Hydraulic and earth engineering – Earth treatment or control – Shoring – bracing – or cave-in prevention
Patent
1996-10-07
1998-05-12
Taylor, Dennis L.
Hydraulic and earth engineering
Earth treatment or control
Shoring, bracing, or cave-in prevention
57217, 4052596, E21D 2002
Patent
active
057496811
DESCRIPTION:
BRIEF SUMMARY
The invention relates to high tensile strand for civil engineering applications, to anchorages formed therefrom and to methods of installing such anchorages. The invention is concerned particularly but not exclusively with such installations for use as mine roof support bolts.
One known procedure for supporting mine roofs is to drill a bore in the roof sufficiently far to reach into stable rock, to secure a high tensile mine roof bolt in the bore in the stable rock and to then carry a support plate engaged with the mine roof surface on the outer end portion of the bolt.
All references to rock are intended to include not only sound rock as such but also other ground materials such as clays or loams or coal.
Products known as ground anchors, soil nails and rock bolts operating on broadly similar principles are also used generally in the civil engineering industry. Although the invention is concerned particularly with mine roof support bolts, it could also be applied more generally to these other products.
A problem arises when the distance available below the mine roof is less than the required length of the roof bolt for anchoring it in stable rock.
An objective of the present invention is to provide means whereby this difficulty may be overcome.
It is already known for example from GB-A-2080845 and EP-A-0250010 to manufacture small diameter strand from a core and outer wires with such dimensions that gaps appear between adjacent outer wires of the strand. Such strand is typically used to provide flexible strength in deformable structures such as car tires where a product is manufactured by moulding a product round the strand. Typical diameters mentioned in the prior art are strand diameters of 0.61 mm or 0.65 mm in one case and wire diameters of 0.35 mm or 0.38 mm in the other case. In contrast strand employed in civil engineering applications typically employs much larger diameters of at least 5 mm diameter although wires as small as 4 mm could be satisfactory.
According to a first aspect of the invention a high tensile strand for civil engineering applications comprises a core wire and a ring of outer wires arranged in a helical pattern around the core wire and in contact with the core wire wherein the number and the diameter of the outer wires in relation to the core wire are such that there are significant gaps between adjacent outer wires.
The simplest kind of commercially available steel strand is known as 7-wire strand and incorporates a core wire and six outer wires surrounding the core wire. Theoretically, a 7-wire pattern of this kind with wires of equal diameters would leave all outer wires in intimate contact with the core wire and in intimate contact with adjacent wires. Core contact is more important than contact between adjacent wires, to ensure that when the strand is gripped, the outer wires are engaged firmly against the core wire. For this reason, the diameter of the core wire is generally made at least 2% and typically about 2.5% or 3% larger than the corresponding diameters of the outer wires.
Preferably in accordance with the present invention as applied to 7-wire strand, the diameter of the core wire is between 5% and 50% greater than the corresponding diameter of the outer wires. Alternatively, the number of outer wires may be reduced, for example to 5 and the core wire may then be the same size as or larger or smaller than the outer wires.
Preferably the outer wires have surface deformations.
According to a second aspect of the invention there is provided an anchorage in rock for a high tensile strand as defined above wherein the strand is anchored in a bore in the rock by a bonding agent, wherein the strand fits in the bore with clearance sufficient to permit easy insertion and to allow flow of bonding agent therearound, the bonding agent also being impregnated into the strand into the gaps between adjacent outer wires.
In a typical case, an end portion of the strand adjacent the surface of the rock carries a support plate engaged with the rock surface to support the local rock.
A
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patent: 5417521 (1995-05-01), Scott
Cassidy Terence
Doidge Jeffrey
Edgar Allen Engineering Limited
Taylor Dennis L.
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