The excavation of a trench can have a significant impact on the buildings in the vicinity, causing them to settle. The main factors contributing to building settlement are soil relaxation and a drop in the water table, both of which can be brought on by trench excavation. In this article, we will explore a rule of thumb for determining the influence of trench excavation on nearby buildings.
Fig.1: Effect of trench excavation on nearby buildings
Trench excavation influence nearby buildings because of two reasons which are:
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Soil relaxation occurs when soil is excavated from the ground, causing the remaining soil to settle or shift into the open space left by the excavation. This can lead to instability in the surrounding area and potentially cause damage to nearby structures.
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Excavation can also cause groundwater levels to lower in the surrounding area. This is because when soil is removed, the void space left behind is often filled with water. As a result, lowering the water table can cause issues such as destabilization of soil, groundwater contamination, and changes in surface water flows.
Soil relaxation due to excavation
Excavating a trench in close proximity to a building can result in soil relaxation and subsequent settlement of the building, as depicted in Figure 1. The process of digging a trench can cause the surrounding soil to loosen and lose its original compactness, leading to a decrease in its bearing capacity. As a result, the building’s foundation may experience settlement, which is the downward movement of the building due to the reduced support from the weakened soil. This settlement can potentially cause structural issues and damage to the building, posing a risk to its stability and integrity. Therefore, when excavating a trench near a building, careful consideration and appropriate measures should be taken to mitigate the potential for soil relaxation and subsequent settlement of the building.
Fig.2: settlement or failure of building foundation due to trench excavation
Figure 2 illustrates the importance of providing sufficient support for the trench to prevent soil relaxation. This is a necessary step to ensure the stability and integrity of the trench. In some cases, excavating the trench away from the foundation may not be a viable option, making it imperative to find alternative methods of support. It is crucial to consider appropriate measures to prevent soil relaxation and maintain the stability of the trench, as depicted in Figure 2. Simply excavating the trench away from the foundation may not always be feasible, emphasizing the need for adequate support to be provided.
Fig.3:Support trench excavation to prevent soil movement
To assess if trench excavation may impact nearby buildings, a simple rule of thumb can be utilized. A line can be drawn from the bottom of the building’s foundation at a slope of 2H (twice the depth of the excavation) to 1V (one horizontal distance). This is illustrated in figure 4. If the trench falls within this line, the engineer should anticipate soil relaxation and potential settlement of the building’s foundation. This serves as a practical guideline to determine the potential impact of trench excavation on nearby structures.
Fig.4: The building is within the line, prevent this condition
If the soil at the project site is identified as being very loose sandy soil, it is recommended to adjust the slope of the line to 3H:1V, as shown in Figure 5. This is different from the standard slope of 2H:1V that is typically used. The increased slope of 3H:1V is necessary to account for the unstable nature of loose sandy soil, which requires a steeper slope for stability. This adjustment in slope will help to ensure that the line is adequately stable and safe in such challenging soil conditions. It is important to carefully assess the soil conditions at the project site and make the necessary adjustments to the slope design to ensure the stability of the line during and after construction. Figure 5 serves as a visual reference for understanding the recommended slope adjustment for loose sandy soil conditions.
Fig.5: line of checking the trench excavation affect on nearby building in very loose sandy soil
Groundwater lowering because of excavation
When digging an excavation, the surrounding areas of the trench will typically experience movement of groundwater, causing it to seep into the excavation. This seepage can lead to a lowering of the water level in the immediate vicinity of the trench, as depicted in Figure 6.
However, this decrease in groundwater level can have negative consequences for the foundation. Specifically, the effective stresses in the area can increase, resulting in settlement of the foundation. In other words, the weight of the foundation can cause the surrounding clay layer to compress and compact, causing the foundation to sink and settle unevenly.
It’s worth noting that the increased effective stress in the clay layer is directly related to the lowering of the groundwater level. As the groundwater level decreases, the weight of the water pressing down on the clay layer decreases, leading to an increase in the effective stress of the layer. This, in turn, can contribute to settlement of the foundation.
Overall, it’s important to carefully consider the impact of groundwater on excavation and foundation projects. Failure to properly account for these factors can lead to serious problems down the line, including foundation settlement and other structural issues.
Fig.6: Groundwater seeps into the excavation