At Governor Gregoire's request, the department undertook an evaluation of slope hazards along the I-90 corridor near Snoqualmie Pass. Two recent rock slope failures, one resulting in three deaths, have prompted great interest by the Governor and Legislators on whether the I-90 corridor widening project (which has some rock-fall areas within) and other rock-fall risk areas should be re-prioritized for earlier repair than is currently planned.
Our review of the unstable slope program, in place since 1995, shows that 100 sites around the state have been mitigated, including nine on Snoqualmie Pass, at a cost of approximately $130 million. In performing this evaluation of conditions and risks of the Snoqualmie Pass slope hazards, we considered these risks in relation to unstable slopes on the rest of the state highway system. Our inventory of unstable slopes that are potential hazards to the highways includes 2,700 slopes.
We have identified six recommendations to put forth for consideration;
- Many of the slope hazards on I-90 not yet mitigated should continue to be assessed and prioritized with the slopes on other highways in the statewide Unstable Slopes Preservation Sub-program (P3). This program is sized at $15 million in the 05-07 biennium.
- Two high hazard - high risk sites in the vicinity of Milepost 50.3 (westbound), near the area of the three deaths in September, be advanced for concept level design immediately. This work will be done with existing funds available for project scoping.
- Three high hazard-high risk sites on I-90 within the project limits of the Transportation Partnership project "Snoqualmie Pass East" project, scheduled to begin construction in 2010. We propose to advance this project by one year to 2009. The current budget spending plan can accommodate this advancement.
- Three high hazard - high risk sites in the vicinity of Milepost 66.0 to 66.58 (westbound), east of the "Snoqualmie Pass East" project should be mitigated as soon as possible. After talking with OFM and the Governor's Policy office we have put forth for legislative consideration that $7 million of state and federal funds be made available for a repair project that can begin as soon as Fall 2006. ($1.3 million of the $7 million pays for the repair work performed in November, 2005 at the Milepost 59 slide).
- Incorporate a hazard reduction (risk management) approach with the ongoing risk removal approach into the Unstable Slope Preservation Sub-program.
- To be able to nimbly react to an emergent and costly situation of rockslides such as have occurred this year, we recommend the Legislature authorize the issuance of $50 million in emergency bonds, appropriated to the Unstable Slope Sub-Program. The previous bonding authorization for this purpose was exhausted in the 2003-2005 biennium.
Download Report
Introduction
Table of Contents, Executive Summary, Introduction (pdf 735 kb)
Section 1
Background on WSDOT’s Unstable Slope Management System (pdf 2.18 mb)
Section 2
2005 Re-assessment of Unstable Slopes between MP 36 and MP 68 on I-90 Snoqualmie Pass (pdf 3.12 mb)
- Introduction
- Interstate 90 and the Geology of the Snoqualmie Pass Area
- Highway Improvements Plans for I-90 and Their Effect on Programming for Unstable Slope Mitigation in the Snoqualmie Pass Area
- The 2005 Reassessment Program
- Summary
- Conclusions and Recommendations
Section 3
Other Problematic Highway Corridors for Unstable Slopes (pdf 3.71 mb)
- Interstate 5
- US Highway 2
- State Route 4
- US Highway 12
- State Route 14
- State Route 20
- US Highway 97
- US Highway 101
(Please note sections 4 - 6 are in the same PDF)
Section 4
Statewide Rockfall Related Accident History (pdf 1.25 mb)
Section 5
Major Highway Closure Due to Slope Failures (1985 to 2005) (pdf 1.25 mb)
Section 6
Avalanche Hazards Along Snoqualmie Pass (pdf 1.25 mb)
- Snow Slide Collision History Discussion for Unstable Slopes Report
(Please note Appendices A and B are in the same PDF)
Appendix A
WSDOT’s Unstable Slope Management System (pdf 1.23 mb)
Appendix B
Guidelines for the P3 Unstable Slope inventory, prioritization process and review (pdf 1.23 mb)