Standard penetration tests are carried out in accordance with BS 5930 and ASTM D1586-92. Standard penetration test are conducted at every 1.5m intervals by driving the split spoon into the soil stratum. This should be by repeated blows of hammer (63.5Kg) falling through 760mm height.
From the test, a penetration resistance or blow count (N-value) is obtained which equals the number of blows to drive the sampler over the depth interval between150 to 450mm. The penetration resistance is recorded for every 150mm penetration. The N-value is reported in blows for penetration of the last 300mm. the first 150mm is regarded as the seating drive while the last 300mm is regarded as the N-value.
The test will be stopped when the sum of the number of blows for increments N-value, it will be N = 50 blows; where the test will achieve N-value < 50 the boring test shall be continued until an appropriate bearing stratum where more than an N-value > 50 is obtained.
Sampling are facilitated by the use of split barrel sampler from which collected samples are recovered by the suction of the non-return air valve in the sampling head. The sample are retained as a small disturbed sample. Where a sample is not retained in the split barrel sampler or when the cutting shoe is replaced by a solid cone, a disturbed sample shall be taken from the test zone. The samples will be removed from the split-barrel and wrap it or seal in a plastic bag to retain moisture. Mark the sample and label it with an identification number, withdrawal depth and withdrawal station. Result of the field standard penetration test are presented in standard format to reflect depth at which test is conducted, number of blows to effect specified penetration and material encountered at each layer.