The results of a series of tests designed to examine the behavior of saturated clay soil under repeated loading are reported. Triaxial tests, under conditions of axial symmetry, were used and the rates of deformation were chosen so as to permit the accurate measurement of pore water pressure at all stages of the tests.It was found that, for any particular consolidation history, a critical level of repeated stress existed. Below this critical level, a state of nonfailure equilibrium was reached in which the stress-strain curves followed closed hysteresis loops. Above the critical level of repeated stress, effective stress failure occurred; and each cycle of loading produced cumulative increases in deformation.An interesting feature of the test results was that a linear relationship between the magnitude of the applied repeated stress and the increase in pore water pressure was found for stress levels below the critical value.

The behavior of gas-laden, soft submarine soils subjected to changes in mean normal and shearing stresses is discussed. Information developed for partially saturated soils is extended to soft sediments. Calculations indicating that gas-laden submarine soils generally have degrees of saturation in situ that exceed ~ 90% are presented. Therefore, it is suggested that insignificant error is introduced in predicting the effective stresses of soft sediments using the standard effective stress equation and neglecting the pore-gas pressure.The presence of gas is shown to permit volume changes of soft sediments under wave loadings. The compressibility of the gaswater pore fluid is quantified. The pore-pressure response, related to the ratio of the compressibility of the pore fluid and soil structure, is shown to be similar to that of fully saturated soils. The relevance of "undrained" shipboard tests to the prediction of slope stability is discussed. It is concluded that the presence of gas leads to undrained strengths, as measured on recovered samples, which are lower than those that occur in situ. The use of these measured strengths in stability calculations leads to conservative predictions of submarine slope stability.

No abstract

A general effective stress method for the prediction of the axial capacity of full displacement piles driven into clay is presented. The method first requires that the initial state of stress in the ground prior to pile driving be defined. The changes in stress associated with the major events in the life of a pile (pile driving, reconsolidation after driving and axial loading) are then estimated from models of pile-soil behavior and added to the initial state of stress to determine the state of stress at failure. Pile capacity is predicted using some of the concepts of critical state soil mechanics described in a companion paper (Kirby and Wroth, 1977). Comparison of predictions with measured capacities at three sites suggest that the effective stress method is promising. The advantages and limitations of the method are described. INTRODUCTION Few would disagree that, in principle, the available shear resistance along the shaft of a pile, Tav is controlled by the effective normal stress on the pile shaft at the time of failure, õ ff, and the effective stress friction angle for soil sliding on the pile material, ?ss' The friction angle, ?ss' can be measured, with difficulty, in the laboratory. However, a satisfactory procedure for prediction of õ ff is not available. A rational procedure for estimating off must begin with definition of the state of stress prior to pile installation and then consider explicitly the changes in stress associated with the major events in the life of a driven pile. These events include pile driving, reconsolidation after pile driving, and pile loading. In other words, prediction of the effective stress at the pile-soil interface, at failure, is a problem of addition: (Calculation available in full paper) Definition of the stress changes for pile driving, reconsolidation, and pile loading is a formidable task. In the past, other researchers have avoided the problem by correlating the available shear resistance at the pile-soil interface (back calculated from load tests) with untrained shear strength of the undisturbed soil and/or vertical effective stress prior to pile driving. The major limitation of such a procedure is Late all the factors influencing pile capacity are lumped into a single, empirically determined, correlation factor. Consequently, there is no basis for extrapolation of correlation factors when soil conditions, loading conditions, pile size, or pile installation methods are different from those included in the load test data from which the correlation factors were derived. An exploratory study has been completed to determine if recent advances in the understanding of soil behavior, and recently developed analytical tools, could be applied to the development of a general effective stress method for the prediction of axial capacity for driven piles in clay. During this study, consideration was given to each of the events in the life of a driven pile. The fundamental concepts of critical state soil mechanics were used to describe soil behavior at large strains.

Existing theories and models describing stress changes and consolidation-time effects around a pile were used to derive in-situ permeabilities and undrained shear strengths from piezometer probe measurements in smectite- and illite-rich soils. Permeabilities derived from piezometer measurements are in reasonable agreement with laboratory measurements, and calculated undrained shear strengths agree well with strength measurements using standard field and laboratory techniques. Undrained shear strengths Su were estimated using insertion pressures Ui, determined from both the 10.2- and 0.8-cm-diameter probes, and the relationship, Ui = 6 × Su. A strength measurement determined with the small diameter probe inserted in the disturbed zone of a previously emplaced 2.5-cm-diameter cylinder showed a significant strength reduction equal to half the value determined for the soil (strength) in the zone unaffected by the implanted cylinder. Differences in decay rates were significant, indicating severe soil disturbance in close proximity to the cylinder. Multisensor piezometer probes (2), 10.2 cm in diameter, were deployed in shallow-water fine-grained smectite-rich soils of the Mississippi delta. Pore-water pressures were measured at subbottom depths of 6.5, 12.6, and 15.6 m. Insertion pressures, time-dependent pore pressure decay, and ambient excess pore pressures were determined. Single sensor piezometers (2), 0.8 cm in diameter, were developed for deep-ocean investigations. Before high pressure testing (55 MPa), probes were inserted in reconstituted illitic marine soil to depths of 16.9 and 26.4 cm below the soil-water interface. Insertion pressures and their decay characteristics were monitored. Significant differences were observed in the pore-pressure decay rates produced by the small and large diameter probes. Decay times for the induced pressures to reach t100 values were on the order of tens of hours for the large diameter probes, whereas the t100 values of the small diameter probes were on the order of minutes. These differences in decay times were a function of the differences in probe diameters (radii) and soil permeabilities.