This paper presents an overview of the principal geotechnical features of the Loma Prieta Earthquake of October 17, 1989 (M_s=7.1). Damage patterns for this event were strongly controlled by geological/geotechnical factors including site-specific ground response, soil liquefaction, and slope stability failures. In addition to describing these features and the associated damages, this paper provides some historical context for the underlying conditions, and summarizes the principal lessons learned.

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Flexural behavior of thin square or rectangular raft foundations resting in smoot hcontact with an isotropic elastic halfspace is examined using a mixed-variational approach. The deflected shape and the flexural moments of the foundation are assumed as even functions of the spatial coordinates x and y and a number of unknown constants. The contact stress distribution at the foundation-halfspace interface is expressed as a direct function of the assumed deflected shape of the foundation. Numerical results are obtained for deflection, flexural moments and contact stress at the interface for different foundation frigidities. Comparison of results with the finite element solutions shows a good agreement for various relative rigidities. The proposed method is computationally efficient and can be used effectively to predict deflections and flexural moments of raft foundations.

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A simplified non-linear analysis is performed to investigate the distribution of vertical displacement in the soil around a friction or floating pile vertically loaded, where contributions of pile base to pile stiffness and to load capacity are negligibly small. A model describing the non-linearity of shear modulus and shear strain is propped based on published laboratory tests data. The non-linear settlement behaviour is also examined by using a hyperbolic relationship between shear stress and shear strain. The results are compared with the linear elastic solutions obtained by boundary element analyses and the observation in published pile load tests. The non-linear analyses result in more concentration of shear strain near the pile shaft surface even under working loads than those of the linear elastic analyses, where the vertical displacements in the soil decrease linearly with the logarithm of a distance in a radial direction from the pile axis at the level of mid-pile depth. For practical application of theoretical elastic solutions, an appropriate equivalent shear modulus is recommended as three times the shear modulus obtained by conventional triaxial tests. It is confirmed that using the equivalent modulus a theoretical solution predicts a good correspondence with in situ measurements of vertically loaded pile.

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The earth dams built with compacted cohesive soils or dense sandy materials are vulnerable to seismically-induced permanent displacements which in some cases may be excessive. Analysis of seismic response involves an uncertainty inherent in the specification of ground motion. In this study, a probabilistic analysis is developed for evaluating seismically-induced permanent displacements in earth dams. The ground motion is modelled as a stationary random process which is defined by the Kanai-Tajimi spectral density function. The dynamic response of the dam to vertically propagating shear waves is formulated in frequency domain. The nonlinearity in shear modulus and damping is incorporated by using an equivalent linear model for the stress strain behavior of soils. The permanent displacement in dams is assumed to be in the form of the cumulative movements of a block of soil mass detached from the body of the dam. Numerical results of an example analysis and a parametric study are presented.

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A critical assessment of stress nonuniformity in hollow cylinder torsional specimens used in the study of soil behaviour under general stress paths is presented. It is shown that acceptable levels of nonuniformity in individual stress components adopted in earlier studies can cause serious and unacceptable nonuniformity in stress ratio across the specimen wall. Stress ratio being the most important variable that controls response of frictional granular materials, concepts of stress nonuniformity are advanced in terms of acceptable stress ratio variation across the specimen wall. Thus the region of general stress space that can be explored for material characterization without introducing serious nonuniformities in stress ratio is delineated together with its interrelationship to specimen geometry.

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A mixed-variational principle is derived for the analysis of axisymmetric flexural interaction of a moderately thick circular plate resting in smooth contact with an isotropic elastic halfspace. In this approach, the transverse plate deflection, the bending moments and the shear force are treated as independent variables and are approximated by power series functions in terms of the radial coordinate and a set of undetermined parameters, which satisfy the boundary conditions applicable to the free edge of the plate. The undetermined parameters are evaluated by utilizing the stationary property of the derived variational functional. Numerical results are presented for various plate rigidities. It is found that, unlike the existing energy-based methods, the deflection and the flexural moments can be predicted by the proposed formulation with the same order of accuracy.

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Cyclic loading may cause settlements in addition to the settlements due to static loads. This paper presents a study of recompression settlements occurring in normally consolidated clay when pore pressure generated by undrained cyclic loading dissipates. The study was based on 18 stress-controlled cyclic direct simple shear tests and 6 conventional consolidation tests on normally consolidated Drammen clay. The specimens were subjected to either one or several series of undrained cyclic loading and drainage. Afterwards, most of the specimens were consolidated further to a vertical stress three times the effective stress at start of cycling. The study showed that undrained cyclic loading and drainage makes normally consolidated clay more resistant to later undrained cyclic loading. The recompression index was influenced by cyclic loading, and for normally consolidated Drammen clay the settlements occurring when the cyclically induced pore pressure dissipates, may be calculated by a recompression index which is 1.5 times the recompression index from a conventional consolidation test. The compression index of specimens which where subjected to undrained cyclic loading and then consolidated past the initial consolidation stresses decreases with increasing cyclically induced pore pressure and increasing cyclic shear strain at the end of cycling.

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Anisotropic characteristics of yield curves of sand in the low and high stress levels were investigated by a series of triaxial drained tests with various stress paths, in particular reference to the stress levels and proportional loading paths. The main results were summarized as follows : 1) The overall shapes of yield curves in the low and high stress levels are roughly similar and the shapes can be approximated by ellipses which are not symmetrical about the stress path during consolidation. 2) The relationship between the tangent slopes of yield curves and stress ratios can be expressed as a unique function of stress ratio, irrespective of the proportional loading path history. 3) Based on the observed yield characteristics, a useful yield function is presented for an anisotropically consolidated sand.

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A new reliability design procedure for slope stability is proposed which can explicitly take into account not only variation of geotechnical data but also the number of samples and the relative location of the samples to the structure. The method is based on Kriging originally developed by Matheron. A case study is carried out taking an embankment on soft clay as an example. The case study clearly illustrated the capability of the proposed model to reflect the factors mentioned above on the calculated failure probability of the slope.

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Some existing analytical methods which have been used in geotechnical engineering are modified to handle the shear strength reduction with time due to softening in fissured, over-consolidated clays and mudstones. These are limit equilibrium analysis of slope stability, calculation of earth pressure and calculation of stress and displacement around a tunnel. The softening effect is incorporated in these analyses using the failure criterion and the time-dependent strength parameters proposed previously by the authors. Examples are presented which emphasize the importance of appreciating the softening effect in analyses and further illustrate that the effect of shear strength reduction with time due to softening could easily be captured with existing analytical methods.

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An investigation is made on the optimum design of a smoothing filter that may be applied in order to reconstruct a subsurface image based on the computer tomography technique (or so-called geotomography). Such a smoothing filter is especially necessary in geotechnical applications where the angle of projection is usually severely restricted. However, hitherto, the most intractable problem encountered when applying a smoothing filter is how to adjust the relative power of the smoothing filter to the observed data. The Akaike-Bayesian Information Criterion (ABIC), which was proposed by Akaike(1980), is applied in order to solve this problem. Some numerical experiments are carried out to examine the effectiveness of the proposed method. It is concluded that the proposed method is very effective in the process of selecting the optimum smoothing filter.

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Laboratory experiments were conducted on an assemblage of glass spheres and on Ottawa sand to evaluate the effect of fabric on stress-strain behavior, volume change, and the maximum dynamic shear modulus. Series of tests on hollow cylindical specimens were performed by a torsional simple shear/resonant column apparatus. After specimens were prepared by the dry tamping method and saturated, they were first isotropically consolidated, and then sheared along different stress paths while keeping the mean effective stress constant. The test results showed that for specimens of both materials, depositional fabric had a slight influence on the stress-strain behavior, and one half-cycle of preshearing created a clear anisotropy on the stress-strain response. In comparisons of the two types of specimens, Ottawa sand showed a slightly stronger depositional fabric effect but was more difficult to induce fabric as a result of preshearing. In general, both materials showed very similar behavior and it may be concluded that the assemblage of glass spheres can be effectively used in studying state and evolution of fabric of this type of granular materials.

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An experimental program was carried out to investigate the effect of landslide (or mudslide) velocity on the drag force applied by the sliding soil on a pipeline crossing the landslide area. Methods currently used in assessing drag forces are reviewed and compared to the experimental results. These results demonstrated that an increase in slide velocity results in higher drag forces and therefore that the assumption of a constant drag coefficient is not valid. The relationship between velocity and drag force was found to be represented remarkably well by a simple power-law equation. This equation was also successfully applied to experimental results reported in the literature.

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This technical note discusses from the viewpoint of "ductility" the performance of two soil improvement methods for liquefaction remediation : sand compaction piles and gravel drains. The sand compaction pile method is judged a viable remedial measure for liquefaction protection because the compacted sand shows ductile behavior in that strain amplitude in the sand increases only moderately when the intensity of earthquake ground motion is increased significantly beyond a design value. For sands having moderately high initial liquefaction resistance, a gravel drain system can improve their capacity to resist earthquake ground motions that are stronger than usual design values. On the other hand, loose sands for which improvement is attempted by the use of gravel drains show nonductile behavior in that the pore pressure in the sand increases considerably even when the earthquake ground motion is slightly increased beyond a design value.

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A simple device named "local deformation transducer(LDT)" was developed to investigate stiffness of soils that can be measured in the laboratory for strain levels ranging from 10^-6 to 10^-2. The theoretical consideration into the non-linear, but uniquely determined, relationship between the gauge strain (i.e., output voltage) and the axial strain is first described. The accuracy of the strain measurement using LDT is then discussed based on the theory and the results of calibration tests. The stability of the output voltage was examined for a few days using a dummy. In isotropic consolidation tests performed on a fine clean sand in triaxial cells, the internal axial strains measured using LDT agreed well with those associated with the condition of no bedding error, which were extrapolated from the results of tests using the specimens with various heights. Besides, in plane strain compression, the internal axial strain measured using the LDT was close to that determined by means of a laser speckle method. The capability of the LDT has been demonstrated in triaxial tests in which the variations of Young's modulus of a gravel, a cement-treated sandy soil and a soft rock were successfully measured for the prescribed range of strains as the specimens were subjected to both monotonic and cyclic loadings.

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In an attempt to study effects of saturation ratio on the strength parameters of partly saturated soils, several series of triaxial compression tests were comducted on samples of silty sands to silty clay which were recovered from several sites of landslides triggered by heavy rainfalls in recent years. The results of the tests on samples compacted to in-situ densities showed that both the angle of internal friction and cohesion tend to decrease significantly with increasing saturation ratio.

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