Tunnel Business Magazine

JUN 2018

TBM: Tunnel Business Magazine is the market leader for North America. TBM is written for leading professionals in all aspects of tunneling and covers project stories, design elements, contracting strategies, legal issues, new technology and more.

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Page 25 of 51

TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // JUNE 2018 F E A T U R E S T O RY dewatering is just going to cause problems. The typical tunneling project today is a sequence of "bathtub excavations" con- nected by tunnels. The bathtubs are relied upon even when the ground conditions are highly favorable for dewatering. The vertical walls of the bathtub are construct- ed with slurry walls, secant piles, soil mix- ing, and steel sheeting. Ideally, a natural, low permeability cut-off stratum exists to act as a bottom of the bathtub, but when that doesn't exist, we must make the bot- tom with jet or permeation grouting. The bathtub approach is not risk-free by any means. A slight imperfection in a deep cut-off in the right (wrong) soil conditions can be catastrophic. Good craftsmanship is more important the deeper one goes. Soil/ structure interaction and compatibility of dissimilar cut-off methods cannot be over- looked. There are a lot of elements of the work that must be executed flawlessly. The recipe for a bathtub disaster is a deep excavation, a slight defect in a cut- off, plus a non-cohesive soil. Looking back on some of the more horrific ground loss events that we have been called in to, there is a common theme – excavation well below the water table, some kind of an unanticipated gap in a cut-off, and surprisingly, a low permeability, but non- cohesive silt. y people disregard the unstable potential of a non-cohesive silt (like Bull's Liver) because they recognize it as a low permeability material. Low permeability material is just assumed to be less susceptible to ground loss. Experi- ence has shown just the opposite. We let our guard down and the tenacity of the groundwater wins. Typically in tunneling, there is little we can do to accommodate leakage and groundwater intrusion without ground loss. A bottom seal can be configured as a deep blanket with soil between sub- grade and the deep blanket to accom- modate some leakage through properly constructed wells or wellpoints. But this option doesn't apply to vertical cut-off walls. When there are defects in cut-offs, unfortunately we aren't usually aware of them until they reveal themselves with a vengeance. The desired response is usu- ally chemical grouting and/or jet grouting, and compaction grouting to replace lost ground. But a high degree of ground dis- turbance is a potential game changer. This is when we call on ground freezing. We usually rely on ground freezing when there is absolutely no room for er- ror, when we are deep and in ground con- ditions that are not amenable to displace- ment or erosion, when we have disturbed conditions or we are attempting to work in and amongst the grout of previous at- tempts. This is where ground freezing excels. It fulfills a need that is difficult or sometimes impossible to fill through other geotechnical methods. But in un- derground construction below the water table nothing is ever 100% guaranteed and even ground freezing has its Achilles' 2 6 Permeation grouting for soil stabilization behind the microtunnel break-in eye.

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