FINAL Site Observations and Recommendations - Wawona Rock Scaling and Mitigation Options Mariposa County CA .pdf

To: Nathan Allen, Project Manager, CFLHD, Lakewood CO

From: Todd Hansen, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Marilyn Dodson, Lead Geotechnical Engineer CFLHD, Lakewood CO

Subject: FINAL Site Observations and Recommendations –Wawona Rock Scaling and Mitigation Options, Mariposa County, California CA FTNP YOSE 14(7)

I. PROJECT BACKGROUND INFORMATION

This geotechnical memorandum recaps the previous rock scaling observations and provides supporting documentation for the proposed slope mitigation project along the Wawona Road.

Previous work on this unstable slope has identified this slope as an active rockfall site starting in 2011, followed by a rockfall mitigation project in August 2017 and an emergency rock scaling operation in May 2019. Currently the slope remains in a marginally stable condition, but has not produced sufficient rockfall to close the roadway since the last site visit in 2019.

The rockfall site is located on Wawona Road (RID 0014) at milepost 7.975, on the right side.

Observations of the slope have noted loose rocks capable of impacting the full width of roadway travel lanes (including potentially impacting passing vehicles) and a historic stone masonry wall supporting the outboard lane. The history of rockfall impacts to the stone masonry wall is not known but by removing the unstable rock above, the risk to this historic structure will be reduced.

These observations are noted below in Table 1 and the conditions prior to the emergency scaling in 2019 are shown in Figure 1 and Figure 2.

Technical Memorandum

Date: June 23, 2020

Figure 1 – Slope conditions in April to May of 2019. Yellow dots indicate rocks of concern.

Yosemite National Park (YOSE) requested a Central Federal Lands Highway Division (CFL) geotechnical engineer to be present during slope scaling on May 29, 2019. Beginning in early April 2019 several YOSE staff shared concerns based on their observations of previously bolted and stabilized rocks. These observations included developing cracks and weathering of soil-like material from behind intact rocks, as well as a grout cap popping off a rock bolt. Following a late May site investigation, CFL provided site details in the Rock Scaling Observations and Mitigation Options memo (2019) which followed up on the roadway improvement options. The site has been a continued source of small rocks into the roadway and the narrow ditch did not have sufficient distance to reduce the rockfall energy. Stabilization discussions between YOSE and CFL considered options to remove the rock, temporarily stabilize the rock or add temporary slope treatments to minimize the potential for rock to reach the road.

II. PREVIOUS INVESTIGATION

This rockfall slope on Wawona Road has been previously classified under Slope ID: YOSE-0014- 7.928-R. Previous rock slope ratings performed in 2011 by CFL personnel and YOSE for this site developed independent rating scores using the Rockfall Hazard Rating System (RHRS) and a simplified rating with input from YOSE Maintenance personnel (CA-PX-YOSE-12-01). The 2011 RHRS score developed by CFL was 550 and the Maintenance score was 4 (based on a “best to worst” scoring of 1 to 5). The following rating and photo from 2011, and following comparison photos give an indication as to how actively this rock slope is eroding.

Slope No. YOSE-0014-7.975-R (Sept-Oct 2011)

Rockfall Occurrence Imminent large, loose rock(s) impacting both lanes; high vehicle risk; high structure risk.

Scaled Slope Length, ft 660 ft Rockfall

Height, ft 25-50 ft

Rock Size Range, ft

2-4 ft

Est. Scaled Volume, cuyd >9 cuyds Inboard Sight

Distance, ft Poor (<275 ft)

Ditch Catchment

None

Rock Distribution

Multiple rocks along top of cut Slope Access

Top of cut, easy access

Slope face with minor upslope exposure, easy access

Estimated Scaling Effort

2+ days, 3+ scalers with pry bars only

Hand jacks and/or inflatable jacks

Work Summary

Long slope with eroding colluvium at top of downstation slope, transitioning to loose, jointed blocks and block stacks toward the upstation end of the cut. Avoid working the high, overhang section – major machine scaling section requiring slope stabilization (bolting). Confine work to colluvial slope and downstation end of cut (jointed rock mass).

Table 1 – Dataset from 2011 CFL Geotechnical Rockfall Hazard Rating Report.

Figure 2 – Slope conditions in Sept-Oct of 2011; RHRS rating of 550.

Figure 3 – Close-up of the area that continued to produce rockfall in Sept-Oct 2011.

Figure 4 – Conditions observed in April-May of 2019, note loss of outlined rock compared to Figure 3 photo above.

Photos comparing the site between 2011 and 2019 are shown above in Figure 3 and Figure 4. The original 2011 photo shows rock in a confined and stable condition within the dashed yellow lines.

The following photo gives an idea of how much the rock slope is exfoliating and losing material onto the road below. Some rock was scaled as part of the stabilization effort in 2017, so the dashed yellow areas are rough estimates of the rockfall that has occurred. In 2017 the stabilization included removing loose rocks and bolting the remaining large boulder and rock shelf. It is anticipated that the erosional loss of rock and confining material occurred after the boulder stabilization. Scaling performed during construction is typically minimal but would not be left in the unstable slope conditions as observed in early 2019.

For review, the options listed below in Table 2 were previously submitted by CFL in 2011 for a recommended park-wide rock scaling program and detail a light approach limited to hand scaling.

The intent of these options is to improve cut slope rockfall conditions while ensuring scaling crew and traveling public safety. Currently the YOSE Scaling Crew and YOSE Maintenance are improving corridor safety for select rockfall sites within the Park.

Remove localized, high risk, imminent rockfall hazards along accessible engineered cut slopes on major park roads.

Work shall generally focus on loose rock high on the slope within the eroding brow of the cut;

however, loose rock on the cut face may be removed if safely accessible.

Limit scaling to safely accessible loose rocks, within the skills and abilities of park maintenance staff.

Never work beneath loose or highly fractured rock, or allow scalers to work within one another’s fall line.

Limit scaling to areas where the traveling public and roadway features can be protected during scaling operations.

Roadway features include pavement, curbing, traffic barriers, retaining walls, drainage structures, utility access, etc. Do not pass traffic with loose rock in the travel lane.

Limit rock removal to hand-scaling methods, including pry bars, jacks, and inflatable jacks.

Defer scaling efforts involving large extents of the cut slope, large rock/volumes, or locations with significant personnel rockfall exposure to future scaling efforts deploying machine scaling.

Scale only areas where hand-scaling will result in improved roadway safety and long-term slope performance.

Do not attempt to scale rock masses where the resultant slope may produce additional rock fall without some measure of slope stabilization.

Restore all ditchlines and drainage features prior to leaving a work site.

Maintain storage capacity for future rockfall debris and maintain drainage.

Table 2 – Recommendations to YOSE for self-performing rockfall mitigation (CA-PX-

YOSE-12-01).

III. EMERGENCY MAY 2019 INVESTIGATION

In March and April of 2019, YOSE staff shared concerns based on observations of displaced rocks, loss of material, undermined rocks and cracks propagating through the slope. These concerns were shared in documents provided by the Park to CFL and further discussed in emails or phone calls.

At the request of YOSE a CFL geotechnical engineer was present to provide site observations in support of the Scaling Crew.

On May 29, 2019 representatives of YOSE and CFL met on site to evaluate the slope and determine what mitigation measures could be self-performed by the YOSE Scaling Crew. CFHLD was represented by Scott Wolfert (highway engineer) and Todd Hansen (geotechnical engineer), while YOSE was represented by Michael Pieper (engineer), Greg Stock (geologist), and the scaling crew of Russ Kish (supervisor) and Justin Buzzard.

With the scaling crew leading the slope investigation all personnel inspected the slope and developed estimates of rock stability. Based on inspection from a lift-basket it was apparent the bolted rock detached from the rock mass above and to the side, but underneath the rock the bolts still appeared to be intact. Rock anchor integrity could only be visually evaluated. The three nested rocks were undermined enough so that a steel chain could be wrapped up under and around them. This lack of confinement material triggered the need for immediate removal of these nested rocks.

Rock bolts for the 2017 slope stabilization called for #8 (1-inch dia.) all-thread bar with bearing plates, washers and hex nut. Based on CFL observations the constructed rock bolts were not installed to design. The drilled holes were undersized, anchors were not sufficiently grouted, bearing plates were not installed and the anchor heads were not recessed into the rock. A bolt that was not constructed per plans is shown below in Figure 5.

Drilling for the bolts was performed with a handheld plugger drill advancing through durable quartzite rock and weathered soil pockets. The slow drill rate can be attributed to the lightweight handheld drilling method which relied on rope positioned laborers to create downforce on the drill bit. Encountering soft soil pockets at depth increased the chance of mudding in the drill bit by having cuttings sticking to the hole sides, trapping the drill steel and forcing abandonment of the hole.

Figure 5 – Failed rock bolt in destabilized boulder above Wawona Road.

The contractor may have reduced the drill bit size for ease of construction but still claimed that drilling the quartzite rock was difficult. A smaller diameter drill hole would not have enough annulus space for grout tube or centralizers. It is unknown how grout was added to the drill hole or if the bar is bonded to the rock. To the left of the bolt is a small grout patch covering one of the two abandoned drill steels stuck in the rock. While some of the rock was scaled during previous construction it is clear from YOSE experience and CFL observations that the bolted rocks and the slope have continued to mobilize.

It appears that following loss of the bearing material beneath, the rock has slipped down around the rock bolts. This is suggested by the popped grout cap which was observed in April of 2019 and was a major indication that the rock was moving. With this continued movement of the rock and the ineffective rock bolts, an alternative mitigation strategy is required to bring the slope back into an acceptable service design life for the safety of the corridor.

IV. GEOLOGIC CONDITIONS

Overall the slope and surrounding area appear to be influenced by the contact between the quartzite and decomposed granite. The quartzite ranges from very hard, intact bedrock to extremely fractured and completely weathered. Completely weathered quartzite has decomposed into soil likely as a result of shearing or hydrothermal alteration along weak discontinuities. This decomposed material is easily eroded and at the time of our site visit, several weak planes were seeping water from flow within fractures. Slope stability of the intact, unweathered quartzite decreases as decomposed rock erodes away creating metastable stacks of fractured rock.

Evidence of a landslide south of the project site supports the variable nature of the stability for the surrounding rock. The geologic map identifies a contact between the Quartzite of Pilot Ridge (Pzp) and the intruded Bass Late Tonalite (Kbl).

Figure 6 – Geologic map of the Yosemite Quad (Peck, 2002).

Rock structure is highly variable in the area with moderate to steeply dipping beds surrounded by vertically oriented foliation within the Bass Lake formation. As shown in the geologic map, the yellow dot is the location of the rockfall. There is a currently inactive landslide on the boundary of the quartzite and tonalite, immediately south of the Wawona rockfall site. As the igneous rock intruded into the quartzite country rock it is likely that tectonic stress and contact metamorphism fractured and altered the country rock. Quartzite is already a product of metamorphism and therefore would not undergo further metamorphism, however intrusion and subsequent alteration of granite dikes could alter into clayey, decomposed granite. This is a possible explanation for the weak structural condition of normally strong rock. These conditions are only mentioned out of consideration for overall programmatic design options, and to help estimate long-term service life of any mitigation option.

The wedge failure in Figure 7 represents the same planar sliding surface as exposed on the south side of this rock cut. Further above the cut face the discontinuity repeats where the slide has exposed the underlying bedrock. Based on observations of the existing rock cut it is assumed that

Kbl blasting for the original road broke back to this persistent discontinuity, and that weathering of that cut has resulted in the current destabilized conditions. Below the wedge failure the toe buttress supporting this rock slab has detached and rotated downward toward the ditch. These detached blocks are underlain into the slope by decomposed granite, which can decrease stability through erosion or hydration of altered clays.

Figure 7 – Wedge and planar failure surfaces dipping into the roadway, May 2019.

Above the rock cut slope brow is heavily fractured, altered quartzite bedrock. As evident in Figure 8 the massive rock to the bottom-left is very hard and intact, typical of the rock cut. In comparison, the rock exposed above to the upper-right is heavily fractured, has undergone alteration and shows indications of previous fault or shear gouging. This change has occurred in-place even though on first observation the overlying materials resembled transported colluvium. The overlying weathered-in-place soils and established vegetation have led to the creation of a transition brow from the rock cut to the existing slope. Trees and shrubs along the road cut have become established in the cut rock faces as well as the slope brow. Slope erosion has exposed tree root-wads along the brow, some of which are supporting large boulders and colluvium upslope.

Figure 8 – Weathered, fractured bedrock in slope brow above existing rock cut, May 2019.

V. ROCK SLOPE REPAIR OPTIONS

Rock slope repair mitigation options are split between removal and stabilization as shown below in Table 3. Rockfall protection options such as fencing or attenuators were not considered due to limited roadway geometry and lack of installation space. The removal option requires excavation of unstable rock. The stabilization option necessitates that rocks held in place remain intact for the lift of the structural reinforcement. Both options are feasible and require significant effort to complete, but stabilization would not reduce the long-term maintenance of the ditch. Other options that were not considered include instrumentation of individual unstable rocks or even a “do nothing” approach. Neither of these are advisable or feasible as permanent options and therefore were not included as alternatives for the site.

Removal Option Methods Stabilization Option Methods

Rock blasting (cushion) Rock Reinforcement (bolts or dowels)

Rock excavation and scaling Structural Buttress

Table 3 – Mitigation options for rock slope repair.

Originally this section of roadway required blasting for the road bench and the stone masonry retaining wall. To reduce the long-term need for road maintenance and reduce the chance of rockfall damaging the stone wall, the rock removal option was selected to create a stable excavation along the road corridor. Removal of individual unstable rocks would likely require specialty contractors. By excavating the unstable rocks and extending the rock cut into the slope beyond the currently observed weathered rock, the cut slope design life can be updated to current design standards.

VI. ROCK SLOPE REPAIR RECOMMENDATIONS

The removal option is recommended because it removes current and potential unstable rocks next to the road. Excavation into the rock slope will require blasting techniques and some heavy excavation equipment capable of ripping or transporting the very strong rocks. Data correlating seismic velocity to rippability is publicly available through equipment manufacturers and as published charts. The Contractor is responsible for selecting appropriate rippability data based on the known seismic velocity of the rock and the anticipated equipment to be used in excavation. If requested by the Contractor, CFL can provide general rippability parameters for information only but the Contractor or the Contractor’s geotechnical engineer is responsible for selecting the rippability data that will be used in support of construction At the base, the designed rock cut is 26-feet wide which will increase the capacity for rockfall storage. Previously there was not any rockfall storage for the existing ditch, so the creation of a buffer zone for catchment and the improvement to the cut slope both improve corridor safety.

The historic Wawona roadway alignment above the cut slope was avoided when developing the proposed rock cut widening. At the closest point, between Station 102 +60 and Station 102+80 the edge of the old roadway is approximately 35 feet (horizontal) from the top of cut. This buffer is intended to prevent unwanted construction impacts or damage to the historic roadway, but does not reduce the need for field construction monitoring.

The typical rock cut is nominally designed with a 0.25H:1.0V back slope, but the final cut face is intended to match the native rock structure. Rock blasting will require production blasting techniques to create the final cut face. Presplitting is not recommended because of the high likelihood that half-cast marks will be left in the final excavation face. A blasting method that will possibly be used is “cushion blasting” or “trim blasting” which is similar to smooth blasting in that the final cut face holes are detonated after the production blast. A pre-blast survey of historic and culturally sensitive resources will need to be performed prior to any blasting activity.

Additional excavation may be accomplished using heavy-duty earthmoving equipment to rip or mechanically scale rock. Long-reach excavators and difficult-access earthmoving equipment may be required for excavation and drilling of the blast holes. This work may require specialty contractors to perform the work on slope, if earthmoving equipment cannot access the blasting bench or slope to be excavated.

VII. LIMITATIONS

Subsurface exploration was not performed as a part of this work. Interpretation of surface and subsurface conditions is based on limited field reconnaissance and surface observations of soil and rock outcrops. The recommendations in this memorandum include interpretations developed by the Government in the process of preparing preliminary repair alternatives. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor.

VIII. SIGNATURES

Prepared by Todd Hansen, Geotechnical Engineer, CFLHD

______________________________________________ Date: 4/15/2020

Reviewed by Dominic Monarco, Geotechnical Engineer, CFLHD

Approved by Marilyn Dodson, Lead Geotechnical Engineer, CFLHD

IX. REFERENCES

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2014). “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.”

Publication No. FHWA-FLH-14-001.

FHWA-Central Federal Lands Highway Division (CFLHD). October 2012. “Yosemite National Park Engineered Slope Rockfall Study, Geotechnical Report CA-PX-YOSE-12-01.”

FHWA-CFLHD. May 27, 2013. “Rock Scaling Field Guide: Wawona Road Yosemite National Park.”

FHWA-CFLHD. June 3, 2019. “Rock Scaling Observations and Mitigation Options: Wawona Road, YOSE.”

Peck, D.L., 2002, “Geologic Map of the Yosemite Quadrangle, Central Sierra Nevada, California:

US Geological Survey” Map 2751, scale 1:62,500.