Devil's Slide Improvement Project

Route 1 from Half Moon Bay Airport to Linda Mar Boulevard in Pacifica,

San Mateo County, California

DRAFT

SECOND SUPPLEMENTAL ENVIRONMENTAL IMPACT STATEMENT/

SECOND SUPPLEMENTAL ENVIRONMENTAL IMPACT REPORT

ALL COMMENTS ON THIS DOCUMENT MUST BE MADE IN WRITING TO:

Robert Gross, Chief
Office of Environmental Planning, South
Caltrans District 4
P.O. Box 23660
Oakland, CA 94623-0660

3.0 ALTERNATIVES


The original 1986 FEIS presented three alternative alignments with various design options to provide a safe and dependable highway in a geologically stable area between the Half Moon Bay Airport and Linda Mar Boulevard in the City of Pacifica. From these alternatives, Caltrans and the Federal Highway Administration selected the Martini Creek Alignment, a two-lane facility with an uphill climbing lane in each direction where grades are 6% or greater, as well as traffic safety design features.

In the 1995 SEIS/R, public comments included references to a tunnel alignment in a slightly different location from the tunnel option referred to in the 1986 EIS (pp.61,62). The original tunnel option was rejected as not a reasonable alternative primarily because construction costs greatly exceeded the cost for the construction of the Martini Creek alignment alternative. At the request of the San Mateo County Board of Supervisors, a tunnel feasibility study was undertaken between May and October, 1996. Based upon the results of that report, and the updated cost estimates for the revised Martini Creek alignment alternative, FHWA and Caltrans determined that a tunnel alternative is a reasonable alternative for the proposed project.

 

3.1 TUNNEL ALTERNATIVE

3.1.1 Design and Cost

The preferred tunnel alternative is a 1,219-m (4,000-foot) long, double bore facility with one lane in each direction. The north approach road is about 457 m (1,500 feet) long, and the south approach road is about 305 m (1,000 feet) long. Proceeding south from Pacifica, the alignment departs from existing Route 1 along a 7% uphill grade, crosses the valley at Shamrock Ranch, passes through a small ravine, enters a tunnel through San Pedro Mountain, and exits the tunnel just south of the Devil’s Slide area where it rejoins the existing highway (see Figures 3-1 and 3-2). The tunnel is basically flat with a 2% grade.

The northerly limit of this tunnel alternative is approximately Kilo Post 64.1 (Postmile 39.8), which is the location where the north approach road connects or joins with the existing highway. The southerly limit of this alternative is approximately Kilo Post 61.0 (Postmile 37.9), which is the south end of the area identified for disposal of excess material from the project. The disposal area is southeast of where the south approach road connects or joins with the existing highway. The tunnel location and alignment is inland of the unstable slide plane area and would meet the project’s purpose and need to bypass the geologically unstable area of Devil’s Slide.

Two tunnel design variations, a 9.1 m (30-foot) wide tunnel (variation A), and an 11.0 m (36-foot) wide tunnel (variation B), are being considered (see Figure 3-3). The total project costs, including both construction and design costs, of tunnel variations A and B are estimated to be $134,900,000 and $137,550,000, respectively. These costs are broken down as follows:

 

Variation Construction Cost Engineering Cost Total

A

$114,000,000 $20,900,000 $134,900,000

B

$116,250,000 $21,300,000 $137,550,000

 

These estimates include the cost of designing and constructing a bridge at the north end of the project across the Shamrock Ranch valley between the existing highway and the tunnel's north portal. Tunnel variation B provides for a separated pedestrian and bicycle access inside the tunnel, while variation A provides a pedestrian/bicycle path outside the tunnel.

Caltrans is always concerned about safety and has always considered the safety of "any and all" users of any facility that is part of the State Highway and Freeway system. This included not only the motoring public but also Maintenance personnel, bicyclist and pedestrians, and occupants of stalled or disabled vehicles.

The existing Highway 1 is a conventional highway with limited shoulder widths but never the less is used by bicyclists. Early in the process, there was a discussion on the cross sectional width of the tunnel and whether or not bicycle access would be provided. Many of those in the bicycle community have concerns regarding any restrictions on bicycles. As mentioned above, safety is a major concern of Caltrans and therefore separated bicycle access was considered in the proposed design of the tunnel project in order to protect bicyclists from high speed traffic within an enclosed space. This of course results in the need for a tunnel with a wider cross section and higher cost. Following much discussion and debate over this issue, Caltrans decided to request that the San Mateo County Board of Supervisors identify their preferred alignment for a bicycle facility associated with the tunnel project.

The San Mateo County Board of Supervisors, in passing Resolution #61060, authorized and directed the President of the Board to notify Caltrans "that the Board's preferred alignment for a bicycle/pedestrian trail be located within the existing Highway 1 Devil's Slide right-of-way, with an alternative alignment around Devil's Slide should it fail". A copy of Resolution 61060 is provided in Appendix B. It was decided to incorporate into the project design, the placement of informational/directional signs directing bicyclists to use the existing Highway rather than the tunnel. In the absence of any ban or restrictions (which would need to be initiated by local government), some bicyclists may still choose to ride with vehicular traffic through the tunnel.

Initially, as part of the tunnel alternative, Caltrans considered utilizing Old San Pedro Mountain Road as a possible alternate bicycle/pedestrian trail around Devil’s Slide. This trail follows a similar alignment as the Martini Creek alignment alternative, but with a northern terminus near Linda Mar school in Pacifica. However, it was determined that this is not an acceptable option for several reasons including the excessive length and steep grades along this alignment that are not conducive to daily bicycle use. In addition, the construction and use of such a trail might result in possible adverse environmental impacts near the saddle area between San Pedro Mountain and Montara Mountain where heavy trail use could potentially affect the endangered San Bruno elfin butterfly. It was also determined that this alternate bicycle/pedestrian trail did not qualify for emergency relief funding—the most likely source of federal funds for this project..

Upon completion of the tunnel construction, Caltrans will relinquish the section of existing Highway 1 right-of-way to the County of San Mateo to own and manage. Maintenance and operations of the property and the bicycle/pedestrian trail will then be within the jurisdiction of the County of San Mateo. The County may choose to operate and maintain this facility or arrange for the management or transfer of the facility to an appropriate public recreational agency.

 

3.1.2 Project Funding

The Transportation Equity Act for the 21st Century (TEA-21) (Public Law 105-178), as approved by Congress and the President in June 1998, addresses the funding for a Devil’s Slide project by providing that emergency relief (ER) funds may be used for whichever alternative is selected. This legislation, at Section 1217(a), not only removes the cap on ER funding from the previous $100,000,000 per state per disaster, but also defines the circumstances at Devil’s Slide to be an "on going and continuing emergency". Under current practices, these provisions will allow Caltrans to request ER funds to be allocated for the Devil’s Slide project when plan, specifications and estimates for the project have been developed and approved.

Since the 1998 State Transportation Improvement Program (STIP) approval process, as amended by Senate Bill 45 (Kopp), was underway prior to the enactment of TEA-21, the final provisions of TEA-21 were unknown while the 1998 STIP was being prepared. To assure against project delays and to maintain continuity for the environmental process, design, and right of way activities, STIP funding for these activities was approved with the adoption of the 1998 STIP.

 

3.1.3 Tunnel System Operations

There are a number of tunnel infrastructure systems that are necessary for safety and operations. These include tunnel control, tunnel surveillance, traffic control, communications, emergency evacuation, environmental monitoring and tunnel maintenance.

The Tunnel Operations and Maintenance Center (OMC) would be located approximately 549 meters (1800 feet) south of the tunnel and would include a control room where the tunnel control computers would be housed. In the event a storm or earthquake breaks communication between the Caltrans Transportation Management Center (TMC) in Oakland and the tunnel, the tunnel could be operated locally. A closed circuit television (CCTV) system would function as a security and safety aid for the Devil’s Slide Tunnel. Pan tilt zoom cameras would be located every 305 meters (1000 feet) within the tunnel and on the first approach variable message signs (VMS) bridge.

The goal of the design of the Devil’s Slide Tunnel is to provide a safe and cost-effective facility. With state of the art surveillance, control, and communications, it may no longer be necessary that the tunnel be under constant supervision. Computers monitor remote sensing devices and an alarm signals if a problem arises. Traffic signals, the radio override system, and the VMS are operated remotely and can provide the communication of valuable traffic information to the motorist in the event of an emergency.

Tunnel Control System

A computerized Tunnel Control System (TCS) would control the tunnel equipment and the traffic. It would have an uninterruptable power supply (UPS) and would be controlled from the Caltrans Transportation Management Center (TMC) in Oakland. The emergency response strategy would be semi-automatic. To speed the response, the TCS computer would provide the tunnel control operator with a pre-programmed set of appropriate responses for each emergency depending on its type and location in the tunnel.

Tunnel Surveillance System

The surveillance system would include traffic sensors, closed circuit television, linear heat detectors, fire alarms, equipment sensors and intrusion alarms for the crosspassages, and refuge rooms.

Traffic Control System

The traffic control system would include VMS and traffic signals as well as an AM/FM radio broadcast system. The VMSs would be placed at the portals and spaced inside the tunnel so that one would be visible to motorists at all times. They would also be placed on the approaches to warn and inform motorists if the tunnel was closed due to stopped traffic, reduced speed or other hazards.

Communication System

The communication system design includes an emergency telephone system with emergency telephones located along the right wall of the tunnel and at both approaches. The linkage between the tunnel and the TMC in Oakland would be provided by leased telephone lines.

Tunnel Maintenance

The tunnel and any associated equipment would be maintained to manufacturer’s standards and would include preventive maintenance. Equipment or elements in the ceiling or walls of the tunnel such as lights, VMS, radio rebroadcast equipment, and linear heat detectors should be designed and located to allow for maintenance without interrupting traffic through the tunnel. During maintenance operations that do require personnel or equipment on the roadway, the VMS would alert motorists of necessary lane closures.

Emergency Evacuation System and Response

The emergency evacuation system would include both crosspassages and refuge rooms. The refuge rooms have separate ventilation, lighting, water, and communication systems. An emergency helipad would be provided on the roadway at each tunnel portal. In situations where there is a threat to human life such as a fire, hazardous substance spill, or a traffic blockage combined with a Carbon Monoxide (CO) level exceeding standards, it may be necessary to evacuate one side of the tunnel.

In all of these cases, traffic in the opposite direction would be stopped and prevented from entering the tunnel. This safety measure would allow motorists to exit through crosspassages without being endangered by traffic in the other bore. Crosspassages are proposed at approximately every 156 meters (511 feet) to provide a means of exit from the tunnel on foot into a safe area. Strobe lights next to the crosspassage doors would identify these exits.

The emergency response plan would be based on local municipal personnel responding to emergency situations. The responsible agencies would be the same as those currently responsible for this section of Route 1 which is located in a steep and undeveloped area between the City of Pacifica and unincorporated Montara. The highway itself is patrolled by the California Highway Patrol (CHP) which estimates a typical response time of 20 minutes to the tunnel site.

The section of Route 1 between Half Moon Bay and Pacifica is patrolled regularly by the San Mateo County Sheriff’s Department and when an accident occurs, a County Sheriff’s Deputy typically is the first to arrive at the scene. The San Mateo County Sheriff’s Department has an office in Half Moon Bay and can typically respond to the tunnel site in approximately five minutes. In the event of an accident, local police or sheriff’s deputies will maintain traffic control and contact other necessary emergency personnel. Caltrans currently has a maintenance station in Half Moon Bay. The closest facility with 24-hour maintenance personnel is the Bay Bridge or San Mateo Bridge Toll Plazas.

Fire Protection

Fire protection facilities would be essentially the same as what currently exists. The new pump station at Route 1 and Linda Mar Boulevard connects to the North Coast County Water District line. This 20.3 cm (8 inch) diameter water line would be continued through the tunnel and rise vertically in a drilled shaft to a 2.7 meter (9 foot) high by 7.9 meter (26 foot) diameter holding tank. A gravel access road would be constructed to the tank location.

The fire alarm system would provide fire detection for the Operations and Maintenance Center, miscellaneous equipment rooms, and the tunnel areas. The system would function as an announcement/alarm system to alert operating personnel and to provide a detailed location of the fire. The systems includes linear heat detectors, smoke detectors, water pressure and fire valve position sensors and manual pull stations.

Ventilation

Ventilation requirements for the Devil’s Slide Tunnel proposal were addressed early in the tunnel feasibility study when the use of jet fans was highly recommended for ventilation in the double bore tunnels (ref consultant). A jet fan unit is an axial fan with a direct drive motor designed to deliver a wide range of thrust capabilities in both the forward and reverse direction. The jet fan system would force air to the downstream (direction of traffic) portal and out of the tunnel, while introducing fresh air into the tunnel at the upstream end.

The key factors which affect the number of jet fans, are whether the tunnel has pedestrian/bicycle path facilities, and the number of traffic lanes. The design option without pedestrian/bicycle facilities allows a higher 120 ppm CO limit. The design option with pedestrian/bicycle path facilities requires a more restrictive 65 ppm CO limit.

In addition, the proposed tunnel design includes a "real time" air quality monitoring system that includes variable message signs that can advise bicyclists when unacceptable air quality conditions exist in the tunnel. It is anticipated that adverse conditions would be infrequent.

Environmental Monitoring

The environmental monitoring system provides data on wind speed, carbon monoxide (CO) and volatile hydrocarbons (HC) to allow operations personnel to react and respond accordingly when an incident or emergency occurs. Although nitric oxide is monitored in the Posey and Webster tubes in Alameda, it is not proposed to be monitored in the Devil’s Slide Tunnel since analysis of pollutant levels in other tunnels has shown that the threshold standard for CO is reached before the threshold standard is reached for nitric oxide. The need to monitor nitric oxide for the Devil’s Slide Tunnel would be investigated and addressed in the final design phase.

 

FIGURE 3-1: Proposed Tunnel Alignment

Figure 3-2

 

FIGURE 3-3: Proposed Tunnel Cross Sections


 

3.2 MARTINI CREEK ALIGNMENT ALTERNATIVE

The proposed Martini Creek alignment alternative is a two-lane controlled access highway approximately 7.2 km (4.5 miles) in length along the Martini Creek alignment (1986 FEIS preferred alignment). Northbound on Route 1 the Martini Creek alignment begins about 0.40 km (0.25 mile) north of Montara near the Chart House restaurant, where it proceeds inland at a northeasterly direction ascending the south side of Martini Creek and crossing it in a northerly direction. The alignment swings northwesterly past a proposed vista point, then northerly to skirt Green Valley and continues to the summit of the San Pedro Mountain saddle. North of the summit the alignment proceeds northwesterly along the flat portion of Shamrock Ranch, swings back in a northeasterly direction on the north side of the ranch, then proceeds northerly to join the existing Route 1 at Linda Mar Boulevard in Pacifica. Since grades exceed 6% on either side of the summit, the design includes climbing lanes for approximately 3.2 km (2 miles) in each direction. A slow vehicle lane is required to extend a short distance over the summit in both directions to allow slower vehicles to regain speed before merging into the single downhill lane. A non-paved vehicle recovery area would be included in the downhill directions for safety reasons (see Figure 3-4).

The Martini Creek Alignment alternative identified in the 1986 FEIS was selected in the FHWA 1986 Record of Decision. In September of 1986, approximately $47 million in special Federal emergency relief (ER) funds and $5 million in other funds were obligated for the Martini Creek alignment alternative when the Federal government approved the plans, specifications and estimates (PS&E) for constructing the Martini Creek alignment project. Recently revised cost estimates for the Martini Creek alignment alternative have increased to $112 million due to inflation and various design changes (see following section).

 

3.2.1 Changes In Design and Cost

Certain aspects of the project design have been modified since the original proposal in 1986 to specifically avoid or minimize impacts to recently discovered sensitive habitat and/or plant and animal species in the project area. Bridge structures have been extended to clear-span valuable riparian and wetland habitat at Martini Creek, Green Valley Creek, the Shamrock Ranch ponds, and San Pedro Creek. A new structure has been included in lieu of a culvert near station 446+00. In addition, modified cut and fill slopes would reduce the disturbance to vegetation, thereby reducing impacts from siltation both during construction and afterwards during heavy winter rains. These same design changes would lessen impacts to anadromous fish populations in San Pedro Creek.

Originally, this was a "balanced" project (i.e. the quantity of fill material would equal the cut material), meaning no excess material would have to be disposed of or imported. The extended bridge structures reduce the need for fill material, thus creating a surplus. Rather than disposing of the material off-site, the profile of the proposed Martini Creek alignment alternative has been increased by approximately 4.5 meters (15 feet) along the alignment in order to keep the project balanced.

Minor alignment shifts at the southern end of the project are also being included to circumvent a stand of Hickman’s Cinquefoil (Potentilla hickmanii), a state-endangered plant species, and to avoid wetland habitat at the southern terminus of the project.

In 1996, Caltrans retained the services of an independent consulting firm, The National Constructor's Group, to prepare a separate analysis of the construction cost estimate for the Martini Creek alignment project. They determined that the project could be constructed for $91,542,669 (National Constructor's Group, 1996). This estimate did not include costs for state furnished materials, supplemental work, contract growth contingencies and engineering costs. These items were expected to add an additional $14,672,854. The total estimated cost would be about $106,215,000.

Caltrans also updated its previous Martini Creek alignment project construction cost estimate of $85,893,645 and included costs for engineering and right of way for a total of $112 million. The increased costs since 1986 also reflect inflation, upgrading of structures to current seismic standards, lengthening of bridges over newly-determined wetlands and red-legged frog habitat, and other environmental mitigation costs.

 

 

FIGURE 3-4: Proposed Martini Creek Alignment

 

3.3 NO PROJECT ALTERNATIVE

The no project alternative, as described in the 1986 FEIS, consists of continuing with the general maintenance and repair of the existing alignment. The periodic slides and slipouts, resulting in road closures of various lengths of time, can be expected to continue. There is a high probability that a seismic event of sufficient magnitude will eventually occur and result in permanent closure of the existing roadway.

 

3.4 ALTERNATIVES CONSIDERED BUT WITHDRAWN

Repair / Maintain Existing Alignment (Slide Dewatering)

The existing alignment alternative discussed in the 1986 FEIS (known as the Marine Disposal Alternative) involved realigning a 1,311 meter (4,300-foot) section of the highway at Devil’s Slide approximately 76 meters (250 feet) inland and uphill of the existing highway. This alternative was rejected primarily due to the additional cost and construction time (approx. $11 million and 1.8 years) involved when compared to the Martini Creek Alignment Alternative. The highway would still be subject to the severe marine and geologic conditions unique to Devil's Slide. This alternative involved disposing of 14.5 million cubic yards of excavated material into the ocean below Devil’s Slide. With the establishment and formal designation of the Monterey Bay National Marine Sanctuary in late 1992, this alternative would not be permitted per regulations of the Monterey Bay National Marine Sanctuary and the National Oceanic and Atmospheric Administration.

Based on an independent draft report by Dr. John Hovland, a local geologist, it had been proposed that the existing highway could be preserved by an extensive effort to drain the active slide and the adjacent land mass (Hovland, 1998). This effort, referred to as "dewatering", can be an effective means of increasing the stability of a slide mass if geological and hydrogeological conditions are conducive to point drainage technology. However, the success of subsurface drainage efforts, whether they be passive horizontal drains, well points employing pumps, or a combination of these techniques, are dependent upon site specific conditions such as: the permeability of the geologic formations; the connectivity and continuity of groundwater reservoirs; the ability to intercept the groundwater with wells or drains, etc.

Based on information provided from previous drilling programs and dewatering efforts at Devil's Slide, Caltrans engineers and geologists had concerns as to the potential success of a dewatering program at Devil's Slide. Although it was the opinion of Caltrans Engineering and Geotechnical Staff that this proposal is not comparable to the certainty of the tunnel alternative in meeting the purpose and need of the project, there was insufficient data to fully resolve the adequacy of dewatering as a permanent solution. .

Senate Bill 45 provided a greater role for local agencies in the cost and scope of projects on the State Highway System. It also required extensive partnering with local agencies in the development of projects. In June of 1997, the San Mateo County Board of Supervisors requested that Caltrans consider the dewatering proposal as a Devil’s Slide project alternative. In response to this request and in conjunction with the Federal Highway Administration, it was determined that in the absence of conclusive cause for rejection, dewatering would be evaluated further as part of the environmental process.

Measure T, the County-wide initiative passed in November, 1996, authorized the county to amend the Local Coastal Program (LCP) to require voter approval for "any alternative to the tunnel, except the repair or reconstruction of the existing road". Dewatering is being considered as a repair of the existing road.

Following consultation between Caltrans geotechnical engineering staff from the District 4 Office and the Headquarters Engineering Services Center (ESC), a plan to further evaluate the dewatering proposal was developed. In November 1997, the ESC Roadway Geotechnical Engineering - North Section initiated a dewatering feasibility study to determine if dewatering Devil’s Slide with deep wells was feasible as a long-term solution to the recurring slide movement within the most active portions of Devil’s Slide. The major component of this effort was the installation of two test wells to provide a means to potentially improve roadway stability prior to the anticipated wet "El NiZ o" winter and also to provide the data and information needed regarding the feasibility of using deep wells to dewater and stabilize this area in order to support and maintain a stable and permanent highway.

Study Area

The study area is western San Mateo County, 2.4 kilometers (1.5 miles) south of the City of Pacifica. Highway 1, at this location, traverses a west-facing coastal cliff and ranges between elevation 100 meters (300 feet) and 137 meters (450 feet) above sea level. The coastal bluff ranges in elevation from sea level to 274 meters (900 feet) above mean sea level (MSL) and has an average slope angle of 45 degrees or greater. This section of highway has had a long history of landsliding and roadway closures.

The history of landsliding dates back in historic time to at least the 1906 San Francisco earthquake. This area is shown on an 1866 topographic map but geomorphic evidence suggests that landslide activity has been ongoing and a part of the normal coastal processes for thousands of years. The difficulties in maintaining a transportation system on this alignment are well documented and precede the construction of Route 1 in the 1930s. The Ocean Shore Railroad, which began construction in 1905, was planned to run from San Francisco to Santa Cruz, California. It crossed Devil's Slide along its route and had a similar history of problems as has Route 1. The railroad project was eventually abandoned in 1921.

Landslide stabilization by removal of groundwater and reducing pore-water pressures has long been a mitigation measure given the right geologic and hydrogeologic conditions. Seepage, buoyancy, and hydrostatic forces act to increase the driving force or lower the restraining forces on the landslide, hence control of subsurface water can be of major importance. Drainage measures that are most often used for stabilization include one, or a combination of the following: drainage blankets and trenches, horizontal drains, drainage wells, pumping wells, drainage galleries, and drainage tunnels. Dewatering of a landslide as a measure of stabilization, applies to any slope where lowering the water table will increase slope stability. However, dewatering would not be effective in stabilizing a landslide mass if the material in the landslide has very low hydraulic conductivity.

Scope Of Work

The information in the feasibility study was based on review of the available geologic and hydrogeologic literature, existing files and reports on or nearby the study location, published and unpublished reports, topographic maps, logs of test borings, and a review of historic and contemporary stereo pair aerial photographs.

In early November, two production wells and four observation wells were drilled and constructed along the roadway bench on the east side of the roadway and near the two ends of the most active portion of the slide mass (see Figure 3-5). Both wells were more than 91 meters (300 feet) deep and open to the formations for most of their lengths so as to intercept any groundwater flow between the elevation of the roadway and sea level. Four piezometers (instrument used to measure the pressure of a fluid) were installed, two near each well, to evaluate the drawdown and cone of influence characteristics of the well points. A 48-hour drawdown test was conducted whereby water was continuously pumped from the well points while water levels in the piezometers were recorded.

The observation wells were constructed with up to six nested piezometers that were isolated from each other and set at increasing depths within the boring. Following the construction and installation of the well system, a ground water pump test was conducted to determine the influence of pumping on ground water conditions within the study area.

General Geologic Setting

Montara Mountain and the Devil's Slide area are located within the California Coast Range Geomorphic Province, which consists of a series of northwesterly trending ridges and valleys formed by compressional tectonic forces.

The geologic units located within the study area consist of two separate rock units in unconformable fault contact. The oldest rock unit in the study area is the Cretaceous Montara Mountain granodiorite. Granodiorite is a coarse-grained plutonic rock consisting of quartz, plagioclase and potassium feldspar, biotite, hornblende, or, more rarely pyroxene. The term applies to all rock intermediate in composition between granite and diorite. The Montara Mountain granodiorite is clearly exposed throughout the southern portion of the study area and the fault contact is thought to form the middle and lower southern edge of the active slide area. The granodiorite is in fault contact with the overlying Paleocene age unnamed sedimentary rock consisting of sandstone, shale, and conglomerate.

The sedimentary rock within the study area is steeply dipping, extremely folded, faulted, and further disturbed by repeated episodes of landsliding. The sedimentary rock units exposed throughout the center and northern portions of the study area were the focus of the feasibility study; all active sliding is occurring within the sedimentary rocks. The fault separating these two units is described as an oblique fault that descends northwest across the cliff face, crosses through the study area between observation wells P-3 and P-4, and continues down the cliff face and offshore. Within the slide area, much of the fault contact has been obscured by colluvium and shallow slide debris both above and below the roadway. This fault contact forms the southern boundary of the active slide area and is believed to play an important role in understanding slide activity in the area, both as a slide surface and as a possible area of subsurface water flow. The fault is considered inactive.

The study area is located within the seismically active San Francisco Bay Area and is within 48 kilometers (30 miles) of five large, active fault zones. These active fault zones all trend in a northwesterly direction and are designated as Alquist-Priolo Special Studies Zones by the California Division of Mines and Geology. These are the San Andreas, Seal Cove-San Gregorio, Hayward, Calaveras, and Rogers Creek fault zones. Due to the proximity to the study area it is anticipated that this area will experience strong ground motion during a maximum credible earthquake (MCE) on any of these fault zones. An MCE is the largest earthquake reasonably capable of occurring based on current geological knowledge. The San Andreas fault is the most notable of these faults and is located 8 kilometers (5 miles) to the northeast. An earthquake on this fault system has the potential to cause the greatest damage to the Devil’s Slide area due to very strong ground shaking.

Rainfall

Rainfall data for the duration of the study was based on information from the rainfall recording station in San Pedro Valley Park just to the east of Devil’s Slide. The rainfall year runs from July 1, 1997 to June 30, 1998. As of May 1, 1998, there had been 59.62 inches of rainfall recorded at San Pedro Valley Park. Based on its geographic location and proximity to the Devil’s Slide area, in a slight rain shadow, it is estimated that at least 60 inches of rain must have fallen on Devil’s Slide and Montara Mountain. Based on historic records, as of the end of May, 1998, this "El NiZ o" winter is the second wettest winter on record. Given this winter’s heavy rainfall, any effects from rainfall should be evident in the groundwater data.

 

 

FIGURE 3-5: Well and Piezometer Location Map

 

Field Tests and Results

The following is a condensed summary of the field tests and the general results (see Table 3-1). For more specific or detailed information, please refer to the Devil’s Slide Dewatering Feasibility Study.

After test wells and new observation wells were constructed and the groundwater table was stabilized, a short-term pump test was performed for each well (W-1 and W-2). The pump tests were conducted during the latter part of December 1997. During this period the wells were monitored for fluctuations in water level. Each pump test was conducted for a continuous 48 hour period followed by a recovery test of 5 hours. The purpose of these tests were to determine:

  1. the rate at which water could be drawn from the subsurface, and
  2. the effect pumping had on ground water in the surrounding area as measured in the observation wells.

 

 

 

 

TABLE 3-1: Short-term Pump Test Results


Well No

Average Pump

Rate

48 hour test

Stabilized

Pump Rate

Transmisivity

(recharge)

Hydraulic

Conductivity

Cone of

Influence

Drop in

Water Level

W-1

6.8 LPM

(1.8 GPM )

1.9 LPM

(0.5 GPM)

0.19 m

(0.63 ft2 /day)

2.2x10-5 cm/sec to

1.0x10-6 cm/sec

 

<14 meters

(45 feet)

No measurable

movement

W-2

1.7 LPM

(0.45 GPM)

 

1.1 LPM

(0.3 GPM)

0.11 m

(0.36 ft2 /day )

 

7.1x10-6 cm/sec

 

12 meters

(39 feet)

No measurable movement

 

In addition to the four newly installed observation wells, two existing observation wells, which were installed in 1988 as part of the previous Marine Disposal Alternative (MDA) Study, were also used to collect subsurface water data. The monitoring wells consist of two drilled holes with as many as three nested piezometers in each hole. These wells are identified below in Table 3-2.

 

TABLE 3-2: Monitoring Wells (1988 MDA Study)


Well Number

Location

Groundwater Conditions

MDA/Pz-2

Southern edge of active slide mass

12 meters West of W-2

Groundwater conditions in the slide mass and the granodiorite

MDA/Pz-3

Northern edge of active slide mass 67 meters North Northwest of W-1 Groundwater conditions in sedimentary units

 

Long-Term Pumping System

The long term pumping system was installed and operational on February 11, 1998 and has been in continuous operation since that time. Review of the MDA Pz-2 and MDA Pz-3 groundwater data from 1988 to May 1998 indicated that there were fluctuations of up to 6 meters (16 feet) and as little as 1 meter (3.28 feet) in the piezometers.

The data suggests that the subsurface water stays within a given range and appears not to vary outside that range regardless of the time of year or amount of rainfall. There have been no large rises in ground water as a result of the winter rains nor a significant drop due to natural draining of the slope. When charted on a graph, the data reflects trends which are gentle, long and drawn out. An analysis of the data suggests that there was no short duration rise in ground water causing the landslide to activate just prior to and after the 1995 failure, which resulted in the roadbed dropping approximately 1.5 meters (5 feet).

The Dewatering Study also considered the effects of the test wells on the stability of the roadway during this past winter when approximately 152 cm (60 inches) of rain fell. With the area of review being an estimated 18.6 hectares (46 acres), approximately 283,702 cubic meters (230 acre-feet) of rain fell in the study area. Given a 30% infiltration rate, approximately 85,111 cubic meters (69 acre-feet) of rainwater has infiltrated into the slide area this year. During the 64 days since pumping began, an average of 37,854 liters (10,000 gallons) of water per day have been removed totaling 2,422,656 liters (640,000 gallons) or 2,418 cubic meters (1.96 acre-feet). The amount of water removed by the pumps is calculated to be 2.89% of the water which has infiltrated the slide.

Due to the small amount of water removed from the system and the limited lateral influence pumping had on the active slide area, it was concluded that pumping produced negligible enhancement of roadway stability over the 1997/1998 winter. In addition, no significant increase in groundwater was noted in any of the observation wells this last winter, or in the MDA sites since 1988. These conclusions would indicate that the slide is not predominately driven by elevated groundwater conditions.

Without indications of elevated groundwater during the winter and spring months, dewatering of the slide mass at Devil’s Slide is not likely to provide greater stability. The process of dewatering is designed to reduce hydrostatic pressure that develops in a subsurface environment as a result of heavy rains. However, if an increased hydrostatic pressure condition does not exist in the slide mass or the subsurface, then dewatering as the sole means of stabilizing a landslide is not likely to be successful.

Conclusions And Recommendations

During the heavy rainfall months of the winter it was confirmed that more water travels through the slide mass as noted in higher pumping rates and higher horizontal drain flows and that these higher flows are directly related to heavy winter rain. However, the additional water has had no apparent impact on the water levels within the slope. It appears that the additional water from heavy rainfall passes through the slide mass to the face of the cliff along the various bedding planes, fracture zone or fault zones in an unconfined manner, like an open channel stream. This additional water present in the subsurface does apply seepage forces and additional weight to the slide mass that along with erosion at the toe of the slide may be the activation mechanism to sliding at Devil’s Slide. However, elevated groundwater conditions in the winter months which act to reduce restraining forces and increase buoyancy of the slide mass do not exist and are not the critical driving force of slide movement at Devil’s Slide. Given this condition, any attempted stabilization of Devil’s slide would require extensive buttressing and erosion prevention at the toe of the slide and an extensive drainage system of dewatering wells and horizontal drains through the entire slide mass to manage and contain winter flows of groundwater.

The dewatering system would need to be able to effectively remove groundwater at a rate equal to the inflow. The best performing well during this "El Nino" winter was W-1 at a rate of 26.1 liters (6.9 gallons) per minute with a radial influence of less than 26 meters (85 feet). This would indicate that a minimum of 89 pumping wells, covering the entire slide mass of 186,150 meters square (2 million square feet), on 46 meter (150 ft) centers would be required to manage and maintain groundwater within the slide mass. This assumption is based on the performance of W-1, which had higher pumping rates and greater lateral influence in comparison to W-2. There would likely be the need for approximately eighty-nine (89) additional pumping wells to effectively manage the winter flows of groundwater throughout the slide mass during the winter.

Horizontal drains would also be needed to remove groundwater from the slide mass and would require extensive effort and expense to not only construct them but to also maintain them. The history of horizontal drains at the site has verified that they are very difficult to construct and to maintain in working order over time. In addition, there has been mixed success in removing groundwater from the slide mass. They have effectively removed water from the subsurface when constructed to an appropriate depth, but there is no clear, measurable indication that they are lowering the water table within the slope. The data suggests that the horizontal drains are removing the water that infiltrates into and passes through the slide mass in an unconfined manner during heavy winter rains and that the flows begin to taper off as soon as the winter rains stop.

Any dewatering system attempted at this location would be based on "risk design", which is a design with unproven or questionable effectiveness. The conditions at Devil’s Slide are not conducive to meeting the basic criteria of dewatering, which is to get the water out of the slide mass and to keep it out as a means of stabilization. Dewatering at that location should be considered only if no other alternative is available. In addition, it is anticipated that any dewatering system installed at the Devil’s Slide is likely to be at least partially if not completely destroyed during future landslide events and would then require heavy maintenance costs and long road closures similar to what has occurred in the past. Past experience with shifting surfaces within the slide would indicate that placement of an extensive dewatering system within the slide would be extremely expensive to maintain, if it could be maintained at all. Furthermore, it would be difficult to construct such an extensive dewatering system considering the topography, geology and accessibility of the area. This effort would likely result in adverse impacts to the environment and local traffic on existing Route 1.

It is recommended that the two existing wells continue to be pumped and monitored over the next year to obtain additional data during the summer and fall months.

The Dewatering Feasibility Study concluded that the groundwater regime within the study area is complex, and dewatering would be extremely difficult. The slide mass has a low to very low hydraulic conductivity and removing groundwater from the slide mass is expected to be difficult and have limited lateral impact on the water table. This limited ability to remove groundwater from Devil’s Slide supports the conclusion that dewatering this slide area is not feasible. Dewatering does not meet the purpose and need and therefore is no longer considered as a viable project alternative.

3.5 STATUS OF NEARBY PROJECTS

There are currently three projects in the vicinity of the proposed project. A slow vehicle lane and safety improvement project is proposed for Route 92 in San Mateo County between Route 35 and Interstate 280. A similar project on the West side of Route 92 between Pilarcitos Creek and Route 35 is currently underway. Together these two projects will provide a continuous slow vehicle lane on Route 92.

The City of Half Moon Bay is proposing to widen route 92 between Highway 1 and the proposed Foothill Blvd intersection, and the San Pedro Creek Flood Control Project is sponsored by the Army Corps of Engineers (COE) and the City of Pacifica.

 

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