The article delves into the different rationales for carrying out foundation dewatering. This process involves removing water from the excavation site to keep the bottom dry, prevent ground water or soil leakage, avoid sand boiling, prevent upheaval failure, and prevent basement floatation. By exploring these different motivations, the article sheds light on the importance of foundation dewatering in construction projects.
Purpose of Dewatering Foundation Excavations
Keep Excavation Bottom Dry
To ensure smooth construction work, it is important to keep the foundation excavation area dry. This is especially crucial when dealing with gravel or sand, as high groundwater velocity can cause water to seep into the area, hindering the construction process. In such cases, dewatering is a common solution that involves lowering the groundwater level to about 0.5 to 1 meter below the excavation. However, when dealing with clay soil, the situation is different. Clay soil is impermeable, meaning water cannot easily travel through it. As a result, groundwater will not pose a problem, and dewatering may not be necessary.
Fig.1: Lowering Groundwater Level
Prevent Ground Water or Soil Leakage
To prevent the potential failure of foundation wall elements caused by significant leakage in an excavation, it is crucial to dewater the excavation area. This is particularly important in sandy or gravelly soil with a high groundwater level, as well as in cases where a diaphragm wall with poorly detailed joints or an improper watertight sheet pile is used to support the foundation wall. When joints are poorly detailed or sheet piles are not properly watertight, water can travel through them, leading to leakage and possible failure of the supporting elements. Therefore, taking measures to dewater the excavation area is necessary to avoid any undesired outcomes.
Prevent Sand Boiling
In the process of excavating sandy or gravelly soil, it is necessary to lower the groundwater level to a minimum of 0.5m. This creates a difference in the groundwater level between the outside of the excavation zone and the inside. If this difference reaches the hydraulic gradient of the soil, sand boiling may occur. To prevent sand boiling, it is advisable to lower the groundwater level outside the excavation area. However, it is important to exercise caution to prevent any settlement from occurring.
Fig.2: Sand Boiling
To Prevent Upheaval Failure
Excavation in clay soil with a permeable layer beneath, such as sandy or gravelly soil, can result in upward pressure from groundwater on the soil layer. If this pressure exceeds the weight of the soil layer, upheaval failure may occur. To prevent such failure, the piezometric pressure of the permeable layer must be reduced by pumping. Figure 3 illustrates this process.
Fig.3: Dewatering used to lower down piezometric pressure of underlying permeable layer to avoid upheaval failure in the excavation
Prevent Basement Floatation
When construction of a building’s basement begins after excavation, there is a risk of floatation in sandy soil. This is because the weight of the basement is initially low, and if the pressure from below is greater than the weight of the structure, it can cause the foundation to heave differentially and float. In the worst-case scenario, the floated foundation may return to its original position, requiring demolition or reconstruction. To prevent this, dewatering should be implemented to mitigate the risk of floatation in the early stages of construction.