Case Histories of Circular Secant Pile Access Shafts for Tunnels-Paper

Paper

Pdf Summary

The paper describes the design, construction, and quality control of circular secant pile access shafts for two tunnel projects: the Ohio Canal Interceptor Tunnel in Akron, Ohio, and the Ohio River Tunnel in Louisville, Kentucky. The shafts ranged from 34 to 51 feet in diameter and from 49 to 107 feet deep. They were temporary excavation-support structures and were socketed into bedrock to control groundwater.<br /><br />The shafts were designed as concrete cylinders that resist lateral earth and water pressures through compressive hoop action. Because the stiff structures allow little movement, designs used at-rest earth pressure. Unreinforced concrete piles were sufficient for the required compressive stresses. Where loading was uneven or openings interrupted the shaft wall, finite element analysis was used; examples included tunnel break-through openings and heavy conveyor-support loads.<br /><br />Maintaining pile overlap was essential to achieve the required wall thickness. The project used a verticality tolerance of 0.5 percent and increased overlap near the surface to account for possible drilling deviation at depth. Verticality was measured with a wireless instrument attached to the drilling equipment, allowing the effective wall thickness to be checked before concrete placement. Rigid casing, powerful rotary rigs, and carefully built guidewalls helped maintain alignment.<br /><br />Concrete volumes were also monitored. Low volumes could indicate incomplete filling or ground collapse, while unusually high volumes could signal weak soils or rock voids. In Akron, organic silts led to concrete takes exceeding theoretical volumes by more than 40 percent on some piles.<br /><br />The projects demonstrate that secant pile shafts can provide self-supporting excavation support and groundwater cutoff in challenging soils and rock, without internal bracing or external tiebacks. Because the walls cannot be directly inspected during installation, reliable construction monitoring and later visual checks during excavation are important to confirm their quality.

Keywords

secant pile shafts

tunnel access shafts

groundwater cutoff

concrete cylinders

hoop action

at-rest earth pressure

pile verticality

finite element analysis

construction quality control

bedrock socketing