Big Creek Historic Bridge

Seismic Retrofit

May 1998-June 2000

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This project was contracted to PKB Construction in April, 1998, and was completed two years later in June, 2000. The goal was to strenthen the bridge to modern earthquake standards without destoying its historic appearance and value. As the profile shows, the designers were successful in retaining most of the original appearance; the only visible sacrifice being the scalloped underside of the bridge which now has a straight-line silhouette. The design includes anchoring the three pier foundations to bedrock with steel cable tendons, a goal which may have been compromised by the complex geology underlying the structure. Nevertheless the overall quality of the work appears to have been excellent. The bridge is held together by a system of steel cables running through tubes encased in reinforced concrete, and anchored at the ends by abutments and columns dug in to the substrate.

CalTrans and PKB were very cooperative in working with reserve staff to minimize disruprion to natural values and reserve activities. One major goal was fully achieved: to finish the project without killing or severely damaging the willows and alders lining the creek bank. In fact, the single largest work project (the foundations of pier 2) took place within 5 feet of several alder and willow trees, all of which survived intact. Thus Big Creek retained its original lining and stream bank characteristics. The most damaging aspect of the work was the disruption of nesting for Black Oystercatchers in 1998, 1999, and 2000. This bird is sensitive to noise and human activity near the nest site. We do not know where or if the birds found an alternative nesting site, nor whether they will return to nest in the cove next year. Since they have nested there every year since the 1980's, we predict that they will re-occupy their former site. Steelhead populations in the creek were unaffected by the work. The workers were careful to contain and properly dispose of cement and other materials, and the site was restored to its original contours after the work. Except for the work zone directly under the bridge, the original soil surface was maintained and we predict that resprouting of root masses will occur next winter. We have gathered lupine and California Poppy seed and have spread it over the site. See also the slide 2000 page for information on reseeding.

One by-product of the bridge work was an archaeological salvage dig competed in spring 2000. The dig produced very interesting findings which are now being analyzed and compiled. The archeologists found well-stratified material, clearly prehistoric, and including a burial and artifacts. After its discovery the burial was covered intact and not disturbed. A re-burial ceremony was conducted by elders of the Salinan Nation, the historic Native American occupants of the Big Creek area. The findings should help shed light on the relationship between shoreline and inland human activity in prehistoric times. See Nature Notes from Big Creek for a brief summary of the findings.


 "Barn Swallow repellers" installed to scare away swallows before they build nests.

 Scaffolding inside pier 2, to install rebar and remove struts spanning the two ouside walls.

 Protection zone for Astragalus plants. These are the host plant of the butterfly Everes amyntula, an uncommon species on the reserve.

 Construction "yard" in highway turnout for contractor hired to do bridge job.

 The old bridge foundation is excavated and the central bridge removed. The outside of the foundation is augmented and built up.

 The maze of rebar includes tubing in which cables run to tie the bridge deck above to the foundation below.

 The bridge deck was reinforced by building a temporary deck underneath and adding concrete beams the full length.

 Tubes run the full length of the beams, which house cables tying the bridge to the new abutments at each end.

 Several drill rigs were used on the job. Drilling and subsequent installation of anchors proved to be the most difficult part of the job, and there is some doubt as to whether the anchors tie in to bedrock, as specified in the plans.

 The old road on the north end of the bridge was re-opened to facilitate work. The original plans called for all work to be made from the bridge deck above, but this was a far more practical solution. Access down this roadway was limited for most of the project.

 The ends of the bridge were dug out and large abutments added. These were tied in to 20' deep anchors shown on the right side of the photo.
 After the abutments were poured the longitudinal cables were installed and tensioned (I was told) to 1000 tons. The cables were then locked in place.

 Sandbags were used to keep the creek from flooding the construction site during winter storms. The site got flooded once by a 7' crest, depositing silt over the work under pier 2.

 Work completed on June 10, 2000. The access road and disturbed work area will be allowed to revegetate from local sources. We will monitor and chop weeds as needed, and have spread local seed as appropriate.
 The pump used to extract water from the creek for construction use. This pump was flooded and destroyed during the February 2000 floods.

Looking down at the achaeological salvage work. The material was well-stratified and yielded artifacts and material for dating the layers.

 The material was obviously prehistoric. Hearth stones are exposed in this photo.

 Huge waves and a high tide on 1-11-01 scoured away the beach sand, resorted boulders, and pounded the kayak locker and the base of the bridge. 1-14-01

 Regrowth is sparse and quite a few seedlings died after germinating in October, but plants are growing. 1-14-01
 California poppy growing under bridge, 1-14-01.

The rains came in December 2001 and created excellent conditions for seedling germination and plant growth. By January 2001 many plants of California Sagebrush (Artemesia californica) and Lizard Tail (Eriophyllum staechadiflolium) were growing in the bare zone along with a long list of other species:

 Lizard Tail Eriophyllum staechadiflolium  Morning Glory Calystegia macrostegia
 California Sagebrush Artemesia californica  Ceanothus thrysiflora
 Bromus (annual grass) sp.  Mustard Brassica
 Clover Trifolium sp.  Burr clover Medicago polymorpha
 Figwort Scrophularia californica  Vetch Vicia sp.
 Water hemlock Conium maculatum  Yarrow Achillea millefolium
 Sow thistle Sonchus oleraceus  California Poppies Eschscholzia californica
 Everlasting Gnaphalium sp.  Yellow Bush Lupine Lupinus arboreus
 Bunchgrass  Coyote Brush Baccharis pilularis
 Hedge Nettle Stachys bullata  


 By far the most common plants recruiting to the bare zone are the sagebrush and the lizard tail (see list above). 1-8-02

 In some areas the number of seedlings is quite large 1-8-02

There are several bunchgrasses growing in the bare zone 1-8-02

 Here is a view of the bare zone from up above on the bridge 1-8-02

By January 2003 the disturbance site has begun to fill in with native vegetation. The only significant exotic species that we have found at the site is Pampas Grass (Cortaderia jubata). About 15 plants were removed by hand. This panoramic photo shows the general progress of regrowth:


 This photo shows a young Pampas Grass (Cortaderia jubata) growing in the cleared area. These plants are newly sprouted this fall. 1-13-03

 This is one of the archaeological investigation trench sites, completely revegetated with Lizard Tail (Eriophyllum staechadiflolium) and other natives 1-13-03



 This is a California brickellbush (Brickellia californica), a native that has recently appeared at the site. It has also become common at the "slide 2000" revegetation zone. 1-13-03

This is a view down the access road that was re-opened during the bridge project. It is regrowing natives. 1-13-03

 Here are some native shrubs growing in the middle of the bare area, including California Sagebrush (Artemesia californica) and Lizard Tail (Eriophyllum staechadiflolium) 1-13-03