IES64446B2 - Method and apparatus for constructing access cavities including manholes to underground services - Google Patents

Abstract

A method for constructing a manhole or other access cavity (10) to underground services comprises excavating the cavity, preserving the underground services (11a) if present, or placing removable plugs to preserve passages for them, introducing a downwardly tapered removable mould into the ground void to define the inner faces of the cavity, filling the ground void up to the limits defined by the mould (and by the plugs if present), with ready-mixed concrete (14), allowing the concrete to begin setting, and lifting out the mould. The mould can be surrounded by a shaped basket (17) of steel reinforcing mesh before filling takes place.

Description

Title: Method and apparatus for constructing access cavities, including manholes, to underground services.

This invention relates to a method and apparatus for constructing access cavities, including manholes, .to underground services.

Access cavities, especially manholes, to underground services are constructed by excavating a void of greater dimensions than the access cavity to be provided, laying a concrete floor therein, optionally supported by a foundation, constructing a walled enclosure above the floor, with provision for services to pass through the walling where necessary, mounting a cover frame on the walling, applying a removable cover or removable covers thereto, and back filling the remainder of the void with earth outside the walling.

Such access cavities are frequently called upon to withstand extremely heavy wear and tear, such as the f passage of road traffic over them in volumes up to those encountered in urban motorways, which traffic <> may include heavy transport vehicles with trailers, digging and earth moving machinery and building machinery. 564446 In conventional practice the walls of the access cavity are erected in concrete blocks and mortar by the manual block laying method, Such walls are vulnerable to lateral stresses, particularly if the back fill is poorly compacted, as is often the case. The cover frame is placed so as to rest on the top surface of the walls, and is bedded thereon in a setting composition, usually a rich cement mortar. As in any non-homogeneous cemented structure, the component parts which did not set in unison tend to separate under mechanical stress and vibration. Thus disintegration may begin with the cover frame coming loose and being laterally displaced, partially exposing the top surface of a wall to contact with vehicle tyres, which then proceed to loosen one block after another. This conventional practice is also labour-intensive and correspondingly costly.

The invention seeks, among its objects, to ameliorate or abolish the above recited drawbacks of the prior art.

According to the invention there is provided a method 25 for constructing an access cavity to underground services, which method comprises (a) excavating a shaped ground void to the approximate external dimensions of the foundation (if any), floor and walling intended to surround and define the access cavity, (b) preserving, during such excavation, the relevant underground services, if present, or positioning removable plugs for the later passage, through the walling, of services not yet laid, (c) introducing a downwardly tapered removable mould into the recess to define the inner faces of the access cavity, (d) filling the space between the ground void and the mould with ready-mixed concrete, to.yield a cavity of which at least the walls are monolithic, (e) allowing the concrete to begin setting, and (f) lifting out the mould.

Preferably the foundation (if any), and the concrete base of the access cavity are cast before the introduction of the mould.

Preferably a shaped basket of steel reinforcing mesh is positioned, before step (d) above, to occupy at least some of the space between the ground void and the mould, so that the invented method yields an access cavity with reinforced concrete walls.

Preferably the reinforcing mesh basket is positioned before the concrete base has set, and is partially driven into said base, so that the reinforcing is continuous from walls to base.

In one embodimeAt of the method, the reinforcing mesh basket is fitted on to the mould and both are lowered together into the excavated recess. The fitting means should be easily released; it may be a ligature which can be conveniently cut. This embodiment avoids any minor distortion of the basket, consequent on manoeuverinq it into the excavation, which might impede a subsequent introduction of the mould.

Preferably the exterior of the removable mould is prior coated with a mould release agent, such as a heavy industrial grease.

Preferably a cover frame is bedded in the top layers of the walls before the concrete sets. In the finished manhole the cover frame serves to receive and support a removable cover or covers.

Accessories should be mounted inside the access cavity on the fresh wall surface immediately after the extraction of the mould. This can be done (by means of masonry nails and the like) at that time with the minimum expenditure of energy. Fixing strength is also enhanced by reason of after-setting of the concrete around the nails etc. Accessories include cable bearers, switchgear, light fittings.

Removable covers, and the frames into which they fit, are commercially available in standard sizes. Access cavities constructed according to the invention are preferably made, likewise, in standard sizes to match the frames.

A wall thickness of 30-50 cm is envisaged for manholes, somewhat less for inspection traps and the like, or when the access cavity will not be exposed to heavy traffic.

The invention also provides apparatus for constructing access cavities, including manholes, which apparatus comprises a downwardly tapered mould shaped to accord with the interior dimensions of a proposed access cavity, and means for supporting the mould from above, whereby the mould can be lowered into an excavated ground void and afterwards lifted out of the moulded access cavity.

The degree of taper is not critical, but it may involve, for example, a difference of about 75 mm in length or width, between the narrower/shorter bottom and the wider/longer top of the mould, for manholes of medium size.

The mould is fabricated for preference from mild steel sheeting, welded together about top and middle horizontal reinforcing frames of square steel tube which construction leaves the mould exterior smooth and unencumbered.

The support means preferably comprises a pair of loops on opposite sides of the mould, projecting upward therefrom, into which loops lifting members of a mobile machine (such as a mechanical digger) can be inserted.

The mould may usefully comprise means for deploying a hazard lamp or sign, for use in cases where the mopld, either in use or* storage, might otherwise constitute a hazard to vehicles or pedestrians.

The mould may in principle have a plan (or horizontal section) of any simple shape that does not impair its withdrawability, but in practice a rectangular plan is convenient. Other shapes, e.g. two parallel sides with round ends, are not excluded, even in practice.

As further apparatus the invention provides a mesh basket welded together from steel reinforcing rod, and dimensionally adapted to occupy at least some of the free space between the ground void excavated according to the method aforesaid, and the removable mould in its position of use, so as to provide a reinforcing skeleton for the walls of the access cavity when they have been cast.

The invention will be better understood from the following description of a particular and preferred embodiment thereof, given by way of non-limiting example only, with reference to the accompanying drawings, in which Fig 1 is a schematic median vertical section through a manhole constructed according to the method of the invention; Fig 2 is a perspective view of a first mould for use in the method; Fig 3 is a perspective view of a second mould, for a one-man manhole, showing also an associated housing for a hazard lamp; Fig 4 is a perspective view of a third mould; Fig 5 shows two details of the mould and lamp housing of Fig 3,' and Fig 6 shows the mould of Fig 2 surrounded by a welded mesh reinforcing basket.

Referring now to the drawings, . a finished manhole 10 (Fig 1) presents, in vertical section, two of its four walls 11, a base 12 and a cover frame 13. The walls 11 are moulded in concrete 14, having been cast between an inner mould (not shown) and an excavation in the ground 16. A basket 17 of reinforcing steel mesh, welded together, surrounds the manhole 10 within the concrete 14. The concrete 14 and the excavation 15 are made wider around the top of the manhole,, and constitute a reinforcing collar or flange 18, itself reinforced by an integral extension 19 of the mesh basket 17. The walls 11 are penetrated by conduit ends 11a for services such as telephone line cables which are to be accessible within the manhole 10.

A first manhole mould 20 (Fig 2) comprises four walls 21 and a base, all fabricated from 5 mm mild steel sheet by welding, with welded-in top 22 and median 23 reinforcing frames of 44 x 44 mm mild steel square tubular stock. Its outer dimensions in millimeters 15 are: base 1105 X 660 top 1181 X 737 slant height 1257 20 It has two welded-in loop handles 24 and a welded-in locating and support platform 25 for a hazard lamp cage (not shown). It is for a two-man manhole and occupies a volume of approximately 1 cubic meter. Its outer surface is smooth and uninterrupted by projections, recesses or perforations.

« A second manhole mould 30 (Fig 3) is for a one-person manhole, being only half as long as the mould of Fig 2 but having the other dimensions of that mould; its volume is about 0.5 cubic meter. It has internal reinforcing frames 32, 33, loop handles 34 and a support platform 35 all similar to the corresponding elements 22, 23, 24, 25 of the mould of Fig 2. A hazard lamp cage 36 is adapted for rest on and padlocked retention by the support platform 35.

Fig 5 shows the support platform 35 and the hazard - lamp cage 36 from Fig 3 on a larger scale, as well as one of the loop handles 34. The handle 34 is of an inverted U shape with the free ends of its limbs welded to a horizontal platform 37, itself a rectangle having one long edge 38 welded to the steel sheet cladding of the mould 30. The handle 34 is additionally welded to the frame 32 by both its limbs.

The support platform 35 comprises a platform plate 39, elements defining a slot 40 and a downturned tab 41 having a perforation 42. It is welded to the frame 32 along two adjacent sides. The lamp cage 36 is a sheet steel box having two opposed faces of steel rod mesh, a horizontal tab 43, a vertically depending tab 44 with a perforation 45, and no bottom. In use thereof, a hazard lamp is switched on and placed within the cage 36, and the latter is then placed on the support platform 35 with its tab 43 engaged in the slot 40, and its depending tab 44 overlapping the similar, downturned tab 41 so that the perforations 42, 45 coincide, for padlocking.

Fig 4 shows a third mould 50 which is generally similar to the first mould 20 of Fig 2, except that a slot 51 has been incised in it through two opposite walls and the bottom. This enables the mould 50 to be lowered into an excavation across which a service member such as a cable (not shown) extends. Thus an inspection cavity can be floored and walled effectively by the method and apparatus of the invention, without any interference with a preexisting through-running service conduit.

Fig 6 shows a mould 60 similar to moulds 20 and 50, v surrounded by a welded wire mesh reinforcing basket 67 occupying an excavation which is not illustrated. The * wire is of 10 mm diameter and the mesh squares are 210 x 200 mm in size. The arrangement as shown is ready for the introduction of ready mixed concrete.

Concrete for use in the method of the invention may usefully be made from a mix of 1 part Portland cement, part sand and 2 parts washed pebble by volume, which when appropriately watered gives a concrete which sets to a compression grade of 35 N/mm2. The mould after each use is cleaned of all surface concrete and regreased before being used again.

It is found in practice that the construction cost of a manhole made by the method of the invention is very significantly lower than the cost of a traditional manhole. The labour content, likewise, is less, and the manufacturing time (apart from the concrete setting time) is shorter, by comparison with the prior art. The finish of the manhole, including the anchorage of the cover frame, is a distinct improvement on conventional practice.

The invention is’ not limited by or to the details of the embodiment described, many of which can be widely varied without departing from the scope of the invention.

Dermot P Cummins

Claims (5)

1. A method for constructing an access cavity to underground services, which method comprises (a) excavating a shaped ground void to the approximate external dimensions of the foundation (if any), floor and walling intended to surround and define the access cavity, (b) preserving, during such excavation, the relevant underground services, if present, or positioning removable plugs for the later passage, through the walling, of services not yet laid, (c) introducing a downwardly tapered removable mould into the recess to define the inner faces of the « access cavity, (d) filling the space between the ground void and the mould with ready-mixed concrete, to yield a cavity of which at least the walls are monolithic, (e) allowing the concrete to begin setting, and (f) lifting out the mould.

2. A method according to Claim 1, which comprises casting the foundation (if any) and the concrete base of the access cavity before the introduction of the mould.

3. A method according to Claim 1 or 2, which comprises positioning a shaped basket of steel reinforcing mesh to occupy at least some of the space between the ground void and the mould, and then 10 filling with concrete, whereby to construct an access cavity with reinforced concrete walls.

4. A method according to Claim 3, which comprises positioning the reinforcing mesh basket before the 15 concrete base has set, and partially driving said basket into said base, whereby the reinforcing is continuous from walls to base.

5. A method according to Claim 3, which comprises 20 fitting the reinforcing mesh basket on to the mould and lowering both together into the excavated recess. Dermot P Cummins