In one of the largest dewatering projects west of the Mississippi River in 2020, immediate work was needed to remove and replace failed culverts running underneath the main runway at McChord Airfield—culverts that were starting to impact the runway itself and the safety of the crews who used the runway.
In early 2020, the U.S. Army Corps of Engineers (USACE), Seattle District, was tasked with repairing the Joint Base Lewis-McChord Airfield runway to its full, 10,100-foot operational length. Subsurface conditions of culverts that bridge the McChord Airfield over Clover Creek were found to have formed depressions over the alignment of the culverts on both the East and West sides of the main runway. Upon further inspection, crews found that both the North and South culverts were deformed or crushed and had been breached. The runway was placed under a limited-use threshold to avoid aircraft trafficking over the pavement above the culverts. Due to the failures and the increasing impacts on the runway, crews deemed this an emergency repair.
The two original 12-foot diameter, 1,800-foot steel culverts were installed in the early 1950s. Their design life was estimated at 50 years and lasted 20 years longer than expected. With the culverts now collapsing, the USACE, Seattle District, made plans to remove the old culverts and install a new concrete arch bridge structure under the entire width of the airfield. In order to make this happen, crews needed to be able to place the footers and other supports about 10 feet under the water table, which meant the entire area needed to be dewatered.
Also, even if the culverts had not failed, they still would have had to be replaced due to a new Washington state law that requires all culverts to allow the salmon to swim freely. Therefore, all project designs for the replacement system had to account for fish passability.
Solution
In April 2020, the USACE, Seattle District, began its search for a dewatering expert and offered the contract to a full-service dewatering subcontractor.
The contractor designed a system to remove the groundwater, as well as the sediment and air found in the soil that consisted of 12-inch rotary wellpoint pumps and their components. The pumps were specifically designed and engineered for wellpoint and sock dewatering with maximum discharge flows of up to 2,500 gallons per minute (gpm). The pumps are capable of an air displacement volume of up to 400 cubic feet per minute from the ground dewatering system and high-vacuum capability of up to 29 inches of mercury. The goal was to move 1,000 gallons of water per minute.
Not only was the contractor tasked with removing the groundwater, but crews had to take extra care to ensure chemicals from the runway usage, such as per- and polyfluorinated substances (PFAS)—a type of flame retardant—were collected and kept out of the discharged water. A carbon treatment filtration system was brought in to remove impurities and chemicals from the groundwater being pulled from the site during the dewatering process before the water was pumped back into the downstream channel. A creek diversion system consisting of multiple large centrifugal pumps was also installed to divert any additional water from Clover Creek around the project.
Results
The dewatering system was installed, and work began in July 2020. The system ran as many as 13 pumps at one time, allowing up to 5,000 gpm to be pumped out. This exceeded the original expectation. The dewatering system lowered the water table for the 1,800-foot-long, 80-foot-wide area and held it open for six months, allowing the footers and other reinforcements to be installed all at once.
Originally, the job was intended to be done in segments. However, once dewatering began, the system enabled crews to speed up their timetables and complete the work in one phase, finishing 29 days ahead of schedule. The system dried out Clover Creek, eliminating the need for the diversion system until the wet season began in November. At that point, work below grade was completed, the dewatering system was demobilized and the Clover Creek diversion system took over.
The contractor was also able to make the project environmentally friendly, focusing on electric instead of diesel.
With the $80 million project completed, the new airfield was made larger than the old culverts and the bridge system was built to handle water flows generated by a 100-plus year storm, compared to the old system designed to handle a 50-year storm flow. The new design also allows for a more natural stream bed, complying with the new Washington state law.