The modulus of elasticity is a function of composition of the soil, its void ratio, stress history and loading rate. In granular soils it is a function of the depth of the strata, while in cohesive soil it is markedly influenced by the moisture content. Due to its great sensitivity to sampling disturbance, accurate evaluation of the modulus in the laboratory is extremely difficult. For general cases, therefore, determination of the modulus may be based on field tests (see A-2). Where properly equipped laboratory and sampling facility is available, may be determined in the laboratory (see A-3).
A-2 Field Determination
A-2.1 The value of shall be determined from plate load test in accordance with good practice [6-2(12)].
where
intensity of contact pressure,
least lateral dimension of test plate,
settlement,
Poisson's ratio, and
NOTE — While this procedure may be adequate for light or less important structures under normal conditions, relevant laboratory tests or field tests are essential in the case of unusual soil types and for all heavy and important structures. Plate load test, though useful in obtaining the necessary information about the soil with particular reference to design of foundation has some limitations. The test results reflect only the character of the soil located within a depth of less than twice the width of the bearing plate. Since the foundations are generally larger than the test plates, the settlement and shear resistance will depend on the properties of a much thicker stratum. Moreover, this method does not give the ultimate settlements particularly in case of cohesive soils. Thus the results of the test are likely to be misleading, if the character of the soil changes at shallow depths, which is not uncommon. A satisfactory load test should, therefore, include adequate soil exploration {see good practice [6-2(2)]} with due attention being paid to any weaker stratum below the level of the footing. Another limitation is the concerning of the effect of size of foundation. For clayey soils the bearing capacity (from shear consideration) for a larger foundation
is almost the same as that for the smaller test plate. But in dense sandy soils the bearing capacity increases with the size of the foundation. Thus tests with smaller size plate tend to give conservative values in dense sandy soils. It may, therefore, be necessary to test with plates of at least three sizes and the bearing capacity results extrapolated for the size of the actual foundation (minimum dimensions in the case of rectangular footings).
A-2.1.1 The average value of shall be based on a number of plate load tests carried out over the area, the number and location of the tests, depending upon the extent and importance of the structure.
A-2.1.2 Effect of Size
In granular soils the value of corresponding to the size of the raft shall be determined as follows:
Where represent sizes of foundation and plate and is the modulus determined by the plate load test.
A-2.2 For stratified deposits or deposits with lenses of different materials, results of plate load test will be unreliable and static cone penetration tests may be carried out to determine .
A-2.2.1 Static cone penetration tests shall be carried out in accordance with accepted standard [6-2(21)]. Several tests shall be carried out at regular depth intervals up to a depth equal to the width of the raft and the results plotted to obtain an average value of .
A-2.2.2 The value of may be determined from the following relationship:
A-3 Laboratory Determination of
A-3.1 The value of shall be determined by conducting triaxial test in the laboratory in accordance with accepted standard [6-2(34)] on samples collected with least disturbances.
A-3.2 In the first phase of the triaxial test, the specimen shall be allowed to consolidate fully under an all-round confining pressure equal to the vertical effective overburden stress for the specimen in the field. In the
PART 6 STRUCTURAL DESIGN — SECTION 2 SOILS AND FOUNDATIONS
second phase, after equilibrium has been reached, further drainage shall be prevented and the deviator stress shall be increased from zero value to the magnitude estimated for the field loading condition. The deviator stress shall then be reduced to zero and
the cycle of loading shall be repeated.
A-3.3 The value of shall be taken as the tangent modulus at the stress level equal to one-half the maximum deviator stress applied during the second cycle of loading.