Hydraulic and earth engineering – Earth treatment or control – Rock or earth bolt or anchor
Reexamination Certificate
2002-07-19
2004-04-13
Lagman, Frederick L. (Department: 3673)
Hydraulic and earth engineering
Earth treatment or control
Rock or earth bolt or anchor
C405S259100, C405S262000, C405S302400
Reexamination Certificate
active
06719498
ABSTRACT:
The present invention relates to slope stabilising means.
There are two well known and much used systems for the stabilisation of vertical or steep faces cut in fresh ground by the utilisation of tensile members installed and bonded into the ground behind the faces.
A soil nail comprises a tensile member generally made of steel, other metal or a composite such as glass or carbon fibre reinforced plastic, installed into a pre-drilled borehole extending at a small angle below the horizontal into the ground and bonded to the ground over its entire length by use of a setting cement or resin grout.
Soil nails are installed as a group in relatively close proximity to each other, typically on a 1 m to 2.5 m square grid at the face of the excavation. The group of soil nails provides the in situ ground with a tensile strength and a shear strength not previously present in the ground.
When a face is cut or excavation is made, there is, without soil nails, a tendency for a zone of soil adjacent to the face to collapse into the excavation (the “active” zone). When soil nails are installed, the collapse of this active zone is prevented by the soil nails tying the active zone into the ground mass behind (the “resistant” zone). However, because the soil nails cannot be artificially pre-tensioned over their length during or immediately after installation, the utilisation of the tensile strength of the soil nail can only be made when the ground in the active zone moves forward in the relation to the ground in the resistant zone. Thus, the soil nails are known as “passive” reinforcement. Hence, forward movement of the active zone of the excavated face and downward movement of the face crest is always associated with the use of soil nails for face retention. In some instances, particularly with buildings or other sensitive structures in the vicinity, such movement is unacceptable.
The alternative system for stabilisation of vertical or steep faces cut in fresh ground is the use of ground anchorages but these must be used in conjunction with a stiff facial structure.
A ground anchorage comprises a tensile member or tendon made of steel, other metal or a composite such as glass or carbon fibre reinforced plastic, installed into a pre-drilled borehole extending at a small angle below the horizontal into the ground and bonded to the ground over only its distal length (“fixed length”), by use of setting cement or resin grout. The distal length over which ground bonding occurs is always sited in the resistant zone of the ground mass some distance from the excavated face. Throughout the active zone, the ground anchorage tendon is completely debonded from the ground by use of grease coating and plastic sheathing (the “free length” of the tendon). This isolates the tendons from the ground and from the grout which may also be placed in this length of the borehole.
However, since the tendon of a ground anchorage is not bonded to the ground within the active zone, it is necessary to provide a ground retaining structure in front of the active zone to which the head of the anchorage tendon is fixed. This structure or structural facing may be of steel (sheet piling, king piling etc.) or reinforced concrete (bored pile, diaphragm wall or structural facing built as excavation proceeds) for example.
At the head of the ground anchorage is an anchor head plate, which transfers load to the structural facing. The benefit of the ground anchorage system is that it allows the tensile member to be artificially pre-stressed prior to the excavation proceeding downward. A stressing jack is placed against the structure and a load is applied to the tendon. This exerts an inward directional force onto the structure and hence to ground behind. The entire tensile force applied to the tendon is transferred through the debonded length of the tendon in the active zone and resisted by the bond capacity of the ground in the resistant zone. Hence, the ground in the active zone is pre-compressed, thus movement of the face and the crest of the excavation is considerably less than that which would take place when utilising a soil nail system. However, the disadvantage of the ground anchorage solution is the high cost associated with the construction of the structural facing through which the entire stabilising load must be transferred.
A description of the use and construction of ground anchorages is to be found in the Code of Practice for Ground Anchorages—BS8081—published by the British Standards institution. This Code also contains suggested terminology.
GB 2223518 describes a single bore multiple anchorage comprising a plurality of unit anchorages, each having a tendon, the tendons being bonded in respective encapsulations at staggered and spaced positions along the bore.
Methods have been utilised in the past for stabilisation of vertical or steep faces cut in fresh ground in which a combination of soil nails and ground anchorages have been considered beneficial.
The present invention provides slope stabilising means comprising a bore containing bore grout and:
(i) at least one ground anchorage means, comprising a tendon having a bond length which is bonded along the bond length within the bore grout and a free length arranged so that there is substantially no adhesion between the free length and the bore grout; and
(ii) at least one soil nail means comprising a tensile member bonded to the bore grout along substantially the whole length of the tensile member.
The present invention has the benefit of directly tying the ground in the active zone to the ground in the resistant zone using the soil nail, at the same time as pre-compressing the active zone ground using the ground anchorage, thus reducing or eliminating face and crest movement. The benefits of both slope stabilising systems can be obtained combined.
The present invention has the additional advantage that, by placing a soil nail and a ground anchorage in a single bore hole, the number of bore holes and components required can be reduced.
The inventors have discovered that, surprisingly, the inclusion of a soil nail in the bore grout of a ground anchorage does not detract from the performance of the ground anchorage. Indeed, by tying the area of ground in the region of the end of the tendon, the soil nail helps to resist the compressive load at the proximal end, allowing a lighter structural facing to be used. Finally, it is found that the inclusion of a ground anchorage does not interfere with the operation of the soil nail.
The present invention further provides a method of stabilising slopes, comprising forming a bore in the ground, and placing in the bore:
(i) at least one ground anchorage means comprising a tendon having a bond length and a free length, and
(ii) at least one soil nail means comprising a tensile member;
grout being fed into the bore whereby the bond length of the tendon of the ground anchorage means is bonded to the grout, the free length of the tendon being arranged to have substantially no adhesion to the resulting grout within the bore, and whereby the tensile member of the soil nail means is bonded to the bore grout along substantially the whole length of the tensile member;
an anchor head being fitted onto the tendon of the ground anchorage means; and
the tendon being stressed and locked with respect to the anchor head.
The Ground Anchorage Means
In a preferred embodiment, there are a plurality of unit anchorages received in a single bore. The bond lengths of tendons of respective unit anchorages are preferably anchored in the bore grout in staggered and spaced relationship along the bore.
The tendons of the ground anchorages may comprise steel, other metal or synthetic polymeric material, for example composite material such as glass or carbon fibre reinforced plastic.
Where steel or other corrodable materials are used, the bond length of the tendon is bonded in an encapsulation comprising a duct filled with resin or cement grout. This encapsulation is bonded in the bore grout whereby the bond length is bonded to the bore grout. Polymeric componen
Keller Limited
Lagman Frederick L.
MacMillian Sobanski & Todd, LLC
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