It is most commonly used in-situ test and normally used for granular soil(cohesionless soils).
This test is used to determine:
1. Relative density
2. Angle of shearing resistance of cohesionless soil
3. Unconfined compressive test
4. Allowable bearing pressure on the basis of settlement criteria
5. Ultimate bearing capacity on the basis of shear criteria
The purpose of this test is to identify the type and the nature of the sub-surface soil at various depths and also to determine the penetration resistance of the soil at various depth of soil.
Name of the equipment which are used in standard penetration test are: Drilling Machine(for Boreholes), Split spoon sampler(for obtaining soil samples), Drop Hammer(63.5 kg weight as per I.S Code), Driving head(Anvil) and Guiding rod.
Procedure for Standard Penetration Test:
1. Digging of borehole of about 55 to 150mm diameter with the help of drilling equipment.
2. Once the digging is done to the desired depth(this desired depth is depend on the area, ground water table and for what purpose(construction/work) the test is done), the drilling tool is removed from bore hole and the split spoon sampler is attach to the drill rod and lower into the bore hole(Watch YouTube Video for more understanding link at the bottom of the page).
3. Attach the weight of 63.5 to the equipment assembly.
4. Lift the hammer upto 750mm and allow it to free fall on the anvil and marked(counted) as one blow.
5. The number of blows of hammer for first 150mm is not counted but after 150 mm of driven the number of blows are required for next 300mm driven depth is counted and noted down in field note book.
6. Take out the split spoon sampler from the borehole and then the sample is examine by the engineer.
Standard Penetration Number = N = Number of required for 300mm of penetration sampler of the soil.
Any value greater than 50% of the average value is rejected and new average value is found out.
Corrections in Standard Penetration Test:(IS 2131-1981)
There are two correction for SPT test:
1. Over Burden Correction
2. Dilatancy Correction or Water Table Correction
Overburden Pressure Correction:
Overburden pressure affects the penetration resistance. It is necessary because the N-value will have effect on it due to confinement of soil at various depth.
Dilatancy Correction or Water Table Correction:
This correction always done after the overburden pressure correction.
Dilatancy correction required only in saturated fine sand or silt. It is more significant in case of fine dense sand (Nc>15) because it has tendency to dilate under rapid loading and -ve pore pressure will develop.
