The standard penetration test is an in-situ test that is going under the class of penetrometer tests. The standard penetration tests are completed in borehole. The test will gauge the resistance of the soil layers to the penetration gone through. A penetration empirical connection is inferred between the soil properties and the penetration resistance. The test is very helpful for deciding the relative density and the angle of shearing resistance of cohesionless soils. It can likewise be utilized to decide the unconfined compressive strength of firm soils.
The necessities to lead SPT are quite simple and are easily available, which makes it one of the advantages of performing this test on soils.
The test is led in a drag opening by methods for a standard split spoon sampler. Once the drilling is done to the ideal depth, the drilling device is taken out and the sampler is put inside the drag gap. By methods for a drop hammer of 63.5kg mass falling through a tallness of 750mm at the pace of 30 blows for every moment, the sampler is crashed into the soil. This is according to IS 2131:1963.
The number of blows of hammer needed to drive a depth of 150mm is checked. Further it is driven by 150 mm and the blows are tallied. Thus, the sampler is indeed additionally determined by 150mm and the number of blows recorded. The number of blows recorded for the main 150mm not mulled over. The number of blows recorded for last two 150mm stretches are added to give the standard penetration number (N).
N = number of blows required for 150mm penetration past seating drive of 150mm.
In the event that the number of blows for 150mm drive surpasses 50, it is taken as refusal and the test is ended. The standard penetration number is rectified for dilatancy correction and overburden correction.
Before the SPT esteems are utilized in exact relationships and in configuration diagrams, the field 'N' esteem must be amended according to IS 2131?1981. The corrections are:
Silty fine sands and fine sands beneath the water table create pore water pressure which isn't effectively dispersed. The pore pressure builds the resistance of the soil and thus the penetration number (N). Terzaghi and Peck (1967) suggest the accompanying correction on account of silty fine sands when the watched esteem is N surpasses 15. The amended penetration number,
Nc = 15 + 0.5 (Nr - 15)
Where Nr is the recorded worth and NC is the amended worth. In the event that Nr not exactly or equivalent to 15, at that point it becomes,
Nc = Nr
From a few examinations, it is demonstrated that the penetration resistance or the estimation of N is reliant on the overburden pressure. On the off chance that there are two granular soils with relative density same, higher 'N' worth will be appeared by the soil with higher keeping pressure.
With the expansion in the depth of the soil, the limiting pressure likewise increments. Therefore, the estimation of 'N' at shallow depth and bigger profundities are thought little of and overestimated individually. Consequently, to account this the estimation of 'N' acquired from the test are remedied to a standard successful overburden pressure. The rectified estimation of 'N' is ?
Nc = Cn N
Here Cn is the correction factor for the overburden pressure.
Safety measures taken for SPT
Pros of Standard Penetration Test
The benefits of standard penetration test are:
Cons of Standard Penetration Test
The constraints of standard penetration tests are: