Hall effect instrumentation for stress and force measurements

Instruments for the measurement of normal stress, shear stress and force in geotechnical engineering laboratory applications are described. These instruments are similar in design to existing strain-gauged instruments, but instead make use of linear output Hall effect sensors. The basis of measurement in all cases is displacement rather than strain. The performance of each instrument is assessed, and it is shown that Hall effect sensors can readily be incorporated in such a way as to produce acceptable levels of accuracy. <br/

The design of diaphragm-type boundary total stress cells

The compliance of boundary total stress cells is normally the primary criterion governing their design. It is widely believed that they will not under-read the pressures applied to them by soil if the ratio of the diameter of the cell diaphragm to the displacement at its centre is not smaller than some threshold value (typically 2000). This Paper reports the results of experimental and finite element studies which show that this approach is unsound, and that boundary total stress cells are more properly designed by taking into account the relative stiffness of the diaphragm with respect to the soil. Care must be taken in the design of calibration experiments, and soil stiffness must be measured locally and at small strains. There is then good agreement between observed cell action factors and predictions made using finite element analyses. Charts are presented which may be used for the design of field and laboratory instrumentation.<br/

An experimental study of the behaviour of embedded lengths of cantilever walls

Laboratory-based 1–g experiments are described which model the embedded length of cantilever walls in sand, and in which the shear and normal stresses between the soil and the wall were measured, together with wall displacements, as the load on the wall was progressively increased to failure. The results show that comparatively large earth pressures, associated with high effective angles of wall friction, are mobilized just below the soil surface in front of the wall. Earth pressures on the retained side, below the centre of rotation of the wall, were smaller than Rankine passive values, confirming Krey's original views on the downwards direction of wall friction at this location. The results fit well within the relatively small available data set for free embedded cantilever walls, and show the trend of increasing bending moment with depth of embedment, given a constant effective angle of friction

Authors’ reply to discussion: a resin impregnation technique for the determination of the density variations in completed specimens of dry cohesionless soil

Author's reply to discussion

Tension pile tests in chalk

This paper reports the results of a series of pull-out tests performed on mini-piles in chalk. Also reported are load–displacement data for similar piles when used in a group, for anchoring large-diameter plate loading test equipment. A comparison is made with the limited data reported in the literature. The observed uplift resistance of these piles is compared with that predicted by existing design methods, based upon SPT, and vertical effective stress. <br/

A resin impregnation technique for the determination of the density variations in completed specimens of dry cohesionless soil

resin impregnation technique has been developed to measure the density variations within laboratory test specimens of dry sand, used for small-scale model testing. A low viscosity, non-toxic resin was selected after a number of trials. The resin is injected in the completed soil specimen using hypodermic needles. The impregnated samples are excavated after the resin has cured, and their density is measured. An advantage of the technique is that the density tests are performed after the test specimen is completed. The resin shows negligible volume change during setting and curing. It also shows a rapid setting time after a period of inactivity, which allows the efficient impregnation of relatively large volumes of sand. Calibration tests have shown good correlation between the densities of impregnated samples and control specimens. Examples of the application of the technique are given. Its use has contributed to the improvement of the uniformity of specimens of dry sand used for small-scale modelling of spill-through abutments and free embedded cantilever walls.<br/