Sunday, November 18, 2012

Soil Investigation.

A detailed study of the subsurface soil should be made as outlined in TM 5-818-1.  The scope of this
investigation depends on the nature and complexity of the soil, and size,   functional intent, and cost of the structure.  These parameters are frequently the consolidated-drained friction angle  N for cohesionless soil, undrained shear strength C for cohesive soil, soil elastic modulus E for undrained loading, soil dry unit weight,
and the groundwater table elevation.  Refer to TM 5-818-1 for guidance on evaluating these parameters. Consolidation and  potential heave characteristics may also be required for clay soils and  the needed parameters may be evaluated following procedures presented in TM 5-818-7.  Other tests associated with soil minvestigation are:

(1) In situ tests. The standard penetration te (SPT) according to ASTM D 1586 and the cone penetration tesi (CPT) according to ASTM D 3441 may be perfonned to estimate stnngth parameters from guidance in TM 5-818-1.

(2) Soil sampling. Most soil data are obtained from results of laboratozy tests on specimens from disturbed and relatively undisturbed samples. Visual classification of soil is necessary to roughly locate the different soil strata as a function of depth and lateral variation.

(3) Location and sampling depth. Borings should be spaced to define the lateral geology and soil nonconformities. It may be sufficient to limit exploration to a depth that includes weathered and fissured material, to bedrock, or to depths influenced by construction. For individual drilled shafts, depths of at least five tip diameters beneath the tip of the deepest element of end-bearing foundations should be investigated. For driven pile groups, a much deeper investigation is appropriate and should extend a minimum of 20 feet or two pile group widths beneath the tip of the longest anticipated pile, or (o bedrock, whichever is less. These depths are the minimum required to provide sufficient data for settlement analysis. The potential for settlement should be checked to ensure compliance with design specifications.

(4) Selection of soil parameters. Results of laboratoiy and in situ tests should be plotted as a function of depth to determine the characteristies of the subsurface soils. Typical plots include the friction angle Ø  for sands, undrained shear slrength Cu  for clays, and the elastic modulus Es. These data should be grouped depending on the geological interpretation of the subsoil of similar types. Each soil type may be given icpresentative values of strength, stiffness, and consolidation or swell indexes for estimating soil settlement or heave. Soil strength parameter could be estimated from established correlations from laboratory testing.

(a) Classification. Soil classification characteristics should be applied to estirnate soil strength arid other parameters from guidance in TM 5-818-1. Data such as gradation from sieve analysis, Atterberg hmits, water content, and specific gravity should be detemined from tests on disturbed specimens. Refer to ASTM D 2487 for soil classification procedures.

(b) Strength. Soil strength parameters are required to evaluate vertical and lateral load capacity. The strength of cohesive soil may’ be determined from triaxial test results performed cm undisturbed soil specimens at confining pressures equal to the in situ total vertical overburden pressure σv. The unconsolidated undrained Q test will determine the undrained shear strength (cohesion) Cu of cohesive soils. The effective friction angle Ø' and cohesion of overconsolidated soils may be determined from results of R tests with pore pressure measurements using a confming pressure similar to the effective overburden pressure Ø'. However, analyses are usually performed assuming eithcr cohcsive or cohcsionless soil. Mean strength values within (he zone of potential failure may be selected for pile capacily artalysis. Refer to TM 5-818- 1 and NAVFAC DM-7. 1, “Soil Mechanics,” for further details.

(c) Elastic modulus. Young’s Ilastic modulus Es, is required for evaluation of vertical displacements of the deep foundation. The Es, may be estimated as the initial slope from the stress-strain curves of strength test results performed on undisturbed soil specimens. The Es, for clay may be estimated from (he undrained shear strength Cu, the overconsolidation ratio, and the plasticity index (PI) shown in Figure 1 -8. The Es typically vanes from 100 to 400 kips per square foot (ksf) for soft clay, 1,000 to 2,000 ksf for stiff clay, 200 to 500 ksf for bose sand, and 500 to 1 ,000 ksf for dense sand.


The value of ks is recommended to be about 40, 150, and 390 ksf/ft for loose, medium, and dense dry or moist sands, respectively, and 35, 100, and 210 ksf/ft for submerged sands after FHWA-RD-85- 106, “Behavior of Piles and Pile Groups UnderLateral Load.” The value of ks is also recommended to be about 500, 1,700, and 5,000 ksf/ft for stiff clays with average undrained shear strength of 1 to 2, 2 to 4, and 4 to 8 ksf respectively.

Figure  1-8. Variation  Kcu  for clay with respect to undrained shear strength and overconsolidation ratio

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