SBC Calculations

Tentative SBC Estimation on Site using VTS Concrete Pocket Penetrometer

The VTS Concrete Pocket Penetrometer is a compact, handheld field instrument used for rapid estimation of Unconfined Compressive Strength (UCS) of cohesive soils. The measured UCS values are further correlated empirically to obtain the Safe Bearing Capacity (SBC) of soil.

This method is mainly intended for preliminary assessment and feasibility studies. The results should be treated as indicative only. Actual SBC depends on several parameters such as:

• Soil stratification and variability
• Groundwater table position
• Foundation depth and width
• Loading type and magnitude
• Drainage conditions

For final design and execution, it is strongly recommended to conduct detailed geotechnical investigations including boreholes, SPT/CPT, laboratory testing and plate load tests as per IS:1892 & IS:6403.

Standard Field Procedure for Pocket Penetrometer Test

1. Instrument Calibration

Use the standard calibration rod supplied with the kit. Apply axial load until maximum dial reading is reached. Repeat at least three times and use the average value to ensure accuracy.

2. Equipment Preparation

Attach penetration rod and loading piston properly. Check smooth movement and ensure the dial gauge is set to zero before testing.

3. Selection of Test Location

Choose a location representative of foundation soil. Avoid disturbed, filled or root-affected zones. Prefer freshly exposed soil.

4. Surface Preparation

Remove loose material and organic matter. Level the surface. Lightly moisten if soil is too dry (avoid oversaturation).

5. Conducting the Test

Insert penetration rod vertically with uniform pressure. Do not jerk or tilt. Continue loading until maximum dial reading is achieved. Record the value.

6. Interpretation of Results

Convert dial reading to UCS using manufacturer chart. Estimate SBC using standard correlations or codal provisions. Apply suitable factor of safety.

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Classical Bearing Capacity Equations

1. Terzaghi’s Equation

Applicable for shallow foundations:

SBC = cNc + qNq + ½ γBNγ

Where: c = cohesion (kN/m²) q = effective overburden pressure γ = unit weight of soil B = footing width Nc, Nq, Nγ = bearing capacity factors

2. Meyerhof’s Equation

Includes shape and depth correction:

SBC = cNc + qNq + ½ γBNγ [1 + 0.4(B/L)]

3. Skempton’s Equation (Clay)

SBC = cNc + qNq + ½ γBNγ [1 + 0.2(B/L)] [1 - Ic/Nq]

Alternate simplified approach:

SBC = Su / F

Su = undrained shear strength F = factor of safety (usually 2.5 to 3.0)

4. Vesic’s Equation

SBC = cNc + qNq + ½ γBNγ [1 + 2(B/Df)] [1 - Ic/Nq]

Df = foundation depth

Important Engineering Notes

• Pocket penetrometer is valid mainly for cohesive soils (clays).
• Not suitable for sands, gravels or very stiff soils.
• Multiple readings should be taken and averaged.
• Always apply adequate factor of safety.
• Use only for preliminary design & cost estimation.