STANDARD PENETRATION TEST
The standard penetration test is the most commonly used in-situ test, especially for cohesionless soils which can not be easily sampled. The test is extremely useful for determining the relative density and the angle of shearing resistance of cohesionless soils. It can also be used to determine the unconfined compressive strength of cohesive soils.
The standard penetration test is conducted in a bore hole using a standard split spoon sampler. When the bore hole has been drilled to the desired depth, the drilling tools are removed and the sampler is lowered to the bottom of the hole. The sampler is driven into the soil by a drop hammer of 63.5kg mass falling through a height of 750mm at the rate of 30 blows per minute (IS – 2131: 1963). The number of hammer blows required to drive 150mm of the sample is counted. The sampler is further driven by 150 mm and the number of blows recorded. Likewise, the sampler is once again further driven by 150mm and the number of blows recorded. The number of blows recorded for the first 150mm is disregarded. The number of blows recorded for last two 150mm intervals are added to give the standard penetration number (N). In other words, the standard penetration number is equal to the number of blows required for 150mm penetration beyond seating drive of 150mm.
Figure: Standard penetration Test
If the number of blows for 150mm drive exceeds 50, it is taken as refusal and the test is discontinued. The standard penetration number is corrected for dilatancy correction and overburden correction.
(a) Dilatancy correction: silty fine sands and fine sands below the water table develop pore water pressure which is not easily dissipated. The pore pressure increases the resistance of the soil and hence the penetration number (N).
Terzaghi and Peck (1967) recommend the following correction in the case of silty fine sands when the observed value is N exceeds 15.
The corrected penetration number ————- (1)
Where is the recorded value and is the corrected value.
(b) Overburden Pressure Correction:
In granular soils, the overburden pressure affects the penetration resistance. If the two soils having same relative density but different confining pressures are tested, the one with a higher confining pressure gives a higher penetration number. As the confining pressure in cohesionless soils increases with the depth, the penetration umber for soils at a shallow depth is underestimated and that at greater depth is overestimated. For uniformity, the N – values obtained from the filed tests under different effective overburden pressures are corrected to a standard effective overburden pressure.
Gibbs and Holtz (1957) recommend the use of the following equation for dry or moist clean sand.
Where is the recorded value and is the corrected value, =effective overburden pressure .
The above equation is applicable for ? 280 .
The ratio should lie between 0.45 and 2.0. If ratio is greater than 2.0, should be divided by 2.0 to obtain the design value used in finding the bearing capacity of the soil.
The correction may be extended to saturated silty sand and fine sand after modifying the according to equation (2), i.e. obtained from equation (2) would be taken as in equation (1).
Thus, the overburden correction is applied first and then the dilatancy correction is applied.