Riverside Badlands Tunnel, Inland Feeder Project - Jacobs Associates

CHALLENGES BETWEEN CONCEPT AND COMPLETION 1003
excellent for tunneling, as the TBM cut the ground easily but the ground was strong
enough to have good stand up time and had sufficient silt and clay fines to prevent water
problems. As anticipated, the formation became less cemented, weaker, and sandier as
the tunnel proceeded north. Eventually the weaker, more pervious ground in combination
with the higher groundwater levels led to significant problems about 1 km (3,300 ft) north
of Live Oak Canyon where flowing ground with groundwater inflows in excess of 25 L/sec
(400 gpm) halted the TBM. The water inflows decreased to about 5 to 6 L/sec (80 to
100 gpm), however, mining could not proceed in such unstable ground conditions.
The GDSR anticipated flowing ground being encountered and provided for the
use of chemical grout (sodium silicate) to control this behavior. SBB planned to install
sleeve port pipes for grouting using the lost point drilling method. Before this could be
done, it was necessary to first stabilize the face and ground around the machine. This
was achieved by chemical grouting through short pipes driven into the face and though
holes cut in the shield. After completing this initial grouting, sleeve port grout pipes 9 m
(30 ft) long were installed ahead of the TBM and chemical grout injected. Then the
head was cleared of cobbles that had collected in the buckets and the head was
turned. Mining recommenced but before a full 1.2 m (4 ft) shove could be completed
the ground flowed into the tunnel and mining had to be stopped. This same chemical
grouting process was repeated several times without success over the next 10 weeks,
probably because the grout was not penetrating far enough into the formation to
stabilize enough ground around the tunnel perimeter.
Considering this situation, MWD investigated the feasibility of installing a deep
well from the ground surface in this area. The landowner was agreeable and
dewatering analyses indicated that a well installed to a depth of about 150 m (500 ft)
would be required to draw the groundwater level down to near the tunnel invert. MWD
directed SBB to install a deep well located about 107 m (350 ft) ahead of the tunnel
heading. In addition, drain holes 45 m (150 ft) long were drilled ahead of the TBM and
a pump was installed in a well drilled in the invert of the tunnel.
The deep well was very effective, producing a flow rate of approximately 27 L/sec
(425 gpm), and the drain holes yielded flows of about 6 L/sec (100 gpm) each. The
combined result of these dewatering measures was an immediate improvement in
conditions at the tunnel heading, and mining operations resumed. A second deep well
was also installed approximately 150 m (500 ft) north of the first well to ensure that the
remainder of the reach could be successfully mined. After a total delay of nearly
12 weeks, mining continued at a rate of 27 m/day (90 ft/day) to Reach 3.
Potential flowing ground conditions were also expected in the San Timoteo
Formation in Reach 2. Because of the effectiveness of the dewatering wells in Reach 4
and the difficulty in grouting from the tunnel, it was decided to use the dewatering
approach for Reach 2. Five wells were installed and they proved to be very effective in
controlling the flowing ground and Reach 2 was mined without incident.
Final Lining
The Contract allowed the contactor to install a final lining of either reinforced
concrete cylinder pipe or welded steel pipe. SBB selected the steel pipe option and
elected to fabricate the pipe in 18 m (60 ft) sections with lap welded joints. The tunnel
was prepared for the pipe laying operation and particular attention was given to
installing a panning system that allowed water to be drained to the invert while
ensuring that the LDCC would not infiltrate and plug the drains. Once proper drainage
was established, the steel pipes were installed in the tunnel using a special pipe carrier
designed by SBB. The pipe was transported and installed at a rate of about 120 m
(400 ft) or 7 sections per day. After installing and welding the pipe sections, stulls were