Molas Pass is located in Southwestern Colorado on US 550. In August 2003, Colorado Department of Transportation (CDOT) began a rockfall mitigation project to alleviate potential dangers caused by rockfall and/or rockslides and to improve the sight distance and reduce the blind corner caused from the rockslope. The work involved rock excavation by production blasting, rock reinforcement with dowels and rock bolts and rockfall mitigation using wire mesh drape. A contractor?s access road was constructed across the upper rock face about 85 feet above the shoulder of the highway to facilitate excavation of the rockslope. The excavation created an unstable highwall specifically a large rock block above the access road. Blasting of the access road exacerbated the rockslope instability of the high wall by day lighting the failure plane and unweighting the toe of the slope of the highwall. The stability of the highwall was aggravated by the excessive backbreak created from blasting. A combination of factors in the blasting such as over-stemming of the shot holes and short timing delays in the firing sequence between the second and back shot hole rows may have led to the severe backbreak. Moreover, the adverse geologic structure appears to have exacerbated the excessive backbreak.
1.1. Background
Excavation and stabilization of unstable rockslopes can present many challenges to the engineer. To reduce some of the uncertainty it is critical that the engineer have a clear understanding of the problem prior to developing a design for the rock excavation, blasting, stabilization and costs.
The following paper describes a case history attendant to analysis of an unstable rockslope resulting from rock excavation and blasting back break problems. In this case the contractor developed a construction, excavation and blasting plan without a clear understanding of the rock engineering problem. As the project progressed, excavation and blasting led to a critically unstable slope. The authors conducted this geotechnical investigation of the rock slope after the Colorado Department of Transportation (CDOT) observed a large kinematically unstable block on the high wall and severe backbreak problems created from blasting.
1.2. Location
Molas Pass is located in Southwestern Colorado on US 550 between Durango and Silverton. In August 2003, CDOT, Region 5 began a rockfall mitigation project at mile marker 68.5 about 1.5 miles south of Silverton to mitigate potential dangers caused by rockfall and/or rockslides and to improve the sight distance and reduce the blind corner caused from the rockslope. At this location, the highway formed a sharp blind corner and pinch point. The overhanging rock not only provided a ramp to pitch rockfall onto the highway; the overhang had a history of catching large trucks (Refer to Figure 1).
