Journal of Geotechnical and Geoenvironmental Engineering

More than 20 years have passed since a Terzaghi Lecture focused on the topic of deep foundations. However, considerable research has been performed, and experience gained, in this subject area in the intervening period. The objective of this paper is to update the earlier references on deep foundations by summarizing results of important recent research on a few aspects of the topic of side resistance, most notably (1) driven piles in saturated clay, (2) driven piles in siliceous sand loaded in compression and uplift, (3) drilled shafts in clay, and (4) drilled shafts in soft rock. It is concluded that, while simple design relations are available for topic (1), much is still to be learned. Under topic (2), the case is made that loading the pile in compression and uplift produces different values of unit side-shearing resistance. Regarding topics (3) and (4), the effects of details related to construction—such as stress relief, moisture migration from the concrete to the geomaterial, borehole roughness, and borehole smear—are shown to be significant. The final point made is that the design of deep foundations is a complex matter that should be addressed in a design context by engineers who are experienced in the observation of pile behavior, theoretical modeling, and the appropriate use of design methods.

Raft foundations enhanced with deep foundation elements (typically piles), simply known as piled rafts, were examined to develop a more integrated, displacement-based, design methodology. Illustrative piled rafts were analyzed using simplified linear elastic and nonlinear plane strain finite element models. The effects of raft and pile group system geometries and pile group compression capacity were evaluated on the “average” and differential displacements, raft bending moments, and pile butt load ratio of the piled rafts. The results were synthesized into an updated, displacement-based, design methodology for piled rafts.

The objective of this research was to understand the mechanism of dynamic replacement (DR) with the view of identifying the attributes needed to effectively implement it. This was achieved by analytically modeling DR of organic soils with sand using the finite-element method. First, this paper describes briefly the finite-element formulation and the calibration of the finite-element model using common laboratory soil tests. Then, model predictions are compared with observations made during a fully instrumented field DR trial. The respective laboratory tests utilized to extract the model parameters certainly confirmed the applicability of the chosen constitutive models for the relevant geomaterials involved in the field DR test. Furthermore, the analytical predictions of the drop hammer displacement and acceleration, as well as pore pressures and lateral displacements in the drop hammer vicinity, agree reasonably well with those observed during the full-scale field DR. Finally, a parametric study is performed to demonstrate the usefulness of the analytical model in predicting the optimum thickness of the initial sand blanket that would produce the maximum treatment benefits.

A full time-domain solution for predicting earthquake ground motion based on the 1D viscoelastic shear-wave equation is presented. The derivation results in a time-domain equation in the form of an infinite impulse response filter. A solution in the time domain has several advantages including causality, direct modeling of impulsive and transient processes, and ease of inclusion of nonlinear soil behavior. The method is applicable to any arbitrarily layered silhouette presented as SH-wave velocity, damping coefficient, and mass density profiles for designated soil intervals. For nonlinear evaluations, an equivalent-linear formulation is incorporated and the standard modulus and damping degradation curves become part of the input set. Input motion can be either rock-outcrop or body-wave motions measured or estimated at the bottom of the geologic profile, and the output is the estimated ground motion time history. Application of the method to vertical array strong motion records from Garner Valley, and Wildlife Site, Calif., shows that predicted surface (and interval) ground motion is virtually identical to that measured. The differences between the results of linear and nonlinear analyses are negligible for most cases. A comparison of the time-domain model with SHAKE shows that SHAKE fails to accurately predict time histories in some situations, whereas the time-domain solution always yields satisfactory predicted surface ground motions.

Piezocone soundings were performed in residual silts and fine sands of the U.S. Atlantic Piedmont geologic province to investigate penetration pore-water pressures and subsequent dissipation characteristics in these “nontextbook” geomaterials. Piedmont residuum has been formed by the in-place weathering of Paleozoic schist, gneiss, and granite. Pore-water pressures were measured at two positions: midface of the cone tip u₁ and at the shoulder or behind the tip u₂. At the standard rate of 2 cm/s, penetration pore-water pressures on the midface element are fairly high positive values (u₁ = +600 kPa ± 200 kPa), yet readings at the shoulder element were negative (u₂ ≈ −90+ kPa). At an increased penetration rate of 20 cm/s, midface and shoulder readings increased in magnitude and were positive (u₁ ≈ +800 kPa and u₂ ≈ +200 kPa); however, measured tip resistance q_c exhibited an unusual decrease, per comparisons with data available in the literature. Upon halt of penetration, both u₁ and u₂ decayed quickly and reached hydrostatic values u₀ within 1–2 min.

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements δL/R² are functions of their dimensionless position x/L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x/L depend only on the wall thickness w; i.e., δ/w = f[x/L]. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of (1) building movements caused by installation of large pile groups; (2) uplift of a pile caused by driving of an adjacent pile within a group; and (3) spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the shallow strain path method analyses.

Specimens prepared from eight natural clayey soils used for clay liners and covers were subjected to cycles of wetting and drying. Volumetric shrinkage strains were recorded during drying. Specimens in which cracks formed during drying were subjected to hydraulic conductivity testing. Results of the study indicate that volumetric shrinkage strains are influenced by soil properties and compaction conditions. Volumetric shrinkage strain increased with increasing plasticity index and clay content, and as the compaction water content increased or decreased relative to optimum water content. Volumetric shrinkage strain decreased with increasing compactive effort. Specimens with the largest volumetric shrinkage strains typically contained the largest number of cracks. Hydraulic conductivity testing indicated that cracking of the specimens resulted in an increase in hydraulic conductivity, sometimes as large as three orders of magnitude.

Soil is typically regarded as a frictional or cohesive-frictional material in limit-state considerations. Often the solution to a limit-state problem (for instance, the bearing capacity of footings) for a purely frictional soil is easier to obtain than the solution for a cohesive-frictional material. A theorem was presented that makes it possible to obtain a solution for cohesive-frictional soil through a transformation of a known solution for purely frictional soil. However, application of the transformation rule based on this theorem is shown to have limitations. This rule appears to be applicable for boundary-value problems where boundary stresses have only normal components and principal stress trajectories are not altered by the transformation. With modern computational tools the correspondence rule is bypassed. However, its applicability is a consequential issue in soil mechanics education, even if of a somewhat historical nature. An example of limit loads on a strip footing is presented, a solution to inclination coefficients is produced, and the consequences of the rule of corresponding states are discussed. Finally, the application of the rule of correspondence in the kinematic approach of limit analysis is investigated. A convenient method is developed for calculations of the energy dissipation rate, which does not require tedious calculations of dissipation on all velocity discontinuity surfaces or in continually deforming regions.

Analyses and experiments are performed to gain further insight into the behavior of unsaturated particulate materials, with emphasis on the pendular stage. First, interparticle forces are computed based on Laplace's equation; soil particles are ideally considered spherical or flat to facilitate the identification of the most relevant factors that affect unsaturated soil behavior. Second, the small strain stiffness is continuously measured on specimens subjected to drying, and changes in stiffness are related to changes in interparticle forces; data show important differences with previously published trends based on remolded specimens. Third, microscale experiments are performed to assess the strain at menisci failure in multiple deformation modes; results indicate that the lower the water content, the lower the strain required to eliminate the effects of capillarity, therefore, while capillary forces affect small strain stiffness, they may not contribute to large strain stiffness or strength. Finally, the rate of menisci regeneration is studied after a perturbation; stiffness recovery decreases with decreasing water content, and full recovery may not be reached when the degree of saturation is low. Several phenomena associated with the evolution of capillary forces during drying are identified.