Paula Wallis, Reporter

It was the target operational date for the San Vicente water pipe in Southern California, but December 4th 2008 passed like so many of the more than 570 previous days, with excavation inching along, crews cursing the impossibly hard ground, and everyone hoping for better conditions with each successive round.

Indication of conglomerate deposits

“Most days, the ground is so hard, we’re just scratching and scraping at the face with little to show but a whole lot of dust,” said Mike Jatczak, Project Manager for the Traylor-Shea JV.

Crews, working in three 8hour shifts, 6 days a week, are on the project’s final and longest drive from Slaughterhouse Shaft west to the Central Shaft or Reach 4E (Fig 1). It has taken the open-faced digger shield 19 months from launch in April 2007 to complete 1.4 miles (2.25m) of its roughly 4-mile (6.43m) drive.

With dogged determination, the CTS machine and its crews continue to chip away at the unexpectedly hard matrix holding the boulders and cobbles of the Stadium and Friars Conglomerate. Some 60% or more than 6 miles of the full tunnel alignment runs through conglomerate, some of which is strongly cemented, which is being blamed for delaying the project by as much as two years.

Still, slight changes in ground conditions have buoyed spirits in recent days with advance rates jumping from an average of 15-20ft (4.5-6m)/day to more than 50ft (15.2m) and there is guarded optimism that conditions are really improving and not just a pocket of softer ground that is raising false hopes as in the past.

“The ground is still extremely hard, but we are moving from Friars Conglomerate into Stadium Conglomerate, which we will be in for most of the remaining 60% of the drive and we’re starting to see some give in the ground, which is encouraging,” said Karen Henry, Project Manager for the San Diego County Water Authority (SDCWA).

In the six working days from December 15 - 20, 2008, contractor Traylor-Shea JV excavated 497ft (151.5m) compared with 144ft (44m) a month earlier in the six work days from November 17 to 22. That’s an average of 82ft (25m) /day compared to 24ft (7.3m)/day four weeks earlier.

Fig1. Longitudinal section of the tunnel, its six geological reaches, and its access shafts

Meanwhile, the CTS1 machine on the project finished its 5.2-mile (8.4m) long drive from Central to West Shaft in September and work is now transitioning to the installation of the steel carrier pipe. Each length of the 8.5ft (2.6m) i.d. welded-steel pipe will be introduced at Central and installed working back from West Shaft. Once the CTS2 machine finishes Reach 4E from Slaughterhouse to Central Shaft, the inner carrier pipe for that 5.8-mile (9.4km) east tunnel section will be installed from the San Vicente Portal and working back from Central Shaft.

This is the final drive of an ambitious and complex 11-mile, 13ft diameter (17.5km x 3.9m) San Vicente tunnel that runs through six geological reaches. It is considered the most multifaceted tunnel currently underway in North America and is testing the experience of engineers.

“This project is the most challenging of those I have been on”, said Jerry Ostberg, Construction Manager for Parsons , which holds the construction management contract. “It’s the most challenging in terms of the number of excavation methods required, which have included drill and blast, NATM, hard rock TBM, and open faced digger shield.”

Tunnel alignment

Drill+blast with a section of NATM were employed against the fractured and weathered water bearing granite of Reach 5 and drill+blast again through the hard rock intrusion of Reach 3. Hard rock TBM was used in the strong igneous and metamorphic rocks of Reach 1 and 6, and open-faced digger shields were selected to handle what was expected to be weak rock, firm to slow raveling above the water table and potentially slow to fast raveling conglomerate below the water table through the long drives of Reaches 2 and 4.

“It certainly has been a challenge”, said Jatczak. “At one point we had six different faces working simultaneously. It was shortly after the open face digger shields began their drives that they hit much harder conglomerate than anticipated, slowing progress to a crawl.”

Tunnel excavation began in 2005 with drill+blast and a section of NATM extending in both directions from Slaughterhouse Shaft through Reach 5. In May 2006, a refurbished Robbins TBM started from the San Vicente Portal to complete the 3,937ft (1.2km) hard granite of Reach 6 to meet with the east heading of NATM from Slaughterhouse. At the same time the first of two new CTS digger shield machines was launched at the Central Shaft to move westward and attack the Stadium Conglomerate of Reach 4.

In April 2007 the TBM, finished with its drive in Reach 6, was recovered and launched again from the West Shaft for its second 4,000 ft (1.2km) drive through the fresh to highly weathered granite and Santiago Peak volcanic rocks of Reach 1. The second CTS machine was also introduced at Slaughterhouse and walked through the drill+blast of Reach 5 to start the west drive of Reach 4 through the conglomerate.

Face at the crown

Shortly after the CTS machines began their individual drives, ground conditions consistently forced the digger shield booms to break up conglomerate that was expected to be well compacted and dense and exhibit behavior ranging from firm to slow raveling above the water table.

“We bid this job based on a program average of 90ft (27.4m)/day,” said Jatczak. “In reality our over-all average is 19ft (5.8m)/day.”

When asked if the GBR was inadequate, Wade Griffis, Construction Administrator for the client defended the site investigation, saying the GBR included twice the number of borings than accepted industry standards.

Regardless, the differing site conditions (DSC) have delayed the project by more than 24 months, and have pushed the cost of the contract from its original $198.4 million, up to $220 million.

Boom traveler and ram

Both the contractor and the client are reluctant to discuss the handling of claims but the SDCWA has acknowledged some of the DSC. “We are making interim payments that we find have merit ahead of claims,” said Griffis.

“We have taken some small issues on other matters to the Disputes Review Board (DRB) and members of the DRB are making routine quarterly visits,” added Jatczak. “ As always, it’s challenging to quantify and reach agreement on the impact of DSC.”

But with roughly 3 miles still to excavate, Jatczak’s primary concern is keeping the CTS machine up and running and supplied with picks. Over its 19-month drive the machine has been down approximately four times for a week or more for maintenance.

“This machine has had a long and hard life and the biggest challenge we have is keeping it in good repair and to keep it from breaking down”, said Jatczak. “We have also experimented with pick tools from various suppliers and found the long, rather sharp, dredging tooth is the most effective”

As a result of softer ground conditions pick consumption is down from a high of 19 per 8 hour shift to roughly 5 per shift. Along with better progress rates and easier ground conditions, the tide may be finally turning for this difficult project as it heads towards its projected on-line date in late 2010.

References

San Vicente stumped by tough conglomerate - TunnelTalk, Sep 2007

San Diego Country Water Authority
Traylor Bros., Inc
J.F. Shea Co., Inc
Parsons