INTRODUCTION

Ripping is a safe, low-pollution, inexpensive soft rock excavation method consisting of fracturing, excavating and carrying by one bulldozer.(Fig. 1(a) It is widely adopted for quarrying which requires mass excavation of bedrock, cutting and grading after blasting, and other jobs. Recently it has become the most general soft rock excavation method.

A big problem with current ripping operations is the reduced production output caused by relatively hard bedrock. The Production output is largely dependent on the bulldozer excavation capacity and the bedrock strength, especially on the number and sizes of joints.

Excavation capacity has been increasing with the trend for larger bulldozers. When considering operation costs, the limit of independent ripper operation can be regarded as 2000 m/sec(for seismic velocity Vp)).

If the bedrock strength exceeds this limit or if ripper production is to be increased, pre-blasting is done, which loosens the bedrock before excavation. This process uses light charges to expand the clearances between the bedrock joints.

Compared with bench blasting, this process uses less charge, resulting in less vibration and scattering of stones. Since explosives are used, this process is restricted by security and safety regulations. Near private houses especially, it is often not approved. In recent years, efforts have been made to study and develop low-pollution bedrock fracturing techniques that do not use explosives. As a result, expanding cement, the hydraulic wedge, hydraulic breaker and other fracture techniques have been developed.

The authors investigated application of hydraulic fracturing techniques applied to bedrock fracture. This paper gives an outline of the tested machine and its operation.

(Figure in full paper)

OUTLINE OF HYDRAULIC-FRACTURE MACHINE

Hydraulic fracturing for geothermal applications is done several thousand meters underground, but here the depth of ripper excavation is only 0.5 to 1.5 m. Effective rock fracturing at this depth is required. To prevent water leaking to the bedrock surface, the fracture should be limited to a small area. To attain the required production, it is necessary to make many fractures within a short time. To shorten the cycle time, a high-speed hydraulic rotary percussion drill is used. Sand is used to seal the pressurizing water. A simple rod and bit mechanism that enables a series of operating steps from drilling to fracturing has been developed.

As a result, it is now possible to make many hydraulic fractures in a short period of time. The mechanism is installed on a full-swing crawler type machine to enhance field operating efficiency.

Fig. 3 shows the bedrock pre-fracturing process of this machine. It consists of four steps as follows:

  • drilling

  • charging of sealing sand

  • consolidating sealing sand

  • fracturing by high-pressure water injection

After fracturing, the rod is lowered and compressed air blows off the sealing sand. If this operation is repeated in the same hole at different depths, multistage fracturing is possible.

Sealing the pressurized water (at 700kg/ cm2 maximum)with common sand is one of the important operations of this machine. Sealing is not influenced by roughness of the drilled hole wall surface.

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