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Section 2: Geotechnical Investigation for Pavement Structures

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Introduction

All transportation systems are built either on, in, or with soil and products from the ground. Soil is arguably the most critical component of the transportation system since most construction is dependent upon project soil properties and characteristics. The characterization and evaluation of soil is critical to the performance of pavement structures. The guidelines provided below will only address geotechnical considerations necessary for the design and evaluation of pavement structures.

This discussion is intended to provide TxDOT personnel, consultants, and contractors, guidance in establishing soil properties and characteristics to be used in pavement design and determine influencing site characteristics that might require modifications to the pavement structure or adjacent works to accommodate those characteristics. In particular, determination of soil strength, applicable modulus (its stiffness), and matrix stability descriptive of a pavement project, a portion of a project, or materials respectively, will be the result of the analysis. From this information, a report should be prepared that documents the findings from the geotechnical investigation.

Applicability

The guidance provided is intended for use by TxDOT personnel, consultants, and contractors involved in the planning, designing, evaluating or construction of soil layers to be used or considered in pavement structures. Although intended for all levels of involvement, the decisions that are generated from an investigation are critical to the performance of the roadway; therefore, it is recommended that the district pavement and/or materials engineer be contacted, to assist and review recommendations.

Background

From project conception through construction and throughout the operation and maintenance phase, geotechnical information is necessary. Geotechnical investigations can be general and cover broad geographic areas, for initial site investigation and determination, or they can be very detailed and specific to identification of properties and characteristics of a single soil as is often done in forensic studies. How to determine what level of investigation is needed and when to perform them is a primary focus of this discussion.

Some of the more frequently asked questions with regard to conducting soil investigations for pavement design are:

  • How do I get started?
  • What information is needed?
  • What test data do I need?
  • At what interval do I need to retrieve material for testing?
  • What test values are acceptable?

There are a number of variables that have to be defined to try to answer these questions. The direction of the investigation is often going to depend on the nature of the project and what engineering properties are desired (e.g., projects on new location, reconstruction, reclamation of roadway materials, and resurfacing or overlay, required cuts or fills, etc.).

Scope of Guidelines

There are numerous geotechnical and soils investigation guides that are relevant to other construction activities; most pertain to evaluations and analyses applicable to structures, slope and global stability, and retaining walls. For information on these subjects, refer to the Bridge Division’s Geotechnical Manual.

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Preliminary Investigation

Preliminary investigations are not complicated investigations, require little time, and are frequently the place to start if no other information or knowledge of the planned roadbed is available. There are a number of resources that planners and designers have available that may be obtained with little effort and are often documents that reside within the office. Although site inspections are frequently conducted in this stage, one might not be necessary depending on the intent and extent of the project, but a visit to the site is encouraged at any time during pavement design development.

The idea is to:

  1. determine what soil types cross the roadbed alignment,
  2. estimate the characteristics and properties of the soils present,
  3. use estimated soil characteristics, properties and potential project geometrics, predict problematic areas, materials, or conditions,
  4. establish a testing plan for roadbed soils.

Project Initiation

Upon approval to proceed with project development, numerous activities begin. Planners and designers should begin their soils investigation at this stage to avert problems associated with poor soils or site conditions. Information of interest include: alignment, type, and scope of the project. Information that is reviewed coincident with project initiation is existing data as discussed later.

  • Alignment
    • Somewhere along the way, the horizontal and vertical alignment of the proposed roadbed will need to be selected. It is preferable to provide input with regard to soil properties and characteristics as early as possible so “informed” decisions may be made.
    • Alignment is important, for it can be influenced by the characteristics of the soils, or when alignment has already been decided, the soils are fairly well defined, subject to verification.
    • Soil morphology, mineralogy, characteristics, and strength will all play a role in what manipulation, modification, or considerations are made in developing the pavement design.
  • Project Type
    • The same information will be needed regardless of project type being planned or designed. The requirements of each project type are differentiated by the extent to which information is available and the influence of roadbed soils on pavement performance.
    • Review of Existing Data can help in defining what information is readily available. It is necessary in all cases to determine what influence the roadbed soils will have or have had on the performance of the pavement structure. Accordingly, preliminary soil data and subsequent subsurface explorations are always recommended.

Project Type

Requirement

New Construction

Requires the greatest effort and time to establish new data or gather information about a roadbed that has not been compiled previously.

Reconstruction

Where a roadway is excavated down to natural subgrade or imported fill material, requirements can be as much or more than New Construction. Efforts will depend on what information has been compiled during the development of the previous pavement structure and the performance history of the roadbed being reconstructed. If prior severe distress was recorded, a detailed investigation may be necessary to provide an explanation.

Reclamation of Roadbed Materials

Where severe distress or roughness is recorded prior to reclamation, soil investigation requirements can exceed those of New Construction. Where there is little severe distress, and reworking the subgrade is not required, the level of detail is substantially reduced. An evaluation of subgrade soils is warranted to ensure material selection, modification of soils, and structural section are compatible and sufficient.

Resurfacing/Overlay

The information required for this pavement type is minimal, but is often driven by the performance of the pavement. It often involves a cursory review of soils data and correlation to roadway roughness characteristics and distress manifestations. Assuming good performance, one may proceed to resurfacing for maintenance requirements or determination of design parameters for structural evaluation. A poorly performing pavement may be the result of roadbed soils; a detailed investigation would be appropriate.



Review of Existing Data

One of the first sources of information that can be easily obtained is data that resides in documents that can be easily retrieved. There has been a significant volume of information compiled by numerous organizations and agencies over the years. A significant volume of data and methods of representing the data are also available within TxDOT internal resources.

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Field Reconnaissance

Field reconnaissance, site investigations/inspections/visits, field surveys and other such terms are commonly used to describe the process of getting out of the office and seeing the physical location of the proposed project. Often, this process identifies features such as soils, pavement phenomena, traffic data, etc., that was either previously unknown or requires confirmation.

Table 3-2: Reconnaissance Areas of Interest

Typical interest in field surveys

Inferences

1. Surface soil exploration

1. Soil classification, estimation of characteristics and properties.

2. Physical layout and alignment

2. Geometrics to determine drainage characteristics, stability of side slopes and cut/fill requirements, steepness and high fills that can contribute to shrinkage cracking.

3. Hydrology

3. Determining drainage conditions and drainage patterns.

4. Topography

4. Cut/fill requirements, stability, drainage.

5. Vegetation

5. Mitigate shrinkage cracking from vegetation in close proximity to roadway edge

6. Geology

6. Mineralogical, pedological (soils) evaluations


  • Surface Soil Exploration There are numerous guides on how to perform quick, field soil explorations. Many "old timers" have rules of thumb regarding appearance, consistency, smell, and taste. Many of these guidelines can be used to broadly identify soil types and characteristics. TxDOT uses a modified version of the ASTM Unified Soil Classification system that is explained more thoroughly in "Tex-142-E, Laboratory Classification of Soils for Engineering Purposes."
  • Size and percentage of particles can determine whether material is coarse or fine grained.
  • Consistency and feel of the soil in a dry state can indicate sand content.
  • The presence of several sizes of particles or whether there are few sizes can indicate a well or poorly graded material.
  • Wet materials that can be rolled into thin ribbons have some plasticity. The effort required to rework and successively roll the material can indicate its toughness. The tougher it is to roll, the higher the plasticity of the material. Generally, the higher the plasticity, the more clay will be present.
  • Wet materials that exhibit hardly any plasticity can be silts or sands or organic materials. Silts can be soft and may roll into a ribbon but will quickly crumble; whereas, sands may not be able to withstand any rolling at all.
  • Organics are normally fibrous, dark grey, and have a musty odor from the decayed matter.
  • Moisture content can give an indication of degree of moisture saturation and propensity for moisture movement within the subgrade.
  • Presence of certain sulfur-bearing compounds such as gypsum or lignite can indicate further testing is required. There are common occasions where two material types are present and are given a dual designation (SC-sandy clay, GC-clayey gravel). This information can be valuable in determining general soil properties as identified in many of the available resources and can serve as a source of data to confirm the data gathered.
  • Physical layout and alignment
    • An idea about the terrain can help determine whether borrow sources might be required, what challenges there might be in providing suitable drainage, and the stability of side slopes. Both fill and materials at roadbed grade level need to be sampled and tested as described in later sections.
    • The presence of other infrastructure can affect the long term performance of the subgrade. Curbs and gutters adjacent to roadways provide special challenges for retaining strength and support from soils in moist environments.
  • Hydrology
    • Water resources are not always evident on every site. This information may have to come from boring logs while conducting subsurface exploration. Seepage and standing water should be noted as it will have a profound effect on project requirements both in managing the condition and structural requirements. Pockets of trapped free water may escape detection until construction reveals their location. The situation may dictate removal using positive drainage measures (french drains, structures, etc.) as necessary. Check drainage from existing local pipe underdrains, culverts or RCP pipes tied into drainage inlets.
  • Topography
    • As with physical layout and alignment, cut and fill sections will require additional consideration whether in terms of revealing existing fill material different than the surrounding subgrade, stabilization, etc., or illuminating a requirement to modify existing or import different materials. Side slope stability in undulating terrain may require special fill materials to ensure pavement stability.
  • Vegetation
    • Vegetation is normally a good thing - having large trees or other vegetation requiring a sustainable water supply if it is close to the roadbed or have roots that are close or under the roadbed are most likely detrimental. There is a greater chance of subgrade desiccation leading to soil shrinkage and possible cracking. This phenomenon is most evident in soils with higher plasticity indexes (“Tex-106-E, Calculating the Plasticity Index of Soils") and large shrinkage potential ("Tex-107-E, Determining the Bar Linear Shrinkage of Soils").
  • Geology
    • From a surface survey, one might be able to identify soil mineralogy, presence of rock, potential for sulfur laden soils and general support potential. As much as the visible evidence of soil layering is useful; the absence of visual evidence also reveals soil characteristics.
    • The presence of rock at the surface can indicate shallow bedrock conditions. How massive or weathered the rock is can indicate subsurface support characteristics. Visual identification of sulfur bearing minerals is problematic when materials are to be chemically modified. The erosion potential of a soil can also indicate support conditions, what drainage patterns exist, and whether specific drainage features will be necessary.

Preliminary Evaluation

Subsequent to the "desktop survey" and site visit, one can piece together data and information to formulate requirements for structural support, subsurface explorations, non-destructive testing, unique or problematic materials.

  • Structural Support
    • From the information gathered in the preliminary stage, it is entirely possible to run a trial pavement design. Soil Conservation Service (SCS) maps may be used to identify the soil series that cross the proposed roadbed alignment. The catalog of soils series data developed jointly between TxDOT and SCS can provide an estimate of the subgrade or natural soil strength, as measured by Texas Triaxial Classification. In turn, the triaxial classification may be used to estimate subgrade modulus which will be used in pavement design procedures. Additionally, the triaxial classification will allow the estimation of base and surfacing requirements as outlined in Modified Texas Triaxial Design Method and as discussed in the design of flexible pavements in this manual.
    • Performing the initial design is not the end of the preliminary design process. From the trial pavement design, the resulting pavement structure should be evaluated for: 1) stability, 2) constructability, 3) cost, and 4) feasibility. It is possible that political, environmental, cultural, and engineering constraints will require that subgrade layers be modified or manipulated in some way to best achieve overall project objectives. Balancing trial pavement structure requirements and project constraints can assist in this process.
  • Sampling Plans
    • Sampling will primarily be from borings, undisturbed samples (Shelby Tubes), test pits, or hand sampling. The frequency at which samples are taken, depths of soils sampled, and the type of sampling required will need to be defined. Based on all existing data, locations should be able to be identified with stationing or some other reference system. Further discussion of sampling requirements may be found under ‘Subsurface Exploration.’
  • Non-destructive testing (NDT)
    • This testing may proceed at any point in the preliminary or design stages. Although the methods and analyses are discussed in Chapter 4, Pavement Evaluation, the objective of this testing is to arrive at modulus values representative of the entire or discrete sections of the roadbed.
    • Testing devices frequently used in Texas, specifically for pavement investigations, include:

Falling Weight Deflectometer (FWD):

Backcalculation of deflection data may be used to estimate the modulus of the subgrade.

Testing should be conducted on surfaced roadways. If testing is for a new location, it is often convenient to estimate the modulus by analyzing data collected on an adjacent roadway or one with a pavement structure that is predicted to be similar to the one planned. Testing is unreliable on unsurfaced materials.

Dynamic Cone Penetrometer (DCP):

It is a stretch to call this testing non-destructive, but there is little disturbance of roadway materials. Several correlations have been made to the rate of driving the rod into subgrade materials. From these correlations, one can estimate the soil stiffness and differentiate layers within 3 ft. of the tested surface, assuming that substantial differences exist. Evaluation of soils containing significant amounts of larger (>1.5 in.) aggregate may be problematic since these aggregates may not be easily “pushed aside,” thereby severely reducing penetration rates.

Ground Penetrating Radar (GPR):

Both ground-coupled and air-coupled units can be used to locate areas of high moisture or differing pavement strata. Since the air-coupled system penetration is limited to a depth of about 24 in., it can only be helpful to investigate shallow subgrade depths, and only where a significant difference in moisture content exists between the base and subgrade. Ground-coupled units can penetrate to great depths, but are, in general, used to investigate unique phenomena such as trench settlement.

Other Advanced Devices and Techniques:

The application of these devices for determining material properties is still being researched. Devices showing potential for implementation include the Seismic Pavement Analyzer (SPA), Portable SPA (PSPA), and Dirt Seismic Pavement Analyzer (DSPA) for soils.



  • Non-destructive testing is not a substitute for soil testing, but the data collected from these activities can establish a confidence that the subgrade is being properly characterized. Much data can be collected and analyzed relative to the time requirements and effort expended on laboratory tests for physical samples. Its productivity allows one to correlate physical sample characteristics and properties to NDT results for application over a broader coverage area.
  • Although the production level is high (compared to laboratory sample preparation and testing), the data collected is representative of one moisture condition. To completely rely on a single measurement at one moisture condition may cause serious errors in determining an appropriate design modulus.

Preliminary Investigation Conclusions

The result of a review of the existing documents and information that is readily available can be as varied as the extent of data obtained. Sometimes this process is a gold mine, and at other times a dry well. Through this process, however, one hopes to at least obtain some of the information useful in defining subsurface investigation requirements and estimate the level of testing that will be required based on characterization parameters given in the following table.

Table 3-3: Project Characterization Parameters Related to a Geotechnical Investigation

Project Characterizations

Soil Characterizations

1. Proposed alignment

1. Geologic model

2. Project type

2. Soil identification

3. Evaluation of project feasibility

3. Estimation of soil characteristics

4. Position of natural drainage features

4. Estimation of soil properties

5. Hydrologic inferences

5. Preliminary stabilization requirements

6. General terrain and some estimate of cuts and fills required.

6. Guidance for subsurface exploration

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7. Plan development for non-destructive testing.



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Subsurface Exploration