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The steep terrain and thin weathered mantle may make many areas of volcanic rock unsuitable forintensive development. Large volumes of excavation, much of it requiring blasting, would be necessary forsite formation, and the resulting slopes may be subject to joint-controlled instability. However, where theserocks occur on flat to gently sloping terrain, their foundation depths are fairly shallow. Recent developmentson Tsing Yi Island have been carried out partly in coarse tuff and shallow stripping depths have beenencountered.Site investigations in the volcanic rocks should be designed to determine the depth and degree ofweathering, the frequency and orientation of jointing and the position and seasonal fluctuations of the watertable. The Standard Penetration Test can be a useful indicator of the depth of successive zones ofdecomposition of the rock mass. Direct shear tests on the discontinuities of Zone C/D rocks, and direct shearand triaxial tests on Zone A/B rocks, can be used to determine the shear strengths of joints and soil matrix. Itshould be noted that, because most failures of insstu material are shallow, the overburden pressure on a failureplane is probably quite low. Representative shear strength parameters should therefore be obtained fromlaboratory triaxial tests carried out at appropriately low confining pressures.3.1H Characteristics of the Metamorphic RocksVery Sittle is known about these rocks within the study area. Dark metamorphosed sulphurous shales andquartzites occur offshore in an area planned for reclamation. These rocks are deeply weathered and occurbeneath, up to 40 m of marine deposits and old alluvium. They decompose to a very dense clayey silt that hasSPT values ranging from 20 up to 120. The upper value indicates that driven piles may terminate within it.It should be noted that the sulphur derived from pyrites may produce an acid groundwater that could reactwith steel piles or concrete. Chemical analysis of the groundwater may indicate the nature and extent of thispotential problem.34