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It is proposed to make corrections to Route 1 in the vicinity of Devils Slide in San Mateo County. Corrections are necessary to eliminate the hazards of narrow roadway, steep grades, sharp curves, heavy superelevations, short tangents, disrupted drainage and falling rock. Improvements are desired to make this-stretch of highway safe for pedestrians and bicyclists and to provide scenic outlooks. In order to assure that the selected solution makes these corrections at the most tolerable cost in funds an? to the environment, studies of a tunnel alternate were requested.

The tunnel would begin at Green Valley, north of Montara, pass under San Pedro Mountain, and emerge near Linda Mar. Single bore and twin bore studies were made. The single bore and twin bore tunnels are identical in length at 4,450 feet. The profile climbs from the south portal towards the north at a 1.89% rate. The twin bore tunnel spreads beyond the portals, so the centerlines are 200 feet -apart.

The single bore is proposed to have a clear width of 46'-0", which will provide two 12'-0" traffic lanes, a 6'-0" median and 8'-0" shoulders left and right. The twin bore alternative will provide two 12'-0" traffic lanes with 6'-0" shoulders left and right in each 36'-0" tunnel.


Geological and engineering investigations indicate that it is feasible to construct either proposed tunnel.

At the south portal, the rock type is a deeply weathered granite; at the north portal, weathered shale. The portions of tunnel immediately adjacent to the portals would be constructed by conventional cut and cover methods until sound rock cover is reached.

The tunnels would be driven through four distinct rock types: granite, arkosic sandstone, highly folded, faulted, and sheared shale, and siltstone. The study presumes that shot crete would provide primary tunnel support for all but about 500 feet at the region where the shales abut the arkosic sandstone. Here the study presumes that steel sets in conjunction with shotcrete would be required.-

An 8-inch thick secondary liner would be used for the entire length of tunnel. This-liner would control ground water, provide the base for the tunnel finish surface, and divide the tunnel into roadway and ventilation areas.

The tunnel study and estimate are- based on the presumption of the most favorable rock conditions. Actual design will require extensive geological investigation. Should this investigation reveal adverse rock characteristics, the cost will radically increase. The estimate given is valid only for August, 1974. Inflation of construction costs has occurred at an unprecedented rate, and the estimate must be increased in kind as time passes. The total construction cost of the single bore tunnel is estimated to be 28,000,000. The twin bore tunnels are estimated to cost 46,000,000.

Tunnels of this length require lighting and forced air ventilation. Power costs for these features are estimated to be 57,600 annually for the single bore and $100,800 annually for the twin bores. A maintenance crew serving 24 hours daily would be needed to operate the lighting and ventilation as well as monitor the tunnel. This crew would consist of about 20 men and cost $396,000 annually.



GENERAL - This report was based upon on-site inspections, observations of the road cuts along Devils Slide, various geologic literature, and aerial photos. A complete study of rock conditions at tunnel grade must be undertaken before actual tunnel design. This will require a large number of borings and a seismic investigation. A pilot drift should be driven.

The twin bore tunnels will have cross-sections as large as the Caldecott, Hazelview Summit and Gaviota Gorge tunnels. The single bore cross-section will be larger than these, approaching the size of thee Waldo Tunnel. It was assumed that there would be no serious construction problems in driving these tunnels as proposed. Rock load and tunnel support requirements given herein are those for idealized geologic conditions.

TUNNEL GEOLOGY - The proposed site lies along a north-south trending line located approximately one-half mile east of existing Route 1. The tunnel line traverses two distinct geologic formations. The northern portion penetrates Paleocene sandstone, shale, and conglomerate. At a point approximately two-thirds through, San Pedro Mountain, the Paleocene rock lies unconformably on Cretaceous granitic rock. The angle of the contact is very steep and may be close to vertical. The Paleocene rocks are highly folded and sheared. The distortion of these sedimentary rocks is apparent in numerous roadway cuts along the adjacent highway.

The nearest known fault is approximately threequarters of a mile northeast of the proposed north portal. This is the Pilarcitos fault along San Pedro Valley. There is no historic movement along this fault. The nearest known active fault is the San Andreas, located approximately four miles to the east.

TUNNEL EXCAVATION - A pilot drift, should be planned. From this drift, the most complete evaluation of rock conditions can be made prior to final design. By locating this drift at the roadway level, drainage from the north heading can be removed by. gravity flow. Ventilation of the headings can be simplified and communications between headings improved.

Both portals should be designed for cut and cover construction to sufficient depth to penetrate the weathered surface material.

Normal hardrock tunnel excavation techniques can be used for the inner tunneling work. From the north, a movable head tunneling machine should be able to mine the relatively soft shales and sandstones. A broad head, multi-toothed "mole" tunneling machine should also be able to mine these rocks. The diameters of the tunnels being proposed are greater than the diameters of "mole" tunneling machines currently in use throughout the world.

Drill and shoot tunneling can be used in the shales and sandstones and this is the most probable method with the granite in the southerly length of tunnel. Controlled perimeter drilling should be able to limit overbreak of rock materially in this granite. overbreak in the folded and fractured sedimentary rocks may be more extensive.

Smaller, more closely spaced perimeter holes should minimize overbreak in all the rock types.

Ephemeral streams flow from springs at both portals. Within the mountain, only a minimum amount of water is likely to be found within the granite. More water may be encountered within the sedimentary rocks. After this perched water is drained, minor amounts of additional water will continue to flow.

ESTIMATED ROCK LOADS - In order to provide a basis for estimating support requirements, loads were calculated for six divisions of rock quality as observed along the tunnel alignment.



405 + 50
            300'       5.8 K/SF          5.6 K/SF
408 + 50
           1650'       3.0 K/SF          2.5 K/SF
425 + 00
            500'       4.0 K/SF          3.5 K/SF
430 + OO
            500'      14.5 K/SF         12.5 K/SF
435 + OO
           1325'       3.0 K/SF          3.5 K/SF
448 + 25
            175'       5.8 K/SF          5.6 K/SF
450 + OO

TUNNEL SUPPORT - The primary tunnel support is presumed to be shotcrete for all but 500 feet. At Station 430+00 to 435+00, shotcrete supplemented by steel sets is presumed. At this area, the rock type is likely to be completely crushed, highly folded and sheared shale. No significant lateral loads are expected.

Shotcrete means pneumatically applied coarse aggregate concrete used to provide support for openings in rock. The rock within a mountain is under load from several causes. The weight of overlying rock, hydrostatic pressure, past geological loadings and changes in temperature- all cause -stress within the rock. When an opening is made, the rock appears to expand, or be pressed, into the opening, causing cracks and separations at the rock surface. Exposure to air in the presence of water hastens this effect. Blocks of rock break out of the ceiling, beginning shortly after the opening is made. This cracking and falling out continues until a redistribution of stresses results in an arch effect in the remaining rock. In the meantime, small blocks dropping can be a hazard to individual workmen, while large blocks falling out can lead to multiple deaths from crushing or entrapment.

Shotcrete is blown against the newly exposed rock of the tunnel bore immediately after blasting. Accelerators are used to result in a high early strength membrane which can absorb the stresses which result when the rock swells into the opening.


CONTINUING COSTS - Tunnels of this length require illumination during both day and night. Daytime lighting must be brighter, with illumination at the portals stronger than in the interior. This permits the eyes of the driver, entering from bright sunlight, to adjust to the darkened interior and back to the bright exterior without momentary blindness. Nighttime lighting should be about one-tenth that needed during the day. Fluorescent strip fixtures are planned for effective lighting with economical power use. Varying intensities are obtained by controlling the excitation of the fluorescent tubes. Slightly more than half of the estimated annual power cost is for lighting.

The atmosphere in the tunnel will be monitored for carbon monoxide. This gas readily replaces oxygen in the human circulatory system. -Long exposure to relatively weak concentrations, or short exposure to heavy concentrations can cause headache, nausea, and death. With 55 miles per hour traffic, a driver would be exposed to the tunnel atmosphere for about one minute. Only an extremely high concentration of carbon monoxide would have any effect at this short an exposure. If the tunnel is not ventilated, a fairly high concentration can develop. Then a mechanical breakdown, a car out of gas, or any other minor mishap would serve to increase the exposure time. Ventilation becomes a necessity. The remaining power costs operate fans to keep carbon monoxide below two hundred parts per million.

A 24-hour maintenance crew is necessary to assure proper working of lights and ventilating system, maintain traffic flow, meet the potential emergency of a fire inside the tunnel, and make the continuing repairs needed to the lighting, ventilating, control; air monitoring and emergency systems. This force would consist of three shifts for a total of 20 men, with slightly larger shift crews during the morning and evening peak periods.

ITEMS OF WORK - The major item of work is removal of rock from the tunnel bore. Within this item is an allowance for overbreak, which means that a contractor is not expected to remove the rock exactly to the neat lines of the tunnel. The overbreak allowance chosen was based on the speculation that the shales, sandstones and granites would break reasonably well, but that the expected highly shattered shale next to the sandstone w9uld be much less controllable.

At either portal, for a distance into the mountain until the cover over the tunnel is equal to one tunnel diameter, a cut and cover technique was planned. The minimum amount of excavation was calculated, assuming vertical walls with one extra foot of clearance to provide room for building the arch section.

Rock support planned was to be developed entirely by shotcrete in the sections where relatively light loads are anticipated. Steel sets in conjunction with shotcrete were planned at the shattered shale section. Shotcrete thickness was approximated using a modified ring tension concept.

The tunnel lining serves to divide the ventilation system from the roadway, control groundwater seepage, provide a smooth surface to lessen damage to impacting vehicles, and give an even surface for reflecting the tunnel lighting. It is not required to give support to the rock. Tunnel finish provides a highly reflective, washable surface to the walls of the vehicle portion of the tunnel. These two items represent initial investments which reduce the continuing cost of the tunnel. By providing a smooth surface in the ventilating sections, a major decrease in fan horsepower is possible. By reflectorizing the tunnel surface, objects in the tunnel become visible by silhouetting against the walls. A safer tunnel is possible with less light.

California Department of Transportation contracts contain an item for contractor's mobilization. This item recognizes the high cost of tooling up for and starting a project. Accelerated payment of the item is made as the contractor begins the project. A contractor may bid up to ten percent of the total of contract items, or 11.1 percent of all the items excluding mobilization.

In all contracts it is the Department's policy to provide a contingency fund to pay for unforeseen work, necessary changes, and justifiable claims. At the time of bidding, this contingency fund is usually five percent. In the early planning stages, a 20 percent figure is needed in order to keep completed designs within the budgeted amount.

ESTIMATED PRICES - The estimated cost of constructing these tunnels is based on calculated quantities gained from preliminary designs. Prices are applied to these quantities by the office of Structures Pricing Sect ion. This section continuously monitors prices bid for construction in California and throughout the nation. Underground work is not contracted as regularly as structure work, and a reliable base of experience is difficult to establish and maintain. several of the items of work involved are reasonably similar to ordinary structure work. For these items, confidence in the estimated price is high. The major portion of the cost, over one-third of the. total, is for rock excavation. The price assigned to this item is highly intuitive.

State of California

M e m o r a n d u m

To : J. J. Kozak
Office of Structures

Attn: R. Reynolds

Date: September 12, 1974

File : 04-SM-l PM 27.8/41.8 04210 - 120771

D T. Cassinelli

Subject: Request for Geologic and Seismic Evaluation

Please prepare a brief report on the geologic and seismic considerations for the bridges and tunnel on the Devil's Slide Bypass Project referenced above. If you require maps or other data please request by phone from D. Heyes at 8-415-597-1458 .

We need the report by mid-October.

Materials Engineer

cc: File(3)

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Please let us know if you spot any errors or omissions. We will happily provide scanned copies of our xeroxed "originals" upon request. [email protected]