load strength index is defined as the ratio of the applied force at failure
to the squared distance between loaded points. This index is related to the
direct tensile strength of the rock by a proportionality constant of 0.7 to
1.0 depending on the size of sample. Useful relationships of point load
tensile strength index to other parameters such as specific gravity, seismic
velocity, elastic modulus, and compressive strength are given in Reference
11, Prediction of Compressive Strength from Other Rock Properties, by
DiAndrea, et al. The technique for performing the test is described in
Reference 9.
c. Classification by Durability. Short-term weathering of rocks,
particularly shales and mudstones, can have a considerable effect on their
engineering performance. The weatherability of these materials is extremely
variable, and rocks that are likely to degrade on exposure should be further
characterized by use of tests for durability under standard drying and
wetting cycle (see Reference 12, Logging Mechanical Character of Rock, by
Franklin, et al.). If, for example, wetting and drying cycles reduce shale
to grain size, then rapid slaking and erosion in the field is probable when
rock is exposed (see Reference 13, Classification and Identification of
Shales, by Underwood).
3. ENGINEERING AND PHYSICAL PROPERTIES OF ROCK. A preliminary estimate of
the physical and engineering properties can be made based on the
classification criteria given together with published charts, tables and
correlations interpreted by experienced engineering geologists. (See
Reference 8; Reference 13; Reference 14, Slope Stability in Residual Soils,
by Deere and Patton; Reference 15, Geological Considerations, by Deere;
Reference 16, Engineering Properties of Rocks, by Farmer.) Guidance is
provided in Reference 14 for description of weathered igneous and
metamorphic rock (residual soil, transition from residual to saprolite,
etc.) in terms of RQD, percent core recovery, relative permeability and
strength. Typical strength parameters for weathered igneous and metamorphic
rocks are also given in Reference 14. Guidance on physical properties of
some shales is given in Reference 13.
Section 6.
SPECIAL MATERIALS
1. GENERAL CLASSIFICATION AND TYPICAL ENGINEERING IMPLICATIONS. See Table
12 for general classification and typical engineering implications of
special materials that influence foundation design.
2.
EXPANSIVE SOILS.
a. Characteristics. Expansive soils are distinguished by their
potential for great volume increase upon access to moisture. Soils
exhibiting such behavior are mostly montmorillonite clays and clay shales.
b. Identification and Classification. Figure 4 (Reference 17, Shallow
Foundations, by the Canadian Geotechnical Society) shows a method based on
Atterberg limits and grain size for classifying expansive soils. Activity
of clay is defined as the ratio of plasticity index and the percent by
weight finer than two microns (2[mu]). The swell test in a one dimensional
consolidation test (see Chapter 3) or the Double Consolidometer Test
(Reference 18, The Additional Settlement of Foundations Due to Collapse of
Structures of
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