Foundation settlement is a serious problem often encountered in structures. If not corrected at right time the entire structure may collapse. So, in this article, we will discuss the various causes of foundation settlement, types of foundation settlement such as uniform, tipping and differential settlement, and its prevention and correction methods.
What Is Foundation Settlement?
Foundation is the lowermost part of a civil structure. It safely transfers the load (or combinations of loads) of the structure to the ground. Foundation stability is one of the major factors in structure performance.
A well-designed foundation distributes the load over the ground such that it doesn’t exceed the safe load-carrying capacity of the ground. Under the load, the ground settles even though it is well capable of carrying the load. The quantum of settlement is under acceptable limits.
Sometimes, due to certain reasons, the foundation settlement is beyond tolerance. Therefore, it is a matter of concern as structural integrity and safety come at a risk.
Why Does Foundation Settlement Occur?
We have developed a basic understanding of the foundation settlement. Now, we will discuss the reasons for the settlement. The understanding of the settlement reasons is important because the potential preventive measures are determined from it.
Some of the very common reasons for the uniform & differential foundation settlements are mentioned below.
1. Expansive Soil
A foundation built on expansive soils often faces the settlement issue. The black soil found in central India is a good example of expansive soil. The montmorillonite mineral of black soil absorbs the water and swells.
The soil volume decreases upon drying and hence foundation made over such soil settles. Moreover, due to swelling and shrinkage, the soil becomes weak and develops voids.
Before constructing the foundation on expansive clays, proper drainage should be prepared. It helps in preventing the saturation of the soil and hence prevention of soil expansion.
2. Non-compacted soil
The foundation built on the non-compacted ground is always a threat to the safety of the structure. The foundation should only be built on hard and compacted strata. The loose soil should be compacted up to 95% of MDD.
To ensure the compaction of soil, it should be ensured that depth more than required shouldn’t be excavated. If done so, the refilled soil should be well compacted by suitable a method.
If the compaction of refilled soil is not possible due to space constraints etc. the plum concrete should be filled up to the foundation bottom level.
3. Change in Moisture Content of Soil
The enormous moisture change can lead to settlement of soil, erosion, or wash out. The reason for moisture change can be improper drainage, leakage of pipe, increase in water table, etc.
Due to an increase in soil saturation, the effective stress decreases. Since the settlement is a function of effective stress, thus the settlement occurs.
Also, the change in moisture content can trigger the expansion of clayey soil as mentioned above in point no. 1.
4. Consolidation – Immediate, Primary & Secondary
The soil settles under a load of the structure. The immediate consolidation occurs as soon as the load is applied. The air voids are expelled from the soil and the particle are rearranged. Only water-filled voids are left afterward.
After the immediate settlement, the primary consolidation starts. The water drains out of the voids. The time taken by the process depends upon the drainage condition of the soil. The time may range from hours to decades.
The cohesionless soil has higher permeability than the cohesive soils and thus offers better drainage. Therefore, the time of primary settlement is significantly lesser for the cohesionless soils.
The secondary consolidation corresponds to the creep nature of the soil under the sustained load.
5. Low Safe Bearing Capacity of Soil
The safe bearing capacity of soil is the load-carrying capacity of soil within the safety limits. Some soils are not capable of carrying the load of the structure.
Hence, offer lower resistance to the settlements. The single-storey residential houses are generally constructed with standard foundation sizes and may face foundation settlement issues.
The borehole sampling and soil test data give an indication of the SBC of soil. The foundation over weak soil is designed based on test data. For a more accurate estimation of the settlement, a plate load test is conducted over the test foundation.
6. Trees & Vegetation
The trees and the vegetation in the vicinity of the structure pose a threat to the foundation. The roots tend to grow and disturb the soil beneath the foundation. Also, the roots absorb moisture from soils near and beneath the foundation.
The roots concentration is more near the surface. Thus, the shallow foundation is more likely to be affected than the deep foundation.
Types Of Foundation Settlement
We have gone through the reasons for the foundation settlement. Let us now understand the types of foundation settlements. It is of three types – uniform and differential and tipping settlement.
1. Uniform Settlement
The uniform settlement corresponds to the overall sinking of the structure. In this type of foundation settlement, all the foundations of the structure settle equally. The complete structure moves down vertically. Hence, the structure remains mostly unaffected by this type of foundation.
The uniform settlement occurs due to low SBC of soil, consolidation of soil, moisture change in soil for the complete area, etc. The characteristics of the uniform settlement are that it has a very low effect on structural integrity.
However, the utilities connected piping, sewer lines, conduits, cables, etc. are disturbed due to the changed elevation of the structure. This leads to twisting, cutting, leakage, etc., and hence costs money to repair.
2. Differential Settlement
When the foundations of a structure have unequal vertical movement, it is called differential settlement. One or more than one foundation experiences a relative movement among themselves. The structure tends to tilt in the direction of a sinking foundation.
The differential settlement of the foundation causes additional shear, bending stress, and torsion in the structural members. It leads to cracks and structural tilt that may ultimately cause failure as well.
3. Tipping Settlement
Other than the uniform and the differential settlement, the structure may also tilt to one side. The tilt is caused by the subsequent settlement of the foundations.
This means, the foundation on one end remains intact, the second grid of foundation settles, the third grid settles more than the third, and so on.
The tipping settlement generally doesn’t cause cracks in the structure. However, upon prolonged tipping settlement, the structure may overturn.
Correction Methods for Foundation Settlement
The settlement of the foundation due to external reasons can be prevented and controlled. Also, the settled foundations can be corrected and prevented from further settlement. The choice of method for correction majorly depends upon the structure, soil type, reason of settlement, and the cost of application.
1. Site Inspection & Correction of External Reasons of Settlement
The site should be thoroughly inspected and the following points should be checked-
- Possibility of Waterlogging.
- Trees and bushes very close to the structure.
- Leakage in underground sewer lines, water lines, HVAC lines, etc.
- Termite or ant burrows, rat holes, etc. near the foundation.
Underpinning is a method to prevent the foundations from further settlement. The method involves the strengthening of the foundation by transferring the load to deeper strata.
The foundation is clubbed with pins embedded in the deeper ground. Underpinning is often done in conjugation with the shoring.
Grouting is a process of injecting a liquid into the soil to protect the soil from erosion, improve load-carrying capacity and reduce water seepage. The grouting can be done in two ways- chemical grouting and jet grouting.
Acceptable Limits For Sttlement
The acceptable limits of concrete foundation settlement are mentioned below-
|Foundation Type||Soil Type||Maximum Limit|
|Isolated||Clayey Soil||75 mm|
|Isolated||Sand or Hard Clay||50 mm|
|Raft||Clayey Soil||100 mm|
|Raft||Sand or Hard Clay||75 mm|
|Pile||–||2% of pile diameter|