GB2024283A

GB2024283A - Underpinning

Info

Publication number: GB2024283A
Application number: GB7921279A
Authority: GB
Original assignee: MCTAY ENG CO Ltd
Current assignee: MCTAY ENG CO Ltd
Priority date: 1978-06-30
Filing date: 1979-06-19
Publication date: 1980-01-09

Abstract

There is disclosed a method for strengthening foundations comprising excavating a hole beneath a portion of the foundation (12) to be strengthened, locating a pile section (13) in the base of such hole, locating an hydraulic jack (16) between the pile section and a portion of the original foundation thereabove, operating said jack to force said pile sections (13) into the ground beneath the foundation, the pressure applied being equal to at least and preferably 50% greater than the calculated normal working load to which said foundation section will be subjected and, if necessary, applying one or more further pile section (13', 13'') on top of the first pile section (13) to form a pile column until such column no longer moves downwardly at predetermined load, removing the jack (16) from the hole and thereafter filling in the hole with suitable load transmitting material such as concrete to form a pile cap. This method is generally applied at regular intervals spaced along the foundation requiring strengthening. The lowermost pile section (13) has a steel end cap (13a), and all sections have a central aperture into which a steel dowel welded to a steel beam (15) is located. The jack (16) is located on the beam (15) and supports a steel plate (17) and an inch of sand/cement (3:) dry packing (18). Packing pieces (19) are wedged between the beam (15) and plate (17) to enable the jack (16) to be removed before filling the hole with concrete. <IMAGE>

Description

SPECIFICATION Method for strengthening foundations The present invention relates to a method for strengthening foundations.

In many existing properties, the foundations thereof require strengthening. The reasons as to why such strengthening is necessary are many and varied. For example, the original footings may not have been dug deep enough and therefore the foundations are of insufficient strength to bear the working load. Possibly, due to the property being built on "bad" ground, the foundations have slowly sunk into such ground and again require strengthening. Further, in recent years, due to extreme weather conditions, the ground beneath the foundations has been caused to move which again necessitates strengthening of the foundations.

One method which has been utilized to date to strengthen, support, reinforce or shore up foundations has been to dig down beneath the original foundation, usually in approximately four foot square sections until "good" ground is reached, the hole is subsequently filled in with concrete up to the original foundation level. Such process is then repeated at regular intervals along the length of and beneath the foundation which requires strengthening.

Such prior art method is reasonable when digging down to three to four foot from ground level.

However, when deeper excavations are required, e.g. up to ten foot or more, then it becomes uneconomical to dig down to such depth.

It is an object of the present invention to provide a method for strengthening foundations which is particularly applicable in the case where the "good" ground is more than three to four feet below ground level and also when working within a restricted space.

According to the present invention there is provided a method for strengthening foundations comprising excavating a hole beneath a portion of the foundation to be strengthened, locating a pile section in the base of such hole, locating pressure application means between the pile section and portion of original foundation thereabove and operating such to force said pile section into the ground beneath the foundation, the pressure applied being equal to at least the calculated normal working load to which said foundation section will be subjected and, if necessary, applying one or more further pile sections on top of the first pile section to form a pile section column until the pile section column no longer moves downwardly at predetermined load, removing the pressure application means from the hole and thereafter filling in the hole with suitable load transmitting material communicating with said pile section or pile section column.

The method described above is then applied at regular intervals beneath the foundation requiring strengthening. The number of such further applications will depend upon the foundation in question, the type of property being treated and the working load which such foundation normally has to support.

The method in accordance with the present invention has the advantage over the prior art method in that it is quicker in operation, it is cheaper than having to dig down to greater depths, there is a saving in the use of concrete etc. as well as time taken to effectuate such method and further, there is virtually always a reduction in the number of excavations required.

As mentioned above, the pressure applied is calculated at least equivalent to the working load and, for safety reasons, generally the pressure is increased to at least 50% above the working load.

The means for applying pressure preferably comprises a hydraulic jack.

The hole is finally filled in with concrete between the pile column and the section of foundation being strengthened, the remaining hole area being filled in with soil or other material.

In one embodiment of the invention, the individual pile sections are installed vertically. However, it is also possible to instal the pile sections at an angle to the vertical to resist horizontal or lateral loads from the structure or from movement or pressure of the subsoil.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows, in section, a pile column inserted beneath an existing foundation section to be strengthened; and Figure 2 shows in side elevation a plurality of foundation strengthening arrangements inserted beneath a property in accordance with the method of the present invention.

As illustrated in Figure 1, a hole 11 is dug beneath a section of an existing concrete strip footing 12 acting as foundation for a property. Afirst pile section 13, which is usually formed from precast concrete and is about sixteen inches in length and approximately six inches square, is then located in the base of the hold 11. Such first pile section 13 is preferbly provided with a pointed steel end cap 13a.

The pile section 13 has a central aperture running therethrough into which aperture is located a dowel bar 14 in order to correctly locate a support beam 15 over said pile section 13. The support beam 15 is thus dowelled into the pile section 13. The beam 15 is formed of structural steel and has a mild steel dowel bar 14 welded thereto. A hydraulic jack 16 is located on top of such beam 15 which jack 16 acts against the section offoundation 12 being strengthened via a mild steel spreader plate 17 and a 1" sand/cement (3:1 mix) dry packing 18. Packing pieces 19 are provided on either side of the jack 16 and are wedged to the underside of the footing and the support beam 15.

When the arrangement has been located as described above, the hydraulic jack is actuated and pressure is applied between the portion of foundation being strengthened and the pile section 13.

When the desired pressure figure has been reached, which is normally approximately 50% above the normal working load, and depending on the property, is usually 5 to 12 tons, it will generally be noted that pile section 13 descends into the ground beneath the base of the hole. When such pile section 13 has been completely pushed into the ground beneath the hole, the hydraulic jack is returned to its inoperative position and the jack assembly removed from the hole. A second pile section 13' is then inserted above section 13 and the jack assembly replaced. Again, the desired pressure is applied via the jack 16 and, depending upon the movement of pile section 13', one or more subsequent pile sections 13", 13"', etc. are utilized to form a pile section column.

A condition is eventually reached whereby no displacement of the uppermost pile section in the pile column takes place, usually over a selected period of a minimum of twelve hours, and this then indicates that the lowermost pile section 13 in the pile column has reached "good" ground, and the column is capable of supporting the calculated working load. When such condition has been attained, the jack 16 is then completely removed, the packing pieces 19 are left in place, and the hole is filled in with either cement or a suitable support member acting as the pile cap and extending between the top of the uppermost pile section and the section of the foundation being strengthened.

As illustrated in Figure 2, there is shown schematically a plurality of pile section columns 21, 21', 21", etc., located beneath an existing foundation 22 in a property 23 and it is noted that such pile columns extend to differing depths which will be consequent upon the level of good ground beneath the property 23.

It will thus be seen that the method in accoredance with the present invention enables a foundation to be strengthened in a much more economical and quicker manner than heretofore. It is also clear that the method of the present invention is equally applicable wherein strengthening of foundation is required wherein there is very little working space beneath such foundation, e.g. when operating beneath and inside a property.

Claims

1. A method for strengthening foundations comprising excavating a hole beneath a portion of the foundation to be strengthened, locating a pile section in base of such hole, locating pressure application means between the pile section and portion of original foundation thereabove and operating such to force said pile section into the ground beneath the foundation, the pressure applied being equal to at least the calculated nsormal working load to which said foundation section will be subjected and, if necessary, applying one or more further pile sections on top of the first pile section to form a pile section column until the pile section column no longer moves downwardly at predetermined load, removing the pressure application means from the hole and thereafter filling in the hole with suitable load transmitting material communicating with said pile section or pile section column.

2. A method as claimed in claim 1, in which the pressure applied is at least 50% above the calculated normal working load to which said foundation section will be subjected.

3. A method as claimed in claim 1 or 2, in which the pressure application means comprises a hyd raulic jack.

4. A method as claimed in any preceding claim, in which the load transmitting material is concrete.

5. A method as claimed in any preceding claim, in which the individual pile sections forming the pile section column at installed vertically.

6. A method as claimed in any one of claims 1 to 4, in which the individual pile sections forming the pile section column are inserted at an angle to the vertical.

7. A method as claimed in any preceding claim, in which the individual pile sections are formed from precast concrete.

8. A method as claimed in any preceding claim, in which each pile section has a central aperture running therethrough.

9. A method as claimed in claim 8, in which a support beam is located over each individual pile section by the provision of a dowel bar which is located in the aperture provided in the pile section.

10. A method as claimed in any preceding claim, in which the first pile section is provided with a pointed steel end cap.

11. A method as claimed in any preceding claim, in which the pressure application means acts against the section of foundation being strengthened via a spreader plate and a sand/cement dry packing.

12. A method as claimed in any preceding claim, in which packing pieces are provided on either side of the pressure application means.

13. A method as claimed in any preceding claim, in which a time period of a minimum of 12 hours is applied to check that the pile section column no longer moves downwardly at predetermined load.

14. A method as claimed in any preceding claim, substantially as hereinbefore described and illustrated in Figure 1 of the accompanying drawings.

15. A method for strengthening foundations comprising applying the method as claimed in any preceding claim at regular intervals beneath the foundation requiring strengthening.

16. A foundation whenever strengthened by a method as claimed in any preceding claim.

17. A structure, the foundation which has been strengthened, substantially as herein before described with reference to and as illustrated in Figure 2 of the accompanying drawings.