What is foundation settlement?
Foundation settlement is a common occurrence where the soil beneath the foundation structure deforms under its weight, causing vertical displacement at the foundation level. This displacement is known as settlement and can be attributed to the reduction of volume air void ratio in the soil. The extent of foundation settlement is influenced by various factors, such as the type of soil and foundation structure. Foundations built on bedrock experience minimal settlement, while those constructed on other soil types, such as clay, may experience significant settlement. For example, the Mexico City Palace of Fine Arts, built in the 1930s, has settled over 15 feet (4.5m) due to the clay soil on which it was constructed. However, in most cases, foundation settlement is limited to millimeter or fractional inch measurements. The speed at which the settlement occurs also affects the structure, and sudden settlement can cause damage.
This article explores the different types of foundation settlement, their causes, and their potential effects on structures. It highlights that quick settlement can cause damages to structures, whereas slow and uniform settlement can often be tolerated. Factors such as soil type, foundation type, and load capacity play significant roles in determining the extent of foundation settlement. While foundation settlement is an inevitable occurrence, proper design, construction, and maintenance of foundations can minimize its impact on structures.
Types of foundation settlement
- Differential foundation settlement
- Uniform foundation settlement
Differential foundation settlement
Differential settlement is a term used to describe the occurrence of settlement at varying rates within different sections of a building. This phenomenon may result from differences in soil type, load distribution, or structural systems within the building. When parts of the building structure settle by significantly different amounts, it can cause various problems such as distortion of the building frame, sloping floors, cracked walls and glass, and malfunctioning doors and windows. Uneven foundation settlement can cause the building to shift out of plumb, leading to the initiation of cracks in the foundation, structure, or finish.
Severe differential settlement is responsible for the majority of foundation failures. In conventional buildings with isolated foundations, a differential settlement of up to 20mm is deemed acceptable. Additionally, a total settlement of up to 50mm is tolerable for the same structures.
Fig.1:Differential settlement
Fig.2:Cracks due to differential settlement
Uniform foundation settlement
Uniform settlement refers to the situation where foundation settlement occurs at a consistent rate across all portions of a building. This type of settlement is most likely to occur when all parts of the building rest on the same type of soil and when the loads on the building and the design of its structural system are evenly distributed. Although uniform settlement generally has minimal detrimental effects on the overall safety of the building, it can still impact the utility of the building. For example, it can result in damage to sewer, water supply, and mains, as well as cause issues such as jamming doors and windows.
Fig.3:Uniform foundation settlement, no cracks development
Fig.4: Difference between uniform and differential settlement
Foundation settlement causes
Direct causes
Foundation settlement occurs as a direct result of the weight of a building, which includes both the dead load and live load. These loads exert a constant force on the foundation of a building, causing it to gradually sink into the ground over time. This sinking can lead to a range of structural issues, including cracks in walls and uneven floors, which can compromise the safety and stability of the building. Therefore, it is crucial to carefully consider the weight of a building and its loads when designing and constructing a foundation. By taking into account these factors, architects and engineers can help to prevent foundation settlement and ensure the long-term stability of a building.
Indirect causes
There are several scenarios where the stability of the ground may be compromised, leading to potential hazards and failures. For example, collapsible soil can pose a serious risk to the success of underground infiltration, as it may cause the soil to settle and collapse, creating voids and cavities. Similarly, when excavations are made adjacent to a foundation, the ground may yield and cause the foundation to become unstable.
Underground tunnels and mines may also be at risk of failure, as the weight of the surrounding soil and rock may lead to collapse if the tunnels are not properly supported. In the case of cavities in limestone formations, the ground may be at risk of collapse if the cavities are not properly reinforced or filled.
During floods or other natural disasters, the stability of the ground may be further compromised. For example, the foundation of a building may be undermined by floodwaters, leading to settlement and potential collapse. Earthquakes can also cause settlement and damage to buildings and infrastructure, particularly if the ground is not properly prepared or reinforced.
Finally, the extraction of groundwater and oil may also lead to instability in the ground. When these resources are extracted, the soil and rock may shift and settle, potentially causing damage to nearby buildings and infrastructure.
Components of total settlement of foundations
Immediate settlement
Immediate settlement, also known as short term settlement, is a phenomenon that occurs primarily in soils with high permeability and low permeability. Coarse grained soils with high permeability and unsaturated fine-grained soils with low permeability are the most common types of soils in which immediate settlement takes place. This type of settlement takes place over a relatively short period of time, typically around 7 days, which means that it can occur and end during the construction process.
Primary settlement
Primary settlement, also known as primary consolidation, is a geotechnical phenomenon that occurs over an extended period of time, typically ranging from one to five years or even longer. This process is commonly observed in saturated inorganic fine-grain soil.
The underlying cause of primary settlement is the expulsion of water from the pores of saturated fine-grain soil. This water movement can result in a reduction in the volume of the soil, which leads to the gradual sinking or settling of the soil surface.
Although primary settlement may take a considerable amount of time to occur, it is an important consideration in geotechnical engineering, particularly in the design and construction of structures on soil. Understanding the mechanisms and factors that contribute to primary settlement can help engineers predict and mitigate potential settlement issues.
Secondary settlement
Secondary settlement is a type of soil settlement that occurs when soil consolidates under constant effective stress. This phenomenon is commonly observed in organic fine grain soils. Unlike primary settlement, which occurs rapidly and shortly after a foundation is constructed, secondary settlement can continue to occur throughout the lifespan of a foundation structure. It is similar in nature to creep, which is observed in concrete materials. Due to the continuous nature of secondary settlement, it is an important consideration for engineers and architects when designing and constructing buildings or other structures on organic fine grain soil. By taking into account the potential for secondary settlement, these professionals can help ensure that the structure remains stable and secure over time.