trail and access road - Central Federal Lands Highway Division

Coastal Erosion Study Report 

Point Bonita Lighthouse Suspension Bridge, Trail and Access Road 

1.0 INTRODUCTION 

This report summarizes the findings of a reconnaissance study of geologic conditions and the 

potential for continued coastal erosion at the site of the Point Bonita Lighthouse Bridge in the 

Golden Gate National Recreation Area north of San Francisco, California (see Figure 1 below). 

This work is being conducted for the Federal Highway Administration, Central Federal Lands 

Highway Division (CFLHD), CA NPS GOGA 433(1), under Task Order DTFH68-06-D- 

00007/T-09-014. 

As outlined in the task order proposal, the purpose of the geologic and erosion assessment was 

defined with the following goals: 

“The work is a preliminary screening study, of both coastal erosion and foundation rockmass 

slope stability hazards potentially affecting the planned service life of the proposed replacement 

bridge, including a cursory, qualitative review of site conditions with respect to overall stability 

of the proposed replacement bridge foundations and underlying rockmass, with the objective of 

identifying specific site conditions (potential fatal flaws) that may require additional evaluation 

in future work. Whether or not potential fatal flaws are identified, modes of failure of the 

foundation rockmass, if apparent, and feasible mitigation alternatives will be identified. The 

modes of unsatisfactory performance of the rockmass may be grouped into three major 

categories: (1) Erosion of the lower portions of the rockmass leading to undermining and 

instability of the overlying portions (structure foundation portion) of the rockmass; (2) Static 

stability of the upper portions of the rockmass separate from erosion of the lower portions of 

the rockmass, and (3) Qualitative predictions of site stability under a seismic event.” 

In addition to the scope described in the Task Order, at the request of the National Park 

Service, additional field observations were made, photographs were taken, and a review of 

previous monitoring data was completed for the trail leading from the southerly tunnel portal to 

the northeasterly bridge abutment, and for the access road from Conzelman Road to the 

northerly tunnel portal (see Photo 1 below). 

1 November 2009



Photo 1. Overview of Point Bonita site showing features. 

2 November 2009



2.0 BACKGROUND 

The historic Point Bonita Lighthouse Bridge is located within the Golden Gate National 

Recreation Area (GGNRA) in Marin County, CA, just northwest of the Golden Gate Bridge, 

and provides access to a lighthouse operated by the United States Coast Guard. 

Figure 1. Vicinity Map 

The bridge was inspected under the National Bridge Inventory System’s (NBIS) Bridge 

Inspection Program (BIP) by the Federal Lands Bridge Office in September, 2007. The bridge 

inspection report was completed and distributed in October, 2007. The report found the bridge 

to be severely corroded and in poor overall condition. Based on the deteriorated condition, the 

bridge is currently load rated for a “two person” limit. The 2007 inspection report 

recommended that the structure be replaced within the next two to three years due to the 

advanced state of deterioration and the dangerous site conditions. 

A draft Bridge Selection Report has recently been completed by HDR Engineering, Inc. That 

report provides recommendations for a replacement bridge and includes a conceptual design. 

3 November 2009



The report further recommends that a study be undertaken to determine the potential for further 

cliff erosion that could impact the stability of the proposed new bridge. 

4 November 2009



3.0 FIELD REVIEW SITE VISIT 

Eric Chase, P.G., C.E.G., and Joel Darnell, Coastal Engineer, both of HDR Engineering, Inc. 

conducted the Field Review Site Visit. The objective of the field review was to document the 

existing geologic conditions around the suspension bridge and along the lower portions of the 

bluffs supporting the bridge. 

The sideslopes at the site are very rugged, and close observation of the sideslopes of Point 

Bonita was not performed due to the oversteepened slopes adjacent to the trail. These 

observations could be performed using proper rock climbing techniques and equipment; 

however this approach was outside of the scope of this study. Data obtained during the field 

visit was limited to observations of slopes and erosion conditions from available viewpoints on 

the mainland and from the path and bridge. Subsurface investigations were not performed. 

The specific field activities conducted during the field visit included the following: 

• Available literature including published geologic maps and reports, unpublished 

consulting reports, trail monitoring data, and aerial photographs were obtained and 

reviewed. 

• Available photos and reports from the literature review were brought to the field for 

reference. Ocean conditions were visually observed, including wave diffraction and 

shoaling, ocean conditions along exposed slopes, and evidence of wave-induced erosion 

and failure within project bounds. 

• Digital photographs were taken of features considered relevant to evaluating current 

geological conditions affecting support for the existing bridge. Photographs were taken 

of the rockmass supporting the bridge abutments and of the underlying slope to ocean 

level. Photographs were taken only from safe vantage points on the adjoining portions 

of the Point. 

• Significant through-going planes of weakness in the diabasic and basaltic rocks were 

recorded in the field on a Light Detection and Ranging (LIDAR)-derived topographic 

5 November 2009



base map. A preliminary geologic map was prepared showing visually-observable 

rockmass slope features. Selected features were photographed and the photos labeled 

(see photos herein and Preliminary Geologic Map, Attachment 1). The reconnaissance 

focused on evidence of past rock slope failures and continuous rockmass discontinuities 

oriented so as to define blocks, wedges, or sheets that are potentially free to move out of 

the rockmass, and which could lead to loss of bridge abutment foundation support. 

• Where accessible without safety equipment, surface geologic data was mapped and 

measured, including rock type, degree of weathering, and fracture, shear zone and joint 

orientation, spacing, continuity, openness, and infilling. The data were subsequently 

transferred to photographs of those portions of the site. 

6 November 2009



4.0 OBSERVATIONS AND DISCUSSION 

On May 27, 2009 Messrs. Chase and Darnell of HDR met with John Ryan of the National Park 

Service (NPS) to perform an engineering geology and coastal erosion reconnaissance 

assessment of the Point Bonita Lighthouse area. The original scope included only the bridge 

abutments and areas immediately adjacent to the bridge. However, Mr. Ryan requested that the 

reconnaissance be expanded to include the access road, tunnel, and trail areas. Mr. Ryan 

indicated areas of concern relative to slope stability and erosion based on his long tenure and 

experience at the site, which encompasses the past 30 years. Mr. Ryan has documented the 

erosion at various places along both the access road and trail, as well as various failures in the 

rock mass supporting the southwesterly bridge abutment (see comparison of 1980 and 2009 

conditions in Photo 4 below). 

Based on available published and unpublished geologic reports and maps and observations 

during the site reconnaissance, the geology of the site consists of a basement complex of 

greenstone comprising metamorphosed diabase, metamorphosed basalt flows and 

metamorphosed pillow basalts (Blake et al., 1985, William Lettis and Associates, 1994, 

Schlocker, J. et al., 1958, and Schlocker, 1974). These bedrock units are locally overlain by 

surficial deposits including talus, landslide debris, marine terrace deposits, and residual soils 

(colluvium). Site geology is presented on the Preliminary Geologic Map, Attachment 1. 

4.1 Geologic Conditions - Suspension Bridge Area 

As described above, the suspension bridge area is underlain by metamorphosed diabase, 

metamorphosed basalt, and metamorphosed pillow basalt. Numerous steeply-inclined 

intersecting planes of weakness are evident in the rocks immediately beneath the 

suspension bridge, as depicted in Photos 2 through 4 and on Attachment 1, Preliminary 

Geologic Map. These planes of weakness reflect tectonic stresses applied to the rock 

both during metamorphism and subsequent tectonic translocation from west to east 

during obduction. Further planes of weakness may have formed due to seismic shaking 

and gravity-induced rock creep. 

7 November 2009



Mr. Ryan indicated that the rock surface beneath the southwesterly bridge abutment has 

been noticeably eroding, and he recalled that the rock at abutment level was nominally 

six feet wider as recently as 15 years ago. Evidence of historic and recent slope failures 

in the middle and lower bluffs can be seen in the following photos. The scar from a 

1937 slope failure event (documented in historic photographs and records provided by 

the United States Coast Guard and NPS) is evident in Photo 1 below (view to 

northwest). Evidence of significant further erosion since the 1937 rockfall event is 

visible by comparison of Photo 2 (1937) with Photo 3 (2009). 

Photo 2. 

Northwesterly view of south side of Point, circa. 1937, showing 

failure of land bridge to lighthouse. 

8 November 2009



Photo 3. 

Northwesterly view of south side of Point, 2009, with scar of 1937 rock 

failure faintly visble. Note that the bridge construction was necessitated 

by failure of the land bridge to the lighthouse. Diabase forms the cliff 

face with pillow basalt exposed at the base near the water line. 

9 November 2009



Photo 4. 

Comparison of 2009 western abutment slope below suspension bridge 

with 1980 slope face. Note block which appears to have failed within 

circled area. 

Through-going planes of weakness (joints and shear zones) are well-exposed beneath 

the bridge and offer insight into the previous mechanisms of local failure. Steeplyinclined 

shear zones have formed the roughly planar surfaces along which the failures 

have released (see 2009 photograph above). Numerous intersecting joints, fractures, 

and shear zones are visible to the left of the recently-failed area, some of which are 

sufficiently through-going to jeopardize the stability of larger blocks of diabase, which 

will likely be next to fail. The most persistent of these weakness planes are depicted on 

Attachment 1. 

Wave erosion is evident as visible undermining at the water line. Two arches and a sea 

cave were noted in the reconnaissance (see Attachment 1), providing further evidence 

of active erosional processes within the wave impact zone. 

10 November 2009

4.2 Geologic Conditions Findings 



This section is intended to address Items 2 and 3 from the task order proposal, outlined 

in Section 1 of this report and re-stated here: 

(2) Static stability of the upper portions of the rockmass separate from 

erosion of the lower portions of the rockmass, and 

(3) Qualitative predictions of site stability under a seismic event. 

The static stability of the upper rockmass appears to be highly variable from location to 

location. As observed during the site visit, the rock is highly to moderately fractured, 

highly to moderately weathered, and is overlain by a variable but generally thin veneer 

of residual soils and colluvium. Significant and geologically rapid degradation of the 

rockmass on which the bridge is founded has occurred in the past 80 years and will 

likely continue. Such degradation may directly influence the stability of the bridge 

foundations (see Photo 4 above). Due to the oversteepened nature of all the sideslopes 

at the site, the continuous undercutting of the lower basaltic and diabasic units by wave 

action cannot be discounted in a qualitative evaluation of slope stability. Observations 

indicate that portions of the slopes are at or near instability (safety factor approaching 

1.0) and may be subject to localized or significant failure, especially during a seismic 

event. A quantitative estimate of the likelihood or significance of these potential 

failures was not included within the scope of this limited study. 

4.3 Coastal Erosion 

4.3.1 Site Conditions 

Wave conditions on May 27, 2009 were typical for the month of May, with 4- to 

6-foot swells from the southwest at periods of 8 to 10 seconds (National Data 

Buoy Center - Station 46237). Winds were moderate at 15 to 20 knots. Wave 

action increased slightly in the afternoon. The tide rose from about -2 ft relative 

to North American Vertical Datum (NAVD) 88 up to about +5 ft relative to 

NAVD88 at high tide (National Oceanographic and Atmospheric Administration 

- Station 9414290). 

11 November 2009



4.3.2 Observations 

North-facing rock bluffs are directly exposed to Pacific Ocean swell. Near the 

water line, ongoing wave undercutting of the bluff face is evident. Active wave 

erosion appears to cease approximately 20 feet above mean higher high water. 

Talus, consisting primarily of diabase blocks, appears to provide some natural 

armoring at the base of the bluff. This armoring is probably only effective 

during relatively calm to moderate wave action. Much of the material at the 

base of the bluff has been broken and repeatedly reworked by waves and 

currents into boulders and large cobbles. Cobbles have accumulated in the 

sheltered areas at the foot of southerly-facing bluffs. 

Erosion and slides of loose sandy material and weathered rock contribute some 

sand and gravel to the beach that fronts the west-facing side of the headland. In 

such a high-energy wave environment, little of these materials remains in the 

intertidal zone, either being buried beneath larger cobble or transported offshore 

and deposited in deeper water following large storms. 

Wave-induced erosion near the water line appears to be a relatively slow 

process, based on comparison of historical and current photographs. Exposed 

pillow basalt formations near the water line formed slopes from nearly-vertical 

to undercut due to weathering by seawater and erosion from constant wave 

action. The foundation of the existing southwesterly bridge tower appeared 

vulnerable to long term erosion and undermining of the rock face by wave 

action. At this location, the rock face is nearly vertical and appears to extend 

from the tower footing to the water line. The northeasterly bridge tower 

foundation is less vulnerable to erosive wave action, being located away from 

the exposed rock face. 

On the east-facing side of Point Bonita, wave action is significantly reduced as 

evidenced by sandy pocket beaches. Weathering and undercutting was observed 

near the water line. Slopes near the water line are generally less steep in these 

12 November 2009



areas. Wave action does not appear to be contributing to the surface erosion 

occurring near the northeast anchor block. 

4.4 Coastal Erosion Findings 

This section is intended to address Item 1 from the task order proposal, re-stated as: 

(1) Erosion of the lower portions of the rockmass leading to undermining and 

instability of the overlying portions (structure foundation portion) of the 

rockmass; 

Our observations related to erosion and undermining of the bluffs at Point Bonita include 

the following. 

• Wave and tidal action are actively eroding and weathering the base of the bluffs and 

rock faces at Point Bonita, particularly on the west side. 

• Existing talus provides some natural armoring at the base of the west-facing bluffs, 

likely reducing erosion rates near the water line. The talus appears to provide 

protection during low to moderate energy wave events, but is unlikely to protect the 

bluffs during high wave periods. 

• Placement of armoring material such as large natural rock to protect the base of the 

bluffs is unlikely to be effective in mitigating bluff erosion and retreat since some 

natural armoring is already in place. Upper bluff weathering will continue to occur, 

even if the effects of wave action are mitigated at the base of the bluff. 

13 November 2009



5.0 NOTES ON GEOLOGIC INSPECTION - TRAIL AND 

ACCESS ROAD 

5.1 Trail 

Two pre-fabricated fiberglass bridges, one approximately 40 feet long and the other 

approximately 80 feet long, have been placed along the trail where on-going erosion 

and sloughing have compromised the structural integrity and safety of the path. Mr. 

Ryan indicated these bridges were installed more than 10 years ago. Mr. Ryan also 

indicated that these footbridge locations are areas of continuing concern from a 

maintenance perspective. An additional area with evidence of downslope creep and 

potential sloughing is found along the trail between 230 and 290 feet west of the west 

tunnel portal. 

At this location the concrete trail pavement is cracked and shows 

evidence of subsidence attributable to downslope creep of surficial soil and weathered 

rock on which the trail pavement is founded. 

In 1996 a number of survey monuments were installed to monitor movement of the 

ground surface along the trail by William Lettis and Associates, Inc. (WLA). A hand 

drawn WLA map, comprising two sheets and labeled WLA Figure 1, Appendix C is 

provided as Attachment 2 hereto. This map illustrates the locations of the trail 

monitoring points discussed below. Measurement data recorded by NPS personnel 

from 1996 through 2003 were provided by John Ryan and were reviewed. The 

measurements consisted of recording the distance between two fixed points at each 

station. The fixed points included brass survey markers, concrete nails, or steel rebar 

driven into the ground. The initial and final readings of the distances between the two 

points for each station, and the corresponding movement rates for each station along the 

trail are presented in Table 1: 

14 November 2009



Table 1. Measurements of Trail Monitoring Stations 1996-2003 

Rate of 

Change 

(inches per 

month) 

Measurement 

Point 

Initial 

Reading 

(Date) 

Final 

Reading 

(Date) 

Net 

Displacement 

TMS-1 63” (6-17-96) 64 ½” (3-5-99) 1 ½” 0.045 

TMS-2 5 ⅛” (6-17-96) 5 7/8” (12-16-03) 3/4” 0.008 

TMS-3 70” (6-17-96) 70 7/8” (12-16- 7/8” 0.010 

03) 

TMS-4 3” (6-17-96) 2 15/16” (7-13- -1/16” 0.0007 

03) 

TMS-5 37” (6-17-96) 36 3/4” (12-16- -3/4” -0.008 

03) 

TMS-6 49 ¼” (6-17- 49” (12-16-03) -1/4” 0.003 

96) 

TMS-7 41 ½” (6-17- 41 3/4” (12-16- 1/4” 0.003 

96) 

03) 

TMS-8 51 ½” (6-17- 51 ½” (12-16-03) No Change 0.000 

96) 

TMS-9 None None N/A --- 

TMS-10 95 ¾” (6-17- 96” (7-13-03) 1/4” 0.003 

96) 

TMS-11 21 ½” (6-17- 21 ¼” (12-16-03) -1/4” 0.003 

96) 

TMS-12 68 ½” (6-17- 68 ½” (12-16-03) No Change 0.000 

96) 


TMS-14 31 3/8” (6-17- 31” (12-16-01) -3/8” 0.006 

96) 



No measurements were recorded for three of the sixteen total stations. The recorded 

target accuracy of measurement (+/- 0.125” to 0.25”) was higher than the measured 

value of net change for all but five of the 13 measured stations. The five stations for 

which the data are therefore considered reliable include TMS-1, -2, -3, -5, and -14. Of 

these five stations, three had measured spacings that became wider and two had 

measured spacings that became smaller. 

15 November 2009



Examination of the descriptions and photographs included with the NPS records also 

provides insight into the nature of the movements recorded for the five stations with 

reliable data, as follows. 

Station TMS-1: The horizontal distance between two steel rebar pins with yellow 

protective caps on either side of the trail was measured. The trail in this location spans 

the head of a shallow colluvium-filled swale, which in 1996 experienced several inches 

of settlement (labeled Slide Area 1 on the attached WLA map, Attachment 2). The 

monitoring data indicate that this section has settled an additional 1½ inches from 1996- 

1999, and may need to be stabilized to keep the trail open to foot traffic. 

Station TMS-2: Two nails set in the paved surface of the trail were measured for their 

horizontal separation. The nails have separated by ¾-inch over a 7½-year period, 

indicating lateral spreading due to slow downslope creep of the underlying surface. 

TMS-2 is located at the head of a second colluvium-filled swale (labeled Slide Area 2 

on Attachment 2). This area may need to be stabilized to keep the trail open to foot 

traffic. 

Station TMS-3: The horizontal distance between a brass survey marker and a piece of 

rebar set on the western (upslope) edge of the trail pavement was measured. The 

readings indicate that the distance has increased by 7/8-inch over a 7½-year period. 

TMS-3 is located very near to TMS-2 in the same colluvium-filled swale (Slide Area 2). 

This area may need to be stabilized to keep the trail open to foot traffic. 

Station TMS-5: This station comprises a yellow-capped reinforcing bar driven into the 

ground on the downslope edge of the trail and a nail located on the upslope (western) 

edge of the trail pavement. This station is located on the southerly edge of the head of 

the colluvium-filled swale in which TMS-2 and -3 are located. The horizontal distance 

between the rebar and the nail decreased by ¾-inch in a 7½-year monitoring period. 

This may be attributable to headward rotation of the rebar due to downslope creep of 

the colluvium, but vertical measurements that would either confirm or preclude this 

possibility were not obtained. This area may need to be stabilized to keep the trail open 

to foot traffic. 

16 November 2009



Station TMS-14: This station comprises a yellow-capped reinforcing bar driven into 

the ground on the downslope side of the 80-foot long fiberglass bridge and a brass 

survey marker set in the foundation of the bridge. The horizontal distance between the 

rebar and the bridge foundation decreased by 3/8-inch in a 5½-year monitoring period. 

This may also be attributable to headward rotation of the rebar due to downslope creep 

of the colluvium, but no vertical measurements were obtained that would help to 

confirm or preclude this possibility. 

In summary, measurements at 13 trail monitoring stations over a period of 5½ to 7½ 

years indicate that continuing downslope creep of surficial soil on which the trail was 

constructed may be ongoing at three locations, namely Slide Areas 1 and 2 and the 

downslope side of the southerly edge of the long, pre-fabricated fiberglass bridge. All 

these locations were identified by WLA as being surficially unstable in 1996. The two 

fiberglass bridges now in use were reportedly installed in areas of recognized erosion, 

creep, or settlement and thus provide a biased record of the overall trail stability. These 

areas continue to degrade due to natural erosion processes. Measurements recorded at 

ten other monitoring stations were inconclusive relative to the expressed target accuracy 

associated with the measurement methods employed by NPS personnel. 

5.2 Access Road 

The access road extends northerly from the northeast tunnel portal to the pipe gate at 

Conzelman Road. The road traverses slope areas above three active landslides. The 

northernmost slide area does not directly impinge on the road alignment, but occupies a 

canyon head between a former housing area and the access road. This slide appears to 

have destroyed a paved access road that formerly led to the housing area. 

Debris from the middle slide rests in a steep and relatively narrow drainage. The access 

road traverses the headscarp area of this shallow failure. Two very old and rusted 

corrugated metal culvert pipes were observed jutting out from beneath the access road. 

The culverts collect water from inlets on and above the access road and discharge into 

17 November 2009



the drainage/middle slide area (see Photo 5 below). Discharge from these drainage 

pipes may have contributed to recent reactivation of the upper portion of the slide mass, 

which now threatens the road shoulder. This area is in need of immediate drainage 

modification and road shoulder reconstruction in order to maintain vehicular access. 

Photo 5. 

Corrugated metal pipes, erosion and undermining 

of shoulder along access road (middle slide area). 

The southernmost slide area is situated on the east side of a bridge approximately 250 

feet uphill of the north tunnel portal (see Photo 6 below). The bridge appears to have 

been constructed to mitigate instability caused by headward migration of the failure 

scarp that had previously undermined the paved access road. It appears that erosion of 

headscarp materials is progressing on the eastern side. 

18 November 2009



Photo 6. Bridge along access road spanning southernmost landslide escarpment. 

Erosional undermining is also progressing toward this bridge location from the western 

side. This western side erosion has been treated with a timber crib wall (see Photo 7 

below), which shows evidence of undermining since emplacement (date unknown). 

Photo 7. 

Timber crib wall beneath western side of bridge along access road. 

Note erosional undermining at base of crib wall and accumulation of 

eroded material on upper cross member. 

19 November 2009



6.0 SUMMARY OF FINDINGS 

6.1 Suspension Bridge Area 

Observations of conditions surrounding the suspension bridge to the Point Bonita Lighthouse 

indicate active wave attack at the base of the rock bluffs on which the bridge is founded. 

Comparison of current conditions, documented in photographs, with archival photographs 

obtained from NPS, along with personal communications with Mr. Ryan regarding his 

observations over a 30-year period, indicates retreat rates at the bluff top of up to five inches 

per year. Photographic evidence indicates active and significant erosion over the past 80 years 

of the rock mass upon which the bridge is founded. For rocks of this type under wave attack 

and weathering processes, the retreat is typically characterized by episodic, sudden rockfall 

events that are variable in size. Accurate predictions of conditions at a future point in time are 

difficult. Placement of appropriately-sized rock armoring such as rip-rap at the base of the 

bluffs on which the bridge, lighthouse, and attendant structures are constructed may help to 

slow the rate of erosion and episodic failure of rock masses in the area, but will not mitigate 

seismically-induced failures or failures induced by factors such as weathering and frost 

wedging. Placement of rip-rap may not be aesthetically acceptable due to alteration of the 

viewscape on the scenic headland. 

The bridge abutments appear to be anchored into the rock to a depth of approximately six feet. 

The southwesterly bridge abutment is in greater jeopardy of being weakened than the 

northeasterly abutment, but both abutment areas are considered highly susceptible to rockfalltype 

failure events that could compromise the future integrity of the abutments and the 

functioning of the bridge. The potential for local and significant failure during a seismic event 

appears to be moderate to high. 

6.2 Trail Area 

Based on field observations and eyewitness accounts, four areas along the trail from the tunnel 

to the northeasterly suspension bridge abutment, including the two pre-fabricated fiberglass 

bridge locations and Slide Areas 1 and 2, continue to be the critical areas from a maintenance 

20 November 2009



standpoint. The bridges, installed over ten years ago, span areas of erosional narrowing of the 

ridge top that have subsequently progressed, according to eyewitness accounts (J. Ryan, 2009, 

personal communication). Retreat rates in weathered rock and minor soil at these locations are 

similarly estimated at a rate of five inches per year per based on observations and discussions 

with Mr. Ryan. Erosional undermining and downslope creep beneath and below the foot path 

in Slide Areas 1 and 2 have caused continuing subsidence and cracking of old concrete and 

asphalt pavement in those locations. These trail areas will require further stabilization in order 

to keep the trail open and safe for pedestrian use. Although the net lateral displacement of the 

trail monitoring stations is generally less than or equal to 1.5 inches over the past decade or so, 

without further mitigation efforts a sudden failure of many tens of feet of trail is possible in any 

given winter rainfall event. Such mitigation efforts can include the installation of pipe and 

batter-boards, precast concrete crib walls, or more robust methods such as a micropile wall, 

soldier piles and lagging or mechanically-stabilized earth wall, depending on the steepness of 

the adjacent and underlying slopes, the extent and thickness of the unstable materials, the 

tributary runoff surface area above the trail at each location, and access constraints. 

6.3 Access Road Area 

The access road from Conzelman Road to the northeast tunnel portal crosses three landslide 

areas. These slides appear to be active, based upon escarpment morphologies, local evidence 

of recent erosion, and eyewitness accounts. Of greatest concern is the area where the road 

transects the escarpment of the middle slide. Discharge from drainage pipes may have 

exacerbated the rate of failure in this area. The shoulder of the road is undermined and the 

pavement is cracking, in part due to root wedging from adjacent trees, but also in part due to 

subsidence related to loss of lateral support. This area may require immediate attention to 

maintain vehicular access. Mitigation of instability in this area might involve construction of a 

mechanically-stabilized earth wall, rock-filled gabions, soldier piles, secant wall, or other 

methods in conjunction with drainage redirection and control, and mitigation of erosion in the 

surrounding slope areas. 

Also of concern is the prefabricated bridge area across the escarpment of the southernmost 

slide, which is being undermined by erosion from both sides of the ridgeline. The rate of 

21 November 2009



erosion at this location, as reported by eyewitnesses, is such that the approaches to the bridge 

and perhaps the bridge itself may be sufficiently destabilized in the near future to preclude 

vehicle passage. It is likely that this bridge will need to be replaced in the next few years. The 

replacement bridge will need to be founded on rock materials at sufficient depth to preclude 

destabilization due to landsliding. Design of the replacement bridge will require a geotechnical 

investigation of the extent and cause(s) of the present ground instability. 

22 November 2009



7.0 CONCLUSIONS AND RECOMMENDATIONS 

7.1 Suspension Bridge Area 

Based on our observations, three primary potential rock mass failure modes exist in the area of 

the suspension bridge. These failure modes include: (1) erosional undermining of the rock 

mass; (2) static (non-seismic) instability of the upper portions of the rock mass; and (3) 

seismically-induced rock mass failures. Feasible alternative mitigation measures for these 

failure modes are discussed below. 

Erosional Undermining of the Rock Mass 

Rip-rap armoring both above and below the mean high water line could be used to mitigate 

wave attack and erosional undermining at the base of the steep slopes below the bridge 

location. Sizing of the rip-rap would be in accordance with coastal engineering methods and 

practices, such as the U.S. Army Corps of Engineers Automated Coastal Engineering Systems 

(ACES), which permits analysis of wave transmission through permeable structures. Other 

armoring methods are also available, including seawalls, rubble-mound breakwaters, and 

various types of revetments. Evaluation of the response of the natural system to any such 

improvements would be required. 

Static (Non-Seismic) Stability of the Upper Portions of the Rock Mass 

Discrete blocks within the diabase unit are defined by a complex system of joints, fractures, 

and shear zones, some of which are visible from the bridge and adjacent footpath areas. 

Steeply-inclined irregular rock masses have failed beneath the bridge location; such failures led 

to construction of the existing bridge. Failures have continued since the construction of the 

bridge, leading to the current, steeply-sloped peninsula on which the bridge now stands. Such 

failures will continue to occur, abetted by effects of wetting and drying cycles, chemical 

weathering, and diurnal heating and cooling. The areas of the two bridge abutments are 

covered by concrete block foundations, but these too are subject to future undermining due to 

on-going rock mass failures. Progressive narrowing and lowering of the top of the rock mass is 

inevitable unless mitigation measures are implemented. Prediction of the timing of such events 

is not feasible, but anecdotal evidence cited above indicates an average retreat rate at the top of 

23 November 2009



the narrow peninsula of about 5 inches per year, occurring as episodic events. Mitigation 

measures could include various types of rock bolts and cable tendons. In rock of this quality 

grouted anchors will generally be required, as mechanical anchors will not provide the pull-out 

resistance needed. 

Wire mesh and shotcrete can also be applied to the rock faces in 

conjunction with rock bolting or cable tendon installation, which can serve to mitigate the 

spalling of smaller rock blocks and masses. The design of such stabilization measures will rely 

upon the results of geologic mapping, and possibly core drilling and laboratory testing of rock 

core samples, as described below for the alternative design aproaches. 

Seismic Stability of the Rock Mass 

The nearest significant active fault is the San Andreas Fault Zone, located approximately 3.6 

miles to the west-southwest of the lighthouse. Other nearby significant active faults include the 

San Gregorio Fault farther offshore to the west, and the Hayward-Rogers Creek-Calaveras 

Fault system to the east of the site. 

Peak spectral ground accelerations between 0.3g and 0.4g at a frequency of 1.0 cycle per 

second have been modeled for the 1906 earthquake on the San Andreas Fault Zone, which had 

a moment magnitude of 7.9. This is considered the maximum credible earthquake (MCE) 

event for the combined four recognized sections of the San Andreas Fault Zone. Prediction of 

the response of the rock mass at the project site to such accelerations is not possible without 

investigation, and no eyewitness testimony is known to exist for how the rock mass behaved 

during the 1906 MCE event. It is considered likely that rockfalls could be triggered by such an 

event, which has a modeled recurrence interval of 361 years. 

Mitigation measures such as those described above for static (non-seismic) stability will also 

serve to mitigate seismically-induced rockfall events. Resistance to expected seismic 

accelerations (i.e., for rock wedge or block stability evaluations) should be considered in 

development of the stabilization strategies. 

Replacement Bridge Foundation Design 

It is our opinion that the replacement bridge foundation design consider use of deeper, 

subsurface elements such as micropiles, in order to carry the bridge loads deeper into the rock 

24 November 2009



mass, in conjunction with rockfall/rock mass failure mitigation methods such as rock bolts or 

tendons to support and retain the adjacent bluff faces. 

It is noted that the suitability of 

subsurface rock materials for foundation support has not yet been evaluated. Other foundation 

support methods such as drilled piers or reinforced concrete caissons will likely not be required 

due to the comparatively light structure loads imposed by a suspension bridge of this size. 

Anchorages for tension members (suspension cables) will likely consist of grouted anchors or 

dowels with sufficient depths of embedment to resist wind and other live loads and seismic 

loads. 

Stabilization of Bluff Faces 

In order to ensure an adequate service life for a replacement bridge, mitigation of the bluff 

faces might include rock bolting or other mechanical stabilization of potential rock wedges, 

poorly supported rock sheets and rock blocks. Grouted anchors will likely be required, as 

mechanical/frictional anchoring elements such as spin-lock anchors would not be 

recommended due to the degraded quality of the rock mass. Rock mass grouting or injection of 

stabilizing compounds is probably not advisable due to the presence of numerous 

interconnected planes of weakness and the steep and exposed nature of the peninsula 

underlying and flanking the bridge location. 

Alternative Design Approaches 

Three alternative options are presented for the project site. Option 1 is the “no project” option, 

in which the suspension bridge, footpath, and access road will not be preserved or maintained. 

In consideration of the ongoing operation and historic significance of the lighthouse it is 

assumed that the “no project” option will not be exercised. 

Option 2 is to proceed to final design and construction with the limited qualitative geological 

information that is currently available. The bridge could be designed and founded on the 

existing rock bluffs, with the recognition that the site geologic conditions likely will impact the 

cost of construction, and thus affect the approach to final design. Post-construction, the NPS 

could closely monitor the bridge site conditions with a contingency plan in-place to close the 

bridge and stabilize the rock mass in the event that continued degradation is observed. The site 

may not experience any significant failure episodes for many years, but the possibility of a 

25 November 2009



sudden failure of a portion of the rock mass, and loss of support for the bridge, will always 

exist. The scope of Option 2 could be expanded to include focused geologic mapping of the 

bluffs and slopes flanking the bridge abutments in the initial phase of construction, by a 

certified engineering geologist representing NPS and qualified for rope and harness work, using 

safety equipment and qualified rigging personnel provided by the Contractor. 

With this 

approach, major exposed discontinuities and fractures could be identified and mapped, and rock 

blocks, sheets, wedges and other features selected for stabilization by rock bolts, anchors or 

other method. This expanded Option 2 would decrease the future potential for rock mass 

failures to occur. 

Option 3 is to proceed with a geotechnical investigation of the rock mass conditions at the site 

to support foundation design for the bridge and design of stabilization of the bluff faces. Such 

an investigation could include the following elements: 

1. Geologic mapping of bluffs, slopes, and locations for suspension cable anchorages 

by a certified engineering geologist with ropes and harness. 

2. Rock coring at each abutment using triple-tube mud rotary methods, to recover rock 

core suitable for geologic logging and samples suitable for laboratory testing. 

3. Downhole televiewer imaging (optical or acoustic televiewer) to assist in 

characterizing frequency, type, and orientation of rock weaknesses as an adjunct to 

rock core logging and geologic mapping. 

4. Preparation of a geologic map and geologic cross sections depicting the findings of 

the investigation. 

5. Rock mass analysis (using the Hoek-Brown or other suitable methodology) to assess 

the overall strength of the metamorphosed, fractured and sheared rock mass for 

support of structure loads, utilizing field data and laboratory test results. 

6. Laboratory testing of core samples to determine unconfined compressive strength, 

shear strength, moisture and bulk density, and to assess the overall mechanical rock 

condition. 

7. Slope stability analyses including toppling analysis, wedge failure analysis, and 

other methods deemed appropriate based on the geologic map and cross sections. 

26 November 2009



8. Preparation of a report of the investigation findings, with recommendations for 

foundation design, and recommended mitigation measures to stabilize the rock 

mass. 

The combination of geologic mapping, subsurface investigation, and slope stability analysis 

would form the basis for development of engineering design measures to mitigate the threat of 

future rock slope failure to the stability of the structure. 

7.2 Trail Area 

The trail extending from the southwesterly tunnel portal to the northeasterly suspension bridge 

abutment is locally unstable. In some areas downslope creep of surficial soils is evidenced by 

cracking of pavement and in recorded measurements of fixed points of reference as described 

above. Appropriate mitigation measures are location-specific, and will range from pipe and 

batter board construction where suitably stable rock materials underlie the creep-affected 

surficial materials, to more deeper-seated measures such as mechanically-stabilized earth walls, 

crib walls, and soldier piles. 

7.3 Access Road Area 

The access road area is affected by landsliding and erosion. Three areas of active landsliding 

on or near the access road were identified in previous investigations, and were re-examined 

during this reconnaissance. The first landslide area is adjacent to the existing access road but 

does not directly impact it. The second landslide area impacts the road where the two 

corrugated metal pipes are located (see Photo 5 above). The third landslide area impacts the 

road where the concrete bridge was constructed (see photo 6 above). Mitigation in these areas 

will first require field investigation to determine the depth of instability, the size and shape of 

the affected mass, the distribution of groundwater and soil/rock moisture conditions, and the 

physical properties of the materials involved in sliding at each location. Once these data are 

obtained mitigation measures for stabilization of the access road can be designed and 

implemented. 

27 November 2009



8.0 REFERENCES 

Blake, M.C. Jr., Barrow, J.A., Frizzell, V.A., Schlocker, J., Sorg, D., Wentworth, C.M., and 

Wright, R.H., 1974, Preliminary Geologic Map of Marin and San Francisco Counties and 

Parts of Alameda, Contra Costa, and Sonoma Counties, California; United States Geological 

Survey Miscellaneous Field Studies Map MF 574 (reprinted 1985), two sheets. 

Ryan, J., 2009, personal communications with Eric Chase (various dates). 

Schlocker, J., Bonilla, M.G., and Radbruch, D.H., 1958, Geology of the San Francisco North 

Quadrangle, California; United States Geological Survey Miscellaneous Geologic 

Investigations Map I-272. 

Schlocker, J., 1974, Geology of the San Francisco North Quadrangle, California; United States 

Geological Survey Professional Paper 782, 109 pp., two Plates. 

United States Department of the Interior, National Park Service, 1997-2003 (Various), Trail 

Monitoring Forms, Point Bonita Trail, Marin Headlands, Golden Gate National Recreation 

Area; unpublished hand written records of survey point measurements taken. 

William Lettis and Associates (WLA), 1994, Geologic and Geotechnical Exploration, 

Potential Pedestrian Trail Access Improvements, Point Bonita Lighthouse, Marin County, 

California; unpublished consulting report submitted to the National Park Service, Golden Gate 

National Recreation Area, January 19, 31pp. plus figures. 

28 November 2009

Attachment 1. Preliminary Geologic Map – Point Bonita Lighthouse Area 

Arch 

Sea 

Cave 

Arch 

29 November 2009

Attachment 2. William Lettis and Associates Trail Monitoring Station Map (1 of 2, WLA 1994) 

30 November 2009

Attachment 2 William Lettis and Associates Trail Monitoring Station Map (2 of 2, WLA 1994) 

31 November 2009

trail and access road - Central Federal Lands Highway Division

Create successful ePaper yourself

Delete template?

Save as template?