Determination of the Soil Failure Zone around a Cone Penetrometer

Abstract

The objective of this paper is to investigate the soil failure zone (zone of influence) arounda cone penetrometer using a pressure sensing mat under laboratory conditions. The investigationwas conducted under (1) constrained conditions using cylindrical split pipe molds and (2)unconstrained conditions using a soil box. These tests used two types of soils Capay clay andYolo loam under two moisture conditions and two compaction levels. These soils were ground and sieved using a #20 sieve. Thus five factors were considered in the study test conditions(constrained versus unconstrained), cylinder diameter, type of soil, soil moisture content and level ofcompaction.<br><br>In the constrained test, maximum radial pressure of 111 kPa was observed in Capay clay of 3.4 to4.3% moisture content with 3 blows of compaction (cone index value of 2040 kPa) when using the 51mm diameter split pipe mold and decreased to 82 and 22 kPa when using the 76 and 104 mmdiameter molds respectively. These maximum radial pressures that developed on the wall of the soilcolumn are associated with the soil failure zone (zone of influence) around a cone penetrometer.The higher the radial pressure the greater the soil failure. The result of these tests suggests that thesoil failure zone (zone of influence) of a cone penetrometer has a radius of 51 mm from the center ofthe cone penetrometer shaft.<br><br>In the unconstrained tests, maximum pressure of 9.01 kPa was observed in the Capay clay of 4.5%moisture content with 3 blows of compaction (cone index value of 550 kPa) and then decreases asthe distance of the cone shaft is further from the wall of the soil column. The zone of influence of thecone penetrometer was observed at a distance of 76.2 mm from the center of the cone penetrometershaft with pressure readings in the range of 0.07 to 0.36 kPa. The pressure levels from these testsare much smaller than in the constrained tests and may suggest that the soil failure zone under fieldconditions is small.<br><br>The over-all results of the constrained and unconstrained tests suggest that the soil failure zone(zone of influence) of the cone penetrometer has a radius of about 76.2 mm from the center of thecone penetrometer shaft. In both tests soil behavior showed similar tendencies that is, increasenumber of blows (higher density of soil) led to higher pressure. On the other hand, increasing soilmoisture content tended to decrease the maximum pressure.