1 INTRODUCTION For over a century explosive blasting has been the primary means used for the excavation of hard rock. Conventional blasting is limited in that it requires special precautions due to the use of explosives and can cause excessive damage to the rock or concrete being broken. In recent years several small-scale methods employing small explosive or propellant charges or specialized mechanical and hydraulic loading means have been proposed as alternatives to conventional blasting. The smaller-scale specialized techniques, while finding many niche applications, have been limited in their ability to break harder rocks or in having undesirable operating characteris- tics. For example, the small-charge explosive and propellant techniques still generate significant airblast, flyrock and toxic fumes. Efforts to develop alternatives to conventional explosive excavation have included water jets, firing high-velocity slugs of water into pre-drilled holes, rapidly pressurizing pre-drilled holes with water or propellant generated gases, mechanically loading pre-drilled holes with specialized splitters, various mechanical impact devices, and a broad range of improvements on mechanical cutters. Each of these methods may be evaluated in terms of specific energy (the energy required to excavate or demolish a unit volume of material), their working environ- ment, their complexity, their compatibility with other excavation operations, and their suitability for automation. A review of the positive and negative aspects of these methods indicates the directions that efforts to develop improved methods might take. 2 BACKGROUND Controlled fracture methods, in various forms, have been proposed for several years as means to excavate or demolish rock and concrete more efficiently. Denisart (1976) proposed the rapid pressurization of a pre-drilled hole by firing a steel piston into a water filled hole such that a preferred (controlled) fracture would be initiated at the hole bottom. By propagating back to the surface from which the hole was drilled, this fracture would efficiently remove a volume of the material. Lavon (1978, 1980) proposed a variety of hydraulic cannons such that a high-velocity slug of liquid (water) could effect an efficient fracturing, excava- tion or demolition upon being fired into a pre-drilled 37th U.S. Rock Mechanics Symposium - Vail, Colorado - 6-9 June 1999 Controlled-foam injection for hard rock excavation Chapman Young Applied Geodynamics Inc., Steamboat Springs, Colorado, USA ABSTRACT: A new non-explosive method to excavate hard rock and concrete has been developed, tested and demonstrated in a mining environment. The method uses high-pressure foam to initiate, pressurize and propagate controlled fracturing in rock. The foam is injected to the bottom of a relatively shallow pre-drilled hole in the rock or concrete to be broken by means of a barrel incorporating a hole bottom sealing method. The high viscosity of the foam (as compared to a gas) combined with its stored energy characteristics (as compared to a liquid) result in consistent and controlled breakage. The pressures required to fracture and excavate rock are significantly less than required in methods based upon the use of small explosive or propel- lant charges. Airblast and flyrock are reduced to very benign levels, allowing the method to be applied in a continuous manner and to be used in urban and other sensitive environments.