic medium. In addition to the foregoing assumption, the structure is also considered to be perfectly flexible. 1. Settlement of Clay Soil The compression index of a clay soil is determined from the log pressure vs. void ratio or log pressure vs. strain consolidation curves. At least one unload reload cycle shall be performed during each consolidation test, to determine the preconsolidation pressure and the recompression index. The unload reload cycle shall preferably be performed beyond the preconsolidation pressure, or in the straight portion of the curve. In the calculation of settlement, the log pressure vs. strain or log pressure vs. void ratio curve is to be used directly in the computations, using the unload reload cycle adjusted to the preconsolidation pressure and the present overburden pressure. This method takes into consideration the stress history of the soil and gives settlement values that are more realistic. 2. Settlement of Sands The settlement of sandy soils may be calculated by empirical methods based on observations. The Schmartmann Procedure which is based on the results of displacement measurements within sand masses loaded by model footings, as well as finite element analyses and deformations of materials with non-linear stress-strain behavior, may also be used. These methods are only approximate but may be used to predict the relative magnitude of settlement under a foundation Time-Dependent Consolidation In clays and silts with low permeabilities, the time rate of consolidation becomes important. The coefficient of vertical consolidation is determined from the time rate data obtained during the consolidation test. The data is plotted using the Taylor Method of square root of time vs. strain or log of time vs. strain to determine the coefficient or vertical consolidation. Vertical permeability tests can be performed in conjunction with the consolidation test at specified load increments. By turning the sample horizontally and trimming it to fit the consolidometer, the horizontal permeability may also be obtained in the same manner. In addition to these, vertical and horizontal permeability tests can also be performed using the triaxial apparatus with specified chamber pressures. These results are utilized in computing vertical and horizontal coefficients of consolidation. The time rate calculation is based on Terzaghi s one dimensional theory of consolidation. It is used to predict the time required for settlement to occur and the necessity of special treatment for accelerating the consolidation. It can also be used to predict the MAY

8 excess pore pressure within the foundation soils. The excess pore pressure measured during construction is compared with the predicted, and may be used to control the rate of placement of the embankments Stability Whenever poor foundation conditions are encountered, the stability of tie embankments becomes critical and the safety factor against failure must be determined. A minimum safety factor of 1.2 is desired during construction. Whenever the safety of existing facilities may be endangered by a failure, this safety factor shall be increased to a minimum of 1.5 or higher. The safety factor may be achieved either by using toe berms or relying on shear strength gain from consolidation 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 and economy. The Authority recommends the use of computers for performing the stability analyses. A large number of failure circles can be analyzed by this method in a short period of time, giving the geotechnical engineer a certain degree of flexibility in his analysis Lateral Earth Pressure In the design of earth retaining structures, lateral earth pressures are computed either by the Rankine Method, which uses the Plastic Equilibrium Theory, or by the Coulomb Method, which uses the Wedge Theory. In both cases, the soil is considered to be at the point of incipient failure. The Coulomb method shall be used if the soil is predominantly granular, otherwise the Rankine method shall be used. A retaining structure must yield at the top, a minimum of 0.001H in order for the soil to develop the full active pressure condition. Before specifying the lateral pressures to be used in design, the type of structure that will be used and whether it can yield enough to develop the active pressure condition must be determined. If not, some value intermediate between the active and at rest condition shall be specified. The strain required to develop the full passive pressure is greater than that required to develop the active pressure condition. Whenever the passive pressure is used in design, consideration shall be given to the possible movement of the structure, and a suitable safety factor applied. The passive pressure shall be ignored in the calculations if the possibility of scour exists. In designing sheet pile cofferdams, consideration shall be given to seepage pressure that may apply. Leakage between the interlocks usually is not large enough to completely relieve these pressures. MAY

9 When designing temporary cofferdams, the above theories shall be used in the sheeting design. However, the struts shall be designed using the earth pressure distributions given by Terzaghi and Peck in Soil Mechanics in Engineering Practice, Second Edition, Article 48 or other methods approved by the Authority s Engineering Department. The stability of the cofferdam shall be checked for uneven loading on opposite sides, as for example when the cofferdam is placed at the edge of a river with the embankment higher on one side than on the other side. Consideration shall also be given to construction conditions that may have an adverse effect on the stability of the cofferdam. An example of this may be excavated material placed on one side of the outside of the cofferdam against the sheeting or live loading from construction equipment. If the cofferdam is not designed for 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 deep excavations that have not been dewatered below the bottom, prior to excavation. The maximum stability number shall be Deep Foundations-Piles The following shall supplement sections ; ; ; ; ; ; ; of the AASHTO LRFD Interim A structure is founded on piles if it is determined that the soil below the footing does not have adequate bearing capacity, if there are soft or loose layers within the zone of influence of stresses from the footing that may produce excessive settlement, or if a cost estimate indicates that it is more economical to use piles. Exhibit 5-1 provides a guide of alternate pile types and LRFD Service I design loads that is based upon past experience on the Authority s roadways and other sources. It should be noted that Exhibit 5-1 is for reference only and shall not be used for actual pile design loads. There may be cases where other types of piles or design loads would be more appropriate. Where the preliminary investigations indicate the need for pile or deep foundations, the recommended type of pile or deep foundation and AASHTO Group I load should be noted on the Structure Foundation Recommendation form (Exhibit 5-2). Other types of deep foundations that may warrant consideration are drilled shafts and caissons. The applicability and potential economy of these alternatives will be a function of the structure configuration and subsurface conditions. MAY

10 If relatively high pile loads are proposed then load tests should be made. The cost of these load tests should be compared to the potential saving by using the proposed higher allowable loads. Steel pile allowable design loads shall be computed on the net cross sectional area after deduction for corrosion losses. Where conditions of excessively corrosive materials are expected, such as meadow mat, peat, unburned carbon, cinders, or other chemically active materials which cannot be economically removed, additional protection of the piles may be necessary. Consideration shall also be given to additional protection in the event of nearby installations producing stray currents causing corrosion losses. Timber piles shall be specified for temporary structures or light loading conditions where the required length of pile is not too great. The natural taper of these piles makes them ideal to be used as short friction piles. Where corrosive conditions or marine animals may attack the wood, the piles shall be pressure treated for protection. Consideration shall also be given to the use of cast-in-place concrete piles and pipe piles, filled or unfilled. If an economic comparison indicates that these piles are more advantageous than timber or steel piles, they shall be used. Where the rock surface is not fully defined or where friction piles are specified, a minimum of two test length piles shall be used at each structure location. The purpose of these piles is to verify the estimated pile lengths prior to ordering piles. These piles may be located within the footing area or, nearby, where the subsurface conditions 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 desired procedure and shall only be used under exceptional circumstances, and then only with prior approval of the Authority s Engineering Department. When it is desired to verify the design capacity of the piles, a pile load test shall be performed either on a test length pile or on another pile driven solely for this purpose. The pile design may allow for some bending to resist lateral loads in the foundation. The maximum bending allowed in the piles is a function of the type of pile (its material properties) and the coefficient of horizontal subgrade reaction of the soils. In no event shall the combined stresses cause an overstress in the pile after allowing for a suitable safety factor. Batter piles shall be used where the lateral loads exceed the capacity of the piles in bending. MAY

11 Deep Foundations-Drilled Shafts The following shall supplement sections ; ; 10.8 of the AASHTO LRFD Interim Drilled shafts shall be considered for foundation support when spread footings cannot be founded on suitable soil or rock strata within a reasonable depth and when piles are not economically feasible due to high loads or obstructions to driving. Drilled shafts shall also be considered when high lateral loads or uplift loads must be resisted and deformation tolerances are small, or as a direct support element for columns used as pier bents. In addition, drilled shafts shall be considered for sign structures to be constructed on existing roadway embankments and cuts. The minimum diameter of a drilled shaft shall be 36 inches and the minimum center to center spacing of any two drilled shafts shall be three times the diameter of the shaft. In addition to the methods outlined in the AASHTO LRFD Interim 2006, the procedures in the Federal Highway publication FHWA-IF shall be followed Foundation Recommendations Designs for roadways, bridges and buildings are based on the soils and foundation recommendations of the geotechnical engineer. The importance of these recommendations cannot be overemphasized, since the performance, as well as the economics of the structures, will in large measure depend on them. The geotechnical analysis and foundation recommendations shall be completed in sufficient time to be incorporated into the Phase B plan submission. These geotechnical studies and foundation recommendations shall be incorporated in a Report, with all relevant back up information, and shall be submitted to the Authority s Engineering Department for review with the Phase B construction plan submittal Roadways When the geotechnical engineer has determined, from an examination of the subsurface profile, that the foundation soils are competent to support the proposed embankment loadings, and that no compressible layers exist within the zone of influence of the stresses, his task is essentially complete. If there should be a deep layer of loose sand or clay type soils, and the proposed embankment stress is greater than the preconsolidation stress, a settlement analysis shall be made. The Engineer shall determine whether the actual embankment quantity placed shall be measured for payment or whether the contractor shall be advised to allow for the settlement in his bid prices. Whatever decision is made should be reflected in the Specifications. MAY

12 Significant sections of the existing New Jersey Turnpike Authority roadways are underlain by soft, weak, compressible soils which include peats, organic silty clays and varved clays. These areas have required special foundation treatment to maintain a stable embankment and minimize roadway settlements. In some areas the Turnpike roadways are in cuts that extend into clay soils which have required underdrains and/or undercutting to maintain stability and a good pavement surface for high speed traffic. These potential problems should be investigated and evaluated as part of the preliminary investigation of embankment foundation and cut areas. If potential problems of stability or excessive settlement exist for a proposed embankment then methods to overcome these problems shall be investigated and evaluated. Several foundation treatments that 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 be given to relative stability, expected post construction settlement and treatment cost. A number of the above listed foundation treatments require close control during construction. Where any such methods are proposed for construction the necessary instrumentation should be included in the construction contract. Also, instrumentation control criteria should be presented in the Report. Some commonly found unsatisfactory foundation areas are outlined below: 1. Surface Marsh and Swamps The materials within this type of deposit can vary from undecomposed meadow mat to organic silt. Also the thickness of these deposits can vary considerably. These materials possess very high moisture content, very low shear strength and extremely high compressibility. In addition to a large hydrodynamic consolidation, they also possess large secondary consolidation characteristics due to creep and decomposition. In general, they are undesirable as foundation materials. Recommended practice is normally to excavate these materials to firm bottom and backfill the excavation with a select granular MAY

13 backfill. Special Subgrade Material, Grade B, Backfill with less than 10% passing the #200 sieve is used for underwater backfill. No Compaction below water level is required. Typical excavation sections are shown in Exhibit Special Man-Made Foundation Problems Problems for roadway cuts and embankments can be caused by a number of manmade features, such as uncompacted fills, dumps, sanitary landfills, and old buried foundations. Usually these conditions cause roadway settlement problems. Often, sanitary landfills can cause a problem involving methane gas. If these conditions are encountered, or suspected, then test pits should be utilized so that the contents can be disclosed. The test pit logs should give a complete verbal description and include photographs. 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 of either sand or silt placed over each lift and rolled. If it can be determined that the sanitary landfill was built under conditions similar to those outlined above, it is sometimes possible to place low embankments, in the magnitude of five feet, on this material. An overload shall be placed above the profile grade and left in place until most of the settlement has occurred, to precompress the material. Before constructing the pavement, the overload is removed and the embankment is proof-rolled using a 50-ton roller with minimum tire pressures of 100 pounds per square inch. Soft or weaving areas shall be removed and replaced with a select granular backfill. Uncontrolled sanitary landfill is not desirable as a foundation material even for low embankments. These materials shall be excavated for their entire thickness and replaced with select granular materials. Typical excavation sections are shown in Exhibit 5-3. In view of the recent emphasis on toxicity, excavation methods may be frowned upon because of the disposal problem. If satisfactory methods of disposal cannot be found, the Engineer may be forced to examine alternative methods of design. One such method is Dynamic Deep Compaction where a heavy weight is dropped, repeatedly, three to eight times at the same spot from a 30- to 120-foot height. In this event, the post-construction settlement shall be estimated and the degree of future maintenance brought to the Authority s Engineering Department s attention. MAY

14 3. Soft Clays and Silts Clays and silts with low shearing strength make poor foundation material. However, with proper treatment, their shearing strength can be increased, thus improving their capacity as a foundation material. Methods commonly used for increasing the shearing strength of soft clays and silts include overloading or overloading in combination with wick or sand drains. These materials have very low permeability and may require a substantial time period for consolidation to take place. The coefficient of consolidation shall be investigated in both the vertical and horizontal directions, and the soil samples shall be carefully inspected for possible horizontal sand or silt layers. All of these can be used as aids in reducing the required consolidation time. With low embankments, where stability using in-situ undrained shear strength is not critical, only overloading may be necessary to consolidate the soft clays and silts. In this case the construction schedule must allow enough treatment time after the embankment is placed for complete consolidation to occur. However, the in-situ undrained shear strength may not be sufficient for the stability of higher embankments, and shear strength gain will have to be accelerated during embankment construction. In this event, wick or sand drains are to be used, with or without toe berms and in some cases lightweight fill may be considered. Overload may be required for complete consolidation to occur within a reasonable time period. Embankments placed on stabilized foundation soils may settle after construction. This post construction settlement may consist of primary and secondary consolidation or secondary consolidation only, depending on the available treatment time. The estimated magnitude of this settlement will be computed in advance and later verified using actual field settlement data. 4. Roadway Cuts Most problems encountered in roadway cuts involve either, or both, high ground water conditions and soft subgrade. Two other problems less frequently encountered are unstable cut slopes and sloughing of cut slopes. These latter two problems are usually encountered when clay or silt soils are encountered in highway cuts. The potential for these problems shall be investigated in all proposed cuts of significant size. Ground water problems can usually be handled by underdrains. A soft subgrade can usually be handled by underdrains and/or undercutting and backfilling with granular soil. MAY

15 Cut slope stability may require one or more of several potential treatments: flattening the slope, control of surface or subsurface drainage or benching. Cut slope sloughing is normally caused by subsurface drainage and is best controlled by controlling the subsurface drainage and/or benching. The evaluation of the potential for these roadway cut problems should be addressed in the Report Structures A subsurface profile should be drawn for each foundation unit of each structure. The foundation recommendations shall be based on this profile, unless the geotechnical engineer has sufficient reason to vary from it. In this event, the reasons shall be stated with the recommendation. Major structures, such as bridges and retaining walls shall be investigated for stability and settlement. Usually the latter will control. For proposed normal size structures and conditions utilizing soil bearing foundations the settlement evaluation may be based on charts correlating settlement with the Standard Penetration Test (SPT) blow per foot on the split spoon sampler. For structures founded on bedrock presumptive bearing values from recognized applicable building codes may be utilized. For large span structures or unusual conditions a more detailed evaluation of proposed soil bearing foundations will be required. Proposed large size box culverts will require at least a preliminary investigation and evaluation. Pipe culverts that are proposed for embankments that are expected to settle shall be evaluated for the effects of any differential settlements. The evaluation of these conditions shall be included in the Structure Foundation Report along with the Structure Foundation Recommendation form (Exhibit 5-2) giving the recommended AASHTO LRFD using Strength Limit States loads. Allowable bearing capacity is computed by methods previously outlined in this Section. Soil bearing foundations may be considered unyielding if the allowable bearing pressure is 3.0 tons per square foot or greater, provided that there are no compressible layers below the footing level that will cause unacceptable settlement. The settlement shall be computed so that the structural designer can take it into consideration if continuous spans are being considered. When embankments are placed on stabilized foundation soils, postconstruction settlement may occur. This settlement may consist of primary and secondary consolidation or secondary consolidation only, depending on the treatment time available. Since the magnitude of MAY

16 this post-construction settlement may affect the type of structure to be chosen, it shall be computed and if necessary, later verified using actual 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 the embankment is one-third to one-quarter of the post-construction embankment vertical settlement.) The abutment shall be designed so that the bearing shoes can be adjusted for this lateral shift. Piles for pile supported footings shall be designed according to the procedures outlined in this Section. The foundation recommendation shall state the type of pile, the design load in tons and the minimum and/or estimated pile tip elevation. Any special treatment required for the piles (e.g. coating for corrosion protection, creosoting wood piles, pile shoes, special pile tips or points, etc.), shall be stated in the foundation recommendations in the remarks section. Piles driven through stabilized foundation soils that are expected to continue to settle after construction will experience drag forces due to the consolidation of these soils. These drag forces can be severe in some cases and may overload piles that have not been designed for this additional load. The foundation recommendations shall also state, in the remarks section, the estimated magnitude of the drag forces, so that the Engineer can take them into consideration Reports Detailed Report submission guidelines are provided in Section 5 of the Procedures Manual. The following is a brief outline of the report requirements. The Engineer is to prepare a Report on all the proposed construction that will be designed. This report is to facilitate the review of contract plans and specifications and to provide a documentation of all potential problems for future reference. The Report can consist of one or more letter reports or formal reports that cover all the proposed construction for bridges, sign structures, retaining walls, major culverts, roadway embankments and roadway cuts, and other significant structures. Three (3) copies of the Report are to be submitted to the Authority s Engineering Department with the Phase B plan submittal. This report should contain the following items: 1. A brief description of the structure or structures. MAY

17 2. A brief description of the soils and foundation conditions. 3. A soils profile showing SPT data, ground water elevations, soil strata and a generalized description of the soils and bedrock encountered. 4. A table of undisturbed soil properties. 5. A brief description of any potential foundation problems and the proposed treatments. 6. Any required criteria necessary for the use of construction instrumentation. 7. For all bridges, retaining walls and major culverts documentation of recommended AASHTO LRFD Strength Limit States allowable loads (Exhibit 5-1) using the Structure Foundation Recommendation form (Exhibit 5-2). 5.3 CONSTRUCTION CONTROL Control of normal methods of construction is covered by the New Jersey Turnpike Standard Specifications, the construction contract specifications and is discussed in the New Jersey Turnpike Construction Manual. The earthwork and structure items noted in these references include compaction, select material gradations, pile load tests and pile driving. Construction of embankments on soft foundations often requires treatments that need special construction control methods with which most Contractors and Field Engineers are not familiar. Consequently, the methods of construction and construction control have to be clearly stated in the plans, specifications and directives to the Field Engineers and Inspectors. The directives necessary for any special control of earthwork construction not included in the contract plans and specification will be included in the Report. Typical methods of embankment foundation treatment that require construction control instrumentation are preload, surcharge, toe berms, controlled rate of construction and sand drain or wick drain treatments. The planning of instrumentation for construction control should consider duplication and location of the instruments because of potential damage due to construction operations. The instrumentation equipment and installation should be included in the construction plans and specifications. The reading of these instruments and reduction of the data will be the responsibility of the Engineer. Instrumentation commonly used on Authority Projects includes A) Settlement Platforms, B) Piezometers, C) Slope Indicators and D) Observation Wells. The need for other instruments, (e.g., strain gauges and load cells), may only occasionally arise in special circumstances and will not be discussed here. MAY

18 5.3.1 Settlement Platforms These devices serve two functions. First, in the stabilization of foundation soils, 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 of primary consolidation, and to predict the rate and magnitude of post construction settlement. Secondly, the magnitude of settlement at the time the final survey is made is used to determine the additional quantity of material placed to compensate for settlement of the subsoils. The design of the settlement platform shall be as simple as possible to facilitate installation, maintenance and observation. An initial reading shall be taken as soon as the settlement platform is installed and before any embankment is placed, and at close enough intervals thereafter to be able to draw a smooth curve through the points. Readings shall be made to the nearest one hundredth of a foot by any satisfactory means Piezometers Piezometers are used to measure pore pressures in the foundation soils during consolidation and stabilization. They are used essentially to control construction operations during embankment placement. In order to facilitate the construction control, the geotechnical engineer shall prepare a chart showing allowable pore pressure vs. embankment height. This chart shall be based 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, manufactured by the Piezometer Research and Development Corporation, to be satisfactory in response to pressures and for ease of maintenance Slope Indicators These devices are essentially early warning devices for measuring horizontal displacements in the weak subsoil strata. They are especially useful in monitoring unstable conditions in cuts or fills. Essentially, these devices consist of a specially designed aluminum or plastic casing installed into the ground with wash boring equipment. The bottom of the casing is anchored in rock or in a very dense soil stratum. If large settlements are anticipated, the sections shall be of the telescoping type to prevent buckling. A specially designed torpedo is inserted into the casing and the angle of inclination at selected depth is measured. The inclination angles are translated into displacement measurements which, by integrating from the bottom to the top produces the displacement profile. The magnitude and rate of displacement are monitored and provide the basis for determining a possible failure condition. MAY

19 5.3.4 Observation Wells These are open perforated pipes placed in the ground to monitor the ground water level. They shall be used wherever the ground water level is critical to construction progress. REFERENCES 1. American Association of State Highway and Transportation Officials, LRFD, 4 th Edition, Interim American Association of State Highway and Transportation Officials, Standard Specifications for Highway Bridges, 17 th Edition, Burmister, D.M., Principals and Techniques of Soil Identification, Proceedings of the Highway Research Board, December, Federal Highway Administration, Drilled Shafts: Construction Procedures and Design Methods, Publication No. FHWA-IF , 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 Terzaghi, K. and Peck, R.B., Soil Mechanics in Engineering Practice, 2 nd Edition, MAY

20 EXHIBIT 5-1 PILE 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: B. Friction Piles HP 8x tons (1) HP 10x tons (1) HP 12x tons (1) HP 10x tons (1) HP 12x tons (1) HP 12x tons (1) HP14x tons (1) HP 14x 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 1. Timber Piles (treated or untreated) tons 2. Cast-in-Place Piles (3) tons tons tons 3. Prestressed Concrete Piles (5) 10 Square tons 12 Square tons 14 Square tons 16 Square tons 18 Square tons (5) for fc = 5,000 psi 4. Steel H Piles HP 10x42 HP 12x53 HP 14x tons tons tons MAY

21 EXHIBIT 5-2 STRUCTURE FOUNDATION RECOMMENDATION SECTION NO. NEW JERSEY TURNPIKE AUTHORITY JOHN DOE ASSOCIATES Consulting Engineers 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 Foundation Unit Ref. Boring No. Boring Elev. Ground Water Elev. * Normal Footing Elev. Soil Bearing Footing Elev. Design Load Tip Elev. ft. TSF Est. Min. Piles/Drilled Shaft Type Design Load 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 MAY

22 EXHIBIT 5-3 TYPICAL EXCAVATION SECTION MAY