How To Build The New Irvington Tunnel

Transcription

1 THE NE IRVINGTON The security of San Francisco s water supply will come one step closer this fall, when the final section of the New Irvington Tunnel holes through. Like the miners who created the existing tunnel nearly 100 years ago, today s miners have successfully and safely overcome challenging ground and plenty of groundwater. NATJ s Kristina Smith reports 10 NORTH AMERICAN TUNNELING JOURNAL

2 W CHALLENGE Above left: Drill and blast works at the Alameda West heading (see map on p15); Top: Roadheader excavation begins at the Alameda West Portal; Above: Pre-excavation grouting at the Vargas Shaft east heading; Above left: Miners hole-through between the Irvington Portal and the Vargas Shaft; Below: Final liner installation under way from the Irvington Portal BACK IN JUNE 2011, when California s New Irvington Tunnel project was in its early stages, David Tsztoo, Project Manager for the owner, San Francisco Public Utilities Commission (SFPUC), predicted what the biggest issue for this job would be: The major construction challenges will be due to groundwater, Tsztoo told NATJ during a visit to site. We have to mine into zones where we will encounter 300 static feet of groundwater. That is a scary proposition. To be conventionally mining in those types of conditions and having a lot of water seepage that could be literally flash flowing into the tunnel is quite a challenge. Some two years down the line, the final portion of the tunnel is almost complete, and groundwater has definitely proven to be the greatest challenge. Controlling it has added 12 months to the original program. There have been a lot more hours expended that we anticipated for all the drilling and grouting, says Dan McMaster, Hatch Mott MacDonald s Construction Manager for the project. Over the course of two years, it all really adds up. The owner is contractually obliged to add days to the contract for probe drilling and grouting delays NORTH AMERICAN TUNNELING JOURNAL 11

3 to the critical path of the tunnel excavation. Running parallel to SFPUC s existing Irvington Tunnel, the 3.5-mile (5.6km) long New Irvington Tunnel will bring water to 2.6 million customers in San Francisco, San Mateo, Santa Clara, and Alameda Counties from the Hetch Hetchy reservoir. Once in operation, SFPUC will have the opportunity to drain the old tunnel in order to inspect its conditions and carry out necessary repairs. Speaking to Tsztoo and McMaster in early July this year, NATJ learned that JV contractor Southland/Tutor Perini had just 1,178ft (359m) of tunnel left to complete. But they were tunneling through a difficult stretch of ground, with a syncline (folded ground) making for complex conditions. The tunnelers were battling with groundwater inflows up to 300 gallons per minute from each 2in (50mm) probe hole. It s taking several days per week to use pre-excavation grouting procedures to grout up to 100ft (30m) in front of the heading, says McMaster. Southland/Tutor Perini has pumped some 6.7 million pounds (3 million kg) of cement grout into the ground over the length of the tunnel. Primary grouting often wasn t sufficient, says McMaster, with secondary and even tertiary grouting required. There were some cases where we drilled and grouted for three consecutive days, says McMaster. After over two years of excavation, the contractor s crews have become very proficient and efficient in how they deal with groundwater inflows, says Tsztoo. In the beginning the mining crews had to go through a learning curve to determine the grout mix, use of packers, how many holes to drill, and where to drill them, to effectively direct the grout and seal off the numerous cracks, fissures and pathways for groundwater intrusion. Like their forebears who mined the original Irvington Tunnel almost a century ago (see box below), miners have fought against ground and water to create this new, seismically resistant water line. Unlike the early miners, however, this crew has had the assistance of three roadheaders, one of them a hefty 55 ton Mitsui Mike, to use alongside drill and blast. The existing Irvington Tunnel is a vital part of San Francisco s Hetch Hetchy Regional water system, supplying around 85% of the city s water. The water comes from the Sierra What history taught us Miners used mostly drill and blast to construct the original Irvington Tunnel between 1928 and 1932, working in from the east and west ends towards the center. Records show that they encountered a variety of challenging ground conditions: running, caving, flowing, raveling and squeezing ground. Though their experiences helped with the planning of the New Irvington Tunnel, site investigation revealed that some of the terminology used then didn t match up with what is used today. For example, what they described as running ground we would call flowing ground. The other big problem was ground water, lots of it: discharge volumes ranged between 150 and 2000 gallons per minute. Unlike their modern-day counterparts, the original Irvington Tunnel miners could not use surface wells for dewatering or nor did they probe ahead of the face and inject preexcavation grouting. So the water had to freely drain into the tunnel, with the miners on many occasions having to construct numerous concrete bulkheads in the headings to stabilize the face. This free inflow of water caused a few problems for local landowners and their wells: The original tunnel was allowed to freely dewater the aquifer so they had a lot of problems with the groundwater wells being dried up as the tunnel progressed underneath the wells and drew down the groundwater in those areas, says David Tsztoo, Project Manager for the San Francisco Public Utilities Commission (SFPUC). There were many well owners who ended up threatening to sue the City because we dried up their wells. The primary support for the existing tunnel was heavy timbers, with an unreinforced concrete lining. The heavy timbers in the original construction, for the most part, may have dry rotted, says Tsztoo. And the lining may have deteriorated from lack of maintenance, which is why the tunnel has almost reached the end of its useful life. Though we do not know what site investigation was carried out before the original tunnel construction, and it is assumed that they were tunneling blind, the miners did seem to know something about the ground they were passing through. At one point, where the tunnel would have run under the line of the current I-680 highway, the alignment kinks as if to avoid an area of ground. I heard a rumor that they wanted to avoid bad ground, so they steered the alignment north of that location, says Tsztoo. As it turned out when we mined from the Vargas Shaft east into that location, we ran into a lot of groundwater, unstable ground and methane gas. It was pretty tough to establish the Vargas East heading. We suspect our forebears knew all that. Despite the undoubted difficulties in mining the original Irvington Tunnel, records also show that the miners back then accomplished it safely, with no deaths or major accidents. That is a testament to the competence and knowledge they had back then. NORTH AMERICAN TUNNELING JOURNAL 13

4 Mountains via the Hetch Hetchy Reservoir and goes through a series of tunnels and pipelines over 100 miles to Sunol Valley where it enters the valley and flows through four buried siphons to the Alameda West Portal. Here it flows through the existing tunnel for 3.5 miles and out of the Irvington Portal into a series of five Bay Division Pipelines in Fremont. But the supply is under threat. The tunnel and siphons lie between two active faults, the Hayward and Calaveras, which means that a major earthquake could damage the infrastructure and potentially disrupt water supply to San Francisco and Bay Area customers. Because it has not been possible to halt the critically needed supply, the existing tunnel has not been inspected since The geology along the tunnel s alignment is varied. The largest proportion of the tunnel s alignment is in Briones Sandstone Formation varying from very weak to very strong rock. There are also sections of Cretaceous sandstone and shale, Tice shale and Oursan sandstone. What makes the geology really interesting is the number of secondary fault zones; at least seven were expected along the length of the tunnel. The records from the original tunnel construction showed that the fault zones consisted of soft clay gouge and moderately to intensely fractured rock, with either squeezing or swelling ground, or both, in the vicinity of the fault zones. The miners also noted running and caving ground at various stations, primarily at or near the faults. The experience of that original tunnel, Left: Excavation of the 115ft deep, 41ft diameter, secant pile wall intermediate Vargas shaft Below: The Vargas Shaft east heading showing spiling and breastboards Below right: Workers at the Vargas Shaft west heading errect steel sets combined with site investigation, convinced designers URS and Jacobs Associates that conventional tunneling rather than a TBM was the best solution for the new tunnel. Because of the number of fault zones along this alignment, they were very much concerned with the squeezing ground and that the TBM could get stopped in those zones, says Tsztoo. There were seven zones, so seven chances to get stuck. And what happens if you get stuck more than once? Four headings to attack Unlike the original tunnelers who worked into the middle from each end, Southland/Tutor Perini have three tunneling sites: the Irvington Portal at the downstream West end, a temporary intermediate construction shaft called the Vargas Shaft 4400ft (1353m) east of there, and at the East end the Alameda West Portal. So, there were potentially four headings to attack at once: one from each end of the tunnel and two from the intermediate shaft. Malcolm Drilling constructed the 115ft (35m) deep, 41ft (12.5m) diameter Vargas Shaft using 76 secant piles. The shaft was complete by March 2011, ready for tunneling to begin. The heading from the Alameda West Portal has been active since the start. Southland/ Tutor Perini started there with one of its two smaller machines, the 38-ton Antraquip AQM-150 roadheader, switching quite quickly to the 55-ton Mitsui Miike S-200. Before moving to Alameda West, the Mitsui had mined 40ft (12m) from the Vargas shaft east to create a tail tunnel, and at 40ft (12m) in encountered the first patch of running ground. Miners also came across something else that would impact the way the project progressed at this point: methane gas. Up until that point, June 2011, the tunnel had been classified as potentially gassy. However, there was an incident where the contractor was welding some steel support inside the tunnel and a pocket of methane causing an ignition. It was an unfortunate incident, says Tsztoo. But it was fortunate that no one got hurt. It was also fortunate that SFPUC had specified all tunneling equipment to be fit for gassy conditions. During the design phase we decided that because of the time it would have taken to convert, we would ask for full on gassy equipment from the get-go, says Tsztoo. The ventilation system did have to be upgraded, however. We knew that specifying full on gassy tunnel equipment would be more expensive but by specifying the equipment, but not the ventilation system, we halved the risk. As it turned out it, was the right call. We saved significantly on time and delays re-tooling the equipment and did not suffer near as huge a delay, says Tsztoo. The project was only put back by about two weeks, says McMaster: one week while CalOSHA, California s health and safety authority, carried out an investigation and changed the tunnel s classification and then a further week to upgrade the ventilation. However, the gassy classification has caused delay over the project s duration. The change meant that assigned personnel had to be monitoring gas at all times at the tunnel heading, and that activities such as surveying can only be carried out during hot work permit periods. Tunnel refuge chambers must also be installed every 5000ft (1524m). While work at the tunnel s eastern end continued, the contractor also mined the western section of the tunnel, working from the Irvington Portal and Vargas West headings, with that section holing through in June At that point, one of the roadheaders moved to Vargas East to start working in towards the Alameda West portal heading. Contractor Southland/Tutor Perini had a 14 NORTH AMERICAN TUNNELING JOURNAL

5 Figure 1: Generalized geological profile of the New Irvington Tunnel (marked in yellow) and the existing tunnel Mill Creek fault zone Fault Unnamed zone C Vargas shaft fault zone Existing Irvington Tunnel Fault zone B New Irvington Tunnel Fault Sheridan Pirate Creek zone A Creek fault zone fault zone New overflow shaft Reach 8 Reach 6 Reach 5 Reach 4 Reach Reach Older quartenary alluvium Claremont formation chert and shale member Reach 2 Reach 1 Briones formation Tice shale Oursan sandstone Claremont formation sandstone member Cretaceous sandstone and shale Fault zone choice of primary support systems. We thought they would go for shotcreting and lattice girders or shotcrete and wire mesh, but they decided to keep everything very simple for the miners and use standard steel ribs with timber blocking throughout, says McMaster. There are four ground classifications set down in the Geological Baseline Report (GBR) ranging from Class I, which is hard rock, to Class IV, which is very unstable rock. Class I requires steel sets up to 5ft (1.5m) centers; Class II sets and Class III at 4ft spacing (1.2m) centers; and Class IV requiring sets at 2.5ft (0.75m) centers. Where the ground has been unstable, the contractor has used a variety of means of presupport to stabilize the heading, such as spilling rods and channel spiles inserted into the crown of the tunnel to allow him to excavate underneath. Other techniques in the toolbox included breasting the front face, using jacks to push off the steel sets and using the top heading and bench method of tunneling. In an industry where TBMs are becoming more and more commonplace, this job required old-school miners who could adapt Figure 2: Location map showing the New Irvington Tunnel project and the existing tunnel Contra Costa Berkley Oakland Hayward Alameda Fremont 680 San Mateo Santa Clara Bay Division Pipelines 1 and 2 Bay Division Pipelines 3 and 4 Existing Irvington Tunnel Irvington Portal Vargas shaft quickly to changing ground conditions. There have been days where we have started with drill and blast, and by that afternoon we have been into Class III and Class IV ground, says McMaster. It was very difficult to find experienced miners - nowadays they are New Irvington Tunnel Alameda West Portal pretty much TBM-oriented and like the idea of working inside a TBM. Southland were able to find a group of experienced guys who then got onboard and trained a number of new guys who came in. Having completed the section between the Irvington Portal and the Vargas Shaft by June 2012, Southland/Tutor Perini decided to install the final liner welded steel pipe in that section. The original specification called for steel pipe only at the points where the tunnel crossed the faults and at the portal locations, with the choice of cast-in-situ concrete or concrete pipe elsewhere, but the contractor elected to use steel pipe throughout. Southland/Tutor Perini were able to offer the steel lining at no extra cost, by doing a deal with supplier Northwest Pipe and taking into consideration the increased efficiencies from having just one type of lining system. SFPUC really jumped at that, says McMaster. It gives them such a quality tunnel. We will never have to worry about water quality from here on out, says Tsztoo Before the steel pipe, installed in 50ft (15m) sections, could go in, areas of the tunnel were shotcreted and drainage installed in the invert, to take groundwater out of the tunnel. After the steel pipe was welded into place, the contractor pumped cellular concrete into the NORTH AMERICAN TUNNELING JOURNAL 15

6 annulus, followed by contact grouting; which involves injecting neat cement through holes drilled through the crown of the tunnel at 10ft (3m) centers. By autumn 2014, the final finish to the entire tunnel will be applied: a skin of 5/8-inch cement mortar, which is spray applied and finished by robotic equipment. Lowering the groundwater There was one particular area along the tunnel s alignment where everyone was expecting challenging times: the Sheridan Fault, below Sheridan Creek, which lies around 4000ft (1200m) in from the Alameda West portal. The challenges expected here were due to the combination of high groundwater pressure and intensely fractured rock. Around the Sheridan Creek area Southland/ Tutor Perini drilled surface wells to draw down the groundwater table before tunneling took place. This allowed the water level to be lowered to 30ft (9m) below the tunnel invert. Perhaps due to the dewatering, passing through the Sheridan fault zone proved to be far less troublesome than anticipated. We were a little surprised that the zone through Sheridan Valley was not as squeezing as we had thought, but that could be to do with lowering the groundwater, says McMaster. We were able to excavate the entire 400ft (120m) Sheridan zone in the dry and we had some of our best mining production to date in that zone. Conversely, in other locations there has been far more groundwater than expected. For instance, 300ft (91m) in from the Irvington Portal, the contractor experienced a sustained inflow of water, far more than the 25 gallons per minute set down in the GBR. We tested the water and it showed that it was coming straight from the existing operating tunnel, which was 75ft (23m) above the new tunnel being excavated, says McMaster. There must have been leaks in the cast-in-place lining. Once all the excavation is complete, it will be time for contractor and client to agree how much of each of the four types of ground there have been and how this compares to the GBR. The jury is still out in terms of the Neighborhood watch There is one aspect of the New Irvington Tunnel that bears little resemblance to the construction of the original tunnel: community relations. Whereas in 1928, the attitude was just to get on with it, the goal now has been to get everyone on board. We started outreach during environmental scoping in 2006, says Betsy Lauppe Rhodes, Regional Communications Manager for the Sunol Projects. Sunol Valley, at the east end of the tunnel, has had no less than 10 construction projects to contend with. The New Irvington Tunnel is the second largest, the Calaveras Replacement Dam being the largest. Among those affected by the construction of the tunnel are people who live close to the Irvington Portal, at the tunnel s west end, which leads right into the city of Fremont. Meaning there are million dollar homes, and a noisy, dusty construction site, existing side-by-side. One of the measures that has helped allay some potential issues has been the erection of a sound barrier. However, rather than a standard wooden structure, which would have cast a shadow onto residents back yards and blocked their views of the mountain, the SFPUC specified a clear polymer sound wall. The sound wall was about five times the cost of a standard wooden sound wall, but it was worth every penny in keeping the peace with the neighbors and encouraging good public relations, says Tsztoo. The other group of neighbors who were potentially affected were local land owners with groundwater wells in the Sheridan Valley. Dewatering wells aimed at reducing groundwater inflows in the tunnel were expected to impact local wells. Specialist groundwater consultants modeled the entire valley and identified up to 34 properties that could be affected. A two-year monitoring program established baseline levels for each property, and then a Groundwater Management Plan was developed for every one of them in case dewatering had an impact. As predicted, some wells were affected. Mitigation measures included lowering water pumps deeper into well casings, performing remedial maintenance on the well systems, providing storage tanks, and commercial water truck deliveries to periodically fill the tanks. A four-inch (100mm) irrigation line also takes water from the dewatering wells to 12 properties which require it for livestock, crops and landscaping. Residents can call a 24 hour helpline, seven days a week, to speak to someone if they have an issue. And Tsztoo and McMaster have made sure that everyone knows what is going on. David and Dan have invested a lot of time and effort, says Lauppe Rhodes. They have been walking the talk, responding to calls and going out there to check wells. Such things are vital if a project is to avoid years of delay. Unlike the original Irvington Tunnel, public relations have been much improved and cordial, says Lauppe Rhodes. Crews hole-through between the Irvington Portal and the Vargas Shaft final quantities of the four ground classifications, and how they compare to the quantities assumed in the GBR, says Tsztoo. The construction contract specified that the quantities of ground classes be evaluated over the entire tunnel alignment, after completion of excavation. With the Sheridan Fault proving less of a problem than planned, and the lack of issues with groundwater in this zone, some might still argue that a TBM would have been a more efficient way to construct the New Irvington Tunnel. But there is no way of saying whether a TBM would have been as successful. There have been numerous patches of squeezing ground, in particular in the heading from Alameda West. And groundwater provided challenging moments in all the headings. Assuming the last, difficult stretch, goes well, traditional mining techniques have served San Francisco s residents well for a second time within the last century. The project has been constructed successfully and safely, with a lower recordable incident rate than the industry average. This job has been very successful with the conventional tunneling approach, says McMaster. 16 NORTH AMERICAN TUNNELING JOURNAL