GB2284673A

GB2284673A - Testing,in situ,the static load-bearing capacity of a pile

Info

Publication number: GB2284673A
Application number: GB9325110A
Authority: GB
Inventors: Kenneth Alexander Cameron
Original assignee: Precision Monitoring & Control
Current assignee: Precision Monitoring & Control
Priority date: 1993-12-08
Filing date: 1993-12-08
Publication date: 1995-06-14
Anticipated expiration: 2013-12-08
Also published as: GB2284673B; GB9325110D0

Abstract

Apparatus for testing, in situ, the static load-bearing capacity of a pile (12) comprises a jack (13) supported by the pile (12) and a beam (20) supported by the jack (13) above the pile upon a bearing (15), said beam (20) being anchored in the region of its ends at a level below that of the bearing (15). The beam is self-stabilising, in that a force applied through the bearing which might otherwise tilt the beam from an upright position is counteracted by reactive restoring forces generated as lateral components of the applied vertical test load. <IMAGE>

Description

Pile-testing apparatus.

The present invention relates to apparatus for testing the static load-bearing capacity of engineering piles.

Piles, usually of concrete, are used for the foundations of buildings and other large structures. The design of a pile is dependent upon the function which it is to perform and the composition of the ground in which it is to be used. It is therefore necessary that a pile design is tested in-situ to determine whether its load-bearing capacity is sufficient for the particular purpose for which it is intended.

Static pile load testing is usually carried out by applying a test force by means of a jack braced t via a spherical bearing, against a beam which is anchored in place at its ends. The problem with using this arrangement of apparatus is that if the anchoring cables used to hold the beam down at its ends become unbalanced, the beam may begin to tilt, whereupon the force of the jack against the beam exacerbates the degree of tilt until eventually the jack is suddenly and forcibly ejected, causing an extreme hazard to nearby personnel and also risking damage to the jack and/or associated measuring equipment.For this reason a testing apparatus using a single beam is not approved for use on many sites, which require that at least one further beam is located above and at right angles to the first beam to counteract any unbalanced forces and thus prevent the beam from tilting. However, since each beam must be anchored at each end to an anchor pile provided specially for the purpose, the requirement for two anchor piles to be provided for each beam used makes the use of multiple beams relatively expensive. This extra expense becomes especially significant when one considers that usually the test piles are prohibited by regulations from being used subsequently for structural purposes.

Against this background it is an object of the invention to provide pile testing apparatus which permits the use of a single beam without the attendant hazard usually associated with such apparatus.

According to the invention, apparatus for testing, in situ, the static load-bearing capacity of a pile comprises a jack supported by the pile and a beam supported by the jack above the pile upon a bearing, said beam being anchored in the region of its ends at a level below that of said bearing. In this form the beam becomes self-stabilising, in that a force applied through the bearing which might otherwise tilt the beam from an upright position is counteracted by reactive restoring forces generated as lateral components of the applied vertical test load.

The jack may be mechanically driven but usually the large forces required to be generated make the use of a hydraulic jack preferable, if not essential. The jack may comprise an integral load cell for determining the applied load or a separate load cell may be used, placed in line with the jack between the beam and the pile to be tested. The apparatus may also comprise means for recording the load applied, the response of the pile to loading or other test parameters which may be required.

The bearing upon which the beam is supported is very preferably a spherical or part-spherical seat bearing so that small variations in the direction of the force applied may be accommodated.

The beam is generally elongate and shaped along its length so as to allow the bearing, beneath the beam, to be at a higher level than the points towards each end of the beam at which it is anchored to the ground. Thus the central part of the beam, to which the reactive force of the jack is applied, may not be aligned, in a vertical plane, with the ends of the beam.

In a preferred form, the beam is symmetrically cranked, so that in use the central part of the beam is parallel to but at a higher level than the two end parts of the beam. In alternative but less preferred forms, the beam has a central part which is not parallel to the end portions.

Examples of such beams include those having an inverted V-shaped or arcuate central portion.

The beam is preferably made from pieces of H-section steel girder, welded together to form a cranked shape, optionally having strengthening straps disposed between each end part of the beam and the adjacent end of the central part of the beam to provide strength to the structure. The beam may, however be formed from other suitable materials, from box-section steel, for example.

The beam is anchored in the region of its ends, preferably by means of threaded tendon bars which are cast into two anchor piles over which the ends of the beam are placed. The beam preferably rests upon supports, which may be concrete or timber blocks, in line with and above the test pile and two anchor piles. The tendon bars or cables which extend upwardly from the anchor piles, are then secured, in tension, to the beam preferably by retaining nuts.

When using conventional straight beams, it is usual to set two tendon bars or cables into each anchor pile so that each may extend upwards past one side of the beam and be secured to a short cross-beam placed adjacent to and above the main beam at right angles to it. The reaction to the jacking force therefore acts downwardly upon the beam where each short cross-beam touches the main beam. This arrangement is used to stabilise, to some extent, the single beam twoanchor arrangement for testing piles. When the apparatus of the invention is used for testing piles1 due to the inherent stability imparted by anchoring the beam below the level of the reaction force bearing upon the beam, a single anchor tendon or cable at or near each end of the beam is sufficient.The beam therefore preferably comprises an aperture extending through the beam from its lower surface to its upper surface through which an anchor tendon or cable may extend. The cable or tendon may then be secured, in tension, to the beam by means of a nut or other fixing device. When threaded tendon bars are used to anchor the beam, the fixing device will preferably comprise a retaining nut.

The apparatus acording to the invention will now be further described, by way of example only, with reference to the accompanying drawings which are:- Fig.l, a view of a conventional testing apparatus in elevation from the front; Fig.2, a view of one preferred embodiment of the apparatus according to the invention, in elevation from the front.

In Fig.l a conventional two-anchor piletesting apparatus is shown, comprising a test reaction beam 10, supported upon blocks 11 above a test pile 12. A jack 13 is located on the test pile and a load cell 14 and spherical-seat bearing 15 are located above and generally coaxial with the jack 13. The beam 10 is anchored by means of a pair of threaded tendon bars 16 set into anchor piles 17 at each end of the beam. Each pair of tendons straddles the beam at its front and back and passes through apertures in a small crossbeam 18 located above and at right angles to the beam 10, to be fastened with a retaining nut 19.

In use, a force is applied by the jack 13 to the pile 12, using the test beam 10 to provide a reaction to the force. The beam is held vertically in place against the upwardly-directed jacking force by the cross-beams 18 and anchor tendons 16; that reaction force thus acts on the beam where the cross-beams 18 touch the beam 10.

However, since the tendon bars 16 of each pair may need to be splayed apart to enable them to encompass the beam 10, they may become unbalanced during the test and thus allow the beam to tilt about its longitudinal axis. Further application of force by the jack upwardly through the spherical bearing serves to exaggerate the tilting motion until the whole assembly may become critically unstable when the beam tilts far enough to allow the jack to be ejected with the force which it had been applying to the pile.

In Fig.2, an apparatus for testing the load capacity of piles according to the invention is depicted. It is similar to the apparatus shown in Fig.l, with the important difference that a cranked reaction beam 20 is used instead of the straight beam 10. The beam 20 is formed by buttwelding lengths of H-section steel for the central part of the beam, welded at each end to twin reversed channel-section end bars. The beam is anchored by two threaded tendon bars, one at each end, which pass vertically through apertures in the beam and are fastened by retaining nuts 19 seated upon washers 21 which bear upon the upper surface of the beam.Using this apparatus7 when the force is applied by the jack 13, through the spherical-seat bearing 15, to the underside of the beam, the beam is self-stabilising with respect to tilting forces which may be set up, because the level at which the force is applied by the jack is higher than the level of application of the reaction to that force, i.e. where the washers 21 bear on the beam 20.

The increased stability of the apparatus comprising a cranked beam over that comprising a straight beam is of major benefit to persons involved in testing the static load-bearing capacity of piles in terms of a greatly reduced risk of causing the jack to eject itself from between a beam and a pile being tested. It also, however, allows a significant saving to be made per test, since the safe use of a single beam in the apparatus according to the invention requires only two anchor piles to be provided and, further, that only a single anchor tendon bar is cast into each anchor pile.

Claims

1. Apparatus for testing, in situ, the static load-bearing capacity of a pile comprising a jack supported by the pile and a beam supported by the jack above the pile upon a bearing, said beam being anchored in the region of its ends at a level below that of said bearing.

2. Apparatus as claimed in Claim 1, wherein the beam is generally elongate and shaped along its length so as to allow said bearing to be at a higher level than the points towards each end of the beam at which it is anchored to the ground.

3. Apparatus as claimed in Claim 2, wherein said beam is symmetrically cranked such that, in use, the central part of the beam is parallel to but at a higher level than the two end parts of the beam.

4. Apparatus as claimed in Claim 3, wherein the beam is made from pieces of H-section steel girder, welded together to form a cranked shape.

5. Apparatus as claimed in Claim 4, wherein the beam comprises strengthening straps disposed between each end part of the beam and the adjacent end of the central part of the beam.

6. Apparatus as claimed in Claim 2, wherein the beam has a central portion which is not parallel to its two end portions.

7. Apparatus as claimed in Claim 6, wherein the beam has a central portion which is V-shaped or arcuate.

8. Apparatus as claimed in any of the preceding claims, wherein said beam comprises an aperture extending through the beam from its lower surface to its upper surface through which an anchor tendon or cable may extend.

9. Apparatus substantially as hereinbefore described with reference to and as shown in Fig.

2 of the accompanying drawings.