Attachment 5: FLH Rock Identification Guidelines 2025

Federal Lands Highway

Rock Description,
Characterization, and
Reporting Guidelines

FLH Rock Characterization Guidelines

August 2025

1.1

1.2

2.1

2.2

3.1

3.1.1

3.1.2

3.1.3

3.2

3.3

3.4

3.5

3.6

3.7

3.8

4.1

4.2

4.3

4.4

4.5

4.6

5.1

5.2

5.3

5.4

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6.1

i

FLH Rock Characterization Guidelines

August 2025

6.2

6.3

6.4

7.1

7.2

ii

FLH Rock Characterization Guidelines

August 2025

1 INTRODUCTION

This guide outlines the standard practice for field identification and classification of rock on Federal Lands Highway (FLH) projects. The intent of this guide is to produce consistent and uniform field descriptions for rock 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 methods used to collect rock data include boreholes, test pits, exposed outcrop mapping, and geophysics, among others, and each can be performed with variable types of equipment and sampling tools. Typically, no single method will collect all the needed data due to various limitations, and assumptions and interpretations will be necessary to properly characterize a site.
The intent of these guidelines pertains specifically to describing rocks based on direct field observations of exploration surface walls, slope cuts, and physical rock 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, sample recovery, sampler type and size, etc., which can eliminate, limit, or preferentially collect materials.
Two important notes to consider:
•

The field identifications in the exploration logs should not solely be an assessment of the collected materials in a sample and should consider the above discussed variables, along with drill action and other observations, that are included as detailed notes.

•

The exploration logs with the detailed notes should be carried forward to assist in: o Selecting appropriate samples for and the types of laboratory tests, and o Digitalization of the exploration for report presentation.

1.2 REFERENCED PROCEDURES

Rock characterization for engineering purposes consists of two basic assessments identified in the 1978 and 1981 International Society of Rock Mechanics (ISRM) publications; one based on the intact rock properties of the rock, and the other based on the discontinuity features. Together,