GEOTECHNICAL POLICIES AND PROCEDURES MANUAL CHAPTER 5 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES

Transcription

1 GEOTECHNICAL POLICIES AND PROCEDURES MANUAL CHAPTER 5 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES

2 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-i TABLE OF CONTENTS 1. PURPOSE INTRODUCTION GENERAL GUIDELINES FOR GEOTECHNICAL INVESTIGATIONS ROADWAY ALIGNMENT INVESTIGATIONS ROADWAY CENTERLINE CUT AND EMBANKMENT INVESTIGATIONS Embankments Over Soft Ground Bridge Approach Embankments Rock Slopes LANDSLIDE INVESTIGATIONS BORROW AREA INVESTIGATIONS STRUCTURE INVESTIGATIONS Bridges Earth Retaining Walls Buildings Drainage Structures (Culverts) Large Load Light and Sign Structures Tunnels Detention Basins FIGURES : Guidelines for Boring Layout : Minimum Requirements for Boring Depth : Guideline Sampling and Testing Criteria REFERENCES... 14

3 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES PURPOSE A subsurface investigation may need to be performed at the site of all roadway construction projects, including: widening, extension, modification and rehabilitation. This Chapter presents guidelines to plan the scope of a geotechnical investigation, including a subsurface exploration and testing program. However, as the requirements and conditions vary with each project, engineering judgment is essential in tailoring the investigation to the specific project. The Manual for FHWA, Subsurface Investigations, NHI Course No , (module ) and the AASHTO Manual on Subsurface Investigations (1988) provide extensive information on planning and conducting a geotechnical investigation. 2. INTRODUCTION A comprehensive investigation program starts with a series of preliminary office studies, such as; a study of project objectives and preliminary plans, review of existing information, identification of geotechnical design issues, formulation of a preliminary exploration and testing plan, and a list of anticipated analyses. Following the office studies, a field reconnaissance should be performed and modifications made, if necessary, to the exploration plan to provide the most useful information. The subsurface exploration program might include both conventional borings and other specialized investigative or in situ testing methods. Subsurface exploration programs should be conducted using a phased approach. This allows the results from critical design areas, or with the most uncertainty, to be performed early in the project. If subsurface information shows materials to be significantly different from those assumed in the planning stages, modifications could be made to the scope of the investigation. Modification to the scope may include boring depths, number of samples, and type of samples. The planning of a geotechnical investigation also includes identification of appropriate laboratory testing and engineering analyses to support geotechnical design needs for the specific project. The results of the investigation are commonly documented in a Geotechnical Report. The amounts and types of data obtained during a geotechnical investigation are often constrained by limitations of time, manpower, equipment, access, or funds. One goal of the investigation program should be to provide sufficient data for the Geotechnical Engineer to recommend the most appropriate and efficient design. Otherwise, more conservative designs with higher factors of safety would be required, which may cost considerably more than a properly conceived exploration program. Another goal should be providing sufficient information for the contractor to bid appropriately and reduce change orders and claims. 3. GENERAL GUIDELINES FOR GEOTECHNICAL INVESTIGATIONS An important step in geotechnical analysis and design is to have an adequate subsurface exploration program. The number, depth, spacing, and type of borings,

4 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-2 sampling, and testing in an exploration program are so dependent on site conditions and the project, that no uniform rule can be established. The Geotechnical Engineer should develop an overall program that addresses the geotechnical issues to the extent justified by the significance of the project elements. The required investigative effort is also dependent on the type and complexity of the design. Therefore, investigation programs are commonly modified as additional information become available. A phased investigation approach may be most beneficial for large projects and/or projects with difficult geotechnical problems. The planning guidelines presented in Tables 6-1 through 6-3 are considered reasonable for obtaining the minimum subsurface data needed for a cost-effective geotechnical design. Table 6-1, Guidelines for Boring Layout, lists the number and location of borings recommended for various types of facilities. Changes in geological stratigraphy could necessitate additional borings. Table 6-2, Minimum Requirements for Boring Depths, details recommendations for planning the depths of exploration holes. Actual geologic conditions could change exploration depths (such as the actual depth to bedrock or hard strata). In some cases, the presence of unsuitable materials such as highly compressible soils or peat deposits could necessitate additional number and depth of borings, possibly in a grid pattern to identify the lateral extent and depth of these deposits. Planning of exploration programs should take into account the data required for the anticipated engineering analyses. The guidelines contained in Tables 6-1 and 6-2 consider only the use of conventional borings. The Geotechnical Engineer may include cone penetration tests, plate load bearing capacity tests, trench excavation tests, geophysical tests, and/or any other appropriate tests as supplementary to or as substitutes for some, but not all, of the conventional boring exploration tests. Table 6-3, Guideline Sampling and Testing Criteria, describes the types and frequency of samples and tests recommended for various applications and subsurface conditions. The following sections provide additional guidelines for specific project phases and design elements. 4. ROADWAY ALIGNMENT INVESTIGATIONS In the early stages of a project, the Geotechnical Engineer may be requested to perform an evaluation of several possible roadway alignments or structure locations. The purpose of this effort is to identify geologic conditions or constraints that could affect the selection decision. This project phase generally does not require extensive subsurface explorations. It is typically limited to preliminary office studies and field reconnaissance where anticipated surface materials are identified and some sampling is performed. General observations should be recorded, including areas of soft soils, organic materials, exposed rock, unstable areas, and other important details. The person performing the field reconnaissance work should be experienced in geological mapping and evaluations. Where time is available, some geotechnical issues may benefit from an extended period of instrumentation and monitoring to measure critical geotechnical parameters, such

5 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-3 as fluctuations of groundwater levels and, in the case of slope stability, the location and shape of the failure surface. A properly conducted study of alignment options can potentially result in significant cost savings, especially if there is flexibility given to the designers to locate the new roadway and structures in the most geotechnically favorable locations. 5. ROADWAY CENTERLINE CUT AND EMBANKMENT INVESTIGATIONS Soil explorations are conducted along the proposed roadway alignment for the purpose of defining the geotechnical properties of foundation materials. This information is used to define the limits of potential Borrow materials or unsuitable foundation materials that could contribute to settlement or slope stability problems. If poor materials are encountered, the subsurface data can assist designers in developing remedial measures and designing stable cut or fill slopes. This information also aides the designer of the pavement section. Roadbed Design Engineers should be consulted to determine locations where soil samples are needed for design of structural section. Criteria for centerline investigations vary substantially, depending on the location of the proposed roadway, the anticipated subsurface materials, and the type of roadway. It is important that the Geotechnical Engineer visit the site to make sure that all portions of the investigation are planned thoughtfully and are accomplishable so duplication of effort does not occur. The overall investigation costs can be reduced significantly if, for example, the information for a structure and the centerline can be obtained from a single boring. In general, borings should be placed at 200- to 500-foot intervals along the alignment, depending on whether the subsurface conditions are variable or uniform, respectively. Borings could be located along centerline or staggered left and right of the centerline, depending on the locations of maximum cuts and fills as well as the interpreted geology. Borings may be spaced further apart if the project does not have significant earthwork or structures and available information indicates the presence of uniform subsurface conditions. Additional borings may be required to define the limits of any undesirable materials or changes in soil stratification that could affect design and construction. In areas of highly variable soil conditions, additional borings should be included in the transverse direction to determine the three-dimensional variability of subsurface materials. For roadway widening projects that provide additional lanes, borings should be placed at the outer edge of the new lane alignment, which may require difficult mobilization on existing slopes. In areas of significant cut or fill, where stability analysis is anticipated, a minimum of two borings should be placed at critical cross-sections to identify the stratigraphy within and above the crest of cuts and beneath and beyond the toe of embankments. In situ testing and instrumentation may be necessary to determine shear strength and groundwater levels over time. Where slope stability is a concern, inclinometer instruments could be installed

6 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-4 during the design phase and used later as a baseline for monitoring slope stability during construction. In all cases, a minimum of three samples per mile, or three per project, whichever is greater, should be obtained for each stratum encountered. Each of the samples representing a particular stratum should be obtained from a different location, with sampling locations spread out over each mile. Samples should be of adequate size to permit classification, moisture content testing, gradation testing, R-Value testing, and Atterberg limits tests. Undisturbed samples should be obtained for any anticipated strength, consolidation, or other specialized testing needs. Borings in areas of little or no grade change should extend to 5 to 10 feet below grade, or drainage pipe or culvert invert level, whichever is deeper. In some cases, including an occasional boring that extends 20 feet below grade is helpful. These deeper borings assist to better define overall geology that could potentially affect design and construction. Deeper borings also apply to projects with deep construction items, such as proposed buried storm sewer systems. Borings typically should include Standard Penetration Tests (SPT). In areas of cuts, borings should extend 5 to 10 feet below the proposed ditch grade. If potentially unstable conditions or materials are encountered at this depth, borings should be extended an additional 10 feet. SPT samples, cuttings samples, undisturbed samples and core samples should be obtained as appropriate for testing and analyses. Strength testing (triaxial tests with pore pressure measurements), consolidation testing, and groundwater instrumentation should be considered to develop parameters for stability and settlement analysis. Some borings for cut slopes in residual soils and slide debris may be drilled to obtain a continuous soil profile for detailed examination of potential weak zones. Materials to be excavated should be evaluated for shrink/swell and for use (R-Value testing) on either the project or disposal. Suitable disposal sites may need to be identified. In areas of fill, borings should extend until all unsuitable materials have been penetrated and the predicted stress from the foundation loading is less than 10 percent of the original overburden pressure. Additional borings may be included to investigate the conditions at the toe of the proposed fill or for in situ tests or speed-drilled instruments installations. A speed-drilled instruments installation boring is a hole that is drilled solely for installing a geotechnical instrument such as vibrating wire piezometer. Although most borings are sampled while being drilled, a speed drilled boring is an unsampled boring used where the stratigraphy is already defined. Because no sampling occurs, the boring is completed much quicker. Probe holes for tests, such as the cone penetrometer, may also be included as needed. 5.1 Embankments Over Soft Ground Investigations for embankments that will be constructed over soft ground comprised of muck, peat, or other very weak deposits, should include deeper and/or additional borings to determine the limits of the deposit, as necessary, in order to properly analyze and

7 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-5 mitigate their effects on embankment settlement and stability. SPT, cuttings, and undisturbed samples should be retrieved in order to classify materials and determine strength and consolidation properties. Some very soft and highly organic materials should be tested in situ with vane shear equipment because retrieving undisturbed samples could be very difficult, and testing disturbed samples typically produces unreliable results. If time allows, groundwater instruments should be installed and monitored to determine how groundwater levels fluctuate throughout the year. 5.2 Bridge Approach Embankments At least one boring should be taken at the point of highest fill. Usually borings taken for bridge abutments are adequate as long as appropriate samples are obtained for both structure and embankment applications. If settlement or stability problems are anticipated, as may occur due to the height of the proposed embankment and/or the presence of poor foundation soils, additional borings should be taken in the longitudinal and transverse directions. The first of these borings should be no more than 15 feet from the abutment. The remaining borings should be placed at 100-foot intervals until the height of the fill is considered insignificant. Borings should be completed at the toe of the proposed embankment slopes and at the embankment centerline. Borings should be continued to a depth at which the proposed stress increase due to the new embankment load is less than 10 percent of the original overburden pressure and unsuitable founding materials have been penetrated. 5.3 Rock Slopes Some road alignments may require cut slopes in rock. The geotechnical investigation should be planned to provide the data and geologic interpretations for the analysis and design of stable rock cuts and an evaluation of the constructability of the proposed cuts. A geologic reconnaissance is essential to map geologic conditions and rock structure, which includes office studies of the geology and aerial photographs, quadrangle maps, etc., as well as the field work. The Rockfall Hazard Rating (RHRS) database should be researched for information concerning past and potential rockfall hazards. If rock slopes currently exist on the project, they should be examined for rockfall evidence and unstable rock structure features. Discontinuities and, to a lesser extent, the intact rock strength control rock slope stability at various cut slope angles. Stereonet projections of structural discontinuities (including rock fractures, joints, bedding planes, faults, foliations, etc.) are an extremely useful technique for evaluating the potential for various types of rock slope instabilities. Subsurface investigations should utilize rock-coring methods where appropriate, using double or triple core barrels to obtain a high percentage of core recovery. The core should be carefully logged, noting all discontinuities and unique features and determining the Rock Quality Designations (RQD). Consider whether to obtain oriented core or utilize in-borehole photography to determine the alignment of rock structure. Borings should

8 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-6 extend a minimum of 15 feet below the planned excavation depths. Deeper borings may be required if potentially unstable strata are encountered near the base of the proposed cuts. Geotechnical parameters for rock include the orientation of discontinuities, the spatial relationship between proposed cut slopes and mapped discontinuities and the resistance to movement (shear strength) along the discontinuities, as well as the overall rock strength and hardness. The strength along discontinuities can be estimated, or specialized tests could be performed. Discontinuity strength is rarely the same in all directions, since even small variations along the joint surface (steps or undulations), known as asperities, strongly influence the potential for sliding along that surface. This affect is most pronounced when the asperities are oriented perpendicularly to the direction of sliding. Since it is difficult to reproduce field conditions in the laboratory for some applications, in situ direct shear tests may be needed. Intact rock strength/hardness can be estimated by point load tests, or determined by performing unconfined compression tests. Groundwater conditions could affect slope design and stability, and therefore should be measured in boreholes. Critical applications may require observation wells or piezometers. Springs and perched water zones should also be mapped. 6. LANDSLIDE INVESTIGATIONS Landslide areas should have been detected in the early stages of the project by means of research and reconnaissance. It is important to conduct a thorough geologic reconnaissance in terrain that might include landslides; otherwise, a landslide may go undetected and not investigated during the field exploration and drilling phases. One landslide reference is the TRB publication on Landslides: Investigation and Mitigation, Special Report 247. To design a landslide remediation, the size and depth of the slide must be known. Inclinometers and piezometers should be installed to accurately define the depth of movement and existing piezometric levels. When monitored over several months or years, this instrumentation can be very valuable in determining the behavior of the landslide and the relationship between periods of active slide movement and seasonal groundwater levels. As a minimum, two instrumented borings should be drilled along the cross-section (axis of movement) of the slide. Larger slides will usually require four or more borings to adequately define the failure shear zone. Borings should extend through the full depth of landslide material, terminating at least 15 feet into underlying stable material. Generally, the boring depths for at least one or two borings should be made even deeper to ensure that an accurate interpretation of the depth of the failure was made and to identify any underlying zones of weakness that could affect the mitigation design. Shallow slides (approximately less than 20 feet deep) can sometimes be effectively evaluated using test pits or trenches, which can expose and allow positive identification of the failure shear zone, its shape and inclination.

9 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-7 Monitoring slide movement can be augmented with a line of survey hubs, referred to as a tagline. The hubs should be placed along the axis of the slide and extend beyond the interpreted limits of movement. A cross line, perpendicular to the slide axis, can also be used. The hubs should be surveyed on a regular basis. Movements should be recorded in the X, Y and Z directions. The results can help define the type of slide, the rate of movement, changes in the slide limits, and areas of greatest activity. The vector sums of the X, Y, and Z movements can be plotted and used to help model the actual shape of the failure surface. Piezometer instrumentation should be designed to accurately record specific groundwater heads that act on the failure shear zone and within the slide mass. It is preferable that several piezometers be installed at different depths to accurately model the groundwater conditions. It is common for the crushing and grinding of materials along the failure zone to create a less permeable zone (aquitard), which can lead to the buildup of artesian water pressure acting on the failure surface. This can dramatically decrease stability. A piezometer should be installed in that zone to determine if this condition is present. Placement of piezometers at specific target areas demands an understanding of the slide geometry, which may require a second mobilization once the inclinometers have shown the actual depth of movement. Simply increasing the depth range covered by the slotted portion of an observation well will not provide good results. In fact, the water level readings will tend to be ambiguous and unusable. This can be avoided if the slotted zone is more targeted and controlled by appropriate seals or if vibrating wire piezometers are used. The benefit of vibrating wire piezometers is that the lag time in response to water level changes is very small and continuous readings can be recorded with a data logger to show groundwater spikes that otherwise might be missed due to their short duration. Sampling in landslide areas often do not follow standard procedures because of the difficulty of identifying shear zones and need of unique types of strength and correlation testing. Often, continuous sampling is desirable to locate the slide shear zone and to obtain ample soils for testing. Undisturbed samples are obtained for shear strength testing (such as triaxial undrained peak shear tests on overburden materials and repeated direct shear tests on shear zone material). If undisturbed samples are not possible to obtain from the suspected shear zones, disturbed samples could be remolded in the laboratory prior to testing. 7. BORROW AREA INVESTIGATIONS Test pits, trenches, and various types of borings can be used for exploration of potential Borrow areas. Samples should be obtained to permit classification, gradation, R- value, compaction, and chemical testing of each material type, as applicable. The extent of the exploration will depend on the size of the Borrow area, the amount of Borrow needed, and the amount of sample required to complete a full suite of specific tests.

10 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES STRUCTURE INVESTIGATIONS The purpose of structure borings is to provide sufficient information about the subsurface materials to design the structure foundations and to provide constructionrelated recommendations. All structure borings should include Standard Penetration Testing (SPT) at regular intervals unless other sampling methods and/or testing are being performed. Undisturbed samples are often obtained to determine shear strengths in addition to material properties (such as moisture contents, unit weight, Atterberg limits, gradation). The borings can sometimes be supplemented with in situ test borings, such as the pressuremeter if field-developed p-y curves are needed for lateral pile analysis. Usually the groundwater level encountered during drilling will suffice for subsequent analysis. 8.1 Bridges Complete at least one borehole at each pier or abutment location. The hole pattern should be staggered so that borings occur at the opposite ends of adjacent piers. Wide pier foundations or abutments that are over 100 feet wide transverse to centerline (roadway width) may require at least two borings, preferably at the extremities of the proposed substructure. For widening of existing structures, the total number of borings may be reduced, depending on the available information for the existing structure. Approximate locations of piers and abutments may be deduced based on experience and a preliminary design concept for the structure when exact support locations are unknown. Borings should be placed at no more than 100-foot intervals along the alignment when exact or approximate support locations cannot be determined. Borings should be continued to a depth that the predicted stress from the foundation and approach embankment loading is less than 10 percent of the original overburden pressure, or until all unsuitable foundation materials have been penetrated and the underlying competent bearing zone penetrated a minimum distance (i.e., 10 to 15 feet into competent bedrock). This depth should be increased, when existence of boulders are possible, to confirm that the rock is bedrock and not a large boulder. If no data is available for predicting the foundation stress, extend the boring until at least 20 feet of bedrock or other competent bearing material (N-values of 50 or greater) is encountered. Additionally, borings should be performed to a depth that the design takes into account scour and lateral loading requirements. When using the Standard Penetration Test, SPT split-spoon samples should be recovered continuously within the upper 20 feet of any boring, and then every five feet down to 60 feet. For deep foundations, an additional zone of sampling every ten feet from 60 feet deep to 100 feet deep is included where SPT samples can be recovered. When cohesive soils are encountered, undisturbed samples should be obtained at 5- foot intervals in at least one boring. Undisturbed samples should be obtained from more than one boring where possible. In situ vane shear tests are recommended where soft clay, peat or other soft or highly organic materials are encountered. Representative undisturbed samples should be obtained in these materials for index testing and possible

11 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES 5-9 laboratory shear strength testing. Chemical tests are required on all new bridge projects. As a minimum, one test should be conducted on each soil that will be in contact with structural steel elements. When rock is encountered, successive core runs should be made with the objective of obtaining the best possible core recovery. The RQD should be determined from rock cores. SPT s should be performed between core runs in soft rock, typically at 5-foot intervals. In the case of a water crossing, samples of streambed materials and each underlying stratum should be obtained for determination of the median particle diameter, D50, for scour analysis. In addition, samples should be obtained to determine Plasticity Index (PI) and particle size distribution for design of filter fabrics. 8.2 Earth Retaining Walls The following are general investigation requirements for retaining wall design: At retaining wall locations, borings should be taken at a maximum interval of one per 100 feet of the wall with a minimum of 2 borings and as close to the wall alignment as possible. Retaining structures with tiebacks or soil nails will need an additional row of borings where the anchor load zone is anticipated. Borings should be continued to depths that all unsuitable founding materials are penetrated, and the proposed stress increase due to the retaining wall structure will be less than 10 percent of the original overburden pressure. Sampling and in situ testing criteria are the same as for bridges. 8.3 Buildings In general, one boring should be made at each corner and one in the center. This may be reduced for small buildings. For large buildings or highly variable site conditions, one boring should be taken at each support location. Refer to building foundation texts for additional guidance in planning the geotechnical investigation. 8.4 Drainage Structures (Culverts) Borings should be taken at proposed locations of box culverts. Trenches or hand augured borings may suffice for smaller structures. For box culverts, borings should extend a minimum of 15 feet below the bottom of the culvert, or until 5 feet of firm, competent material is encountered, whichever is deeper. For smaller structures, exploration holes should extend at least 5 feet below the bottom of the structure, or until 5 feet of firm, competent material is encountered, whichever is deeper. Chemical testing must be performed for each site. Material from each stratum above the invert elevation should be tested. For drainage systems parallel to roadway alignments, tests should be performed at 1,500-foot intervals along the alignment.

12 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES Large Load Light and Sign Structures One boring should be made at each designated location. Borings should extend approximately 40 feet into suitable soil, or 5 feet into competent rock. Deeper borings may be required for cases with higher torsional loads, or if large boulders are anticipated. Other criteria are the same as for bridges. 8.6 Tunnels Due to the extreme variability of conditions under which tunnels are constructed, investigation criteria for tunnels should be established for each project on an individual basis. Refer to tunneling texts for detailed guidance, or consult with an expert in tunneling. 8.7 Detention Basins Test pits and trenches typically are adequate for the investigation of proposed detention basins. Samples should be obtained to permit classification, gradation, Plasticity Index, unit weight, moisture content, R-value, compaction, permeability test, and/or chemical testing of each material type, as applicable. The extent of the exploration will depend on the size of the detention basin area. Field tests may include infiltration tests, as applicable.

13 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES FIGURES 5-1: Guidelines for Boring Layout Geotechnical Features Bridge Foundations Boring Layout For piers or abutments less than 100 feet wide, provide a minimum of one boring with the hole pattern staggered so that borings occur at the opposite ends of adjacent piers. For piers or abutments over 100 feet wide, provide a minimum of two borings, one at each end of the pier or abutment. Additional borings should be provided in areas of erratic subsurface conditions. Retaining Walls A minimum of two borings should be performed for each retaining wall. For retaining walls more than 100 feet in length, the spacing between borings should be no greater than 200 feet. Retaining structures with tiebacks or soil nails will need an additional row of borings where the anchor load zone is anticipated to estimate lateral loads and anchorage capacities. Include additional borings outboard of the wall line to define conditions at the toe of the wall as needed. Roadways The spacing of borings along the roadway alignment generally should not exceed 200 to 500 feet. The selected spacing and location of the borings should be based on the geologic complexity and soil/rock strata continuity in the project area, with the objective of defining the vertical and horizontal boundaries of distinct soil and rock units within the project limits. Cuts A minimum of one boring should be performed for each cut slope. For longer cuts, the spacing between borings along the length of the cut should generally be between 200 and 400 feet, as needed, based on the complexity of the geology. At critical locations and high cuts, provide a minimum of two borings in the transverse direction to model the existing geological conditions for stability analyses. Embankments Use criteria presented above for cuts. Culverts A minimum of one boring at each major culvert. Additional borings should be provided for long culverts or in areas of erratic subsurface conditions. Note: This table is based on the Subsurface Investigations Manual (FHWA/NHI ). Also see FHWA Geotechnical Checklist and Guidelines (FHWA-ED ).

14 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES : Minimum Requirements for Boring Depth Areas of Investigation Bridge Foundations* Recommended Boring Depth All borings should be extended below the estimated scour depth. Spread Footings For isolated footings of breadth L f and width 2B f, where L f 2B f, borings should extend a minimum of two footing widths below the bearing level. For isolated footings where L f 5B f, borings should extend a minimum of four footing widths below the bearing level. For 2B f L f 5B f, the minimum boring length should be determined by linear interpolation between depths of 2B f and 5B f below the bearing level. Deep Foundations In soil, borings should extend below the anticipated pile or shaft tip elevation a minimum of 20 feet, or a minimum of two times the maximum pile group dimension, whichever is deeper. For piles bearing on rock, a minimum of 10 to 15 feet of rock core should be obtained at each boring location to verify that the boring has not terminated on a boulder. For shafts supported on or extending into rock, a minimum of 10 to 15 feet of rock core, or a length of rock core equal to at least three times the shaft diameter for isolated shafts or two times the maximum shaft group dimension, whichever is greater, should be extended below the anticipated shaft tip elevation to determine the physical characteristics of rock within the zone of foundation influence. Retaining Walls Extend borings to depth below final ground line between 0.75 and 1.5 times the height of the wall or to where the net increase in soil stress is less than 10% of the existing effective stress in the soil at that depth. Where stratification indicates possible deep stability or settlement problem, borings should extend to hard stratum. For deep foundations, use the criteria presented above for bridge foundations. Roadways (minimal grading) Cuts Embankments Culverts Extend borings a minimum of 10 feet below the proposed subgrade level. Boring should extend a minimum of 15 feet below the anticipated depth of the cut at the ditch line. Boring depths should be increased in locations where base stability is a concern due to the presence of soft soils or weak zones, or in locations where the base of the cut is below groundwater level to determine the depth of the underlying pervious strata. Extend borings a minimum depth equal to twice the embankment height, unless a hard stratum is encountered above this depth. Where soft strata are encountered which may present stability or settlement concerns, the borings should extend to hard material. Use criteria presented above for embankments. Note: This table is based on the Subsurface Investigations Manual (FHWA/NHI). Also see FHWA Geotechnical Checklist and Guidelines (FHWA-ED ). Requirements for highway bridges are based on AASHTO Standard Specifications for Design of Highway Bridges.

15 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES : Guideline Sampling and Testing Criteria Sand-Gravel Soils Rock Silt-Clay Soils Ground Water Soil Borrow Sources Quarry Sites SPT (split-spoon) samples should be taken at 5-foot intervals or at significant changes in soil strata. Continuous SPT samples are recommended in the top 15 feet of borings made at locations where spread footings may be placed in natural soils. SPT jar or bag samples should be sent to lab for classification testing and verification of field visual soil identification. Continuous cores should be obtained in rock or shales using double or triple tube cone barrels. In structural foundation investigations, core a minimum of 10 feet into rock to insure it is bedrock and not a boulder. Percent core recovery and RQD value should be determined in field or lab for each core run and recorded on boring log. SPT and undisturbed thin wall tube samples should be taken at 5-foot intervals or at significant changes in strata. A sufficient number of samples, suitable for the types of testing intended, should be obtained within each soil layer. Take alternate SPT and tube samples in same boring or take tube samples in separate undisturbed boring. SPT jar or bag samples should be sent to lab for classification testing and verification of field visual soil identification. Tube samples should be sent to the lab for consolidation testing (for settlement analysis) and strength testing (for slope stability and foundation bearing capacity analysis). Field vane shear testing is recommended to obtain in situ shear strength of soft clay, silt, and well-rotted peat. Water level encountered during drilling, at completion of boring, and at 24 hours after completion of boring should be recorded on boring log. In low permeability soils, such as silts and clays, a false indication of the water level may be obtained when water is used for drilling fluid and adequate time is not permitted after hole completion for the water level to stabilize (more than one week may be required). In such soils a plastic pipe water observation well should be installed to allow monitoring of the water level over a period of time. Seasonal fluctuation of water table should be determined where fluctuation will have significant impact on design or construction (e.g. Borrow source, footing excavation, excavation of toe of landslide, etc.). Zones of artesian water and seepage should be measured and recorded. Exploration equipment that will allow direct observation and sampling of the subsurface soil layers is most desirable for material site investigations. Equipment consisting of backhoes, dozers, or large diameter augers is preferred for exploration above the water table. Below the water table, borings can be used. SPT samples should be taken at 5-foot intervals or at significant changes in strata. Samples should be sent to lab for classification testing to verify field visual identification. Groundwater levels should be recorded. Piezometers or observation wells should be installed to monitor water levels where significant seasonal fluctuation is anticipated. Rock coring should be used to explore new quarry sites. Use of double or triple tube core barrels is recommended to maximize core recovery. For riprap source, spacing of fractures should be carefully measured to allow assessment of rock sizes that can be produced by blasting. For aggregate source, the amount and type of joint infilling should be carefully noted If assessment is made on the basis of an exiting quarry site face, it may be necessary to core or use geophysical techniques to verify that the nature of the rock does not change behind the face or at depth. Core samples should be sent to lab for rock quality tests to determine suitability for riprap or aggregates. Note: This Table is based on FHWA Geotechnical Checklist and Guidelines (FHWA-ED ).

16 GEOTECHNICAL INVESTIGATION PLANNING GUIDELINES REFERENCES AASHTO, Manual on Subsurface Investigations, 1988 ASCE, Subsurface Investigation for Design and Construction of Buildings, Manual and Report on Engineering Practice No. 56, 1976 Department of the Navy, Soils Mechanics Design Manual, 7.1, NAVFAC DM-7.1, Naval Facilities Engineering Command, 1986 FHWA, Advanced Course on Slope Stability, Vol. 1, FHWA- SA-94, 1994 FHWA, Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications, FHWA-PD , 1985 FHWA, Driled Shafts for Bridge Foundations, FHWA-RD , 1993 FHWA, Geotechnical Engineering Notebook, (Sections added when needed) FHWA, Soils and Foundations Workshop Manual, 2nd Edition, FHWA HI , 1993 FHWA, Subsurface Investigations, NHI Course No , FHWA-HI , 1997 FHWA, Subsurface Investigations Geotechnical Characterization, Reference Manual for NHI Course No , FHWA-NHI , 2002 FHWA, Geotechnical Instrumentation, FHWA-HI , 1998 FHWA, Manual on Design and Construction of Driven Pile Foundations, FHWA-HI and 014, 1996 FHWA, Rock Slopes: Design, Excavation, Stabilization, Turner-Fairbank Research Center, FHWA-TS Highway NCHRP, Recommended Guidelines for Sealing Geotechnical Exploratory Holes, National Cooperative Highway Research Program, NCHRP Report 378. Peck, R.B., Hanson, W.E., & Thornburn, T.H., Foundation Engineering, 2nd Ed., Wiley, Terzaghi, K, and Peck, R.B., Soil Mechanics in Engineering Practice, 2nd Ed., Wiley, 1967 TRB, Landslides: Investigation and Mitigation, Special Report 247, 1996