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Module 1 : Site Exploration and Geotechnical Investigation

Lecture 4 : In-situ tests [ Section 4.1: Penetrometer Tests ]

Table 1.2 Here K value varies 0.1 to 1.0. It depends of soil type

Sr. No.	Soil type
1	Silt, sand silt & slightly cohesive sand silt mix	0.1 to 0.2
2	Clean fine sand to medium sand & slightly silty sand	0.3 to 0.4
3	Coarse sand & sand with gravel	0.5 to 0.7
4	Sandy gravel & gravel	0.8 to 1.0

Dynamic cone penetration test (DCPT)

General

The aim is to determine the effort required to force a point through the soil and so obtain resistance value which corresponds to the mechanical properties of the soil. The preliminary use is in cohesionless soils when static penetration test is difficult to perform or dynamic properties of the soil are of special interest.

Procedure

The test set up shown in fig. 1.8. The standard cone is connected to the drilling rod. The driving head with the guide rod is connected and properly fixed on the top of the drilled rods. This complete assembly is kept in position with the cone resting vertically on the ground where the test is to be carried out. For the circulation of the bentonite slurry the pumping unit of the bentonite slurry is properly connected to the guide rod through flexible tube. The cone is driven into the soil by blows of 65 Kg hammer falling from a height of 750mm. The blow count for every 30cm penetration is made to get a continuous record of the variation of the soil consistency with depth. The sufficient circulation of the bentonite slurry is necessary for elimination of the friction on the rods. Sometimes the bentonite slurry is not used when the investigation is required up to a depth of 6m only.