When most people think of rehabilitation they usually mean not much more than a makeover, and then rarely more than once. In the case of the Cal Park Hill Tunnel near the north shores of San Francisco Bay, this disused infrastructure link is about to commence a third and double life following its second reconstruction since originally driven as a single-track rail tunnel in 1886. This time reconstruction was necessary not only to transform its use, but also secure a roof collapse and other structural instability in the area.
The renewed passageway under California Park is now ready to accept pedestrian and cycle traffic on one side while the other side is set to start installation next year for SMART—the Sonoma Marin Area Rapid Rail Transit.
Both passages will facilitate residents’ travel from San Rafael and other communities on one side of the Cal Park Ridge to the Bay area side and, in particular, the Larkspur Ferry to San Francisco and other Bay-side communities. This is also adjacent to the famous San Quentin federal prison.
History
Cal Park Hill Tunnel carried a single-track railroad from 1886 to 1924, when it was widened to accommodate two tracks. It carried passenger traffic until the 1950s and freight until the 1990s.
One of the tracks was removed in 1970 but continued tunnel use was halted by a fire in the 1990s fuelled by timber supports in the tunnel, resulting in the loss of around 30 per cent of the support. The fire burned on through the night and caused structural weakness that resulted in a collapse practically through to the surface and 300ft (100m) behind the south portal.
Then a portion of the tunnel some 8ft (2.4m) long near the north portal caved in due to rotting timber supports. This ‘sinkhole’ even carried a tree down with it, while two smaller sinkholes had appeared near the south portal.
The tunnel has therefore been out of use ever since. Jacobs Associates were engaged to provide inspection and rehabilitation design services for the south end of the tunnel immediately following the fire, but to a limited extent for mainly safety reasons. In 1994 and 1995 the consultants were again involved to design repairs to the north portal and to install a new security gate. This was followed in 1998 by conceptual cost estimates for tunnel repair and stabilization to prevent further deterioration and collapse.
State of affairs
The tunnel is 1,100ft (335m) long, 21ft (6.4m) high, and was wide enough for the double-track railroad. It runs in the Franciscan Formation that, in this area, tends to contain more hard material although still very mixed and structurally weak. The materials are generally weak shale, sandstone and some bands of hard greywacke, although the tunnel collapses also contained soil mixed with the rock.
Fortunately the early post-fire actions above prevented total collapse and allowed the successful reconstruction. In the collapsed zones the mixture of hard and soft material made it very difficult to excavate safely without further structural deterioration.
An additional concern was the proximity of the 101 Highway that runs close to and practically parallel to the Cal Park Hill Tunnel. Caltrans (the California state transport authority) wanted no settlement to the carriageway and so stabilization of the embankments at the tunnel approaches as well as within the tunnel itself were high priorities.
Reconstruction
In 2008 Jacobs Associates was engaged to provide construction management services including project management, resident engineering, field inspection, independent quality control testing and office engineering. Ghilotti Construction won the contract to carry out site clearance, temporary support, ground stabilization, tunnel reconstruction to design and regrading of portal approaches for both the multi-use pathway and the railroad.
Due to the fire incident the owner and project managers were keen to remove all timber from the tunnel and to use only steel and concrete in its reconstruction.
Resident staff engineer with Jacobs Associates Erin Clarke explains that the contractor, Ghilotti Construction, was responsible for temporary support for working safety but that the type and degree of permanent support required was determined by the engineering team according to the conditions discovered. With the ban on timber, there is a reliance on steel arch ribs, sprayed concrete and in situ concrete.
“The contractor used mainly shotcrete over one to four sets of steel ribs at 4ft (1.22m) centers. There were also voids behind this support that had to be filled. The standard shotcrete support was two layers of steel-fiber reinforced concrete, each 4in (100mm) thick. There was up to 6ft (1.8m) of overbreak behind the steel sets in places. In these areas we used Stay-form panels, sprayed shotcrete over them and then filled behind.”
AMICO Stay-form is produced by the Alabama Metal Industries Corp in galvanized steel sheets, ribbed and expanded, used as formwork, which is left in place. The special ‘Shot-form’ is a mesh specially designed for sprayed concrete to minimize pass-through and reduce rebound.
With the new grading of pathway and railroad bed carried out in advance of tunnel work, the reconstructed tunnel is now increased from 23ft (7.0m) high above grade to 28ft (8.5m), and 33ft (10.1m) total width. This allows for a 6in (150mm) layer of hardcore aggregate at the carriageway.
Pedestrian passageway
The pedestrian and cycle way passage of the tunnel is of sophisticated design with many features and equipment to improve the environment and security, so to encourage use. The division between this and the rail passage is built of concrete block masonry (CMU). This is designed to resist train noise at the full planned speed of 45 miles/h (72 km/h).
The rectangular section of the passageway changes to a transitional arch at each portal for user protection and light intensity transition. The opposite (rock) surface of the passagway is sprayed concrete with a trowel finish to remove rough surfaces.
A cable-carrying passage in the crown is formed of a deck of lightweight glass fiber-reinforced concrete. Two switchgear and control rooms have been formed along the side of the tunnel with adjacent cable-duct access points. These are made fully waterproof by a bentonite seal and have air conditioning installed. There is also an emergency power supply back up.
Equipment in the multi-use passage includes emergency phones (two linked to stations in each direction), a linear heat detector also linked in both directions, fire-fighting equipment, cctv cameras, and six fans in the crown for maintenance ventilation to remove any fumes, etc. Other communications include automatic dial emergency call out and a ‘leaky feeder’ radio booster for use by maintenance personnel and police Wi-Fi.
The path continues on the surface for a total of 2 miles (3.2km).
Rail route
The SMART rail link received county approval in March and is still on schedule for installation in the tunnel next year. The full route will eventually run 75 miles (120.7 km) to the north linking with Sonoma, famous as the place of first declaration of Californian independence from Mexico. From the south tunnel portal a new station will be built at Larkspur Ferry terminal.
Expectant users
When T&T North America visited the Cal Park Hill Tunnel there were (unsolicited) conversations on the Larkspur Ferry from San Francisco about when the tunnel pathway would eventually be opened— evidence of an expectant public with many wanting to be first through. But Clarke has been reported to say that she is determined to be first through on her bike!
Portal plaque indicates the dates of the last reconstruction-1924-and the latest-2010 The rail compartment of the Cal Park Tunnel showing sprayed concrete sidewall support and the concrete masonry divider to the pedestrian side (right) [Photo: Jacobs Associates – Sue Bednarz] Looking south through the completed multi-use pathway passage [Photo: Jacobs Associates – Sue Bednarz] Cal Park Tunnel south portal showing multi-use pathway transition structure [Photo: Jacobs Associates – Sue Bednarz]
