Professional Contributions

Restoration of an Urban National Park Service Setting

Three Case Studies

William Godwin, PG, CEG

Figure 1. Vicinity and geologic map by Blake et al (2000). GGB – Golden Gate Bridge. No scale.

Introduction

This paper presents the geologic aspects of recent restoration efforts on National Park Service (NPS) facilities that allow reuse by members of the public in addition to geoscientists. The three case studies presented examine the background and geologic basis for remediation and restoration of infrastructure that provides both historic and current access to these areas. The three cases studies, whose locations are shown in Figure 1, consist of the Point Bonita Lighthouse Bridge replacement, the Baker Beach Disposal Area (BBDA) removal, and the Presidio Parkway replacement of Doyle Drive.

History and Geologic Setting

The Golden Gate National Recreation Area (GGNRA) is one of the largest urban national parks in the world. Established in 1972 to bring “parks to the people,” GGNRA’s nearly 76,000 acres of land and water extend north of the Golden Gate Bridge to Tomales Bay in Marin County and south to San Mateo County in coastal California. Most of the GGNRA occupies the coastline and was originally developed to support military fortifications and shipping navigation.

The subject area is located in California’s Coast Ranges geomorphic province, an approximately 500-mile-long series of northwest-trending mountain ranges (2,000 to 6,000 feet elevation) and valleys that generally trend subparallel to the San Andreas Fault. The geomorphology of the Coast Ranges province is controlled predominantly by deformation (faulting, folding, and shearing) associated with subduction of the Farallon Plate beneath the North American Plate and movement between the Pacific and North American Plates along the San Andreas Fault system.

The geology of the Coast Ranges province features several terranes, which are distinct blocks or assemblages of Cretaceous and Jurassic, igneous and sedimentary oceanic rock scraped off the Farallon Plate during its subduction and metamorphosed (or not) to varying degrees. These terranes and associated mélange zones are collectively called the Franciscan Complex.

In the Bay area, the Franciscan Complex is divided into the eastern and the central belts, with the older eastern belt lying structurally higher and being of higher metamorphic grade than the central belt. In Figure 2, William Elder (geologist with the National Park Service) shows that San Francisco and the Marin Headlands contain three of these terranes, from oldest to

Figure 2. Simplified geologic model by Elder (2001). No scale. youngest, the Alcatraz, Marin Headlands, and San Bruno Mountain. Separating the terranes are the Hunters Point and City College mélange zones, which are primarily composed of sheared serpentinite and shale with scattered blocks of greenstone, chert, graywacke, and high-grade metamorphic rocks.

Baker Beach Landfill Removal Project (2003–09)

The Presidio of San Francisco was maintained by the United States Army for 200 years before base closure transferred ownership to the National Park Service (NPS) in 1994 and administration to the Presidio Trust (Trust) in 1998. The Trust oversees public infrastructure, restoration and leasing of historic buildings, maintenance and restoration of open spaces and native ecosystems, and clean-up of contaminated Army-era sites.

The Presidio of San Francisco occupies the far northern portion of San Francisco, California and is comprised of steep, seaward facing cliffs to the west and rolling hills that give way to tidal lowlands on the eastern portion that fronts on the San Francisco Bay (see Figure 1). The Baker Beach Disturbed Areas (BBDA) were identified as potential Army-era disposal sites warranting further investigation under the cooperative agreements in place between the Trust and the NPS. Four BBDAs (1, 1A, 2, and 2A) are situated on sea cliffs that are as high as 270 feet (80m) above the beach. Relief varies from vertical bedrock faces at the top of the cliff, typical 1:1 slopes below the cliffs, and 1:2 lower slopes where landslide deposits have accumulated (see Figure 3).

Figure 4. Geologic and exploration map of BBDA-1 (MACTEC, 2007).

Site Geology

Basement rocks lie at shallow depths and consist primarily of serpentinite (ultramafic), graywacke sandstone, and lesser amounts of greenstone (altered volcanic), which have been tectonically deformed. Overlying these rocks are younger, unconsolidated or poorly consolidated deposits including the Colma Formation, landslides, dune sands, slope debris and ravine fill (colluvium), beach deposits, and artificial fill. Of particular interest for the project were the artificial fill and the underlying colluvium and landslide deposits, which are nestled amongst historically significant former gun batteries (Marcus Miller and Boutelle) as shown within BBDA-1 in Figure 4.

Subsurface Investigation

Prior to beginning the subsurface investigation, a Sampling and Analysis Plan (SAP) was prepared and submitted for approval to the California Department of Toxics and Substance Control (DTSC), the lead regulatory agency. The location of subsurface exploration and sampling points in BBDA-1 are shown in Figure 4.

Twenty-one test pits were excavated by AIS Construction using specialized equipment (Spyder, Model No. S2). Fifteen soil borings were drilled using standard and limited access drilling rigs. Soil samples were analyzed for: Polyaromatic Hydrocarbons (PAHs); California Code of Regulations (CCR) Title 22 metals, Organic Pesticides, Polychlorinated Biphenyls (PCBs), Total Petroleum Hydrocarbons (TPH) quantified as diesel and fuel oil, and Volatile Organic Compounds (VOCs).

A structural condition survey was performed at four historic Batteries to establish a baseline condition prior to debris removal or slope disturbance. After the initial assessment, slope inclinometers and survey reference points were installed near the structures most likely to be compromised by construction.

Because of the historical significance of the West Battery and Endicott period structures, some of which are partially buried, a detailed trenching program was instituted. Detailed archaeological trenching was performed on the bluff top within the historic architectural fill of the magazines and batteries.

Construction and Debris Removal

The construction remediation contract was awarded to AIS Construction in 2005. After all permitting and regulatory agreements had been met, the contractor mobilized to the site and began removal activities of debris on the slopes. The contractor used a series of long-reach excavators, Spyder excavators, and conveyors to bring excavation debris fill and surficial soils to the top of the bluff, resulting in a clean rock surface, best shown in Figure 5. A sorting and loading operation was carried out at the bluff top.

The quantity of debris removed was substantially more than estimated with 54,000 cubic yards (cy) removed versus an estimate of 29,400 cy after a conversion from tonnage

trucked offsite. The project was completed and documented in a construction report (MACTEC, 2008).

Point Bonita Lighthouse Bridge Replacement Project (2011–12)

The Point Bonita Lighthouse is located on the Marin Headlands in Marin County and is the third-oldest lighthouse on the West coast. The lighthouse was built in 1855 as a navigational aid for ships traversing the Golden Gate channel. The lighthouse was moved to its current location in 1877, and it is the only lighthouse in America that can be reached only by crossing a suspension bridge, which was designed to be a replica of the Golden Gate Bridge. Prior to 1937, the primary access to the lighthouse was via a pathway and associated hand-dug tunnel, measuring 6 feet high by 4 feet wide and 118 feet in length, along a narrow peninsula as there was no access via the swift Pacific Ocean 124 feet below. Despite two major renovations, half a century of corrosive salt air took its toll on the approach bridge and a replacement was needed by 2010.

Site Geology

According to geologic mapping by HDR (2009), the bluffs surrounding the bridge location are composed of Cretaceous metadiabase, metabasalt, and meta-pillow basalt exhibiting a low degree of metamorphism. The meta-pillow basalts form the base of Point Bonita and can clearly be seen from the bluffs at the site. The metadiabase overlies the meta-pillow basalts and directly underlies the western end of the bridge (Figure 6). Both the western bridge tower and anchor block are founded in the metadiabase. The metadiabase is weathered, hard, and moderately fractured. Wedge failures along fractures produces over-steepened slopes and potentially unstable conditions at the bridge site. A significant slope failure in 1937 led to replacement of the bridge in 1954 by the US Coast Guard.

Rock Stabilization and Replacement

Later Investigations determined that the east abutment of the bridge did not appear to pose any imminent rock stability issues and therefore bluff stabilization was not needed in this area. However, the west abutment did

Figure 6. Lower: view northwest of geology and access to Lighthouse Point by suspension bridge in 2009; Upper: previous pathway at time of 1937 slope failure (photos in HDR, 2009). Figure 7. Completed Point Bonita Lighthouse bridge showing ground anchorage and basalt arch at left

require rock stabilization and reinforcement. Along the north side of the western abutment, rock slope failures had occurred and left an unstable overhanging rock mass. The recommended slope treatment was a high strength stainless steel mesh net with tensioned rock bolts of staggered length and pattern.

Flatiron was the general contractor hired to construct a near-identical replica of the existing bridge. The new structure is a two-hinged suspension bridge with asymmetrical straight backstays. The replacement bridge has a 156-foot main span with a 55-foot approach span on the west side and a 33-foot approach span on the east side. A Geobrugg stainless steel anchored SPIDER S4-230 mesh system was installed to stabilize the rock face.

PHOTO COURTESY OF THE NATIONAL PARK SERVICE

Presidio Parkway Project (2011-15)

The Presidio Parkway project is a replacement of Doyle Drive, a 1.6-mile segment of Route 101 in San Francisco that is the southern access to the Golden Gate Bridge (GGB). The GGB connects Marin and San Francisco counties and provides a major regional traffic link between the peninsula and North Bay Area counties. The existing access structure, built in 1936, did not meet current highway standards and was seismically deficient. The new Presidio Parkway is a six-lane facility, which features a high-viaduct between the Park Presidio Interchange and the San Francisco National Cemetery (Presidio Viaduct), shallow cut-and-cover tunnels past the cemetery to Battery Blaney (Battery Tunnels), and cut-and-cover tunnels from the Main Post to east of Halleck Street (Main Post Tunnels).

Site Geology

The project extends through variable terrain that includes Franciscan bedrock of mélange, sandstone and colluvium in the higher elevations and a complex sequence of old fill, dune and marsh deposits, and old bay clay along the lower elevations. This project was adjacent to Crissy Field, part of an extensive 127-acre backdune marsh that drained Tennessee Hollow watershed to San Francisco Bay. Over many decades and culminating with preparations for the 1915 Panama-Pacific International Exposition, the marsh was filled, and the resulting land was used by the United States military (WLA, 2000) (see Figure 8). From 1997–2000, forty acres of natural habitat were restored including an 18-acre tidal marsh and twenty-two acres of dune and dune swale habitat (see Figure 9). These deposits are all subject to liquefaction and severe ground motion in a large earthquake from the nearby San Andreas and Hayward fault zones.

Construction

A public-private partnership was established to design and construct this project, led by the Kiewit/Flatiron/HNTB design-build team. One key design requirement for the low lying areas was the

Figure 8. In 1935, Doyle Drive was under construction and would divide the Main Post in the park and Crissy Field, which was then an Army airstrip.

IMAGE COURTESY NATIONAL ARCHIVES AND RECORDS ADMINISTRATION.

Figure 9. Presidio Parkway under construction, view south with new Main Post Tunnels at right and Low Causeway at left. Figure 10. Riato CDSM drill rigs working areas for new interchange with Palace of Fine Arts in background

need for ground improvement of the liquefiable soils. A Cement Deep Soil Mixing (CDSM) method was implemented to provide a stable base for both at grade design elements in addition to deeper foundations (Chen et all, 2014) as shown in Figure 10.

Cut-and-cover tunnel foundations were prepared on the bluff tops underlain by weathered Franciscan bedrock. Foundations for the high viaducts were 9-foot-diameter bored shafts that in certain cases extended more than 200 feet below grade, as shown in Figure 11.

Conclusions

Access to historically significant structures and complex geologic terrain within an urban setting of National Park Service land was enhanced by investments in new infrastructure and restoration of existing infrastructure. These investments allow the public to interpret geologic highlights of the GGNRA in an environment that has had a significant reduction in toxic compounds and seismic risk.

PHOTO BY W. GODWIN

References

Blake, M.C., Jr., Graymer, R.W., and Jones, D.L., 2000, Geologic map and map database of parts of Marin, San Francisco, Alameda, Contra Costa, and Sonoma Counties, California: U.S. Geological Survey Miscellaneous Field Studies, MF-2337, v. 1.0. Chen, Wei-Yu, Bro, Andrew, Vahdani, Shahriar, and Herlache, Andrew, 2014, Drilled pier foundation with ground improvement using cement deep soil mixing for highway bridges, in Proceedings, DFI/EFFC 11th International Conference on Piling and Deep Foundations, Stockholm, Sweden. Elder, W.P., 2001, Geology of the Golden Gate Headlands, in Stoffer, P.W., and Gordon, L.C., eds., Geology and Natural History of the San Francisco Bay Area; a 2001 National Association of Geoscience Teachers Field-Trip Guidebook: U.S. Geological Survey Bulletin 2188, chapter 3, p. 61-86. Fugro Consultants, Inc., February 6 2012, Final Geotechnical Design Report, Presidio Parkway Project, San Francisco, California, 04-SF-101, 1, Post Miles 8.2/9.8, 6.9/7.1, Contract Number 04-1637U4, prepared for HNTB Corporation. HDR Engineering, Inc., November 2009, Point Bonita Lighthouse Suspension Bridge, Trail and Access Road, Coastal Erosion Study Report. CA NPS GOGA 433 (1), prepared for Federal Highway Administration, Central Federal Lands Highway Division (CFLHD). MACTEC Engineering & Construction, Inc., December 2008, Construction Completion Report, Baker Beach Disturbed Areas 1 and 2A Landfill Removal, Presidio of San Francisco, California, prepared for The Presidio Trust. _____, July 2007, Final Remedial Design Document and Remedial Action Work Plan, Baker Beach Disturbed Areas 1 and 2A, Presidio of San Francisco, California.