Geotechnical investigations furnish information that can be used to develop an optimal geotechnical design for a project, one that eliminates overconservative design parameters and minimizes instances of underdesign. Overconservative designs increase project costs without significant benefits. Underdesign can result in failures that delay a project, increase construction costs, increase the difficulty and cost of maintenance, and potentially endanger the public. Optimizing designs produces an acceptable balance between costs and risk.

Geotechnical investigations can help determine the causes of problems that occur during project construction and maintenance and identify corrective actions. Solutions can then be developed so that construction or use of a facility can resume in a safe, reliable, economical, and efficient manner.

Subsurface investigations are performed to gather data for geotechnical investigations and may include drilling, sampling, geophysical testing, laboratory testing, and monitoring of soil and rock conditions.  The data from the subsurface investigation informs geotechnical analyses and the resulting construction recommendations.

Guidelines for conducting geotechnical and subsurface investigations are available (see Section 5). The application of these guidelines should be tailored so that investigations meet the needs of individual projects.

Geotechnical investigations directly influence a project’s development, contract documents, construction processes, and long-term performance. Items influenced by geotechnical investigations include:

• Design and Construction Requirements — Investigations provide information that influences roadway design (e.g., location and grade), identify subsurface issues that may impact construction, and inform procedures that address those issues.
• Right-of-Way — Investigations inform the setting of rock, soil cut, and embankment slopes and thus right-of-way needs.
• Structure Foundation Types — Investigations inform the selection of pile or spread footing foundations for support of structures, selection of pile types, the quantity or size of foundation needed, and the bearing of the structure on soil or bedrock.
• Pavement Design — Investigations influence pavement design by providing information on subgrade materials, whether materials should be treated or stabilized, and if rock from roadway excavation is available to build subgrades.
• Geohazards — Investigations identify geohazards that may be present within the roadway footprint such as caves, sinkholes, hazardous materials, and undesirable geologic formations. See Geohazards article.
• Sustainable and Weather-Resilient Infrastructure — Investigations provide information that can improve infrastructure resiliency when exposed to hazards such as floods, scour, erosion, earthquakes, and landslides.
• Pay Quantities — Investigations provide information that directly influences the types and quantities of pay items. This includes but is not limited to rock excavation, common excavation, pile lengths, pile quantities, subgrade stabilization, retaining wall types, and foundation stabilization methods.
• Long-Term Performance — Projects without a geotechnical investigation are more likely to be underdesigned which can result in poor performance, increased maintenance requirements, safety concerns, loss of use, and greater life cycle costs.

Project managers can learn more about initiating and understanding geotechnical investigations in the Highway Knowledge Portal article, Geotechnical Investigations – Where to Begin and How to Proceed.

Geotechnical investigations may increase project costs, but they add value. It is much cheaper to conduct a geotechnical investigation than to accommodate unknown ground conditions (e.g., unknown pile lengths, unknown excavation, unsuitable foundation soils, poor subgrade materials, geohazards). The goal for any geotechnical investigation is to generate savings during construction and maintenance that exceed the cost of the investigation.

In addition, indirect costs related to subsurface conditions (e.g., claims, change orders, cost overruns, user costs) can sometimes exceed the direct costs.

Table 1 lists factors that influence the cost and scope of geotechnical investigations. Conducting geotechnical investigations along transportation corridors can be difficult because they are linear and traverse broad areas with highly variable subsurface conditions.

A few researchers have put forward recommendations for how much money should be spent on geotechnical investigations as a percentage of a total project award. However, no major highway and transportation organizations (e.g., AASHTO, TRB, FHWA), have published guidelines or specifications which address this issue.

Mott MacDonald and Soil Mechanics, Ltd. (1994) evaluated 58 transportation projects in the United Kingdom to study the impact of geotechnical investigations on construction cost overruns. While they did not identify a direct correlation between geotechnical investigation expenses and cost overruns, even modest increases in geotechnical investigation costs as a percentage of the project award significantly decreased the probability of cost overruns.

Boeckmann et al. (2016) noted that improvements and investments in several areas reduce claims, change orders, cost overruns, and substantially improve design efficiencies, including:

• Subsurface investigation practices
• Communication
• Training
• Agency personnel
• Alternative investigation techniques and equipment

The Division of Structural Design Geotechnical Branches can provide project managers cost estimates for geotechnical investigations to help them set realistic project development budgets. Since budgets are used to develop KYTC’s Six-Year Highway Plan, cost overruns can affect the timing of delivery of other projects in the Plan.

Unlike steel, concrete, and other engineered materials, in situ soil and rock do not neatly conform to standards nor do they have homogeneous characteristics. Soil and rock properties vary across a project site, and sometimes the variations can be significant. Since these materials are below the earth’s surface and not readily visible, defects, anomalies, and geohazards can be hidden from even the most thorough investigation.

Conventional drilling and sampling typically provide coverage for 0.002% or less of a project’s surface area. Only where a boring is performed do engineers have certainty about subsurface features, and that certainty can be reduced if the drilling is performed incorrectly.

Understanding the subsurface characteristics of areas not captured by borings requires interpretation, interpolation, and inference. While other subsurface investigation techniques can fill in knowledge gaps, use of these methods must be balanced with concerns related to budget, time, and benefits. Therefore, the experience, knowledge, and judgement of a geotechnical professional is essential for understanding subsurface data as they provide guidance that reduces project risk.

Human actions also present risks and are also a primary cause of claims, change orders, and cost overruns according to studies by Baynes (2010), Clayton (2001), and Moorhouse and Millet (1994). These include:

•  Inexperienced project staff

• Failure to effectively communicate risks

• Failure of the client or contractor to follow recommendations

• Choice of contracting method

It is impossible to eliminate all geotechnical-related risks regardless of the extent of the subsurface investigation, nor is it economically feasible to attempt to do so. Remember, the goal is for savings realized during construction and maintenance as a result of conducting the geotechnical investigation to exceed the cost of the investigation itself.

Geotechnical personnel develop a subsurface investigation plan that is tailored for project-specific scope, location, and known subsurface conditions. The plans use previously developed mapping, plans, and geotechnical studies (when available) to refine and supplement the investigation.

 5.1 Guidelines for Conduct of Subsurface Investigations

 The following documents provide guidelines for conducting subsurface investigations:

•  Manual on Subsurface Investigations (AASHTO & TRB)
• Geotechnical Engineering Circular No. 5 – Geotechnical Site Characterization (FHWA)
• Geotechnical Engineering Circular No. 14 – Assuring Quality in Geotechnical Reporting Documents (FHWA)
• Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications (FHWA)
• Soils and Foundations Reference Manual, Volume I & Volume II (FHWA)

These guidelines provide recommendations on topics such as exploration locations, sampling, laboratory testing, analyses, and reporting. When developing the scope of a geotechnical investigation, these guidelines should be viewed as a starting point, not as rules that must be adhered to strictly. All scopes should be tailored to the project type, location, requirements, and geologic environment. KYTC’s Geotechnical Guidance Manual provides further guidance on customizing a scope based on the topography, geology, and the state of practice in Kentucky. It is important for an experienced geoprofessional to evaluate the project and prepare an appropriate subsurface investigation plan.

Given recent developments in three-dimensional (3D) subsurface modelling, building information modelling (BIM), and digital plan delivery, it may be necessary to update traditional subsurface investigation guidelines to ensure that subsurface environments are modeled in an appropriate and accurate manner. This is critical for generating trustworthy information to guide the design and construction of a project. Using these tools can introduce risks because data can be inappropriately modeled, interpolated, or interpreted without input from an experienced geoprofessional.

Investigation plans may also incorporate a monitoring program. Some common monitoring items are groundwater levels, landslide movements, settlement, and structure distress. Monitoring informs decisions about new construction or corrective actions taken to address maintenance or construction problems.

5.2 Load and Resistance Factor Design (LRFD)

The use of AASHTO LRFD Bridge Design Specifications in structure design can also impact the scope of a subsurface investigation. These specifications account for risk, which is reflected in resistance factors used for foundation design.

Resistance factors are selected based on the scope of the investigation, techniques used in the investigation, and the analytical methods used. By performing additional investigation, using certain investigation techniques, using certain analytical methods, and applying certain construction control processes, risk can be reduced. With reduced risk the resistance factors may be increased, allowing more of the foundation capacity to be utilized. This can reduce the cost of foundation construction significantly.

5.3 Desk Studies

 When developing the scope of a subsurface investigation, the first step is to review available information, including:

•  Existing geologic and soil mapping
• Other maps and data sources (e.g., mine maps, geologic hazards, wells)
• Previous KYTC geotechnical studies and reports
• Archived and as-built plans

Through a desk review, geoprofessionals can identify problematic geologic formations, soil types, hazards that may impact project location and design, and determine the scope of the subsurface investigation. Information from archived plans and as-built plans may provide information (e.g., boring information, soil tests, pile lengths) that can be reused. KYTC also maintains databases of archived geotechnical reports, boring logs, and laboratory testing. Archived information can supplement or reduce the scope and extent of subsurface investigations.  After reviewing this information, the geoprofessional prepares preliminary subsurface investigation plans.

5.4 Field Reconnaissance

The next step in the process is field reconnaissance. Geoprofessionals inspect the terrain and existing conditions to identify issues that could influence the scope of geotechnical investigations and the project design. Online mapping services can aid and supplement field reconnaissance. Potential issues to identify during field reconnaissance include:

•  Previously unidentified geologic hazards
• Land use concerns
• Utility locations
• Site accessibility issues
• Areas where non-traditional subsurface investigation techniques may be helpful

The geotechnical investigation plan can then be adjusted to address these items.

Online satellite maps, street views, and photo logs can supplement and sometimes replace field reconnaissance efforts.  Geoprofessionals can view these images during the desk studies to gain a better understanding of the site and possible impediments to conducting the subsurface investigation.

 5.5 Scope Adjustments

A geotechnical investigation may conclude that additional subsurface information should be obtained. Investigations commonly identify unforeseen geologic conditions, soil types, groundwater conditions, geologic hazards, and other issues that may impact project design and construction. Additional borings, lab tests, instrumentation, non-traditional investigation techniques, and engineering analyses may be needed to obtain the information needed to appropriately design, specify, and construct a roadway or structure without undue cost, claim, and delay risks.

In some cases, conventional drilling and sampling techniques need to be supplemented with less common data retrieval or monitoring methods. Refer to the Highway Knowledge Portal article, Geotechnical Investigations – Subsurface Investigation Tools and Techniques for more information on tools and methods for performing geotechnical investigations and monitoring.

The extent and cost of a geotechnical investigation vary by project type, size of project area, geologic setting, and other factors. Engineers and geologists carefully develop a subsurface investigation plan tailored for each project utilizing available mapping and data. While it is not possible or economically feasible to eliminate all geotechnical risks from a project, a well-planned and performed geotechnical investigation helps reduce the probability of cost overruns, claims, change orders, delays, construction problems, and maintenance issues.

American Association of State Highway and Transportation Officials (AASHTO), AASHTO LRFD Bridge Design Specifications, 9^th Edition, ISBN No. 978-1-56051-738-2, Washington, D.C., 2020, 1912 pp.

American Association of State Highway and Transportation Officials (AASHTO), Manual on Subsurface Investigations, 2^nd ed., AASHTO, Washington, D.C., 2022.

Baynes, F. J., “Sources of Geotechnical Risk,” Quarterly Journal of Engineering Geology, Vol. 43, 2010, pp. 321-331.

Boeckmann, A. Z. and J. E. Loehr, NCHRP Synthesis 484, Influence of Geotechnical Investigation and Subsurface Conditions on Claims, Change Orders, and Overruns, The National Academies Press, Washington, D.C., 2016, 76 pp. https://doi.org/10.17226/21926

Clayton, C.R.I., “Managing Geotechnical Risk: Time for a Change?” Geotechnical Engineering, Proceedings of the Institution of Civil Engineers, Vol. 149, No. 1, 2001, pp. 3-11.

Federal Highway Administration (FHWA), Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications, FHWA ED-88-053, Washington, D.C., 1988, Revised 2003. https://www.fhwa.dot.gov/engineering/geotech/pubs/reviewguide/checklist.pdf

Kentucky Transportation Cabinet (KYTC), Geotechnical Guidance Manual, Frankfort, KY, 2005. https://transportation.ky.gov/StructuralDesign/Interim%20Guidance%20Manual/Geotechnical.pdf

Loehr, J. Erik, A. Lutenegger, B. Rosenblad, and A. Boeckmann, Geotechnical Engineering Circular No. 5 – Geotechnical Site Characterization, FHWA NHI-16-072, Federal Highway Administration, Washington, D.C., 2016. https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi16072.pdf

Moorehouse, D.C., and R.A. Millet, “Identifying Causes of Failure in Providing Geotechnical and Environmental Consulting Services,” Journal of Management in Engineering, Vol. 10, No. 3, 1994, pp. 56-64.

Mott MacDonald and Soil Mechanics, Ltd., Study of the Efficiency of Site Investigation Practices, Project Report 60, Transport Research Laboratory of the Department of Transport, Crowthorne, Berkshire, UK, 1994, pp. 63.

Rix, G.J., N. Wainaina, A. Ebrahimi, R. C. Bachus, M. Limas, R. Sancio, B. Fait, and P. W. Mayne, Manual on Subsurface Investigations, The National Academies Press, Washington, D.C., 2019, pp. 372. https://doi.org/10.17226/25379

Samtani, N.C., and E.A. Nowatzki, Soils and Foundations Reference Manual, Vol. 1 & 2, FHWA-NHI-06-088 and FHWA-NHI-06-089, National Highway Institute, Federal Highway Administration, Washington, D.C., 2006. https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi06088.pdf and https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi06089.pdf

Sheahan, J., A. Zdinak, and J. DiMaggio, Geotechnical Engineering Circular No. 14 – Assuring Quality in Geotechnical Reporting Documents, FHWA-HIF-17-016, Federal Highway Administration, Washington, D.C., 2016, pp. 93. https://www.fhwa.dot.gov/engineering/geotech/pubs/hif17016.pdf