Mountain anchor

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Anchored section joint

A rock anchor is a construction element that is used in mining and tunneling to keep underground cavities open. They are there on shock or the ridges in the mountains introduced (the rock) and each consist of a long pole (or rope), which is anchored to the mountain-side end with an anchor foot in the rock, and (at the protruding end with a plate anchor head ) holds the rock. Degradation routes that are secured by bolting, it is called in mining anchoring leg. The most important anchors are listed in DIN 21 521.

history

The first theoretical beginnings of anchoring in mining date back to 1913. In that year, a patent specification (No. 302909) was submitted by Stephan, Fröhlich and Klüpfel. The patent was entitled "Method for intercepting and securing the hanging wall and the impacts in mining without support from below". In this invention, which was forgotten and had no practical use, the anchor heads were connected to one another using wire ropes and U-irons . In 1934, Keeley described the stabilization of a mining cavern using wedge slot anchors. In 1942 and 1943, anchoring was successfully used in the English mining industry. Anchoring was introduced in American mining between 1943 and 1950 and, from 1951, also in European mining. In the Saar mining industry in 1960, nine percent of all new excavation routes were driven as pure anchor routes. At the same time, anchors were started to be removed in the Ruhr mining industry, but this was not able to prevail over the long term.

Basics

The task of the anchors is to connect several layers of rock. The individual rock layers are put together in such a way that they behave like a coherent rock slab. This prevents the individual layers from peeling open and reduces the risk of breaking into the cavity. Around the cavity, anchors called tension elements in the form of rods or ropes are installed in radial bores and tensioned. The anchors have a positive effect on the stability of the roof .

Anchor types

There are structurally different types of anchors. Most rock anchors are made of steel, but there are also plastic anchors, which are usually made of fiber-reinforced plastic. Basically, these anchors consist of an anchor head, anchor foot and anchor rod. Anchors with expansion elements, composite anchors and friction anchors (friction pipe anchors) are distinguished according to the type of fastening in the rock. Rope anchors are also used in potash mining and civil engineering. There are special sliding anchors for stretches where great convergence is to be expected. There are also self-drilling anchors.

Anchor with expansion elements

There are several different anchors with expansion elements, the expansion sleeve anchor, the wedge socket anchor, the double wedge anchor and the slot wedge anchor.

Expansion sleeve anchor

The expansion sleeve anchor has a thread at both ends of a steel rod, on the mountain side a cone and the expansion sleeve are screwed on, so that the cone expands the expansion sleeve when the screw connection is tightened and thus wedges itself with the rock. On the joint side, an anchor plate (a steel plate measuring approximately 20 × 20 cm) is placed over the anchor, which is tightened with a nut. As a result, the anchor is under tension and the expansion sleeve cannot loosen.

Wedge socket anchors

Wedge socket anchors consist of an anchor rod with one end upset in a wedge shape. A partially slotted expansion sleeve is located above this wedge-shaped end. At the other end of the anchor rod there is a thread with a fastening nut. When the fastening nut is tightened, the wedge-shaped upset end is pulled into the expansion sleeve. A hydraulic tensioning device is used to set the anchor. Due to the long expansion sleeve, these anchors are also suitable for softer rock layers and offer good adhesion. However, these anchors are not stealable.

Double wedge anchor

Double wedge anchors have a two-part expansion body, which is formed by two diagonally cut cylinder halves. In this armature, the two cylinder halves lie against one another with their cut surfaces. A wedge body is located in the expansion body. A smaller wedge is screwed to the anchor rod. One of the two wedges is ribbed to increase friction, the other wedge is smooth because it slides along the wall of the bore. There is a stop for the wedge body on the anchor head. When the anchor rod is turned, the wedge body moves downwards and pulls itself against the second wedge. Due to the construction of this anchor, the wedge body can only move transversely, which means that the anchor becomes stuck in the rock. However, with this type of anchor only the two opposing surfaces of the double wedge support. In some applications, however, the holding force achieved by spreading the double wedge anchor is insufficient. Double wedge anchors are only used occasionally due to the small effective adhesive surfaces. The advantage is that these anchors can be stolen.

Slot wedge anchor

The slot wedge anchor has a slot at the end of the rod with a wedge at the end. When the anchor is driven in, the slot is widened by the wedge, which anchors the anchor in the rock. Slot wedge anchors are simple and inexpensive. Since they only have a small contact area on the borehole wall, these types of anchors can only be used in solid rock such as sandstone or sand slate. In order for the anchor to hold securely, the drill hole depth must be measured precisely. Due to the design of the anchor, the anchor can only be fastened in the borehole if the wedge is driven into the slot when it is pushed into the deepest part of the borehole, thereby driving the expanding jaws apart. The wedge can be driven in with a pick hammer with a special attachment piece.

Expansion anchor

These anchors consist of a steel tube that is closed on one side and folded lengthways. At the open end there is a threaded connection through which water can be pressed into the anchor under high pressure. After being inserted into the borehole, the anchor is pressurized and expanded until it rests firmly on the inside of the drill hole and claws there. After removing the water pressure, the thread can be loaded.

Chemical anchor

There are three types of compound anchors, cement mortar anchors, synthetic resin or adhesive anchors and friction anchors. Adhesive anchors and cement mortar anchors are suitable for soft rocks.

Cement mortar anchor

In the case of cement mortar anchors, the borehole is first filled with cement mortar to the depth of the borehole using a hose . Then the anchor rod is pushed into the filled borehole. As a rule, these anchors are inserted without pre-tensioning. If anchors with pre-tensioning are required for cement mortar anchors, a fast-hardening mortar is used instead of normal cement mortar in the area of ​​the force application. In the front area of ​​the borehole, a stable cement suspension is then introduced. However, these anchors are less suitable for anchoring in the overhead area. Insertion mortar anchors are available for anchoring in the overhead area. These consist of two perforated sheet metal pipes that are first filled with a stiffer cement mortar. Then the two halves are tied together with wire and inserted into the borehole. The anchor is then hammered into the sleeve prepared in this way. During this process, the mortar penetrates through the perforations into the space between the borehole wall and the sheet metal pipe sleeve. However, the use of insert mortar anchors is very complex.

Adhesive anchor

With adhesive anchors, a two-component adhesive cartridge is pushed into the anchor hole, which is then destroyed when the anchor is screwed in. This mixes the adhesive and hardens shortly afterwards. Then an anchor plate is attached with a nut in the same way as the expansion sleeve anchor. There are steel anchors and plastic anchors. The curing time of the two-component adhesive is 20 to 30 minutes, depending on the ambient temperature. Adhesive anchors are used for soft rocks where the anchor can be pre-tensioned just half an hour after the anchor has been inserted.

Friction tube anchor

Pipe anchors consist of an oversized, longitudinally slotted pipe. On the mountain side, it is slightly conical in order to be able to introduce it into the borehole. The end of the pipe at the end is flanged and reinforced with a welded ring made of round steel. The anchor plate for this type of anchor is convex in shape. The installation is done by first pulling the plate with the convex side towards the joint end of the anchor over the latter, then the anchor is pushed into the anchor hole with the drill carriage or drill . The pipe anchor holds solely through its material tension. In contrast to expansion sleeves and adhesive anchors, re-tensioning is not possible.

Rope anchor

Rope anchors consist of any type of wire rope . The rope of the anchor is between 4.5 and 10 meters long and has a diameter of 20 to 22 millimeters. Unused or used wire ropes are used. Used ropes have the advantage that they stretch less than unused ropes. In addition, used ropes are cheaper. Compared to anchors with rods, rope anchors have a higher load-bearing capacity. There are different types of rope anchors. In one type of rope anchor, the anchor end is provided with a rigid end or sleeve part. This rigid part allows a plate and nut to be attached to the anchor. So that the rigid end is firmly connected to the rope, it is connected to the rope by casting or cold pressing. Such anchors have a predetermined length and must be prefabricated according to the depth of the borehole. There are also rope anchors in which the wire rope has an anchored rotating part at one end. This rotating part is inserted into the borehole to secure the anchor. At the other end of this rope anchor, a rigid part is attached to which the anchor plate is attached with a nut.

Sliding anchor

The sliding anchor has a sliding control element on the anchor rod. This control element has an opening through which the anchor rod is inserted. The slide control element is mounted in a so-called slide body cage. In order to be able to set a more precise breakaway force, sliding anchors are also built with sliding bodies that have several recesses. This also gives the sliding anchor a higher clamping force. The recesses are each made in a separate cross-sectional plane of the sliding body. As a rule, sliding bodies with three recesses are mounted in a sliding body cage; with larger anchors, sliding bodies with more recesses are also possible. Due to their special construction, sliding anchors enable defined sliding in the anchor borehole, thereby protecting the anchor from overload.

Self-drilling anchors

Self-drilling anchors are anchors that have a drill bit at one end. With this drill bit, the holes can be created with the anchor itself. After the drill hole has been created with the anchor, the drill hole and the surrounding subsoil are grouted with cement mortar. The anchor together with the drill bit remains in the borehole. This type of anchor is used when building tunnels in loose rock.

Mode of action

With rock anchors, the effective anchor force is very closely linked to the movements of the surrounding rock. Depending on the mode of action, a distinction is made between individual anchors and system anchors. Individual anchors are used to secure individual blocks of stone. Furthermore, they also serve to maintain the shape of the excavation and the load-bearing capacity of the rock during excavation . System anchors are used to activate the vault effect of an excavated cavity. By inserting the system anchors, a rock support ring is created, that is, a rock casing that surrounds the route cavity. Tensions in the rock in the area of ​​the cavity are shifted deeper into the rock. This rock support ring counteracts the loosening of the rock and increases its own load-bearing capacity. In order for the anchors to be able to provide sufficient load-bearing capacity, the rock must have sufficient cohesion. Rocks with numerous potential or effective interfaces and mountains with low shear strength cannot be anchored.

Installation

Depending on local conditions, anchors are brought in together with wire mesh to form an anchor-wire mesh composite system. A basic prerequisite for the proper installation is a profile-appropriate blasting out of the road cross-section when driving by blasting . But it is also possible under certain mountain conditions, e.g. B. in salt mining, possible to insert anchors without wire mesh. Depending on the area of ​​application, a distinction is made between initial anchoring and subsequent anchoring. The first time anchoring takes place after the teeing off and robbing . The post-anchoring takes place either at the same time as the initial anchoring or after a certain period of time the cavities have been in place. Longer anchors are used for post-anchoring to absorb thicker detachments . As a result of the post-anchoring, the anchor setting density is increased overall. After drilling an anchor borehole, the respective anchor is driven into the borehole and screwed in accordance with the installation instructions. This process is repeated until all anchors are set.

monitoring

Special measuring anchors are installed at regular intervals so that the functionality and load-bearing capacity of the anchors can be monitored accordingly after installation. These measuring anchors are either equipped with colored markings or with an extensometer. The measuring anchor regularly, depending on the setting per shift or working day, by the competent supervisor means ruler measurement checks. In certain, predetermined time intervals a control of the measurement carried out by the anchor, in addition mine surveying . In addition, regular convergence measurements are carried out.

Individual evidence

  1. ↑ a b c Wolfgang R. Dachroth: Handbook of building geology and geotechnics. 3rd edition, Springer Verlag Berlin Heidelberg New York 2002, ISBN 3-540-41353-7 .
  2. ↑ a b c d e Walter Bischoff , Heinz Bramann, Westfälische Berggewerkschaftskasse Bochum: The small mining dictionary. 7th edition, Verlag Glückauf GmbH, Essen 1988, ISBN 3-7739-0501-7 .
  3. ↑ K. Kovári: History of Shotcrete Construction, Part III ( Memento of the original from April 27, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (accessed on December 16, 2011; PDF; 507 kB).  @1@ 2Template: Webachiv / IABot / www.igt.ethz.ch
  4. ↑ a b c d e f g h i j k l Carl Hellmut Fritzsche: Textbook of mining science. Second volume, 10th edition, Springer Verlag, Berlin / Göttingen / Heidelberg 1962.
  5. ^ Joachim Huske: The coal mining in the Ruhr area from its beginnings to the year 2000. 2nd edition, Regio-Verlag Peter Voß, Werne, 2001, ISBN 3-929158-12-4 .
  6. ↑ a b Horst Roschlau, Wolfram Heinze, SDAG Wismut (Hrsg.): Knowledge storage mining technology. 1st edition. German publishing house for basic industry, Leipzig 1974, pp. 90–96.
  7. ↑ a b Axel Hausdorf: Numerical investigations into the stability of chamber ridges in salt mining with special attention to a system anchorage with elasto-plastic-strengthening anchor characteristics and different anchor preload values. Dissertation [1] (accessed December 2, 2011; PDF; 11.0 MB).
  8. ↑ a b c d Alexander H. Schneider: Security against collapse in underground mining. ETH dissertation No. 14556, Institute for Geotechnics, vdf Hochschulverlag AG at ETH Zurich, Zurich 2002, ISBN 3-7281-2872-4 .
  9. ↑ a b c d e Ernst-Ulrich Reuther: Textbook of mining science. First volume, 12th edition, VGE Verlag GmbH, Essen 2010, ISBN 978-3-86797-076-1 .
  10. ↑ Marc Ladner: Study on expansion sleeve anchors in tunnel construction. Dissertation Archived copy ( Memento of the original dated November 1, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (accessed December 15, 2011). @1@ 2Template: Webachiv / IABot / library.eawag-empa.ch
  11. ↑ a b European patent specification rock or concrete anchors. Patent No. EP0861986 (accessed December 15, 2011).
  12. ↑ Two-step hollow bar composite anchor for glue cartridges and glue granulate. Patent specification, patent number DE102006011652A1 September 20, 2007 (accessed November 15, 2011).
  13. ↑ Boltex friction tube anchors ( Memento from January 2, 2014 in the Internet Archive ) (accessed on January 15, 2016; PDF; 927 kB)
  14. ↑ Patent document: Reibrohranker EP 2480760 A2 (accessed on January 15, 2015).
  15. ↑ rope anchor. Patent specification, document identification DE69230145T2 March 9, 2000 (accessed on December 16, 2011).
  16. ↑ Improved sliding anchor. European patent specification, patent number EP 2 087 203 B1 (accessed December 16, 2011).
  17. ↑ a b c d Hans Kilmer: Anchor route driveway on the Minister Achenbach mine. In: Our company No. 31, company magazine for the companies of the Deilmann Haniel Group, August 1982 [2] (accessed on December 16, 2011; PDF; 8.0 MB).
  18. ↑ a b Egon Hoffmann: First excavation of a dismantling section in an arched cross-section with the exclusive use of anchors on the Emil Mayrisch mine. In: Our company No. 21, company magazine for the companies of the Deilmann Haniel Group, May 1978 [3] (accessed on December 16, 2011; PDF; 8.9 MB)