Abstract
The need to provide an index of subsurface soil behaviour which would include allowances for intact soil strength, soil texture and its matrix properties, discontinuity frequency, groundwater condition together with stress condition at the depths encountered has led this author to develop a soil mass classification system to isolate in a qualitative manner categories of soil which, under a particular set of engineering constraints, will behave in a similar way. The principle on which this classification system is based is that the selected parameters which are important in determining soil mass behaviour can be determined quickly and easily by a field geologist or engineer. For a particular soil stratum, a numerical assessment is made within designated limits of the quality of the soil as it relates to a particular parameter. These numbers are then summed to give an index number which can be related to the behaviour of soil mass culminating in the prediction of excavation procedures and support system density for a 3⋅5 m diameter hand excavated tunnel. The classification provided very good feed-back for the NATM tunnelling method, including stand-up time and support requirements.
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