Devils SlideDevils Slide

Lee BowersLee BowersRoss MaguireRoss Maguire

LocationLocation

•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 HazardsCalifornia 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 ProblemThe Problem• This section of route 1 This section of route 1

replaced the steep, narrow replaced the steep, narrow and dangerous Pedro and dangerous Pedro Mountain Road in 1937.Mountain Road in 1937.

• The first Landslide occurred in The first Landslide occurred in 1940, destroying much of the 1940, destroying much of the road.road.

• The road has suffered from a The road has suffered from a cycle of regular destruction cycle of regular destruction and reconstruction due to the and reconstruction due to the landslides.landslides.

• These landslides have thrown These landslides have thrown the stretch of road into the the stretch of road into the public eye – a more public eye – a more permanent solution is desired permanent solution is desired by all.by all.

The ProblemThe Problem

• Since construction, the Since construction, the route has suffered from route has suffered from several geological several geological problemsproblems

• Despite improvements in Despite improvements in drainage and pavement drainage and pavement strength, road still subject strength, road still subject to long closures.to long closures.

• 1995 closure – 158 days.1995 closure – 158 days.

• Cost to repair - £2,983,000Cost to repair - £2,983,000

• This cycle will continueThis cycle will continue

GeologyGeology

The region consists of north-The region consists of north-westerly trending ridges and westerly trending ridges and valleys, a result of tectonic valleys, a result of tectonic forces.forces.

The cliff face is characterised by The cliff face is characterised by steep, eroded slopes with steep, eroded slopes with gradients up to 70%gradients up to 70%

3D model shows difficulties this 3D model shows difficulties this terrain presents to the route, terrain presents to the route, running straight through some of running straight through some of the steepest cliffs in the region.the steepest cliffs in the region.

GeologyGeology Two geological ‘units’ exist –older Two geological ‘units’ exist –older

Montara Mountain granodiorite Montara Mountain granodiorite (quartz, feldspar biotite).(quartz, feldspar biotite).

Younger Sedimentary rock Younger Sedimentary rock (sandstone and shale). (sandstone and shale).

There is an oblique fault between There is an oblique fault between these two units, descending these two units, descending northwest across the cliff face.northwest across the cliff face.

The sedimentary rock is subject to The sedimentary rock is subject to active sliding where the sedimentary active sliding where the sedimentary rocks are exposedrocks are exposed

Any landslide has a catastrophic Any landslide has a catastrophic effect on the road above.effect on the road above.

Possible Options?Possible Options?

No Alternative?No Alternative?

- High risk

- Massive maintenance costs

- Not a long term option

Slide Dewatering?Slide Dewatering?

A Tunnel?A Tunnel?

A Bypass?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 SolutionsAdopted Solutions 19581958; Process for a bypass began; Process for a bypass began

19731973; Local environmentalist group first suggested ; Local environmentalist group first suggested construction of a tunnelconstruction of a tunnel

19751975; Bypass project ended due to public opposition; Bypass project ended due to public opposition

19791979; Tunnel idea dropped by the Caltran in the late; Tunnel idea dropped by the Caltran in the late

19861986; Caltran pushed for a bypass again; Caltran pushed for a bypass again

19951995; Documents emerged that the showed Caltran ; Documents emerged that the showed Caltran over-estimated the cost of tunnel constructionover-estimated the cost of tunnel construction

19951995; Caltran ordered to re-evaluate cost of tunnel; Caltran ordered to re-evaluate cost of tunnel

19961996; 74% of San Mateo County voted in favour of ; 74% of San Mateo County voted in favour of the tunnel the tunnel

19961996; Caltran changed its opinion, viewing the tunnel ; Caltran changed its opinion, viewing the tunnel as the best environmental, economic and popular as the best environmental, economic and popular alternative!alternative!

The TunnelThe Tunnel

Mitigation MeasuresMitigation Measures Noise and dust controlsNoise and dust controls Traffic restrictionsTraffic restrictions Erosion control measuresErosion control measures Environmentally Sensitive Areas off-limitsEnvironmentally Sensitive Areas off-limits

Underground structures resist seismic forces better than Underground structures resist seismic forces better than surface structuressurface structures

Only small defects likely to occurOnly small defects likely to occur

Construction started in 2005Construction started in 2005 Completion expected in 2010Completion expected in 2010 Existing route continues to be Existing route continues to be

maintained…maintained…