Hydraulic and earth engineering – Earth treatment or control – Rock or earth bolt or anchor
Patent
1993-06-08
1995-07-18
Taylor, Dennis L.
Hydraulic and earth engineering
Earth treatment or control
Rock or earth bolt or anchor
52698, 411387, E21D 2002
Patent
active
054335582
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to self-tapping rock bolts and to self-tapping and self-drilling rock bolts.
Rock bolts are designed to provide support resistance for excavations in rock, such as underground and surface mines, tunnels, cuttings, etc. They are an extremely effective way of supporting rock excavations and hence they have achieved high acceptance in both the mining and civil engineering industries.
Rock bolts come in many shapes and sizes, and two main types are solid rock bolts or tubular rock bolts. Solid rock bolts (e.g. deformed bar, dywidag, expansion shell, slot and wedge, etc.) have a solid central core to the bolt which provides the bolt with high tensile and shear strength characteristics. Tubular rock bolts on the other hand (e.g. split-sets, swellex, etc.) rely on the strength of the "tube" itself and hence normally have lower tensile and shear capacity than solid rock bolts.
Solid rock bolts maximise the ratio of cross sectional area of the rock bolt to cross sectional area of the rock bolt hole. Solid rock bolts therefore not only provide high tensile and shear strength capacity but also provide high tensile and shear stiffness characteristics. However, all solid rock bolts have a smaller cross sectional area than the cross sectional area of the hole in order to allow the rock bolt to be inserted into the rock bolt hole. As an example, solid rock bolts used in underground coal mines in Australia have a nominal diameter of 21.7 mm and are inserted into a borehole with a nominal diameter of 27 mm. There is therefore an annulus of approximately 2 mm between the surface of the rock bolt and the surface of the inside of the hole.
Solid rock bolts can be anchored into the rock bolt hole in two main ways, namely, with a cement or a chemical resin grout and with a mechanical locking device such as an expansion shell or a slot and wedge anchor.
In the case of a cement or resin grout, the grout forms a bond between the surface of the rock bolt and the internal surface of the hole. Therefore solid rock bolts used in this way often have a "rough" surface to increase the bond between the bolt and the grout (e.g. deformed bar, dywidag, T bolt, etc.).
However, little attention is given to the bond between the grout and the internal surface of the borehole. The process of drilling the rock bolt hole itself does create "roughness" on the internal surface of the hole, but this is not generally planned or designed in existing solid rock bolt systems. The only consideration is:given to the annulus size (i.e. the distance between the rock bolt and the wall of the hole), which is normally kept to a minimum (as indicated above), but this is primarily done to reduce the total amount of grout required rather than to increase the stiffness of the bolt/grout system.
Resin grout anchors normally use chemical cartridges or "sausages" to provide sufficient grout to anchor the rock bolt in the hole. The length of these sausages can be varied to change the length of the anchor so that in practice the rock bolt can be point anchored or fully encapsulated or somewhere in between these two extremes. The support response required and the rock type determine the length of grout anchor used but in normal circumstances the minimum length is 400-500 mm. Therefore, the bond between the rock and the grout is equally as important as the bond between the bolt and the grout.
Solid rock bolts with mechanical anchoring systems are designed to force a mechanical device or part of the bolt itself against the sides of the borehole by using either axial or rotational movement of the bolt. The most common examples of mechanical anchoring systems are expansion shells or slot and wedges and these normally provide a single point anchoring system at the end of the rock bolt hole. Therefore the surface profile of the solid rock bolt has no effect on the bolt capacity and in most cases these bolts are made from plain bars. Under extremely high loads these anchors tend to slip along the hole and these bolts can therefore accommodate consid
REFERENCES:
patent: 1288893 (1918-12-01), Holmes
patent: 4271554 (1981-06-01), Grenell
patent: 4697969 (1987-10-01), Sparkes
patent: 4946314 (1990-08-01), Gruber
patent: 5074729 (1991-12-01), Oba
BHP Engineering Pty Ltd
Taylor Dennis L.
LandOfFree
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