Before starting any building work, it is crucial to understand the strength of the ground. The bearing capacity of soil helps to determine how much weight the ground can bear without collapsing or causing damage. This calculation will help to build structures that can be safe and secure for a long period.
Several formulas, test methods, and types are available to evaluate the bearing capacity of soil. In this Brick & Bolt blog, we share the best practices for increasing the strength of the soil to help you take immediate action if you have any soil-bearing issues.
What is the Bearing Capacity of Soil?
Bearing capacity, in simple terms, means the capacity of the soil to bear the loads applied above the ground. Three main factors influence the bearing capacity of any soil in construction: shear strength, the soil’s density and the type of soil. The embedment depth of the applied load can also influence the soil’s strength.
It refers to the maximum pressure the soil can handle before it shifts or fails. If the soil is too weak to support the weight, the load can be spread over a larger area using reinforced concrete foundations to improve balance and strength.
The Types of Soil Bearing Capacity
Several types of soil bearing capacity are as follows:
Ultimate Bearing Capacity (qu)
It is the maximum pressure the soil can handle before it fails.
Net Ultimate Bearing Capacity (qnu)
It is the ultimate bearing capacity minus the overburden pressure (weight of soil above the foundation).
qₙᵤ = qᵤ – 𝛾Df.
Where 𝛾 = unit weight of soil,
Df = depth of foundation.
Net Safe Bearing Capacity (qns)
It is the net ultimate bearing capacity divided by a safety factor to ensure stability.
Formula: qns = qnu / F, where F = 3 (usual value).
Gross Safe Bearing Capacity (qs)
It is the ultimate bearing capacity divided by a safety factor.
Formula: qs = qu / F.
Net Safe Settlement Pressure (qnp)
It is the pressure the soil can hold without causing too much settlement.
Net Allowable Bearing Pressure (qna)
This is the pressure used for foundation design.
If qnp > qns, then qna = qns. Otherwise, qna = qnp.
Soil Bearing Capacity Formula
The general formula for ultimate bearing capacity is:
qᵤ = c’Nc + γDNγ + 0.5γBNγ
Where:
qᵤ = Ultimate bearing capacity
c’ = Cohesion of soil (strength of soil without friction)
Nc, Nq, Nγ = Bearing capacity factors
D = Depth of foundation
B = Width of foundation
γ = Unit weight of soil
Understanding the Formula
- Cohesion (c’) is the strength of soil that does not depend on friction.
- Nc shows how soil cohesion affects bearing capacity.
- Nq depends on the internal friction angle (ϕ) of the soil.
- Nγ = (Nq − 1)tanϕ, which adjusts based on the soil’s weight and the pressure above it.
This formula helps to calculate the ultimate bearing capacity. After that, engineers use other steps to find the net allowable bearing capacity (qₙₐ) needed for foundation design.
Soil Bearing Capacity Test Methods
Some Soil Bearing Capacity test methods can be useful to find the soil-bearing capacity as discussed here.
Standard Penetration Test (SPT):
The soil’s strength can be identified using this SPT test method. It works well for fine and coarse soils, but it is not very reliable for clay and gravel. In this test, a split-barrel sampler goes into a borehole. A hammer blows the sampler multiple times. The N-value shows how many blows the sampler needs for 30 cm penetration.
Cone Penetration Test (CPT):
The extent to which the soil resists penetration can be traced using this CPT test. A cone-shaped probe pushes into the ground at a steady speed. The test records the resistance of the soil. CPT gives detailed information about soil properties.
Plate Load Test:
The plate load test checks how much weight the ground can hold. A round steel plate presses into the soil with increasing weight. The test observes how much the soil settles under pressure. This test works well for soils with large particles and firm textures.
Pressure Meter Test:
The pressure meter test finds how the soil changes shape under pressure. A cylindrical probe goes into a borehole. Then, it inflates to push against the soil. This test measures how much the soil deforms.
Vane Shear Test:
The vane shear test measures the strength of soft soils. A vane-shaped tool goes into a borehole and rotates at a steady speed. The test records how much force is needed to turn the vane. This helps measure the soil’s shear strength and its ability to hold weight.
How Can the Soil’s Bearing Capacity Be Increased?
The soil’s bearing capacity can be increased by following the below steps:
1. Strengthening the Foundation
A deep foundation is stronger because the soil below can hold more weight. But if the soil gets too wet at deeper levels, digging deeper is not a good idea.
2. Soil Drainage
Excess water weakens sandy soil. To drain out excess water, drains need to be incorporated into the foundation area.
3. Compression of the Soil
Packing the soil together makes the particles tighter and stronger. This lessens movement and makes the soil stronger.
4. Restraining the Soil
Sheet piles (heavy metal sheets) may be driven into the soil to prevent movement. When the piles are driven in, the soil is compacted tightly, making it stronger.
5. Widening the Foundation
A wide foundation spreads the weight over a larger area, reducing pressure. However, you can only make the foundation wider to a certain limit.
6. Replacing Weak Soil
If the soil is weak, it should be replaced. Dug out the topsoil and add the stronger materials like sand, gravel, or stone.
7. Grouting
Drill the soil with small holes and pump the grout (a cement-like substance). This fills the gaps and cracks and gives the soil extra strength.
Conclusion
In summary, this article will guide you on soil bearing capacity and its calculation method. Understanding the strengths and weaknesses of the soil helps to ensure safe, secure, and effective structures that safeguard every penny of your investment.
As it is a tedious process, let the work be done by professionals, which may work better. If you need any professional guidance, we at Brick & Bolt offer insight and tailored solutions for your queries. For guidance with your construction plan, contact us today freely, and our experts will share their knowledge for your construction project.