Construction Dewatering


Construction dewatering is a significant aspect of many construction projects, particularly in wet geographies and when the work involves below-ground excavation. “Dewatering” refers to the process of removing excess water from the soil or groundwater at a construction site to create a dry and stable environment suitable for building. This is crucial for ensuring the safety and structural integrity of the construction project.

In this article, we’re taking some time to look closer at the means and methods behind construction dewatering.

Purposes of Construction Dewatering:
– Provide stable working conditions at the construction site.
– Prevent water from undermining or damaging the foundation of the structure.
– Prevent the mixing of water with construction materials, such as concrete, which requires dry conditions to cure properly.
– Reduce the risk of groundwater-related problems, like water contamination, soil erosion, or landslides.

Dewatering Methods

Sumps and Pumps

The sumps and pumps method is the most basic form of dewatering. This involves digging sumps (pits) in the construction area, which naturally collect water due to gravity. Pumps are then used to remove the water from these sumps. This method is particularly effective in areas with low to moderate groundwater levels.

Ideal for shallow excavations and small-scale construction projects where the water inflow is relatively low.

It’s simple, cost-effective, and easy to install. It requires minimal technical expertise and can be quickly set up.

Limited effectiveness in areas with high groundwater levels or in soils with low permeability. It can also be less efficient if the inflow of water is very high, as the sumps might not hold all the water long enough to be pumped out, or the pumps may not be able to keep up.

Wellpoint Systems

Wellpoint systems involve a series of closely spaced shallow wells, known as wellpoints, installed around the perimeter of an excavation site. These wellpoints are connected to a common header pipe, which is in turn connected to a pump, usually at ground level.

Commonly used in medium to large construction projects, especially where the excavation depth is moderate, and the soil is permeable.

Highly effective for lowering groundwater levels in sandy or gravelly soils. The system can be scaled and adapted easily by adding more wellpoints for larger projects. It allows for precise control over the groundwater level.

Efficiency decreases in clay or very fine soils. Installation and operation require more expertise and equipment than simple sumps and pumps. Regular maintenance is necessary to ensure the system’s efficiency

Deep Wells

Deep well dewatering involves drilling wells, typically around the perimeter of the excavation site, and installing submersible pumps at the bottom of these wells. Each well operates independently, which makes this method highly adaptable to varying site conditions.

Ideal for large-scale projects like dam construction, deep basement excavations, and tunneling where significant water inflow is expected.

Suitable for deeper excavations and can handle larger volumes of water. It works effectively in a wide range of soil types, including those with low permeability.

The method is more expensive and time-consuming to set up than wellpoints or sumps. It requires specialized equipment and expertise in drilling and pump installation.

Eductor Systems

Eductor systems, also known as ejector systems, use high-pressure water and venturi nozzles to create a vacuum that helps to draw water out of the ground. This method is particularly effective in soils with very low permeability, where other dewatering methods might struggle.

Often used in deep excavation projects where traditional dewatering methods are not effective, such as in clay soils or where the excavation goes below the regional water table.

Can be used in a variety of soil types, including those with very low permeability. It’s effective at greater depths compared to wellpoints.

The system is complex and requires a high level of expertise to design and operate. It’s also more expensive due to the energy requirements for maintaining high water pressure.

Each dewatering method has its specific applications, advantages, and limitations. The choice of method depends on various factors, including soil type, depth of excavation, groundwater conditions, and project scale. It’s often necessary to use a combination of these methods to effectively manage water at a construction site.

Water Disposal and Treatment

The water extracted during dewatering often needs to be treated to remove sediments or contaminants before being released back into the environment or into the municipal drainage systems. We discussed this is more detail in a former article about dewatering.

Environmental Considerations

Compliance with environmental regulations is critical in dewatering processes to prevent ecological damage. Care must be taken to avoid lowering the groundwater table excessively, which can affect nearby water bodies and ecosystems. The states of Oregon and Washington have strict policies and guidelines in place for managing erosion and sediment control, which ties closely into dewatering strategies.

Types of Dewatering Monitoring and Control

Continuous monitoring in construction dewatering is critical to ensure the effectiveness of the dewatering process and to prevent any potential issues such as excessive drawdown, structural instability, or environmental impacts. Achieving continuous monitoring involves several strategies and technologies:

Water Level Monitoring

Piezometers and Observation Wells: A piezometer is a geotechnical sensor used to measure pore water pressure (piezometric level) in the ground. Designed to measure pore water pressure in soil, earth/rock, foundations and concrete structures. These are installed in and around the construction site to monitor the groundwater levels continuously. Piezometers can provide real-time data on water level changes, which is crucial for adjusting the dewatering systems accordingly.

Automated Sensors: Advanced sensors can be used to provide continuous, real-time monitoring of water levels. These sensors can be linked to data loggers and remote monitoring systems, allowing for off-site monitoring and immediate response to changes.

Pump and System Performance Monitoring

Flow Meters: Installed in the dewatering system to monitor the rate of water being pumped out. These help to ensure the pumps are operating efficiently and effectively.

Remote Monitoring Systems: Many modern dewatering systems come equipped with remote monitoring capabilities, allowing for real-time tracking of pump performance and immediate detection of any malfunctions or inefficiencies.

Soil and Structural Stability Monitoring

Inclinometers and Tiltmeters: These are used to monitor any movement or deformation in the soil or nearby structures. This is particularly important to ensure that the dewatering process is not negatively impacting the structural integrity of the construction site or adjacent buildings.

Vibration Monitors: To detect and monitor any vibrations caused by dewatering pumps or other construction activities, ensuring they remain within safe limits.

Environmental Monitoring

Water Quality Testing: Regular testing of the water being pumped out is essential to ensure that it does not contain harmful levels of pollutants or sediments before it is discharged or treated. Having an onsite Certified Erosion and Sediment Control Lead (CESCL) is advised so that testing can be regularly performed.

Ecological Impact Assessments: Monitoring the impact of dewatering on nearby water bodies, vegetation, and wildlife habitats to ensure compliance with environmental regulations. These assessments are done with ecology surveys through a geotechnical engineer.

Data Management and Analysis

Centralized Data Management Systems: Collecting data from various monitoring tools into a central system for analysis. This helps in identifying trends, predicting potential issues, and making informed decisions. Management can be as simple as notes in a book, or a software solution.

Predictive Analysis Tools: Advanced software can be used to analyze the collected data to predict possible future scenarios or issues, allowing for proactive management of the dewatering process.

Use of Drones and Aerial Surveillance

Drones: Equipped with cameras and sensors, drones can be used for aerial inspections of the site, offering a comprehensive overview of the dewatering process and its effects on the larger area.

Continuous monitoring of construction dewatering is a multifaceted approach that involves a combination of physical tools, sensors, data analysis, and environmental assessment. It ensures not only the effective management of the dewatering process but also the safety, stability, and environmental compliance of the construction project.

Final Thoughts

Construction dewatering is a complex process that requires careful planning and execution. It often involves the collaboration of geotechnical engineers, environmental experts, and construction professionals to ensure that it is done effectively and sustainably. The failure to properly set up these systems can be catastrophic not just for the site but for the areas surrounding it, impacting wildlife, ecosystems, residential and business areas and more.

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