ABSTRACT:

This paper describes fundamental studies of the application of the ultrahigh pressure water jet for rock drilling. The study of the application was carried out aiming to speed up rock drilling. In-situ drilling tests, for the actual rocks at the site, were carried out to find the most suitable pattern for the bit for several kinds of rock properties up to 60m underground.

INTRODUCTION

For grouting to improve rock bed, boring through the full depth of the rock bed is required and, at present, rotary boring machines, which enable boring to a large depth and are adaptable to various geological conditions, are used for boring through the rock bed.

On the other hand, water jet technology is attracting attention in various fields as the result of technical developments in recent years, such as the development of a compact high pressure water generating unit and the improvement of the high pressure swivel (rotary joint in piping) which has minimum hydraulic pressure loss. On this background, we have developed a rock bed boring machine utilizing the cutting ability of water jetting for the purpose of improving the boring speed.

OUTLINE OF THE SYSTEM

While conventional boring machines bore through the rock bed by pressing button Chips, which are mounted on a rotary bit into the solid rock so as to break off chips of rocks, this water jet machine bores through the rock bed by injecting pressure water through a rotating bit. This system consists of a bit, a rod, a boring machine, an ultrahigh pressure water generating unit, etc. as shown in Figure 1.

In the course of developing the water jet boring machine, the developing procedure was as follows:

  1. the characteristics of the water jet from injection nozzle

  2. the borability of the rock

  3. the rods of double-tube structure

  4. a high pressure swivel which has minimum pressure loss

  5. improved rod joints

(Figure in full paper)

After in-door drilling tests were performed on rock samples, in-situ drilling tests were done on a typical rock bed.

DESIGN OF THE BIT

In order to design the shape of the bit and the location of the injection nozzles and chips, the jet characteristics tests and cutting tests were perfomed. The results of the jet characteristics tests with a single nozzle and cutting tests on rock samples also made clear the structure of the jet and cutting parameter (cutting width and cutting depth).

Jet characteristic tests with a single nozzle

Tests were performed to determine the pressure attenuation rate and the jet spreading diameter versus the distance on the ultrahigh pressure water jet axis.

The outlet pressure is maintained approximately equal to the nozzle injection pressure within the short length called "the initial section length". Figure 2. shows the initial section length plotted against the injection pressure (95–340MPa). These tests were performed under the conditions as shown in Table 1.

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