Construction of Two Compression Ring Shafts Utilizing Cutter Soil Mixing and Jet Grout Bottom Seal-PPT Presentation

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The project involved replacing a 30-inch PGE gas line in Newark, California, using deep shafts constructed with cutter soil mixing (CSM) and jet grouting. The work included a 30-foot-diameter jacking shaft and a 20-foot-diameter receiving shaft, with no dewatering permitted.<br /><br />CSM panels, approximately 48 inches wide and 50 feet deep, formed compression rings around both shafts. The jacking shaft also included breakout and reaction panels to accommodate microtunnel-boring equipment and jacking loads of up to 400 tons. Jet-grouted columns formed watertight bottom plugs: 30 columns for the jacking shaft and 14 for the receiving shaft. The columns were designed to be 8 feet in diameter, with plug thicknesses of 18.5 and 16.5 feet, respectively, and a target unconfined compressive strength of 200 psi.<br /><br />Ground conditions included silty clay, silty sand, clay, clayey sand, and sand, with upper and lower aquifers. Testing was performed before construction to verify mix designs, strength, column diameter, and design assumptions. Quality control included wet-grab sampling, coring, hydrophones to assess jet-grout column diameter, inclinometer checks of drilled-hole verticality, and monitoring and reporting installation parameters.<br /><br />Key challenges included restricted site logistics, variability in achieving the CSM compression ring, preventing water infiltration through the jet-grout bottom seals, and a tight schedule. The project’s lessons were to assess the feasibility of construction solutions during bidding and to establish quality-control and reporting procedures before work begins.

Keywords

gas line replacement

Newark California

cutter soil mixing

jet grouting

microtunneling

deep shafts

compression ring

bottom plug

quality control

dewatering restriction