When the amount of soil or its capacity to support loads changes, commonly because of loading, consolidation or changes in moisture content, the ground surface moves down and this process is known as soil settlement. It is a basic concept in geotechnical engineering and construction as it has the ability to affect the safety and stability of structures made on the soil. In order to keep structural integrity, lower the chance of damage, and make sure that foundations are planned correctly, it is important to understand soil settlement.
This blog will explain soil settlement, the issues it can create and possible solutions. This blog will give you a good understanding of soil settlement.
What is the Meaning of Settlement in Soil
The process of soil settlement happens when the ground surface goes down as a result of a decrease in soil volume, usually caused by applied loads, changes in moisture content or long-term consolidation. The soil particles are compressed by the weight of loads or constructions and this results in vertical shrinkage and surface depressions. Depending on the type of soil, permeability, and the magnitude and time of the applied load, settlement can happen instantly or gradually over time.
Differential or uneven settlement can result in more serious structural problems than uniform settlement. The magnitude and uniformity of the downward movement identify how settlement affects structures. Uneven settlement can result in unequal load distribution that results in structural problems like tilting, cracking or even the collapse of pavements, roads and structures.
Types of Settlement
There are various types of settlement, all of them having different characteristics and distinct causes. It is important to understand these types of settlements to study the behaviour of soil under different conditions and design proper foundations.
1. Immediate Settlement
2. Consolidation Settlement
3. Secondary Consolidation Settlement
4. Differential Settlement
Let us look at all these types of settlements in detail.
1. Immediate Settlement
Immediate settlement can happen as soon as the load is applied to the soil and is also known as elastic settlement. In over-consolidated clays or granular soils (like sands and gravels), that has high permeability and low water content it will allow immediate reaction to loading, then this type of settlement is normal.
When a load is placed on the soil, the particles compress, and the soil quickly settles without much delay. The magnitude of immediate settlement depends on the type of soil, the applied load, and the soil’s ability to compress. It usually happens in the first few hours to days after the load is applied and is often a smaller fraction of the total settlement.
2. Consolidation Settlement
Consolidation settlement is a gradual change that happens over time, especially in compressible soft soils like clay. On placing a load on such a soil, initially the pore water in the soil is displaced, followed by movement of soil particles closer as excess pore water pressure diminishes. Understanding the difference between compaction and consolidation of soil is crucial for engineers to predict settlement behavior. Reaching equilibrium may take several months to years depending on soil type and load.
3. Secondary Consolidation Settlement
After the primary consolidation settlement, secondary consolidation settlement may continue at a slower rate. This type of settlement occurs when soil particles rearrange themselves over time to relieve internal stresses. After primary consolidation is done, soil particles have plastic adjustments and rearrangements under a continuous load, which is the main reason for secondary consolidation, also known as creep. However, biological and chemical processes also have a role in secondary settlement in some soils, especially those with a high organic content. While it is generally a smaller component of total settlement, it can still be significant in some cases.
4. Differential Settlement
Uneven settlement in structures is known as differential settlement, and it occurs when a structure’s foundation settles unevenly. It is one of the most dangerous types of settlement because it can lead to serious and costly damage such as cracks in walls, misalignment of doors and windows, tilted buildings, and everything in between. This occurs due to the different rates of soil settlement at either end or different load-bearing soil strata possessing different attributes. The bearing capacity of soil varies significantly across different soil types, contributing to differential settlement patterns.
For structures with rigid or continuous foundations, differential settlement is more problematic because even small changes in settlement over the structure’s footprint can result in large pressures that can cause cracks, distortions, and structural damage. In the worst case it could entirely destroy the building. Variations in the soil conditions under various sections of a foundation are the primary reason for differential settling, but it can also be created by poor construction methods, uneven loading or environmental factors.
Causes of Soil Settlement
The cause of soil settlement can be broadly divided into natural and anthropogenic (human-induced) factors:
1. Load Application
One of the main reasons for settlement is the application of loads like those from buildings, roads, or other structures. When a structure imposes the load on the soil below, it increases stress within the soil mass, leading to compression and following settlement. The type of load, the magnitude of the load, and how it is distributed all influence the extent of settlement.
2. Soil Type and Compaction
The degree of settlement is greatly affected by the type of soil. Soils having clay, silt, or peat are more compressible than the ones that are sandy or gravelly. The density, plasticity and moisture content all change its ability to compress under stress. In construction, it is important to prepare the soil properly because poorly compacted soil settles more easily than well-compacted soil. Proper soil investigation methods help identify soil characteristics before construction begins.
3. Moisture Content
The behaviour of the soil underweight depends on its moisture content. Water loss or absorption can happen because of changes in the volume of the soil. For example, clay soils have the ability to swell when wet and shrink when dry creating settling or heaving. If the soil is very moist or saturated with water, it can become weaker and settle more when it is under stress.
4. Soil Erosion or Excavation
Erosion and excavation can change the structure of the soil and can result in settling. Differential ground settlement or uneven structural instability can come from unsupported voids or reduced lateral support due to soil erosion or removal, mainly below or adjacent to a structure. In addition, erosion brought by wind or water can damage the structure of the soil and allow the ground surface to settle.
5. Chemical Reactions
Chemical processes that make the soil lose volume like the decomposition of organic matter or the dissolution of soluble minerals like gypsum can result in settlement. For example, voids that cause settlement can be created when soluble minerals, like gypsum, dissolve in the soil.
Issues Caused by Soil Settlement
Soil settlement can cause a range of issues, both functional and structural. These are:
1. Cracking of Structures
Uneven soil settlement can have an effect on roads or building foundations. Cracks in the walls, flooring and other structural elements can happen from this. Roads can have uneven pavement that is unsafe for cars due to surface deformation and cracks.
2. Structural tilting
Tilting can make windows and doors to settle, making them difficult to use. In serious cases, this could result in the collapse of a building or other structures.
3. Reduced Load-bearing Capacity
Excessive or uneven settlement can change the distribution of loads and even go above the load-bearing capacity of the soil, which can affect the performance of a foundation. A settlement can sometimes result in the failure of the foundation which can cause the structure built on it to fall.
4. Increased Maintenance Costs
Buildings that experience a lot of settlement need routine maintenance, repairs and inspections to confirm structural safety. This increases the cost of maintaining the property with time. Underpinning or foundation remodelling is needed in certain situations which can be costly.
Solutions to Address Soil Settlement
Various engineering strategies can be used to reduce or address soil settlement depending on the type of soil and project requirements.
1. Soil Improvement and Compaction
Before construction soil improvement methods like mechanical compaction, chemical stabilisation or grouting can be used to increase the strength of soil and reduce its compressibility. Soil stabilisation techniques are essential for improving soil properties and preventing excessive settlement. This increases the soil’s ability to resist settlement under load.
2. Deep Foundations
Reducing settlement problems can be done by using deep foundations, like caissons or piles. These foundations transfer structural loads to deeper more solid or less compressible rock or soil strata, reducing settling in softer surface layers.
3. Monitoring and Early Detection
Measuring soil settlement over time with instruments like inclinometers, piezometers and settlement plates helps in early detection of the problem and quick repair to prevent serious structural damage. Comprehensive soil testing is crucial for understanding soil behavior and predicting potential settlement issues.
4. Soil Consolidation
Preloading and creating vertical drains are two methods used to speed up the consolidation process and prevent constant settling when working with clay or other compressible soils. Pore water can be quickly removed from the soil using the vacuum consolidation method or surcharge loading. To improve pore water drainage all these methods can be combined with prefabricated vertical drains.
Conclusion
Soil settlement can have a great effect on infrastructure and development if it is not properly controlled during design and construction. Engineers and other experts can develop and put into practice efficient solutions to remove or reduce these risks by having a complete understanding of all types of settlement, their causes and the problems they can create. The long-term safety and stability of structures can be achieved by managing soil settling using proper foundation systems, constant monitoring and suitable soil preparation.
Hemali Patel