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The objective of this paper is to investigate the soil failure zone (zone of influence) around a cone penetrometer using a pressure sensing mat under laboratory conditions. The investigation was 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 and Yolo 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 of compaction.

In the constrained test, maximum radial pressure of 111 kPa was observed in Capay clay of 3.4 to 4.3% moisture content with 3 blows of compaction (cone index value of 2040 kPa) when using the 51 mm diameter split pipe mold and decreased to 82 and 22 kPa when using the 76 and 104 mm diameter molds respectively. These maximum radial pressures that developed on the wall of the soil column 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 the soil failure zone (zone of influence) of a cone penetrometer has a radius of 51 mm from the center of the cone penetrometer shaft.

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 as the distance of the cone shaft is further from the wall of the soil column. The zone of influence of the cone penetrometer was observed at a distance of 76.2 mm from the center of the cone penetrometer shaft with pressure readings in the range of 0.07 to 0.36 kPa. The pressure levels from these tests are much smaller than in the constrained tests and may suggest that the soil failure zone under field conditions is small.

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 the cone penetrometer shaft. In both tests soil behavior showed similar tendencies that is, increase number of blows (higher density of soil) led to higher pressure. On the other hand, increasing soil moisture content tended to decrease the maximum pressure.