Anchorage

The scheme of the anchor

ANCHORAGE (EN: anchorage, roof bolting; DE: Ankerausbau, Gebirgsanker, Anker; FR: boulonnage, ancrage; ES: anclaje; RU: анкерная крепь) is the mining roof support, the major element of which is the metallic, concrete, polymer, or wooden rod (anchor), which is fixated within the hole (borehole).

It is intended for the strengthening of the massif of the rocks, and for the increasing of the stability of its exposures by the method of the fastening of the rock layers, which are different in strength. The first facts about the usage of anchors for the strengthening of the mining workings refer to 1900, when wooden wedge-slit anchors have been used at the coal mine within the Upper Silesia. The experiments with the usage of the steel anchors for the strengthening of the drifts have been conducted within the Netherlands, Germany, United Kingdom, USA yet before the 1st World War of 1914-18. The industrial usage of the anchorage has started after the 2nd World War of 1939-45.

During the underground development of the deposits of minerals, they use anchorage for the strengthening of the capital, preparatory, and cleaning mining workings (independently of the shape, of the cross section, and of the time of service) alone or in combination with the framed roof supports; they also use it as the method of combat with the heaving of the rocks of the soil, for the strengthening of the coal or rock mass, which subsequently should be destructed by the cutting-loading combined machine (polymer and wooden anchorage), for the prevention of the squeezing of the coal within the retreating clearance stopes, for the suspension of the pipes for various purposes, and for the fixation of the mining equipment. They distinguish anchorages with the fixation of anchors at the bottom part of the hole, or of the borehole (point fixation), with the help of the various mechanical locks, and along the entire length or the significant part of it (continuous fixation) by the chemical compositions, which are based on the synthetic resins, by the cement (sand-cement) mortars, with the help of the energy of explosion. There has got the distribution within the USSR the anchorage with the point fixation of the anchors (such as ShK, AK-8, AD-1, AP-2, ES-2), which is appropriate to use within the rocks with the strength for the uniaxial compression not less than 29 megapascals. It consists of the metallic anchor (Figure 1) with the length of 0.8-2.5 metres and diameter of 20 millimetres, which is having at one end (in the joint part) the wedge-shaped head, at the other end the screw thread, of the two half-couplings, of the baseplate, and of the tension nut.

The scheme of the fixation of the anchor

It is appropriately to use the anchorages with continuous fixation of the anchors within the weak unstable rocks with the strength for the uniaxial compression of less than 29 megapascals; there is not excluded the usage also within the stronger rocks. During the fixation of the anchors by the chemical composition (Figure 2), there are introduced into the borehole the necessary quantity of the ampoules with chemical fixative (resin and hardener), and then the rod, which is rotated with the help of a drill or perforator, and is moved to the bottom of the borehole.

The shells of the ampoules rupture, and their contents are intermixed. After the hardening of the chemical composition, and after the fixation of the anchor, they install the baseplate, and create preliminary tension with the help of the nut. During the fixation of the anchors (which are made of concrete) with the cement (sand-cement) mortars, the latter are introduced into the borehole within the ampoules or by the special pump. Anchors, which are fixated by the energy of explosion (at the stage of the industrial mastering), represent themselves as the metallic pipe, which is filled with explosive substance. The tubular anchor, which is placed into the borehole, expands itself after the blasting of the charge of the explosive substances, thus taking the shape of the borehole, and strongly fixates itself. The bearing capacity of the anchor is: 49-69 kilonewtons with the point fixation, 147-196 kilonewtons with the continuous fixation. The usage of anchorage permits to decrease by 2-2.5 times the quantity of work for the strengthening (compared with the framed roof supports); the consumption of the fixation materials is significantly decreasing itself. Within the underground transport and hydraulic engineering, anchorage serves for the stabilization of the massif of the rocks during the process of construction, and sometimes during the operation of the underground structure.

The usage of anchorage is possible within both strong hard rocks, and disturbed semi-hard rocks, providing the sufficiently straight contour of the working. Anchorages may be used in conjunction with other types of the roof support: polygonal, arched, made of the sprayed concrete. They use for anchorage metallic anchors with locking devices (wedge-slit and strut ones), reinforced concrete ones (stuffed, pumped, "perforated" ones), and polymer-concrete, which are fixated along the entire depth of the hole (Figure 3, a, b, c).

The scheme of installation of anchorsThe scheme of installation of anchors

Within the workings of the large cross-section, they install the preliminarily stressed reinforced concrete anchors (Figure 3, d), which interact with the massif of the rock till the manifestation of deformations within it. Reinforced concrete and polymer-concrete anchors may be included into the composition of the permanent roof support of the underground workings. They situate anchors predominantly within the vaulted part of the working: along the radial direction within the homogeneous cracked rocks, across the strike of the layers and cracks within the layered rocks; the interval of installation along and across the tunnel is identical. In order to avoid the local falls of the rock, they suspend between the anchors, along the contour of the working, the steel mesh with the holes of 0.05x0.05, 0.1x0.1 metres in size, and sometimes install metallic catchers. The length of the non-stressed anchors is 1.5-4 metres, of the stressed ones is 5-15 metres. The carrying capacity of the non-stressed metallic anchors is 59-78 kilonewtons, of the reinforced concrete ones is 98-118 kilonewtons, of the stressed ones is 294-980 kilonewtons and more. The major advantages of anchorage (compared with the arched roof support): great possibilities of mechanization of the underground works, economy (for 1 metre of tunnel) of 300-1500 kilograms of metal, 0.7-2.5 cubic metres of timber.

Anchorages are widely used within the foreign countries.

Anchored fixation of pipelines