The project consists of 28 km of tunnels and 42 km of pipelines and is scheduled for completion in 2004. The three tunnel contracts, Arrowhead East and West Tunnels and the Riverside Badlands Tunnel, are currently under construction.
The Arrowhead East and West Tunnels are at the northern section of the Inland Feeder project and when completed will extend through the foothills of the San Bernardino Mountains under United States Forest Services (USFS) lands. The Arrowhead East tunnel is approximately 9.1km long while the Arrowhead West Tunnel is approximately 6.4km in length.
Who got the contracts?
Both tunnel contracts were awarded to a Shank-Balfour Beatty JV for $88.4M and $87.6M respectively. Bechtel Infrastructure is the designer for both tunnels with Jacobs Associates as the sub-consultant for tunnel design. The tunnels are being constructed under issuance of a Special Use Permit issued by the United States Forest Service.
Tunnel excavation is being undertaken by Shank-Balfour Beatty designed 5.48m-diameter TBMs specifically designed and configured to perform efficient excavation in the intensely disturbed gneiss, schist, marble and quartz monzonite formations. The tunnels will require excavation in rock with compressive strengths of up to 200 MPa. In addition to the challenging geological composition a number of major faults including the San Manuel, Arrowhead Springs and the north branch of the San Andreas will be crossed. Water heads of up to 300m are anticipated. The design allows for pre-excavation and post excavation grouting to control water inflows and protect ground water features.
Arrowhead East commenced excavation in May 1998, but in April 1999, after 2.4km had been excavated, tunnelling was stopped and an intensive grouting programme initiated to reduce water inflows. Grouting has been completed but tunnelling has not recommenced. The USFS is currently evaluating the MWD’s plans for proceeding and allowing tunnelling to continue.
Riverside Badlands Tunnel
The Riverside Badlands Tunnel is 12.6km long and has an excavated diameter of 4.85m. The construction contract was awarded to Shank-Balfour Beatty for $113M in July 1998 and tunnel excavation commenced in December 1999.
The designer is a JV of Daniel, Mann, Johnson, & Mendenhall with Woodward-Clyde Consultants, and construction management is being undertaken by Hatch Mott MacDonald. At the end of March 2000, 4428m of tunnel (35 per cent) had been completed. Ground conditions range from alluvium and soft rock to hard metamorphic rock with groundwater pressure heads of up to 101m. It is being constructed between the San Andreas and San Jacinto Faults, one of the area’s most active seismic regions.
The tunnel alignment runs approximately North-South from the Opal Ave Portal, just East of the City of Redlands, to Gilman Spring Portal, South of the State Route 60 at the north-east edge of the San Jacinto Valley. The tunnel has a slight southward down-grade of between 0.03 and 0.2 per cent and at the deepest point the tunnel is 260m below the surface.
The final 3.65m i.d. pipe will be welded steel with thickness ranging from 12mm to 25mm, and has been designed for the ground loads at depths up to 260m, external groundwater pressure of up to 993kPa and an internal hydrostatic pressure of 745kPa. In addition the design considers seismic ground motions from an earthquake with a maximum magnitude (Mw) of 8 along the San Andreas Fault. The steel pipe is being manufactured by Schuff Steel Company of Phoenix, Arizona in 18.3m lengths.
The design calls for the annular space between the pipe and the initial tunnel support to be filled with low-density cellular concrete with a minimum compressive strength of 2.76 N/mm². Currently, pipe production is well advanced and Schuff had fabricated more than 6300m of pipe by the end of March 2000.
The profile and geological section of the Riverside Badlands Tunnel is shown in Fig 2 and a description of the various formations to be encountered along the tunnel are summarised in Table 1.
Three distinct formations will be encountered; alluvium, metamorphic rocks and a weak sedimentary rock know as the San Timoteo Formation.
The alluvium is located in the San Timoteo and Live Oak Canyons and consists of medium dense to very dense silty and clayey sands; medium dense to dense, poorly graded, and well-graded sands; and stiff to very stiff, lean sandy clays and silts. Some gravel layers are also anticipated. Above the water table the ground conditions are expected to be running to slow ravelling and below the water table flowing to slow ravelling. The water table is approximately 20m above tunnel invert so dewatering of the alluvium prior to tunnel excavation will be carried out.
The San Timoteo Formation consists of sandstone, conglomerate sandstone, and siltstone /claystone that are uncemented to weakly cemented. The formation can be sub-divided into the Lower, Middle and Upper Members with the Upper Member containing higher percentages of conglomerate and conglomerate sandstone than the Middle Member which is more fine grained and contains a greater percentage of siltstone/claystone. The Lower Member contains a large percentage of sandstone and conglomerate sandstone with little claystone/siltstone. Unconfined compressive strengths range from 0.05MPa to 15 MPa with the Upper Member, generally, exhibiting higher strengths.
Clasps within the sandstone conglomerate range from extremely weak to moderately strong with strengths up to 300MPa. Although the tunnel is to be driven below the water table the low permeability San Timoteo Formation is not expected to yield high water inflows except at fault zones. In these zones probing and pre-excavation grouting will be implemented prior to excavation.
The metamorphic rocks are generally gneissic in composition. The northern reach also contains a variety of granitic and volcanic dikes. The rock strengths of the gneiss is generally moderately strong to strong with unconfined compressive strengths ranging from 3.2 to 164MPa. The southern reach is generally less strong with unconfined compressive strengths ranging from 3.2 to 50.5MPa. The unconfined compressive strengths of the dike rocks in the northern reach range from 18.5 to 139.1MPa.
The northern reach is expected to be highly fractured and contain numerous sheared, brecciated, faulted and crushed rock zones. In this northern reach, the Banning fault will be encountered which will consist of an approximately 300m wide, highly brecciated, crushed rock, sand and clay gouge zone where significant water inflows can be anticipated.
Construction
The option existed to carry out the tunnel excavation from any of four locations. Shank-Balfour Beatty made the decision to excavate the complete tunnel with a single TBM from the southern portal at Gilman Springs. This portal was excavated in alluvium to a maximum depth of 68 m, with the portal excavation sloped at maximum grades of 1 in 1.5 . The excavation was configured so that a full face of the Upper San Timoteo formation was exposed for the commencement of TBM excavation. The portal has been developed with a roll over dump to empty the muck cars and a conveyor system to transport the material to the on-site disposal area.
At the northern end of the tunnel, the Opal Avenue portal has also been developed ready to receive the TBM.
Although the contractor has chosen not to carry out tunnel excavation from either of the two intermediate construction sites at San Timoteo Canyon and Live Oak Canyon, ventilation and emergency access shafts have been constructed. The 6.7m diameter shafts are approximately 38m deep and constructed with steel ribs at 1.2m centres and 90mm of shotcrete between the ribs. The shafts stop short of the tunnel crown by approximately 1.2m. Break through to the tunnel will be made after the TBM has passed.
A number of options for initial support were identified in the design documents including steel ribs and lagging, steel liner plate and pre-cast concrete segments. Shank-Balfour Beatty chose pre-cast concrete segmental lining as a single method for the whole alignment. The pre-cast concrete ring consists of four 1.2m wide segments 200mm thick. The concrete strength is 27 N/mm2 and the segments are lightly reinforced for handling. The ring is designed to take a maximum of 2.8 diameters of rock load.
The segments are erected in the tail shield of the TBM and expanded against the excavated surface as the machine advances. Hydraulic jacks expand the ring, which is then held in position by two screw jacks. The rings are re-expanded and the screw jacks re-tightened at the rear of the TBM trailing gear.
Tunnel boring machine
The TBM has been specifically designed and fabricated by Shank-Balfour Beatty. The machine is 4.85m in diameter and is fully shielded. The body and head were fabricated in Japan by Hitachi Zosen to Shank-Balfour Beatty’s design and the final fabrication completed on the site. The fully breasted cutterhead is configured with 24 face and 8 gauge cutters, each of which are 420mm in diameter and changeable from the rear. The thrust capacity per cutter is 22.25tonnes.
Muck is loaded from the face with a screw conveyor and then by belt conveyor to muck cars. Twelve 200-mm thrust-jacks with a total thrust capacity of 1,700 tonne advance the machine by pushing off the pre-cast concrete segments which are erected in a tail shield. Nine hydraulic motors producing maximum torque of 117tonne drive the head. The machine is similar to the TBM that was used in the Arrowhead East tunnel and as result of experiences on that excavation the following changes were made:
1) The cutterhead power was increased to allow increased rev/min thus providing more efficient excavation in the stronger rock formations;
2) The number of gauge cutters was increased from four to eight and all were made adjustable;
3) The shield was tapered 25mm in diameter from front to back to provide relief in squeezing ground conditions;
4) Modified anti-roll devices were installed to provide an increased ability to control TBM roll in the stronger rock formations.
Current progress
Excavation rates in the Middle and Lower San Timoteo formations and in the southern metamorphic rock have been excellent with an average advance rate, to date, of 27m/shift. The ‘D’ fault at the southern interface of the Middle and Lower San Timoteo has been successfully traversed and although a 30m increase in water head was expected the impermeable nature of the formation limited water inflows, and grouting was not required. Total water inflow has averaged approximately 5.8litre/min per 100m of tunnel.
The next major challenge for the project will be the excavation of the very soft clays, silts, sands and gravels of the alluvium soils in San Timoteo Canyon and preparations are well advanced to commence.
The shafts have already been excavated in the San Timoteo Formation adjacent to the alluvium. Dewatering wells have been installed and grouting equipment prepared in the event that grouting is necessary. In addition, compaction grouting is planned under the Union Pacific Railroad that runs on the south side of San Timoteo Canyon.
Currently the project is approximately six weeks ahead of the original baseline schedule and the expected completion is significantly before the Contract end date of July 2003.
Related Files
Overall layout of the Inland Feeder Project
The Badlands