In situ tests:
In case of soft sensitive clay and coarse grained cohesionless soils it is almost impossible to recover a satisfactory UDS. That’s why to solve this problem many methods have been derived in which from the field tests performed in situ we can determine the engineering characteristics of these deposits.
Following are some of the field in situ tests performed.
- Standard penetration test, SPT, (ASTM D1586)
- Static cone penetration test , CPT, (ASTM D341)
- Field vane shear test, FVT (ASTM D 2553)
- Plate load test, PLT (ASTM D1194)
- Pressure test (PMT)
- Standard penetration test, SPT, (ASTM D1586):
This in situ test was invented in 1927 and was standardized by ASTM in 1958. For the estimation of relative density and the angle of internal friction of granular soil, silty sands etc.
In this test a standard spilt barrel of about 500mm is driven into the ground and then a hammer weighting is dropped from a distance of 0.76 m. In this way it is driven in to a testing depth of 0.15 m.
A number of correlation have been developed between STP resistance of the soil and certain soil properties, though the test is empirical. These correlations are acceptable for free draining soils and are often used for the preliminary designs. These correlations are comparatively very good. For the soils of low permeability the correlations are just a rough estimate and may be used with the other tests as a conjuction only.
In 1978 a scientist named Schmertmann presented a detailed discussion on the possible errors on SPT results. A lot of work has been done on it large no. of studies have been presented which shows a great variation in the equipments and the procedures used in this standard test. Due to the presence of gravel or cementation operational procedures influence the blow counts. The value of the SPT resistance of the soil does not have any reflection on fractures and slicken side of the clays,
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