Ballinger Shaft Portal Area Stabilizations

Shoreline, Washington


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 east 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 Challenges

Soils outside the portal areas were variable and consisted of distributed glacially consolidated sand and clay. The contractor was concerned that water and soil intrusion into the shaft might occur under the high hydrostatic pressures (up to 5 bar) as the TMB mined into and out of the shaft. Variable soils and high groundwater table made conventional grouting very risky.


The Solutions

The freezing outside the two portal areas was accomplished by drilling holes and installing freeze pipes in these holes from inside the horizontal shaft. Chilled brine was then circulated through all the freeze pipes to freeze a solid mass of stabilized soil. The high hydrostatic pressures at 200 feet in depth made drilling slower and more difficult; however freeze pipes were successfully installed within the budgeted time.


The Results

A solid block of frozen soil was created 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 intrusion into the shaft.

 

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