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The relationship between soil and the eventual structure is as old as civilisation. You may have often wondered about the fact that the foundation and soil are comprised of totally different constituents, yet (when combined) create wonders together. When it comes to any structure’s integrity, it has its limitations based on the capacity of the ground that it is founded on. So, whenever a structure is erected on the soil, it is essential to check its capacity limits, which is known and estimated as safe bearing capacity or SBC. In fact, this is the foremost soil testing method that needs to be carried out prior to any construction. 

Failure of the soil is eventually determined based on two primary criteria: (1) Shear Capacity and (2) Settlement Criteria. 

Safe bearing capacity (SBC)

SBC can be defined theoretically as the permissible load that the soil can endure, such that it neither fails in shear nor exceeds the permissible settlement limits. A soil’s SBC is successfully defined in terms of allowable application of pressure. The unit of SBC is kN/m^2 or kip/ft^2. This is also referred to as Net Bearing Capacity or Allowable Bearing Capacity of the soil.

When it comes to the foundation’s size and type, both are determined on the type of the structure as well as the type and capacity of founding soil. So, it is absolutely necessary to estimate the SBC in each footing design to ensure complete safety.

To put it in simpler words, if we are asked to carry some cement sacks, how much load we can successfully carry without breaking down or falling will be considered as our safe bearing capacity. This is precisely the case with soil, too.

Ultimate bearing capacity

SBC is determined from the soil’s ultimate bearing capacity, which is the absolute maximum pressure at which soil commences to fail. It is also known as Gross Bearing Capacity. Ultimate bearing capacity is basically an unfactored capacity. When successfully divided by a factor of safety, we get SBC.

A wide range of field and laboratory tests are used to determine a soil’s ultimate bearing capacity. Some of these soil testing types are briefly discussed below.

Direct shear tests are utilised to estimate the ultimate capacity of sandy soil. In addition, triaxial shear tests are generally preferred for clayey soils, whereas unconfined compression tests are reserved solely for clayey soils.

Field tests to determine SBC

• Plate load test

This is a popular load test that is used to ascertain the ultimate bearing capacity and the settlement of soil under a given load. This test offers the highest accuracy rate when it comes to sandy and clayey soils. Squarish or circular plates of certain defined dimensions are placed on the ground. Settlements that correspond to steadily increasing loads are eventually recorded to gauge the soil’s bearing capacity.

• Drop weight method

This soil testing method is considered to be reliable when it comes to directly obtain the SBC. A pit of depth that is equal to the depth of the foundation is prepared, and a square cube of a known weight is dropped onto it from a height that is predetermined. The dimension’s depth made by the square-cube is accurately measured. The cube’s weight is then multiplied with the height of the fall in order to obtain the soil’s ultimate bearing capacity.

• Standard penetration test (STP)

When it comes to STP, the number of blows needed to penetrate a standard tube into a particular soil is measured. The penetration number or the resistance value is used to determine the ultimate bearing capacity.

SBC values of different soil types

After calculating the ultimate bearing capacity of the soil (from any one of the above tests), we can work out the SBC of the soil using the following equation:

SBC=Ultimate Bearing Capacity / (Cross-Sectional Area × Factor of Safety)

When it comes to soil, the factor of safety is generally considered as 3.

On a final note:

It is essential to determine the load that the soil can carry safely prior to designing a structure. Soil structure interaction plays a key role in the design as well as maintenance of any structure, whether large or small. Incompatibility may lead to serious disasters as well.