Standard Penetration Test (SPT) and Bearing Capacity Estimation - Part 01

There are many in-situ test methods to obtain carrying capacity of footings. (to obtain C and phi values)
1) SPT - Standard penetration test
2) CPT - Standard penetration test
3)VST - Vane shear test
4) PMT - Pressure meter test

From above testing methods SPT and CPT 2 tests are widely use. when considering the cost SPT is cheap but CPT is expensive. but considering the time consumption, SPT take quite time but time consumption for CPT is low.

SPT - Standard penetration test

Procedure 

For this test we need borehole machine which will includes rods, casings, drilling bits etc. and also split spoon sampler to collect samples, hammer (63.5Kg) and hammer running rod (760 mm) drop.

First locate the machine in required place to do the test. Then stat bore hole drilling. In bore hole test we take measurements generally from 1.5 m intervals. First drill bore hole upto 1.5 m and insert the Split spoon sampler with attached rod to the hole to take the sample. after installing of the sampler, we need to drop the hammer to the rod. Hammer is raised 760mm to the up word and drop freely to the rod, the process is repeated for a total distance of 450mm. the number of hammer blows for each 150mm interval is recorded.

The blow counts for the last 300 mm of penetration are summed and it is take as blow counts (N). Fist 150 mm are not used for the N value, because it will disturb by the drilling process.

 The blow counts obtained from the SPT as measures previously may be affected by the overburden pressure and the ground water table. therefore we need to use corrected N value.

 


CN < 2, if CN>2 use CN as 2
q' = Effective overburden pressure in K Pa, at the depth where N is measured
( q' = Df*gamma soil - Ud)