When a sinkhole develops, anything on the surface or in the soil could end up in the aquifer and contaminate the drinking water. Geotechnical Remediation According to the geotechnical testing, the soil consisted of multiple layers: trash, sand, clay, silt, limestone, sand, limestone with large voids, sand and finally continuous limestone down to about 230 ft (70.1 m) with blow counts greater than 50. Earth Tech, LLC performed compaction grouting down to the last continuous layer of limestone for stability, with depths ranging from 210 to 295 ft (64 to 90 m). Earth Tech compaction grouted points surrounding the sinkhole perimeter in consecutive stages: 3 calibration points, 63 primary points, 46 secondary points and 30 tertiary points for a total of 142 points. The primary points were located nearest to the sinkhole. Sinkhole at the Southeast County Landfill remaining points with good soil conditions. At any one time, Earth Tech had about three to four strings or 4,000 linear ft (1,219.2 m) of casing on the ground ready for use. During compaction grouting, there was a large variability on the amount of grout required at each point. Some points only required 3 to 4 cu yd (2.3 to 3.1 cu m) of grout. However, other points took over 60 cu yd (45.9 cu m) of grout to fill the large, hidden cavities in the limestone. The Challenges Earth Tech faced many challenges on this jobsite. The biggest challenge was the remote location of water — critical to the compaction grouting work. The closest water source was a 3 acre pond with 2 to 3 ft (.61 to .91 m) of standing water, which was 3,000 ft (914 m) away from the jobsite. The only other local water source was a 1 acre pond with 6 to 7 ft (1.8 to 2.1 m) of stand- ing water, which was 6,000 ft (1829 m) away. Earth Tech depleted both of these ponds and temporarily stored the water in 20,000 gal (75.5 cu m) tanks for the compaction grouting. Stabilization and Trash Cleanup When the sinkhole developed, the top priorities were to stabilize it and prevent contamination of the groundwater. In the first stage of remediation, a local specialty contractor stabilized the sinkhole by drilling diagonal shafts that reached under the bottom of the sinkhole then pumping grout into them. This was completed in April 2011. Next, the waste from the sinkhole area was relocated and the area was backfilled and covered with sod to create a safe work environment. Once this was completed, only a minor depression was visible in place of the previous sinkhole. The site looked like a sloping grassy field and the trash odor was no longer an issue. In the third stage, geophysical and geotechnical investigations were performed and a detailed remediation plan was sub- mitted to FDEP in April 2012. The agency was involved to help ensure that no con- taminated materials leached into the aquifer during the final geotechnical remediation. Minor depression visible in place of the previous sinkhole Earth Tech used two Soilmec SM-20 rigs with a combination of 5.5 in and 4 in (140 and 102 mm) diameter casings, all 0.5 in (12.7 mm) thick. For the hard rock areas within the limestone that contained chert with a strength of up to 20,000 psi (138 MPa), the 5.5 in (140 mm) diameter casings were used with tricone roller drill bits. Approxi- mately two thirds of the entire job used these larger casings, while the 4 in (102 mm) diameter casings were used for the Earth Tech also had a very limited work area of only 20 to 30 ft (6.1 to 9.1 m) sur- rounding the compaction grouting perime- ter, and this work platform had to be very stable despite the uneven slopes it was built on. In addition, the weather added to the challenge by washing out the work platform during heavy storms; twice within 4 weeks of each other. Earth Tech became very adept at building and rebuilding the work platform to continue its remediation work. DEEP FOUNDATIONS • SEPT/OCT 2014 • 97
