Large Diameter Auger Excavation
A Case Study
The Alabama Air National Guard former JP-4 underground storage tank (UST) site is located at 579 Westgate Parkway in Dothan, Alabama. The former UST was utilized by the Air National Guard to store and dispense petroleum products. The tank was removed at an earlier date and the current consultant, CH2MHill received approval to conduct a source removal to address residual petroleum contamination.
Fortis Environmental Group, LLC and Tri-Con Inc. were selected to implement source removal using a combination of large diameter auger (LDA) and conventional excavation. Source removal activities comprised of removing and segregating “clean” overburden soil and petroleum-impacted soil. Shallow soil was addressed through conventional excavation and deeper soil was removed using the LDA drilling rig. Major project activities are discussed in the following paragraphs.
Prior to mobilization to the project site, Tri-Con personnel prepared a variety of deliverables, including a detailed written work plan and an activity hazard analysis for all work items. Once these deliverables were approved, Tri-Con mobilized their LDA rig, excavators, loaders, concrete plant, and all other necessary equipment to the facility. All equipment and materials delivered to the site were inspected and approved prior to start of work. Due to the tight schedule, Tri-Con personnel began clearing and grubbing, setup erosion controls, and removed pavement cover while other crew members set up the concrete plant, traffic routes, and staging areas.
During the setup stage, two potential problems were noted: the presence of a water line through the excavation area and a lack of available water at the concrete plant location. Both issues were addressed by removing and redirecting the existing utilities to meet the project needs.
Because the excavation area was located on a hill-side, an LDA shoring wall was installed on the up-hill side of the
excavation to prevent cave-in during the conventional excavation portion of the work. The conventional excavation activities included the segregation of soils and preparation of a work surface for the LDA drilling rig. The LDA excavation activities began at the bottom of the conventional excavation and extended to 37 feet below land surface in some areas. During the Large Diameter Auger excavation, it was discovered that the former tank pit had not been compacted and contained a significant amount of concrete and debris. These two factors prevented continued LDA excavation in that specific area due to caving caused by the lack of compaction and an inability to advance casing due to the debris. The problem was solved by installing a shoring trench along the two tank pit walls. Combined with the surrounding LDA borings, this formed a complete ring of flowable fill shoring around the tank pit and allowed the Tri-Con crew to excavate the former tank pit with a long-reach excavator.
Post Office Box 12998