J.H.W. COUNSELL (SYMONDS GROUP LTD)
P. A NOSSITER (M.J. GLEESON GROUP PLC) - England.
ABSTRACT: This paper describes the planning, design and construction of a scheme to widen and strengthen a historic multi-span-masonry arch bridge across the River Thames. The history of a river crossing at this point is discussed and the features that make Edward Lapidge's structure of 1828 a part of the English national heritage are described. In order to deal with modern day traffic levels it has been necessary to develop a scheme to widen the structure and strengthen the existing arches. This has had to be carried out in a manner that maintains the bridge's attractive appearance and recognises all local environmental conditions.
1.0 INTRODUCTION AND HISTORY
Kingston Bridge, London, UK ( Figure 1 above ), is a strategic crossing point over the River Thames in South West London. It carries a total of approximately 50,000 vehicles per day with some 2000 vehicles per hour crossing in each direction at peak times.
There has been a crossing of the Thames at Kingston since Roman times. It is believed that the first bridge of timber construction was built in AD43 following the Roman invasion of England. Over the centuries it was rebuilt and repaired on several occasions.
The wooden bridge was eventually replaced in 1828 by a masonry arched bridge, Figure 2, which was widened on the south side in 1914. Spanning the river are five elliptical arches with Portland stone facades with a bold cornice and balustraded parapet. Semi-circular cut waters carry flat panelled piers surmounted by balcony projections that form recesses in the balustrading. Above the springings the 1914 bridge is separated from the 1828 bridge by a small gap which can only be seen from the river. The façade of the 1914 bridge is a replica of the 1828 original using similar Portland stone and ornamental features.
Figure 2
The nearest alternative road crossing points on the Thames are at Richmond, 7 km downstream, and Hampton, 4 km upstream.The bridge is part of the national heritage and in 1951 was granted listed status. This places extra control on any work that is carried out to maintain the structure and also any alterations.
2. SITE CONSTRAINTS
The bridge is an integral part of the road network serving Kingston town centre, the wider area of south west London and beyond. It is therefore essential that any work on the bridge does not disrupt the flow of traffic and if unavoidable should be carried out only during off peak times.
The river at the bridge is non-tidal with flow being controlled at Teddington weir 4 km downstream. The west bank of the river lies within the Thames flood plain so ground levels in this area cannot be increased because of the risk of flooding elsewhere during high flow conditions. Although no longer used by any significant commercial traffic, the river is a valuable amenity used by pleasure craft particularly in the summer season. Navigation must be maintained whilst carrying out any maintenance work to the bridge
The areas surrounding the bridge are environmentally sensitive with parkland on the west side and the historic market town of Kingston on the east. Restaurants and cafes overlook the river on the Kingston side. Both sides have riverside footpaths and are within designated conservation areas recognising their historical and visual importance with the bridge acting as a focal point.
The bridge carries numerous services including electricity and telecommunication cables. A majority of these are located in the south side footway.
3. INSPECTION AND LOAD ASSESSMENT
Kingston Bridge has been the responsibility of The Royal Borough of Kingston Upon Thames since 1986. In September 1993, consulting engineers Travers Morgan were commissioned to inspect the condition of the bridge and carry out a load assessment.
Prior to carrying out the load assessment detailed inspections were undertaken to examine the bridge's condition. The inspections revealed numerous defects including:
1. Displaced and cracked brickwork in the hidden voids over the piers. (These internal voids are shown in Figure 3).
Figure 3 - Internal Voids
2. Lack of effective drainage and waterproofing causing penetration of water through the bridge fabric.
3. Open joints, loss of mortar and minor cracking to the brickwork arches.
As a result of the inspections, exceptionally heavy vehicles (over 38 tonnes) were prohibited from crossing the bridge and a programme of regular inspections instigated to monitor its condition.The subsequent load assessment identified a number of elements of the bridge with a reduced load bearing capacity as shown in Table 1 below.
Table 1: Assessed Load Carrying Capacity
| Element |
Assessed Load Carrying Capacity(tonnes) |
|
|
1828 |
1914 |
|
|
Bridge |
Bridge |
|
| Pier Voids | 3 | 40 |
| River Arches | 3 | 3 |
| Richmond Approach Arch | 3 | 3 |
| Kingston Approach Arch | 10 | 13 |
The imposition of a permanent 3 tonne weight limit was rejected due to the severe disruption it would cause to traffic, local businesses and the community as a whole. Access to Kingston town centre by public transport would also be severely restricted if buses were banned from crossing the bridge. It was therefore decided to carry out investigations to identify methods of strengthening the bridge. Whilst the investigations were being undertaken the bridge would continue to be monitored.
4. STRENGTHENING
An established technique for strengthening arches is to construct new external arches to support the existing. Alternatively, the strengthening could be contained within the fabric of the bridge which upon completion would be hidden from view. In deciding which method to adopt for Kingston Bridge priority was given to maintaining the appearance of the bridge and hence the external method was rejected.
A comparative study was undertaken to assess the relative technical merits of five different strengthening techniques. As part of this process close consultations took place with English Heritage who, whilst not objecting to the principle of strengthening, requested as little interference as possible with the existing internal fabric of the bridge. The study concluded that, on balance, a concrete composite saddle was the preferred option as it preserved a majority of the internal fabric and in particular the hidden voids over the piers.
The strengthening scheme would require work to be carried out from the carriageway surface with a reduced number of traffic lanes and for traffic to be diverted around the working areas. Studies identified that two traffic lanes were the maximum that could be kept open during the works which would mean a lane reduction in each direction. To assess the implications of this on the area-wide road network a detailed traffic analysis was carried out. This showed substantial impact over the whole area and the option to reduce the number of traffic lanes to one in each direction was rejected. Investigations were then undertaken to identify possible means of maintaining four lanes of traffic.
5. TRAFFIC SOLUTIONS
Studies were undertaken to examine temporary bridge and widening options in detail. This exercise involved widespread consultations with affected parties including the public, businesses, local authorities, and English Heritage. An environmental impact study was also carried out to assess and compare their likely impact on the area.
A permanent widening scheme was eventually chosen due to the reduced environmental impact and also the opportunity to introduce permanent facilities for buses and cyclists.
6. DETAILS OF WIDENING
The widening is on the upstream side of the 1914 bridge and adds 6.6 m to the current width of 17.5 m. A section through the widened bridge is shown in Figure 4.
The river spans match the existing arrangement of 5 elliptical arches symmetrical about the centre line of the central arch.
Figure 4
The new piers comprise reinforced concrete stems founded on a reinforced concrete pile cap with bored cast in-situ concrete piles. The piers are faced with natural Portland Stone.
Precast concrete arch units are used for constructing the spans of the widened bridge to minimise the disruption to river traffic during construction. Lightweight materials have also been specified to minimise the loading on the foundations and hence reduce potential settlement.
The precast units forming the spans of the widened bridge are made continuous over the crown using coupled reinforcing bars with in-situ concrete. These precast units are faced with masonry and brick to match the existing arch profile and finishes. An in-situ lightweight reinforced concrete saddle is cast on top of the precast units to form a composite section. Starter bars project from the extrados of the precast units to provide continuity with the saddle.
The haunches of the arch are filled with a low density foamed concrete fill overlain with half a metre of higher density foamed concrete below the carriageway surfacing. Reinforced concrete spandrel walls are cast in-situ on top of the arches and faced with new Portland Stone to match the existing 1914 elevation.
7. DESIGN ANALYSIS
7.1 Widened Bridge
The spans of the widened bridge were analysed using a LUSAS 2-dimensional finite element model. Each span was idealised as being restrained at each end by the piers. To accommodate shrinkage and thermal effects movement joints are incorporated at the end of each arch.
The transverse stiffness of the arch was ignored in the analysis. A plane stress analysis of unit width was therefore carried out using plane stress elements. In addition the stiffening effects of the spandrel walls were not taken into account in the design of the composite reinforced concrete arch barrel. A local transverse analysis was carried out to determine the forces in the spandrel walls.
End support conditions were modelled to simulate the degree of fixity provided during the construction of the bridge and also in the permanent situation. The analysis therefore considered the three pinned, two pinned and fixed situations.
7.2 Strengthening
The superstructure was modelled globally as a five span masonry arch using the program ARCHIE. This analysis was used to investigate the out of balance forces in the piers and hence to check their stability during the construction phases and when strengthening is completed.
The analysis of the lightweight reinforced concrete saddle and brick arch composite behaviour was based on a non-linear analysis of the central river span. A plane stress two dimensional finite element model of unit width was developed using LUSAS. The material model for the existing brick arches assumed no tensile stresses. The reinforced concrete saddle was modelled using linear elastic properties based on uncracked section behaviour.
The behaviour of the finite element model was benchmarked against a single span ARCHIE model by comparing the order and position of hinge formation under a point load positioned close to midspan.
8. RIVER WORKS
Cast in-situ bored piles were designed on the basis of mobilising skin friction within the London clay substrata. Experience of this material in the London area is extensive and it is generally uniform and consistent. The 600 mm diameter concrete piles were reinforced to accommodate the possible lateral loading on the pile group arising from differential live loads on adjacent arch spans.
Piles were sleeved through a new 2 m thick base of mass concrete installed at the bottom of the cofferdams. This base acts as a seal to the clay formation and prevents softening of the upper zones of the clay . It also helps resist the effects of heave following excavation of the cofferdam. Once widening works are complete this mass concrete also provides resistance to scour effects.
9. NEW ARCHES
The restrictions on temporary works in the river and the potential for disturbance to navigation were the factors that primarily led to the choice of precast reinforced concrete for the main structural members of the new arches. In addition there is the advantage of factory conditions for manufacture which was utilised in the design by incorporating the brickwork facing to the soffit to match the existing bridges as an integral part of the precast unit.
Twelve units are cast for each span, each single unit being a half span and contained within dimensions which can be carried on UK roads without special escort arrangements. Figure 5 shows details of the units.
Figure 5 - Precast Arch Units
10. CONSTRUCTION.
The contract to widen and strength the bridge was awarded to M.J. Gleeson Group Plc in October 97 with a construction period of 39 months under the NEC 2nd Edition, Engineering & Construction Contract.
10.1 Access & Stone Removal
The area of the construction site is extremely restricted by the River Thames, by the close proximity of the protected tree area owned by Hampton Court Palace on the Richmond side and by numerous Commercial properties on the Kingston side. Access to the site is only available via a narrow temporary access across a tree protection area, to the riverbank immediately south of the bridge. All equipment and materials have been delivered to site via this access.
The planning consent for the bridge involved the re-use of as much of the original Portland Stone facade as possible from the southern face. To this end the embrasure and cutwater stone together with the cornice, bottles, plinths and copings were surveyed, plotted and numbered. After careful removal these were taken to store off site ready for re use in exactly the same configuration but on the newly widened part of the bridge at a later stage.
10.2 Cofferdam Construction To Piers
Cofferdams could not be completely closed and had to be sealed onto the existing piers by the installation of bulkheads cut to the profile of the brick and stone piers. 9m long (20W) sheet piles were driven to provide a 4m penetration into the London clay. Then excavation and placing of concrete to seal the base was undertaken underwater to prevent the need for a bottom frame. After installation of the top frame the cofferdam was pumped out to expose the existing base. This revealed a steel caisson which had been used for the construction of the foundation to the 1914 structure as shown in figure 6.
. 
Figure 6
This caisson was not indicated on the original as-built drawings and proved a major problem due to the extensive use of steel plate in its construction. The piles had to be sleeved through this caisson. Numerous methods were attempted to break through the steel and concrete construction to provide a 650mm-dia hole to allow the piles clearance from the existing base. These methods included thermic lancing, high pressure water jetting, diamond stitch drilling, percussive drilling and rock drilling with bursting compound. The most successful method was the traditional compressor and breaker together with financially motivated operatives. The lack of space within the cofferdams precluded the use of an auger rig and the piles were installed by tripod rig restricting output to one pile per rig per day.
The pier construction above the piles was heavily reinforced concrete with new 200 mm thick Portland Stone cladding to the sides. The original cutwater stone was returned to the front of each pier. Portland Stone Roach bed was used below the splash zone and Whitbed above.
10.3 Pre-Cast Units
The design called for the arches to be pre-cast concrete with brick slips and Portland stone bands to match the existing arches. The shape of the southern face of existing structure had to be accurately determined by undertaking a detailed laser survey. In addition, the Stonework subcontractor, Universal Stone, was given the task of drawing and plotting every stone in the voussoir face. These plots were used for the stone manufacture. To ensure that an accurate representation was achieved these two surveys were overlaid and a tracing made of the true shape of the arch with the exact location of each voussoir stone. This tracing was then supplied to the pre-cast manufacturer Macrete (Northern Ireland) and used to manufacture the timber moulds for the pre-cast arches. When the moulds were made Macrete sent timber templates to site and these were erected on the bridge face to check the shape prior to casting. This proved to be a time consuming, but worthwhile exercise as it produced extremely accurate results in the shape and alignment of the final units.
Because of the dimensional constraints which required matching the existing arch geometry the units were manufactured to much tighter tolerances than normally required for precast beam construction. In addition to the shape of the units great care had to be exercised over the width of each unit. The horizontal alignment of the new voussoir was not necessarily parallel with the existing voussoir face. Consequently the first unit to be installed against the existing face was tapered to accommodate any mis-alignment and to produce a uniform joint. The pre-cast units were match-cast against the preceding unit to ensure extremely close fitting of the units when installed. Figure 7 shows one of the 80 pre-cast units being lifted into position by the 50 tonne crawler crane mounted on a unifloat pontoon. The design required a 20mm gap between the units at the crown and that they be supported until the concrete placed in the pocket (see figure 5) had reached the required strength. This support was achieved by the use of Mabey Towers and tie rods.
Figure 7
Once the Pre-cast units had been installed the heavily reinforced saddle slab could be constructed. To reduce the weight on the pre-cast arches the saddle slabs and spandrel walls are cast with 40N Lytag concrete with a max oven dry density of 1800Kg/m3 and max fresh wet density of 2000Kg/m3. The shape of the units and the requirement to balance loading over the arches called for complex curved shutter as shown in figure 8.
Figure 8
A waterproofing membrane was spray applied to the saddle slabs and spandrel walls and the remaining void filled with foam concrete with a density of 700Kg/m3 to within 0.5m of the road construction and topped off with foam concrete of 1400Kg/m3 density. A 46-way service duct was cast into the foam concrete and, in some spans within the reinforced saddle slab. These enabled the diversion of services from the existing structure to allow access for the strengthening works in the later phases. With the road construction laid the traffic could be diverted and work could start on the strengthening of the existing bridges.
10.4 Strengthening Work
Figure 9
1828 Bridge showing voids
The listed building consent was granted subject to English Heritage approval of the contractor’s detailed method statement for the part demolition of the voids in the 1828 structure and the installation of the saddle slab. In figure 9 the existing small void can be seen on the right of the photograph. At this location the void has been removed completely and supports have been installed. Only the brickwork to enable the saddle slab to be constructed has been removed from the intermediate void.
The reinforced slab is bonded to the extrados of the brick arch by epoxy grouted stainless steel hoops drilled 300mm into the 600mm brickwork. In total there are 6500 hoops which act in shear to transfer load into the brickwork arch and down into the piers. Lightweight construction has again been used to reduce the load on the existing foundations. The saddle slabs are cast with Lytag concrete and foam concrete is used to backfill to the underside of the road construction.
10.5 Stone Facing
As much stone as possible was recovered from the existing structure but not the original voussoir and spandrel wall stones which form an integral structural part of the bridge. As a consequence in excess of 2300 individual items were manufactured for the new structure from Portland Stone and installed to exactly replicate the original facade.
11. CONCLUSION
The solution to strengthen Kingston bridge has had to take into account a wide range of issues beyond the pure engineering. By permanently widening the bridge a balance is believed to have been achieved between the need to minimise disruption to users of the bridge and to preserve the character of the bridge and its surroundings. Construction has had to recognise these issues and methods have been adopted which preserve of the fabric of the historic structure. In addition, the development of the scheme has required close liaison with all affected parties including commercial interests, government bodies, local authorities and the public. The client, Royal Borough of Kingston, has striven to ensure that all these parties have been closely involved and kept informed of progress both during design and construction.