Installing A Wastewater Pump Station Over An Artesian Groundwater Aquifer With A Piezometric Water Head 19ft Above Ground Surface

BACKGROUND
Lakehaven Water and Sewer District is located between Seattle and Tacoma in King and Pierce Counties, Washington. The District spans an area of 35 square miles over several small cities including the majority of the City of Federal Way and serves a population of approximately 112,000 people. The District provides water and sewer services, including approximately 350 miles of sewer mainline, 27 pump stations, and two water resource recovery facilities. This project includes the design and construction of a new 9.3 mgd submersible pump station (PS 33B) located on a constrained site surrounded by wetlands, a stream, and challenging subsurface conditions. The new pump station replaces an aged and at-capacity existing wastewater collection system pump station PS33A located approximately 500 feet away. This presentation highlights the unique design and construction elements, including lessons learned, of installing a large collection system pump station on a small site with unsuitable materials, artesian aquifers, and adjacent stream and wetlands.
DESIGN ELEMENTS
The project site for the new pump station is an approximate 95-foot wide by 100-foot long parcel located at the south edge of a busy residential roadway bordered by Hylebos Creek and wetlands to the east and wetlands to the west. Major design components located on the pump station site include a building with separate electrical room, restroom, and generator room, an aboveground diesel fuel tank, a skid-mounted odor control system, electrical transformer, influent sewer manhole, wet well, valve vault, flow meter vault, and ancillary components. The pump station capacity will be provided in two phases. Initially, the pump station will have a capacity of 2.4 mgd and will discharge through the existing 10-inch diameter force main. Ultimately, this pump station will be expanded to a capacity of 9.3 mgd and include 4 total (3 duty, 1 standby) 3.1 mgd pumps. To facilitate this expansion, approximately 9,000 lineal feet of parallel 14-inch and 18-inch diameter force mains will be installed to a new discharge location in the gravity sewer system.
Nine geotechnical borings were performed at the site and identified three different pressurized aquifers within 40 feet of existing grade. The lower aquifer is a significant artesian aquifer encountered at 37 feet below grade. This confined aquifer is known by local hydrogeologists as the Redondo Milton Channel (RMC) and creates a water head extending 19 feet above ground surface if punctured. The subsurface soils from 0 to 30 feet depth include a slightly pressurized middle aquifer followed by an upper aquifer with groundwater at grade. It was also determined that the top 7 feet of soil at the project site were structurally unsuitable and would need to be replaced. In addition, the sloped site topography requires the site to be built up and level, and contained within a 6-foot tall reinforced interlocking block wall around the site perimeter. The most critical design challenge was maintaining global stability of the site by ensuring that the lower aquifer would be protected and not punctured during all phases of construction of the new improvements. Design requirements implemented to protect the lower aquifer included: - Limiting open excavation volumes during removal and replacement of unsuitable soils. - Adding temporary fill to the site extending two feet above proposed grade during installation of the wet well. - Installing the wet well via a 28-step sink-in-place caisson method that included dewatering two upper aquifers, maintaining high water levels inside the caisson, and diver assistance. The installation maintained a minimum design safety factor of 1.5 against puncture of the lower aquifer. Refer to Figure 1.
,b>CONSTRUCTION AND LESSONS LEARNED
Successful installation of the 21-foot deep sink-in-place caisson took approximately 8 weeks, three weeks longer than planned. Challenges and lessons learned along the way included: - Progress of the caisson slowed to four to six inches per day as the caisson reached within two feet of its finished elevation. The pilot holes predrilled along the perimeter walls that were backfilled with pea gravel to facilitate sinking could have been drilled deeper for more efficient sinking production at the final stages. In addition, steel sheets could have been driven on the outside walls simultaneously with the sinking to reduce the amount of material sloughing into the caisson, which also slowed sinking progress. - A water level (higher than the level of groundwater outside the caisson) was maintained inside the caisson during the sinking to add weight as soil was removed to maintain the design safety factor above the lower aquifer. Maintaining this water level proved to be more difficult and required much more water than anticipated. At one stage of caisson sinking, a nearby fire hydrant was required to supply water to the inside of the caisson at a rate up to 100 gpm. To keep water from escaping the project site and eroding the adjacent wetland and stream, a 2-inch and 3-inch submersible pump recirculated water back to the caisson from the upper aquifer at approximately 40 gpm and 225 gpm, respectively. Refer to Figure 2. - Once the caisson was sunk and prepped for sealing with the underwater concrete mud slab, the wood two-by-fours that protected the PVC waterstop in the perimeter of caisson walls had to be removed by the divers; a task planned to be completed in a couple hours. However, since the wood had been underwater for the last six weeks, it had swelled and took the divers over three weeks to chip out. A non-expanding formwork material could have been used or an alternative chamfer design at the waterstop would have facilitated easier removal of the formwork underwater. Refer to Figure 3. The above challenges and lessons learned were specific details that occurred at key stages of the caisson installation. Prior to construction, a pre-installation meeting was held with all involved parties to review the complete caisson installation process. However, the main lesson learned is that for unique, uncommon construction methods such as this, meetings should be held prior to each key stage of installation so the finer details can be discussed, questioned, and hopefully resolved ahead of time.