The project team responded rapidly to alleviate environmental concerns and determined that any groundwater impacts could be effectively contained by the zone of influence of the plant-production wells, which daily pumped on the order of 8 million gal. of water from the aquifer for use in making concentrated phosphates. They confirmed that as long as these wells were pumped, contaminated ground water would be contained and captured on-site. In fact, the production wells, acting as recovery wells, have prevented any contaminants from migrating off property. Shortly after detecting rising levels of contamination in the plant-production wells, projected peak chemical concentrations in the recovered ground water were reliably predicted, providing the owner with the opportunity to implement reverse osmosis in a timely manner to efficiently consume the impacted water in the concentrated phosphate plant.

The subsurface exploration program overcame serious difficulties related to providing safe access to the erosion cavity by using angle drilling from the edge of the scarp. A cross-hole seismic survey was conducted to assist in defining the location and extent of the cavity in the confining unit beneath the stack. A team of experts assembled to assist in formulating the remedial approach included consultants and specialty contractors from throughout North America. Remediation involved injecting nearly 4,000 cu yd of concrete 400 ft beneath the surface through 50 grout-injection casings. The project incorporated numerous "firsts," including multi-level phased angle drilling and grouting sequences. The structural integrity of the confining unit was restored, and any further leakage of pore water from the phosphogypsum stack was eliminated.

More than 100 grout mixes were tested to obtain a special mix that was pumpable, would not segregate or bleed, was compatible with acidic pond water and would exhibit the desired strength and hydraulic conductivity over a wide range of slumps. Because of the corrosive nature of the pore water, steel casings had a very limited life span. The operations had to be planned to complete drilling and grouting as expeditiously as possible after installation of a surface casing. Round-the-clock operations were planned and coordinated to expedite the repair work and to preclude grout from flowing from an injection casing toward another hole that was still being advanced or that had not up to that point been grouted. This task was particularly challenging because of the very limited work area and the close proximity of a large number of inclined casings. Maximum flexibility was required to allow for changing.

drilling/grouting procedures and grout mixes to accommodate changing conditions. An on-site concrete batch plant and ready-mix trucks were mobilized to provide added flexibility. The erosion cavity in the confining unit was infilled with cemented gypsum blocks that had collapsed from the stack. Concrete was injected at high pressure not only to fill the voids but also to cause the grout to flow via hydraulic fracturing. Because of the intimate bonding between the concrete and gypsum, the resulting "gypcrete" exhibited excellent strength and hydraulic properties. A multistep drilling methodology was implemented to advance each of the 50 grout casings to inclined lengths of up to 450 ft. Three casing sizes (9, 6.75 and 4.5 in. nominal diameter) and two types of rigs were used. They included a rig geared for production drilling and a recently developed rig (mobilized for the first time ever on this project) used to obtain large-diameter core samples to define the limits of the cavity, verify grout placement and prepare a pilot hole for setting the grout pipe.

Precise angle drilling was required to ensure that the grout casings were terminated within the cavity at the proper elevations. A Fotobore directional survey performed inside the casings detained precise bearing and inclination. Any deviation from the target inclination and bearing of a casing had to be accounted for in planning future grout holes to achieve complete grout coverage. In an upstage grouting sequence, each casing was slowly extracted while grouting operations were in progress. A vibratory hammer assisted in the extraction process so as to minimize the potential for the inclined casing getting stuck in grout. Phased grouting operations progressed from deeper to shallower target levels in the confining unit.

The first objective was to seal the throat of the cavity in order to minimize grout losses. Installation of a temporary relief casing provided ample time for any freshly installed grout plug to cure and gain strength prior to being subjected to increased hydraulic pressures. Stuck casings that could not be retracted were perforated using hydroblasting equipment, then abandoned by injecting liquid grout under pressure. Extensive monitoring of the water table and of piezometric levels determined the progress and effectiveness of the remedial work aimed at plugging the erosion cavity. Grouting operations began in December 1994. By late February 1995, water began filling the cavity, indicating that the hole had been plugged, and grouting under high pressure was continued until April 1995.

There has been no impact on ground-water resources beyond the plant site. The remediation filled the cavity and plugged the hydraulic connection between the gypsum stack and the Floridan Aquifer. Contaminant levels in the plant-production wells have since exhibited a systematic downward trend. Ground water and potentiometric surfaces have returned to presinkhole levels. After the cavity was remediated, the hole in the stack was refilled with sedimented gypsum, restoring the stack's permeability and original appearance. The owner of the plant is IMC-Agrico Co., Mulberry, Fla. Ardaman & Associates, Orlando, Fla., were the engineers on the remediation project. Hayward Baker, Inc., Tampa, was contractor for production drilling and grouting. The core-drilling contractor was Boart Longyear Co., Wytheville, Va. The project was submitted by District #10 director Roger L Jeffery. -John Prendergast Reprinted from Civil Engineering, July 1996.