Abstract:
A large liquid natural gas processing plant on the Gulf Coast of Texas required three below grade detention ponds to store storm water at the site for flood mitigation. The ponds ranged in size from 185 feet x 335 feet to 329 feet x 933 feet in area. Challenging geotechnical conditions, high ground water and weather-related issues impacted the decision to use closely spaced, large diameter deep mix method (DMM) columns for support of the pond slopes.
The geologic conditions at the site consisted of very soft clay overlying varying thicknesses of increasingly stiffer clays and a water table at approximately two feet below grade. Short term and long-term responses of the soils were evaluated for each pond to determine the optimal DMM column configuration for the required factor of safety for global slope stability. Analysis indicated that 5 to 6 rows of 10 foot diameter DMM columns installed to a depth of 30 to 35 feet below grade would be required. In areas where 10 foot diameter DMM columns would not fit due to obstructions, 6 foot and 8 foot diameter columns were utilized.
Due to poor site conditions a work platform was required to support the construction equipment.  DMM columns were installed around the perimeter of each storm water detention pond in tangent rows with overlaps at the corners to create a relatively uniform mass for stability. DMM column compressive strength was verified via UCS testing of remolded grab samples and in-situ vane shear tests. In addition to the DMM columns a perimeter line of sheet piles were installed to limit seepage into the ponds.