The OCI Wyoming mine has the world’s largest and purest deposits of trona: a mixture of sodium carbonate (soda ash), sodium bicarbonate, and water. Sodium carbonate is used widely in industry, including high purity applications such as food (e.g., baking soda) and specialty chemicals.
Since 1962 the mine has generated enormous piles of tailings. Subsurface groundwater contaminants from these tailing piles were increasingly seeping into the mine’s settlement pond, increasing refining costs, OCI needed a way to keep the tailings’ subsurface runoff out of the settlement pond. Furthermore, since the mine has at least 50 more years of trona reserves, OCI required a solution that would extend past the mine’s anticipated lifespan.
Barr Engineering determined that a cut-off wall 3,500 feet (1067 m) long, ranging in depths of 17 to 45 feet (5.2 to 13.7 m), was required to keep the tailings’ subsurface runoff from reaching the settlement pond. Barr specified a Trench-Cutting-Remix-Deep Wall (TRD) to create a underground, continuous, homogenous, 18-inch (46 cm) wide soil-cement wall. However, the salinity of the surrounding soil will shorten the life of the soil-cement wall.
Fortunately, Barr had previously used CMI’s synthetic sheet piling and was familiar with its 50-year plus lifespan. By driving CMI’s weather able rigid vinyl sheets into the middle of the soil-cement filled trench, the resulting hybrid wall would have a lifespan well beyond that of a wall created with just one or the other of its components. In other words, when, after some decades, soil salinity or other elements corrode the OCI Wyoming soil-cement sides of the TRD, the interior PVC sheets will virtually be just beginning their service life.
Based on Barr’s familiarity — as well as a personal visit by an OCI representative to CMI’s state-of-the-art manufacturing plant — CMI was selected to be the supplier of the synthetic sheet piling.
Being the first of its kind, the hybrid wall construction presented a unique challenge to the team: how to allow for long breaks during the cement pour? That is, the sheets could only be inserted into the soil-cement slurry when it was still relatively soft — weekend or days-long service breaks would create an impenetrable barrier at the last sheet installed.
CMI worked closely with Barr’s geotechnical contractor, Hayward Baker, to create a new, patenting-pending solution: the CMI Slurry Void Device. (See next page for SVD details.) CMI designed and built the SVD in just two weeks to meet Hayward Baker’s tight installation schedule.
OCI has installed groundwater monitors to detect any further migration of contaminated subsurface water from the tailings pile. Preliminary results indicate that the hybrid cut-off wall is containing the pile’s groundwater, making it easier, faster, and less expensive for the mine to refine its trona using the settlement pond.
Wall Specification: ShoreGuard SG 325
The TRD operator cuts the trench while remixing the rocky soil with cement, creating a homogenous, continuous soil-cement wall.
The ABI MOBILRAM, fitted with a CMI PileClaw® mandrel, drives a 45-foot (13.7 m) long SG-325 sheet into the center of the 18” (46 cm) wide soil-cement pour.
For long breaks, CMI’s Slurry Void Device is inserted all the way into the soil-cement filled trench, with a slot for the male lock end of the last-installed sheet. The SVD is pumped full of sepiolite slurry to displace the soil-cement mix and the SVD is removed. When work resumes, the TRD backs up to the last sheet, removing any hard material between it and the soft column of sepiolite. The TRD changes direction and resumes its trenching and remixing.
The sheet installation process: SG-325 sheets are driven into the TRD’s 2-hour old soil-cement pour using PileClaw® technology and bentonite slurry.