Tunnel Business Magazine

AUG 2017

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Cutter soil mixing walls were an economical and faster construction technique for micro-tunneling access shafts on PG&E's gas pipeline replacement project. Holding Back the Ocean Sixteen thousand vehicles now bypass eets every day to access the P ectly from In- terstates 395 and 95. Trucks and cars pass through two parallel tunnels completed in 2014 (one in each direction) that travel beneath Biscayne Bay, connecting the auseway on Watson Island with P dge Island. The twin 42-ft diameter, 4,000-ft long P unnel bores pass through highly permeable native silty sands and into multi-layered sands, cemented sands and highly porous limestone. iscayne Bay. The groundwater table is within a few feet of the ground surface, so proper dewatering techniques were essential to the project. In general, tunnel mining can proceed through groundwater and saturated soils. But groundwater must be controlled at the oints. Under these conditions, groundwater control through pumping was simply not feasible. olm designed and constructed a watertight alternative to traditional dewatering using soil mixing. The tem- porary excavation support system at the tunnel ends served as access points for the and also helped to keep ground- water out of the construction area. Par walls were installed and reinforced with W-beam soldier piles and strand anchors. Across the access face, perpendicular to the alignment, a grid of unreinforce panels was installe about 9 ft by 4 ft, and up to 50 ft deep. A 12-ft diameter unreinforced lean mix concrete drilled shaft was then installed in the center of each grid element, result- ing in a monolithic, watertight block. That took care of the walls, but high groundwater still exerts uplift pressure on the excavation bottom. To counteract the uplift, a grid of drilled shafts was in - stalled along the shallow end of the exca - vation and micropiles in the deeper por- tions. The drilled shafts and micropiles were tied into the concrete tremie slab to create a relatively solid, watertight work- ing surface. The lesson learned from the P - ami Tunnel is that creating relatively wa- tertight entry and exit points is the only way to build tunnels at sites with a high groundwater table and permeable soil conditions. Using innovative construc- tion techniques are often the only way to do that. Tunneling Takeaway Groundwater control techniques, both through extraction and exclusion, are an essential component of most tunneling projects. Today's larger and more powerful ground improvement equipment have enabled a huge advance in ground im- provement techniques. Larger elements, drilled shafts, and groundwater extrac- tion wells, can be installed more econom- ically. Advances in quality control tech- niques offer more confidence that ground improvement systems are built and will perform as intended, reducing the risk to tunneling operations. Every tunneling project is unique. The major takeaway is that tunnel owners and prime contractors should rely on their ground improvement subcontrac- tor's expertise to bring both innovation and proven techniques to bear when wrestling with groundwater conditions. For tunneling, it's all about control. F E A T U R E S T O R Y TUNNELINGONLINE.COM 22 TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2017 About the Authors Matt Kennedy is Malcolm Drilling Dewa- tering Division Manager and was project manager on the Seattle State Route 99 tunneling project (email: mkennedy@ malcolmdrilling.com). Ihab Allam, PE, was Malcolm Drilling's project manager on the PG&E pipeline replacement project. Nick Turus, PE, managed Malcolm Drilling's work on the Port of Miami Tunnel project.

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