Each brittle rock has a characteristic critical impact energy at which fractures develop extensively. This threshold energy level depends on the size distribution of flaws within the rock. The impact energy required for fracture development has been estimated and verified experimentally for a variety of rocks. A single impact below the critical energy level deforms the rock elastically and causes minor crushing. Crushing results from stress concentration at the bit tip. At critical energy, cracks propagate through the rock. Above the critical energy, crushing and cracking are the dominant failure modes. Rebound measured with a special fracture hammer on a variety of rocks correlated with ultrasonic pulse velocities as regards the indication of fracture development at critical impact energy. The impact energy required to cause fracture was marked by a sharp decrease in the hammer rebound. Point load tests showed up to 70 percent decrease in rock strength about the fracture energy level. The radius of the fractured zone delineated with ultrasonic tests ranged from 4 to 7 cm;The effectiveness of percussive impacts as a means of reducing rock strength is proven in this investigation. Results have significance in mechanical excavation of hard and abrasive rocks. Hard rock trenching requires higher cutting forces than available rotary trenchers can exert. Based on obtained results, a rational design is proposed for a percussion trencher. This trencher would prefracture and hence, weaken hard rocks by percussive impacts before drag bit action. Recommendations have also been made on the field use of the special impact hammer which was developed for this research. Its robustness and portability make it ideal for the measurement of the in situ impact strength of rocks.