This paper outlines how the London Borough of Brent came to utilise sewer technology to strengthen culverts carrying the highway. This technology avoids the disruption and costs associated with traditional strengthening methods.
The programme of assessment of highway structures resulting from the new 40 tonnes vehicles allowed on the highway from 1st January 1999 has highlighted the poor state of many structures on the British highway network. The strengthening or replacement of structures following these assessments can easily cause traffic chaos on today’s highly trafficked roads. In urban environments such as London and other major cities, such disruption has a serious effect on local businesses and the travelling public.
The London Borough of Brent is the Highway Authority for a part of North West London including such areas as Wembley, Kilburn, Willesden and part of the massive Park Royal industrial estate. The North Circular Road bisects the Borough and events at the Wembley Stadium can cause the area to be a bottle neck.
Several culverts carrying a variety of watercourses under the highway were identified by the Borough’s assessment programme as being severely under strength. Strengthening schemes were designed which replaced the sub-standard roof with one on piles but these were not progressed for several years due to lack of available funding.
In the 1995/6 financial year, funds became available for the strengthening of part of the structure carrying the Wealdstone Brook under the junction of the busy Kenton Road with Woodcock Hill and Kenton Lane. The structure straddles the boundary with Harrow and the cost of the works was shared between the two boroughs. The junction is complex and under traffic signal control and an elaborate traffic management scheme was necessary for the works. This involved diverting traffic in one direction via a narrow residential street, in order to allow the contractor, J Murphy and Sons Ltd, possession of half of the main road for construction works. On completion of half the works, the traffic was switched to the other side.
Following
two months of preliminary utilities works, the main works were completed
in three months, during which time they featured regularly on local radio
stations because of traffic delays. Complaints were received from businesses
over a mile from the works because of loss of trade. 
The
signing and coning of diversions and road closures were continually vandalised
by irate motorists and required constant maintenance and renewal by the
Contractor.
The commissioning client, StreetCare Service Development Unit, was determined
that similar disruption should be avoided if at all possible for future
similar works. Existing plans for the strengthening of three subterranean
brick arches carrying the A404 over the old course of the River Brent were
discarded. Proposals for strengthening were invited from several contractors
with the requirement that there would be minimal disruption to traffic with
the successful 
tender.
The ICE Design and Construct Conditions of Contract was used as the basis
for the contract. The submission accepted was that by J Browne Construction
Co. Ltd. The scheme involved sinking an access shaft in an adjacent open
space outside the highway boundary onto the adjacent culvert. The central
arch of the structure was then lined with large diameter concrete jacking
pipes and the annulus backfilled with foamed concrete. The two side arches
were abandoned and backfilled with concrete. The solution was appropriate
in this location due to the small residual flows which could easily be accommodated
in the new pipe. All works were carried out within the confines of a small
compound off the highway and muck away was from a side road.
The Borough was subsequently faced with a more complicated problem at Dudden Hill Lane, the A4088, where two culverts, one carrying the Mitchell Brook and the other carrying a small stream, joined under the main carriageway. The limiting of disruption to traffic on the road and local residents was of utmost importance as London Underground had recently reconstructed a nearby bridge. The traffic management associated with their work had been in place for over a year with resulting serious delays to traffic and inconvenience to local residents.
A meeting was held between StreetCare’s term consultant, AFM Consulting, J Browne Construction and Paul Williams acting on behalf of the client to devise a method of carrying out the works whilst minimising the effect on traffic. This meeting resulted in a decision to adopt a method which had previously been used for strengthening sewer culverts for some time but to our knowledge had not been used in a highway application.
The method involves lining the culvert with specially manufactured GRP liners which relieve the existing roof slab of the bending moment. These are manufactured in two sections for ease of handling and assembly. They are inserted into the culvert and shotcreted into position. The loss in section of the culvert and reduction in flow capacity that would be expected is compensated for by the improved flow characteristics of the GRP compared to the original concrete walls and floor.
Visits to the factory where the liners were to be manufactured and examination of the Water Research Council (WRC) methods used in the design convinced Brent officers that the use of this method would fulfil the criteria set. These were to strengthen the culvert to the required standard, cause minimal disruption to traffic and keep within budgeted project costs.
In the WRC model, the GRP lining works in a composite action with the existing concrete roof slab in such a way that the bending moment is taken by the GRP with the existing defective slab being put into compression. The existing sub-standard reinforcement is near the new neutral axis and no longer a critical factor. For high tensile materials such as GRP, failure will generally be due to shear at the interface between the GRP and the shotcrete and not due to tension. Therefore the limitation on the composite design is the ability of the panel/grout interface to resist shear during bending. The panels are therefore fabricated with a high friction shear key.
The scheme for Dudden Hill Lane involved diverting the minor culvert to a new junction with the Michell Brook under an adjacent residential road. In order to do this, a road closure on the side road was necessary which allowed the installation of a new culvert composed of conventional concrete culvert sections. The redundant length of the original minor culvert was then abandoned and backfilled with foamed concrete. The side road was re-opened and the remaining work restricted to a small compound area. The main culvert was subsequently lined with the GRP sections which were shotcreted into place. Other than the short closure on the side road, the works resulted in no disruption to traffic. The whole contract was completed in eight weeks with the lining of the main culvert taking only one week.
The benefits of this method of strengthening include the ability to commence on site a short time after the decision to carry out works. This pre-contract period is required for the fabrication of the moulds and the manufacture and curing of the GRP units. The use this method enables work to be carried out at short notice as no lead in time needs to be allowed for the processing of traffic orders and for Statutory Undertakers to programme and complete their works. Following delays on major schemes elsewhere in London, money was made available to Brent in November 1997 by LoBEG, the London Bridges Engineering Group. Despite there being only four and a half months remaining, it proved possible to strengthen a further three culverts before the end of the financial year. Using conventional methods this time scale would have been impossible to meet.
The flexibility of the system for use in a variety of different shaped culverts was demonstrated when it was used to strengthen the Ealing Road and Chaplin Road culverts. The original concrete culvert was rectangular with a large width in relation to its height. The strengthening here consisted of the installation of two liners thereby effectively halving the span and creating a central pillar.
In the case of one of the three schemes completed, the second phase of the Kenton Road culvert, the flow carried is the Wealdstone Brook which, as a main river, is under the jurisdiction of the Environmental Agency. Thus their consent was needed before lining the culvert. The Agency accepted that the improved flow characteristics would compensate for the reduction in section and granted their consent. Difficulties in obtaining E.A. approval often prohibits the use of other methods of strengthening which involve the loss of section in a culvert. The works were completed in five weeks, a stark contrast to the total of five months for the works to phase one of the same culvert described above.
The economics of using this method are also very convincing. The cost of phase two of Kenton Road was £164,000 for the strengthening of 41m of double culvert. In contrast, phase one had cost approximately £190,000 for 25m of double culvert including the extensive works to statutory undertakers’ plant. If the costs of disruption to traffic were taken into account, the savings due to lining would be even more pronounced.
The writer believes that this method has proved to be very successful for this application and has much to offer bridge and highway engineers, especially in urban areas and locations with high traffic flows. It is suitable for sites with minimum access, has little or no effect on traffic, is almost noiseless, is economical when compared to traditional methods, is quicker than conventional methods and perhaps most important, does not disrupt existing utilities’ underground plant. Use of this and similar innovative solutions can lessen the bad publicity that the industry receives from the carrying out of essential maintenance works.
Client StreetCare Service Development Unit, London Borough of Brent
Engineer for the Client Eur Ing Paul A Williams BSC CEng MICE MIHT
Consulting Engineers AFM Consulting
Main Contractor J Browne Construction Co. Ltd
Lining Sub Contractor Civation Ltd
Biography of Author
Paul A Williams BSc(Hons), Eur Ing, C Eng, MICE, MIHT
A native of South Wales, following a degree in Civil Engineering at Surrey University, Paul spent a period in Southern Africa working for a major local contractor, Roberts Costruction Pty Ltd. On his return to the UK he spent the next eight years in Local Government with periods at West Glamorgan County Council and the London Borough of Hackney until 1985 when he joined R Travers Morgan & Ptnrs, later Travers Morgan Ltd, where he became Divisional Director in the Highway Maintenance Division. He has worked as an independent Consulting Engineer for the last four years undertaking a variety of commissions for several London Boroughs.