EP0074195A1

EP0074195A1 - Kentledge apparatus

Info

Publication number: EP0074195A1
Application number: EP82304321A
Authority: EP
Inventors: Alastair Aitken Sinclair
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-08-22
Filing date: 1982-08-17
Publication date: 1983-03-16
Also published as: GB2104137B; GB2104137A

Abstract

Kentledge apparatus having an open-ended chamber which is closable by placement on or penetration into soil or other base material. An air pump for evacuating the chamber in order to create a differential pressure across the wall of the chamber thereby to provide a force holding the chamber against the soil, an abutment fixed to the chamber and a jack or other driving means at contact with the abutment for exerting a force towards the soil, the force being counteracted through the abutment by the chamber.

Description

This invention relates to kentledge apparatus.
Kentledge in civil engineering is the provision of a static load for backweighting or counterweighting purposes, especially in construction work, for example in driving piles and testing their effectiveness. The kentledge apparatus can be used to exert about 1½ times the load which the pile will bear in use, in order to show that it is adequate for the purpose.
At the present time, a number of methods are used to provide a static site load or kentledge facility. These commonly include the following:-

(i) Concrete kentledge blocks built upon a grid of fabricated steel beams.
(ii) Soil anchors either in the form of soil screws or deep embedded plates.
(iii) Rock anchors drilled and grouted into the underlying bedrock.
(iv) Tension piles where the "pull-out" value of the piles is mobilised.
(v) Sectional water tank load supported by a rigid steel framework.

Method (i) is common but extremely costly due to the difficulties of transportation, handling, and erecting of the heavy blocks involved and the time taken to do so. For example, the blocks have to weight about 200 tons if a 200 ton force is to be applied. Methods (ii), (iii) and (iv) require specialist contractors to carry out the work using costly tools and procedures and again the time taken to accomplish this is considerable. Method (v) has the disadvantage that the value of the load capable of being achieved is not high nor can it, in practical terms, be easily increased.
According to the present invention there is provided kentledge apparatus comprising walling defining an open chamber closable by placement on or penetration into base material, means connected with the chamber for evacuating the chamber of air, abutment means secured to the walling and driving means engaging the abutment means and arranged so that on actuation it exerts a force in a direction towards the base material, the reaction of said force being opposed by the abutment means.
The chamber may be in toroidal form, being annular in cross-section, in which case the abutment means can be in the central aperture of the toroid, and the driving means providing the force on the abutment can extend through the aperture. In-site investigation work, for example, Dutch Cone Penetrometer or Continuous Flight Auger equipment could be mounted on a beam disposed across the top of the chamber so that the probe or auger tube passes through the central hole into the subsoil below. The reaction of a hydraulic cylinder used to drive the penetrometer or auger is taken by the beam and thence by the chamber which is kept in a continuous state of evacuation. This facility can, in areas where site surface conditions are poor or where a larger than normal reaction is required, be more convenient than the more usual lorry mounted location or concrete block type of backweight.
In a similar way, conventional precast piles could be driven or tested through the central holes of the toroidal chamber using a long stroke hydraulic cylinder reacting through a structural frame connected to the top of the chamber wall. In appropriate site conditions, such an application can result in major benefits. The types of piles driven by this means can be commercially available types or purpose designed and developed to suit the plant.
Alternatively, different shapes of chamber may be used, and/or varying numbers of externally-interconnected chambers. An especially advantageous form of the apparatus has a number of spaced chambers interconnected by one or more external beams or other framework forming the abutment means. Preferably the driving means, which may for example be a hydraulic or other type of jack, be disposed on the framework centrally of the chambers. In this way a force exerted by the driving means is counteracted by the framework which then distributes the reaction equally around the chamber or chambers. By providing a number of chambers it can be easier to transport and handle the apparatus than if a single large chamber is used.
In the case of a number of chambers, these may be articulated in their connections so that in use the chambers can be used at differing heights, and the chambers can be evacuated and sunk into the ground one at a time without disconnecting the framework.
In use the chamber is placed on or pressed through the surface of base material, for example ground into which piles are sunk or soil to be tested by a probe, so as to close the chamber which is then evacuated by an air pump or other evacuation means. The pressure differential between the interior of the chamber and the ambient atmosphere causes a downward force to apply on the chamber, and this force opposes the applied upwards force caused by the reaction from the driving means. The apparatus thus provides the required backweight or counterweight.
The chamber wall can be of precast concrete, for example a welded fabrication, or other suitable construction. It may be in one piece or formed as a number of interfitting sections.
The chamber wall may be domed opposite its opening in order to withstand the pressure difference mosteffectively, although an alternative arrangement is to provide a sagging membrane, or a flat plate with strengthening members.
The means for limiting penetration through the surface may be for example one or more flanges, such as flanged plates, extending from the chamber wall at the desired distance from the open end. The plate could be disposed so that in use it lies in a plane parallel to the plane of the surface to be penetrated. On evacuation of the chamber, therefore, the apparatus may sink into the ground under the effect of external air pressure until the plate lies flat on the ground surface, thus resisting further penetration. Continued evacuation then establishes a kentledge load whose magnitude depends on the plan area of the chamber.
Examples of applications for apparatus of this invention are:-

(i) Pile Driving - A hydraulic pile driving rig can be connected to the chamber wall or walls to drive precast concrete piles through the central hole into the sub-soil below. A number of commercially available piles or any purpose designed types can be driven by this means. The kentledge apparatus provides the backweight against which the hydraulic rig can react to provide the force to push the pile into the sub-soil.
(ii) Site Investigation - Using a similar arrangement to (i) above, some types of site investigation equipment which require backweighting can be mounted on the chamber wall. A notable example of such equipment is the Static Cone Penetrometer which is normally vehicle mounted or otherwise fixed to the ground using screw anchors. Continuous tube samples can also be obtained from cohesive or semi-cohesive sub-soils using a hydraulically driven sample tube.
(iii) Load Testing - Piles driven by conventional means or by the apparatus described in (i) above can be tested utilising this apparatus. In a similar manner, plate loading tests of sub-soils can be carried out provided that the depth to the plate is greater than the radius of influence of the applied vacuum below the vessel.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 is a sectional side view of kentledge apparatus of this invention;
Fig. 2 is a sectional side view on A-A of Fig. 3 of a furtherembodiment of the kentledge apparatus of the invention; and
Fig. 3 is a plan view, party schematic, of the apparatus of Fig. 2.

Referring to Fig. 1, the apparatus in this embodiment is shown in use in driving or testing the load on a pile 1. The upper end of the pile 1 is engaged by a bearing face of a hydraulic jack 2 which depends from and is fixed to a rigid framework 3 in the form of side-by-side beams 4. Rigid struts 4A extend from the beams 4 to the inner side walls of a toroidal kentledge vessel 5 which is fabricated from steel plate.
The vessel 5 is annular in plan and is closed at its upper end by a domed structure 6 extending round the vessel. The inner and outer side walls of the vessel 5 are cylindrical and open at their lower end, at which they carry tapered and replaceable cutting edges 10 to allow easy penetration into soil 9.
Tubing 7 passes through a sealed connection into the interior of the vessel 5, being connected at its other end to an air pump (not shown).
Inner and outer circumferential flanged plates 8 extend around the side walls of the vessel 5 at a distance from their upper ends in order to prevent excessive penetration of the vessel 5 into the soil, and a second tube 11 passes through a sealed connection into the vessel 5 to lie on the surface of the soil 9. This tube 11 is connected to a water pump and serves on actuation of the pump to remove ground water from the inner annular space of the vessel 5 when required by site conditions.
In use the vessel 5 is placed in position with its cutting edges 10 resting on or slightly embedded in the soil 9, and the air pump is actuated to evacuate the interior of the vessel 5, which is closed at its lower end by the soil, through the tubing 7. As vacuum develops within the vessel 5 a differential pressure builds up across its walls, due to constant atmospheric pressure, and this pressure acting on the domed structure 6 pushes the vessel downwards into the soil 9 until it reaches the position shown in Fig. 1 in which the flanged plates 8 engage the soil surface and prevent further penetration. The air pump continues to operate as necessary to maintain a required degree of evacuation of the vessel 5, which is thus firmly anchored to the soil 9 by air pressure.
With the hydraulic jack 2 engaging the top of the pile 1, the jack 2 is actuated to exert a downwards force on the pile, the reaction to this force being taken by the beams 4 and thence, through the struts 4A, by the vessel 5. Thus, the vessel 5 is subjected to an upwards force by the action of the jack 2 on the pile 1, this force being opposed by that resulting from the differential air pressure across the vessel wall.
In this way a force far in excess of the weight of the kentledge apparatus can be exerted on the pile 1.
After use the embedded vessel 5 can be easily withdrawn from the soil 9 by reversing the air pump to inject air into the vessel 5 through the tubing 7 until a greater pressure of air exists within the vessel than outside it.
This reversed differential pressure thus pushes the vessel upwardly out of the soil 9.
Referring now to Figs. 2 and 3, four steel kentledge vessels 5 with internal stiffening are provided, each being cylindrical and flat-topped, and open at their lower ends. The flat plates 12 extending across the tops of the vessels 5 are strengthened to prevent them buckling under pressure in use.
The vessels 5 are each 3 metres in diameter and 10mm in wall thickness and are arranged in two pairs spaced, in this example, 6 metres apart, the vessels in each pair being 4 metres apart, so that the overall form of the arrangement is rectangular in plan. The vessels in each pair are interconnected by a universal decking beam 13 welded to the upper face of each vessel 5, each decking beam being 457mm X 191mm X 67kg/m. A universal loading beam assembly 14 of two side-by-side beams each being 914mm X 305mm X 253kg/m extends between the decking beams 13, being welded to them mid-way along their lengths. Web stiffeners 15 are provided on the assembly 14. At the mid-point of the loading beams 14 an underside spherical seating is secured which connects the beams 14 with a hydraulic jack 2 through a dynamometer 16 and spacers 17. The jack-2 is adapted to engage the upper end of a pile 1, and a pile test capping 18 is provided for the purpose.
Each of the vessels 5 has an external annular bearing plate 8 around it, each plate being 0.3 metres in width but may vary depending on the bearing capacity of the soil and disposed at an adjustable predetermined distance from the lower end of the vessel 5 depending on the type of subsoil into which it is embedded. One of the parameters determining this predetermined distance being the porosity of the subsoil.
Air lines 7 extend one to each vessel interior, a hand-operated valve 19 being provided in each line adjacent the vessel to allow shut-off in case of line failure, and each line leads through an independently-controllable mechanical valve 20 into a manifold communicating with duty and stand-by vacuum units 21, 22. Each unit 21, 22 is actuated-and controlled pneumatically, hydraulically or electronically from a control console 23.
Sensors 24 are disposed within each vessel 5 and connected to the console 23 to give a read-out of the air pressure within each vessel 5.
The operation of this apparatus is similar to that of the Fig. 1 apparatus, except that in the present case either all the valves 20 can be opened simultaneously so that the vessels 5 are evacuated simultaneously, or only one valve 20 may be opened to allow more effective actuation of a single vessel at a time; when that vessel is in its final position its corresponding valve 20 may be closed and each of the others opened in sequence to drive the vessels individually into the soil 9. Once all the vessels 5 are in position all four valves 20 can be opened to maintain the desired degree of vacuum.
An advantage of the apparatus of Figs. 2 and 3 over that of Fig. 1 is that it is more easily transported and handled, as four vessels of 3 metres diameter can be carried on a lorry without special limitations, whereas a single vessel having the same cross-sectional area would be of much greater diameter and therefore form a very large load. Further, in use the four vessels can be sunk to differing depths if necessary and can be interconnected through articulated points.
The four vessels of Figs. 2 and 3 together weigh 5.9 tons, the loading beams 14 weigh 3.6 tons and the decking beams together weigh 1 ton, so the overall weight of the apparatus is about 11 tons. At 70% evacuation of the air from the vessels 5 the apparatus can resist a force of 200 tons on the pile 1.
In some cases the site on which the apparatus is to be used may be impenetrable, for example having a concrete surface, and if so the lower ends of the vessels 5 may be fitted with flexible seals to engage the surface. On evacuation the vessels are thus held down on the surface by external air pressure, as described above, without penetrating through the surface.
Modifications and improvements may be incorporated without departing from the scope of the invention.

Claims

1. Kentledge apparatus comprising walling defining an open chamber closable by placement on or penetration into base material, means connected with the chamber for evacuating the chamber of air, abutment means secured to the walling and driving means engaging the abutment means and arranged so that on actuation it exerts a force in a direction towards the base material, the reaction of said force being opposed by the abutment means.

2. Apparatus according to claim 1, wherein the driving means is a jack which is secured to a reaction member on the walling.

3. Apparatus according to Claim 1 or 2, wherein means are provided on the walling for limiting penetration of the walling into the base material..

4. Apparatus according to claim 3, wherein the limiting means is a flange extending laterally from the walling for engaging the surface of the base material.

5. Apparatus according to any one of the preceding claims, wherein the walling defines a toroidal chamber.

6. Apparatus according to any one of claims 1 to 4, wherein walling is provided defining a plurality of interconnected chambers.

7. Apparatus according to claim 6, wherein each of the' chambers is cylindrical.

8. Apparatus according to claim 6 or 7, wherein the chambers are interconnected by a framework which comprises the abutment means.

9. Apparatus according to any one of the preceding claims, wherein the driving means is disposed centrally of the apparatus so that the reaction of said force exerted on the abutment means is distributed substantially-equally around the chamber or chambers.

10. Apparatus according to any one of the preceding claims wherein the driving means exerts said force on a pile, probe or auger.