ACCESS TO A MINERAL DEPOSIT

Access to a mineral deposit is afforded by

(1)  Haul roads and steps       — in the case of an opencast mine

(2)  Incline and adit                — in the case of shallow underground mine.

(3) Pit of shaft — In the case of deep underground mine.

Incline and adit

A mineral bed which is at a shallow depth can be approached by an incline or adit. An incline is a sloping road driven from the surface to the deposit through the alluvium and the rocks overlying the mineral deposit. Unless it is for locomotive, an incline is usually steeply dipping at 1 in 4 or 1 in 5. The normal practice is to have an incline along the true dip of the deposit though the shape of the property may some times necessitate its drivage along an apparent dip. In the case of steep beds, usually with gradients of 1 in 4 and steeper, a haulage incline is along the apparent dip to have milder inclination of the haulage road. Inclines which touch a coal seam at a vertical depth of 30 m are common in Singareni group of mines. The trend nowadays is to approach mineral bed upto a depth (vertical) of 200 m by inclines.

If a deposit is at a higher level than the general ground level and outcrops, the access to it is by a level or a slightly rising roadway from the ground level. This type of nearly level roadway is known as Adit. Kurasia, Chirimiri and some other underground mines are approached by adits.

The size of an incline is usually 4.2 m wide x 2 m high if it is for endless haulage or a belt conveyor With rope haulage beside it. Inclines for travelling only may be as small as 2m X 2m

Pit or shaft

Access to a mineral deposit at depth is by a well, known as pit or shaft. Deposits at depths of 30m and more have been entered by shafts in this country. If the strata overlying the vein or seam are soft, and excavation cost is not high, an incline may be driven to enter deposits upto about 30 m depth. Such long inclines at steep gradients of 1 in 4 or 1 in 3 are common at Kolar gold field and Mosabani mines and are known as inclined shafts. In Indian coal mining practice, the term inclined shaftis not used and the word "shaft" invariably connotes a vertical shaft.

Depending upon their purpose shafts are referred to as main shaft or auxiliary shaft. Main shafts are designed for hoisting mineral to the ground surface. Auxiliary shafts are generally for man-winding, ventil4tiohVmaterial transport, stowing or filling material, etc. though the main shaft may, in some cases, serve some of these purposes.

An inclined shaft in metal mining practice is rarely in the ore body proper which normally does not have uniform angle of dip. It is a standard practice to drive an inclined shaft nearly parallel to the dip of the ore body but in the country rock of the footwall as the hang wall collapses after stoping (Fig 6.2). In Kolar Goldfield, vertical shafts are located at a distance varying between 100-150 m from the lode in the footwall with the intention that they should not be affected by the stresses or strains which develope in the 'rock when the ore is excavated by stoping. The stresses may affect the verticality of the shaft, and in an extreme case, may result in its collapse.

In horizon mining the coal seams are approached by long level stone drifts starting from centrally situated shafts, (fig 6.3) for example, at Sudatndih Colliery (BCCL), some mines with steep seams in North Eastern Coalfields Ltd. Horizon mining is a standard practice in many- metals mines where the lodes are steep.

Where two or more deposits have to be worked by underground mining methods, one deposit is entered by an incline or pit from the surface. The same incline or pit may extend to the lower deposit. Sometimes the lower deposit is entered from underground workings by

(a) A stable pit (sometimes called secondary shaft), or      

(b) A cross measure drift.

A staple pit or blind pit (shaft) is a vertical pit from one underground working place or road to another underground working place or road but does not extend up to the surface.

A cross measure drift (also called drift for brevity) in a coal mine is an underground roadway through stone connecting- two or more coal seams.

It may be horizontal or inclined. The term drift in metal mining practice has a different meaning, as explained in later chapter. Cross measure drifts in coal mines are usually of moderate length but in metal mines, some cross measure drifts are quite long. The noteworthy example is Bullen's incline at Kolar Goldfield, serving the lower levels of Orgaum section of Champion Reef Mine. It is 1520 m long and extends from 40th to 88th level It is equipped for raising men and large tonnage of ore at high speed.

 

It is desirable to plan sinking of 2 shafts at least upto the first level in a metalliferous mine or to first horizon in coal mining whereby production can start from one shaft while the other one is being deepened.