In late 2017, construction began on a new MSE wall to expand a portion of the waterfront at Point Defiance Marina. The site consists of hundreds of feet of loose slag mine tailings placed in the waterfront area as part of the ASARCO Lead and Copper Smelter operations, which took place for several decades in the 1900’s. In addition to the surrounding area’s highly toxic soil, the adjacent sea water creates an electrochemically aggressive environment.
The wall is about 430 feet long and reaches a height of more than 15 feet, with a geometry that includes eight inward and outward bends. Its foundation sits at a maximum depth of three feet below the extreme high tide elevation of 6 feet. Obstructions behind the wall face include drainage structures, an electrical vault, and an abutment for a 25-foot gangway extending to the boat docks.
Considering the corrosive environment and the need to skew soil reinforcements away from obstructions in the fill volume, GeoStrap® was chosen for it’s advantage over galvanized steel and geogrid. Concrete facing panels were used for their durability against tide action and waves, ensuring that the shoreline will be protected in this location for years.
Frequent rainfall and high tides challenged the construction of the wall. At the time of placement of the bottom row of concrete panels, the water level was at the top of the leveling pad. Aggregate backfill with low fines eased construction in the rain, as it allowed water to freely flow through and did not require a high compaction effort. This type of fill is free-draining, which will also help prevent hydrostatic pressure from accumulating behind the wall during the life of the structure.