Journal of Geotechnical and Geoenvironmental Engineering

Centrifuge and conventional model studies in which short pile foundations were subjected to large overturning moments are reported. The centrifuge tests were carried out on small model piles with lengths ranging from 40 to 100 mm and diameters from 20 to 50 mm embedded in dense and loose cohesionless soil. Lateral pull was applied at levels between 40 and 320 mm above the ground surface. Tests were performed at accelerations between 12 and 50g to simulate the behavior of prototype piles ranging from 0.125 to 1 m in diameter. Soil packing, pulling height, pile geometry, and ground surface profile are shown to influence moment-carrying capacity significantly. Centrifuge tests were also carried out on embedded walls to establish empirical shape factors for potential use in conjunction with two-dimensional theoretical analyses. These shape factors were found to increase significantly with pile embedment ratio. Conventional tests at unit gravity performed on 100-mm-diameter piles ranging in length from 200 to 500 mm generally confirm the centrifuge work. Prototype moment limits derived from the centrifuge results compare reasonably well with predictions from the design methods of Broms, McCorkle, and UIC/ORE.

Tailings produced during mineral processing contain up to 75% or more water and are pumped into settling ponds for disposal. They often become a pollution source due to acid generation. While tailings have been used in underground backfill, the use is very limited at present. This paper presents research results on direct solidification and strengthening of the dilute tailings slurry without dewatering. Two types of tailings were tested at water/binder ratios of up to 4.5 using a special high-water binder. Adequate strength was achieved by solidifying the tailings as is. To further improve the strength, sands and fly ash were used as reinforcement materials or partial replacement of the binder. For oil sands tailings, the 3-day strength was increased by up to 114% with 10–15% additives. For hard rock tailings, up to 30% strength gain was achieved with 15% fly ash addition, and the strength was increased by up to 36% at 10% binder replacement and no strength reduction was observed at 20–25% replacement. These results indicate that higher strength can be achieved with the proper amount of additives and that the backfill process can be simplified and the operation cost reduced.

Simplified analytical models are developed for the lateral harmonic response of single piles and pile groups in layered soil. Pile-to-soil interplay is represented by a dynamic Winkler formulation based on frequency-dependent springs and dashpots. For pile-to-pile interaction, the wave field originating from each oscillating (“source”) pile and the diffraction of this field by the adjacent (“receiver”) piles are considered. The response of single piles and pile pairs is evaluated both numerically (through a transfer-matrix formulation) and analytically (introducing an efficient virtual-work approximation). Closed-form solutions are obtained: (1) for the impedance of single piles; (2) for the dynamic interaction factors between two piles; and (3) for the “additional” internal forces (“distress”) developing in grouped piles because of pile-to-pile interaction, a phenomenon frequently ignored in current methods of analysis. Both swaying and rocking vibrational modes are considered. The effect of pile length and soil layering on the impedances and interaction factors is studied. The predictions of the model are in agreement with earlier results, while its simplicity offers a versatile alternative to rigorous solutions.

Recent improvements in seismological source modeling, analysis of travel path effects, and characterization of local site effects on strong shaking have led to significant advances in both code-based and more advanced procedures for evaluating seismic demand for structural design. A missing link, however, has been an improved and empirically verified treatment of soil-structure interaction (SSI) effects on both the strong motions transmitted to structures and the structural response to these motions. This paper describes analysis procedures and system identification techniques for evaluating inertial SSI effects on seismic structural response. The analysis procedures are similar to provisions in some building codes but incorporate more rationally the influence of site conditions and the foundation embedment, flexibility, and shape on foundation impedance. Implementation of analysis procedures and system identification techniques is illustrated using a building shaken during the 1994 Northridge earthquake. The analysis procedures predict the observed SSI effects accurately. A companion paper applies these analyses to empirically evaluate SSI effects using available strong motion data from a broad range of sites and then develops general conclusions regarding SSI effects on seismic structural excitation and response.

System identification analyses are used to evaluate soil-structure interaction effects for 77 strong motion data sets at 57 building sites that encompass a wide range of structural and geotechnical conditions. Kinematic interaction effects on the “input” motion at the bases of structures are found to be relatively modest in many cases, whereas inertial interaction effects on the structural response to these motions can be significant. To quantify inertial interaction effects, fixed- and flexible-base modal vibration parameters are used to evaluate first-mode period lengthening ratios $\tilde{\mathit{T}}/\mathit{T}$ and foundation damping factors $\tilde{\zeta }{}_{0\text{.}}$ The response of some structures is dominated by inertial interaction (e.g., $\tilde{\mathit{T}}/\mathit{T}$ ≈ 4, $\tilde{\zeta }{}_0$ ≈ 30%), whereas others undergo negligible soil-structure interaction (e.g., $\tilde{\mathit{T}}/\mathit{T}$ ≈ 1, $\tilde{\zeta }{}_0$ ≈ 0). Simplified analytical formulations described in the companion paper by Stewart et al. are used to predict inertial interaction effects. The predictions are found to be reasonably accurate relative to empirical results, with some limitations for deeply embedded and long-period structures. A collective examination of the empirical and predicted results reveals a pronounced influence of structure-to-soil stiffness ratio on inertial interaction, as well as secondary influences from structure aspect ratio and foundation embedment, type, shape, and flexibility.

The limit-equilibrium method is commonly used for slope stability analysis. However, it is well known that the solution obtained from the limit-equilibrium method is not rigorous, because neither static nor kinematic admissibility conditions are satisfied. Limit analysis takes advantage of the lower- and upper-bound theorems of plasticity to provide relatively simple but rigorous bounds on the true solution. In this paper, three-noded linear triangular finite elements are used to construct both statically admissible stress fields for lower-bound analysis and kinematically admissible velocity fields for upper-bound analysis. By assuming linear variation of nodal and elemental variables, the determination of the best lower- and upper-bound solution may be set up as a linear programming problem with constraints based on the satisfaction of static and kinematic admissibility. The effects of pore-water pressure are considered and incorporated into the finite-element formulations so that effective stress analysis of saturated slopes may be done. Results obtained from limit analysis of simple slopes with different ground-water patterns are compared with those obtained from the limit-equilibrium method.

The static and seismic bearing capacity problem of shallow strip footings is investigated. Two kinematically admissible failure mechanisms M1 and M2 are considered in the framework of the upper-bound method of the limit analysis theory. The M1 mechanism is symmetrical, and it permits the calculation of the bearing capacity in the case of no-seismic loading. It is composed of a triangular active wedge under the footing and two radial shear zones composed of a sequence of rigid triangles. The M2 mechanism is nonsymmetrical and is composed of a single radial shear zone. This mechanism permits the calculation of the bearing capacity in the presence of seismic loading. Quasi-static representation of earthquake effects using the seismic coefficient concept is adopted. The solutions obtained are rigorous upper-bound ones in the framework of the limit analysis theory. The numerical results of the static and seismic bearing capacity factors are presented in the form of design charts for practical use in geotechnical engineering. These results are compared with results of other authors.

The air pressure in the unsaturated subsurface changes dynamically as the barometric pressure varies with time. Depending on the material properties and boundary conditions, the intensity of the correlation between the atmospheric and subsurface pressures may be evidenced in two persistent patterns: (1) The amplitude attenuation; and (2) the phase lag for the principal modes, such as the diurnal, semidiurnal, and 8-h tides. The amplitude attenuation and the phase lag generally depend on properties that can be classified into two categories: (1) The barometric pressure parameters, such as the apparent pressure amplitudes and frequencies controlled by the atmospheric tides and others; and (2) the material properties of porous media, such as the air viscosity, air-filled porosity, and permeability. Based on the principle of superposition and a Fourier time-series analysis, an analytical solution for predicting the subsurface air pressure variation caused by the atmospheric pressure fluctuation is presented. The air permeability (or pneumatic diffusivity) can be quantitatively determined by using the calculated amplitude attenuations (or phase lags) and the appropriate analytical relations among the parameters of the atmosphere and the porous medium. An analysis using the field data shows that the Fourier time-series analysis may provide a potentially reliable and simple method for predicting the subsurface barometric pressure variation and for determining the air permeability of unsaturated zones.