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Laboratory Testing and Interpretation of Rock Properties

John Poullain, P.E.

Course Outline

This four hour online course discusses guidelines and criteria for laboratory testing of rock and the interpretation of rock properties. Basic concepts of rock behavior and the selection of appropriate tests for the design of structures and foundations in or on rock are considered. Frequently used rock tests include those used to establish index properties, determine strength, permeability and durability of rock. Index property tests range from moisture content, specific gravity to unit weight and strength tests include the unconfined and triaxial compressive, tensile and direct shear tests. Durability tests include the slake durability index for wetting and drying and tests of rock freezing / thawing. Also considered is quality assurance for laboratory testing which includes the storage, handling and selection of specimen samples. The AASHTO and ASTM designations for the most frequently used laboratory tests are provided.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Intended Audience

This course is intended for civil engineers and project engineers.

Course Introduction

The design of building foundations, cuts, fills and slopes requires an understanding of rock strength and characteristics and consideration of problem types of rock and how rock masses behave under imposed loads. Laboratory tests and in-situ field-testing provide the information required to evaluate the subgrade rock conditions and to determine measures to take for proposed construction and construction activities. These criteria and guidelines are important so the appropriate tests are selected especially since the laboratory tests can be expensive but not nearly as expensive in case of a project failure.

Rock falls and rockslides may develop from cyclic freezing and thawing in rock joints and fractures. The fall may generate rolling and leaping of rock fragments, which must be considered for highway excavations. Protective measures depend on rock characteristics and discontinuities and include bench cuts, retaining walls, and cribs, tie bars, and protective facing or adequate distances from the toe of cut if ROW is not restrictive. Protective measures will depend on the characteristics and fissures of the rock mass. The surfaces of easily weathered rock can be protected with unite applications. In sandstone for example fractures occur easily, limestone often split according to the calcite crystal planes and igneous rock that had surface cooling can have complex fissuring.

Potential for a rockslide is not readily predicted unless cracking or motion of some surfaces occurs. Gravity and subsurface water are the two most important slide-producing agents. Gravity may cause sliding in cases of massive rock beds underlain by planes of weaker rock, in alternating competent and incompetent rock, or if the rock is badly weathered or fissured. Other rockslides are produced from increased loads, undercutting of the foot of a slope, or deep excavations of a channel next to the rock slope.

Preventative measures against rockslides depend on subsurface investigation and interpretation of rock properties. Detrimental loads should be removed if already inplace, weak material at the foot of the slope may be replaced with stronger material and properly compacted. Surface drainage should be controlled. Subsurface water is more difficult to understand and varies with the time of year, year to year and may reach an area in an indirect path. Undercutting the toe of slope should never be allowed unless preventative measures to maintain slope stability are installed.

Rock processed for aggregate used in road subbases, railroad ballast, concrete, and erosion protection must resist weathering, applied loads, and erosion from water attack. Weathering easily deteriorates porous aggregates that are easily broken or swell when saturated. Unsound aggregate includes shale, friable sandstone, clayey rock and some chert. Freezing and thawing destroys aggregate and concrete composed of unsound aggregate.

Because of the variety of rock types and applied soil mechanics problems there are also many types of tests, laboratory and on site, for determining the engineering properties of rock. Before the laboratory tests can be requested the design engineer must define the purpose for a testing program for himself and the laboratory personnel. Not only must laboratory personnel be well trained and conscientious, laboratory facilities must also provide for a variety of tests with well-maintained testing equipment and samples fulfilling these specific needs. Accurate measurements are of great importance and the test equipment must be properly maintained, otherwise the test results will be valueless and misleading. Poorly constructed and maintained equipment such as triaxial, uniaxial and shear test equipment with maladjusted or worn o-rings or membranes will produce serious test errors.

Rock samples must be handled and stored with care following established standards. Samples should be inventoried, examined and tested as soon after receipt. Sometimes especially for large testing programs it may become necessary to store the samples for days or weeks. If samples are stored for long periods of time the undisturbed samples should be protected against damage or changes in water content by maintaining at temperatures close to those required at the project. Stored samples may undergo physical and chemical changes no matter how carefully stored.

The existing subgrade may have poor strength or instability due to excess clay, expansive clays, silts, fine sands, discontinuities, limestone sinkholes, bentonitic shales or high watertables. Existing rock properties such as voids must be known to protect against potential settlement from the design bearing capacities. There are problem rocks such as tailings and shales, which have structures that will crumble when exposed to air or water and when saturated are greatly reduced in strength. Such rock masses may expand in volume due to stress and deformation and the hydraulic integrity is altered and permeability increased.

The following text errors should be noted:
a. Page 10-15. First paragraph, Figure 10-3d should have read Figure 10-10.
b. Page 10-26. First paragraph, Table 10-7 should have read Table 10-8

Course Content

The course is based on Chapter 8 and 10 of the US Dept of Transportation FHWA publication FHWA NHI-01-031, "Subsurface Investigation-Geotechnical Site Characterization", (2001 Edition, total of 42 pages), PDF file and the course paragraph "Course Introduction". Chapter 3, Pages 3-27 through 3-29 of FHWA NHI-01-031 publication. These pages are not part of the quiz but have been included to describe the Rock Quality Designation (RQD) procedure used for evaluating rock. RQD values are required for the rock classification equations discussed in the text.

The links to the parts of both documents are as follows:

FHWA NHI-01-031, "Subsurface Investigation-Geotechnical Site Characterization", Chapter 8, (2001 Edition, 12 pages)

FHWA NHI-01-031, "Subsurface Investigation-Geotechnical Site Characterization", Chapter 10, (2001 Edition, 30 pages)

FHWA NHI-01-031, "Subsurface Investigation-Geotechnical Site Characterization", Chapter 3, Pages 3-27 to 3-29, (2001 Edition, 3 pages)

Terms and Definitions

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

This course should serve as a guide for testing rock in the laboratory and to better understand the variety of tests, purposes, and their advantages and limitations. Basic concepts of rock behavior are discussed to better understand the purposes of tests and the measurements of various values for rock indexes, characteristics and strengths. Laboratory test data is necessary to meet the needs of the type of construction and plan the measures to provide the necessary strength, slope stability and bearing capacity with the existing rock masses. Tests for index properties include moisture content, specific gravity, and unit weight. Strength tests include triaxial, compressive, and shear. The importance of quality assurance for laboratory testing is stressed including the proper storage, handling and selection of rock specimens for testing programs. This course should serve as a guide for selecting laboratory tests needed to determine the characteristics of rock masses and foundations and interpret the properties of rock in order to protect against potential settlement or ground movement.

Related Links

For additional technical information related to this subject, please refer to:
Information and applications describing subsurface investigations, boring, sampling methods, soil identification, coring rock, preparation of logs and borehole viewing means.
Logging while drilling provides real time information while drilling by using various resistivity, nuclear and acoustic means to define the subsurface conditions.
Lists over 50 software packages, shareware or commercial as noted, for soil boring logs and subsurface profiling.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.