The Pacific Northwest is home to some of the wettest areas in the United States, with rainfall averages of 36” per year in the city of Portland. Construction sites that may be subject to rainfall or water tables near ground level will utilize dewatering processes to safely remove the water to a location outside of the area of work.
Here at Perlo, we assume that all months between October and June will involve wet weather construction but pay close attention to the weather year-round. When we are evaluating dewatering methods, a variety of factors are considered to find the best method from an efficiency and cost standpoint, as well as minimizing the environmental impact of our efforts.
Every construction site’s technique for dewatering will be different, which makes the topic an important one to pay attention to.
What is dewatering?
Dewatering is the process of removing surface or ground water from a particular location. Most construction work cannot occur in areas with significant water ponding, so techniques have been designed to move water out of each area for the duration of construction. The process typically involves sloping the areas of work to drain water away, pumping surface water to another location, or drilling a series of well-points into the ground around the area of work and pumping it to another location in order to artificially lower the water table while work is occurring.
Some form of pumping is typically utilized for all deep foundation work, pipe zones, utility trenches, and manhole structures, as these items are placed beneath the surrounding ground level.
What methods exist for dewatering?
There are several methods utilized on jobsites depending on the specific needs and soil conditions:
This method includes the use of digging trenches or channels and using gravity to direct the water away from the area of work.
Placing a sump pump directly into the ponding water areas and redistributing the water to another location.
Drilling a series of small wells around the area of work, and utilizing pumps to pull the water out of the soil and lower the groundwater level. In this case the water is in the ground as opposed to pooling or ponding on the surface in a specific area.
Generally used for very large construction sites with deep excavation requirements, these wells are larger and require big machinery to install, but can be quite effective at removing water from the area.
In each of these cases, the water is removed from one area and must be taken to another location, which is typically onsite.
Methods for water redistribution
Dewatering is heavily monitored by local, state and federal guidelines. Generally speaking, water from a construction site is not supposed to leave the site itself, especially if it has not been cleaned of sediment or chemicals. As with the methods for removing water from the ground, the soil types that exist on the site will often determine how the removed water should be re-distributed. For instance, in the case of porous or gravelly soils, the water may be able to be pumped to a particular area on site and allowed to percolate back into the ground. But less porous soils, like clay soils, may not accommodate this option. Here are some of the many ways water can be redistributed:
Redistribution to forested or densely vegetated areas
A combination of the three methods
Sometimes the volume of water or lack of redistribution areas will require that water be pumped into the storm or sewer lines. In these cases, the water must be treated for sediment and chemicals before it can go into the public lines.
Jurisdictional requirements for dewatering
Similar to what we saw in the Navigating the Permit Process article, achieving dewatering permits is dependent on the rules and regulations of each individual jurisdiction. Most will require drawings be provided with engineering to demonstrate how the water will be treated.
Plans for dewatering will be based on a report developed by a geotechnical engineer. This will allow the engineering and construction teams to understand the site conditions and determine how best to capture site water for redistribution. The plans must be reviewed and approved by the local jurisdictions. Often, multiple plans must be in place as contingencies for multiple weather conditions and all options must be pre-approved.
If the water cannot be distributed onsite, it will need to go into the storm water or sanitary sewer lines. In these cases, additional filtering and testing are typically required before the water is released into these public lines.
Contractors have responsibility to ensure that water is treated and redistributed correctly. A Certified Erosion & Sediment Control Lead (CECSL) inspector must be onsite to inspect the systems and maintain site records. This individual has test kits so that we can test the water onsite to verify turbidity and pH levels. The testing must be completed at the point where the water discharges into the outflow point.
Jurisdictional inspectors will check documentation and complete independent tests to verify the quality of the water. If a contractor doesn’t maintain the dewatering system according to the rules, their site can be shut down until the work is corrected.
Permanent construction dewatering systems
While construction dewatering is part of our everyday construction practices, permanent dewatering systems are often installed inside buildings with underground parking structures, basements or those with active springs underneath. In these cases, the building will have a sump pump system that pulls water from the lower level and distributes it into the storm water or sanitary sewer system. Building managers will need to maintain the system to ensure proper working order for the life of the building to avoid flooding lower levels.
Best practices for dewatering
The most critical piece of installing and maintaining construction dewatering is to plan ahead, review and adjust the plan frequently and remain flexible when required. This scope of work involves coordination from all parties working onsite, close communication with inspectors and the local jurisdiction and diligent attention to the system to ensure it is operating properly no matter the site and weather conditions.
If you have an upcoming project and would like to know more about what will be involved in completing the construction work, contact our office at 503.624.2090 or use our contact form here.