ICE - Institution of Civil Engineers - Virtual Library

Current research work into the behaviour of piled foundations in liquefiable soils has concentrated on the response to lateral inertial and kinematic loads. It has recently been demonstrated in the literature, however, that an alternative mechanism exists, whereby piles carrying significant axial loads may become unstable as lateral soil restraint is lost owing to earthquake-induced soil liquefaction. In the work presented here and in a companion paper, this effect is examined in greater depth, particularly regarding the integration with and effects on the lateral response of piles such that the two effects can be treated under a unified framework. Dynamic centrifuge testing was used to investigate pile instability by examining the behaviour of pile groups with lateral deflections. Amplification of these deflections was observed to occur during liquefaction, although currently available simple idealisations of soil behaviour were found greatly to over-predict these amplifications when compared with the experimental observations. The response of the soil to such large-deformation events was determined for use in more sophisticated numerical modelling presented in the companion paper. Relative pile–soil flexibility was found to have a strong influence on amplifications occurring in the tests. Ultimately, at high axial load or amplification, unstable collapse (bifurcation) was found to occur both during earthquake shaking, and afterwards owing to excess pore pressure migration altering the strength of the supporting soil surrounding the piles.

Dynamic centrifuge modelling has been used to observe amplification of lateral displacements and unstable collapse in pile groups under the action of significant axial load and in liquefiable soils with low stiffness. The popular pseudostatic non-linear p–y analysis method has here been developed to account for such instability effects using the Riks post-buckling analysis method, here implemented in ABAQUS. The numerical method was validated against centrifuge model tests presented in the companion paper and pre-collapse amplification effects and collapse loads were found to be well predicted. The model was extended to examine the response of 2 × 2 pile groups formed from 0·5 m diameter piles with bending properties typical of steel and reinforced concrete and with initial deflections indicative of those that may occur during lateral spreading as a result of strong shaking. These analyses demonstrated the vulnerability of the flexible concrete piles to significant amplification and unstable collapse even at moderate loads, while for the stiffer and stronger steel piles, amplifications are only 10–20% even at maximum typical working loads, and therefore practically negligible in the design of piles in liquefiable soils.

The behaviour of laterally loaded piles is considerably influenced by the uncertainties in soil properties. Hence probabilistic models for assessment of allowable lateral load are necessary. Cone penetration test (CPT) data are often used to determine soil strength parameters, whereby the allowable lateral load of the pile is computed. In the present study, the maximum lateral displacement and moment of the pile are obtained based on the coefficient of subgrade reaction approach, considering the non-linear soil behaviour in undrained clay. The coefficient of subgrade reaction is related to the undrained shear strength of soil, which can be obtained from CPT data. The soil medium is modelled as a one-dimensional random field along the depth, and it is described by the standard deviation and scale of fluctuation of the undrained shear strength of soil. Inherent soil variability, measurement uncertainty and transformation uncertainty are taken into consideration. The statistics of maximum lateral deflection and moment are obtained using the first-order, second-moment technique. Hasofer–Lind reliability indices for component and system failure criteria, based on the allowable lateral displacement and moment capacity of the pile section, are evaluated. The geotechnical database from the Konaseema site in India is used as a case example. It is shown that the reliability-based design approach for pile foundations, considering the spatial variability of soil, permits a rational choice of allowable lateral loads.

This paper presents the results of an experimental and analytical study conducted to assess the measurement depth and volume of vibratory roller-measured soil stiffness. Test beds were constructed in lifts with controlled thickness and instrumented with stress and strain sensors. A composite assessment of roller-measured stiffness on individual lifts throughout construction revealed that measurement depths ranged from 0·9 to 1·2 m using two common-sized vibratory rollers. For overlying material layer thickness greater than the measurement depth, roller-measured stiffness was found to be a function of the overlying material only. Otherwise, roller stiffness was a function of the two materials. Measurement depth was not significantly influenced by the layer material stiffness ratio. The sensitivity of roller-measured stiffness to thin lifts of material (e.g. 0·2 m) was found to depend on the stiffness ratio of the two materials. Data from in-ground instrumentation supported by elastic theory revealed that, at the experimentally observed measurement depths, both the total and cyclic stress and strain were approximately 10% of their maximum values. An analysis of cyclic strain profiles from three different soils revealed that the measurement depth is weakly dependent on excitation force amplitude. For the two rollers used, measurement depth ranged from 0·8 m under low excitation force to 1·2 m under high excitation force.

Experimental measurements have been carried out with a new, accurate, triaxial device (Triaxial StaDy) to determine the quasi-elastic properties of dry Hostun sand. The experimental investigations consist in successively applying very small axial cyclic static loadings (strain single-amplitude cycle, ε _sa ≈ 10^–5 m/m) and four types of wave, generated by piezoelectric sensors (compressive and shear waves in the axial and radial directions), at different levels of the stress–strain curve. The use of both static and dynamic results allows for an entire description of the anisotropic elastic compliance tensor at a given stress–strain state to be made. The evolutions of this tensor during triaxial loading and unloading are presented. The isotropic and deviatoric triaxial stress paths considered underline the effects of inherent and induced anisotropy respectively. The different methods used to set the sample involve clearly different kinds of anisotropic elastic behaviour, which are probably indicators of different granular structure organisations.

Laboratory consolidation tests for a two-layer soil system under oedometer conditions were conducted under both vacuum pressures and surcharge loads. The effects of the order of soil layers on the behaviour of vacuum consolidation of the two-layer soil system have been investigated by comparing vacuum pressure and surcharge load induced consolidations. Under one-way drainage conditions, for both a surcharge load and a vacuum pressure, the order of soil layers only influences the rate of consolidation but not the final settlement. When a layer with a relative lower value of hydraulic conductivity (k) is located immediately adjacent to the drainage boundary, the consolidation rate is slower. However, for vacuum pressure applied under two-way drainage conditions, the order of the soil layers not only influences the rate of consolidation but also the magnitude of settlement.

Numerous research studies have been carried out in the recent past to determine the influence on the resilient modulus of railway ballast of parameters such as stress history, number of loading cycles, density, grading, fines content, maximum grain size, aggregate type, particle shape and moisture content. However, the influence of particle breakage on the resilient modulus has not been described in depth. The current study highlights the influence of particle breakage on the resilient modulus of latite ballast (commonly used in railway track in New South Wales, Australia). Aggregate degradation is represented by the ballast breakage index (BBI). Laboratory test results indicate that the BBI has a profound influence on the resilient modulus of ballast. The resilient modulus increases with BBI, irrespective of the effective confining pressure. In addition, a simple hyperbolic relationship has been proposed between the resilient modulus M _R and the bulk stress φ for latite ballast.