FINAL LABE 10(1)_Site Visit Geotech Obsv.pdf

To: Jim Kerrigan, Project Manager, CFLHD, Lakewood CO

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

Through: James Arthurs, Acting Lead Geotechnical Engineer CFLHD, Lakewood CO

Subject: Geotechnical Site Observations – LABE 0010(1) Main Road Repaving, Tulelake, Siskiyou County, California

I. PROJECT BACKGROUND INFORMATION

This geotechnical memorandum summarizes the site visit and assessment of construction concerns for Lava Beds National Monument (LABE) which took place on June 2 and 3, 2021. Located in Tulelake, California the monument is known for geological, historical and a high desert environment. Over 800 known lava tubes and caves are found on the 47,000 acre-monument, along with many volcanic vents. Basalt lava and clinker eruptions over the past half million years from the Medicine Lake shield volcano created an 850-square mile volcanic region, making it the largest Cascade volcano in areal extent.

This unshelved 3R project previously identified concerns operating heavy equipment over lava tubes prompting a site visit from the CFLHD geotechnical engineer. Research by DeMarco (2004) presented analysis and risk of operating equipment over the lava tubes, information which is heavily relied upon in this memorandum considering no investigation was performed for this trip.

Previous research and work identified options to allow operating equipment over the tubes, which will be discussed in a later section. In addition to the paving concerns, LABE staff raised concerns for a road cut which produces rockfall into the roadway. This road cut was only observed and no slope investigation was performed to evaluate rockfall hazard or risk.

II. SITE OBSERVATIONS AND RECOMMENDATIONS

Site Review Memorandum

Date: June 22, 2021

A. Lava Tube Stability

The area of greatest concern is the Cave Loop Road route 0210, with multiple popular caves running under the roadway alignment. Based on the LABE provided map of Cave Loop road there are at least 14 named lava tube entrances for visitor ingress and egress as shown in Figure 1. These tubes are also identified habitat for roosting bats, meaning preservation of the features is of paramount interest.

As DeMarco (2004) presented in a constructability evaluation the “geophysical methods were largely impacted by the vertically and laterally discontinuous nature of the rock mass. Variations in lava unit density and continuity made it difficult to discern large openings from “blisters” or low-density scoria beds, and very difficult to determine the actual depth to located features.”

This means that the basalt lava flows and subsurface conditions are so varied throughout that it is very difficult to know where voids or weak rock may be encountered at depth without exploratory drilling, which is prohibited under the Presidential protection orders for lava tube environments. These highly variable conditions at depth, combined with the rock formation discontinuities (flow boundaries, fractures, joints, stress cracks, etc) are documented by DeMarco as reasons for needing to reconsider the heavy construction equipment weight or operating frequency.

Possible solutions previously discussed include additional ground penetrating radar, temporary shoring inside caves under the Figure 1 – LABE Cave Loop Map road, operating reclaimer equipment in excess of 30 Hz (if possible), using lightweight reclaimer equipment, removing pavement with laborious conventional methods or even requesting special permission to perform small probe borings.

1. Paving over Lava Tubes

Heavy equipment has previously operated on top of the lava tubes throughout the monument’s history, and traffic has operated without any sort of cave-in incident ever since. The geologic region of LABE, nestled between the Cascade volcanoes and the Basin and Range Province, has experienced severe volcanic and tectonic activity throughout its existence. Just over 50 miles east of LABE is the western extent of the Basin and Range, which has experienced Mw ~7 earthquakes between 180 and 16.8-thousand years ago (Badger and Watters, 2004). Earthquakes of this magnitude are capable of creating the lava tube breakouts seen in LABE and initiating the 2.9-km3 Foster Creek landslide in the nearby Summer Lake basin.

Considering the tectonically active area and historical performance of the roadway, the CFLHD geotechnical engineer concurs with DeMarco’s previous comments during a project review:

“It was determined that the likelihood of lava tube failure is very remote, as construction loading is marginal compared to typical overburden loads, tube openings are typically “welded” from past lava flows, and arching geometries are suitable for withstanding substantial applied loads for the common range of opening dimensions present at the Monument.”

All considered, the pavement reclamation and 3R project is anticipated to be of minimum impact to the tubes beneath. Caution should still be exercised by the Contractor and they must be made aware of the potential risk of operating heavy equipment over ground with known and unknown lava tubes and voids.

B. Rock Cut Stability

Along the Main Park Road route 0010, the rockfall source identified by LABE personnel is between Milepost 9.682 to Milepost 9.769 in the left shoulder. This section of road cut is part of the original construction and has not been modified since circa 1960’s or earlier. The rock in the cut is variable with large blocky basalt outcroppings and rubbly clinker flow deposits exposed.

Assuming the basalt density is 185-lb/ft3 and largest blocks range in size from ¼-cu yd to ½-cu yd this would equate to 1,300-lb to 2,500-lb blocks. Large blocks represent the least probable and highest risk to the roadway because there is no catchment ditch. Additionally, the rock cut is not at a constant slope (i.e. 0.25H:1.0V) which creates launch points for rocks falling from height.

Launch points allow a falling rock to increase angular momentum (rotation) and deflect out into the traffic lanes.

Reviewing the slope and pavement, no impact marks were noted in the asphalt and few rocks were on the roadside. A handful of rocks that are detached from the rock slope are easily identifiable and could be considered for spot scaling. Figure 2 below shows a screen capture from the NPS RIP database and a detached rock that LABE Maintenance shared concern regarding its continued stationary placement. Treatments of a rock cut slope such as this one are varied, and are presented in depth below.

Without directly mitigating the existing rock cut slope, the other treatment that can be considered is to deepen the ditch and remove vegetation to create more catchment for rockfall to collect. A narrow catchment ditch will only capture the lower sourcing rocks because the higher sourcing rockfall will bounce out from the face if allowed to develop momentum. This treatment alone would not reduce the threat of rockfall from the cut slope and would only function for as long as the catchment area was kept clear.

Figure 2 – Spot scaling example at LABE 0010 Main Park Road, MP 9.714 (left)

1. Rock Slope Options

Rock slope repair mitigation options are split between removal and stabilization as shown below in Table 1. 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 life 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 because it only treats spot locations. Bolts and dowels only treat the individual blocks or stacked rocks in a particular area, as opposed to excavation back into the rock slope to create a new face and catchment ditch along the entire treated length. Other options that are considered include basic photogrammetry and even a “take no-action” approach. While the no-action and monitoring options may be effective for a short-term, they are not permanent solutions. In the future, a treatment method is required to address continual weathering of loosened rocks or possible emergency events such as regional declared disasters.

Table 1 – Mitigation options for rock slope repair.

Removal Option Methods Stabilization Option Methods Monitoring

Rock blasting (cushion) Rock Reinforcement (dowels) Basic Photogrammetry

Excavation and scaling Draped Mesh (upper brow) No-action

2. Future Slope Treatment (Optional)

It is apparent this section of roadway required blasting to establish the current road bench. Any work to improve this section of road will require specialty contractors, ranging from rock scaling to rock blasting, therefore when considering future cut slope treatments, it will be more cost effective and reduce overall risk if the removal option methods are selected. Scaling of individual unstable rocks will last up to 5-years and stabilization with steel bar or mesh will perform to material design life (50 to 75-year). By excavating the unstable rocks and extending the rock cut into the slope beyond the currently observed weathered rock, the cut slope can be updated to current design standards and provide a catchment ditch for material to collect, reducing the overall risk and need to clear rock debris. For future planning purposes, the rock removal option is recommended to provide the best value for reducing maintenance and longest service life.

Figure 3 – LABE 0010 Main Park Road, MP 9.783 (left)

3. LABE Self-Perform Rock Slope Treatment

For information only, the options listed below are included in case LABE is able to self-perform or independently obtain a rockfall scaling company or heavy equipment contractor. The options listed below in Table 2 have been provided to other NPS Parks with more severe cut slope issues and therefore it assumes an internal rock scaling program exists in the Park and details a light approach limited to hand scaling, not mechanical.

Table 2 – Recommendations for self-performing rockfall mitigation.

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

Focus scaling on loose rock in the eroding and detached block sections; loose rock supporting rock nests above should be left alone unless more invasive mitigation is readily available.

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 move below one another. Only mechanically scale from the side and keep equipment out of impact area.

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 individual rock removal to hand-scaling methods (prybar etc) or if possible, lasso with a high-strength aramid fiber rope.

Scaling with equipment, heavy chain or steel cable should only be performed by experienced operators.

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 the risk of exposure is less than the potential 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 or a heavier scaling method.

Establish a ditchline and catchment features prior to leaving a work site.

Maintain storage capacity for future rockfall debris and maintain drainage.

Other options to exist for LABE to self-perform rock scaling. Wrapping a cable or heavy chain around individual unstable rocks and pulling with backhoes or other conventional equipment is an established technique. Sweeping the slope by hanging a heavy chain, blast mats or tires attached to a cable is another effective method, but requires an excavator and practiced operator to perform safely. Please contact the CFLHD Geotechnical Engineer for further questions.

III. 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 lava tubes and pavement condition. The recommendations in this memorandum include interpretations developed by the Government in the process of reviewing a shelved project. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgement of the Contractor or LABE staff with knowledge of the subsurface lava tubes.

IV. SIGNATURES

Prepared by Todd Hansen, Geotechnical Engineer, CFLHD

Reviewed by James Arthurs, Acting Lead Geotechnical Engineer CFLHD, Lakewood CO

V. REFERENCES

Badger TC, Watters RJ. Gigantic seismogenic landslides of Summer Lake basin, south-central Oregon. Geological Society of America Bulletin. 2004;116(5-6):687-697. doi:10.1130/B25333.1.

DeMarco MJ. Highway Constructability Criteria for Shallow Lava Tubes, Lava Beds National Monument, Tulelake California. Master’s Thesis, University of Colorado, Denver. Nov 2004.

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.