Shani Wallis, TunnelTalk

On going to press (late April 2001), bid documents for continuation of the two Arrowhead Tunnels on the Inland Feeder project for the Metropolitan water District Southern California (MWD) were ready to go out. The long awaited "Special use" permit to restart TBM tunneling under its property was expected any day from the US forest service and an earliest possible bid date for the one large $200+ million estimated redesigned contract was planned for May or June.

Arrowhead East working site

Once awarded, the successful bidder will order two new TBMs for the contract and will set about fabricating segments for the heavy duty bolted and gasketed precast-concrete steelskinned carrier pipes of the 41km (25.7 mile) water supply pipeline will be installed.

Redesign and rebid of two original contracts was initiated back in April 1999 when tunneling was stopped to address high ground water ingress. After excavating 2.4km (8,000ft) of the first 9.2km (30,500ft) Arrowhead East Tunnel outflows measured at the portal had reached 5,500 litre/min (1,450gpm) - much higher than the 2,275 litre/min (500gpm) limit interpreted by the third parties from the EIR (Environmental Impact Report).

The two 5.4m (17.8ft) o.d. tunnels pass through the San Bernadino Mountains east of Los Angeles beneath property owned by the US Forest Service and adjacent to land owned by the San Manuel Tribe of Native American Indians. The heavily reinforced steel-sleeved precast concrete carrier pipes of the conduit, designed by Jacobs Associates as specialist tunnel sub-consultants to the MWD's project engineers Bechtel, are watertight and will form the world's longest impermeable structure. Water ingress during excavation, to a contract specified limit of 15,140 litre/min (4,000gpm) was to be controlled by grout injection.

Thr four 'junk' segments of the expanded primary lining design with a joint at each axis and the expansion jacks in the crown joint

When tunneling stopped in April 1999, high level discussions between MWD and third parties continued, while down in the tunnel, contractor Shank/Balfour Beatty was instructed to start a program of cut-off grouting.

"It wasn't that any one incident of inflow was excessive," explained John Townsend, of Hatch Mott MacDonald and the part of initial DMJM/Woodward Clyde Arrowhead East construction management team.

"It was rather that cumulative ingress through the primary lining and measured at the portal was higher than envisaged by the Forest Service".

From April to October 1999 more than 700 ton of different types of grout were injected laterally through the "Junk" segments of the contractor's primary lining over a 1,168m (3,832ft) length of tunnel from the end of the TBM's 55m (180ft) trailing backup. This cut-off grouting successfully reduced outflows to 1,000-1600 litre/min (270-350gpm) with about 100 litre/min (30gpm) of the continued flow came through the face, around the TBM shield, and along the backup zone where lateral grouting was impossible. The remainder came through pressure relief valves installed to control Hydrostatic loading on the concrete segments.

30 BAR POTENTIAL PRESSURE

Despite the results, concerns about cumulative inflows with continued tunneling persisted and alternatives had to be investigated. "One possible alternative was to sacrifice the two Arrowhead Tunnels and take the pipeline in open trench through the low lands and urban area to the west of the mountains," said Daniel Tempelis, recruited from private industry to manage the Inland Feeder construction program for MWD. "This however required further environmental review and cost estimates of environmental mitigation against strong public objection could prove prohibitive."

The solution acceptable to the third parties and adopted by MWD broad of directions in April 2000 was total redesign of the Arrowhead Tunnels to incorporate a bolted, gasketed primary segmental lining to limit water inflows during excavation.

Stockpile and close-up of the 6.1m(20ft) long × 4.36m o.d. (3.6m i.d.) steel-skinned reinforced concrete carrier pipes of the impermeable water supply conduit

This redesign is technical challenge. The new bolted and gasketed lining has to withstand potential Hydrostatic water head pressures of 30 bar generated by possible direct fault and fissure connections to a head of groundwater equal almost to the maximum equal almost to the maximum 275m (900ft) overburden above the Arrowhead East Tunnel. "As far as I am aware, a lining to that specification has never been designed before," said Townsend. "The one-pass gasketed lining on the Storebelt subsea railway tunnel in Denmark, with the highest hydrostatic lining specification to date, was designed to meet 8 bar hydrostatic pressures and the gaskets were tested to 16 bar."

Leading gasket manufacturers are working with Jacobs Associates, the lining designer, to develop a new prototype gasket but at such high pressures a completely watertight lining cannot be expected. "The heavily reinforced precast concrete of the five segments and the key in each 1.2m (4ft) wide x 330mm (13in) thick ring of the bolted primary lining is designed to withstand the high water load pressures and the loads exerted by the stiffness of the gasket," said David Crouthamel, leader of the Jacobs design team, "but some seepage through the joints, through any cracks, and through the steel insets of the grout holes must be anticipated. Also, the TBMs for the job cannot be closed against such high pressures. There will be inflow from the face through the non-pressurized TBM cutter-heads and around the shields. Given the conditions, all we can do is retard inflows through the initial segmental lining to the accepted level of 4.5 litres/min (1gpm) per 30m (100ft) of tunnel, and a total flow of 2,250 litre/min (500gpm) at the portals of the two approximately 6.5km (21,500ft) long tunnels. Once the internal steel-skinned carrier pipe is installed and backfilled, the facilities will be sealed and the water table will recover and remain at its natural levels. Maximum water head on the Arrowhead East Tunnel rises to 335m (1,100ft) for the final impermeable works. Conditions on the Arrowhead West Tunnel under a reduced overburden are less severe at 167m (550ft) head during construction and 215m (700ft) on the permanent works."

Initial discussions considered the possibilities of modifying the two TBMs designed and built for the Arrowhead Tunnels by Shank but this proved technically too demanding and uneconomical. The diameter of the Shank machines is too small. The internal diameter of the tunnel must remain 4.9m (16ft) to accommodate the inner carrier pipe and the 330mm (13in) thick non-bolted, non-gasketed junk segments of the expanded primary lining used by Shank/Balfour Beatty for the original tunnel contracts. The new TBMs need to be more robust and more powerful and require a heavy-duty segment erector.

Water ingress and grout injection operations through the expanded primary lining of 'junk' segments in the Arrowhead East Tunnel

The TBMs designed for the tunnels by Shank have a screw conveyor but they were not pressurized and were used principally to control extraction of material from the face. The new TBMs might also have a screw conveyor for the same purpose but pre-grouting ahead of the face will remain a key element in controlling water inflows. The new TBMs are therefore required to have two independent grouting stations. They must also have facilities to drill 12 grout holes through the shield around the cutterhead for forward fan grouting, and another six grout holes directly into the face and without needing to turn the cutterhead.

Both new machines must be equipped to handle large water flush flows - particularly the machines for the Arrowhead East Tunnel. This will now be a downhill excavation. The uphill portal was used by the suspended Shank/Balfour Beatty drive and is now inaccessible through the installed linings.

Due to the unexpected delays and the need to redesign the Arrowhead Tunnels, it was determines that continuing the impractical. A change order was therefore issued to delete further tunnel excavation from the contracts and Shank/Balfour Beatty was paid for work completed to date. In addition, Shank/Balfour Beatty was instructed to remove the TBM from the Arrowhead East Tunnel and install the internal permanent pipe in the 2.4km (8,000ft) heading to prevent any further drainage. As the TBM could not be withdrawn through the primary lined tunnel, it had to be dismantled in-situ. A special ventilation system was then required in the dead-end tunnel to provide adequate ventilation for welding together the steel skinned carrier pipes. The Shank/Balfour Beatty TBM was on site for the uphill Arrowhead West Tunnel drive but had not been launched. The new single-shielded TBMs will push off the bolted lining and pea gravel and grout will fill the annulus.

Rock conditions along the tunnel comprise a variety of igneous and metamorphic rocks with strengths of up to 250Mpa (36,000psi) UCS and with zones of faulted ground.

With no used machines in the market to match the required specifications, the new 5.5m (18ft 8in) diameters machines will be purpose built and are expected to cost between $10-12 million each. One bidder for the initial contract did propose using a bolted and gasketed lining as the only way to control water ingress, but the bid was considerably higher than the lowest conforming bid which MWD, as a public authority, is compelled by law to accept.

"Although more specific in performance criteria, the contract will leave to the contractor decisions about the detailed design of the TBMs and the segmental lining, including the choice of gasket and the bolt configuration the casting yard and producing the segments will be on the critical path for the new contract," he added.

Fig 1. Plan of the Inland Feeder and Diamond Valley Reservoir project

Cost of redesign and rebid

The decision to redesign and rebid the continuation works - for which both Shank and Balfour Beatty are permitted to compete - has been an expensive move for MWD. Expenditure on the Arrowhead East and West Tunnel Contracts to date has reached $160 million against the combined lowest bids from Shank/Balfour Beatty in February 1997 and June 1998 of $175.6 million ($88 million for Arrowhead East and $87.6 million for Arrowhead West). Payment to Shank/Balfour Beatty include procurement of two TBMs; mobilization of both contracts and all construction sites; excavation of 2.4km (8,000ft) of tunnel with the interior carrier pipe installed; some $40 million for the post-construction cut-off grouting operations in the Arrowhead East heading; mobilization and production of more than 20,000 segments for the primary lining (14,000 of which have been destroyed); and an added 2.85km of permanent carrier pipeline installed in trench work at portal sites. It also includes, under a separate contract, procurement by MWD of all the internet carrier pipes for the 9.2km (30,500ft) Arrowhead East Tunnel. Specifications for the Arrowhead West carrier are different and fabrication had yet to start.

Time wise, the Arrowhead Tunnels' redesign has caused at least three years' delay. Under the original program, the Arrowhead East and West contracts were to be completed by mid-2002 with the Inland Feeder coming on line by early 2004. Commissioning is now delayed until 2007.

However, commissioning of the system to transfer water from the California State Aqueducts into the Diamond Valley Reservoir is not needed strategically until 2012. The new reservoir meanwhile is being filled from aqueducts using a pumping station near the reservoir and its intake tower. The reservoir is presently over half full despite the project delay.

Cost wise, even with redesign and rebid of the Arrowhead Tunnels, the Inland Feeder will come in under the project's approved $1.188 billion budget at about $981 million. Construction of the reservoir also came in under its $2 billion budget. Millions of dollars however will have been spent twice over.

To maintain a tighter control on progress, performance, quality and project management it is proposed that a system of partnering be adopted for implementation of the redesigned Arrowhead East and West tunnels contract. "It was suggested that an 'alliance' format be adopted, "said Tempelis, "but the opportunity to adopt the concept has passed. We do practice a form of partnering on MWD contracts but once the new Arrowhead Tunnels contract is awarded we may consider entering a broader partnering program with the successful low bidder."

Getting the Arrowhead Tunnels back into excavation after the long hiatus is much anticipated.. it will also involve appointment of a new construction management team. The original CM contracts were also cancelled when redesign and rebid of the construction a contract was adopted by MWD.

Working site at the south portal of the 12.8km (8 mile) Badlands Tunnel

Meanwhile, Shank/Balfour Beatty is making positive progress on the 12.8km (42,000ft) Badlands Tunnels, the third and longest tunnel on the Inland Feeder. Being the incumbent tunneling contractor, Shank/Balfour Beatty was successful in submitting the lowest of six bids for the project's Badlands contract at $112 million in February 1999, narrowly under marking the next bid by $200,000.

Using a Shank-designed TBM manufactured by Hitachi Zosen and similar in design to the machines procured for the Arrowhead Tunnels, the joint venture has achieved progress rates of between 55-60m (180-200ft) through soft sandstones, claystones and conglomerates. The machine was launched in December 2000 and had reached the 9.8km (32,088ft) mark by mid-March 2001 working 24h/day, 7days/week with two 8.5h mining shifts/day. Hole through to complete the 12.8km (42,000ft) drive, with the expanded segmental primary lining, is anticipated for summer 2001. By current estimates, the contract, with the internal steel-sleeved carrier pipe installed, will be completed in late 2002, some nine months ahead of the July 2003 contract target.

"This contract had a realistic schedule from the start," said John Townsend, who moved from the suspended Arrowhead works to be resident engineer for the Hatch Mott MacDonald construction management team on the Badlands Tunnel. "There were however potential areas for delay. The 4.8m (16ft) o.d. tunnel runs mainly through dry sandstone and clay stone deposits but does pass beneath two sand and gravel alluvial valleys, beneath the water table. The first was encountered in May 2000 and installation of a surface dewatering system with chemical grouting from the surface through the 30m (100ft) overburden, did cause minor delays to TBM advance. Before meeting the second, a more efficient dewatering system was installed ahead of excavation and the TBM advanced quickly through the 150m (500ft) section." During contracts in mid-March for this project update, the tunnel was passing through the wet, sheared and faulted conditions of the banning fault and extensive pre-grouting was again being undertaken. "By turning the cutter head it is possible to drill eight forward grout holes through the muck buckets," explained Daniel McMaster, assistant resident engineer for Hatch Mott MacDonald," and probe holes, drilled to check for inflows, are grouted according to contract schedules with micro fine and type 3 cement grouts. More than 1.5 million pounds of cement grout has been injected to retard water flows on this occasion. It will be slow going until the TBM moves into drier, more favorable conditions when progress will improve again sharply."

Transportation of the permanent carrier pipe into the host tunnel

Designer for the Badlands Tunnel is the DMJM (now merged with Fredric R Harris) and Woodward Clyde (now URS) JV and Hatch Mott MacDonald took over construction management. "With suspension of the two Arrowhead Tunnel contracts, we had three construction management teams when we only needed one," explained MWD's program manager Tempelis. "We cancelled two and retained one." Hatch Mott MacDonald was CM on the Arrowhead West contract.

Unlike the Arrowhead Tunnels, the technical demands of the Badlands Tunnel are much reduced. Maximum overburden is 262m (860ft), the material for the most part is softer and drier and the maximum head of water pressure predicted at tunnel level is 100m (330ft). Dewatering from the surface is permitted and water ingress limits are less stringent at a maximum 10,030 litre/min (2,650gpm) from a potential four construction sites. As a result the tunnel dimensions are different. The internal diameter of the conduit's steel sleeved carrier pipe is the same at 3.6m (12ft) i.d. but the pipe is less thick allowing for a smaller host tunnel, a smaller diameter TBM and a smaller outer diameter for the Badlands primary lining. For this reason, segments produced by Shank/Balfour Beatty for the Arrowhead Tunnel and had to be scrapped.

Meanwhile back on the closing works for the Arrowhead East job, Shank/Balfour Beatty has completed installation of the carrier pipe in the 2.4km excavated section of tunnel and is expected to vacate the site by November 2001.

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