by Daniel G. Gibbons, PE, and Payal R. Vora, LEED AP
Construction site dewatering is used to locally control and lower groundwater levels in the vicinity of the excavation. This is typically done to maintain soil conditions needed during, and occasionally after, construction—for example, to keep soil dry enough for construction loads, installing building materials, or safe working conditions. Dewatering requirements for structural needs differ from those for below-grade waterproofing of the building. This article defines and describes ‘site dewatering’ during construction from the perspective of below-grade waterproofing design and installation, and highlights the differences in dewatering requirements between structural and waterproofing needs.
For below-grade construction, an adequately dry project site is an important prerequisite for providing a well-constructed below-grade waterproofing system and, ultimately, a dry interior space. This often requires water sources to be controlled with a properly designed dewatering system. To understand construction dewatering and how it can affect below-grade waterproofing, it is important to consider both the theoretical and practical aspects of how below-grade water can enter an excavation.
At a basic level, a site that includes a below-grade interior space needs to be dry enough to allow the waterproofing system to be installed, covered, and stabilized without being damaged by water. For the waterproofing system, the specific period in question extends from the completed substrate preparation for the below-grade waterproofing through to when the structural engineer notifies the contractor the construction dewatering can be turned off.
From a structural perspective, construction dewatering is required to provide an open and stable space to construct a building and prevent buoyant forces from lifting the structure until sufficient dead load is in place. (In addition to dead load, structural engineers may also use structural tie-down elements to resist buoyant forces). Although dewatering impacts both structural stability and waterproofing, the requirements for each differ. In practical terms, although lowering the groundwater level may satisfy a project’s structural requirements, the presence of standing water—or even isolated pockets of water in the excavation—can adversely impact the installation of the below-grade waterproofing and the reliability of its future performance.
In other words, the dewatering requirements for waterproofing are more stringent than the structural requirements of dewatering. Additionally, fine-tuning the system to achieve adequately dry waterproofing conditions is essential to the performance of the below-grade waterproofing and relative success of the project.
Theoretical basis dewatering
The modern understanding for how water flows through permeable soils was developed in the 19th century by Henri Darcy. (For more information, see Chapter 3 of Groundwater Lowering in Construction—A Practical Guide to Dewatering, by Pat Cashman and Martin Preene). Known as Darcy’s Law, it is expressed as:
Q = k·i·A
In this equation, ‘Q’ equals the groundwater flow rate, ‘k’ equals soil permeability, ‘A’ equals the cross sectional area through which the water flows, and ‘i’ equals the hydraulic gradient, expressed as:
Here ‘dh’ equals the difference in total hydraulic head between the upstream and downstream ends of the flow path, and ‘l’ equals length of the flow path through the porous soils which the water flows.
The variables in Darcy’s Law help explain what groundwater flow is based on. Granular soils like gravel and sand have a higher permeability than silts and clays, and will result in higher groundwater flow, all other things being equal. Similarly, the higher the difference between the groundwater elevation at a project site and the elevation where water is being pumped out as part of the dewatering system, the higher the groundwater flow rate into the excavation.