Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is a dynamic penetration test that is carried out at site. This test is the most frequently used subsurface exploration drilling test performed worldwide. The test will measure the resistance of the soil strata to the penetration undergone. The test shows its extremely usefulness 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.

Fig: Standard Penetration Test (SPT) in Field

Tools for Standard Penetration Test (SPT)

The equipment’s used to conduct SPT are:

1. Standard Split Spoon Sampler (Outer dia. 50.8mm, inner dia. 35mm and length 650mm)
2. Drop Hammer weighing 63.5kg(140lb)
3. Guiding rod
4. Drilling Rig.
5. Driving head (anvil).

Procedure for Standard Penetration Test:

The test is conducted in a bore hole by means of a standard split spoon sampler .Once the drilling is done to the desired depth, the drilling tool is removed and the sampler is placed inside the bore hole. By means of a drop hammer of 63.5kg(140lb) falling from a height of 750mm(2.5ft), the sampler is driven into the soil. The number of blows of hammer required to drive a depth  of 150mm is counted. Further it is driven by 150 mm and the blows are counted. Similarly, the sampler is once again further driven by 150mm and the number of blows recorded. Total 450mm length will be driven and the number of blows required for these penetration will be recorded. The number of blows recorded for the first 150mm not taken into consideration and the number of blows recorded for last two 150mm(150mm+150mm =300mm) intervals are added to give the standard penetration number (N). 

If the number of blows for 150mm drive exceeds 50, it is taken as refusal and the test is discontinued. 

Corrections in Standard Penetration Test:

Before the SPT values are used in empirical correlations and in design charts, the field ‘N’ value have to be corrected as per IS 2131 – 1981. The corrections are:

1. Dilatancy Correction
2. Overburden Pressure Correction

Advantages of Standard Penetration Test

The advantages of standard penetration test are:

1. The test is simple and economical
2. Test can be applied for variety of soil conditions
3. The test provides representative samples for visual inspection, classification tests and for moisture content.
4. Actual soil behaviour is obtained through SPT values
5. The method helps to penetrate dense layers and fills

Disadvantages of Standard Penetration Test

The limitations of standard penetration tests are:

1. Test is costly and time consuming.
2. The samples retrieved for testing is disturbed.
3. The results will vary due to any mechanical or operator variability or drilling disturbances.
4. The test results from SPT cannot be reproduced
5. The application of SPT in gravels, cobbles and cohesive soils are limited

How to determine Bearing Capacity of soil from SPT Value

Bearing Capacity of soil from SPT Value 

Bearing Capacity of Soil: Bearing Capacity is the capacity of soil to support the loads applied to the ground. Here we will discuss how to determine bearing capacity of soil from SPT value.

Why Bearing Capacity is Important?

Bearing capacity is important due to the following reasons:

1. Structural Stability

Prevents Settlement:

Adequate bearing capacity ensures that the foundation does not settle too much under the weight of the structure. Excessive settlement can lead to uneven floors, cracks in walls, and other basic issues.

Avoids Shear Failure:

If the soil's bearing capacity is exceeded, the ground can shear and collapse, leading to disastrous structural disappointment.

2. Safety

Ensures load transfer:

Proper foundation design based on soil bearing capacity guarantees that loads from the structure are viably exchanged to the ground, keeping up the integrity and safety of the building.

Mitigates the Risk of Collapse:

Understanding and designing for the proper bearing capacity helps prevent the chance of sudden collapse due to soil disappointment.

3. Cost Effectiveness

Optimizes Foundation Design:

Knowing the bearing capacity allows engineers to design the most efficient and cost-effective foundation. Overdesigning (utilizing too many expansive establishments) can be costly, whereas underdesigning can lead to structural issues and expensive repairs.

Reduces construction costs:

Accurate evaluation of bearing capacity can lead to the selection of appropriate foundation types and sizes, reducing unnecessary material and labor costs.

4. Life span of the structure

Guarantees Strength:

Establishments planned with legitimate thought of bearing capacity contribute to the long-term strength and benefit life of the structure.

Minimizes Upkeep:

A well-designed establishment decreases the need for visit support and repairs due to foundation-related issues.

Plans for Future Loads:

Knowing the bearing capacity permits the planning of establishments that can accommodate future extensions or extra loads without compromising structural integrity.

Read:Types of Foundation

How to Determine Bearing Capacity of Soil From SPT Value: The Standard Penetration Test (SPT) is an in-situ test which is carried out in borehole to determine soil bearing capacity . This test measured the resistance of the soil strata to the penetration undergone. SPT value can determined by counting the number of blow required to penetrate the last 12in (300 mm approx.) of a 18 in.( 450 mm approx.) sampler. After getting this value we can easily determine the bearing capacity of soil by following formulas:

Meyerhof's Formula:

For footing width 4 feet or less:

Q_a= (N/4) / K

For footing width greater than 4 feet:

Q_a = ( N/6) [( B+1)/B]² / K

Bowle's Equation:

For footing width 4 feet or less: 

Q_a= (N /2.5) /K

For footing width greater than 4 feet: 

Q_a= (N/4) [( B+1)/ B]² / K. 

Where, 

Qa= Allowable bearing capacity of soil in, kip/sft   

N = SPT Number Below the Footing

B = Width of footing in foot 

D = Depth from ground level to the bottom of footing,  in foot

K=  1+ 0.33( D/B) which will be less than or equal to 1.33