Journal of Geotechnical and Geoenvironmental Engineering

Lateral dynamic experiments were performed on small-scale pile groups at two different sites, one with stiff soil and the other with soft soil. The pile diameters and spacings were varied to create spacing to diameter (s/d) ratios of between 2.25 and 15.0. The results showed the group interaction effects to be negligible at s/d ratios greater than 12 for the stiff site, and greater than 16 for the soft site. The experiments are the first to alter both the pile diameter and the pile spacing and confirm that the spacing-to-diameter ratio is an important parameter when calculating group interaction effects. The dynamic testing was found to cause a permanent nonlinear response in the groups. A simple linear elastic theoretical analysis was then developed to predict the dynamic response of the groups. The analysis produced good estimates of the experimental results for a constant value of Young's modulus for each site. The values of Young's modulus used in the modeling were found to be typical of those for lateral loads on pile foundations.

The evaluation of wave-induced seabed response is particularly useful for geotechnical and coastal engineers involved in the design of foundations for offshore structures. However, most previous theoretical investigations available considered only isotropic behavior in a porous seabed, even though the behavior of anisotropic soil has been reported in the literature. In this paper, an analytical solution for the standing wave-induced soil response in a cross-anisotropic seabed is proposed. Based on the analytical solution presented, the wave-induced liquefaction potential can then be estimated. The numerical results indicate that the soil response in a saturated cross-anisotropic seabed of infinite thickness depends on both the wave condition and soil properties. This result contradicts the conclusion obtained from the conventional isotropic models. The effects of the anisotropic parameters on the wave-induced liquefaction potential are also discussed in detail.

Plane-strain, finite-element, and finite-difference computations of the bearing-capacity factor, N_γ, have been made for smooth and rough footings, for friction angles, ϕ, in the 30°–45° range, and dilation angles, ψ, in the 0 ≤ψ≤ϕ range. The finite-element analyses were based on a tangent stiffness approach. The commercially available code, fast Lagrangian analysis of continua (FLAC), was used for the finite-difference analyses. It was found that these methods may be used to obtain reliable N_γ values provided they are used cautiously. To obtain high-quality solutions, however, it is necessary to review the results critically, and, if necessary, alter the control parameters (e.g., mesh dimensions, number of calculation increments) being used. Computational difficulties were found to increase with increasing ϕ, and with increasing difference between ϕ and ψ, and the significant of these difficulties with regard to the use of the two computational techniques is discussed here. Curves relating N_γ to ϕ are presented, and compared to previously published relations. It is shown that commonly used values of N_γ, which have generally been based on associated plasticity calculations, are unconservative for real soils, for which ψ<ϕ.

Quantitative evaluations are performed on two failure mechanisms, namely, deep-footing and wide-slab mechanisms, that dominate the bearing-capacity characteristics of sandy ground reinforced with horizontal reinforcing layers. The improvement contributed by reinforcement, creating a quasi-rigid, wide earth slab immediately under the footing to the bearing capacity is analyzed. The maximization of the bearing-capacity increase of a footing is a matter of optimizing the depth and width of the quasi-rigid earth slab. The results of a total of 105 model tests, including reduced-scale model tests and some centrifuge tests under 15 g, are analyzed using calibrated internal friction angles of sand and an experimentally verified failure mechanism in reinforced sandy ground, namely, the deep-footing mechanism. Back-calculations are performed on the load-spreading angle α, which represents the total effect of the deep footing and wide slab for all the tests. Both linear and nonlinear multiple-variable data regressions are performed to find the relationships between the effect of reinforcing and the factors that control the scheme of reinforcement. The analytical procedure and equation derived in the present study substantiate the prediction of the ultimate bearing capacity of sandy ground reinforced with horizontal reinforcement.

This paper focuses on the seismic response of liquefaction sites by evaluating the transient shear strains and strong-motion characteristics at Treasure Island during the 1989 Loma Prieta earthquake (M_W= 6.9). These strain levels are used to model soil and buried pipeline deformations observed in the Marina District of San Francisco during the same earthquake. Predicted and observed pipeline damage compare favorably, thereby indicating that transient strains played a critical role in lifeline performance. A simple model for liquefaction sites shows that shear modulus reduction caused by increased pore pressure can amplify either acceleration or transient displacement, depending on soil properties and deposit thickness. Liquefaction of thick, loose sand deposits can attenuate accelerations while amplifying long-period (>4 s) displacements. The model provides a quantitative means of characterizing the phenomenon of ground oscillation that has been observed at liquefaction sites.

Investigations of liquefaction sites, combining subsurface mapping and evaluation of liquefaction damage, are summarized for four areas in San Francisco affected by 1906 and 1989 earthquakes, where more than 950 borehole and sounding records were collected and analyzed. The maps developed from this database provide a comprehensive picture of subsurface conditions with substantial practical value. Postliquefaction settlement, horizontal displacement caused by lateral spread, and earthquake damage to buried pipelines are shown to be related closely with the thickness of underlying liquefiable soil. The influence of surface and liquefiable layer thicknesses on liquefaction damage is evaluated. Mapping liquefiable layer thickness is shown to be an excellent means of locating areas of potentially severe liquefaction, which is adapted easily to geographic information systems (GIS) for planning and design purposes.

Characterization of the long-term mechanical response of geomembranes used in waste-containment facilities is crucial to designing base liner and cover systems that perform satisfactorily. To investigate the long-term mechanical response, strain-controlled multiaxial tension testing was performed over a fourfold variation of strain rate using a newly developed multiaxial tension-test apparatus capable of performing constant strain rate and constant stress creep tests. This device subjects a geomembrane specimen to multiaxial stress states and allows for the development of strain conditions that vary from plane strain at the clamped edges to balanced biaxial at the center. Results from testing high-density polyethylene (HDPE) specimens indicate that the secant modulus and strength decreases considerably at strain rates appropriate for long-term field applications. The strength of HDPE measured at a typical laboratory strain rate of 1% per minute can be more than twice the strength predicted at a strain rate of 1E-6% per minute, which may be representative of field performance for a typical 30-year design life. A hyperbolic model and the Singh-Mitchell creep model (which was originally formulated for soils) are shown to capture the time-dependent mechanical response of HDPE well.

The potential benefits of geosynthetic reinforced soil foundations are investigated using large-scale model footing load tests. A total of 34 load tests were performed to evaluate the effects of single and multiple layers of geosynthetic reinforcement placed below shallow spread footings. Two different geosynthetics are evaluated: a stiff biaxial geogrid and a geocell. Parameters of the testing program include the number of reinforcement layers, spacing between reinforcement layers, the depth to the first reinforcement layer, plan area of the reinforcement, the type of reinforcement, and soil density. Test results indicate that the use of geosynthetic reinforced soil foundations may increase the ultimate bearing capacity of shallow spread footings by a factor of 2.5.

The load-displacement behavior of horizontally loaded vertical anchor plates as reported in 128 published field and laboratory investigations has been studied. Laboratory and field results documented in the literature have been collected for shallow anchor plates with a variety of configurations and embedded in different types of sands. A pullout capacity factor and a geometry factor were established. These factors are dimensionless and incorporate all the parameters that significantly affect the behavior of horizontally loaded vertical anchor plates. Several attempts have been made to plot the collected data in a form such that a general trend can be observed. This data was incorporated in a generalized form to predict the ultimate horizontal pullout resistance of vertical anchor plates in terms of the influencing parameters. Another formula was proposed to calculate the displacement associated with the applied load level. The proposed expressions are of good benefit to the practice since they provide an easy approximate estimation for parameters of relative complex nature, e.g., the ultimate pullout resistance and the associated displacement. The accuracy of these expressions are within reasonable limits especially since they are based on laboratory and field results obtained from tests in a wide variety of sands and conducted by several investigators under a diversity of settings and conditions.