Riverside Badlands Tunnel, Inland Feeder Project - Jacobs Associates

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CHALLENGES BETWEEN CONCEPT AND COMPLETION 1001 Table 5. Comparison of estimated and actual grouting quantities Reach Feature 2 Reservoir Canyon Fault, Flowing ground Cement (kg) 163,400 (360,000 lbs) 3 Banning Fault 817,200 (1,800,000 lbs) 4 Flowing ground 8 Flowing ground 10 Fault D 272,400 (600,000 lbs) Totals Estimated Quantities Microfine (kg) 163,400 (360,000 lbs) 817,200 (1,800,000 lbs) Sodium Silicate (L) 3,917,500 (1,035,000 gal) — — 1,449,700 (383,000 gal) — — 870,600 (230,000 gal) 1,253,000 (2,760,000 lbs) 272,400 (600,000 lbs) 1,253,000 (2,760,000 lbs) Cement (kg) 26,700 (58,750 lbs) — 1,060,600 (2,336,030 lbs) Actual Quantities Microfine (kg) 108,100 (238,170 lbs) 383,400 (844,510 lbs) Sodium Silicate (L) 591,000 (156,140 gal) 0 0 210,600 (55,640 gal) 0 0 0 — 0 0 0 6,237,700 (1,648,000 gal) 1,087,200 (2,394,780 lbs) 491,500 (1,082,680 lbs) 0 801,600 (211,780 gal) Figure 5. Comparison of estimated and actual groundwater inflows
1002 2003 RETC PROCEEDINGS <strong>Tunnel</strong>ing Challenges For the most part ground conditions encountered in the tunnel were as described in the GDSR. However, difficult conditions were encountered creating challenges that had to be overcome. Alluvial Valleys—San Timoteo and Live Oak Canyons. The tunnel had to cross two alluvial valleys at San Timoteo and Live Oak Canyons. The Contractor was required to design and implement appropriate groundwater control measures for mining through the saturated alluvium. Feasible approaches outlined in the GDSR included dewatering, use of an EPB tunneling machine, and jet grouting. Chemical grouting methods were considered to be only marginally applicable due to the cost and high fines content of the alluvium. SBB elected to dewater the alluvium and installed 23 dewatering wells in San Timoteo Canyon and 10 wells in Live Oak Canyon. The wells were typically spaced 15 m (50 ft) apart alternating on each side of tunnel centerline and were designed to extend 3 to 7.5 m (10 to 25 ft) below tunnel invert. Dewatering started in San Timoteo Canyon (Reach 7) 8 weeks prior to the anticipated TBM arrival date. However, well yields were low and piezometer readings indicated that groundwater levels were not lowered sufficiently as they ranged from about 1.5 to 6 m (5 to 20 ft) above the tunnel crown when the TBM arrived at Reach 7. SBB considered that the ineffective dewatering was due to the ground having a lower permeability than expected, and decided that the ground could be controlled with the TBM by operating it in a partially pressurized mode (i.e. quasi-EPB) utilizing the screw conveyor. Unfortunately, the TBM was halted soon after entering the alluvium when silty sands flowed through the TBM and into the heading. At this point, it was obvious that tunneling could not proceed without a more positive means to control the flowing ground and SBB decided to chemically grout the rest of the alluvium in this tunnel reach. Drilling of vertical grout holes in a 3-hole pattern was initiated by as many as 5 drill rigs. Holes were drilled on 2.1 m (7 ft) centers, and chemical grout (sodium silicate) was injected through sleeve port grout pipes in several stages to treat the tunnel zone. Within ten days of initiating the grouting work, mining resumed and the grouting production allowed mining to proceed on a single shift basis in which an average of 18 m (60 ft) of tunnel was excavated per day. While the chemical grouting operation in San Timoteo Canyon was successful, it was expensive and required complete surface access along the tunnel. Because similar ground conditions were anticipated ahead in Live Oak Canyon (Reach 5), and because the performance of SBB’s dewatering system in Reach 5 had also indicated insufficient dewatering, there was concern that chemical grouting might also be required in this canyon. However, conflicts with existing residences and nearby water wells created difficulties for a chemical grouting approach. In addition, experience gained from the installation of a new water well for one of the residents suggested that the permeability might be higher than originally anticipated in this canyon. Recognizing the constraints to grouting, the actual performance of SBB’s dewatering system, and the apparent higher permeability of the alluvium, SBB was directed to install two additional wells extending some 24 m (80 ft) below tunnel invert and into the San Timoteo Formation. These deeper wells produced an average flow of 14 L/sec (225 gpm) each and quickly lowered the groundwater level in the narrow canyon. By the time the TBM arrived at Reach 5, the groundwater level had been lowered to below the tunnel invert. As a result, mining proceeded through the alluvium in Reach 5 without incident at an average rate of about 60 m/day (200 ft/day) [Table 4]. Reach 4—San Timoteo Formation. North of Live Oak Canyon, in Reach 4, the groundwater level in the San Timoteo Formation gradually rises from about 15 m (50 ft) above the tunnel crown to over 60 m (200 ft) at a location several thousand feet north of the canyon. South of Live Oak Canyon, in Reach 6, the San Timoteo Formation was
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Riverside Badlands Tunnel, Inland Feeder Project - Jacobs Associates

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