SBC Calculations

Tentative SBC Estimation on site using VTS Concrete Pocket Penetrometer Soil Testing Equipment Mini Penetrometer Kit 

yogendra borse

The VTS Concrete Pocket Penetrometer Soil Testing Equipment Mini Penetrometer Kit  is a portable instrument that can be used to estimate the unconfined compressive strength (UCS) of soil, which can be used to determine the Safe Bearing Capacity (SBC) of soil.Kit provides only an estimate of the SBC of soil for estimate preparation of project. The actual SBC may vary depending on the soil properties, groundwater conditions, loading characteristics, and the geometry of the foundation. It is always recommended to conduct a detailed site investigation and laboratory testing to determine the actual safe bearing capacity of soil.at the time of execution.

Here are the steps to use the VTS Concrete Pocket Penetrometer to estimate the SBC of soil:

Calibrate the instrument: 

    Use the calibration rod provided with the kit to calibrate the instrument. The calibration rod has a known diameter and known compressive strength. Insert the rod into the penetrometer and apply a load until it reaches the maximum reading. Record the reading and repeat the process at least three times. Calculate the average reading and use it as the calibration value.

Prepare the equipment: 

    Assemble the VTS Concrete Pocket Penetrometer by attaching the loading piston and the penetration rod to the body of the instrument. Ensure that the instrument is calibrated and that the dial gauge is set to zero.

Select the test location: 

    Choose a test location where the soil is representative of the soil conditions at the site. The test location should be free of any obstructions and should have a level surface.

Prepare the soil surface: 

    Clear the soil surface of any debris or vegetation. Level the surface and moisten the soil to a consistency that allows for easy penetration.

Perform the test: 

    Insert the penetration rod of the VTS Concrete Pocket Penetrometer into the soil vertically. Apply a constant load on the loading piston until the dial gauge reaches a maximum reading. Record the maximum reading as the penetration resistance of the soil.

Estimate the SBC: 

    Use the estimated UCS value to estimate the Safe Bearing Capacity (SBC) of the soil based on empirical correlations or the geotechnical design codes. These correlations may be available in the manual or can be obtained from other sources.Use a correlation chart or table to estimate the Safe Bearing Capacity (SBC) of the soil based on the penetration resistance. The correlation chart or table should be based on the soil type and conditions at the site.

To determine the Safe Bearing Capacity (SBC) of soil from shear strength, we can use the following methods and equations:

The Terzaghi's Bearing Capacity Equation:

This method is used for cohesive soils, and it is based on the Mohr-Coulomb theory. The equation for Terzaghi's bearing capacity is:

$${SBC} = c N_c + q N_q + \frac{1}{2} \gamma B N_{\gamma}$$

Where,

c = Cohesion of the soil

q = Vertical stress at the foundation level

γ = Unit weight of the soil

Nc, Nq and Nγ are the bearing capacity factors

The Meyerhof's Bearing Capacity Equation:

This method is used for both cohesive and cohesionless soils. The equation for Meyerhof's bearing capacity is:

$${SBC} = c N_c + q N_q + \frac{1}{2} \gamma B N_{\gamma} \left[1 + 0.4\left(\frac{B}{L}\right)\right]$$

Where,

c = Cohesion of the soil

q = Vertical stress at the foundation level

γ = Unit weight of the soil

Nc, Nq and Nγ are the bearing capacity factors

The Skempton's Bearing Capacity Equation:

This method is used for cohesive soils, and it is based on the effective stress principle. The equation for Skempton's bearing capacity is:

$${SBC} = c N_c + q N_q + \frac{1}{2} \gamma B N_{\gamma} \left[1 + 0.2\left(\frac{B}{L}\right)\right] \left[1 - \frac{I_c}{N_q}\right]$$

Also another formula is 

$${SBC} = {{S_u} \over F}$$

Where,

Su = Undrained shear strength of the soil

F = Factor of safety

The factor of safety depends on the type of structure being supported, the consequences of failure, and the uncertainty in the soil parameters.

The Vesic's Bearing Capacity Equation:

This method is used for both cohesive and cohesionless soils. The equation for Vesic's bearing capacity is:

$${SBS} = c N_c + q N_q + \frac{1}{2} \gamma B N_{\gamma} \left[1 + 2\frac{B}{D_f}\right] \left[1 - \frac{I_c}{N_q}\right]$$