Rock Scaling Recommendations Logan Creek Drive Cut Slope US 50, Milepost DO 8.47 to DO 8.61 Douglas County, Nevada January 2008 MATERIALS DIVISION
STATE OF NEVADA DEPARTMENT OF TRANSPORTATION MATERIALS DIVISION GEOTECHNICAL SECTION GEOTECHNICAL REPORT ROCK SCALING RECOMMENDATIONS LOGAN CREEK DRIVE CUT SLOPE January 2008 DOUGLAS COUNTY, NEVADA Prepared by: Jeffrey A. Palmer, Ph.D., P.E. Principal Geotechnical Engineer Reviewed by: Parviz Noori, P.E. Assistant Chief Materials Engineer - Geotechnical Approved by: Dean Weitzel, P.E. Chief Materials Engineer
Project Overview The project consists of scaling loose rocks from a single slope above the roadway on US 50. Anchoring rocks or using rockfall meshes and fences are not being considered at this time. All scaled rocks and debris are to be removed from the project site. NDOT Maintenance forces will clean the bottom of the slope to remove any existing debris prior to work. Payment quantities are expected to be for scaled rock only and in-place soil on the slope should remain on the slope during the scaling process. Scaling work is to be done in a manner that keeps future erosion on the slope to a minimum. The slope is to be scaled prior to an overlay project planned to start in the spring or summer of 2008. The scaling project is planned to take approximately two weeks to scale plus one week to mobilize equipment and personnel to the site. The Rockfall Hazard Rating System (RHRS) study and report prepared by the University of Nevada (1993) identified this slope as Slope Number 13. This slope received a score of 305 and was the seventh from the top on the list of 62 slopes rated and ranked by Overall Detailed Rating Scores, where the slope at the top of the list has the greatest rockfall hazard potential. The slope is approximately forty feet high at its peak, consisting of granodiorite bedrock having a geologic character that is mainly erosional. The two factors that contributed the most to the RHRS score were the Average Vehicle Risk (based on the slope length and traffic speed limit) and Site Distance (based on reduced visibility due to the existence of a horizontal curve of the road at this location). The report recommends mitigating the slope by scaling mostly on the top and ends of the slope. The majority of the scaling work to be accomplished is at the crest of the slope where a soil cover layer has been exposed due to construction of the cut slope. This can be seen in the photos in this report. There are many rocks within the soil cover layer that are eroding out of the bedrock below the soil cover layer. Most of the slope face below this soil cover layer is bedrock and does not need scaling. 1
Site Location The slope is located just south of Logan Creek Drive on US 50 near Lake Tahoe. It is located from milepost DO 8.47 to DO 8.61 in Douglas County, Nevada on the east side of the road. The road at this location has four lanes, two northbound and two southbound. A pullout area exists on the west side of the road for about one-third of the slope length, directly across from the center of the slope. Recommendations Photos 4 through 8 show areas with circles around groups of rocks recommended for scaling. Some rocks that are circled in the photos may stay in place if determined during scaling attempts that they are firmly attached to bedrock. Other rocks on the slope that are not circled might need scaling, which have to be determined during construction and approved by the NDOT Engineer prior to scaling. There is a house that was built on top of rocks on the backside of the hill that may be impacted if the large rocks on the top of the hill above the slope are excessively disturbed. Caution must be exercised to not disturb the large rocks on top of the hill above the crest of the slope. Rocks scaled from the slope might possibly travel across the road and down the slope on the west side of the road. Several houses are located directly below the highway that could potentially be damaged if a rock were to travel over the road, down the west side and impact the house. A temporary moveable barrier must be placed on or at the edge of the roadway to control moving rocks. The moveable barrier must be of sufficient height and strength to capture nearly all material coming off the slope. Typical barriers used for roadway are not adequate for this application. Placing the moveable barrier in the center of the road would allow for quick cleanup of material from the road on the south bound side after scaling. Stopping traffic will only be allowed for a limited period of time, as specified in the Special Provisions for this project. The barrier allows traffic to continue quickly by keeping rocks off of the south bound travel lanes. 2
Rock splitting techniques should be expected for use, however no blasting, noticeably loud techniques or techniques that cause rocks to be thrown in the air are allowed. All methods of work shall be reviewed and accepted by the Engineer prior to use. The approximate volume of material expected to be generated by scaling is 400 cubic yards. The work will require specialized equipment and manpower including scalers in harnesses hanging from ropes attached to the top of the slope face (repelling), use of a man lift bucket to allow scalers to reach the slope face or other means that can place scalers directly on the slope face. Scaling may be performed by using hand tools such as power tools (hydraulic or other types of rock splitters approved by the Engineer, jackhammers, etc.), pulling cables, crowbars, prybars, jacks and shovels. Excavation using heavy equipment may be used, only if necessary, for slope sections where removing rocks, such as a boulder or overhanging bedrock outcrop, using hand scaling is not practical. Scraping the slope face or dragging heavy objects on the slope face is not allowed. Slope scaling should start at the top of the slope and proceed down slope while removing rocks. All material on the slope face that is loose, hanging or creating a dangerous condition will need to be removed or stabilized during or after completion of that section of slope. References 1. Application of the rockfall Hazard Rating System To the Rock Slopes Adjacent to US 50 and State Route 28 On the East Side of Lake Tahoe, Nevada (November 18, 1993), by Robert Flatland, Advisor Robert J. Watters, University of Nevada, Reno. 2. Rockfall Hazard Rating System Participant Manual, Publication No. FHWA SA-93-057, November 1993, National Highway Institute. 3
Photos 1 and 2. US 50 slope just south of Logan Creek Drive. 4
Photos 3 and 4. North end of slope showing recommended rocks to be removed. 5
Photos 5 and 6. Center of slope showing recommended rocks to be removed. 6
Photos 7 and 8. South end of slope showing recommended rocks to be removed. 7