The fourth bore of the Caldecott Tunnel is being undertaken by the California Department of Transportation (Caltrans) in association with the Contra Costa Transportation Authority (CCTA) to relieve congestion on State Route 24 (SR 24) through the Berkeley Hills between Oakland and Orinda/Walnut Creek in the eastern margin of the San Francisco Bay Area. The new fourth bore is located north of the three existing Caldecott Tunnel bores on SR 24. The selection of the tunnel alignment was based on technical considerations, including avoiding the Slope stability issues that exist north of the exiting bores as well as third party considerations, which included impacts on existing property owners and recreational facilities.
The project tunneling elements include:
• The mined tunnel portion of the fourth bore is 990.4m long. This portion has a horseshoe shape with excavated dimensions of approximately 15m in width and 11 to 12.3m in height (see figure 1). These dimensions were selected to accommodate the fourth bore roadway clearance envelope 5.1m in height; jet fans; the travel way, which includes two traffic lanes that are 3.6m wide and two shoulders that are 3m and 0.6m wide; a 1-m-wide emergency walkway located adjacent to the south shoulder; and a curb adjacent to the north shoulder.
• Seven cross passages between the fourth bore and the existing bore no. 3. The horseshoe-shaped cross passages are 32.7 to 44.1m long. The excavated dimensions of the cross passages are about 4.2m wide and 4.7m high.
The geology of the alignment is characterized by northwest-striking, steeply dipping and locally overturned marine and non-marine sedimentary rocks of the Middle to Late Miocene age. The western end of the alignment traverses marine shale and sandstone of the Sobrante Formation (see figure 2). The middle section of the alignment traverses chert, shale, and sandstone of the Claremont Formation. The eastern end of the alignment traverses non-marine claystone, siltstone, sandstone, and conglomerate of the Orinda Formation.
The fourth bore alignment will encounter four major inactive faults. These faults strike northwesterly and perpendicular to the tunnel alignment. In addition to these inactive faults, the active Hayward fault is located 1.4km west of the Caldecott Tunnel. This is the closest major fault to the project site, capable of producing a magnitude 7.4 earthquake.
The structure of the rock mass units along reaches of the alignment is expected to vary from being blocky in the best ground, down to a disintegrated or crushed condition in the poorest quality rock. Average values of unconfined compressive strengths for the different geologic units along the tunnel alignment are expected to vary from 9.6MPa (1,400 psi) to 48.5MPa (7,000 psi) in the various geologic units along the alignment. Rock mass ratings (RMR- Bieniawski, Z.T. [1989], Engineering Rock Mass Classifications, Wiley, New York) are expected to vary between 20 and 65 along the alignment.
Tunnel design
The design and construction of the fourth bore are based on the sequential excavation method (SEM), also called the New Austrian Tunneling Method (NATM). Jacobs Associates of San Francisco prepared the NATM design for the fourth bore. ILF Consultants of Oakland, California, prepared the NATM design for the cross passages between the fourth bore and bore third bore.
The ground classification process for the NATM design was twofold: identification and characterization of rock mass types (RMT) along the alignment having similar mechanical characteristics, and identification of ground classes based on similarity of anticipated ground behaviors of each RMT in response to excavation. An appropriate support category was then developed for each ground class. For example, Ground Class One comprises all RMTs along the alignment that require Support Category I. Ground Class Two correlates to Support Category II, and so on. Individually, the support categories address sets of similar ground behaviors, and as a whole they address all anticipated ground behaviors along the alignment. The ground classes were the basis of design for the initial support categories. Actual ground classes along the alignment, and hence support category application, are determined during construction based on probe drilling ahead of the face, geologic mapping of the tunnel, and tunnel monitoring.
NATM excavation sequences and support designs were developed for four support categories and three subtypes (IA/B, IIA/B, IIIA/B, and IV) corresponding to the ground classes. The overall excavation and support sequence design utilizes a top heading and bench. The top heading excavation is full face with a sloping core or face dowels providing face support. Advance lengths in the top heading vary between one and 1.8m. The bench excavation design allows for full width, split bench, and center cut options, depending on the support category and the lag maintained between the top heading and bench. Advance length in the bench is twice the advance length in the top heading. A minimum lag between the top heading and bench is required to ensure equilibrium of the top heading under biaxial loading before additional loading is introduced as a result of the excavation of bench drifts. The advance lengths are primarily controlled by anticipated ground stand-up time and the size of the drift.
Convergence-confinement analyses were performed using Fast Lagrangian Analysis of Continua (FLAC) to determine the required thickness of shotcrete lining and the length of rock dowels in the various support categories. The FLAC analyses simulated the excavation sequence, installation of perimeter rock dowels, the sequence of shotcrete application, and the strength/stiffness gain of the shotcrete with time. The analyses also incorporated an elastic-plastic material model to simulate the inelastic behavior of shotcrete. The models were used to estimate the moments and thrusts that develop in the shotcrete lining and the loads that develop in the rock dowels. The factored loads in the shotcrete lining were plotted on thrust-moment interaction diagrams to verify that the loads are less than the factored capacity of the lining. FLAC3D models of the full NATM excavation and support operation in each support category were used to estimate the amount of relaxation that occurs in the ground ahead of drift headings, evaluate face stability, estimate the required bench lags, and evaluate pre-support performance.
Depending on ground conditions along the alignment, the initial support system includes 200 to 300mm fiber reinforced shotcrete (FRS) initial lining, 50 to 100mm of face sealing FRS, fiberglass face dowels, drill-and-grout and self-drill-and-grout rock dowels, lattice girders, and spiles in various combinations. In addition the portal areas are pre-supported with pipe canopies, and invert arches are required in both the top heading and bench for limited reaches of poor ground anticipated adjacent to the west portal and central portions of the alignment.
The support design also includes additional support measures, as necessary to address local variations in ground conditions or behaviors. These additional support measures are installed when measured convergence exceeds warning levels or when specific ground conditions or support system behaviors are observed as defined by the design.
The Caldecott fourth bore uses a double lining system consisting of an initial support system (discussed above) and a cast-in-place reinforced concrete final lining (figure 1). A waterproofing membrane with a geotextile backing layer for drainage will be installed between the initial support and the final lining. The initial support system is designed to carry the ground loads that develop during construction, while the cast-in-place reinforced concrete final lining is designed to carry long-term ground loads and any additional loads due to interior finishes or equipment anchored to the lining.
The initial and final linings will function as a combined support system in the long term. Over time, after the completion of construction, a portion of the ground load carried by the initial support system will be transferred to the final lining because of deterioration of the initial support system.
The final lining will also accommodate seismic deformations and provide a durable and sound tunnel lining. In accordance with general Caltrans practice for ‘important’ facilities on lifeline routes such as State Route 24, the seismic design for the tunnel is based on the Safety Evaluation Earthquake (SEE) and a lower-level Functional Evaluation Earthquake (FEE). The project uses a 1,500-year return period for the SEE event and a 300-year return period for the FEE event. Seismic demands do not control the thickness of the final lining, despite the close proximity of the project to a major active fault and seismic design criteria corresponding to an earthquake with a 1,500-year return period and a peak ground acceleration of 1.2g.
Construction methods
Tunnel excavation from the east portal is being performed using a Wirth T3.20 heavy duty road header as the primary excavation equipment. The Wirth road header was shipped in parts from the manufacturer in Germany to the Port of Oakland and assembled on the jobsite. The selected road header is rated to effectively cut rock with UCS strengths up to approximately 150MPa (21,750 psi). The contractor has chosen a heavy duty road header to avoid a potential switch to a drill and blast operation which has contractual limitations to prevent impacting third parties. So far, all encountered rock formations have been successfully excavated by the road header. A RDH 200DH Drillmaster and a 200DH Boltmaster are being used for drilling. This drilling equipment is being used for spiling, face bolting, and probe drilling ahead of the face as well as for drilling radial rock dowels. The muck is loaded either directly into RDH Haulmaster trucks by the road header’s conveyor belt or is loaded by a Sandvik LS175 load haul dump.
The major excavation equipment at the West Portal is a CAT 330 hydraulic excavator with an Antraquip cutterhead attachment. Mucking operations are currently utilizing a Sandvik LS175 load haul dump.
The gassy classification of the tunnel imposed special requirements on the tunneling operations, including a requirement for explosion proof equipment, and stringent ventilation requirements and safety procedures.
Personnel experienced in SEM tunneling on the Contractor’s side are responsible for selecting the appropriate support category based on the ground conditions and observed or anticipated behaviors. Approval of support selection by the Engineer is based on an independent assessment by the Owner’s SEM team, led by the Owner’s NATM Engineer, Gall Zeidler Consultants of Walnut Creek, CA and supported by the designer of record, Jacobs Associates. The ground conditions are classified and predicted based on the preconstruction investigation program; detailed geological documentation of each tunnel face; systematical probe drilling ahead of the face that incorporates a data logger system to monitor key drill parameters; deformation monitoring with convergence measurements, extensometers, and surface settlement points.
Excavated materials suitable for reuse are disposed of at Treasure Island in the San Francisco Bay. Excavated materials contaminated with natural hydrocarbons are disposed of at the Keller Canyon Landfill in Pittsburgh, CA.
Current progress
The project was awarded on November 10, 2009, to the general contractor Tutor Saliba Corporation (TSC) of Sylmar, CA. The surface clearing, retaining wall work, and preparatory work in the portal areas began on January 13, 2010. TSC is advancing the tunnel from portal two (east portal) (figure 2). Tunnel turn under at the East Portal commenced on August 12, 2010, after installation of a pipe canopy. The top heading passed from the Orinda Formation into the Claremont Formation (figure 2) in the first week of March 2011. In the beginning of May 2011, the top heading excavation from the East Portal passed the point of the highest overburden at roughly 450m. Support categories installed in the Orinda Formation were primarily IB, IIA, and IIB. A minor portion was supported with support category IIIA. Local overbreaks at the tunnel crown, instabilities at the tunnel face and partially higher convergences were addressed with the contractual additional support measures, such as spiles, face dowels, and rock dowels. The ground conditions encountered to date have generally been as predicted in the Geotechnical Baseline Report.
On average, roughly two rounds, or cycles, per 24-hour working day have been achieved for the top heading excavation so far. The total progress rate depends on the advance length per round as well, which varies from 1m, 1.4m or 1.8m per advance.
The tunneling from the west portal in the Sobrante Formation (figure 2), which is expected to comprise roughly one third of the complete tunnel length, as well as the mining of the cross passages, has been subcontracted to Foxfire Constructors, of San Clemente, CA. The pipe canopy installation at portal one (west portal) was finished in the first week of March 2011, and the tunnel turn under at the west portal started on March 7.
Since the beginning of May the top heading from portal one has advanced approximately 30m through the most challenging geology that will be encountered along the tunnel alignment that requires construction of a temporary shotcrete invert within the top heading.
The tunnel is scheduled to be opened to traffic in the later part of 2013.
Construction challenges
The most significant construction difficulty encountered to date has been installation of a 55m pipe canopy at portal one (west portal) to the required line and grade.
Several challenges on the heading from portal two (east portal) have been successfully tackled. These include:
• Additional support rock dowels installed over 50m behind the top heading face to control cracking of the initial lining that occurred when the face had advanced approximately 70m from the location of the cracks – swelling of the ground in response to the changed groundwater regime or groundwater loads on the shotcrete lining are two of the postulated causes for the cracking;
• A high flush flow event at the heading;
• The use of face-supporting fiberglass dowels in lieu of the face-supporting core in Support Category III;
• Maintenance of over 430m of slickline to supply shotcrete at the face from a pump at portal two.
Figure 1, typical section of the Caldecott fourth bore Figure 2, geological profile of Caldecott bore fourth bore The final lining will accommodate seismic deformations and provide a durable and sound tunnel lining The tunnel is scheduled to be open to traffic in the later part of 2013
