WY NP GRTE Multi(1) Field Report from 2023-09-23.pdf

158377846

FIELD REPORT

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661

Project No. Site Visit Date(s) / Field Report Date WY NP GRTE Multi(1) 9/25/2023 / 5/29/2024

Project Name WFLHD Representative GRTE Safety Tech Assistance North Park Road Report

Eric Lim Sr. Geotechnical Engineer

Contractor Weather / Conditions n/a Clear, ~60s F

Site Visit Observations

We visited the reported landslide and maintenance problem area at MP 15.6 on Highway 89/191/287 John D. Rockefeller Parkway, approximately 7.8 miles north of the intersection with Colter Bay Village Road. We understand that this area of the roadway has been settling and has been repeatedly patched with additional asphaltic concrete.

The approximate GPS coordinates of the patched area are 43.989173° N, 110.668137° W.

The approximate site location is shown in Figure 1 – Vicinity Map. Figure 2 - Site Features Map from 2020, shows the slide area distinguished by the darker pavement patch, as well as nearby riprap and gabion revetment areas along the lake shore.

We walked to the site along the beach from the Jackson Lake Overlook pullout along the highway approximately one-half mile south of the slide area. Just south of the slide area, we observed an approximately 500-foot long area of riprap lining the beach and lower portions of the roadway embankment. We also observed what appears to be remnants of a gabion revetement structure just north of the riprap area. Photos of the riprap and gabion revetment are shown in Figures 3 to 6. Using the aerial photograph history function of Google Earth, we were able to find a photo from 2015 that shows the gabion structure placed in an arch-shape in this area, as shown in Figure 7.

Figure 1 – Vicinity Map

Figure 2 - Site Features Map from 2020

Riprap Limits

Gabion Revetment

Visible Patch Limits

Jackson Lake

Yellowstone

Jackson

Figure 3 – Riprap on beach

Figure 4 - Closer view of riprap

Figure 5 - Gabion revetment

Figure 6 - Closer up of gabion revetment

Figure 7 - Aerial view of riprap and revetment areas, 8/2015 (Google, 2024)

At the slide area, we observed that the downslope side of the pavement has been settling, with multiple pavement layers measuring approximately 26 inches in thickness, as shown in Figure 8 and 9. We observed travelling vehicles bounce moderately over the area, especially on the south end of the patched area, which is shown in the foreground in Figure 10.

Figure 8 – Slide embankment area facing north

Figure 9 – Slide embankment area facing southeast eric.lim Callout Note thick layers of previous pavement patches eric.lim Callout Note thick layers of previous pavement patches

Figure 10 – Slide area facing northwest

Horsetails and other water loving vegetation were observed on both sides of the road in the slide area, indicating a consistent water source or seepage conditions. Minor seepages or small stream were also visible on the east side of the roadway. At least one small culvert (~18” diameter) was observed in the patch area. The slopes above and below the slide area were gentle to moderate and ranged on the order of 1V:4H to 1V:2H. The area is moderately to heavily treed with moderate to dense understory.

Approximately 750 feet northwest of the slide area, we observed a near vertical erosion area along the lakeshore, as shown Figure 11. We interpret this area to be possible slide materials based on the presence of an exposed large boulder with titled bedding surrounded in a soil matrix.

Figure 11 – Steep bank NW of slide area with buried rock material

MP15.6 Landslide History

The landslide has been known to Grand Teton National Park for many decades, as documented in Geotechnical Report No. 12-04, Geotechnical Investigation, WY PRA-GRTE 13(5), dated April 19, 2004. The total slide length was reported to be approximately 560 feet along the roadway; however, a 190-foot long section required excessive maintenance efforts due to repetitive movement. Vertical movements were up to 18 inches at the time of the report. One boring was drilled below the roadway (approximately 72 feet west of centerline) and an inclinometer was installed. Subsequent inclinometer readings indicated a distinct shear zone at approximately 54 feet in depth, which is approximately 65 feet below the roadway.

The landslide type was described as a long radius arch rotational failure. Several conceptual repair options were discussed, including: In Situ Soil Modification; Stone Columns; and a Tied-back Diaphragm Wall. Additional explorations were recommended for full mitigation design.

Of particular note in GR12-04, is that the Aerial Photo map in Appendix A shows the possible slide limits and active lobe a few hundred feet to the south of the currently observed patch/roadway settlement area, as indicated on Figure 12. The difference may be a mis-mapping, due to the lack of precise location measurements, such as GPS devices, at the time of the geotech report in 2004.

Figure 12 – Mapping Conflict from GR12-04.

Recommendations for Further Study

We understand that the subsurface investigations can begin in 2025, and construction is currently planned for fiscal year 2028. We plan to perform additional field reconnaissance in the summer of 2024, including attempting to locate the previously installed instrumentation performed previously, and rectify the potential mis-mapping of the slide from the 2004 geotechnical report.

Several borings will likely be proposed. At least three in cross-section down the fall line of the slope (perpendicular to the roadway) are recommended to develop the landslide cross section for design of mitigation measures and potential instrumentation installation. Several other borings are recommended along the roadway to the north and south to help characterize the width of the landslide. Several borings may be done in pairs: one boring in the roadway for pavement investigation and actual roadway subgrade conditions; another off the roadway to install instrumentation that is accessible after maintenance and patching of the pavement.

Figure 13 shows a potential boring configuration for preliminary consideration.

Current Patch Area

Figure 13 - Potential Boring Locations