Dewatering can be defined as the process of draining rainwater or groundwater from an excavated area before construction can begin. Usually, dewatering pumps, with high air-handling capacity are used to remove the water from the excavated site. This water is often let out into wetlands, lakes, or other water bodies, which have lower groundwater levels.
Trenchless technology also has solutions for dewatering such as using horizontal wells, perforated pipes, or wellpoint systems, situated around the construction site to remove the water prior to or during the excavation. Water in an excavation needs to be drained effectively to stabilize the soil so that construction work can proceed..
Trenchlesspedia Explains Dewatering
Problems such as encountering groundwater during excavation have to be dealt with in many open-cut construction methods as well as in trenchless methods. In many places where there is a shortage of time and high cost related to traditional dewatering, horizontal directional drilling is carried out to place perforated pipes below the water table.
This method was undertaken in Melbourne, Australia, to install a dewatering system for the AU$6 billion Level Crossing Removal Project. This helped to create a comfortable environment to carry out bulk excavation work below the train lines without disruption of services.
Dewatering is also used where the soil requires stabilization. Other methods such as compaction, ground freezing, and addition of material to the soil are also used.
Trenchless Dewatering Methods
- Wellpoint dewatering: consists of small diameter wells that are connected to a header pipe and a wellpoint pump. The pump draws up water through notches in the wellpoint by creating a vacuum in the header pipe.
- Horizontal drains method: a flexible, perforated pipe is installed horizontally using a trenching machine. The un-perforated end of the pipe is brought to the ground surface and attached to a wellpoint suction pump.
- HDD wells: used where permanent dewatering solutions are required like those needed beneath existing structures.
- Deep wells: consists of a series of bored wells fitted with submersible pumps at the bottom. Each well is capable of creating a cone of depression around itself while drawing water to the surface.
- Vacuum wells: uses a vacuum pump at the surface in addition to the submersible pumps at the bottom.
- Eductor system: works to control pore water pressure. Where wellpoint and deep wells may not be effective in dewatering, eductors can be used to stabilize the slope and the base of the excavation.
Adverse Effects of Dewatering
Ill-managed dewatering processes can cause seepages, inflows, and instability of the excavation. It can also cause erosion of the side slopes of the excavation, or excess groundwater pressure on retaining structures from hydrostatic loads, leading to delays or even failure of the project.
Letting out water in the open can also cause erosion of surrounding soil. It can also have an adverse effect on surrounding infrastructure causing settlement and damage to property.