GB2258872A

GB2258872A - Cast-in-place manhole liner apparatus and method.

Info

Publication number: GB2258872A
Application number: GB9118136A
Authority: GB
Inventors: David Jonathan Shaw
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-08-22
Filing date: 1991-08-22
Publication date: 1993-02-24
Anticipated expiration: 2011-08-22
Also published as: GB9118136D0; GB2258872B

Abstract

A manhole is constructed by securing a series of vertical elongate cage-like spacers 2 in spaced circumferential positions around a precast concrete liner 1 by means of bands 9. A plastics mesh former 10 is then placed around the spacers and the resulting gap between the liner 1 and the former 10 is filled with concrete. <IMAGE>

Description

MANHOLES TECHNICAL FIELD OF THE INVENTION This invention relates to the construction of vertical shafts in the ground which, for convenience, are referred to herein as "manholes".

BACKGROUND Manholes are often constructed using a precast concrete liner formed in a number of sections which are held together by an outler layer of concrete which is cast around the liner in situ to hold the sections together.

Normally, the concrete layer is poured into the gap between the liner and re-usable steel shuttering, which is subsequently removed after the concrete has achieved its initial set. However, the use of such shuttering has a number of inherent disadvantages. Firstly, the shuttering is expensive to obtain. Secondly it is difficult to place and accurately position in the hole, requiring the use of heavy lifting machinery. Thirdly, the steel shuttering is bulky and heavy to transport and handle, requiring regular oiling or other protection against corrosion.

For speed and general convenience, the steel shuttering is sometimes omitted and the hole is simply back-filled around the pre-cast liner sections with rubble, earth or concrete, or a mixture of these. However, if an inadequate amount of concrete is used there is a danger of the precast sections moving with settlement. On the other hand, if a high proportion of concrete is used this is extremely expensive.

It has been proposed to use plastics mesh in place of steel shuttering, but this has not proved successful since the mesh has a tendency to move as the concrete is poured.

An aim of this invention may be viewed as being to provide an easily handled, inexpensive and effective alternative to steel shuttering.

SUMMARY OF THE INVENTION The present invention proposes a method of constructing a manhole comprising securing a plurality of spacers at mutually spaced positions about the external surface of a precast liner, placing a sheet-like former about the liner to be supported a predetermined distance therefrom by the spacers, and placing a concrete mix in the resulting gap between the liner and the former.

The invention also provides apparatus for use with a precast liner in forming a manhole, comprising a plurality of spacers for attachment at mutually spaced positions about the external surface of the liner, in combination -with a sheet-like former for location about the liner to be supported a predetermined distance therefrom by the spacers.

The spacers are preferably preformed cage-like structures which allow the concrete mix to pass through them and form a united mass when set. The spacers are preferably of elongate form and are secured substantially vertically to the liner. The spacers are preferably secured to the liner by bands which encircle the liner.

The sheet-like former preferably comprises a mesh. The former may be of galvanised or other rust resistant flexible metal sheet, but for low cost, plastics sheet may be used. The former may be left in position after the concrete has hardened.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is exemplified in the accompanying drawings, in which: Figure 1 is a partially cut-away perspective view of a manhole of the invention in the course of construction, Figure 2 is a plan view of the manhole, and Figure 3 is a perspective view of a suitable form of spacer for use in construction of the manhole.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Fig. 1, a liner is formed by a set of stacked precast concrete rings 1, each of generally cylindrical shape. (The rings may however be of other cross sectional shapes such as square or hexagonal for example.) Each ring has a series of axially disposed elongate spacers 2 secured in circumferentially spaced positions around its girth by means of connecting bands 9.

The spacers can be seen in end view in Fig. 2 and in perspective view in Fig. 3. The number of spacers can be varied, but for a circular manhole of around 1 metre in diameter five, six or seven spacers, equally circumferentially spaced, will normally suffice. Each spacer is formed as an open framework of steel wires.

Although the spacer may take various forms, in the present example the spacer includes a pair of parallel base wires, 3 and 4, and a pair of parallel top wires, 5 and 6, which are mutually closer spaced than the base wires 3, 4 and are held rigidly away from the base wires by generally U-shaped connecting wires 7.

Although not essential, the base wires may conveniently carry a series of loops 8 through which the fixing bands 9 can be threaded, thereby providing positive vertical location of the spacers 2 relative to the fixing bands 9. The same effect could however be achieved by providing pairs of closely spaced connecting wires 7 between which the bands 9 are a close fit.

The bands 9 are lengths of metal and/or plastic strip which are looped circumferentially around the respective ring 1 and tightened into frictional engagement with the ring. The ends of the band are joined together by any convenient means, e.g. by using suitable compression connectors. This operation may conveniently be carried out before the rings are lowered into the hole in which the manhole is to be constructed.

When the rings are correctly positioned in the hole a sheet of plastics mesh 10 is placed around the rings.

The sheet is separated from the rings 1 by the spacers 2, to which the sheet may, if required, be secured by wire ties or the like. A concrete mix is then placed in the gap between the sheet and the rings. Because of the open frame-like construction of the spacers the concrete mix is able to penetrate into the framework of the spacers to form an integral mass. As the concrete is placed air is expelled from the gap via the mesh of the sheet 10. A vibrating poker may be used to ensure that there are no voids in the concrete.

Since the sheet is relatively inexpensive it can be left in place and the hole can be back-filled around the sheet almost immediately, without waiting for the concrete mix to achieve its initial set.

Claims

1. A method of constructing a manhole comprising securing a plurality of spacers at mutually spaced positions about the external surface of a precast liner, placing a sheet-like former about the liner to be supported a predetermined distance therefrom by the spacers, and placing a concrete mix in the resulting gap between the liner and the former.

2. A method according to Claim 1, in which the spacers are preformed cage-like structures of elongate form and are secured substantially vertically to the liner by placing bands around the liner.

3. A method according to Claim 2, in which the bands are threaded through the spacers.

4. A method according to any preceding Claim, in which the sheet-like former comprises a mesh and is left in position after the concrete mix has hardened.

5. Apparatus for use with a precast liner in forming a manhole, comprising a plurality of spacers for attachment at mutually spaced positions about the external surface of the liner, in combination with a sheet-like former for location about the liner to be supported a predetermined distance therefrom by the spacers.

6. Apparatus according to Claim 5, in which the spacers are preformed cage-like structures.

7. Apparatus according to Claim-5 or 6, in which the spacers are of elongate form.

8. Apparatus according to Claim 7, in which the spacers are secured substantially vertically at spaced circumferential positions about a precast liner.

9. Apparatus according to Claim 5, 6 or 7, in which the spacers are secured to the liner by bands which encircle the liner.

10. Apparatus according to Claim 9 as appended to Claim 6, in which the bands are threaded through the spacers.

Ii. Apparatus according to any of Claims 5 to 10, in which the sheet-like former comprises a mesh.

12. Apparatus according to any of Claims 5 to 11, in which the former is of plastics sheet.

13. Apparatus according to any of Claims 5 to 12, in which the gap between the former and the liner is filled with concrete.

14. A method of constructing a manhole, substantially as described with reference to the drawings.

15. Apparatus for use with a precast liner in forming a manhole, substantially as described with reference to the drawings.