Journal of Geotechnical and Geoenvironmental Engineering

Numerous field and laboratory research projects have examined loading conditions, total soil stresses, and excess pore pressures during and following pile installation. Recently, a large number of seismic retrofitting projects for bridges in California have revealed the need for well-documented field tests evaluating the effect of pile installation on the static and dynamic properties of soft clays. This paper documents the geotechnical field investigation and field monitoring program, which included measurements of excess pore pressure and horizontal deformations at three radial distances from a full-scale closed-ended pile driven into a deep deposit of Young Bay Mud. Significant excess pore pressures developed as a result of pile driving and they are a function of distance to the pile wall. Lateral deformations decrease with increasing distance from the pile and can be approximately described by the cylindrical cavity expansion method. Inclinometer measurements show lateral deformation towards the pile wall as the excess pore pressure dissipates. These measurements are essential in the calibration and validation of analytical techniques to predict changes in properties of the foundation soil.

A full-scale closed-ended pile was driven into a deep deposit of soft clay that was instrumented with inclinometers and pore pressure transducers at three radial locations and three depths. This paper presents the results and interpretation of both field measurements of shear-wave velocity and the laboratory testing program performed on pre-pile and post-pile “undisturbed” specimens. A companion paper provides full details of the site investigation, field measurements of excess pore pressure, and the deformation field around the pile. Shear-wave velocity profiles at four radial distances were obtained as a function of time following pile driving using the suspension logging method. Compressibility characteristics for this soil were determined through one-dimensional constant rate of consolidation tests carried to very high stresses. Shear strength testing included anisotropically consolidated undrained triaxial tests performed on specimens at two confinement levels to study the effect of fabric and evolving anisotropy. Direct simple shear testing was performed on specimens in their normal vertical orientation, and rotated 90° to observe changes in structure/fabric orientation after pile installation.

Frictional resistance along the exterior of an embedded structure or structural element develops through relative displacement at the interface. An understanding of how surface topography influences interface strength and deformation behavior is required to develop comprehensive interface models for soil-structure analyses, to develop interface design methods and for producing enhanced construction materials. This paper presents the results of an investigation to quantify the influence of surface topography on shear stress and volume change behavior of dilative granular material interface systems. The root spacing, asperity spacing, asperity height, and asperity angle of machined, idealized surfaces are systematically varied. Direct interface shear test results using Ottawa 20/30 sand and glass microbeads show that maximum interface efficiency for these materials is achieved for a asperity spacing to median grain diameter ratio between 1.0 and 3.0, and an asperity height to median grain diameter ratio greater than 0.9. An asperity angle of 50 degrees or greater yields maximum efficiency for any given asperity spacing or height. The results suggest that interface behavior is governed by predictable geometric and mechanical relationships that are applicable to more complex manufactured surfaces.

Numerical computations using FLAC are reported to evaluate the soil-bearing capacity for circular smooth and rough footings. The effect of nonassociativity of the soil is investigated. The results indicate a decrease in the bearing capacity-factors value when the soil displays high nonassociativity. The bearing capacity factors $N{\gamma }^{\prime }$ obtained from the computations are significantly lower than those reported by Bolton and Lau in 1993. The results compare favorably with experimental data reported by de Beer in 1970. The sources of possible inaccuracies in numerical simulations are discussed.

Geotechnical engineers face several challenges when designing structures over soft soils. These include potential bearing failure, intolerable settlement, large lateral pressures and movement, and global or local instability. Geosynthetic-reinforced and pile-supported earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. The inclusion of geosynthetic(s) in the fill enhances the efficiency of load transfer, minimizes yielding of the soil above the pile head, and potentially reduces total and differential settlements. A numerical study has been conducted to investigate pile-soil-geosynthetic(s) interactions by considering three major influence factors: the height of the fill, the tensile stiffness of geosynthetic, and the elastic modulus of pile material. While current methods have not fully addressed important effects of the geosynthetic stiffness and pile modulus on the soil arching ratio, numerical results suggested that the stress concentration ratio and the maximum tension in geosynthetic increase with the height of the embankment fill, the tensile stiffness of geosynthetic, and the elastic modulus of the pile material. The distribution of tension force in the geosynthetic reinforcement indicated that the maximum tension occurs near the edge of the pile.

Previous papers have reported on the performance of a recently developed nonlinear ground response analysis code, SPECTRA, with reference to the prediction of the free-field response at a Large-Scale Seismic Test site in Lotung, Taiwan during the M6.5 earthquake of May 20, 1986. Two more major earthquakes of different characteristics shook this test site later that same year, a M6.2 earthquake that occurred on July 30 and a M7.0 earthquake that occurred on November 15. The present article analyzes the free-field responses recorded by a downhole array from these latter two events using the code SPECTRA and a widely used equivalent linear analysis code SHAKE. The studies focus on the relative accuracy and sensitivity of the two codes with respect to the variations of the input material parameters, using time histories, acceleration response spectra, Fourier acceleration amplitude spectra, and Arias intensities as criteria for the comparison. The two codes captured the general wave form of the acceleration histories well, but there was a general tendency for both codes (particularly SHAKE) to underpredict the Arias intensities of the earthquakes.

The mechanical response of a solid continuum changes drastically as the deformation evolves from a diffuse state to a highly localized state. For this reason the subject of strain localization has received much research attention lately. This paper investigates the impact of strain localization in the form of strong discontinuity, or displacement jump, on the limit strengths of retaining walls supporting an elastoplastic backfill. The analysis focuses on the propagation of strong instability in active and passive loading using a recently developed strong discontinuity finite element model where the elements are enhanced to accommodate the presence of displacement jumps. Specifically, the analysis applies to dilative frictional material that is susceptible to shear banding. For the retaining wall problem, strong instability is shown to initiate at the ground surface and propagate downward at an angle that depends on the state of stress at the onset of localization.

An approximate approach has been proposed for the analysis of nonlinear responses of vertically loaded pile groups. The nonlinear pile-soil-pile interaction is modeled by hyperbolic load-transfer functions. To evaluate the practical applicability of the proposed approach, a back analysis of a number of published case histories referring to single pile loading tests was carried out. Discovered by the back analysis were the more appropriate values of the main model parameters. To extend the solution of single pile to the case of a pile group, an equivalent pier interacting with surrounding soil by means of the hyperbolic load transfer functions was considered. The load-settlement response of the equivalent pier can be calculated using solutions for the response of a single pile, but, to take into account the group action due to pile-soil-pile interaction, load-transfer functions have been modified. The behavior of three full scale instrumented pile groups was studied, and the computed results using the proposed model compare favorably with the measured values. The performed study of the three field tests on pile groups indicates that the load-deformation behavior, leading up to failure, can be reasonably predicted.

A procedure for estimating the undrained stress-strain behavior of sand from drained self-boring pressuremeter and seismic piezocone penetration tests is proposed in this paper. The procedure offers an inexpensive alternative to laboratory testing and avoids the uncertainty of the empirical methods based on index measurements such as the Standard Penetration Test blow count and the tip resistance in a Piezocone Penetration Test (CPTU). To check its validity, the proposed procedure was used to infer the undrained triaxial stress-strain curves and the results were compared with laboratory triaxial tests on undisturbed samples. The undrained limit equilibrium stability of a dike was also assessed using the inferred stress-strain behavior to illustrate the usefulness of the procedure. The result of the stability analysis was found to be in qualitative agreement with the observed performance of the dike during a recent field experiment attempting to trigger static liquefaction.

An important design parameter of reinforced soil structures is the friction mobilized between the soil and reinforcement elements, i.e., the pullout friction. The most commonly adopted method to identify this friction is a special test setup, i.e., the pullout test. Compared to the results of the pullout test, the direct shear test gives much smaller values. In this paper the mechanism of interaction between a soil and rigid planar as well as nail reinforcement is investigated. It is found that the mobilized friction between soil and reinforcement is influenced by the elastic parameters of the soil and its dilatancy angle. A simple approach is proposed from which the pullout friction can be estimated from the direct shear coefficient of friction between soil and reinforcement and the friction angle and dilatancy angle of the soil, all of which can be determined by direct shear tests. The results of the proposed model are in good agreement with results of pullout tests from the literature.