Chalk is a soft white limestone rock which is composed mainly of calcium carbonate in the form of skeletal remains of minute sea creatures, together with clay and other mineral impurities. Unlike other limestones0t was deposited in calcite rather than argonite rich muds and has retained its original high porosity and softness due to the absence of re—crystallisation of argonite to calcite.
While the chalk in Britain was deposited as a relatively homogeneous mass, subsequent tectonic movements, weathering, the effects of glaciation and accompanying deep freezing, and near surface deformations, have resulted in the development of variable macro—fabric features. The large— scale mechanical behaviour of the Chalk is strongly influenced by the frequency, orientation and openness of discontinuities. It is extremely difficult to obtain useful samples for laboratory testing, except for intact specimens from zones where the discontinuities are widely spaced. Detailed visual descriptions, combined with large— scale in—situ measurements of the load deformation characteristics are leading to a better appreciation of the factors which control the large—scale deformation and strength properties of the chalk. The procedures currently used in the U.K. are based on the extensive investigations made at Mundford (see Fig 1) by W H Ward et al (1968). Among the many aspects requiring further study are the degree to which the relationship between deformation properties and descriptive grades obtained at Mundford can be applied to other sites.
The site investigations for relief roads around Luton (approximately 100 km SW of Mundford, see Fig 1) included a reasonably comprehensive series of large diameter in— situ plate tests directly comparable with those made at Mundford. The availability of heavier loading frames also allowed the tests to be taken to failure. This paper gives some of the preliminary findings from these tests against the background of those from Mundford.
The classification system used in the present investigation is that developed during the investigation at Mundford (see Table 1).
(Table in full paper)
(Figure in full paper)
During the last decade deep in-situ loading tests on 865 mm diameter plates in 900 mm diameter boreholes have been developed at the Building Research Station (BRS) to measure the load deformation and bearing capacities of representative volumes of stiff clays and soft rocks (Ward et al, 1968; Marsland, 1971, 1977). The essential features of these tests have been the considerable efforts made to remove disturbed material from the test surfaces, to reduce errors in the measurements of both loads and settlements, and in the more recent tests, to keep the interval of time between unloading due to excavation and re-loading to a minimum.
The early tests at Mundford (Ward et al, 1968) were carried out by jacking against a simple beam (maximum capacity 100 tonnes) which was held down by tension piles.
