When a large sinkhole, approximately 150 feet in diameter and extending to a depth of 320 feet, opened up on the side of a phosphogypsum stack at PCS Phosphates’ White Springs facility, remediation was needed to stop hydraulic communication between the low pH surficial aquifer and the underlying Floridan limestone water supply aquifer. Based on a unique approach to the work, Moretrench was retained by the owner to implement a sequenced, stage-up pressure grouting program to effect the remediation. The ultimate objective was to re-establish a hydraulic seal through the “Green Clay” stratum which served as the confining unit between the surficial and deep aquifers.
Geotechnical Engineers Ardaman & Associates had identified five discrete key levels within the sinkhole cavity: the throat at the cavity base at El. -90 feet NGVD; top of limestone at El. -30 feet NGVD; bottom of confining unit at El. +10 feet NGVD; mid confining unit and top of green clay at El. +40 feet NGVD; and top of confining unit at El. +70 feet NGVD. The intent of the grouting program was to completely seal the throat of the sinkhole, fill the cavity from the bottom up, and re-establish the seal within the green clay.
At each target elevation, Moretrench used sonic drilling methods to install large diameter surface casings to within close proximity of the sinkhole cavity. Sonic drilling was then performed to penetrate the disturbed ground within the sinkhole, permitting continuous sampling to map the sinkhole limits, determine the degree and nature of infilling, and also verify the presence of grout as work proceeded. In several locations triple-barrel coring was performed to supplement the sonic samples. The boreholes were surveyed to determine their exact location.
Low mobility grouting (LMG) techniques were utilized for batching and injection of a mortar- like, sand-entrained grout into the formation; however, the grout was specifically formulated to be more fluid than typical in order to travel through the sinkhole cavity and displace the loose infilling material. The grout mix consisted of sand, cement, flyash and bentonite, plus several chemical admixtures, and was batched on-site at a central batch plant capable of altering the proportions of any of the grout components on demand. Throughout grouting operations, an automated, real-time data collection system measured and recorded grout flow and injection pressures. Over the course of the project, more than 30 grout pipes were installed and approximately 4,000 cubic yards of grout injected to depths as great as 300 feet.
Prior to the drilling for grout pipe installation, additional drilling had been performed to install piezometers by means of which water levels were monitored to confirm that separation of the upper and lower aquifers had been re-established.