HACETTEPE UNIVERSITY MINING ENGINEERIN DEPARTMENT Name: Cem LAFCI ID: 20521066 LECTURE:TUNNELING and SUPPORT DESIGN LECTURER:Prof. Dr. Bahtiyar UNVER
INTRODUCTION BOLTING THEORIES TYPE OF ROCK BOLTS, VARIATIONS and COMPRESSION ZONES REINFORCEMENT MECHANISM OF ROCK BOLTS
Previously timber props or cribs used In 1927, rock bolt was used in metal mine In 1943 Weigel proposed the basic concepts
According the Weigel theory; some of his idea are still acceptable. These are: Support weakened rock below the natural arch line Bolt weak, thin strata to create thicker, stronger strata Bolt early in the mining
Advantages of rock bolting: Reduce storage Decrease size of the openening Prevents any appreciable deformations by quick installation Improving ventilation by lowering resistance Provides freedom for trackless vehicles Providing natural supports
Increasing trend of rock bolting
The theories used to explain bolting mechanism vary from place to place and sometimes are elusive. But there is 3 basic mechanism: Suspension Beam building Keying
SUSPENSION EFFECT of ROCK BOLTING
BEAM BUILDİNG EFFECT of ROCK BOLTING
KEYING EFFECTof ROCK BOLTING
Bolts are installed inclined to the roofline and perpendicular to the fracture plane: Where; σb= min. Axial stress bolt must be provide σp= Horizontal stress α= Angle between the normal to the fracture plane and the horizontal plane; ɸ = Friction angle of the fracture plane.
When the bolt is installed perpendicular to the roofline: Where; σb= min. Axial stress bolt must be provide σp= Horizontal stress α= Angle between the normal to the fracture plane and the horizontal plane; ɸ = Friction angle of the fracture plane.
Conventional Bolt Type Special Bolts
Based on the basic anchor types, bolts can be categorized into: Point-anchored Full-length grouted
Many types of rock bolts are developed by combining of these two basic bolt types to meet the specific support requirements and geological or geotechnical conditions.
There are at least seven different Bolt types commonly used support tailgates: Full length grouted Combination Point-anchored Tensioned with T channel Truss Rebar with truss Double locks
Special bolt types such as: Cable bolts Trussers Split sets are designed to counteract such condition
Work better than steel bars Less costly Flexible Great facilating material handling Bolt length is no limited by opening geometry Flexibility also allowed horizontal movement( thus reducing tendency to shear)
Cables using the reinforce ground before mining Typical highstrength cable bolt
Cable bolts are widely used to stabilize slopes
Compression zone created by trussers
It consists of thin wall steel tube 1,5 inches Installation is very quick Stress imposed on the tube is not very large 3,5 million split sets are used in metal mines per year
The effectiveness of the bolt in mobilising the inherent strength of the rock mass depends on several parameters The presence laboratory investigation is aimed at verifying the effect of bolt inclination, type of bounding( full- bounded length and point- anchored) and level of tension on the performance of rock bolt.
The shear strengt and dilation versus shear displacement for this joint can be presented as
The orientation of bolt to the joint plane has pronounced effect on shear resistance offered by rock bolt. The stabilizing action of a tensioned (active) bolt intersecting a joint can be explained using
The main advantage of tensioned bolt is that it increases shear- stiffness of the bolted joint as soon as it is installed. Fast-setting resin permits tensioning of the bolt a few minutes installation.
Point anchoraged rock bolt is limited to hard rocks and for use as temporary reinforcement. When cement or resin is fills annulus around the full length of the bar as means of anchorage, the bolt is called full bonded bolt. Full-bonded bolt requires only a small amount of movement in the rock mass to mobilize its anchorage capacity.
The anchorage length (L) of rock bolt to maintain stability is calculated based on the following formula: Where, F is factor safety (2<F<3); Tt is ultimate load of rock bolt; D is fixed anchor diameter and τ u/t is ultimate bond stress.