Soil Bearing Capacity

The Introduction

One of the most significant and essential concepts in construction and engineering is soil-bearing capacity, which is the support from soil reaching the subgrade level. This is important in the design of safe and efficient foundations. Therefore, the article will delve into what is meant by soil-bearing capacity, why it is essential, and how much capacity may be computed.

What is the bearing capacity of the soil?

Bearing capacity refers to the maximum load per unit area the ground can withstand without failure. This is important in determining the type of foundation that a building or structure must support. The structure could fail if the soil cannot support the loads imposed.

Soil Bearing Capacity

Why ground-bearing pressure is useful

The importance of ground-bearing pressure is that it will either promote the stability of a structure’s foundation or maintain the said stability. When that force is applied to the soil, where the pressure is significant enough so that the bearing capacity of the soil is exceeded, the soil will give way and be deformed. That will be the scenario in which the building settlement is not even, leading to cracking, more extensive damage, and even collapse. It thus honestly assesses the bearing capacity to avoid all these problems and ensures the structure’s long and safe life.

The soil-bearing capacity types

Bearing capacity is generally divided into three categories:
Ultimate bearing capacity, net safe bearing capacity, and allowable bearing capacity. Understanding these types helps you choose a suitable foundation design.

What is the ultimate bearing capacity of soil?

Ultimate bearing capacity of soil: 

This is the maximum pressure the soil can withstand before failure or collapse. It is the theoretical limit beyond which the soil will pass before undergoing significant deformation. This is crucial in helping one establish safety margins for computing the suitable ultimate bearing capacity in the foundation design.

What is the allowable bearing capacity of the soil?

Allowable bearing capacity is the maximal given pressure under which the soil may be placed without failure or set up past a level regarded as permissible. It is estimated by applying a safety factor over the ultimate bearing capacity. Due to the uncertainties in soil properties and the loading conditions in design, the safety factor is worked out so that the foundations are safe and stable.

Types and formulae of bearing capacity

Several methods and formulas exist for determining the bearing capacity of various soil types. These consist of empirical, analytical, and field tests. Regardless of their advantages and disadvantages, the method chosen is determined by the site’s specific conditions and soil type.

How To Find Out The Soil Bearing Capacity

Bearing capacity is the safe capacity of the soil, determined by undergoing soil tests and behavior and applying the correct formulae. The following sections will describe the procedures for calculating bearing capacity for clay and granular soils and some common values for different types of soils.

How to find the bearing capacity of clay soils

To calculate the bearing capacity of clay soils, the following formula is commonly used: \[ q_u = c \cdot N_c + q \cdot N_q + 0.5 \cdot \gamma \cdot B \cdot N_\gamma \] where \( q_u \) is the ultimate bearing capacity, \( c \) is the cohesion of the soil, \( \gamma \) is the unit weight of the soil, \( B \) is the width of the foundation, and \( N_c, N_q, N_\gamma \) are bearing capacity factors that depend on the shape and depth of the foundation.

Bearing capacity of granular soils calculation procedure

The equation for determining the bearing capacity of the granular soils is:

\[ q_u = q. N_q + 0.5. {\gamma}. B. N_{\gamma} \]

where overburden pressure at the base level \( = q \), unit weight of the soil \( = \gamma \), width of the foundation \( = B \), bearing capacity factors peculiar to granular soils \( = N_{q}, N_{\gamma} \).

Average soil-bearing capacity values

Soil-bearing capacity values are typical values related to the type and state of the soil. Some usual ranges are: 

  • Soft clay: 50–100 kPa
  • Medium clay: 100–200 kPa
  • Stiff clay: 200-400 kPa
  • Loose Sand: 100–200 kPa
  • Compact sand: 200–600 kPa
  • Gravel: 300–1000 kPa

Methods of calculating the bearing capacity

The bearing capacity of soil is calculated by using several methods, which include:

Standard Penetration Test (SPT): 

A field test that estimates soil bearing capacity through developed resistance against penetration.

Plate Load Test: 

This test applies a load to a plate sitting over the ground’s surface to measure the settlement and deduct the bearing capacity.

Cone Penetration Test (CPT): 

This test involves driving a cone penetrometer directly into soil resistance measurements at various levels to obtain an extensive profile of soil properties.


The soil’s bearing is an essential element in foundation design, as an accurate assessment ensures that stable ground is offered against settlement and structural cracks when a structure is placed on it. Various methods and equations can assess the bearing capacity for different kinds of soils, giving high assurance for construction works.

By Admin

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