How to Calculate the Safe Bearing Capacity of Soil

Understanding the concept of Safe Bearing Capacity of Soil

The safe bearing capacity indicates the maximum load that can be resisted by the soil without undergoing bearing failure. The safe bearing capacity of the soil has been expressed by the pressure per unit area which the soil can safely supported.

Importance of safe bearing capacity of soil

It is important to measure the ground-bearing pressure in the construction unit as it helps to ensure that the soil has the capacity to support the construction structure without any kind of failure or excessive settlement in case of load placed on the ground. During the design phase of the construction project, the estimation of the soil-bearing capacity of the underlying part of the soil is important. In the case of not estimating the soil-bearing capacity properly before the beginning of the building construction projects, there are chances of catastrophic consequences including the collapse of the building foundation at the later stage of the construction project.

Factors impacting the bearing capacity of soil

The type and composition of soil

The bearing capacity of the soil has been varied by the different types and composition of the soils. For example, cohesive soil has lower load-bearing capacities rather than non-cohesive soils.

The moisture content of the soil

The strength of the soil has been also affected by the water content in the soil. The bearing capacity of the soil can be reduced by the excessive wet condition or excessive dry condition of the soil.

Distribution of the load

The distribution of the building’s load also plays an important role in measuring the load-bearing capacity of the soil.

Depth of the foundation

The depth of the soil at which the building foundation is placed also significantly impacts the bearing capacity of the soil.

Types of soil

Based on the cohesion, soil can be categorized into two types which are:

Cohesive and non-cohesive soil

Cohesive Soils

These types of soil are clay and silt. Cohesive Soils possess those particles that can stick together due to the actions of electrostatic forces. This type of soil has the tendency to retain water. Thus, this type of soil becomes very hard once it is dried. It has lower load-bearing capacity as compared to non-cohesive soils.

Non-Cohesive Soils

This type of soil contains gravel and sand. These non-cohesive soils possess particles that can not strongly adhere. Due to the presence of this type of particle, the non-cohesive soil can drain well. In addition, non-cohesive soil has higher bearing capacities than cohesive soils.

The construction project team should understand the types of soil of the construction site properly to make the proper foundation designs for the building structures.

How to measure the bearing capacity of soil

Various testing methods have been used by the engineers to estimate the safe bearing capacity of the soil which include:

Laboratory Testing

This type of testing is involved in the analysis of the soil sample under controlled conditions. This testing helps to assess the properties of the soil as well as assessing the bearing capacity of the soil. This laboratory testing process is crucial to get more precise calculations.

In-Situ Testing

This type of test can be conducted directly on the construction site. The real-time data has been provided by the in-situ testing which is used in the engineering practices. In in-situ testing, the Cone Penetration Test (CPT) and Standard Penetration Test (SPT) are employed.

Types of soil’s bearing capacity

The bearing capacity of soil has more specific types which are:

Ultimate bearing capacity (qᵤ)

The maximum vertical pressure that is applied to the ground surface has been indicated by the ultimate bearing capacity. At this point, a shear failure mechanism has been developed in the sporting soils.

Net ultimate bearing capacity (qₙᵤ)

In the meantime by increasing the net pressure at the base of the foundation, there is a shear failure of the soil. The results of the net bearing capacity of the soil can be measured by multiplying the foundation depth along with the soil’s weight and subtracting it from the ultimate load-bearing capacity of the soil.

The weight of the soil has been denoted as 𝝲

The foundation depth is detonated as D

The ultimate bearing capacity is qᵤ

In this context, the formula for net ultimate bearing capacity (qₙᵤ) is = qᵤ - 𝝲Df.

Net safe bearing capacity (qₙₛ)

This can be measured by dividing the net ultimate bearing capacity by the safety factors. The factors can be enhanced to limit excessive settlements as per the requirements. The formula for measuring net safe bearing capacity (qₙₛ) is = qₙᵤ / F.

Gross safe bearing capacity (qₛ)

The gross safe bearing capacity of soil can be estimated by dividing the ultimate bearing capacity of the soil by the safety factors. The formula for estimating the soil’s gross safe-bearing capacity (qₛ) is = qᵤ / F.

How to prevent foundation failure by improving soil-bearing capacity

Incidents like foundation failure can be mitigated by the engineers by employing soil improvement techniques and ensuring the proper designs of the building foundations.

Cohesive and non-cohesive soil

Techniques for soil improvement

There are several techniques including stabilization of the soil, compaction, and geosynthetics which help to enhance the bearing capacity of the soil.

To learn more, watch the following video tutorial.

Video Source: Tensar, a division of CMC

Making proper foundation design

Engineers have to make proper foundation designs that can distribute the load of the building evenly. This proper design is essential for preventing excessive settlement and building failure.