Differential settlement of foundations

June 6, 2024 Roberto Magalhães

Different foundation settlements are related to many construction areas and soil conditions. In addition, other environmental changes, external influences such as ground vibrations, etc. can also cause subsidence of this type.

What is differential settlement?

It is an uneven/irregular settlement of foundations due to ground movements, changes in soil conditions and external factors that affect the stability of foundations.

If the foundation sits evenly, there will be no major problems if the entire structure sits in the same way as the ground floor. However, if the structure's settlement is significant, but uniform, problems such as: B. Damage to the connections with other structures, differences in height in the connection to the outside, etc.

It is therefore necessary to avoid different foundation settlements and keep foundation settlements within limits or minimize them.

Causes for different settlements

Incorrect foundation sizing

The design of surface foundations is based on the allowable load capacity.

Foundation area = column load (SLS) / allowable load capacity

The support load at the serviceability limit state (SLS) is divided by the allowable load capacity of the soil to obtain the area of the foundations. This is the beginning of the foundation construction. This way we keep the pressure under the foundation at the same level.

If the pressure under all footings is similar, there is a high probability of similar footing settlement as long as the ground conditions remain the same for all footings.

However, if we accidentally or intentionally change the pressure under the foundation, different settlements will occur in the foundation.

source sound

This type of soil modification is accompanied by a change in moisture content.

As the moisture content of the soil increases, the clay increases in volume. When foundations are built on these soils, they are subject to upward movement unless there is a mechanism to prevent the movement.

Ground movement can lift buildings and begin to shrink as soil moisture levels decrease. Most of the time, this movement can be observed in different areas where expansive soil is present.

Special attention is required when planning shallow foundations in these soil types.

Poorly compacted soil

If necessary, the soil can be filled during construction in the area where the foundation will be built. This can be done to improve the soil. This requires uniform and sufficient compaction of the soil under the foundation.

If soil compaction varies from location to location, different settlements may occur. To avoid differential settlement of the foundations, it is necessary to carry out random compaction checks.

Soil with low support capacity

Soils with low bearing capacity and deviations from the permissible load limit can lead to settlement of the foundation under different loads.

Change in soil profile

The soil profile can change from one place to another. It can be vertical or horizontal. Different soil conditions are the reason for different settlements.

Before planning, the condition of the soil must be adequately assessed.

Change in soil water pressure/groundwater level

Changes in groundwater levels can lead to settlement of foundations. This problem can arise when deep excavations are made to build foundations on the adjacent property.

Continuous drainage near the structure may result in foundation settlement if groundwater recharge is not adequate or drainage must be controlled by continuous support down to the rock or impermeable layer.

vibration

Vibrations caused by the environment can affect parts of the structure. If these are felt in parts of the structure, settlement of the foundation may occur.

Trees and vegetation

If there is vegetation on the ground where the foundations are built, this can cause the foundations to settle over time as they decompose.

Additionally, if there are trees nearby, their roots can lift up and damage the foundation. Therefore, special care is required for these trees.

This is how you can recognize different settlements

Signs of differential foundation settlement can be identified as the formation of cracks in the structure and damage to non-structural elements such as brick walls.

Some of the following deficiencies can be highlighted.

Damage to walls and floors
Cracks in the foundation
Locking doors and windows

How to repair the foundation

There are modern conventions and methods for repairing foundations.

Repair procedures are categorized according to supporting measures.

Screw Abutments, Compression Abutments, Plate Abutments
Repair through concrete support
Blasting process
Micropiles

This will avoid differential settlement

Prevention is better than the cure. Therefore, necessary measures should be taken at the initial stage rather than making any repairs or modifications to the structure/floor.

Knowledge of soil conditions before planning and construction

The designer and construction team must be accurately informed about ground conditions.

The planner can only decide whether geotechnical investigations should be carried out, to what extent, what parameters are required, etc., if he has certain knowledge of the terrain conditions.

On the other hand, the construction team must know the soil conditions to be able to plan construction accordingly. Lack of knowledge of soil types, etc. leads to many construction problems.

Pre-project geotechnical investigation

The most common approach is to carry out a geotechnical survey prior to planning. Planning must be based on real terrain conditions and not on hypothetical values.

Correctly assess load capacity

Based on the data obtained during the geotechnical investigation, the bearing capacity of the soil is calculated. Due to incorrect interpretation of test data, it may happen that we end up determining an incorrect load capacity.

The planner must know the type of soil and possible load capacity values. Otherwise, different foundation settlements may occur if we plan the foundation with the wrong values.

tension beam

It is common practice to provide a tie beam to control the settlement of the structure.

The use of tension beams is recommended for swollen soils. These can absorb the upward movement of fundamentals up to a certain level.

Tension beams must be designed to withstand bending and shear stresses caused by upward movement.

Soil improvement

Whenever necessary, the soil must be improved to control the sinking of the foundations. Worn areas of the floor can be treated or replaced as needed.

Check the water table during construction

If construction is carried out close to a structure and water is continually sucked in, the foundations may settle. Furthermore, depending on the length of the structure, different settlements may occur and create different levels of groundwater.

With continued drainage, the water table drops. This reduces water pressure under the foundation. This can lead to the foundation settling. On sandy soil, the foundation may lie differently.

To avoid problems of this type, support could be continued up to the hard layer or rock surface and the influence of groundwater withdrawal, groundwater recharge, etc., could be adequately assessed. These methods and evaluations must be carried out under the guidance of experienced and qualified engineers in the field.