Aesthetic attainment on highway projects must be accompanied by a rational process of evaluating what is necessary visually and what is not. A few drill hole traces are generally not deleterious visually and evidence improved slope stability and reduced rock mass disturbance. In cooperation with the land administration agency, the highway department should use visual simulations and visual prioritization to arrive at measurable, objective criteria for slope roughness and drill hole trace retention.
Criteria for visual enhancement fall into the short range and long range perspectives. The short range perspective includes textural enhancements that are important if the viewer is nearby and will hold the view for an extended period. Consideration should be given to accentuating ledges and pockets on the ends of cuts where the potential for rock launching is less and the visual benefit is greater. The long range perspective may be more significant to the driver because it is generally available for viewing over an extended travel time. The long range perspective is best served through enhancements that mimic natural landforms such as desert varnish stain, slope rounding, slope warping, and slope laybacks at drainages. The excavation can be shown on the plans and included in the earthwork estimates so it is accomplished at a competitive unit rate. Controlled blasting techniques can be used to enhance the long range enhancements if drill hole traces can be allowed.
Several blasting approaches are available for the creation of enhanced cut slopes in rock. The choice will depend on rock fracture density, rock hardness, and designed slope angle, as well as aesthetic criteria. Controlled â€œcushionâ€ blasting has been effective in creating rugged cut slopes with natural appearing ledges and pockets, but needs careful design and continuous evaluation starting with a test blasting program, to be effective in changing geologic conditions. Special tightened step drilling and horizontal drilling can be used to advantage where slope angle or access require them. Horizontal holes offer the potential for elimination of hole traces in massive rock, as long as the depths of the holes can be adequately controlled.
About the Author
Robert (Bob) Cummings, P.E. of Saguaro GeoServices, Inc. of Tucson, AZ has considerable experience in geotechnical engineering for transportation, heavy construction, and mining. Mr. Cummings has provided geological engineering and geotechnical engineering services to federal, state and local transprtation athorities on both urban freeway and rural highway development projects, involving materials testing, geotechnical exploration in rock and soil, pavement design, retaining walls, drilled shafts, and heavy earthwork in rock and soil, the last including development of rock blasting procedures in sensitive scenic and infrastructure settings.