A Guide to Construction Dewatering and its Methods

What is Construction Dewatering?

Construction dewatering refers to the removal of water from the ground in order to dry out an area so that construction can take place. This process is used to remove water from hardened concrete, soil, or other materials and enable the reuse of construction materials as well as assess soil suitability for construction purposes. Dewatering often takes place after a period of rain or flooding, but it can also be used to remove water from a structure that is being demolished.

Why is it Necessary?

The purpose of dewatering is to remove water from a material or structure so that it can be reused or disposed of safely. Without this process, many materials would be unusable or pose a safety hazard.

Construction dewatering can be used for a variety of purposes, such as the following:

• Lowering the water content of slurry prior to transport

• Enabling the dewatering of construction materials for reuse

• Assessing the suitability of soil for construction purposes

This is a critical step in construction that can significantly impact the environment, so it’s essential to carefully consider several factors when planning for construction dewatering. Some of these factors include:

• the type of soil;

• the depth of the water table;

• the length of time needed for dewatering; and

• the volume of water to be removed.

With careful planning and execution, construction dewatering can be a successful and effective way to ensure a construction project proceeds as planned. Once the site is dry, excavation support such as timber shoring helps keep trench walls stable.

4 Major Construction Dewatering Methods

There are a variety of methods that can be used for dewatering, each with its own advantages and disadvantages. Each method works best for specific soil types and excavations depending on the groundwater conditions, soil permeability, depth of excavation, and the required rate of water removal. The most common methods are:

Sump Pumping

Sump pumping is one of the most common and economical methods for dewatering. It works by allowing the groundwater to seep into the excavation area. Once it is collected in the sumps, it will be then pumped out. This method of dewatering is most effective for shallow excavations with permeable soils like sand or gravel.

Advantages of using this method include:

• Low Cost and Simplicity: Provides minimal setup and easy operation

• Flexibility: Suits varying excavation shapes and soil types

• Ease of Installation: Uses simple equipment and basic tools that don’t require specialized expertise or complex machinery

Wellpoint Method

The wellpoint method uses a series of small wells placed along the excavation area. Each wellpoint is connected to a riser pipe, which is attached to a single vacuum pipe and header pump. The pump removes groundwater from the site, and after it is treated to eliminate contaminants and unwanted materials, the water is released from the site.

Advantages of using this method include:

• Reusability: Enables relocation and reuse of wellpoints, riser pipes, and pumps, reducing overall project costs

• Safety: Guarantees extracted water is properly treated before being released into the environment

• Efficiency: Sustains a dry excavation area by continuously lowering the groundwater table

Deep Wellpoint Method

With the deep wellpoint method, the groundwater level is lowered below the excavation level by using boreholes and submersible pumps. The excavated area is surrounded by wells of 15 to 20 cm in diameter, where groundwater naturally flows into these wells and is pumped out. As a result, the groundwater level in the excavated region drops, effectively reducing the water level within the site.

Advantages of using this method include:

• Handles large volumes of water: Manages high water levels efficiently, being the ideal method for sites with highly permeable soils like sand, gravel, or fractured rock

• Provides continuous operation: Maintains stable groundwater levels throughout long construction periods, ensuring consistent dewatering performance

• Requires fewer wells: Achieves the same groundwater drawdown with fewer deep wells than wellpoint systems, simplifying installation and reducing surface clutter

Eductor System

Like the wellpoint method, the eductor dewatering system also use a well, only the difference between the two lies in the mechanism used to draw water. Eductor wells differ primarily in that they pull water using high-pressure water rather than a vacuum to draw it out.

Advantages of using this method include:

• Uses a closed-loop system: Minimizes water consumption by reusing the same high-pressure water within the system

• Prevents boiling and instability: Prevents base heave, boiling, and other instability issues in excavations by controlling pore water pressure effectively

• Dewaters deep excavations: Achieves drawdown at depths up to 30 meters, making the method suitable for deep construction works where vacuum systems are ineffective

5 Steps for Efficient Dewatering

Construction dewatering is an integral step in excavations, foundation works, tunneling, and other below-ground construction activities. The process generally involves the following stages:

1. Assess water conditions: Begin by studying the water at the site to find out where it comes from Note that this can vary from groundwater or rain, so make sure to test it for any dirt or harmful materials that can lead the foundation to corrode over time.

2. Obtain permits: Obtain all required permits before starting and conducting the dewatering process. This ensures everything is in compliance with local and national environmental regulations.

3. Select the appropriate method and equipment: Ensure that the right method and materials are used to ensure effective dewatering. This is essential for ensuring both the quality and safety of the dewatering process.

4. Water treatment: Clean the water released by using filter bags, sediment tanks, or settling basins to remove dirt, sand, and pollutants. Doing this helps ensure it meets environmental standards and does not do the site any damage in the long run.

5. Discharge and monitor: Pump the treated water to an approved safe area or drainage system. After,  monitor the surrounding area’s reaction to the new water released for any damaging effects on soil stability, vegetation, or nearby water bodies.

Benefits

There are many benefits to dewatering on a construction site, such as the following:

• Promotes worker safety: Construction dewatering helps promote worker safety as it eliminates various health risks, such as the removal of potentially contaminated waters, and lessens slips and falls.

• Ensures stable soil and work area: Efficient and properly done construction dewatering helps prepare the site for excavation and cement footing, while also preventing mudslides, unstable foundations, and bogging, which causes equipment failure.

• Prevents unnecessary delays: By preparing the construction site through dewatering, workers can avert unnecessary delays on the site caused by flooding, equipment failures, and many more.

• Enhances longevity of work equipment: Preparing a construction site by dewatering it first removes excess moisture that can affect the equipment on-site.

• Environmentally friendly: Dewatering helps lessens the impact on the environment because it removes contaminated waters and prevents soil erosion and any site hazards in the area.