GET has been providing soils explorations and geotechnical engineering studies for additions and improvements at Mobile Area Water and Sewer’s C.C. Williams Wastewater Treatment Plant on McDuffie Island since 1987. Recent improvements have included four 110-foot concrete cast-in-place clarifiers, a new headworks structure, an effluent pumping station (PCEPS), ancillary structures related to operations, a biosolids building with a dedicated truck bay unloading area and a new 2-story chlorine process building. Soil borings performed for these studies ranged in depth across the site from 50 to 100 feet.
GET investigations found the site to be generally underlain by soft compressible soils. Consolidation tests were performed on undisturbed samples of cohesive soils recovered from the site that indicated that the in situ soils to depths of approximately 40 feet were moderately compressible. Because of the potential for settlement, the decision was made to support the four-110-foot diameter concrete tanks (clarifiers) on augercast pile foundations. Areas of other major structures were preloaded for several months to cause anticipated settlement to occur prior to construction. Once the preload program was complete, structures were then supported on shallow foundations. Lightly loaded structures and/or those tolerant of a modest amount of long-term settlement were supported on shallow foundations after improvement of the near-surface soils.
GET provided engineering oversight and support through the construction of the phases of improvements as well as the construction materials testing of soils and concrete materials including pile load test and production pile installation monitoring.