Time-dependent (viscous) characteristics of the shear stress and shear strain behaviour of water-saturated or airdried specimens of clean sands (Hostun and Toyoura sands) observed in a series of drained plane strain compression tests are presented. The overall shear stress-shear strain relationships were very similar in a set of monotonic loading tests performed at constant axial strain rates that were different by a factor of up to 500. Despite the above, significant viscous effects on the stress-strain behaviour were observed, a) when the strain rate was changed stepwise or at a constant rate, b) at creep and stress relaxation stages, and c) immediately after loading was restarted at a constant strain rate following a creep stage. One type of constitutive model was developed in the framework of the general three-component model to simulate these behaviours. According to this model, the strain is decomposed into elastic and irreversible components, while the stress is decomposed into time-independent and dependent components. This model was developed to simulate such experimental results in that the time-dependent stress component changes not only when the irreversible shear strain rate changes but also when loading continues at a certain constant irreversible shear strain rate, while these viscous effects decay with an increase in the irreversible shear strain. The rationales for the structure of the proposed model obtained from the experiment are presented. It is shown that this model can simulate well the experimental observations described above, although they were obtained under certain limited test conditions.

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To evaluate the capability of a centrifuge physical model to catch the main aspects of the shaft friction mobilisation of non-displacement piles in sand, a series of centrifuge tests was carried out by axially loading an instrumented model pile embedded without displacement, in homogeneous dry sand. Three model piles with different roughness were combined with two silica sands (Toyoura and FF) with different grain size. After a description of the experimental set-up, test results were analysed with reference to the current practice (β method). Moreover, the effects of the normalised pile skin roughness, R_n, on the shear stress mobilised were investigated. Finally, the shaft friction mechanisms observed from centrifuge tests, were interpreted by direct shear tests on sand/rigid plate interface under constant normal stiff-ness.

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Two types of consolidation tests using a large consolidometer equipped with Linear Vertical Displacement Trans-ducers (LVDT), pore pressure transducers and total earth pressure cells were carried out. With these instruments, deformation behavior as well as pore pressure responses were monitored throughout the tests. The first type used two-step high pressure loading at 100 and 190 kPa. The second type used incremental step loading starting from an applied pressure of 12 kPa. In the high pressure loading test with 100 and 190 kPa, there was no pore pressure dissipation noted from half a day to more than 10 days, depending on the locations of the pore pressure transducers. However vertical displacement was measured during this period with no pore pressure dissipation. In the low pressure step loading test, the slurry continued to compress without any noticeable dissipation of excess pore pressure. The gain in effective stress was much lower than the applied pressure although large settlement had occurred. The lower bound values of undrained shear strengths measured by laboratory vane and fall cone were in direct proportion to the gain in effective stress. In addition, particle migration was evident from the laboratory measurements.

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Manuscript was received for review on May 31, 2000. Conventional theories for predicting the capacity of a single pile in sand have not been able to support the undisputed experimental results, which originated the concept of the critical depth (Kerisel, 1961; Vesic, 1967; Tavenas, 1971). Furthermore, the predictions obtained from these theories are wide (Poulos and Davis, 1980). An axisymmetric model was developed to predict the capacity of a single, vertical pile in sand, subjected to axial loading. The proposed model incorporates salient features previously neglected in conventional theories, viz, the interaction between the shaft and the tip resistance, coupled with a punching shear mechanism as a unique mode of failure. This model signifies a distinct departure from existing methods, which calculate the shaft resistance, based on a mechanism independent of tip resistance. The model is also capable of accounting for the effects of the sand density, the initial lateral earth pressure and the relative depth and the roughness of the pile shaft. In the proposed model, the concept of the critical depth is theoretically established, through a variable failure mechanism and the degree of shear mobilization along the pile-soil interface.

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Understanding the anchor-soil interface behavior is essential to the determination of the anchor pullout capacity and prediction of the deformation of the anchor-reinforced system under working load conditions. Numerous anchor pullout tests under both lab and field conditions have generally yielded extremely high apparent interface strength, which cannot be explained by the conventional interface friction theory between the anchor material and the soil. In this paper, the mechanism and the phenomena of the anchor-soil interaction were studied, and the soil dilatancy due to shearing was regarded as the main factor contributing to the increase of the anchor-soil interface friction. Considering this effect, along with the adoption of a cylindrical shear deformation pattern of the soil, anchor-soil interface models have been developed for hardening and softening behavior, respectively. Using the developed interface model, the or-ward and backward calculation algorithms have been formulated and applied to predict the anchor performance for the given interface parameters and to determine the interface parameters from the given anchor pullout test, respec-tively. Furthermore, the prediction of the anchor pullout performance was compared favorably with two hypothetical cases, two laboratory test results, and one field case.

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Ring shear and shear box tests were used to investigate the relationship between volume change and particle break-age during the shearing of two sands. One sand was a carbonate sand which was sheared under a high confining stress to examine whether, in the region of compressive shearing behaviour due to particle breakage, the breakage would ever cease and the soil reach a stable grading. The other sand tested was a quartz sand that was sheared at low confining stresses, to investigate whether a dilatant sand would also be subject to particle breakage. In both cases breakage was found to continue to very large strains, with no evidence of a stable grading being reached within the range of strains used. While the breakage was very small for the quartz sand it was large for the carbonate, emphasising that any definition of a critical state by means of conventional triaxial or shear box testing would be approximate only, because of the limited strains that they allow.

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One of the main mechanisms of drilled shafts in enhancing the stability of the soil slope is through soil arching, in which the interslice forces transmitted to the soil slice behind the shafts are reduced. This paper presents a finite element analysis technique for quantitatively studying the soil arching mechanisms associated with the drilled shafts tabi-lized soil slope. The modeling techniques and the constitutive relationships of the soils are described in detail. By performing a series of numerical studies, the load transfer characteristics due to soil arching are quantified for both cohe-sive and cohesionless soils. Among the parameters investigated, the ratio of shaft spacing, s, to the shaft diameter, d, was found to exert the greatest influences on the development and intensity of soil arching. Practical design tables have been developed to relate the arching-induced stress transfer to the s/d ratio, shaft diameter, and soil strength parameters. It was found that the smaller the s/d ratio and the higher friction angle of cohesionless soils, the more soil stresses are being transferred to the drilled shafts due to soil arching. The cohesive soils have greater tendency for soil arching as shown by a small cohesion value needed for fully developing the soil arching. The propensity of cohesive soils to creep may negate the arching to some extent.

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Drilled shafts have been used as an effective means to stabilize a soil slope with marginal safety factor. A limit equilibrium based slope stability analysis technique is presented in this paper that would allow for the determination of the safety factor of the reinforced slope and the forces acting on the drilled shafts. Specifically, the finite element naly-sis generated load transfer characteristic curves were incorporated into the traditional method of slice approach to account for the soil arching effects. Mathematical formulation of the proposed analysis method is given in detail, fol-lowed by validation of the approach with other analysis methods. Examples of the slopes with or without the drilled shafts are given to illustrate the reasonableness of the solution provided by the proposed approach. The efficiency of using drilled shafts to stabilize a slope is discussed by examing the influence of the shaft location, shaft size and spacing on the calculated safety factor. Finally, a practical case involving the use of the proposed approach is presented.

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The viscous aspects of the stress-strain behaviour of saturated and air-dried clean sands in drained plane strain com-pression (PSC) and saturated clean sand and soft clays in undrained triaxial compression (TC) are presented. Common as well as different viscous features among the different geomaterials are addressed. The general three-component model is used as the framework for constitutive modelling, in which the total strain rate ε is decomposed into elastic and irreversible components ε^e and ε^ir while the stress σ is decomposed into inviscid (non-viscous) and viscous ompocnents σ^f and σ^v. In the simplest model (called the new isotach model) among those described in the paper, σ^f is a non-linear function of ε^ir, while σ^v is a non-linear function of ε^ir and always proportional to σ^f for primary loading. This model is relevant to kaolin for the full pre-peak range and a reconstituted low-plasticity clay (Fujinomori clay) at small strains, both in undrained TC. The model is modified to simulate the viscous effect that decays with ε^ir, as observed with clean sands and a natural soft clay. It is shown that the second type of model (called the viscous evanescent model and the TESRA model) simulates well the above-mentioned behaviour, not only during primary loading, but also at unloaded conditions. The model is further modified to simulate the behaviour of Fujinomori clay whereby the rate at which the viscous effect decays gradually increases with ε^ir (the general TESRA model). The viscous components σ^v of the three models can be represented by a set of common equations, and the other models are specifically simplified versions of the general TESRA model.

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After the arrest of cone penetration in clays and silts, excess porewater pressures decay with time until Δu=0 and hydrostatic conditions prevail. A dissipation model is developed and initial porewater pressures are formulated in terms of cavity expansion theory and critical-state components, indicating the derived coefficient of consolidation (C_h) is a function of stress history (OCR), effective friction (M), and rigidity index (I_r), as well as the probe diameter. Both OCR and I_r are assessed theoretically from the CPTu results. The governing rate of dissipation can be expressed by a second order differential equation and solved explicitly in closed-form. The framework is unique in that both mono-tonic decay and dilatory response (initial increase and then decrease of Δu with time) are handled by the approach. The model results show good comparison with laboratory data, as well other currently accepted methods of C_h determination.

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