Trend in Sinking Vertical Mine Shafts Using a Pilot Borehole and a Review of Equipment Used for Drilling Large-Diameter Boreholes

N. F. Kosarev, V. G. Lukyanov

(Presented by the scientific seminar of the Department of Labor Protection and Mining Engineering).

At present, the most promising method for sinking vertical mine shafts is shaft sinking with the use of a pilot borehole, which provides higher technical and economic indicators in comparison with the conventional method. When shallow shafts are sunk by the conventional method, low commercial advance rates are achieved, since the time required for equipping the shaft considerably exceeds the time spent on its sinking. Therefore, in recent years, in both foreign and domestic practice (including in Kuzbass), there has been a tendency toward sinking vertical shafts along a borehole.

Because shaft sinking with the use of a pilot borehole for lowering rock to the horizon is still only slightly widespread in the USSR, there is at present no equipment intended for these purposes, and, unfortunately, none is being designed. Recently the development of drilling technology for drilling vertical boreholes has proceeded in the following directions:

1) when drilling deep boreholes, the tendency is toward reduced and small diameters;

2) when drilling boreholes and shafts for ventilation purposes, on the contrary, the tendency is toward large diameters (0.5–2.5 m for boreholes and more than 2.5 m for shafts);

3) when drilling blast holes in open pits, the tendency is toward high drilling rates for shallow and small-diameter boreholes.

The available literature sources do not contain a complete description of existing rigs suitable for drilling a pilot borehole, nor information on their technical and economic indicators; therefore, selecting the most suitable rig for specific geological conditions presents certain difficulties. This is further complicated by the fact that shaft-construction organizations do not have any rigs, while the administrations of the Soyuzshakhtoosushcheniya trust have only percussion-cable rigs that permit drilling boreholes with a final diameter of no more than 600 mm only to a depth of 150 m, or low-power rotary drilling rigs with which only small-diameter boreholes can be drilled.

Since there are practically no special domestic rigs for drilling boreholes for lowering rock to the horizon, the following standard equipment may be used in drilling such boreholes:

1) rigs for drilling water wells;
2) rigs for drilling oil wells;
3) rigs for sinking ventilation wells and ventilation shafts;
4) rigs for drilling coal chutes, ore passes, etc.;
5) rigs for drilling blast holes in open pits.

In characterizing existing rigs, certain terminology will be encountered; brief definitions of it are given below.

In drilling practice the following schemes are used for cleaning boreholes of cuttings or dust:

1) direct flushing, when the clean flushing agent enters through the drill string, washes the bottom hole, and exits with the cuttings through the outer annular space;

2) reverse flushing, when the flushing agent enters through the outer annular space and exits through the drill string;

3) combined flushing, when the flushing agent enters the borehole through the drill string, then, in the tool, passes into the outer annular space, washes the bottom hole as in reverse flushing, and exits into the outer annular space;

4) drilling with air blowing—the same as direct flushing, except that air is used instead of water.

According to the operating principle of the drilling unit, the following are distinguished:

1. Percussive:
   a) cable-tool rigs;
   b) grab rigs;

2. Rotary:
   a) shot-drilling rigs,
   b) core-drilling rigs;

3. Vibratory;

4. Thermal;

5. Explosive.

According to the location of the rigs, they are divided into:

1) surface rigs—for drilling from the surface;
2) underground rigs—for drilling from underground workings.

Rigs for drilling water wells often employ a cable-tool action, usually of the UKS type. Most of these rigs have relatively low drilling rates, usually not exceeding 40 m/rig-month. The best drilling performance with cable-tool rigs was achieved by the Burvodstroy trust, which drilled a large number of wells 400 mm in diameter and about 120 m deep in 6 working days when operating in two shifts. Abroad, cable-tool drilling rates reach 600–1000 m/rig-month. Very good results have been obtained with rigs of the French firm Benoto. Thus, in Milan, Benoto rigs drilled wells 77 to 105 m deep and 1.05 m in diameter in various rocks: clayey soil, coarse pebbles, clayey sands, and conglomerates; beds up to 1.0 m thick containing basalt were also intersected. The sinking of a 77-meter well and its casing with pipes was carried out in 86 hours. The deviation of the well from the vertical direction was only 40 mm.

Equipment for drilling oil wells has a large weight, since it is intended for drilling deep wells. The cost of rotary drilling rigs is usually several times higher than the cost of cable-tool equipment. However, even with the great weight of rotary drilling rigs it is impossible to drill a large-diameter borehole, since the largest diameter of three-cone bits (standard H554-56) is 490 mm. In foreign practice, they often use

cutters with a diameter of 143 mm and reamers with diameters of 143/270, 270/406, 406/610, 610/812.

In domestic practice, equipment for drilling oil wells is adapted for drilling large-diameter boreholes and shafts—these are the Shchspotnev–Ivanov reactive-turbine rigs, etc.

Equipment for driving ventilation boreholes and ventilation shafts is less widespread, since it is at the stage of industrial testing and is very expensive. These include the UZTM, UKB-3.6, TM-2.3 rigs, etc.

In core drilling, the following methods of breaking off and extracting the core are currently known:

1) by means of a collet grab;

2) by blasting small charges of directional explosive and extracting with a collet grab;

3) by mechanical undercutting with special cutters mounted in a collet grab or bit;

4) under its own weight.

The first method is used when drilling in stable but weak rocks. After the core has been drilled through over the full height of the bit, the drill string is pulled out of the borehole and, in its place, a collet grab is lowered to break off and extract the core.

The second method is used when drilling in stable and strong rocks, when it is impossible to break off the core with a collet grab or bit. The core is broken off by a group explosion of a series of charges lowered on the collet grab. A similar method of breaking off and extracting the core was used by the “Zeni” company when drilling a borehole 1.5 m in diameter and up to 140 m deep in West Virginia (USA).

The third method consists in undercutting with special devices consisting of chains, ropes, and sprockets located directly in the lower part of the bit. During normal operation these devices are placed between the walls of the drilling cylinder; when the bit is rotated in the reverse direction they are actuated: they undercut, pick up the core, and hold it during extraction.

The fourth method is used when drilling raise boreholes in weak and poorly stable rocks.

Equipment for drilling ore passes and coal-discharge furnaces is usually intended for drilling in soft rocks, rock salt, and coal at a drilling depth of 50–70 m, which often does not satisfy mine builders. These rigs include BNM-860, UKV-0.8, LBS-4, BSA-6, etc.

Equipment for drilling blast holes is intended for drilling shallow, small-diameter holes in open pits; therefore it can find only limited application.

At present there is a tendency toward the manufacture of rigs on which both cable-tool and rotary drilling could be used. Such rigs make it possible to change the drilling mode depending on mining and geological conditions and are undoubtedly progressive. In the Soviet Union, a similar ABS-1 unit underwent industrial tests and gave good results: thus, with a borehole diameter of 660 mm and a depth of up to 105 m, the average mechanical drilling speed was 4.16 m/h, the average round-trip speed was 2.18 m/h; and with a diameter of 500 mm and a depth from 105 to 310 m, the average mechanical drilling speed was 5.77 m/h, and the average round-trip speed was 3.16 m/h. According to calculations, the annual savings from operation of the unit will amount to as much as 68 thousand rubles, depending on the conditions of application and the drilling methods compared. Similar units are also available abroad.

A category of equipment used for drilling boreholes that differs somewhat from drilling rigs consists of reamers and drilling combines. Their special feature is that they are, in fact, only the working member that breaks the rock. Rotation is transmitted to the reamer through the drill string of the rig in conjunction with which the reamer operates. Thus, the RUP-2 and RUP-3 reamers operate in combination with the SBU-3m machine; drilling combines operate in a similar manner; the KBU-1.2—40 combine can operate in combination with the “Uralmash-6E,” BU-75, and other installations.

It is necessary to create such reamer designs that could be used with any types of rigs and that would be included in the drilling-rig assembly and supplied by the manufacturing plant at the customer’s request.

Vibratory drilling is used in hard rocks. Existing SBU-55m and UVB-25 rigs make it possible to drill boreholes up to 30 m deep and up to 150 mm in diameter at considerable rates. Thus, the UVB-25 rig, in granite with a strength coefficient \(f=6\), achieved a drilling rate of 18 m/h, and at a strength of \(f=16\), 2.8 m/h. However, such borehole diameters and depths cannot ensure the lowering of rock to the level.

The STB-1 and SBO rigs—thermal drilling—also provide small diameters and depths and cannot be used for drilling boreholes for lowering rock to the level. It is possible that these types of drilling are promising.

There are known, basically, two varieties of explosive drilling—drilling with jet explosive drills and drilling using explosive cartridges. In both cases, rock destruction occurs without the usual mechanical rock-breaking tool. Jet explosive drills are used to drill boreholes 100–250 mm in diameter in hard and very hard rocks to depths of up to 100 m. In one of the boreholes drilled using explosive cartridges, in the interval 2250–2745 m in hard silicified limestones, an advance per one run of the tool of up to 20 m was obtained, with a trip speed of up to 0.60 m/h, as compared with an average advance per bit of about 3 m and a trip speed of 0.15 m/h obtained under similar conditions in a borehole drilled by the turbodrilling method.

Explosive drilling (both using cartridges and charges of explosive without casings) is promising.

For drilling boreholes for lowering rock to the level, it is necessary to create such types of rigs that would cover all mining and geological conditions within the system of a trust or at least of a combine.

In our opinion, two basic types of equipment are necessary:

1) rigs for drilling boreholes from the surface;

2) rigs for drilling boreholes from a mine level, the latter being subdivided into:

a) rigs for drilling from the bottom upward,

b) rigs for drilling from the top downward.

Requirements imposed on the first type of rigs:

1) a high drilling rate for boreholes with a final diameter of not less than 600 mm and a depth of up to 400 m;

2) easy transportability;

3) low weight of the rig;

4) ease of assembly, disassembly, and maintenance;

5) the rig must not allow significant deviation of the borehole.

Requirements imposed on the second type of drilling rigs.

The rig must:

1) be easy to disassemble and assemble into separate blocks of small dimensions and weight, so that they can be easily transported through mine workings;

2) have small dimensions allowing a borehole to be drilled from workings with a cross section of up to 8 m²;

3) have explosion-proof equipment;

4) have a dust-suppression or dust-capturing device;

5) provide the possibility of phase drilling; the number of phases should depend on the strength of the rocks;

6) be easy to service;

7) not permit significant deviation of the borehole.

The creation of such rigs will make it possible to reduce expenditures on equipment and to increase the commercial rates of sinking vertical shafts.