d. Factors Affecting Tests. Various testing and material factors that
may affect cyclic strength as determined in the laboratory are method of
specimen preparation, difference between reconstituted and intact specimens,
prestressing, loading wave form, grain size and gradation, etc. For details
on cyclic testing, see Reference 21, A Review of Factors Affecting Cyclic
Triaxial Tests, by Townsend. For the nature of soil behavior under various
types of dynamic testing see Reference 22, The Nature of Stress-Strain
Behavior for Soils, by Hardin.
4. EMPIRICAL INDICATORS. The empirical relationships given here are to
be used only as indicators and not in final design. Design involving
dynamic properties of soil must be done only under the direction of
experienced personnel.
a. Shear Modulus. In the absence of dynamic tests initial estimates of
shear modulus, G, may be made using the relationships found in Reference 23,
Shear Modulus and Damping in Soils: Design Equations and Curves, by Hardin
and Drnevich, and Reference 24, Soil Moduli and Damping Factors for Dynamic
Response Analyses, by Seed and Idriss.
b. Poisson's Ratio. Values of Poisson's ratio ([upsilon]) are
generally difficult to establish accurately. For most projects, the value
does not affect the response of the structure sufficiently to warrant a
great deal of effort in their determination. For cohesionless soils,
[upsilon] = 0.25 and for cohesive soils [upsilon] = 0.33 are considered
reasonable assumptions. See Reference 25, Foundation Vibration, by Richart.
c. Liquefaction of Coarse-Grained Soils. Liquefaction has usually
occurred in relatively uniform material with D+10, ranging between 0.01 and
than 25 blows per foot. Liquefaction is more likely to be triggered by
higher velocity than by higher acceleration. These characteristics may be
used as a guide in determining the need for dynamic testing. The potential
influence of local soil conditions (depth of stratum, depth of groundwater
table, variation in soil density, etc.) on shaking and damage intensity must
be carefully evaluated. See References 26, Earthquake Effects on Soil
Foundation Systems, by Seed, and Reference 27, A Practical Method for
Assessing Soil Liquefaction Potential Based on Case Studies at Various Sites
in Japan, by Iwasaki, et al. A surcharge reduces the tendency of a deposit
to liquefy.
Section 7.
TESTS ON COMPACTED SOILS
1. UTILIZATION. Compaction is used to densify soils during placement to
minimize post-construction consolidation and to improve strength
characteristics. Compaction characteristics are determined by moisture
density testing; structural and supporting capabilities are evaluated by
appropriate tests on samples of compacted soil.
2. MOISTURE-DENSITY RELATIONSHIPS. The Proctor test or a variation is
employed in determining the moisture-density relationship. For cohesionless
soils, Relative Density methods may be more appropriate.
7.1-153
Change 1, September 1986
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