M.C. Matthews

The geotechnical value of ground stiffness determined using seismic methods

Abstract Geotechnical design routinely requires that the in situ strength, stiffness and permeability of the ground be determined. Most satisfactory designs for constructions such as buildings, excavations and tunnels ensure that an adequate margin of safety is maintained, and, under these conditions, measurements of the stiffness of the ground are required so that movements in the ground, both during and after construction, can be calculated. Over the past two decades the careful back-analysis of the behaviour of the ground around constructions such as tunnels and excavations has repeatedly shown that the in situ stiffness of soils and rocks is much higher than was previously thought, and that stress—strain behaviour of these materials is non-linear in most cases. Numerical analyses, using finite element and finite difference computations and field observations, have demonstrated that when margins of safety are adequate the strain levels in the ground around retaining walls, foundations and tunnels are small, and typically of the order of 0.0-l%–0.1%. Improved measurements in the laboratory have confirmed the non-linear stress-strain behaviour of soil, and shown that stiffness is much higher when measured locally and at small strain levels than when determined using conventional laboratory techniques. The realization that strain levels around construction are small, and that field stiffnesses are much higher than previously measured in the laboratory, has led us to re-appraise the value of stiffnesses derived from field seismic geophysical methods. Such methods allow stiffnesses to be determined on representative volumes of the ground, and at the in situ stress state, and for this reason may provide valuable data. This paper reviews the importance of stiffness in geotechnical design and how seismic methods are used in ground stiffness investigations.

Locating dissolution features in the Chalk

Dissolution features are common in the Chalk and may result in differential or collapse settlement of foundations if undetected. Dissolution pipes and cavities may be easily missed by conventional drilling methods. Probing and geophysical methods of investigation offer an attractive alternative due to their ability to cover large areas rapidly and thus minimize cost. The success of geophysical methods depends on many factors, principally the size of the feature in relation to the depth of burial and the cover material. This paper describes a study of dynamic probing and a number of geophysical methods used to locate dissolution features at two sites with contrasting ground conditions. The first site contained a bowl‐shaped doline over a clay‐filled dissolution pipe beneath a relatively thin soil cover. At the second site there was a thick, predominantly granular cover material that contained cavities which had migrated from dissolution pipes in the chalk below. Ground truth data from trenching was obtained to provide a basis for evaluating the investigation methods used. The ability of both dynamic probing and geophysical methods to locate and map dissolution features is discussed.

Large diameter plate tests on weathered in-situ chalk

The prediction of the settlements of spreadfoundations on in-situ chalk is best carried out using empirical methods, based on the simple behavioural model developed by Burland & Lord (1970) using results from plate tests. However, the data available to support this approach remain somewhat limited, stratigraphically, geographically and in terms of size and level of loading. This paper reports the results of nine large (1.8 m) diameter plate tests carried out on three weathered chalks with widely different intact densities and strengths. The results are interpreted in terms of the Burland and Lord model, and despite the differences in the chalks at these sites, are found to be broadly in agreement with the parameters deduced on the basisof the pioneering work carried out at Mundford by Wardet al. (1968). In addition the paper provides someadditional data on creep settlements, confirming that the long-term settlements on chalk can be considerably larger than those predicted on the basis of short-term loading tests.

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.

A flow slide in central Italy

The paper describes a slope failure in granular colluvium, together with its geomorphological, seismic and geotechnical setting. The predictions from conventional geotechnical slope stability analyses, taking into account topography, pore pressure and seismic effects, are compared with the observed pattern of instability. It is concluded that the failure was a flow slide and resulted from a combination of factors. Geotechnical limit equilibrium stability analyses of entire slopes are rarely able to predict the smaller-scale initiation events leading to flow slides, because these are controlled by local topography, ground and groundwater conditions. Nor are they able to predict the speed and run-out of such slides.