ROOF BOLTING IN COAL MINES

foreign flexible plastic strips represent stratified mine roof the wooden blocks at the ends represent supporting pillars or ribs inserting the plastic strips into the model simulates the roof of a mine entry next a weight is placed on the strips causing maximum deflection in the roof's back this is similar to the SAG that occurs in thinly stratified mine roof when the SAG exceeds the elastic limit of the roof rock fractures develop and a roof fall usually occurs if the strips are bolted together with continuous or individual plates placed at the exposed ends of the bolts a strong Consolidated beam is formed now when the weight is superimposed on it virtually no sag or deflection occurs although methods of reinforcement may be modified as conditions require the basic principle and end result are essentially the same the weak roof is supported by bolts secured in strong overlying strata for example bolts may be inserted at an angle rather than vertically in the plastic strips and when the weight is applied no appreciable sag occurs to determine sag and roof movement Bureau Personnel use a differential sag station an open spring is inserted into a regular bolt hole the spring with a piano wire attached is left at the back of the hole additional Springs with wires attached are placed at different elevations [Music] the wires are tensioned with a five pound weight and the distance from the brass base plate and the crimped metal ferrule is measured with a standard dial gate base readings are recorded and additional ratings are taken at regular time interface folding materials shown here are typical of those used in most American coal mines the first systematic roof bolting installations were made with one inch diameter slotted type bolts and suitable wedges effective bolt Anchorage depends on proper hole size depth wedge thickness and the force used to drive the bolt over the wedge [Music] with the slotted type bolts the prongs are tightly compressed against the wall of the borehole to attain suitable Anchorage elbow to installed by expanding the Shell with a threaded tapered plug the shell is forced outward against the borehole wall as the plug is screwed into the shaft to prevent widespread uncontrolled adoption of roof bolting as early as 1949 some states required permits from their mining departments before any trial installation could be started standard specifications for roof bolting materials were adopted at a meeting initiated by the Bureau of Mines along with recommendations to develop safeguards against the dust Hazard created by Ruth Drilling foreign to install roof bolts in a mine a roof control specialist appraises the mines roof conditions to see if good Anchorage can be obtained and what type and length of volts should be used [Music] for testing an expansion shell roof Bolt the diameter of the hole is measured with a gauge at the proposed Anchorage Horizon then a roof bolt assembly complete with a pulling collar bearing plate and an expansion unit is inserted into the hole and tightened testing equipment is placed on the collar and an initial load is applied by a hydraulic ramp an extensometer inserted between the pulling rod and mine floor measures bolt movement as the load is gradually increased until the desired limit is reached or until continuous displacement of the bolt assembly occurs without further increasing the load when this happens the ultimate Anchorage capacity of the assembly has been reached [Music] the standard roof bolt Anchorage testing procedure developed by the Bureau of mines and accepted by the American mining Congress is normally used in these tests [Music] section 302 of the federal Coal Mine health and safety act of 1969 requires each operator to furnish the Bureau of mines a copy of their roof control plan the plan must show the type of support and spacing approved by the Secretary of the Interior and must be reviewed periodically at least every six months thank you an important advantage of roof bolting over timbering is that by eliminating vertical supports in a mine opening the maneuverability of mobile equipment is greatly improved note the freedom from obstruction here [Music] mobile Transportation equipment such as shuttle cars can be maneuvered faster and safer where roof bolts are installed particularly at intersections another Advantage is that roof bolts are less likely to be dislodged by blasting than conventional roof support in working areas that have been roof folded loading efficiency is improved without sacrificing safety Some Coal beds once considered unminable can now be mined safely and economically with bolted roof cuts of coal can be cleaned up faster and more thoroughly thereby reducing accumulations that could be ground into dangerous explosive dust after a thorough examination has been made of the newly exposed roof temporary supports are installed before it is bolted foreign holes for roof bolts are usually drilled with Stoppers or rotary drills in this mine a percussion stopper drill is used some drills utilize both rotation and percussion when Drilling in soft strata the rotary action is used for hard strata the machine can be switched readily to percussive action foreign [Music] hydraulically operated rotary drill is very flexible in that the speed and pressure can be varied over a wide range this is an advantage when roof rocks have different hardnesses are encountered [Music] in an effort to control dust hazards created by drilling operations various types of drill dust collectors were tested in the bureau's experimental coal mine many features of today's modern roof drills with integral dust collecting systems resulted from such tests this approved drilling machine channels the dust down through the hollow drill Steel [Music] with continuous mining machine operations close coordination of activities is required to avoid interfering unduly with machine performance sometimes this is accomplished by mounting roof drills on the side of the miner so that bolts can be installed while the miner is operating foreign if the drills are not mounted on the machine bolting cannot be done simultaneously with the cutting and loading of coal in this case the machine Must Be Stopped periodically and moved back to permit bold installation when the machine operator reaches the last row of bolts [Music] where Minds use the half face Advanced system coal is alternately extracted from half the face width and then from the other half the roof area adjacent to the mining machine can be bolted then the machine moved over while the area vacated is bolted by coordinating the face Advance with the bolting cycle the machine operator always works under supported roof [Music] with a boring type continuous Miner an arch store elliptical opening is left which substantially increases the resistance of the roof to Sag and fall in this arched entry the bearing plates and bolt heads are sometimes recessed into the roof to prevent them from interfering with starting a cross-cut or pillar lift [Music] although the principle of roof bolting is fairly simple each mine requires individual study to determine the proper plan and materials for bolting open-end pillar work may require a more elaborate bolting plan with supplementary supports consisting of crossbars as well as metal reinforcement such as steel channels steel ties and wooden blocks such materials Aid in securing the roof between bolts [Music] Minds will sometimes deviate from the common forefoot pattern of roof bolts special bolting patterns may be required for wide intersections often longer bolts may be needed to reinforce the regular pattern although bolting provides one of the best means of support it is not the sole cure for all roof control problems supplementary timbering may be necessary in some mines to provide adequate roof support foreign s is continually conducting research on roof bolting in Laboratories and in the field one phase is aimed toward developing techniques and equipment to appraise the safety and efficiency performance of individual bolts [Music] mining industry and roof boat manufacturers are directed toward evaluating new bowling equipment and improving the quality of roof bolting materials [Music] in this test a specially designed roof bolting machine with a spring-loaded tightening wrench is used to obtain torque tension relationships and frictional constants bolts are tightened in a manner simulating actual underground conditions and bolt loads are recorded using pressure gauges and a hydraulic rack here a standard direct reading dial wrench is used to obtain torque readings in order to determine the torque tension relationship numerous tests are conducted to confirm results underground research is conducted in torque tension analysis a standard mine roof bolt is installed with a u-type bolt tension indicator inserted between the bearing plate and the mine roof the bolt is then tightened in the normal Manner and the bolt tension in pounds is Red from the gauge [Music] [Applause] the next step is to determine bolt tightness using a torque wrench foreign tension relationship is obtained the bolt is loosened until the u-type cell can be removed and then it's re-tightened [Music] the stratoscope is another device used to determine the stability of mine roof it was developed to detect and accurately measure strata separation above mine openings here a nine-foot borehole is being explored visually sometimes they are photographed to provide a permanent record the extent of strata separation caused by roof sag AIDS in predicting impending roof failures roof molding is more of a narc than an exact science but the Bureau of Mines conducts research in an attempt to place it on a more scientific basis this centrifuge is designed to facilitate research in analyzing Earth's stresses above mine openings particularly embedded strata and bolted areas scaled down sample sections of mine roof strata are tested in the centrifuge to obtain data on the holding effect of various bolting patterns over the years we have learned a great deal about bold installations but there are many imponderables such as the quality of rock to be supported variations in Bedding roof unconformities and pillar strength there are unknown factors with respect to rock stratification strength and anchoring characteristics each individual situation must be investigated and studied carefully foreign with roof boating has been due to the Cooperative effort of several agencies among them are the manufacturers of roof bolting equipment and materials mining companies who willingly applied techniques devoting time effort and money to achieve the desired Act labor organizations and state Departments of mines for their willingness to adopt Innovations and make necessary changes in their mining regulations and finally much credit must be given to the work from themselves who actually did the bolting and worked under the roof that was Bolten the ultimate objective of roof control through bolting techniques and applications is to make the nation's Minds safe places in which to work to this end a substantial effort of the United States Bureau of mines has been and will continue to be dedicated foreign