Attachment 4: FLH Soil Identification Guidelines 2025

Federal Lands Highway

Soil Description,
Interpretation, and
Reporting Guidelines

FLH Soil Description, Interpretation, and Reporting Guidelines

March 2025

1.1
1.2

2.1
2.2

3.2.1

3.2.2

3.3

3.4

4.1
4.2

4.3

4.4

4.5

4.6

4.7

4.7.1

4.7.2

4.7.3

4.7.4

4.7.5

4.8

3.1
3.2

5.1
5.2

5.3

6.1
6.2

6.3

6.4

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FLH Soil Description, Interpretation, and Reporting Guidelines

March 2025

ATTACHMENTS

APPENDIX A

Example Field Soil Boring Logs

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FLH Soil Description, Interpretation, and Reporting Guidelines

March 2025

1 INTRODUCTION

This guide outlines the standard practice for field identification and classification of soils on Federal Lands Highway (FLH) projects. The intent of this guide is to produce consistent and uniform field descriptions for soil by FLH personnel and consultants working on FLH projects.

1.1 PURPOSE AND INTENT

The purpose of performing geotechnical investigations is to characterize the in situ subsurface conditions underlying a site. Some common explorations used to collect soil and groundwater data include boreholes, test pits, cone penetration test (CPT) probes, and geophysical surveys, among many others, and each can be performed with variable types of equipment and sampling methods. Typically, no single method will collect all the needed data due to various limitations. Assumptions, interpretations, and extrapolations will be necessary to properly characterize a site. The intent of these guidelines pertains specifically to describing soils based on direct field observations of exploration surface walls, existing slope cuts and outcrops, and physical soil samples collected by advancing and/or driving samplers. Determining in situ subsurface conditions from boreholes can be particularly challenging due to, for example, the drill rig type, drill method, sample intervals, sampler type and size, etc., which can eliminate, limit, or preferentially collect soil constituents. Since soil identification, for engineering purposes, is based on the distribution and behavior of the fine-grained and coarse-grained soil constituents, this absence and/or excess will cause erroneous soil classification of the in situ subsurface characterization unless critical interpretations are made during and after field data collection based on sample reviews, laboratory tests, and the holistic assessment. Prior to performing the field investigations, a desktop study of the area should be performed to determine the mapped surface and bedrock geology stratigraphy, presence of geologic hazards, review past geotechnical data and reports in the vicinity, and other available information.