Excavation Shoring for a Gas Holder under a Public Roadway: Why Put a Square Peg in a Round Hole?
Excavation Shoring for a Gas Holder under a Public Roadway:
Why Put a Square Peg in a Round Hole?
Heather Prentice and Brian Trent (Consumers Energy)
Fred Simmons and Mark Klemmer (Arcadis US, Inc.)
Mrs. Prentice is the Director of Environmental Compliance, Risk Management and Governance at Consumers Energy with approximately 18 years of environmental investigation and remediation work experience. Additional experiences include project management, developing remedial site closure strategies, evaluating site characterization data, development of bid documents, and overall responsibility for corporate Superfund and contaminated sites liabilities. She received a Bachelor of Science in Civil Engineering from Ohio Northern University with a focus on environmental impacts.
Removal of a 100-foot diameter gas holder was complicated by a utility corridor running over the top of the structure, an existing building foundation 15 feet to the north, a gas regulator station with high-pressure gas mains to the south, and a Michigan Department of Transportation highway to the west. In addition, the holder excavation demanded a tight time table as the gas regulator station needed to be taken out of service during the excavation and placed back in operation before winter. Traditional sheeting and shoring methods of similar depths require either external or internal bracing (tiebacks and whalers). In this case, external bracing would have required a much larger footprint due to the length of tiebacks required for bracing. Internal bracing requires straight runs of sheeting rather than a circular installation for whaler installation, resulting in over-excavation of soil, increased cost for transportation and disposal, as well as careful excavation of soil between and under bracing that slows down the pace of excavation and backfill. To eliminate bracing across the excavation, a shallow trench (approximately 4-feet) was excavated around the interior of the sheeting where 1-inch diameter re-bar was installed perpendicular and parallel to, as well as attached to, the top of the sheeting. Concrete was then poured to fill in the trench, forming a reinforced concrete ring designed to stiffen the sheeting and hold back earth pressures. An inclinometer was installed to monitor ground deformations throughout the excavation activities to verify that the concrete ring was performing as designed and ensuring a safe excavation. By changing the shape of the shoring support, the excavation area was reduced by approximately 7,000 square feet saving $500,000 in additional sheet piling, water management, and restoration.