Tunneling and
Shafts
SoilFreeze has extensive experience in providing shoring for deep shafts, tunneling machine
access, deep manhole construction, equipment recovery, and mining are all common applications for frozen soil shafts. SoilFreeze maintains well-tested protocols for keeping shoring walls watertight when penetrated, most commonly by tunnel boring machines. Shaft construction may also be called for in densely populated urban developments and limited overhead construction, such as below power lines or under elevated roadways. SoilFreeze partners with highly skilled, local pipe installers to minimize social disruption during installation and maintenance.
Design considerations: The deeper shafts go into soils with high groundwater, the greater the anticipated hydrostatic pressure. A critical design element, therefore, becomes how deep to install the freeze pipes (past the excavation depth) to create an unfrozen “plug” below the base of the excavation. Only the most specialized and experienced geotechnical engineer will be able to analyze the natural soils to determine the water resistance needed to a degree of certainty. Since frozen soil, like concrete, is strongest when in compression, cylindrical shafts are the most popular shape.
Examples of deep shaft construction are SoilFreeze’s San Pedro and Harbor Island shafts in Los Angeles, CA. Four shafts were constructed, two 14-foot diameters and two 24-foot diameters. Each shaft was excavated to a depth of over 90 feet in beach sand with silt layers. The shoring was designed to reach a depth of 120 to 140 feet to provide a watertight “plug” in the bottom, taking into account the 80 feet of hydrostatic pressure.
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The Project
An 18-foot diameter tunnel boring machine became stuck 300 feet below a residential neighborhood; two miles into a four mile long tunnel construction project. The high ground water pressure at the 300 foot depth made repair from inside the TBM impractical. An innovative but somewhat risky solution requiring a second, smaller diameter TBM, coming from the opposite direction, to be driven into the stuck TBM’s empty shell was the only feasible way to complete the project.
The Challenge
Disturbed glacial sand and clays and high groundwater pressure surrounding the stuck TBM had to be stabilized. The stabilization and “sealing” around and under the stuck TBM had to be completed from a residential street 300 feet above with minimal disruption and noise. Soil stabilization needed to protect workers inside the stuck TBM while they dismantled the conveyors, bulkhead walls, etc., leaving only the steel shell in the ground under 5 bars of water pressure.
The Solution
SoilFreeze was able to create a continuous, variable, water-tight structural mass of frozen soil around and below the front section of the TBM. Our patented and special designed zone freeze pipes were installed in order to concentrate the freeze to only the lower 50 foot “zone” on the top, sides and in front of the TBM, which used less power. Single phase “residential power” could therefore be used instead of noisy diesel generators. To freeze the soil directly below the TBM, a few short freeze pipes were installed from inside the tunnel.
The Results
The soils around the stuck TBM were completely frozen creating structural support and sealing out groundwater throughout the two month TBM dismantling process. The contractor was able to drive the smaller 16-foot diameter rescue TBM though the stabilized (frozen) soil to successfully complete the tunnel.
Brightwater TBM Rescue
Case Examples:
Ballinger Shaft Portal Area Stabilizaitons
The Project
The Ballinger shaft is a concrete-lined structure originally designed to receive a 17-foot diameter TBM through a West portal at the end of its 2 mile mining operation. Because of tunneling difficulties with the TBM east of the Ballinger shaft a second portal was constructed at the bottom of the 200-foot depth shaft. That portal allowed the TBM to be refurbished and then launched out to complete the tunnel project.
The Challenge
Variable soil conditions outside the portal area consisted of distributed glacially consolidated sand and clay. The contractor was concerned that water and soil might intrude into the shaft under the high hydrostatic pressures (up to 5 bar) as the TMB mined into and out of the shaft. Variable soils and a high groundwater table made conventional grouting too risky.
The Solution
By installing our freeze pipes from inside the horizontal shaft through drilled holes, SoilFreeze was able to freeze the outside areas of the two portal. Chilled brine was then circulated through all the freeze pipes to freeze a solid mass of stabilized soil. Even though the high hydrostatic pressures at 200 feet in depth made drilling slower and more difficult, our freeze pipes were successfully installed within the budgeted time.
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The Results
A solid block of frozen soil was created at a distance of 50 feet (West) to 100 feet (East) outside the shaft after freezing for about 6 weeks. The frozen soil successfully stabilized the soils outside each tunnel and allowed the contractor to mine into and out of the shaft and refurbish the TBM, with no water or soil intrusions into the shaft.