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Section 2: Geotechnical Investigation for Pavement Structures

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2.1 Introduction

Soil is arguably the most critical component of any transportation system, since all transportation systems are built either on, in, or with soil and products from the ground. The characterization and evaluation of soil is critical to the performance of pavement structures. The guidelines provided herein will only address geotechnical considerations necessary for the design and evaluation of pavement structures.

These guidelines are prepared to provide department personnel, consultants, and contractors with guidance in:

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  • Determining soil properties and characteristics to be used in pavement design. These properties include, but are not limited to, soil strength, applicable modulus (or stiffness), and volumetric stability of a pavement structure; and
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  • Determining the influencing site characteristics that might require modifications to the pavement structure or adjacent works to accommodate those characteristics.

From this information, a report should be prepared that documents the findings from the geotechnical investigation.

2.1.1 Applicability

The guidance provided is intended for use by department personnel, consultants, and contractors involved in the planning, designing, evaluating, or construction of soil subgrade to be used or considered in pavement structures.

Although intended for all levels of involvement, the decisions that are made from an investigation are critical to the performance of the roadway. Contact the district pavement engineer (DPE) and/or materials engineer for further assistance and recommendations.

2.1.2 Background

It is important to determine the levels of investigation and when to perform them. From project conception to construction and throughout the operation and maintenance phases, geotechnical information is essential. Geotechnical investigations can be very general and cover broad geographic areas, such as an initial site investigation. They can also be very detailed and specific, such as identification of properties and characteristics of a single soil, as is often done in forensic studies.

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

There are a number of variables that must be defined in an attempt to answer these questions. The direction of the investigation often depends on the nature of the project and the engineering properties desired (e.g., projects on new locations, reconstruction, reclamation of roadway materials, and resurfacing or overlay, required cuts, or fills).

2.1.3 Scope of Guidelines

These guidelines are intended solely for pavement applications.

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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2.2 Preliminary Investigation

Preliminary investigations require little time and are frequently the place to start if no other information or knowledge of the planned roadbed is available. A number of resources are available to planners and designers and may be obtained with little effort, such as documents that reside within the office.

Site inspections are frequently conducted and encouraged in this stage as a means of optimizing the pavement design to reflect site conditions.

The idea is to:

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  • determine the soil types along the roadbed alignment;
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  • estimate the characteristics and properties of the soils present;
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  • use estimated soil characteristics, properties, and potential project geometrics to predict problematic areas, materials, or conditions; and
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  • establish a testing plan for roadbed soils.

2.2.1 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 includes: alignment, type, and scope of the project. Existing data should be reviewed at the beginning of the project as discussed later.

2.2.1.1 Project Alignment

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  • At some point, the horizontal and vertical alignment of the proposed roadbed must be selected. Providing input about soil properties and characteristics as early as possible is preferable so informed decisions may be made.
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  • Alignment is important because it can be influenced by the characteristics of the soils. When alignment has already been decided, the soils are fairly well defined, subject to verification.
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  • Soil morphology, mineralogy, characteristics, and strength will all play a role in what manipulation, modification, or considerations are made in developing the pavement design.

2.2.1.2 Project Type

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  • The same information will be needed regardless of the project type being planned or designed. The requirements of each project type are differentiated by the scope of the information available and the influence of roadbed soils on pavement performance. A review of existing data can indicate the type of information readily available.
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  • In all cases, determining the influence the roadbed soils will have or have had on the performance of the pavement structure is necessary. As a result, preliminary soil data and subsequent subsurface explorations are recommended at all times.

Project Type

Requirement

New Construction

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

Reconstruction

When a roadway is excavated down to natural subgrade or imported fill material, requirements can be equal to or more than ‘New Construction’ projects. Efforts will depend upon the information 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

When 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 often driven by the performance of the pavement. This project type 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.



2.2.2 Review of Existing Data

In addition to data available from the department resources and references, a significant volume of information has been compiled by other numerous organizations and agencies over the years, as shown in Table 3-1.

Anchor: #i1009693Table 3-1: Existing Data Sources

External Data Sources

Internal Data Sources
  1. USDA, Natural Resources Conservation Service
  2. Bureau of Economic Geology, University of Texas
  3. US Geodetic Survey (USGS) maps
  4. Aerial photographs
  5. Maps
  6. Commercial Internet
  7. Engineering library files
  8. Topographic
  9. Satellite images
  1. Previous project data files
    • planning document
    • pavement design
    • boring data
    • surveys
  2. Maps
    • official state travel
    • control-sections
    • county
  3. New PMIS (PA) with mapping functions


2.2.3 Field Reconnaissance

Field reconnaissance, site investigations/inspections/visits, field surveys, and other such terms are commonly used to describe the process of traveling to the physical location of the proposed project. Often, this process identifies features, such as soils, pavement phenomena, and traffic data, either previously unknown or requiring confirmation as shown in Table 3-2.

Anchor: #i1009703Table 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, drainage patterns, and water table proximity

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 evaluations


2.2.3.1 Surface Soil Exploration

There are numerous guides on how to perform quick field soil explorations. Many experienced pavement engineers have rules of thumb regarding appearance, consistency, smell, and taste of soils. A number of these guidelines can be used to broadly identify soil types and characteristics.

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  • Size and percentage of particles can determine whether material is coarse or fine-grained.
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  • Consistency and feel of the soil in a dry state can indicate sand content.
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  • The presence of several sizes of particles or whether there are few sizes can indicate a well or poorly graded material.
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  • Wet materials that can be rolled into thin ribbons have some plasticity. Generally, the higher the plasticity, the more clay will be present.
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  • 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.
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  • Organics are normally fibrous, dark grey, and have a musty odor from the decayed matter.
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  • Moisture content can give an indication of degree of moisture saturation and propensity for moisture movement within the subgrade.
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  • Presence of certain sulfur-bearing compounds, such as gypsum or pyrite, can indicate further testing is required.

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.” When two material types are present, it is common for the materials to be 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.

2.2.3.2 Physical layout and alignment

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  • Terrain features can help determine whether borrow sources might be required, the potential challenges in providing suitable drainage, and the stability of side slopes. Both fill and materials at roadbed grade level should be sampled and tested as described in later sections.
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  • 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.

2.2.3.3 Hydrology

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  • Water resources are not always evident on every site. This information may come from boring logs while conducting subsurface exploration. Seepage and standing water should be noted as these conditions will have a profound effect on project requirements—both in managing the condition and structural requirements.
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  • Pockets of trapped free water may escape detection until construction reveals their location. The situation may dictate removal using positive drainage measures (e.g., French or trench drains, structures) as necessary. Check drainage from existing local pipe underdrains, culverts, or RCP pipes tied into drainage inlets.

2.2.3.4 Topography

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  • 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.
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  • Side slope stability in undulating terrain may require special fill materials to ensure pavement stability.

2.2.3.5 Vegetation

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  • Although vegetation is normally beneficial, having large trees or other vegetation requiring a sustainable water supply close to the roadbed or roots that are close or under the roadbed are most likely detrimental. These conditions create 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”).

2.2.3.6 Geology

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  • From a surface survey, identifying soil mineralogy, presence of rock, potential for sulfur laden soils, and general support potential is possible. As much as the visible evidence of soil layering is useful, the absence of visual evidence also reveals soil characteristics.
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  • 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 critical when materials are to be chemically treated using calcium-based additives. The erosion potential of a soil can also indicate support conditions, existing drainage patterns, and whether specific drainage features will be necessary.

2.2.4 Preliminary Evaluation

Subsequent to the site visit, combine data and information collected to formulate requirements for structural support, subsurface explorations, non-destructive testing, and unique or problematic materials.

2.2.4.1 Structural Support

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  • From the information gathered in the preliminary stage, it is entirely possible to develop a trial pavement design. Soil maps generated from the Natural Resources Conservation Service Web Soil Survey may be used to identify the soil series that cross the proposed roadbed alignment.
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  • From the preliminary 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 in some way to best achieve overall project objectives. Balancing the requirements of the preliminary pavement structure and project constraints can assist in this process.

2.2.4.2 Sampling Plans

Sampling will primarily be taken 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 should be defined. Based on all existing data, locations should be able to be identified with GPS (Globing Positioning System), stationing, or some other reference system, such as TRM (Texas Reference Marker System). Further discussion of sampling requirements may be found under “Subsurface Exploration.”

2.2.4.3 Non-Destructive Testing (NDT)

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  • This testing may proceed at any point in the preliminary or design stages. The objectives of this testing are to evaluate the existing pavement structure and determine modulus values representative of the entire or discrete sections of the roadbed. The methods and analyses are discussed in Chapter 4, Pavement Evaluation.
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  • Testing devices applicable to a geotechnical investigation frequently used in Texas are included in Table 3-3.
Anchor: #i1040965Table 3-3: Non-Destructive Tests for Geotechnical Investigations

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 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 be particularly helpful: 1) when investigating shallow subgrade depths and 2) in locations where a significant difference in moisture content exists between the base and subgrade.

Ground-coupled units using lower frequency antennae can penetrate to great depths, but are generally used to investigate unique phenomena, such as utility trench settlement. The nature of “ground coupling” also reduces production speed and is not suitable for project length surveys in most cases.

Other Advanced Devices and Techniques:

The Total Pavement Acceptance Device (TPAD) is a multi-function evaluation vehicle combining the attributes of continuous deflection measurement, air-coupled GPR, high definition video, and global positioning. Data are used to determine uniformity of support.

The Veris Soil Mapping System is used as a screening tool to measure the soil conductivity covering large areas. High conductivity values may be an indication of soluble sulfates present in the soil. It may be used at any stage of subgrade preparation and embankment construction to measure the conductivity within 2-4ft from the surface. Test results may be plotted on a color-coded map to illustrate the conductivity values. Areas of high conductivity may be selected for sampling and testing of sulfates in accordance with Tex-145-E.



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  • 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 correlation between physical sample characteristics and properties to NDT results for application over a broader coverage area.
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  • Although the production level is high compared to laboratory sample preparation and testing, the data collected is representative of one moisture condition (existing at the time of testing). To rely completely on a single measurement at one moisture condition may be misleading in determining an appropriate design modulus.

2.2.5 Preliminary Investigation Conclusions

Review of existing documents and information can be as varied as the extent of data obtained. It may or may not yield valuable information. Through this process, however, the goal is 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 Table 3-4.

Anchor: #i1009743Table 3-4: 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

 

7. Plan development for non-destructive testing.



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2.3 Subsurface Exploration

2.3.1 General

A comprehensive subsurface exploration plan is necessary to communicate the intent and level of testing that may be required. Effectively communicating these requirements not only ensures that required data is obtained, but it serves as a plan to minimize resources expended.

2.3.1.1 Proposed Testing

Communication with lab personnel can help determine the volume of material that might be required to perform the type and number of tests desired. Since there are limited in-house resources and funding often defines outsourcing, it will be necessary to minimize the number of tests and still obtain the level of data required to fully describe project site characteristics. Costs are typically 0.5%-1.0% of the project estimate.

2.3.1.2 Location

As simple as it seems and obviously critical, locations for sampling have to be specifically identified and communicated to field personnel. Identify not only the geographical location of samples to be taken, but the depth schedule of sampling at each location as well.

2.3.1.3 Sampling Method

The two sampling methods most often used are disturbed sampling, sometimes called bulk sampling, and undisturbed sampling. Each has its advantages depending on the tests being performed. Since bulk sampling rapidly provides sufficient material for laboratory testing, it is most commonly used. Undisturbed sampling is most commonly used to identify existing engineering properties and to make recommendations to the designer.

The two primary sampling techniques used in pavement material analysis are disturbed and undisturbed. Each is descriptive of the amount of disruption of the soil matrix from its natural or in situ state.

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  • Disturbed

    Disturbed samples are frequently referred to as bulk samples. The materials are generally collected with a power auger with helical flights that raise the materials to the surface for collection. This method is efficient because a great amount of materials can be collected in a short amount of time.

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  • Undisturbed

    Undisturbed samples are not frequently requested. For the most part, these samples are collected by contract geotechnical services. The advantage of having these samples is the ability to test materials with (relatively) little disturbance, at the moisture content and density which it was extracted.

2.3.1.4 Frequency of Sampling

Sampling frequency depends on the level of investigation, uniformity of soils, and the potential for detrimental reaction from chemical stabilization. General recommendations for various soil conditions are listed in Table 3-5.

Anchor: #i1043315Table 3-5: Recommended Sampling Frequency

Uniform

0.5 to 1.0 mile

Non-uniform

0.25 to 0.5 mile

Highly variable

1,000 ft. to 0.25 mile

Potential sulfate bearing and soil organic content

500 ft.


2.3.1.5 Depth of Sampling

Sample materials continuously to a depth of at least 15 ft. in areas with high moisture fluctuations. Where excavations will exceed this depth, sampling should be conducted to finished subgrade depth plus 2 additional feet.

When materials change physical characteristics, a new bulk sample should be taken.

2.3.2 Material Evaluation

TxDOT’s laboratory testing procedures contain the methods and processing requirements to accomplish each procedure. It is not the intent to repeat those methods in this document, but procedures used frequently are listed in Table 3-6 and briefly discussed.

Anchor: #i1009786Table 3-6: Typical Laboratory Soil Test Procedures

Test Category

Test

Test Method

Significance

Visual Identification

Soil Classification

Tex-142-E

Use as a check to verify assumed soil properties

Index Properties

Particle Size Analysis

Tex-110-E

A quantitative determination of the distribution of particle sizes

  

Moisture Content

Tex-103-E

Determines natural subgrade moisture for use in drainage and soil suitability analyses

  

Plasticity Index

Tex-106-E

Defines the amount of moisture a material can hold without turning into a liquid, gives an indication of the potential volume change of the material, assists with classification, potential construction/stabilization characteristics, and a measure that has been correlated to numerous engineering properties

  

Potential Vertical Rise

Tex-124-E

Swell potential of subgrade soils

  

Moisture Density Relationships

Tex-114-E

Compaction control purposes during construction can provide stronger, more durable materials

Strength Properties

Triaxial Strength

Tex-117-E

Strength of subgrade materials

Chemical Properties

Determining Sulfate Content in Soils

Tex-145-E

Soil analysis to determine the presence and the quantity of soluble sulfates that could have detrimental reactions with chemical (calcium-based) soil additives

  

Soil Conductivity

Tex-146-E

Field and lab detection of sulfate bearing soils

 

Soil Organic Content

Tex-148-E

Determining soil organic content

  

Soil pH

Tex-128-E

Determining the alkalinity or corrosivity of soils

  

Soil Resistivity

Tex-129-E

Corrosivity of subgrade soils


2.3.2.1 Suitability

It is essential for the design engineer evaluating laboratory data to set minimum acceptable criteria. From a pavement design standpoint, any material in place should be either suitable or modifiable to a suitable state; additional thickness of pavement layers will be able to compensate for most soils. Since there are time constraints, political influences, costs, and other such criteria that often influence the judgment regarding a soil’s suitability, this approach is often not feasible. There is not one criterion that can determine what is acceptable. All factors must be weighed and trial designs made with each alternative considered.

2.3.2.2 Swell potential

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  • The “Guidance on Potential Vertical Rise” memo (paraphrased below) is intended to encourage cost-saving measures by not treating or replacing soil as a potential vertical rise (PVR) mitigation technique except for roadways where risk and comfort are of the highest importance. Consideration of PVR for design purposes is restricted to districts with areas of high soil moisture fluctuation and high plasticity index.
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  • Test method, Tex-124-E, “Determining Potential Vertical Rise,” is the recommended procedure for determining PVR. A 15 ft. soil column is recommended for the analysis to determine PVR. The maximum allowable amount of PVR for design is 1.5 in. for main lanes (2.0 in. for frontage roads, when allowed), or less conservative (higher allowable swell) as established by individual district standard operating procedures (SOP). NOTE: The lower the PVR, the more conservative the design.
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  • A pavement structural design proposing to include PVR mitigation strategies will require the approval of MNT – Pavement Asset Management unless the proposal meets all of the following four criteria:

    If the proposal meets all of the criteria above, the pavement design with the PVR mitigation strategy will be submitted to the Maintenance Division for review only.

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  • PVR Treatment Strategies. Where the calculated PVR of the in situ soils exceeds the allowable for sections meeting the above criteria, mitigation of swell may be accomplished by one or more of the following methods:
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    • Chemical soil stabilization, in accordance with the “ Guidelines for Modification and Stabilization of Soils and Base for Use in Pavement Structures.” The target additive content must be designed to provide a permanently stabilized subgrade soil layer in accordance with the applicable test method for the type of additive under consideration.
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    • Undercut, remove, and replace expansive soils with select fill subbase. Select fill subbase should be placed for a depth of 2 ft. directly beneath the last structural pavement layer. Avoid friable, low plasticity materials, such as sands or loams, since these materials lack shear strength and will act as free moisture conduits that can further exacerbate shrink/swell potential of underlying high PI materials.
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    • Mechanical reinforcement with geosynthetics, such as geogrid, can be utilized when the top soil strata (1 to 3 ft.) is non-expansive and is underlain by expansive soils. For this situation, practical and economic considerations typically prohibit chemical treatment or undercutting to these depths. Use geogrid in the base course layers and/or use a thicker base course to compensate for any minor movement. Typically, when this occurs, the PVR only exceeds the maximum allowable limit by a small amount.

    A design considering or incorporating PVR mitigation will be allowed if it is proposed (optional and not a requirement by the department) by a design-build or CDA (Comprehensive Development Agreement) firm to address a pavement maintenance clause.

2.3.2.3 Feasibility of Chemical Treatment

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  • Research has shown the potential for detrimental effects of introducing calcium-based additives into sulfate bearing soils. A protocol has been proposed and is discussed in the “ Guidelines for Treatment of Sulfate-Rich Soils and Bases in Pavement Structures.” The protocol evaluates the potential for the occurrence of detrimental reactions after the introduction of a calcium-based additive. If chemical treatment mitigation techniques are not successful, the alternative course of action may be to:
    • replace sulfate bearing soils
    • leave untreated and modify pavement layers
    • leave untreated, modify pavement layers, and add geogrid if necessary, or
    • dilute problematic soils to a level of acceptability.


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  • Research has shown soil organic contents above 1% may reduce the effectiveness of calcium-based additives, and long-term strength of the treated soil may be reduced or not achievable using reasonable quantities of additive.
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2.4 Treatment Guidelines

Satisfactory pavement performance is largely attributed to a good foundation that provides adequate strength and stability. Base and subgrade layers serve as the foundation of pavement structures. Structurally, base and subgrade layers must provide adequate strength and must distribute loads uniformly and effectively. This structural capacity is obtained by optimizing material engineering properties, ensuring adequate confinement and drainage. When widening or rehabilitating an existing pavement structure, it is essential to match the existing typical section when possible. Frequently, in situ soils and local base materials do not meet the material engineering properties required for good pavement foundation performance. Texas has some of the most expansive soils in the country, which cause distresses in many pavements around the state. Also, a large portion of pavement construction performed today consists of rehabilitating existing roads, which frequently contain reclaimed subgrade and/or base material layers that are inadequate for current or future traffic loading demands. In order to achieve needed engineering properties, subgrade soils and engineered materials (select fill and flexible base) frequently require treatment.

Material properties are improved by incorporating chemical additives, such as lime, cement, fly ash, emulsion, or asphalt. These additives, or a combination of these additives, are effective when the material is designed and applied properly. Proper design and application of materials with additives will minimize premature failures of the material and pavement structure.

“ Guidelines for Modification and Stabilization of Soils and Base in Pavement Structures” is a document outlining the proper methodology of selecting, designing, and evaluating treated soils and base courses for pavement structures. This document also provides some basic knowledge on the various treatment methods, the goals of treatment, and the mechanisms of each treatment method.

When soils and base contain soluble sulfates, use the “Guidelines for Treatment of Sulfate-Rich Soils and Bases in Pavement Structures” to identify the feasibility for treatment and construction considerations for incorporating chemical additives.

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2.5 Geotechnical Summary Report for Pavement Design Development

Upon completion of the field investigation and laboratory testing program, the geotechnical engineer will compile, evaluate, and interpret the data and perform engineering analyses for the design of pavement foundation layers. Additionally, the geotechnical engineer will be responsible for producing a report that presents the subsurface information obtained from the site investigations and provides specific technical recommendations. An example of a geotechnical design report is shown in Table 3-7.

Since the scope, site conditions, and design/construction requirements of each project are unique, the specific contents of a geotechnical design report must be tailored for each project. In order to develop this report, the author must possess detailed knowledge of the facility. The report must identify each soil and rock unit of engineering significance and must provide recommended design parameters for each of these units. A summary of the analysis of all data is required in the report to justify the recommended index and design properties.

Groundwater conditions are particularly important for both design and construction; these conditions should be carefully assessed and described. For every project, the subsurface conditions encountered in the site investigation should be compared with the geologic setting to better understand the nature of the deposits and to predict the degree of variability between borings.

Anchor: #i1009866Table 3-7: Geotechnical Report

Table of Contents

1. Introduction

2. Scope of Work

3. Site Description

4. Field Investigation

5. Discussion of Laboratory Testing and Significance

6. Site Condition and Geologic Setting

a. Regional Geology

b. Site Geology

7. Discussion of Findings

a. Soil and rock properties

b. Ground water conditions and drainage

c. Chemical analysis

d. Organic analysis

e. Swell characteristics

f. Reactivity with chemical additives

8. Analyses of Data

a. Soil and rock strengths and moduli

b. Characteristics and properties of chemically treated soils

c. Determination of in situ material properties, if applicable

9. Conclusions and Recommendations

a. Feasibility and use of native materials

b. Recommendations regarding borrow materials

c. Chemical treatment of native or borrow materials

d. Modulus values and strengths of native or borrow materials

10. References

List of Appendices

Appendix A – Site Plan

Appendix B – Geologic Model (or schematic)

Appendix C – Boring Location Plans

Appendix D – Boring Logs

Appendix E – Laboratory Test Results

List of Figures

List of Tables



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