This paper describes methods of obtaining undisturbed samples of weathered rocks for shear strength tests with particular reference to triple and double tube core barrels. It also discusses the use of seismic refraction methods to determine the rippability of weathered rock when excavation is necessary and especially when determining the suitability of the material as a source of fill.
Hong Kong, with its high value of land, is a mountainous territory with some of the highest population density in the world. The main geotechnical problems are those associated with slopes, caissons, retaining walls and reclamation of land (Brand & Phillipson, 1981). Many sites are developed by making steep cuts in hillsides, composed predominantly of granites and volcanic rocks in varying states of decomposition. In order to optimise the angle of slope of these cuts, shear strength parameters are obtained from closely controlled testing of undisturbed samples. The samples are commonly obtained by triple tube rotary coring using water as the flushing medium. A comparison of some of the triple tube sampling methods and their effectiveness in different types of material is given.
Many borrow areas are developed on the hillsides of Hong Kong and the borrow is placed in the sea to form reclaimed land. Vast amount of fill are required annually and there is a constant need to locate suitable sites. Most of these sites are in remote hilly areas in the New Territories. In order to rapidly evaluate the depth of weathering and rippability of the decomposed rock a comprehensive seismic refraction survey, with control drillholes, has recently been carried out. The results of this survey are presented here.
A simplified geological map of Hong Kong is shown at Fig. 1 from which it can be seen that, with the exception of some sedimentary rocks to the North, the Colony rock formation is of igneous origin, of two types, granitic and volcanic. The significant distinction between the two types, for engineering purposes, is the grain size which is shown at Fig. 2.
In-situ decomposition of the rock mass by water percolation in the joints leads to more rapid weathering of the granites due to their larger grain size and wider joint spacing than in the volcanics. However, the volcanics having been formed earlier have been subjected to weathering over longer periods, and weathering to depths considerably deeper than for the granites has been recorded. The weathered rock is classified by grade, in the Geotechnical Manual for Slopes (Geotechnical Control Office, Public Works Department, Hong Kong), for engineering purposes. An additional column is given in Table 1 indicating typical values obtained from Standard Penetration Tests.
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(Table in full paper)
Geotechnical analyses require strength parameters from effective stress tests and hence the samples obtained from investigations must be of good quality and be representative of the mass from which they are taken if safe, economical design is to be achieved.