Print this page In Situ Subsurface Testing
John Poullain, P.E.
Course Outline
This four hour online course provides guidelines for in situ testing subsurface formations. In situ tests are used to determine the stresses and deformation behavior of rock masses and delineate critically weak seams within rock masses. Basic concepts of soil and rock behavior together with the selection of appropriate tests for construction projects are considered. In situ testing consists of geotechnical and geophysical tests. Frequently used geotechnical tests for shear strength, bearing capacity, stress conditions, deformability and liquefaction susceptibility include the standard penetration, cone penetration, direct and vane shear tests. Strength tests include unconfined compressive, triaxial, and shear. Also covered are geophysical tests, mechanical wave and electromechanical methods, used to delineate subsurface conditions, top of rock mass conditions and for locating buried objects, pipes, and cavities. Problem soils such as clay shales and rock formations with weak seams and jointed masses are discussed for consideration of construction projects. Guidelines for testing procedures and storage and handling of specimen samples along with the ASTM designations for the most frequently used in situ tests are noted.
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:
- Be familiar with testing procedures and overview comments explaining the commonly used in situ geotechnical tests;
- Be aware of the advantages and disadvantages of the commonly used geotechnical tests used to determine the engineering properties of subsurface formations;
- Understand the types and applicable uses for geophysical tests, i.e. mechanical wave and electromagnetic methods, for delineating subsurface conditions and locating buried objects;
- Be familiar with the behavior of soils under load and commonly used soils mechanics terms;
- Understand the index properties of soils like Young's Modulus and specific gravity, which characterize soils and determine their basic properties;
- Understand the selection and assignment of appropriate tests for a project as determined by the imposed load, type of construction and existing subsurface formations;
- Be familiar with the types of problem soils and rock masses to consider for foundation, embankment, and excavation design;
- Have an understanding how certain soil properties such as creep and stiffness are affected under imposed loads;
- Be aware of general guidelines for sampling of subsurface materials;
- Consider consolidation, swell and collapse potential of soils and how test results may be affected;
- Consider how fluid flow potential for rock masses is tested;
- Have an understanding of quality assurance procedures and guidelines for obtaining accurate test results; and
- Be aware of
the ASTM and AASHO designations for the most frequently used tests.
Intended Audience
This course is intended for civil engineers and project engineers.
Course
Introduction
The design of building foundations, excavation, fills and slopes requires an understanding of soil strength; soil characteristics and consideration of problem soils and how soil behaves under imposed loads. In situ tests along with laboratory tests provide this information. These criteria and guidelines are important so the appropriate tests are selected especially since the tests can be expensive but not nearly as expensive in case of a project failure.
Because of the
large variety of soils and large variety of applied soil mechanics problems
there is also a large variety of soils tests, in situ and laboratory, for determining
the engineering subsurface formations. Before the tests can be requested the
design engineer must define the purpose for a testing program for himself and
the testing personnel. 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 will
produce serious test errors.
Purposes for
In Situ Testing
In situ tests are the best means to determine the properties of subsurface materials
and may be the only means in some cases. Applicable tests and purposes for in
situ testing include:
a. pressure tests
determine leakage in rock masses and are used for estimating grout requirements
b. electromagnetic methods are used to detect subsurface cavities, buried tanks
or pipes
c. determination of the in situ stresses and deformation conditions of jointed
rock masses
d. determination of soft or weak seams, top of rock, weathered rock and stratigraphy
e. determination of liquefaction susceptibility of formations
f. estimation of loads on tunnels and rock burst susceptibility in excavations
g. used where other tests are not effective
h. help to reduce conservative and costly assumptions solely based on theoretical
designs
i. provide data when samples are disturbed, especially soils, which can affect
the deformation properties
j. tend to average out the effects of complex subsurface interactions
Guidelines for
In Situ Testing of Rock
a. conduct nondestructive tests prior to destructive testing
b. recognize disturbances and fractures caused by coring procedures
c. ends of intact rock cores should be carefully trimmed
d. store samples to protect against drying, overheating, freezing
e. document frequency, spacing conditions, filling of discontinuities
f. photo document cores, and fracture patterns
g. maintain equipment calibration
Not only must personnel be well trained and conscientious, testing facilities must provide for a variety of tests with well-maintained testing equipment and samples fulfilling the specific needs. Test equipment for strength and shear should have proper fittings, o-rings and membranes. Soil 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 soil samples for days or weeks, but not longer than 15 days If possible. If samples are stored longer then undisturbed samples should be protected against damage or changes in water content by maintaining at temperatures close to those required at the project. Rewaxing and relabeling may be required. Nevertheless stored soil samples may undergo physical and chemical changes when stored too long no matter how carefully stored and resealed.
Soil particles are rearranged and densified to improve the soils' engineering properties of strength, permeability and compressibility. The existing subgrade may have poor strength or instability due to excess clay, expansive clays, silts, fine sands, voids, collapsing soils or high watertables. The existing soil properties must be known to protect against potential settlement with the required bearing capacities. There are problem soils such as loess, hydraulic fills and tailings, which have collapsing or low-density structures, and when saturated have large decreases in volume and loss of strength. Other soils, which contain clays such as bentonite, can expand and increase in volume when exposed to water. Expansive soils however can shrink or decrease in volume when water is not present. There are also dispersive clays so named because the soil particles are not structurally sound and can easily disperse or detach and erode in still water.
Compaction or mechanical
stabilization is one of the oldest means of soil stabilization.
Mechanical stabilization may achieve the desired results by blending two soils
and/or mixing with admixtures. If suitable soil was located within a feasible
haul distance, blending the soils together could effect an improvement in the
existing soil. However the soil blending would introduce ROW, hauling and handling
issues to consider. Using chemical or bitumen additives to improve a soil is
another possibility but handling and excavation of the existing soil would also
have to be considered. Certain soils because of their chemical nature, organic
or high acid compounds may not be responsive to these stabilization methods
and may be corrosive to steel reinforcement. Often the soils are not readily
distinguished by their classification or physical properties. Corrositivity
and pH tests will determine the chemical and organic content of the soil if
these are suspected problems.
The following text errors should be noted:
a. FHWA NHI-01-031:
The Figures mentioned in the text, from Figure 5-8 (page 5-10) and Figure 5-33
(page 5-34) are incorrect and each one should be increased by one digit. The
Figures are shown correctly in the text.
b. FHWA NHI-01-031: Figure 5-32 mentioned in the text should read Figure 5-37.
Course
Content
The course is based on Chapter 5 (pages 5-21 through 5-30) of
the US Army Corps of Engineers Manual EM 1110-1-1804, "Geotechnical Investigations",
(2001 Edition, 10 pages), PDF file. The course is also based on Chapter 5 of
the US Dept of Transportation FHWA publication FHWA NHI-01-031, "Subsurface
Investigation-Geotechnical Site Characterization", (2001 Edition, 37 pages),
PDF file.
The links to the parts of both documents are as follows:
A brief list of "Terms and Definitions" shown in the text material has been included as a study aid for the course:
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 in situ testing and to better understand the variety of tests, purposes, and the advantages and limitations of each. Basic concepts of soil and rock mass behavior are discussed to better understand the purposes of tests and the measurement of strength and stress values to characterize the subsurface formations. In situ test data is necessary to meet the needs of construction and plan for the necessary strength, slope stability and bearing capacity based on existing subsurface conditions. In situ tests include SPT, FVS, CPT, direct shear, plate bearing, flatjack and hydrofracture. Strength tests include triaxial, compressive, shear. The importance of quality assurance for in situ testing is stressed including the proper storage and handling of samples for laboratory testing programs. This course should serve as a guide for selecting those tests needed to determine the characteristics of subgrade formations in order to protect against potential settlement, liquefaction or ground movement for proposed construction.
Related Links
For additional
technical information related to this subject, please refer to:
http://www.haywardbaker.com/
Information and applications describing construction methods for ground improvement,
structural support and earth retention and necessary materials. Provides solution
tools for problem soils and project applications.
http://www.usace.army.mil/publications/eng-manuals/em1110-1-1804/chap5.pdf
Describes lab tests for pervious and fine-grained cohesive soils and the soil
characteristics tested for.
http://geosystems.ce.gatech.edu/Faculty/Mayne/papers/index.html
Subsurface
testing methods and testing equipment and presentations, papers and case studies
by faculty members of Georgia Institute of Technology are presented.
http://www.vulcanhammer.net/download/laboratory_field.php
Offers geotechnical downloads for various manuals from FHWA, Dept. of the Interior,
US Army Corps of Engineers and papers from collages and PE's. Case studies,
recent developments and downloadable software are available.
Quiz
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.
