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What is the Standard Penetration Test

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.

Instruments for Standard Penetration Test

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.

  • Standard Split Spoon Sampler
  • Drop Hammer - 63.5kg
  • Guiding rod
  • Drilling Rig
  • Driving head (anvil)

Technique for Standard Penetration Test

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.

Corrections in Standard Penetration Test

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:

  1. Dilatancy Correction
  2. Overburden Pressure Correction

Dilatancy Correction

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

Overburden Pressure Correction

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

  • Split spoon sampler must be in acceptable condition.
  • The cutting shoe must be liberated from mileage
  • The stature of fall must be 750mm. Any change from this will influence the 'N' esteem.
  • The drill rods utilized must be in standard condition. Twisted drill rods are not utilized.
  • Prior to directing the test, the base of the borehole must be cleaned.

Pros of Standard Penetration Test

The benefits of standard penetration test are:

  • The test is straightforward and conservative
  • The test gives agent tests to visual assessment, characterization tests and for dampness content.
  • Real soil conduct is acquired through SPT values
  • The technique assists with infiltrating thick layers and fills
  • Test can be applied for assortment of soil conditions

Cons of Standard Penetration Test

The constraints of standard penetration tests are:

  • The outcomes will differ because of any mechanical or administrator changeability or drilling unsettling influences.
  • Test is exorbitant and tedious.
  • The examples recovered for testing is upset.
  • The test results from SPT can't be reproduced
  • The use of SPT in rock, cobbles and durable soils are restricted
What is the Standard Penetration Test