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Comparative Analysis of Dewatering Techniques in Construction Sites: Surface vs. Groundwater Challenges

Dewatering Challenges

Surface water on construction sites can come from rain, runoff or nearby bodies of water, such as ponds, lakes or rivers, along with other areas impacted by rainfall. The primary challenges associated with surface dewatering include:

1. Volume and Flow Rate: High volumes and flow rates during heavy rains or flooding can overwhelm dewatering systems.
2. Debris and Contaminants: Within surface water, there is often debris, sediment and contaminants that can clog pumps and damage equipment.
3. Environmental Regulations: Compliance with environmental regulations is necessary to prevent negative impacts on local ecosystems and bodies of water.
4. Accessibility: The sources of surface water can be difficult to access, requiring specialized equipment.

Groundwater dewatering focuses on removing water from under the earth’s surface. Challenges here include:

1. Water Table Variability: Groundwater levels fluctuate which can affect the predictability of dewatering efforts.
2. Soil Conditions: Different soils, like sand or clay, require different dewatering techniques.
3. Seepage and Recharging: Groundwater tends to seep back into excavated areas, creating the necessity for continuous pumping.
4. Permitting and Impact: Extracting groundwater may require permits as it can impact local water tables and wells.

Dewatering Management

Effective surface water management is essential to prevent delays, protect structures, ensure worker safety and not incur unforeseen costs.

1. High-head and High-volume Pumps: These centrifugal pumps are the ideal solution for various dewatering jobs.
2. Trash Pumps: Designed to handle water with debris, these rugged pumps have larger impellers and casing to prevent clogging. Their heavy-duty construction provides longer life, economical operations and lower fuel costs.
3. Diaphragm Pumps: These pumps are designed to be highly resistant to abrasion and handle liquids containing a large amount of sand, solids and stringy, fibrous materials.

Groundwater presents different challenges that often require more complex systems to lower the water table and keep excavations dry.

1. Wellpoint Pumps: Rotary wellpoint pumps have been trusted by contractors for more than 50 years. These pumps are capable of high air handling, large water volume and high vacuum capability.
2. Vacuum-Assisted Pumps: Combining vacuum assistance with centrifugal pumping, these pumps are durable and highly efficient for removing groundwater in various soil conditions.
3. Submersible Pumps: These pumps are a powerful answer for when high heads or lifts become a factor on job sites, making them the perfect solution for deep well applications.

Case In Point

With sustained winds exceeding 150 mph and more than 30 inches of rain falling, Hurricane Maria tore a path of destruction through Puerto Rico in September 2017, leaving an island of people in fear of catastrophic flooding. In the aftermath of the storm, it was immediately clear there was a significant issue with the Guajataca Dam. The dam’s spillway began to collapse, and its valves were destroyed, causing an outpouring of water that resulted in major erosion and necessitated the evacuation of 70,000 homes. There was also a tremendous fear the dam could fail and flood homes downstream.

The U.S. Army Corps of Engineers began mobilizing teams to help save the community. In less than 24 hours, a Thompson Pump team was on the ground. The team determined 10 complete systems consisting of 18-inch centrifugal pumps would be needed, due to their automatic self-priming, flows up to 11,000 GPM, heads up to 130 feet, suction lifts up to 28 feet and large solids handling up to four inches.

The pumps were placed and began lowering the dam’s water level, thus alleviating the possibility of major flooding. The team then went a step further and used the pump systems to reroute water from the dam’s reservoir to a canal feeding three water treatment plants. This allowed clean water to be made available to the community for the first time in weeks.

For more than a year, the pump systems continued to operate, allowing the community to recover and rebuild, and perform hurricane mitigation to prevent any future flooding issues.

In one of the largest dewatering projects west of the Mississippi River in 2020, immediate work was needed to remove and replace failed culvers running underneath the 10,100-foot operational length main runway at Joint Base Lewis-McChord Airfield. To replace the two original 12-foot diameter steel culverts, crews needed to place the footers and other supports about 10 feet under the water table of Clover Creek, which meant the entire area needed to be dewatered.

The U.S. Army Corps of Engineers Seattle District worked with a local dewatering subcontractor to design a dewatering system to temporarily lower the groundwater so the culvert removal and replacement could be completed. 12-inch rotary wellpoint pumps were selected due to their unique performance capabilities, having been specifically designed and engineered for wellpoint and sock dewatering with maximum discharge flows of up to 2,500 gallons per minute, air handling capabilities of up to 400 cubic feet per minute and high-vacuum capability of up to 29-inch Hg.

The system was so efficient it lowered the water table and held the excavation open for six months, allowing all reinforcements to be installed at once.

Conclusion

Managing dewatering on construction sites requires understanding the specific challenges posed by surface water and groundwater and choosing the right combination of methods and equipment tailored to each challenge. By addressing these factors, construction projects can achieve efficient water management and ensure a safe, efficient and productive construction environment.