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.

Issue link: http://digital.tunnelingonline.com/i/986315

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Page 23 of 51

ever you let your guard down, it will act, and often with swiftness and power. As long as the properties of soil and wa- ter remain constant (which we can proba- bly count on for a long time) groundwater will remain the 1 source of problems in underground construction. Could this be due to the Ron Heuer groundwater tenacity or the fact that there is a very wide range of factors and conditions that aggregate successful or unsuccess- ful groundwater control? It's probably a combination of the two. With tunneling work particularly, we put our methods to the test. We go deep. Sometimes really deep. We tra- verse variable ground conditions many times along uncompromising alignments which cannot be varied to avoid unfavor- able conditions. In tunnels, shafts, adits, breakouts, cross passages, etc., we never have to space to accommodate "additional measures." It is an unforgiving environ- ment and tunneling rarely allows a "work around" when there is a problem. We typi- cally cannot afford a "hiccup" in the work. That's why the groundwater control ap- proach on a project is so critical. Before the common use of the pressur- ized fac tering was a huge component of most soft-ground tunneling projects. The open face digger shield, for example, would struggle when perched or residual water was encountered in poten- tially running ground conditions, and cat- astrophic soil run-ins would occur when an isolated pocket of undrained sandy soil would be encountered by surprise. In easily dewaterable ground like thick deposits of sand and gravel, it was like a tunneler's dream. If there was little risk of off-site adverse effects (consolidation of compressible soils or moving plumes) or the ground could be dewatered with- out concern about difficult changes in geology, perching layers, or recharge from open water or even utilities, a dewatering approach was the lowest risk option. The greatest risk hinged on the effectiveness of the dewatering program. That risk re- mains today with sequentially excavated tunnels; however, the third-party impacts are less likely because the ground is in- variably of lower permeability or "tighter" as that condition is a prerequisite for the stand-up time that goes hand-in-hand with the sequential excavation. It seems as if today there are more concerns about groundwater lowering perhaps because we are driving tunnels through some of the most congested urban areas, there is contaminated groundwater everywhere, and there is always the perception that TUNNELINGONLINE.COM F E A T U R E S T O RY 2 4 Jet grouting for sewer encapsulation. Two levels of wellpoints for dewatering of TBM launch box. TBM: TUNNEL BUSINESS MAGAZINE // JUNE 2018

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