105 - Geotechnical Investigations

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105.1 Introduction

The purpose of this section is to establish Department policies and procedures for geotechnical investigations, including subsurface investigation (e.g., test borings, piezometers, in-situ testing, sampling), soil/rock laboratory testing, and report preparation guidelines to be used on the foundation design of Delaware bridges, associated earth retaining structures, and other highway structures.

105.2 Terms

ASTM Standards – ASTM International standards. Most of the standards referred in this section are part of Volume 4.08 Soil and Rock (D420 – D5876).

AASHTO Standards – American Association of State Highway and Transportation Officials (AASHTO) standards.

Bedrock – Consolidated rock underneath surface soil deposits. Bedrock exposed at the surface is known as rock outcrop. For subsurface exploration purposes, bedrock is typically defined at auger refusal (or any other penetration technique refusal), not to be confused with very dense residual soil, isolated boulders, or cobbles.

Boulders and Cobbles – Rounded fragments of rock, cobbles are typically bigger than 3 inches, while boulders are bigger than 12 inches (approximately average sizes). These particles represent obstructions for drilling and should be carefully identified to avoid confusing them with bedrock during subsurface investigations.

Decomposed Rock – Weathered rock due to physical and chemical processes. Typically considered as an Intermediate Geomaterial (IGM).

FHWA GEC-5 – Abbreviation for FHWA-IF-02-034 Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties (2002).

FHWA NHI-01-031 – Abbreviation for FHWA NHI-01-031 Subsurface Investigations – Geotechnical Site Characterization Reference Manual (2002), which supersedes the AASHTO Manual on Subsurface Investigations (1988).

Intermediate Geomaterial (IGM) – A material that is transitional between soil and bedrock in terms of strength and compressibility. Careful consideration should be given to IGM to avoid over predicting their strength and under predicting their compressibility.

Organic Matter – Decomposed material in soil derived from organic sources such as plant remains. Typically unsuitable for foundations based on low strength and high compressibility. Muck is a deposit of soil with a high content of organic matter, typically unsuitable for foundations.

Rock Mass Rating (RMR) – A geomechanical classification system for rocks. It expresses the quality of bedrock with one index based on the most relevant parameters, such as the intact rock strength, spacing and conditions of joints, and groundwater conditions.

Rock Quality Designation (RQD) – A measure of the degree of jointing or fracture in a rock mass. It is measured as the cumulative length of the drill core fragment having lengths of 4 inches or more, divided by the entire drill core length. It is expressed as a percentage.

Unsuitable Material – Refers to soil and rock deposits that are unsuitable for geotechnical applications because of low shear strength and high compressibility. This includes weak, highly plastic clays, organic soils, and soft weathered rock (if considered for Deep Foundations).

105.3 Subsurface Investigations

A subsurface investigation is typically defined as the investigation program performed to geotechnically characterize a site. It encompasses many aspects, such as a literature search and review of available published information regarding soil and geology maps, a site reconnaissance, and often in-situ testing to define a geotechnical model. A laboratory testing program is also associated with the subsurface investigation, typically performed on samples recovered during drilling operations.

The absence of a thorough geotechnical investigation or inadequate data may result in a foundation system with a large factor of safety, which may be unnecessarily expensive; an unsafe foundation; and/or construction problems, disputes, and claims.

A proper subsurface investigation should include structural borings. The common methods of advancing structural borings are auger drilling on soil and rotary coring (mostly for recovering rock cores). Auger drilling provides a disturbed soil sample that can be used for material characterization purposes. Undisturbed samples are typically obtained using a thin-walled sampler referred as a Shelby tube. Shelby tubes are commonly used for obtaining undisturbed samples of cohesive soils; they are not very effective for retrieving samples in granular soils. Rotary coring provides a rock core sample that can be used for laboratory testing.

The term “structural boring” is used throughout this section to refer to test borings performed for subsurface investigations at structure locations. These borings should not be confused with other types of borings, such as probe holes advanced only with the purpose of confirming top of rock elevation, dewatering holes advanced to lower the water table, piezometers to monitor groundwater table fluctuations, or any other kind of hole drilled with a different purpose. Note that there are also test borings performed for subsurface investigations on roadways, they are referred to as “roadway borings” and are not covered in this section.

As a boring is advanced in soil, Standard Penetration Tests (SPTs, ASTM D1586 – 1) are performed. See FHWA GEC-5 for detailed information regarding the SPT procedure.

Other in-situ test techniques can be used with or without borings, such as:

  1. Cone Penetrometer Tests (CPT/CPTU/SCPTU) (ASTM D 5778)
  2. Flat Dilatometer Test (DMT)
  3. Pressuremeter Test (PMT) (ASTM D 4719)
  4. Vane Shear Test (VST) (ASTM D 2573)

These in-situ tests do not provide samples, but directly measure soil resistance that can be correlated with shear strength, deformation modulus, and pore water dissipation. These methods can be used if the geotechnical designer believes they will provide useful information that cannot be provided by the regular SPT tests.

Common geophysical test methods that may be considered include:

  1. Seismic Methods: seismic refraction, spectral analysis of surface waves (SASW), and multi-channel analysis of surface waves (MASW)
  2. Electrical Methods: electric resistivity imaging, electromagnetics (EM), ground penetrating radar (GPR)

Although these methods are not typically used in most bridge projects, they could provide useful geological information almost impossible to obtain with regular borings. They are frequently used to detect anomalies in soil and bedrock. Additional information regarding subsurface exploration methods and in-situ testing may be found in FHWA GEC-5, as well as FHWA’s Every Day Counts 5 initiative “A-GaME” (https://www.fhwa.dot.gov/innovation/everydaycounts/edc_5/geotech_methods.cfm) and the SHRP2 R.

The geotechnical investigation should provide sufficient information to be used by the designer for the tasks described in the following subsections.

105.3.1 Estimating Soil and Rock Properties

Soil properties can be estimated from existing correlations with the SPT "N" values and other in-situ tests, such as pocket penetrometer tests and VSTs on cohesive soils.

The SPT is the most commonly used test in subsurface investigations. It is used to determine N-values. The N-values and other in-situ test results from the SPT can provide an indication of soil density, consistency, friction angle φ, and shear strength. N-values must be corrected for effective overburden pressure and hammer efficiency in order to use empirical correlations to develop preliminary values for friction angle and shear strength. See A10 – Foundations for more information regarding correcting N-values and correlating them with soils physical properties.

Rock properties can be estimated from retrieved rock cores using the RQD and the rock type. Other common rating systems such as the RMR should be used to estimate the rock mass shear strength.

Note that bedrock is typically expected only in northern New Castle County. The designer can refer to the Delaware Geological Survey website (http://www.dgs.udel.edu/) for additional useful information.

Rock coring is to be performed using a double tube, wire-line preferred NX core barrel, 2 1/8 inches inside diameter. Different core barrel lengths are available, for example 5 and 10 feet. The Department preference is to use a maximum length of 5 feet to avoid potential damages to the long cores that may result in lower RQD values.

For consultant design projects, the designer shall photograph and store the rock cores until construction is completed. After construction is completed, the cores shall be provided to the Delaware Geological Survey.

105.3.2 Estimating Ground Water Table Elevation

The subsurface investigation should determine the groundwater table elevation by measuring the water depth in the structural borings immediately after completion and a minimum 24 hours after completion. The 24-hour reading is typically needed to establish the groundwater table elevation. There are cases for which it may not be needed because the location of the water table is evident, for example in soils next to or below streams or in soil borings having only dry samples. The water depth readings can be correlated with the moisture description from the retrieved samples and laboratory moisture content tests.

Short-term monitoring typically consists of obtaining water depth readings immediately after completion (0 hour) and 24 hours after completion. The 0-hour reading is not always reliable because water may have been introduced into the hole as a result of coring operations or uncontained surface runoff. The 0-hour reading is commonly supplemented by the 24-hour reading. For most cases, the 24-hour reading is considered to be reliable because any disturbance to the local groundwater table should have stabilized after this period. If 24-hour readings are to be obtained, the Department preference is to install perforated screen pipe in the test boring hole after drilling is completed.

There are special cases that require additional short-term monitoring, normally at 48-hour and 72-hour increments. A few examples requiring this kind of short-term monitoring include drilling on clays with very low hydraulic conductivity where local groundwater disturbances may take longer to stabilize and penetrating confined aquifers with artesian pressure. For these cases, the Department preference is to use an open standpipe piezometer.

Because the groundwater elevation may vary throughout the year, the designer may request short- and long-term groundwater elevation monitoring. Short-term monitoring is typically performed at 24-hour, 48-hour, and 72-hour increments. Long-term monitoring requires installation of monitoring wells at the site.

Accurate groundwater level information is needed for estimation of soil densities, determination of effective soil pressures, and preparation of effective soil pressure diagrams. Water levels will indicate possible construction difficulties that may be encountered during excavation and the degree of dewatering effort required. This information is also needed to identify potential liquefiable sands, also known as “running sands,” as discussed in Section 210 – Foundations.

105.3.3 Estimation of Bearing Capacity

Bearing capacity for shallow and deep foundations systems on soil and/or rock should be evaluated based on the results of the subsurface investigation and laboratory test programs. A10 – Foundations presents the different methodologies used to calculate bearing capacity on soil and rock for both service and strength limit states. For stream environments, the geotechnical analysis of bridge foundations shall be performed on the basis that all streambed material in the scour prism above the total scour line has been removed.

105.3.4 Estimation of Settlement

Magnitude and rate of settlement should be evaluated based on the results of the subsurface investigation and laboratory testing program. In general, granular materials and stiff fine- grained soils exhibit elastic settlement. Elastic settlement occurs rapidly during construction or shortly after. See A10 – Foundations for more information regarding estimation of elastic settlement.

Fine-grained soils (clays and silts) with a soft to medium stiff consistency usually exhibit consolidation settlement. Parameters describing the consolidation behavior (magnitude and rate of settlement) can be estimated based on results, such as SPT N values and pocket penetrometer readings. However, the Department recommends obtaining these values from a 1-D consolidation test (ASTM D2435) using undisturbed soil samples. See A10 – Foundations for more information regarding estimation of consolidation settlement.

105.3.5 Estimated Depth of Unsuitable Materials

The subsurface investigation and laboratory test programs should provide sufficient information to determine the depth of unsuitable materials, such as weak fine-grained layers and soft/weathered bedrock. The foundation system should be designed either to work with these constraints, proving that enough bearing resistance is available at an acceptable level of settlement, or bypass these layers and bear on underlying competent strata (i.e., deep foundations). Quantities for over excavation (undercutting) and backfilling will be estimated based on the depths of unsuitable materials.

Deep foundations are often used to bypass weak/soft compressible strata and transmit the foundation loads to more competent underlying layers. In these cases, settlement of the weak/soft soils surrounding the piles should be evaluated for settlement and associated downdrag.

105.3.6 Global Stability

Global stability (also known as overall stability) of substructures, retaining walls, and embankments should be evaluated based on the results of the subsurface investigation and laboratory test programs. See A10 – Foundations and A11 – Abutments, Piers, and Walls for more information regarding estimation of global stability against circular and planar failures.

Per A11 – Abutments, Piers, and Walls, a minimum factor of safety of 1.3 shall be used when geotechnical parameters are well defined and the slope does not support or contain any structural element. A minimum factor of safety of 1.5 shall be used where geotechnical parameters are based on limited information, or the slope contains or supports a structural element. These factors of safety are equal to the inverse of the specified resistance factors by the load and resistance factor design (LRFD) design methodology (F.S. = 1/φ).

105.3.7 Corrosive Environment

The subsurface investigation should provide sufficient information to ascertain any deleterious elements of the existing subsurface soils. The effects of corrosive soils and groundwater must be taken into account in the design of the foundation. The soils investigation shall provide the following minimum information to determine the potential deterioration to footings, driven piles, and drilled shafts:

  1. Soil pH, sulfate, and chloride contents in soil and groundwater and moisture content;
  2. General soil profile, including type, variation, depth and layering of fill and undisturbed natural soils, and groundwater level;
  3. Previous land use;
  4. Soil resistivity (laboratory test on soil samples); if evaluation of data with respect to criteria in Section – Corrosion and Deterioration indicates a potential corrosion problem, a field resistivity survey may be warranted; and
  5. If foundations are located in open water, a representative water sample should be analyzed for chlorides, sulfates, bacteria, pH, and the velocity should be measured.

105.3.8 Lateral Squeeze

Bridge abutments and similar structures supported on pile foundations installed through soft soils that are subjected to unbalanced embankment fill loading shall be evaluated for lateral squeeze. Lateral squeeze could also occur at the toe of slope embankments even without a structure. Refer to Section – Lateral Squeeze for more information.

105.4 Subsurface Investigation Request

Material and Research (M&R) is responsible for performing the subsurface investigation and laboratory testing program for in-house design projects. The designer should request test borings and in-situ field testing through M&R to be performed at selected locations. For consultant design projects, the consultant is responsible for the subsurface investigation program.

105.4.1 Request for Test Borings

Borings should be requested from M&R by completing the Soils/Rock Testing Program request form available on the DRC (Figure 105-2). For consultant design projects, the consultant should consider following the same process and procedures for their own subsurface investigation program. Note that the consultant will be responsible for all permits, maintenance of traffic, and other required coordination when obtaining the borings or field testing.

The request should be accompanied by the following:

  1. Location map showing the site with respect to the general area.
  2. Plan of the existing or proposed structure showing the approximate locations of the proposed substructure units and the borings requested. The plan should show as a minimum:
    1. Existing right-of-way limits and access.
    2. Location control points to assist the boring crew in accurately locating structural borings by station and offset, northing/easting, and/or latitude/longitude; and to record ground surface elevations.
    3. Any known underground and/or overhead utilities.
  3. Depth of structural borings, including boring termination criteria.
  4. In-situ testing at depths and borehole locations.
  5. Design schedule.
  6. Boring request form.

Depending on the size and complexity of the project, a meeting between the designer and M&R may be practical to discuss the scope and schedule of the proposed project. A two-stage boring schedule may be desirable for larger projects: an initial program followed later by an extensive program based on the results of the initial work.

The layout, number, and depth of structural borings depends on the local geology and proposed substructure foundations. Each project site should be treated individually and the investigation should not follow a specified format. The following are general guidelines that can be modified depending on specific circumstances. See FHWA GEC-5 for additional information regarding recommended boring layouts and boring termination criteria. Quantity and Location of Structural Borings

The specific number of structural borings depends on the complexity of the structure, the anticipated subsurface conditions, and the level of risk that can be tolerated for the structure. For example, although two borings are typically considered to be enough for a culvert, or in some cases, for a small single-span bridge, two borings may not be sufficient for another single-span bridge where conditions significantly change at each substructure. The number of borings per substructure should be determined based on anticipated subsurface conditions rather than the geometry of the substructure.

The following are median values, not minimum values. Median values refer to representative/average cases. Median values are recommended for project sites with limited subsurface conditions information. For example, the only information available comes from a literature search, such as soil maps, oil/gas/water wells, and geologic mapping.

The designer can increase or decrease the number of structural borings depending on the specific project and the available subsurface information at the site. For example, the designer can decrease the number of borings if old borings were drilled at the site, or if the project is located in close proximity to another structure where uniform subsurface conditions have been identified. Similarly, the designer can increase the number of borings if the subsurface investigation for an adjacent structure revealed non uniform soil/rock conditions across the site. In preparing the request, the designer should consider the following guidelines for borings:

  1. Borings should be obtained in the following median quantities:
    1. Two borings shall be obtained per abutment; this number should only be reduced if the designer is confident uniform conditions exist across the substructure. For example, the abutment is 40 feet long and local experience indicates the presence of uniform strata.
    2. One boring shall be obtained per wingwall; more borings may be needed if the adjacent borings for the abutment show non-uniform conditions across the site or the wingwall is longer than 40 feet.
    3. Two borings shall typically be obtained per pier; as for the abutment this number can be reduced if the designer is confident uniform conditions exist across the substructure.
    4. Two borings shall be obtained for pipes, culverts, and three-sided rigid frames. The borings shall be located at the inlet and outlet of these structures and shall be staggered.
    5. Two borings shall be obtained for retaining walls and similar structures (such as ground-mounted noise walls) up to 100 feet in length. For longer wall structures, additional borings should be added at 100-foot intervals.
    6. One boring shall be obtained for each ancillary structure foundation.
  2. Borings should be within 20 feet of the proposed footprint of the substructure.
  3. The borings for adjacent footings should not be located in a straight line but should be staggered at the opposite ends of adjacent footings, unless multiple borings are taken at each footing.
  4. Where rock is encountered at shallow depths, additional borings or other investigation methods such as probes (borings without samples) and test pits may be needed to establish the top of rock profile. Understanding the hardness of the rock is also important for rock excavation for spread footings and rock sockets. Additional rock samples may be required in areas where the hardness of rock varies or has not been established.
  5. Where muck, organic soils, weak, and/or unsuitable materials are encountered at shallow depths, additional borings, test pits, or other investigation methods (probes, cone penetrometers) may be needed to determine the required over excavation quantities or ground improvement.
  6. The number of borings required and their spacing depend on the uniformity of soil strata and the type of structure. Erratic subsurface conditions require close coordination between M&R and the designer. Under non-uniform conditions, additional borings may be necessary.
  7. Where spread footings are being considered, the designer should request that the driller take continuous samples. For deep foundations, continuous sampling may not be necessary while penetrating competent strata but should be provided while crossing weaker soils.
  8. The Department recommends that the designer visit the site with the driller prior to and/or during drilling operations. Depth of Structural Borings

The following are recommended criteria for boring depth termination. They should be used as general guidelines only. Termination of borings will depend on the encountered conditions:

  1. For pile foundations on soil, the designer must have soils information extending at least 10 feet below the estimated pile tip elevation. Initial borings should extend to a depth that allows the geotechnical designer to perform preliminary analyses to estimate an approximate tip elevation. Termination criteria for subsequent borings can be refined based on the results of these preliminary analyses. Examples of termination criteria for initial borings are:
    1. Twenty to 30 feet below the top of the first hard layer to ensure that the layer is of sufficient thickness. The hard layer is defined as having an N-value of 20 or more for 20 feet.
    2. For shallow deposits where the material provides limited resistance (N-value is less than 5 for fine-grained soil, 10 for coarse-grained/cohesionless soil) above the hard layer, the boring should extend a minimum of 30 feet or to refusal (N-value ≥ 50 blows/1⁄2 foot). If the weak/unsuitable material extends for a significant depth and a hard layer cannot be encountered, contact the Department Geotechnical Engineer.
  2. For pile foundations on rock, terminate borings at least 10 feet into competent rock. If top of rock is weathered/soft, consider extending and terminating borings 10 feet into underlying competent strata.
  3. For drilled shafts, terminate borings a minimum of 10 feet below the estimated pile tip elevation but no less than two times the drilled shaft width.
  4. For spread footings on soil, terminate borings below the proposed bottom of footing elevation at a minimum depth of 1.5 times the estimated footing width. If unsuitable soils are present at this depth, extend borings to more competent strata. If top of rock is encountered within 1.5 times the footing width, consider terminating borings a minimum 10 feet into competent rock. Less than 10 feet of rock requires the approval of M&R.
  5. For spread footings on rock, terminate borings a minimum of 10 feet into competent rock or 1.5 times the estimated footing width. Extend borings if voids or unsuitable soil seams are encountered in bedrock. Terminate borings in competent bedrock.

105.4.2 Boring Logs

Boring logs should contain the following information:

  1. General information: State and Federal project numbers, the bridge number, the location of the boring, start/finish dates, the surface elevation, the equipment used, the sampling method, and water level readings.
  2. Sample information: Sample number, sample depth, hammer blows per 6 inches, descriptions of the material in the samples, the amount of material recovered in each sample, the laboratory soils AASHTO classification, and RQD results.
    1. A typical soil description consists of:
      1. Water content (dry, moist, wet), apparent consistency (fine-grained soils) or density (granular soils), color, soil type, and AASHTO group name (Group Index). Example:
        • Wet, stiff, gray silty clay with trace fine to coarse sand and fine gravel. A-7-6 (19).
    2. A typical rock core description consists of:
      1. Rock type, color, hardness, degree of weathering, bedding/foliation thickness, and discontinuities spacing. Example:
        • Gneiss, grey, medium hard, moderately weathered, intensely foliated, closely fractured.
  3. The locations of undisturbed samples are designated with the sample numbers. Any other information is listed under “Remarks.”

Boring data are entered into a graphics design file using the Department's Boring Sheet program so designers can access it with computer aided design and drafting (CADD). The boring logs shall be included in the Contract Plans.

DelDOT uses the AASHTO classification, as displayed in Figure 105-1, as the primary classification system. See AASHTO M145 for the AASHTO soil classification system.


105.4.3 Coordination for Soils/Rock Testing

The designer should review the results of the test borings as soon as they are received to ensure that the borings are adequate and to give M&R as much time as possible to perform any additional tests.

For in-house design projects, the designer should work with M&R to develop the soils/rock laboratory testing program and to select the correct soil and rock samples to be tested. M&R has the capability of performing most of the soil/rock tests commonly required for bridge projects; however, M&R is not equipped to perform every test defined by AASHTO. Private testing laboratories can be used to perform other tests, if warranted.

To finalize the desired soil/rock testing program, the designer shall submit the Soils/Rock Testing Program request form (Figure 105-2) presented on the DRC – Pavement Materials Tab.


Click here to download an Excel version of the Soil Boring Request Sheet

NOTE: Direct Shear tests – specify confining stresses and target unit weight for remolded samples

The sections below provide guidance on the typical soil and rock tests used by the Department. See FHWA GEC-5 for more information regarding laboratory tests used in the estimation of properties of soil and rock. Typical Soil Tests

Soil properties can be estimated based on laboratory test results on disturbed and/or undisturbed samples. Disturbed samples obtained by SPT or directly from drilling cuttings, should be used only for material characterization tests such as, but not limited to:

  1. Soil classifications (D4318, AASHTO T88, T89, T90, ASTM D6913)
  2. Moisture content determination (AASHTO T265, ASTM D2216)
  3. Atterberg Limits (AASHTO T89/90, ASTM D4318)
  4. Specific gravity (AASHTO T100, ASTM D854)
  5. Standard and modified Proctor tests (AASHTO T99, T180, ASTM D698, D1557 )
  6. Direct shear test on remolded granular soils (AASHTO T236, ASTM D3080)
  7. Corrosion potential on soil: pH, chloride content, sulfate content, minimum resistivity on soil (AASHTO T288, T289, ASTM D4972, CalDOT 422, CalDOT 417)
  8. Determination of organic content in soils by loss of ignition (AASHTO T267)

Undisturbed soil samples obtained by using Shelby tubes or other acceptable methods should be used for laboratory testing to determine soil parameters used directly in geotechnical design. The tests mentioned above are still applicable to undisturbed samples. Some of the additional recommended tests on undisturbed soil samples are:

  1. In-situ unit weight and void content of undisturbed soil samples (AASHTO T233)
  2. One-dimensional consolidation (AASHTO T216, ASTM D2435)
  3. Swell test of undisturbed samples (ASTM D4546)
  4. Unconfined compression of cohesive soil (AASHTO T208, ASTM D2166)
  5. Unconsolidated-undrained triaxial test (AASHTO T296, ASTM D2850)
  6. Consolidated-undrained triaxial test (ASTM D4767)
  7. Consolidated-drained triaxial test (ASTM D7181)
  8. Direct shear test on undisturbed soil samples (AASHTO T236, ASTM D3080)
  9. Permeability of soil, constant or falling head (AASHTO T215, ASTM D2434, D5084) Typical Rock Tests

The unconfined compression strength of the intact rock can be estimated from laboratory tests depending on the quality of the retrieved rock core samples:

  1. For samples having a sufficient length to diameter ratio, use the unconfined compression test (ASTM D7012).
    1. The Department will allow the use of the former unconfined compression strength test method correction for samples less than 2L:1D (ASTM D7012).
    2. The Department also allows the use of the point load testing (ASTM D5731) for samples less than 2L:1D, with prior approval from M&R.

105.5 Geotechnical Report

For in-house design projects, the Geotechnical Report is prepared by M&R. For consultant design projects, the consultant is responsible for the Geotechnical Report. The objective of a Geotechnical Report is to provide a preliminary summary of the subsurface investigation data and laboratory testing programs to be used to evaluate the need of additional investigation programs and develop feasible foundation alternates.

At a minimum, the Geotechnical Report should present the following information:

  1. Plan view of the structure showing the location of the borings
  2. Boring logs
  3. Available laboratory test results
  4. An evaluation of the encountered subsurface conditions including:
    1. Depth, thickness, and variability of soil strata
    2. Depth to groundwater
    3. Identification and classification of soils
    4. Shear strength, compressibility, stiffness, permeability, frost susceptibility, and expansion potential of encountered soils
    5. Depth to rock, identification and classification of rock, rock quality (i.e., soundness, hardness, jointing, resistance to weathering, and solutioning), compressive strength, and expansion potential
    6. Preliminary soil and rock parameters to be used in design (these parameters are limited to the laboratory test results). The Geotechnical Designer will develop additional parameters.

105.6 Foundation Report

A Foundation Report is required for all structures and is prepared by the designer. The objective of the Foundation Report is to provide the information collected during the subsurface investigation and laboratory testing programs and to present the recommended foundation type, foundation recommendations, general site preparation criteria, and other final design considerations, including final soil and rock design parameters for structural use.

Foundation Report requirements are divided into two categories, Standard and Concise. A Standard Foundation Report shall be submitted except as noted in Section 105.6.1 – Concise Foundation Report.

At a minimum, the following sections should be included in a Standard Foundation Report and should be presented in the following order:

  1. Report Narrative:
    1. Section 1 – Introduction: project location, project purpose, project description
    2. Section 2 – Geologic and Geographic Setting: general topography, regional soils data, regional geologic data, including relevant findings from a literature search, soils maps, oil/gas/water wells, geologic mapping, and structural contours.
    3. Section 3 – Subsurface Investigations: discussion of subsurface investigations, subsurface descriptions and general site findings, including encountered depth, thickness and variability of soil strata, depth to groundwater, identification and classification of soils, depth to top of rock, and rock description.
    4. Section 4 – Laboratory Testing: discussion of laboratory tests performed and summary of test results and analysis, including:
      1. Classification and corrosion potential of soils
      2. Shear strength, compressibility, stiffness, permeability, frost susceptibility, and expansion potential of encountered soils
      3. Identification and classification of rock, rock quality (i.e., soundness, hardness, jointing, resistance to weathering, and solutioning), compressive strength, and expansion potential
    5. Section 5 – Data Interpretation and Analysis: presentation of design parameters, analysis and final design considerations, including:
      1. Soil and rock parameters to be used in design
      2. Determination of bottom of footing/pile cap elevation
      3. Evaluation of foundation alternates (may not require calculations)
      4. Shallow vs. deep foundations: bearing capacity, lateral capacity, settlement, external stability, global stability considerations
      5. For shallow foundations: general consideration regarding consolidation settlement, time rate of consolidation, need for preloading, quarantine period
      6. For deep foundations: general consideration regarding settlement of piles, settlement of pile group, settlement of surrounding soils, downdrag forces, potential driving obstructions, presence of boulders
      7. Constructability issues, construction sequence, need for temporary shoring
    6. Section 6 – Foundation Recommendations: final foundation recommendations, including:
      1. Foundation type
      2. Bottom of footing/pile cap elevation
      3. Scour considerations and scour countermeasures
      4. Corrosion protection (i.e., special cement type concrete, epoxy coated rebar, consideration of sacrificial steel thickness for foundation elements design).
      5. For shallow foundations:
        1. Recommended factored bearing capacity
        2. Expected magnitude and time rate of settlement
        3. Differential settlement
        4. Quarantine period if necessary
        5. Any necessary overexcavation of unsuitable materials below the bottom of footing elevation
        6. Specified required backfill material
      6. For deep foundations:
        1. Type and size of piles/shafts (and any other deep foundation system)
        2. Estimated pile/shaft lengths and minimum pile tip elevation
        3. Pile driving methods and termination criteria, including drivability, dynamic monitoring with signal-matching (Pile Driving Analyzer [PDA] with Case Pile Wave Analysis Program [CAPWAP]), and restrike
        4. Need for special pile tip reinforcement if expecting obstructions
        5. Factored pile/shaft structural resistance
        6. Factored axial geotechnical resistance: side friction, end bearing
        7. Factored horizontal pile/shaft resistance (if necessary)
        8. Estimated individual pile/shaft settlement, estimated pile/shaft group settlement
        9. Estimated downdrag forces (if applicable)
        10. Pile batter (if required)
      7. Site preparation criteria:
        1. Recommendations for over excavation (undercutting) of soft/unsuitable materials, preloading, quarantine period, and monitoring/instrumentation program; whether excessive settlement is expected
        2. Recommendations for temporary shoring, cofferdam protection
        3. Provisions for dewatering of excavations, diverting of surface water
        4. Recommendations regarding special treatments for global stability (overall stability)
        5. If pertinent, results of seismic characterization
  2. Appendix A – TS&L Plan: Provide a general plan view of the proposed structure as described in Section – Type, Size, and Location Submission Requirements. The plan should indicate the proposed substructure locations and location of borings. Include all pertinent information, such as location of temporary shoring where applicable and scour protection, if necessary. Provide an elevation view of the proposed structure showing bottom of footing/pile cap elevation, estimated pile tip elevations, stream bed elevation, required overexcavation, and backfill limits.
  3. Appendix B – Typed Boring Logs
  4. Appendix C – Plotted Boring Logs (Structure Plan Boring Logs)
  5. Appendix D – Core Box Photographs (as applicable)
  6. Appendix E – Geotechnical Calculations and Computer Output
  7. Appendix F – Laboratory Testing
  8. Appendix G – Subsurface Soil/Rock Profiles with Boring Logs (as applicable for long structures of 200 feet or greater length)
  9. Appendix H – Special Provisions and Geotechnical Details
  10. Appendix I – Maps: Typically includes location map, aerial map, topographic map (USGS 7.5min Quadrangle Map), soils map, and geological map.

The designer should review FHWA ED-88-053 Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications (2003) for other pertinent items.

105.6.1 Concise Foundation Report

A Concise Foundation Report may be submitted for projects that are determined to be of reduced risk because of their scale, site conditions, or overall complexity. Approval of the Bridge Design Engineer is required prior to proceeding with the preparation and submission of a Concise Foundation Report in lieu of a Foundation Report. Example projects types that may be considered for submission of a Concise Foundation Report include, but are not limited to, culverts, ancillary structures, short (less the 8 feet in height) retaining walls used for limited grade separation (i.e., not supporting live loads), and short single-span bridges.

A Concise Foundation Report shall include the information as outlined in Sections 105.6(1)a– d, (2), (4), (6), (7), and (10).

105.6.2 Foundation Report Submittals

Two copies of the Foundation Report should be submitted for review and approval by the Bridge Design Engineer. Additional copies may be requested for major, unusual, or complex bridges to be submitted to FHWA for its review and comment when applicable. Electronic submission of the report may be acceptable if previously approved by the Bridge Design Engineer.

105.6.3 Quality Assurance and Quality Control

The objective of a QA/QC process is to self-correct omissions and errors during the geotechnical design of substructures. Refer to Section 101 – Introduction for QA/QC requirements.

105.6.4 Geotechnical Design References

Geotechnical design should be in accordance with this Manual and the current AASHTO LRFD. For information not included in these documents, refer to the following references. In the case of contradicting information, priority will be given in the following order:

  1. DelDOT Bridge Design Manual
  2. AASHTO LRFD Bridge Design Specifications
  3. FHWA Design Manuals
  4. Transportation Research Board (TRB) Design Manuals
  5. Naval Facilities Engineering Command (NAVFAC) Design Manuals
  6. USACE Design Manuals
  7. American Society of Civil Engineers (ASCE) Publications

105.7 References

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

FHWA, 2002a. FHWA-IF-02-034 Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties, April.

FHWA, 2002b. FHWA NHI-01-031 Subsurface Investigations – Geotechnical Site Characterization Reference Manual, May.

FHWA, 2003. FHWA ED-88-053 Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications, February.