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