Typical area along highway that is subject to rock slides.

This project is located on the east side of the Sierra Nevada mountain range in northwestern California very close to the Nevada State line. Construction involved a 1.1km (.7mile) stretch of State Route 89 along Emerald Bay near Cascade Lake and Eagle Falls on the southwestern shore of Lake Tahoe.

The purpose of this project was to upgrade a stretch of the existing narrow twolane section of Route 89 to a more modern twolane cross section. The primary goal of the proposed design was to address slope stabilization and erosion control issues to reduce rock slides that frequently closed this section of the highway, which is the only allweather route around the west side of Lake Tahoe. Secondary goals of the project were to improve safety, enhance roadway drainage, and minimize potential negative impact on the water quality of Lake Tahoe.

This portion of Route 89 was originally constructed in 1930-31 to provide a link along the west side of Lake Tahoe between the two major eastwest routes in the region: State Route 70 on the north and U.S. Route 50 on the south. With the construction methods available at that time, the road closely followed the lay of the land, following ridge lines and older wagon trails along the edge of the lake. This resulted in a number of very dramatic vistas from the route, particularly in the area around Emerald Bay.

Environmental and Design Issues and Constraints

The physical location of the project, combined with the climatic conditions of the region, presented a number of unusual issues and constraints that had to be addressed during the design and construction phases of the improvement.

Unstable Geology

The area adjacent to the western side of Lake Tahoe is subject to relatively frequent earthquakeinduced landslides and rock falls. The basic geological structure of the area is fractured granite with steep slopes and little ground cover. Indeed, the primary rationale for the project was the desire to alleviate continuing slope instability problems.

Limited Construction Season

This section of State Route 89 is closed during the winter months (typically from midNovember through the end of February) because of continuing major snowfall accumulations that make plowing impractical. Moreover, with several sections of the route lacking shoulders or guardrails, allowing motorists to use the facility, even if adequate snow removal could be provided, would not be advisable. The route is, however, subject to heavy tourist use in the summer and early fall months, and a significant percentage of the traffic is motor homes and buses. These factors combined to limit the available construction period to the spring and late fall months of the year. Additional time constraint problems arose, because the road had to be totally closed for a period of approximately 6 weeks for blasting, boulder removal, and major earthwork.

The Tahoe Regional Planning Agency

A major issue associated with the project was the requirement to maintain the unique visual characteristics of the project area. This task is, in part, the responsibility of the Tahoe Regional Planning Agency (TRPA), a congressionally mandated bistate (Nevada/California) agency that is organized to maintain and enhance the scenic and aesthetic qualities of the Lake Tahoe region.

Some areas within the project limits were identified as TRPA Stream Environment Zones that drained directly into Lake Tahoe. About 0.09 ha (.22 acres) of these zones would be disturbed during the project.

Actions Taken to Resolve Issues

Roadway Design Elements

In order to relieve the rock slide problems that had necessitated the project, the back slope angles were reduced and a number of the largest granite boulders were removed. Slope netting was used in some locations to provide stability until vegetation could be established. Doublethickness retaining walls, terraced rock catchment walls, and trenches were used over the length of the project to provide additional protection areas between the problem slopes and the improved roadway.

Further Reading:
   State Route 89--Emerald Bay, South Lake Tahoe