Text 9. Rock pressure

The development excavations and the excavations in the orebody may become dangerous if no support be used to resist rock pressure. The greater the depth below surface, the greater will be the pressure due to the weight of rock. Rock pressure may result also from movement along fault planes and along lines of weakness in the rock foundation. Rock pressure is the cause of rock-bursts, falls of hanging walls, and the premature caving of workings; the main proportion of underground accidents may be attributed to these causes.

The roof of an excavation in rock is not subjected to a pressure equal to the weight of the whole of the overlying mass; were it so, it would be impossible to mine to greater depths than a few hundred feet below surface. Rock pressure is vertical; but the lines of pressure from the rock far above an excavation will tend to combine into resultants which follow the line of an arch; the weight of rock above the arch is supported by the solid ground adjoining the excavation. Otherwise the roof of an excavation at say 4,000 ft. below surface, would be subjected to a pressure of 330 t. per square foot in a rock such as quartzite and 360 t. per sq. ft. in a rock such as hornblende schist.

According to its physical and mechanical properties a rock may be: homogeneous, as granite; stratified, as quartzite; laminated, as slate; schistose; broken or weakly-consolidated; unconsolidated, as sand.

Causes of heavy ground. There are three principal causes of heavy ground: “ground flow”, sometimes referred to as “earth pressure”, is present to a more or less degree in all underground openings and is particularly troublesome when the openings are made through soft materials such as dikes, slates, fault gouges or ore. “Ground subsidence”, another cause of heavy ground, may be due to the proximity of mining operations or to weakness of the bedding or cleavage planes in certain types of formations because of faulting or lack of adhesion, permitting the material to slip on itself and become unstable when an opening is made in it. The third, “ground swelling” or expansion is due to the exposure of an excavation to air or water or both.

Type of Support and Factors Affecting its Choice. The above mentioned principal causes of heavy ground are present in most deep mines, but each must be contended with in a somewhat different way. The problem is not how to support, but rather how to cope with these types of heavy ground so that the supporting members – whether of rigid steel, yielding steel, concrete, or conventional timber – will give the maximum life before replacement is necessary.

There are many factors which enter into the choice of the type of support such as: location, the distance taken either horizontally or vertically from the center of disturbance; the orientation of the opening, whether it runs perpendicular or parallel to planes of weakness; the use to which it is to be put, that is, if it is going to be subject to blasting and the impact of scrapers, etc.; the time you expect to hold it open; the shape and size of the opening; and possibly others.

(2590)

NOTES:

• premature caving - преждевременное обрушение, обвал;

• hornblende - роговая обманка, амфибол;

• ground flow - течение пород;

• gouge - глинистая примазка жил, глина трения;

• ground subsidence - оседание пород;

• adhesion - связность породы;

• ground swelling - разбухание, вздутие, вспучивание пород.