Published October 2010 in Trenchless Technology Magazine

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Microtunneling Used in Contra Costa Water District AIP Project
— Oct 01, 2010

The Contra Costa Water District (CCWD) in Concord, Calif., is building its new Alternative Intake Project (AIP) near its existing pumping facilities in the Delta region east of the San Francisco Bay. The new AIP project will provide an alternative untreated water intake facility at Victoria Canal, where the water quality is typically better than that at the current intake location on Old River.

The AIP project will pump untreated water directly into CCWD’s existing Old River pipeline facilities, which convey untreated water to the District’s Los Vaqueros Reservoir and Contra Costa Canal. This new project will divert up to 250 cu ft of raw water per second from the new intake and pumping plant through a new 72-in. diameter pipeline. A majority of the pipeline was installed by cut-and-cover, except for a key 900-ft undercrossing of Old River in which microtunneling was used to construct a 96-in. diameter, 1-in. thick sidewall steel casing at the existing Old River Intake Facility.

The crossing required a 92-ft deep jacking shaft and a 50-ft deep receiving shaft. The tunnel crossed underneath two river levees managed by different reclamation districts. Levees in the Delta region are generally recognized as marginally stable structures prone to settlement and occasional failure. Water levels in the river are typically 10 to 15 ft higher than the adjacent farmlands and a levee failure would be catastrophic.

To maintain levee integrity, crack-stopper walls were required and settlement due to shaft construction and tunneling operations was limited to 1 in. Levee performance during shaft construction and tunneling was monitored via a vast array of surface settlement points and multistage extensometers, inclinometers and vibrating wire piezometers.

The Delta region is at the confluence of several major rivers that drain the Central Valley from the north, east and south. Poorly consolidated to unconsolidated sediments in the upper 150 ft characterize the near-surface geology of the region. The near-surface sediments consist of relatively thin deposits of peats, organic soils and fills (typically the levee structures) overlying the alluvial soils that consist of variable mixtures and thicknesses of sand, silt and clay. The deposits have highly variable consistencies, ranging from soft to stiff and loose to hard. Sandwiched in the upper 130 ft of the alluvial deposits are two confined aquifers presenting over 30 psi of pressure.

The aquifers influenced the design and construction of the jacking shaft. Launch of an Akkerman – SL74 microtunnel boring machine (MTBM) was complicated by a confined aquifer opposite the tunnel eye consisting of very dense to hard sand. The aquifer is capped by medium stiff clays. The alignment transitioned out of the hard aquifer sand into medium stiff clays within 100 ft of the shaft.

Difference-Maker

The cutter-soil-mixing (CSM) method — a construction technology relatively new to the United States — was used to construct 50- and 92-ft deep watertight shafts in  difficult ground conditions. This was the second known application of this technology in the United States for construction of microtunnel shafts and the first known application using shotcrete-reinforced walls.

CSM was first introduced in Europe in 2003 and has been used in Europe, Asia and Canada. Prior to the CCWD project, the CSM method had been previously used on two other U.S. projects.

The shafts for this project penetrate soft saturated silts and clays, loose-to-dense sands and a confined aquifer. Caissons constructed in the wet were considered the most feasible method for the ground conditions. Approximately 180 ft into the drive and about 130 ft beneath the river, the MTBM developed a “crabbed” condition as the machine transitioned out of the dense sand aquifer into the significantly softer clayey deposits above the machine. This compromised the o-ring seals at the pipe adapter ring and further advancement was not an option.

A casing and machine retraction plan was devised to withdraw 30 ft of the installed 96-in. steel casing. In addition to static load pulling the casing and static load pushing at the face of the machine imparted by slurry pressure, the retraction plan featured a 24-in. Taurus pneumatic pipe bursting hammer installed inside the casing near the MTBM. The retraction plan included several modifications at the pipe adapter joint to ensure the machine would not separate from the pipe string during retraction. The modifications consisted of Dywidag bars to keep the machine and the pipe string cinched together, keeper brackets across the joint and hydraulic jacks. Casing retractions are generally unprecedented and even more so for the 96-in. diameter casing used on this project.

The retraction design and process were underscored by the team’s commitment to the partnering process put in place at the start of the project. The microtunneling portion of the project was completed in October 2009.