The Claremont Tunnel is the main water transmission facility of the East Bay Municipal Utility District (EBMUD). The 5.4km tunnel is located beneath the Oakland-Berkeley Hills between EBMUD’s Orinda Water Treatment Plant (WTP) and the Claremont Center in Berkeley, California. The tunnel conveys up to 175M gallons per day of treated water from the Orinda WTP to over 800,000 customers in Richmond, Oakland, San Leandro, and surrounding communities.
Originally constructed between 1926 and 1929, the Claremont Tunnel has a 2.96m diameter horseshoe cross-section and a nominal 300mm thick concrete final lining. The majority of the tunnel lining was not reinforced, and only a small portion of the final lining was contact grouted to fill in voids between the lining and the surrounding ground.
The tunnel crosses the Hayward Fault approximately 43m below Tunnel Road in Berkeley, and is highly vulnerable to damage from major earthquakes along the fault. In a moment magnitude (Mw) 7.0 earthquake, the tunnel must accommodate up to 2.45m of horizontal offset and approximately 165mm of vertical displacement along the primary trace of the fault, and up to 0.7m of sympathetic movements within a 302m secondary fault zone straddling the primary fault. In addition to discrete fault offset, 328mm of active fault creep had to be accommodated within the design. Such displacements would cause collapse of the existing tunnel lining and likely block water flows for many months until repairs could be completed.
In 1994, EBMUD approved a US$189M Seismic Improvement Program (SIP) to proactively reinforce and protect its water system from the damaging effects of earthquakes, and avoid severe disruptions to its customers’ water service. The Claremont Tunnel Seismic Upgrade Project became a key element of the programme. Completion of the tunnel as T&TI went to press is the last major piece of the SIP. The programme will have met its goal of ensuring that sufficient water is available to customers following a major earthquake.
During the design phase of the project, contact grouting and lining repairs were planned for the existing tunnel and construction of a 516m Bypass Tunnel was planned as a replacement for the most vulnerable part of the tunnel – the section that crosses the Hayward Fault Zone (HFZ). A 151m access adit was planned at Claremont Center to provide construction access to the Bypass Tunnel and future access for maintenance.
The Bypass Tunnel features a special vault section spanning the active strand of the Hayward Fault and is designed to accommodate large earthquake movements along the fault (figure 1). This article generally discusses the design and construction of the Bypass Tunnel but focuses particularly on the construction of the vault section and the ground condition challenges encountered. For details about other portions of the project, which included contact grouting and repairs to the existing tunnel, and a thickened tunnel lining through the adjacent Subsidiary Fault Zones (SFZ), refer to Caulfield et al. 2005[1].
Project construction began in July 2004. The Bypass Tunnel and Access Adit were excavated between September 2004 and October 2005; vault excavation occurred between late April and mid-August 2005. The final tie-in connections to the existing Claremont tunnel were made while it was taken out of service in December 2006. Project completion is scheduled for June 2007.
Geology and ground conditions
The anticipated HFZ location and width were based on regional USGS maps, surface mapping performed and borings drilled for this project, historic logs for the BART Oakland-Berkeley hills transit tunnels and the original Claremont tunnel, and fault creep documented during prior inspections of the existing tunnel. Within the anticipated 302m HFZ, the maximum primary displacement zone, or active strand of the fault, was expected to span a length of 19.7m of the Bypass Tunnel alignment. The rest of the HFZ consisted of subsidiary fault zones[2,3].
On the west side of the main active strand, the tunnel was excavated through alluvial and colluvial Quaternary sediments as well as Franciscan mélange and silica carbonate rock. East of the main active strand, serpentinite was encountered; at first, it was pervasively sheared and crushed, but it became more blocky and less deformed at the eastern end of the side drifts[4,5]. The serpentinite encountered during the construction of this project contained small amounts of naturally-occurring chrysotile asbestos and heavy metals, which required special handling and disposal of muck, as well as personal protective equipment for construction personnel.
Additionally, it was determined during the design that the tunnel would be classified as ‘gassy’ by the California State OSHA Mining and Tunneling Unit, based on documented gas intrusion in the original Claremont Tunnel construction. This required that equipment used for the tunneling had to be rendered non-flammable, spark arrested, or acceptable to Cal/OSHA.
Seismic offset vault design
The Bypass Tunnel was designed to withstand offset due to seismic activity and remain in service to provide essential water for service immediately after an earthquake. The size of the vault section was based on the anticipated fault zone width and anticipated offset. It is a 35.5m section of the tunnel alignment with finished inside width of 5.4m. This section can accommodate the entire 2.8m lateral design offset in one location. However, shear fuses were incorporated into the final concrete lining to provide weak zones, encouraging rupture every 3.6m along the vault, in order to provide some residual structural capacity that will help facilitate repair and removal of debris after the earthquake.
A steel carrier pipe was included in the design to keep rock and concrete debris out of the water flow. The pipe is 27.9m long, has an inside diameter of 1.9m, with a 760mm wall thickness, and is lined and coated with cement mortar. If the vault completely collapses in a seismic event, this carrier pipe will support the full overburden load of the Slope above it. It is designed to pass the essential lifeline flow of at least 130M gallons of water per day.
The initial support and excavation sequence was intended to allow an enlarged opening to be constructed in crushed and squeezing ground (as defined in the Geotechnical Baseline Report[3] and based on Terzaghi’s ground classifications), and to protect the surrounding rock from erosion following offset. Egg-shaped side drifts approximately 3.28m wide and 4.6m high were designed to be excavated for the entire length of the vault and backfilled with concrete prior to vault excavation. Following seismic offset, these side drifts are intended to protect the surrounding rock from potential erosion caused by water passing through the tunnel, and to mitigate intrusion of the surrounding rock material into the water flow (figure 2).
Following backfill of the side drifts, the vault was anticipated to be excavated using top heading and bench construction sequencing. The steel sets for initial support were designed with a special shape so that they would transfer ground load to the concrete in the side drifts, rather than to the crushed and squeezing ground. They were designed to be installed on 1.2m centers, but a provision was made in the contract requiring that the contractor furnish jump sets, or additional sets enabling installation on 650mm centres, for the entire length of the vault to protect the tunnel excavation from squeezing ground.
Enlarged vault in challenging ground
Excavation of the Bypass Tunnel and the vault was primarily completed by mechanical means with an Alpine AM 75 roadheader (and AM 50 roadheader for the tie-in connections). Where harder rock was encountered, excavation was occasionally supplemented by drill-and-blast. In some areas of softer ground, IHI mini-excavators fitted with bucket, hoe-ram, and Voest-Alpine mini-roadheader attachments were used in lieu of the roadheader. Cover over the tunnel ranged from 14.8m to 75.6m, and water inflow in the excavation heading was less than 30 gallons per minute during the entire excavation. The main active strand of the fault was essentially dry.
The first activity for the construction of the vault was widening the bypass tunnel in a 6.5m long inbound transition from Sta. b9+49 to b9+29. Custom steel sets were fabricated by American Commercial on a slightly different spacing than was called for in the design. The transition required the contractor to modify the mining cycle, and it was challenging to use even the mini-excavators to mine a face wider than the previously installed steel set. Spiling was also required over the entire 6.5m long transition from the last Bypass Tunnel steel set, as well as over the outbound transition from Sta. b8+01 to b7+81.
Ground support at vault turnunder
The side drifts and vault were required to begin at approximately Sta. b9+29. The design called for a nominal 4.9m pillar to be maintained between the two side drifts. The contractor installed approximately 200mm to 255mm of steel fibre reinforced shotcrete on the entire tunnel face, and further reinforced the pillar with welded wire fabric pinned up with 1.2m split sets. Grouted R32S hollow, self-drilling spiles were also installed over the last transition steel set at Sta. b9+29, between approximately the 10 o’clock to 2 o’clock positions. Spiling was not required in the remainder of the main vault. Following the installation of initial and presupport, the left side drift excavation began.
The shape of the side drifts was modified slightly from that shown on the drawings to provide clearance for ventilation lines and allow continued use of Wagner ST 3.5 Scooptrams (LHDs) for muck removal. Two IHI mini-excavators were used to mine the side drifts. Their electrical and exhaust systems were modified by the contractor to conform to the Cal/OSHA gassy tunnel permit. The side drifts were excavated in a top heading and bench sequence, with the bench taken out every third 1.2m round, or every 3.9m. They were supported with 2.6m grouted fibreglass rock dowels and steel fibre reinforced shotcrete. The fibreglass rock dowels in the interior top heading area were subsequently removed during excavation of the top heading.
Modified ground support
The instrumentation and monitoring program for this portion of the project included convergence points read by tape extensometer on a daily basis. In early June 2005, readings taken from the convergence points installed at Sta. b8+55 and Sta. b8+30 in the left side drift showed that the side drift was converging at a slightly accelerating rate. It was assumed that this convergence extended no further that Sta. b8+80, as the convergence point at that station showed virtually no movement. The contractor quickly completed the excavation of the remainder of the drift to Sta. b8+01, and placed concrete backfill in approximately the bottom 0.98m of the drift. The convergence was arrested immediately, and the remainder of the drift was backfilled with concrete. Because this convergence likely indicated the presence of squeezing ground, the EBMUD engineer required installation of jump sets in the vault between Sta. b8+82 and Sta. b8+01 (figures 3 and 4).
Sequence changes
The original contract documents specified that the left side drift would be excavated first. Then, while the left side drift was being backfilled, the right side drift would have been excavated. However, during construction, the combination of reasonably stable ground conditions and the desire to make up time in the schedule drove a change in the contract which allowed the drifts to be excavated simultaneously with a minimum lag distance of 9.8m between excavation faces. Once the drifts were completed, the 6.5m wide vault top heading was excavated for the entire vault length, and the bench was removed from the heading end (Sta. b8+01) backward to Sta. b9+29.
Geological mapping was important to verify the locations of the Hayward Fault primary strand and the vault, and the final location of the carrier pipe which would later be installed. The side drifts and the vault were mapped by geologists from Geomatrix Consultants and EPC Consultants during the brief time between excavation and placement of initial support shotcrete[5]. This mapping of the Hayward Fault primary strand verified the location of the vault and was used to determine that the optimal location for the carrier pipe was about 4.2m west of its design location (originally centered about Sta. b8+55; moved to b8+68).
Conclusions
The Claremont Tunnel Seismic Upgrade Project has safeguarded the water lifeline of over 800,000 EBMUD customers. The project offers assurance that an essential, adequate water supply will be available immediately after a major earthquake.
The project also revealed new information about the Hayward Fault. Much information about the fault has been developed over the last several decades through studies, geologic investigations, and construction projects. However, as this project demonstrated, there is still much to be learned, as evidenced by the need to shift the carrier pipe some 4.2m to match the approximate center location of the Hayward Fault primary strand.
The ground conditions encountered during tunneling for this project may have added incrementally to what is known about constructing tunnels through active faults. More likely, the project exposed how much more there is to learn about in-depth fault ground conditions, and why innovative, practical means are necessary to overcome the challenges they pose to tunnel construction.
Left and right side drift excavation Drift Roadheader in vault top heading Roadheader Mining the Vault bench Mining Tunnel face at Sta.b8+80 Tunnel Cast-in-place concrete and reinforcing in Vault. Inside geometry was slightly modified so the contractor could reuse the Bypass tunnel steel forms Cast
