Devils Slide Lee Bowers Ross Maguire

Location Route 1, California. 20km south of San Francisco. ‘‘Callibro Highway’ links Montara to Linda Mar. Route runs through several areas of ‘natural beauty’. ‘Devils Slide’ runs along coastal route that is vulnerable to landslides.

California Landslide Hazards California has a history of frequent landslides throughout the state. Most of the Coastline is in a Moderate or High Landslide Incidence region. ‘Devils Slide’ is in a region of High Landslide Incidence. Not only do they occur often, these landsides can be quite severe. Incidence and Susceptibility Low Medium High High susceptibility, Low incidence

The Problem This section of route 1 replaced the steep, narrow and dangerous Pedro Mountain Road in This section of route 1 replaced the steep, narrow and dangerous Pedro Mountain Road in The first Landslide occurred in 1940, destroying much of the road. The first Landslide occurred in 1940, destroying much of the road. The road has suffered from a cycle of regular destruction and reconstruction due to the landslides. The road has suffered from a cycle of regular destruction and reconstruction due to the landslides. These landslides have thrown the stretch of road into the public eye – a more permanent solution is desired by all. These landslides have thrown the stretch of road into the public eye – a more permanent solution is desired by all.

The Problem Since construction, the route has suffered from several geological problems Since construction, the route has suffered from several geological problems Despite improvements in drainage and pavement strength, road still subject to long closures. Despite improvements in drainage and pavement strength, road still subject to long closures closure – 158 days closure – 158 days. Cost to repair - £2,983,000 Cost to repair - £2,983,000 This cycle will continue This cycle will continue

Geology  The region consists of north- westerly trending ridges and valleys, a result of tectonic forces.  The cliff face is characterised by steep, eroded slopes with gradients up to 70%  3D model shows difficulties this terrain presents to the route, running straight through some of the steepest cliffs in the region.

Geology  Two geological ‘units’ exist –older Montara Mountain granodiorite (quartz, feldspar biotite).  Younger Sedimentary rock (sandstone and shale).  There is an oblique fault between these two units, descending northwest across the cliff face.  The sedimentary rock is subject to active sliding where the sedimentary rocks are exposed  Any landslide has a catastrophic effect on the road above.

Possible Options?  No Alternative? - High risk - Massive maintenance costs - Not a long term option  Slide Dewatering?  A Tunnel?  A Bypass? - Mountainous area - In-land alternative - Climbing Lanes required over steep gradients - Large visual impact - Route runs through sensitive animal and plant habitants - Cheapest and quickest option - Low impact on the local environment - Direct route - Lower dependency on unstable coastal geology - ~5 year construction time - More expensive - Depends upon; - “Draining of the active slide” - Permeability of geologic formations - The ability to extract groundwater with wells and drains - Continuity of groundwater reservoirs - Previous drilling and dewatering efforts have proved unsuccessful

Adopted Solutions  1958; Process for a bypass began  1973; Local environmentalist group first suggested construction of a tunnel  1975; Bypass project ended due to public opposition  1979; Tunnel idea dropped by the Caltran in the late  1986; Caltran pushed for a bypass again  1995; Documents emerged that the showed Caltran over-estimated the cost of tunnel construction  1995; Caltran ordered to re-evaluate cost of tunnel  1996; 74% of San Mateo County voted in favour of the tunnel  1996; Caltran changed its opinion, viewing the tunnel as the best environmental, economic and popular alternative!

The Tunnel  Mitigation Measures  Noise and dust controls  Traffic restrictions  Erosion control measures  Environmentally Sensitive Areas off-limits  Underground structures resist seismic forces better than surface structures  Only small defects likely to occur  Construction started in 2005  Completion expected in 2010  Existing route continues to be maintained…