Ordinary methods of shaft sinking are not suitable in some cases and special methods have to be adopted under the following conditions:
1. Loose or unstable ground, such as sand, mud, etc.
2. Excessively watery strata.
The special methods are: 1. The Piling System 2. Caisson Methods 3. Freezing Method 4. Cementation Process 5. Drilled Shafts.
Method # 1. The Piling System:
This method is known as simply “piling” or “sheet piling and is suited sinking through loose deposits of sand, mud, or alluvium near the surface upto a depth of 20 m. Interlocking steel piles, 6 m to 10 m long, are used and they are practically water-tight. Additional lengths may be available by welding or riveting two or three lengths of piles.
At the surface, the piles are set up to form a ring and then they are hammered down in rotation, each member being driven a few metres at a time by a direct-acting steam piling hammer. As the piles descend in the loose ground, the latter, enclosed by the piles, is excavated and cleared up, but it should be remembered that the bottom ends of the piles are kept sufficiently ahead of the excavation to prevent inrush of water or loose sand. When the excavation reaches strong rock, permanent lining is constructed and the sinking then proceeds in the manner already described for normal conditions.
Method # 2. Caisson Methods:
The methods can be divided into three classes:
This consists of a cylindrical well of brick work, 0.3 m to 0.4 m in thickness over a.m.s. ring having a steel cutting shoe. The shaft is excavated and the drum sinks down gradually by its own weight. As the drum sinks down, further brick work is added on the top.
A compound sinking drum consisting of brick work surrounded by 13 mm thick steel plates is sometimes used to resist uncertain tensile stresses. Concrete sinking drums also can be used. Care must be taken to see that the drum descends vertically and with this object additional weights may be placed over the drum.
This is commonly adopted where the strata consists of alternate tough and loose ground arid also when the drop shaft refuses to sink further due to very high skin friction. In these cases sinking is carried out with the help of hydraulic rams which force down the cast iron drums. This method can be used for depth upto 60 m.
This method is adopted when there is a danger of ground filing up the shaft or where there is considerable inrush of water under a small head. Compressed air is led into the chamber formed by means of a partition, 1.8 to 2 m above the cutting shoe compressed air keeps back the water and sand. An air lock is mounted on top of the partition as a passage for men material. The limit of the pressure of the air is 4 kgf/cm2 beyond which persons cannot work. This method cannot be used for depths of more than 30 m.
These caisson methods are commonly adopted for the construction of foundations for bridges, tall buildings, etc.
Method # 3. Freezing Method:
This method is used when the sinking is proceeding through an unstable or friable strata with heavy inrush of water, or sand connected with inflow of water and essentially involves the formation of a large block of frozen ground in the water-bearing strata. The frozen block prevents the influx of water into the shaft.
The whole process can be divided into three operations:
I. The first operation consists of drilling holes, usually 150 mm diam. at 2.2 to 3 m intervals around the shaft from the surface or from a fore shaft. The holes, after drilling, are to be lined with special tubes and care should be taken to see that all the holes are vertical.
II. Inside the holes special small tubes are inserted to enable the cold brine (solution of CaCl2) to be circulated. Cold brine, while circulating in the holes, extracts the heat from the surrounding strata and the circulation of brine is continued till a wall of ice of sufficient size is formed. Sinking and lining is carried out in the normal way after the formation of ice wall.
III. The third and final operation is thawing which consists in removing the ice wall by sending hot brine through the existing holes.
This method is very rarely used in India.
Method # 4. Cementation Process:
This process can be used in all cases of shaft sinking, particularly in any fissured water bearing strata except in running sand or lose ground. It can be successfully applied in sinking even when the inrush of water is heavy.
Treatment of ground around the shaft is carried out to achieve one or more of the following objectives:
(1) To stabilise the collapsing ground,
(2) To reduce the inflow of ground water,
(3) To avoid flooding,
(4) To prevent sand “boiling”.
The operation is usually carried out in 2 phases, one before the sinking and the other after shaft lining. Ground conditions usually dictate the pattern of treatment.
The pre-sinking treatment reduces the surprise-stoppages of the sinking due to unfavourable ground conditions. Further, by reducing the amount of water inflow it not only saves expenditure on the dewatering pumps but substantially enhances the rate of sinking and the quality of the work.
On occasions post-cementation treatment may be necessary to have improved working conditions in the mine. Otherwise humidity in the underground excavation would create serious ventilation and corrosion problems.
The method consists in drilling the holes and then injecting a slurry of water and cement under pressure through the holes till they are completely sealed off. In the past injection was done at low pressures like 6 kgf/cm2 but it has been proved that high pressure of the order of 300 kgf/cm2 can be used successfully. The water cement ratio can be changed according to the requirements.
A process known as pre-silicatisation, which reduces the friction of the rock to the passage of cement is necessary in certain types of rocks. Extra holes are drilled for the purpose and are treated first with silicate of soda and then with aluminium sulphate.
This process of treating the holes with the chemicals is known as silicatisation. The holes to be treated with chemicals are known as “product-hole” and their number is usually three times that of cementation holes.
After cementation of holes the shaft sinking proceeds in the usual manner.
Method # 5. Drilled Shafts:
The drilling of mine shafts by suitable machines is the most progressive method of shaft sinking adopted in Russia and other advanced countries of the west. The process is completely mechanised and all the sinking work can be carried out by sequence controlled automatic machines so that there is no need to keep any men at the shaft bottom.
Basically a shaft borer is similar to a large tunnel-boring machine. A cutting head equipped with roller bits continuously excavates the entire shaft cross-section at once. The removed fine muck is pumped as slurry to the surface or dropped through a pilot hole for underground removal. In many cases sprayed or concrete is used as initial lining.
The machines used in Russia for such shaft sinking are suitable for shaft diameters 6.2 m to 8 m (excavated diameter) upto a depth of 800 m through rocks of medium strength like those available in coal mining localities. The average speed is 100 m per month. The shaft is drilled in 3 stages; a pilot hole 1.2 m dia. is drilled to the full depth with a tricone bit; thereafter the reamer enlarges the hole to 3.6 m and then to 6.2 m.
The reamer is rotated by the drill pipe through a square section pipe at the surface, operated by electric motors. During drilling, the hole and shaft are full of mud-water mixture (mud flush) which exerts a pressure within the shaft and takes the place of temporary lining.
It also flushes the reamers, removing the broken rock to the surface with the help of an air-lift pump. The permanent shaft lining is built after the entire length of the shaft has been widened to its full diameter.
Actual capital expenditure for boring per metre of shaft by the technique of shaft boring is higher than by the conventional drilling blasting-mucking method and other special methods of sinking. But the cost is likely to be reduced with more knowledge of the ‘knowhow’ in handling of rigs.
Such machines are not used in Indian mining practice.
The current trends in the design of the shaft boring rigs are:
(1) Drilling of shot holes to a depth of 6 m.
(2) Increasing the hoist speed for deep shafts to 15-18 m/sec.
(3) Automation of sinking process.
The progress available with shaft sinking machines may be appreciated from the following performance of a machine manufactured by WIRTH, a leading manufacturer in West Germany. In early 1983, one 7 m diam. shaft was sunk in Alabama, U.S.A., completing the sinking of the shaft, 650 m depth, in 6 weeks. Sinking rate at one stage was 35 m/day. The period is inclusive of the time spent on shaft-lining.
The Company manufacturer’s machines which have achieved sinking rates of 7 m in a 7- hr. shift. The machine is adopted for cuttings-removal system and works in conjunction with the company’s pilot hole machines for sinking of blind shafts. The cuttings are removed upwards.