2.4USEOFOBSERVATIONALMETHODOnce an appropriate value for the design has been selected, it is possible to complete the design andproceed to construction.There is one final step that can be performed to validate the data andpossibly improve the accuracy of the selected value. This three step process involves: (1) using thedesign value and the actual estimated loading to predict a field response; (2)systematically monitorthe field performance, and (3)“back calculate"the actual property of interest.This process ofprediction, monitoring, and reassessment is known as the Observational Method (Terzaghi and Peck,1967).Twoexamplesof this techniquefollow:In soil, the use of piezometers to monitor the rate of pore pressure dissipation and measuredsettlements of a large area fill can result in a more accurate estimate of the compressibilityand time rate of consolidation characteristics of soft soils as well as provide information tomaximize the rate offill placement.In rock,the useof instrumented rock bolts and displacementmonitoring instrumentation canprovide valuable information regarding the kinematics of block stability and the strength ofthejointedrockmass.The Observational Method is an invaluable aid and ideally should be a part of every geotechnicalproject. Sadly, this approach is often overlooked due to budget concerns, and in many cases is noteven considered by the design engineer.Where appropriately used, the Observational Method canhave significant benefits not only to the project at hand, but also for other projects in the areabecause a full-scale assessment of the engineering properties can be made.It is stronglyrecommended that the Observational Method be included as a basic tenet of all projects.9
9 2.4 USE OF OBSERVATIONAL METHOD Once an appropriate value for the design has been selected, it is possible to complete the design and proceed to construction. There is one final step that can be performed to validate the data and possibly improve the accuracy of the selected value. This three step process involves: (1) using the design value and the actual estimated loading to predict a field response; (2) systematically monitor the field performance; and (3) “back calculate” the actual property of interest. This process of prediction, monitoring, and reassessment is known as the Observational Method (Terzaghi and Peck, 1967). Two examples of this technique follow: • In soil, the use of piezometers to monitor the rate of pore pressure dissipation and measured settlements of a large area fill can result in a more accurate estimate of the compressibility and time rate of consolidation characteristics of soft soils as well as provide information to maximize the rate of fill placement. • In rock, the use of instrumented rock bolts and displacement monitoring instrumentation can provide valuable information regarding the kinematics of block stability and the strength of the jointed rock mass. The Observational Method is an invaluable aid and ideally should be a part of every geotechnical project. Sadly, this approach is often overlooked due to budget concerns, and in many cases is not even considered by the design engineer. Where appropriately used, the Observational Method can have significant benefits not only to the project at hand, but also for other projects in the area because a full-scale assessment of the engineering properties can be made. It is strongly recommended that the Observational Method be included as a basic tenet of all projects
CHAPTER 3PLANNINGASUBSURFACEINVESTIGATIONANDLABORATORYTESTINGPROGRAM3.1INTRODUCTIONTo evaluate soil and rock properties required for geotechnical design related to transportationprojects, subsurface investigation and laboratory testing programs are developed and executed.Thedata collection efforts associated with these activities should occur early in the project, failure toconduct an appropriately scoped investigation and laboratory-testing program will result in potentialdata gaps and/or the need to re-mobilize to the site for supplementary testing. Data gaps can causesignificant delays in the project and can potentially lead to either an overconservative and costlydesign or an unconservative and unsafe design.It is, therefore, imperative that the subsurface andlaboratory testing programs be carefully planned to ensure that the information collected in the fieldand the laboratory will be sufficient to develop soil and rock properties for design and constructionThis chapter will present general guidelines related to the development of subsurface investigationand laboratory testing programs for the evaluation of soil and rock properties. Since the selection ofsampling and testing methods will be driven by the scope of the project and geologic conditions,critical project related issues must be understood prior to field and laboratory planning activities.For heterogeneous deposits and special materials (ie., colluvium, organic soils, etc.) greater effortwill be required during the planning stage of the investigation to assess the applicability of specifictools and sampling devices.3.2PLANNING THE SUBSURFACE INVESTIGATION AND LABORATORYTESTINGPROGRAM3.2.1GeneralPlanning subsurface investigation and laboratory testing programs requires the engineer to be awareof parameters and properties needed for design and construction, as well as to understand thegeologic conditions and site access restrictions.Specific steps include: (1)identify data needs; (2)gather and analyze existing information; (3)develop a preliminary site model; (4)develop andconduct a site investigation; and (5) develop and conduct a laboratory-testing program. Specificplanningsteps areaddressed inthefollowingsections.3.2.2IdentifyData NeedsThe first step of an investigation and testing program requires that the engineer understand theprojectrequirements and the siteconditions and/orrestrictions.Theultimategoal ofthisphase isto10
10 CHAPTER 3 PLANNING A SUBSURFACE INVESTIGATION AND LABORATORY TESTING PROGRAM 3.1 INTRODUCTION To evaluate soil and rock properties required for geotechnical design related to transportation projects, subsurface investigation and laboratory testing programs are developed and executed. The data collection efforts associated with these activities should occur early in the project; failure to conduct an appropriately scoped investigation and laboratory-testing program will result in potential data gaps and/or the need to re-mobilize to the site for supplementary testing. Data gaps can cause significant delays in the project and can potentially lead to either an overconservative and costly design or an unconservative and unsafe design. It is, therefore, imperative that the subsurface and laboratory testing programs be carefully planned to ensure that the information collected in the field and the laboratory will be sufficient to develop soil and rock properties for design and construction. This chapter will present general guidelines related to the development of subsurface investigation and laboratory testing programs for the evaluation of soil and rock properties. Since the selection of sampling and testing methods will be driven by the scope of the project and geologic conditions, critical project related issues must be understood prior to field and laboratory planning activities. For heterogeneous deposits and special materials (i.e., colluvium, organic soils, etc.) greater effort will be required during the planning stage of the investigation to assess the applicability of specific tools and sampling devices. 3.2 PLANNING THE SUBSURFACE INVESTIGATION AND LABORATORY TESTING PROGRAM 3.2.1 General Planning subsurface investigation and laboratory testing programs requires the engineer to be aware of parameters and properties needed for design and construction, as well as to understand the geologic conditions and site access restrictions. Specific steps include: (1) identify data needs; (2) gather and analyze existing information; (3) develop a preliminary site model; (4) develop and conduct a site investigation; and (5) develop and conduct a laboratory-testing program. Specific planning steps are addressed in the following sections. 3.2.2 Identify Data Needs The first step of an investigation and testing program requires that the engineer understand the project requirements and the site conditions and/or restrictions. The ultimate goal of this phase is to
identify geotechnical data needs for the project and potential methods available to assess theseneeds. During this phase it is necessary to:identify design and constructability requirements (e.g., provide a grade separation, transferloadsfrombridge superstructure,provideforadry excavation),identify performance criteria (e.g., limiting settlements, right of way restrictions, proximityofadjacent structures)and scheduleconstraints,identify areas of concern on site and potential variabilityof local geologydeveloplikelysequenceandphasesofconstruction;identify engineering analyses to be performed (e.g., bearing capacity, settlement)identify engineering properties and parameters required for these analyses;evaluate methods to obtain parameters and assess the validity of such methods for thematerial type and construction methods, andevaluatenumber of tests/samples needed and appropriatelocationsforthemAs an aid to assist in the planning of site investigation and laboratory testing, table 1 provides asummary of the information needs and testing considerations for various geotechnical applicationsA discussion of specific field and laboratory test methods is provided in chapter 4.3.2.3Gather and Analyze Existing InformationBefore any equipment is mobilized to the site, existing data for the site,both regionally and locallyshould be evaluated as a logical first step in the investigation.This is an important and inexpensivestep that is often overlooked.There are many readily available data sources that can be used toidentify major geologic processes that have affected the site, site history, geologic constraints, man-made features, and access issues.The planning step can be extremely cost effective and productive.Existing data will provide information which can reduce the scope of the subsurface investigationhelp guide the location of testing and sampling points, and reduce the amount of time in the field dueto unexpected problems. For example, historical aerial photographs can be used to identify an areawhere fill had been placed, where a landslide scarp exists, or major geologic structures such asfaults, bedding planes, and continuous joint sets.A list of potential information sources along withthetype of information availableispresented intable211
11 identify geotechnical data needs for the project and potential methods available to assess these needs. During this phase it is necessary to: • identify design and constructability requirements (e.g., provide a grade separation, transfer loads from bridge superstructure, provide for a dry excavation); • identify performance criteria (e.g., limiting settlements, right of way restrictions, proximity of adjacent structures) and schedule constraints; • identify areas of concern on site and potential variability of local geology; • develop likely sequence and phases of construction; • identify engineering analyses to be performed (e.g., bearing capacity, settlement); • identify engineering properties and parameters required for these analyses; • evaluate methods to obtain parameters and assess the validity of such methods for the material type and construction methods; and • evaluate number of tests/samples needed and appropriate locations for them. As an aid to assist in the planning of site investigation and laboratory testing, table 1 provides a summary of the information needs and testing considerations for various geotechnical applications. A discussion of specific field and laboratory test methods is provided in chapter 4. 3.2.3 Gather and Analyze Existing Information Before any equipment is mobilized to the site, existing data for the site, both regionally and locally, should be evaluated as a logical first step in the investigation. This is an important and inexpensive step that is often overlooked. There are many readily available data sources that can be used to identify major geologic processes that have affected the site, site history, geologic constraints, manmade features, and access issues. The planning step can be extremely cost effective and productive. Existing data will provide information which can reduce the scope of the subsurface investigation, help guide the location of testing and sampling points, and reduce the amount of time in the field due to unexpected problems. For example, historical aerial photographs can be used to identify an area where fill had been placed, where a landslide scarp exists, or major geologic structures such as faults, bedding planes, and continuous joint sets. A list of potential information sources along with the type of information available is presented in table 2
Table1.Summary of information needs and testing considerations for a range of highway applications.Field TestinglEngineeringRequired InformationLaboratory Testing"GeotechnicalhsarsEvaluationsforAnahsesShallowbearing capacitysubsurface profile (soil, groundwater, rock)1-D Oedometer tests..vane shear test.FoundationsSPT (granular soils).settlement (magnitude & rate).shear strength parameters..direct shear testsshrink/swell of foundation soilscompressibslity parameters (including consolidation,CPT..triaxial tests*(natural soils or embankment fill)shrink/swell potential, and elastic modulus)dilatometer.grain size distribution.chemical compatibility of soil andfrost depthrock coring (RQD)Atterberg Limits..concretestress history (present and past vertical effective.nuclear density.pH, resistivity testsfrost heave-stresses).plate load testingmoisture content.chemical composition of soil.scour (for water crossings)geophysical testingunit weight..depth of seasonal moisture changeextreme loadingorganic content.unit weightscollapse/swell potential tests.geologic mapping including orientation and-rock uniaxial compression.characteristics of rock discontinuitiestest and intact rock moduluspoint load strength testDriven Pile pile end-bearingsubsurface profile (soil, ground water, rock)SPT (granular sois).triaxial tests.Foundationspile load test.pile skin frictionshear strength parameters.interface friction tests..horizontal earth pressure coeficients.CPT:settlement.grain size distribution..down-drag on pile.interface friction parameters (soil and pile).vane shear test.1-D Oedometer tests.lateral earth pressurescompressibility parameters.dilatometer.pH, resistivity testschemical compatibility ofsoilandchemical composition of soil/rockpiezometersAtterberg Limits-..pileunit weightsrock coring (RQD)organic content:.-driveability.presence of shrink/swell soils (limits skin friction)moisture content.geophysical testingunit weight.presence of boulders/ very hard.geologic mapping including orientation and.layerscharacteristicesofrockdiscontinuitiescollapse/swell potential tests..scour (for water crossings)slake durability..vibration/heave damage to nearby:rock uniaxial compressionstructurestest and intact rock modulusextreme loadingpoint load strength test() Corresponding AASHTO and ASTM Standard references are provided in chapter 4 for field and laboratory tests.12
12 Table 1. Summary of information needs and testing considerations for a range of highway applications. Geotechnical Issues Engineering Evaluations Required Information for Analyses Field Testing(1) Laboratory Testing(1) Shallow Foundations • bearing capacity • settlement (magnitude & rate) • shrink/swell of foundation soils (natural soils or embankment fill) • chemical compatibility of soil and concrete • frost heave • scour (for water crossings) • extreme loading • subsurface profile (soil, groundwater, rock) • shear strength parameters • compressibility parameters (including consolidation, shrink/swell potential, and elastic modulus) • frost depth • stress history (present and past vertical effective stresses) • chemical composition of soil • depth of seasonal moisture change • unit weights • geologic mapping including orientation and characteristics of rock discontinuities • vane shear test • SPT (granular soils) • CPT • dilatometer • rock coring (RQD) • nuclear density • plate load testing • geophysical testing • 1-D Oedometer tests • direct shear tests • triaxial tests • grain size distribution • Atterberg Limits • pH, resistivity tests • moisture content • unit weight • organic content • collapse/swell potential tests • rock uniaxial compression test and intact rock modulus • point load strength test Driven Pile Foundations • pile end-bearing • pile skin friction • settlement • down-drag on pile • lateral earth pressures • chemical compatibility of soil and pile • driveability • presence of boulders/ very hard layers • scour (for water crossings) • vibration/heave damage to nearby structures • extreme loading • subsurface profile (soil, ground water, rock) • shear strength parameters • horizontal earth pressure coefficients • interface friction parameters (soil and pile) • compressibility parameters • chemical composition of soil/rock • unit weights • presence of shrink/swell soils (limits skin friction) • geologic mapping including orientation and characteristics of rock discontinuities • SPT (granular soils) • pile load test • CPT • vane shear test • dilatometer • piezometers • rock coring (RQD) • geophysical testing • triaxial tests • interface friction tests • grain size distribution • 1-D Oedometer tests • pH, resistivity tests • Atterberg Limits • organic content • moisture content • unit weight • collapse/swell potential tests • slake durability • rock uniaxial compression test and intact rock modulus • point load strength test (1) Corresponding AASHTO and ASTM Standard references are provided in chapter 4 for field and laboratory tests
Table 1.Summaryofinformationneedsandtestingconsiderationsforarangeofhighwayapplications(continued)Ficld Testing"GeotechnicalEngineeringRequired InformationLaboratory TestingIssuesEvaluationsforAnalysesDrilled Shaftshaft end bearingsubsurfiae profile (soil, ground water, rock).technique shaff1-D OedometerFoundationsshaft skin frictionshear strength parametersshaft load testtriaxial tests-.constructabilityinterface shear strength friction parameters (soil andvane shear testgrain size distribution....down-drag on shaftshaft)CPT..interface friction testscompressibility parameters.quality of rock socket..SPT (granular soils)pH, resistivity tests-horizontalearth pressure coefficientslateral earth pressuresdilatometerpermeability tests..chemical composition of soil/rocksetlement (magnitude & rate).Atterberg Limitspiezometersunit weightsgroundwater seepage/ dewatering.rock coring (RQD)moisture contentpermeability of water-bearing soilsunit weight.presence of boulders/ very hardgeophysical testing.layerspresence ofartesian conditionsorganic content-scour (for water crossings)*presence of shrink/swellsoils (limits skin friction)collapse/swell potential testsextreme loading.geologic mapping including orientation androck uniaxial compression testcharacteristics of rock discontinuitiesand intact rock modulus*degradation of soft rock in presence of water and/or airpoint load strength test(e.g., rock sockets in shales)slake durabilityEmhankmeatssettlement (magnitude & rate)subsurface profile (soil, ground water, rock).nuclear density1-D Oedometerandtriaxial tests:bearing capacitycompressibility parametersplate load test..Embankment.slope stability..iest flldirect shear testsshear strength parametersFoundationsCPT..grain size distributionlateral pressureunit weights.intermal stabilitytime-rate consolidation parameters.SPT (granular soils)Atterberg Limits.-borrow source evaluation (availablehorizontal earth pressure coefficientsdilatometerorganic content:.quantity and qualiy of borow soil)interface friction parameters.vane shearmoisture-density relationshiprequired reinforcementpullout resistance.rock coring (RQD)hydraulic conductivitygeologic mapping including orientation andgeophysical testinggeosynthetic/soil testing..shrink/swellcharacteristics ofrock discontinuities.shrink/swell/degradation of soil and rock fillslake durabilityunit weightExcavations andslope stabilitysubsurface profile (soil, ground water, rock).test cut to evaluate stand-uphydraulic conductivityCut Slopesshrink/swell propertiestimegrain size distribution.bottom heave.unit weights.liquefaction..piezometersAtterberg LimitsCPT.dewateringhydraulic conductivity.triaxial tests.SPT (granular soils).lateral pressuretime-rate consolidation parametersdirect shear tests::.soil softening/progressive failureshear strength of soil and rock (includingvane shear..moisture contentdilatometerdiscontinuities).slake durabilitypore pressuresrock coring (RQD)geologic mapping including orientation and.rock uniaxial compression test..characteristics of rock discontinuities.in siturock direct shear testand intact rock modulus:geophysical testingpoint load strength test.() Corresponding AASHTO and ASTM Standard references are provided in chapter 4 for field and laboratory tests.13
13 Table 1. Summary of information needs and testing considerations for a range of highway applications (continued). Geotechnical Issues Engineering Evaluations Required Information for Analyses Field Testing(1) Laboratory Testing(1) Drilled Shaft Foundations • shaft end bearing • shaft skin friction • constructability • down-drag on shaft • quality of rock socket • lateral earth pressures • settlement (magnitude & rate) • groundwater seepage/ dewatering • presence of boulders/ very hard layers • scour (for water crossings) • extreme loading • subsurface profile (soil, ground water, rock) • shear strength parameters • interface shear strength friction parameters (soil and shaft) • compressibility parameters • horizontal earth pressure coefficients • chemical composition of soil/rock • unit weights • permeability of water-bearing soils • presence of artesian conditions • presence of shrink/swell soils (limits skin friction) • geologic mapping including orientation and characteristics of rock discontinuities • degradation of soft rock in presence of water and/or air (e.g., rock sockets in shales) • technique shaft • shaft load test • vane shear test • CPT • SPT (granular soils) • dilatometer • piezometers • rock coring (RQD) • geophysical testing • 1-D Oedometer • triaxial tests • grain size distribution • interface friction tests • pH, resistivity tests • permeability tests • Atterberg Limits • moisture content • unit weight • organic content • collapse/swell potential tests • rock uniaxial compression test and intact rock modulus • point load strength test • slake durability Embankments and Embankment Foundations • settlement (magnitude & rate) • bearing capacity • slope stability • lateral pressure • internal stability • borrow source evaluation (available quantity and quality of borrow soil) • required reinforcement • subsurface profile (soil, ground water, rock) • compressibility parameters • shear strength parameters • unit weights • time-rate consolidation parameters • horizontal earth pressure coefficients • interface friction parameters • pullout resistance • geologic mapping including orientation and characteristics of rock discontinuities • shrink/swell/degradation of soil and rock fill • nuclear density • plate load test • test fill • CPT • SPT (granular soils) • dilatometer • vane shear • rock coring (RQD) • geophysical testing • 1-D Oedometer • triaxial tests • direct shear tests • grain size distribution • Atterberg Limits • organic content • moisture-density relationship • hydraulic conductivity • geosynthetic/soil testing • shrink/swell • slake durability • unit weight Excavations and Cut Slopes • slope stability • bottom heave • liquefaction • dewatering • lateral pressure • soil softening/progressive failure • pore pressures • subsurface profile (soil, ground water, rock) • shrink/swell properties • unit weights • hydraulic conductivity • time-rate consolidation parameters • shear strength of soil and rock (including discontinuities) • geologic mapping including orientation and characteristics of rock discontinuities • test cut to evaluate stand-up time • piezometers • CPT • SPT (granular soils) • vane shear • dilatometer • rock coring (RQD) • in situ rock direct shear test • geophysical testing • hydraulic conductivity • grain size distribution • Atterberg Limits • triaxial tests • direct shear tests • moisture content • slake durability • rock uniaxial compression test and intact rock modulus • point load strength test (1) Corresponding AASHTO and ASTM Standard references are provided in chapter 4 for field and laboratory tests