(3) Stress Path Method. The stress path method is based on
modelling the geological and historical stress conditions as they are known
to influence soil behavior. To apply the method, stress history is
determined and future stresses are computed based on actual construction
plans. The stresses are modelled in a set of triaxial or similar strength
tests (see Figure 6). Details of this procedure are found in Reference 20,
Stress Path Method, Second Edition, by Lambe and Marr.
Section 6.
DYNAMIC TESTING
1. UTILIZATION. Capabilities of dynamic soil testing methods and their
suitability for various motion characteristics are shown in Table 7 (from
Reference 10). Dynamic testing is needed for loose granular soils and soft
sensitive clays in earthquake areas, for machine foundation design, and for
impact loadings. Only a brief description of tests follows. For further
guidance on testing procedures, see References 10 and 11.
2. RESONANT COLUMN TEST. The resonant column test consists of the
application of sinusoidal vibration to one end (termed the active end) of a
solid or hollow cylindrical soil specimen. The other end is known as the
passive end. Compression waves or shear waves are propagated through the
soil specimen to determine either Young's modulus (E+s,) or shear modulus
(G). Moduli are computed from the resonant frequency of the cylinder. For
example, in the case where passive end platen is fixed, the lowest frequency
for which the excitation force is in phase with the velocity at the active
end is termed the resonant frequency. Damping is determined by turning off
the excitation at resonant frequency and recording the decaying vibration.
3. CYCLIC TESTS. Currently, these are the most commonly used methods of
evaluating the Young's modulus, shear modulus, damping, and liquefaction
potential of coarse-grained soils.
a. Cyclic Triaxial Compression Test. In triaxial testing of saturated
soils, cell pressure is maintained constant while the axial stress is
varied.
b. Cyclic Simple Shear Test. Simple shear equipment has also found
wide use in cyclic testing. The non-uniform stress conditions in simple
shear may cause failure at a lower stress than that which would cause
failure in situ. Measurement or control of lateral pressure is difficult
in simple shear tests.
c. Cyclic Torsional Shear. Cyclic torsional simple shear tests on
hollow samples offer the capability of measuring lateral confining pressure.
In hollow cylinders stresses within the specimen are more uniform, though
the specimens are difficult to produce. Also, tapered hollow cyclinders
have been used in torsional cyclic tests.
7.1-151
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