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The Idiots' Guide to Highways
RAISING IRONWORK FOR BITUMINOUS OVERLAYS
Method of raising street ironwork when overlaying with bituminous material
Plastic Gully Pot Formers
Specifications relating to street ironwork
General notes to lifting ironwork
Link to page showing ironwork lifting using high density plastic gully "formers" and mastic asphalt
METHOD OF RAISING STREET IRONWORK WHEN OVERLAYING WITH BITUMINOUS MATERIAL
the considerable depth of excavation that is necessary to
"lift" a large manhole.
The surrounding road pavement has been cut back to a sound vertical joint.
The manhole casting is raised on thin engineering bricks or special "tiles" for finer adjustment.
The bricks and tiles must be of engineering quality.
It is usual these days to use "fast set" cementitious compounds as the mortar, and setting times can be as little as 15/20 minutes, and this allows raising the ironwork on the day of the bituminous surfacing.
Although "normal" mortar is quite acceptable in new construction, or where there is a road closure of sufficient duration to allow the mortar to reach full strength.
|In this example the casting is being raised by thin engineering bricks on only one side to allow for the camber of the road, the remainder of chamber top is just receiving a bedding layer of mortar.|
|The whole manhole cover
is now successfully raised to its new height, i.e. the height of the
finished surface course.
It is now required for the all loose material to be removed from the excavation and for bituminous material to be hand placed around the ironwork in successive layers and fully compacted to prevent any later settlement of the surrounding bituminous material due to compaction by traffic.
The above most important operation is often overlooked and is a contributory factor to early failure of ironwork.
|The manhole frame has
now been lifted to the level of the finished road surface and it just remains for the mortar to harden, and this
will not be long, possibly two hours for full curing.
I forget what the actual final compressive strength is but it is impressive, far in excess of what is actually needed, but you have to use the correct amount of water and you have to complete the work before the mortar starts to "go off".
Please note that the vertical joints in the road pavement are in good order, cut back into sound road pavement.
These joints, on this job, were painted with hot 50pen. grade bitumen.
Also note that all loose material has been sucked from the excavation by the on site sweeper, this is very, very important.
Do not allow the common practice of sweeping all the loose material, on the area to be paved, into this excavation.
This is so clean because I had insisted that it was "cleaned" for a second time after one of the gang had done just what I described.
It is now important to fill the excavation by hand with hot binder course material to existing road level prior to the surface course being laid, and to compact it completely.
Efficient compaction of good, hot, bituminous material around the ironwork contributes greatly to preventing any movement in the cover.
The surface course will be laid as one complete course with no joints around the ironwork.
this photograph of another manhole you are able to see the layer of thin
engineering bricks that have raised the manhole cover the
required amount for the thickness of the bituminous
Fast set compound will now be grouted all around the manhole casing, as above, to secure it firmly.
Once the mortar has hardened and at the time of paving all loose material must be removed and suitable bituminous material hand placed and compacted prior to the actual surfacing.
cluster of stop-valve covers that have been raised.
The excavation will be sucked clean with the road sweeper attachment.
When all loose material has been removed bituminous material will be hand laid around the boxes and compacted with a hand rammer to existing surface level, prior to the paver laid bituminous mat being applied.
The correct placing of bituminous material around raised ironwork and its full compaction is an extremely important part of the ironwork raising procedure.
PLASTIC GULLY POT FORMERS
Gullys created using plastic formers, as shown in the pictures below, will only have any strength and be capable of carrying the iron gully and wheel load of traffic passing over the ironwork if the former is correctly and completely surrounded with suitable structural strength concrete.
If the concrete is not correctly placed then any amount of correct ironwork placement will not prevent it from failing as the plastic former flexes beneath it.
I am not saying that a hole as big as is shown on the images should be filled with concrete, but sufficient to completely surround all the plastic former, including underneath, and to create a wide enough platform for placement of gully surround.
It is usual to have a minimum specification for concrete strength and surround dimensions in your contract document.
It is usually necessary to fill the gully pot with water when placing the concrete around it, if the concrete is at all fluid, or the former will "float" out of position.
"Testing" the concrete surround can be done after placement of the concrete surround, if you were not on site at the time of placement, by probing with a length of sharpened steel reinforcing bar.
Some would say this "unofficial" testing method is rather drastic, but it will have no effect if the gully has been correctly constructed.
RELATING TO STREET IRONWORK
BS 497 : Specification for manhole covers, road gully gratings & frames for drainage purposes :
Part 1 : Cast iron and cast steel
N.B. THIS SPECIFICATION WAS WITHDRAWN WITH EFFECT FROM 30th JUNE 1995, and is superseded by,
BS EN 124:1994:Gully tops and manhole tops for vehicular and pedestrian areas : Design requirements, type testing, marking, quality control
The specification includes such criteria as loading, design and dimensions.
BS EN 124:1994 was so difficult to understand BS 7903 was introduced to help make things a little less confusing.
BS 7903:1997:Selection and use of gully tops and manhole covers for installation within the highway
Highways Agency Guidance Document - HA 104/2 - Chamber Tops and Gully Tops for Road Drainage Services
GENERAL NOTES TO LIFTING IRONWORK
The term ironwork used in relation to highways maintenance and construction means gulleys (drains), manholes, stop-tap covers, inspection chamber covers, etc. etc..
All these products will be manufactured to the appropriate British Standard with regards to approved designs, dimensions and strength.
The actual specification and testing of ironwork items is a separate issue to the setting or lifting of such products, but without the correct setting of manholes covers and gully gratings they will be subject to unnecessary high levels of loading that may be above their design criteria and therefore will be the cause of early failure.
To achieve maximum effectiveness and strength ironwork must be lifted / installed / bedded in the correct way, poor workmanship in installation is a big cause of ironwork failure.
Ironwork that has been lifted poorly will "move" and settle, making the road unsafe, and cause premature failure of the pavement construction / surfacing / overlay.
Obtain a copy of the scheme design or overseeing organisation specification for full details of how the particular piece of ironwork should be lifted.
However, the basic requirements amount to good practice, the chamber shall be soundly constructed and pointed (if brickwork), some chambers / gulley pots are pre-made in concrete.
The gully pots that are preformed in plastic MUST be completely surrounded with fresh concrete, according to the specification, if the gully is to achieve the correct load carrying strength.
Failure to achieve this total surround of appropriate, fresh, concrete is often a cause of the settlement of ironwork.
Ironwork in general shall be bedded on an approved mortar, quite often a rapid setting mortar,
e.g. a strength of 10n./mm. squared at 2 hours.
At the time of lifting it is not always known what will be required to successfully raise the ironwork to the new level, so a broad selection of materials and items should be carried by the lifting gang, e.g. fast set mortar, engineering bricks, engineering quality "tiles", steel strips to support ironwork, etc..
The frame of whatever item of ironwork you are lifting must be completely supported by the method of lifting/setting you have adopted.
Ironwork can be lifted before or after the wearing course has been laid, but it is usually preferred to lift before final surfacing if possible as this allows more thorough inspection of the ironwork lifting, and it avoids extra joints in the surfacing material and allows better compaction of the bituminous surface course.
It is usually a requirement of the specification, e.g.'s
BS 4987:Part 2 and BS 594:Part 2, ( BS EN 13108 : Parts 1 & 4 and BS 594987 from the 1st.of January 2008)
that vertical faces of manhole covers and gulleys are cleaned and painted with a thin uniform coating of 50 or 70 penetration grade hot bitumen or cold applied thixotropic bitumen compound of similar grade before coated macadam/hot rolled asphalt is laid.
Safety of operatives and the public shall be paramount at all times, during the lifting of ironwork, and suitable signing and guarding must be in place while the work takes place.
The ironwork may be in a lifted condition for a while before surfacing takes place and while traffic is allowed to use the highway.
Appropriate signing and guarding must be in place to inform the driving public of any hazard.
try to refer to as few commercial sites as possible in compiling
my site, but when a site offers useful or interesting information
about a subject I make an exception.
Useful information on the manufacture and specification of inspection covers and gratings can be found by pressing, -------> HERE
Useful information, and illustrations, on gully pot chambers and associated pipes and pipework can be found by pressing, HERE
It is sad to state that in my experience when I have been asked to investigate the failure of road ironwork, which usually means nothing more than the lifting of the grating and sticking your head down the chamber to have a good look, it has been the standard of workmanship that has been to blame for the failure.
This lack of workmanship seems especially prevalent when road gullys are raised to allow for an overlay of bituminous surfacing.
In this situation it is often the case that the ironwork is raised correctly to the right level, i.e. the surface level of the finished wearing course, but with a minimum of effort.
When you look at the standard of "lifting" it leaves a lot to be desired in terms of correctly placed and mortared in brick work and quarry tiles ( quarry tiles are used for small lifts ) to form a strong load transferring bed for the gully.
The finished bituminous surfacing will look fine for a while, but then you will see the tell tale cracking start to appear around the gully showing that the supporting brickwork between the gully pot/chamber and gully was not correctly constructed when raised.
And, I will repeat once more it is most important for all loose material to be removed from around the lifted ironwork and suitable bituminous material to be hand placed and fully compacted before the main paver laid surfacing is performed.
Do you know I have even seen loose material, on the old road surface, in front of the paver, removed by sweeping it in to the cavity around the lifted ironwork, and then "suits" wonder why early ironwork failure occurs, and if there is no "supervision" on site it is likely to keep happening.
Correct procedures in placing hot-mix bituminous materials around lifted ironwork adds considerably to the strength of the ironwork.
Again as I have said many times in the pages of this website there is no substitute for good site supervision to ensure work is correctly carried out.
that can be achieved in a "paper"
exercise by removing good on site supervision is no a saving at all, there will
result a considerable cost to the quality of your highway network, and a
significant monetary cost of future repair and maintenance.
Removing qualified and experienced on site supervision may look good on the accountants balance sheet but that is the only place where it will look good.
Contractors who perform the work correctly, and there are some excellent, conscientious gangs who specialise in this type of work have nothing to fear, in fact they will benefit because contractors with poor working practice when gully lifting will be exposed by good site supervision and should be removed from the tender list.
If the overseeing organisations do not penalise contractors /suppliers who perform badly and do not support contractors/suppliers who provide a good service, the quality of all products and service supplied to maintain highway networks will follow a downward spiral.
I will also make a separate and strong point that when "plastic former" gulley pots are used, they are meant to be surrounded by a substantial amount of good quality concrete. If you do not follow, and enforce good engineering practice you will have gulley failures that are nothing to do with how well the actual ironwork is set.
Do not tell me bad practice does not happen, it does, you need strong on site supervision, if you do not penalise the contractors that do it badly you are not supporting the contractors who do it correctly.
And, "endless" subcontracting does not help, with everybody taking their "10%" until the amount of money remaining is just not sufficient to allow the correct procedures to take place, not if the contractor who actually does the work wants to make a profit.
It is a "real" world out their, you had better understand it.
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