Bender elements have surfaced as versatile transducers for low-strain testing of soils in a variety of cells and load conditions. However, lack of guidelines leads to different implementations among laboratories. The authors present an interesting evaluation of effective length and travel time determination. The purpose of this discussion is to contribute complementary information, summarizing our experience with bender elements in various short-term and long-term tests (a typical installation and examples of application are described in Fam & Santamarina (1995)).

The stiffness of soil at very small strain Go is a useful parameter for characterizing the non-linear stress–strain behaviour of soil for monotonic loading and it is required for analyses of the dynamic and small strain cyclic loading of soils. Tests were carried out on fine-grained soils in a hydraulic triaxial cell fitted with bender elements and with local axial gauges. From the results of these tests simple expressions were obtained which describe the variation of G>o with current state in terms of the current stress and overconsolidation ratio. The parameters in these expressions were found to depend on plasticity index. The simple expressions for Go were found to apply generally at larger strains, with the values for the parameters also depending on the current strain. Values of Go measured in laboratory tests on reconstituted London clay agree well with values measured in tests on undisturbed samples and in field tests which make allowance for the different states in the various tests. La rigidité Go d'un sol, sous très faible déformation, est un paramètre intéressant qui permet de caractériser la non-linéarité du comportement en contrainte-déformation de ce sol lors d'un chargement monotone et d'analyser les cycles de chargement dynamique à faible déformation. Des essais ont été réalisés en cellule triaxiale hydraulique sur des sols finement grenus équipés de capteurs en flexion et de jauges axiales locales. Les résultats obtenus au cours de ces essais ont permis de définir des relations simples donnant la variation de Go en fonction de la contrainte courante et du degré de surconsolidation. Les paramètres dépendent de l'indice de plasticité. Une expression simple de Go est applicable à de plus fortes déformations, les paramètres étant alors en plus fonction de la déformation courante. Les valeurs de Go mesurées en laboratoire sur de l'argile de Londres reconstituée sont en bon accord avec celles obtenues sur des échantillons intacts où lors d'essais in-situ et rendent compte des différents états rencontrés lors des différents essais.

It is well recognised that the dynamic interaction between structure, foundation and supporting soil can affect significantly the seismic behaviour of buildings. Among other effects, embedded and deep foundations can filter the seismic excitation, causing the foundation input motion (FIM) to differ substantially from the free-field motion. This paper presents a theoretical and numerical investigation on the filtering effect induced by rigid massless embedded foundations. Based on the results of dimensional analysis and numerical simulations, it is shown that the problem can be reasonably described by two sole dimensionless groups, namely: (i) H/VS, relating the wave length of the signal to the embedment depth of the foundation, and (ii) the aspect ratio of the foundation, B/H, where B is the foundation width in the polarization plane. New simplified and physically sound expressions are derived for the kinematic interaction factors, = uFIM/uff0 and = FIMH/uff0, which are frequencydependent transfer functions relating the harmonic steady-state motion experienced by the foundation to the amplitude of the corresponding free-field surface motion. Standard methods for using these functions in the evaluation of the FIM are critically reviewed, with reference to both static and dynamic procedures for the seismic design of structures.

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Small-strain stiffness of reconstituted clay compressed along constant triaxial effective stress ratio paths INTRODUCTION Any calculation of ground movements around an engineering structure requires a knowledge of soil stiffness. Even at relatively small strains, the stress±strain behaviour of soils is highly non-linear and the stiffness can vary signi®cantly over the range of strains of interest in civil engineering. The assessment of reliable stress±strain relationships for soils, for both static and dynamic deformation problems, requires a correct evaluation of the shear stiffness at small strains, G 0 , and of the shape of the stiffness degradation curve (e.g. see Burghignoli et al., 1991). By assuming that the mechanical behaviour of the soil is linear elastic inside a relatively small