Subsurface Soundings

The method of subsurface soundings consists of measuring the variation in the resistance offered by the soil with the depth by means of a tool known as penetrometer. The penetrometer may consist of a 50 mm diameter mild steel cone fitted loosely to a steel rod or it may be a tool known as Standard Split Spoon Sampler. The sampler can be split into two parts longitudinally for removal of the soil sample which get filled up in its tube when, it is driven in the ground. The penetrometer is driven in the ground with the help of blows from a 650 N weight falling from a height of 750 mm. The number of blows required to drive the penetrometer into the ground through a distance of 300 mm is termed as Standard Penetration Resistance or SPT of the soil and it is designated as N. The ‘N’ values of soil are thus determined at different depths. The resistance offered by the soil to penetration is co-related with the engineering properties of soil like density, consistency (strength), permeability and bearing capacity. Curves co-relating the N values with bearing capacity of certain types of soil have been drawn and are available for ready reference.