Modular block faced, reinforced soil bridge wingwalls, utilising lightweight expanded clay fill Bloc modulaire à face, pont et murs d'aile avec sol renforcé, utilisant des granulas léger d`argile expansée comme matériau de remblai. C G Jenner Chief Civil Engineer - Tensar International Ltd, Blackburn, UK
P G Wills Group Manager Structural Solutions – Tensar International Ltd, Blackburn, UK
S Karri Director- P & S Consulting Engineers Ltd, Lytham St Annes, UK
L. Blundell Business Development Manager- Maxit UK, Runcorn, UK
ABSTRACT The design and construction of a new elliptical arch canal bridge confronted engineers with a number of problems. The aesthetics of the structure needed to be in keeping with the 200 year old canal it was crossing, whilst the design needed to consider the very weak ground conditions of the area. The consulting engineer utilised conventional brick arch design for the bridge sitting on reinforced piled foundations. However, the 4 wingwalls were designed using the established technique of geogrid reinforced soil, utilising lightweight expanded clay fill materials for the first time in Europe. Engineers have successfully used reinforced soil structures faced with precast concrete modular blocks since the 1980’s and the chosen system includes a stainless steel tie easily fitted during construction, allowing the addition of an attached brick skin, matching in with the brick arch structure. The choice of the lightweight fill provided new challenges for both designer and contractor. RÉSUMÉ Un certain nombre de problèmes ont confronté les ingénieurs lors de la conception et la construction d'un nouveau pont-canal à arc elliptique. L'esthétique de la structure devait être en accord avec celle du canal vieux de 200 ans qu'elle croisait ; Tandis que la conception devait prendre en considération les conditions très faibles du sol. L'ingénieur conseil a utilisé la conception conventionnelle de l`arc de brique pour le pont, reposant sur des fondations sur pieux en béton armé. Cependant, les 4 murs d'aile (wingwalls) ont été conçus en utilisant la technique établie du sol renforcé (stabiliser) par geogrid, en utilisant des granulas léger d`argile expansée comme matériau de remblai pour la première fois en Europe. Les ingénieurs ont employé avec succès des structures de sol renforcées de bloc modulaire de béton préfabriqué à face, cette dernière a été utilisée depuis les années 80; Le système choisi inclut une tige d'acier inoxydable facilement adaptable pendant la construction, permettant l'addition d'une peau de brique qui copie l`esthétique de la structure existante en arc de brique. Le choix de granulas léger comme matériau de remblai a apporté de nouveaux défis pour l'ingénieur et l'entrepreneur. Keywords: geogrid, reinforced, soil, lightweight, expanded clay, bridge, wingwall, modular, concrete, block 1 INTRODUCTION The Grand Union canal was constructed over 200 years ago in the South East of England. Its 137 mile main line from Birmingham in the Midlands to the Capital was once the busiest canal in the country. The canal now passes through part of the new town of Milton Keynes in North Buckinghamshire. As the town has developed over the past forty years, improvements have been made to the historic waterway that passes through the town. One of the
most recent modifications undertaken is the addition of a pedestrian footbridge over the canal in the Water Eaton area of Milton Keynes. The addition of this footbridge will allow direct access for the residents of Water Eaton to a large park on the other side of the canal. The existing crossing was remote from the residents and therefore little used. The local planners required that the completed structure needed to be in keeping with the historic surroundings of the canal. To achieve the planning requirement the contractor opted to build the ellipti-
cal arched structure using traditional brick construction rather than the more popular concrete structure clad with brick. The site chosen for the location of the bridge had a number of problems associated with it. The first issue to be overcome was that the foundation soils on both sides of the canal were weak and did not provide adequate bearing capacity if standard footings were adopted for the abutments and wingwalls. The client’s consulting engineers considered a number of different solutions. The first was to dig out the soft soils and backfill using a more suitable, better quality fill. This option was quickly rejected, as access to the park side of the site was severely restricted. Therefore excavated spoil and imported fill could only be transported using small front dumpers. The programme implications and cost of import and export would not fit within the time-frame and budget for the project and was therefore rejected. The second option reviewed was the use of piled foundations for the abutments and the wingwalls. The cost of this option was also beyond the budget, therefore this was also discounted.
duction in the bulk density of the fill material. As a consequence the reduced load from the soil on the chosen retaining wall would be greatly reduced. However, it would still be necessary to use piling to achieve adequate bearing for a conventional reinforced concrete wall.
Figure 2. Layout plan showing wingwall locations
The consulting engineer therefore, looked at alternative retaining wall systems in combination with the LWA fill. One of these alternatives considered was the use of reinforced soil techniques. The use of reinforced soil allowed for reduced bearing pressures when compared to conventional walls. The large footprint and flexible nature of a reinforced soil structure adopts the Meyerhof pressure distribution to transfer the loads to the foundation soils, rather than a trapezoidal distribution with high toe pressures which would be exerted by a stiffer concrete structure. The use of reinforced soil techniques and the LWA would allow the wingwalls for the structure to be constructed on the existing foundation soil without the need for piling. This combination allowed the project to come in below the budget set by the client. Figure 1. Location Map
The consulting engineers therefore looked at systems that could offer lower imposed pressure on the existing foundation soils. In order to do this they proposed a lightweight expanded clay aggregate (LWA) manufactured by Maxit Ltd. The LWA is manufactured by heating and firing natural marine clay in a rotary kiln to 1150°C. The process transforms the clay into various sized lightweight ceramic granules, which have a hard ceramic shell and a porous core. The bulk density of this material is in the order of 3.5kN/m³. This would lead to an 80% re-
In addition, the extreme lightweight nature of the fill allowed it to be pneumatically blown across the canal thus solving access problems for fill transportation to the park side of the bridge. 2 THE CHOICE OF A PRACTICAL SOLUTION Various proprietary reinforced soil systems are available on the market. Due to the requirements of the project, some additional features were necessary for the reinforced soil system finally chosen. The first of these features was that the wingwalls had to
match in with the bridge structure in terms of appearance. To achieve this a brick facing would need to be easily and securely added to the face of the reinforced soil wall. Also the project was to be adopted by the local government authority, which required the reinforced soil structure be designed in accordance with the UK Highways Agency Design Manual for Roads and Bridges, BD70/03. This standard requires that any proprietary wall system used should have independent approval and certification. The consulting engineers contacted Tensar International Ltd a UK geogrid manufacturer and specialist in reinforced soil design. After initial discussions it was decided to utilize their TW1 Link wall system. This is an earth retaining system that has a precast concrete modular block face incorporating a feature that allows a stainless steel tie to be built-in during construction. Subsequently a brick or masonry skin may be attached to the face of the reinforced soil structure by incorporating the stainless steel ties in to the mortar beds. The chosen wall system comprises of a uniaxially orientated high density polyethylene (HDPE) geogrid positively connected to a dry laid concrete block system using a unique high efficiency polymer connector. The high connection strength of the system is integral to the design of the structure and as such has been independently assessed and published in the BBA certificate. An added advantage of the modular block earth retaining system was the components parts could be easily transported to the park side of the bridge and then handled in to their final position. This was of vital importance as access for any heavy lifting equipment was limited. In addition, using the precast modular concrete block facing meant that no curing time or formwork was necessary, saving precious time from the programme. Similar earth retaining wall systems had been used for more than 25 years previously, but always using conventional fills. With this project it was to be used for the first time with LWA. Therefore the design for this structure needed to be carefully considered. 3 THE DESIGN Reinforced soil modular block walls have been designed and constructed internationally for more than 20 years. However, all the structures have been designed and constructed using conventional fills, the majority granular in nature. The main concept of a reinforced soil structure is that the reinforcement
allows the soil to act as a single body of soil; in effect creating a mass gravity retaining structure.
Figure 3. Typical cross section through the structure
For this particular structure the effects of using an extremely lightweight fill reinforced with geogrids was unknown. Therefore the design process needed to be carefully considered to ensure that a stable structure would be constructed. In the UK, reinforced soil structures used in bridge applications need to be designed in accordance with BD70/03 incorporating BS8006:1995. Wall structures designed in accordance with these standards, incorporating geogrids classified as ‘extensible’ utilise the tie-back wedge design method. However the design standards are based on experience of conventional fills. In order to design this hybrid structure with confidence, the design was broken down into the constituent parts to establish how the use of lightweight fills would affect each individual aspect. 3.1 External stability The first aspects to be considered were the global effects. These are primarily considered to be sliding, bearing and overturning. Adequate safety factors to sliding were first considered with the reinforced fill being the LWA and the backfill (or retained soil), it being an imported granular fill material. The mass of the reinforced soil block provides the resistive force to sliding with the driving force coming from the active wedge in the retained soil. Due to the low density of the LWA, it was found that the length of the geogrids reinforcement would have to be extremely long to provide an adequate factor of safety against sliding. This length was considered to be impractical for construction purposes. Therefore a rethink as to how check the design for sliding resistance was necessary. As the fill material in the reinforced soil block could not be changed due to bearing capacity considerations it was not possible to alter the resistive force of the soil block. Therefore only changes to
the driving force could be considered. The driving force on the back of the reinforced soil wall is provided by the active wedge of the retained soil. This is a function of the weight of the retained soil and its angle of shearing resistance. By using a better quality backfill material, the angle of shearing resistance would also increase. Increasing the angle of friction of the backfill did have the desired effect on the length of the reinforced soil block but not significantly enough to ensure that construction would be practical. The only alternative left was to use a lowdensity backfill. As the approach ramp to the bridge was to be constructed with the wall it seemed practical that the LWA could be used as the backfill material also.
demonstrated that an increase in the bulk density of the fill required greater strength from the geogrids reinforcement. However, in all calculations the lowest creep rupture strength geogrid from the available range was all that was necessary.
Checking the sliding calculations using this philosophy provided an adequate factor of safety for sliding with a practical length of geogrids reinforcement. After sliding had been assessed, bearing was next considered. Due to the low bearing of the soils this was carefully thought through. The use of the LWA provided a factor of safety that was substantially greater than what was required. Settlement was considered separately by the scheme designer and due to the low weight of the soils used across the full base width of the structure, settlement was considered to be minimal. Global stability was analysed using a standard Bishops method of slices, circular analysis. This provided an overall factor of safety in access of 1.5, which is the target for retaining walls. 3.2 Internal stability The external stability of the lightweight fill and a modular block wall system was considered to be adequate. Therefore using BS8006, the internal stability of the system needed to be analysed. This initially proved to be a problem as the interaction between the HDPE geogrids and the lightweight clay fill material was unknown. To assess whether the design of such a system was achievable, it was initially decided to use an interaction factor based on experience and previous test results on other materials. An interaction factor of 0.75 was adopted for the initial design. This was to be confirmed prior to final design of the structure. The specification for the LWA showed that the bulk density of the material could vary dependant on the amount of saturation. The bulk density could vary from 3.75kN/m3 to 6kN/m3. As part of the analysis all the various combinations for the bulk density of the reinforced fill and the backfill were considered. The result of numerous calculations
Figure 4. The lightweight aggregate and HDPE geogrid
Once it was established that the system could be designed, testing was undertaken to establish the interaction factor between the geogrids and the LWA. The particle size proposed for the construction was a 10mm to 20mm, which would breakdown slightly during placement and compaction. In order to establish the interaction factors, two 300mm shear box tests were commissioned. The first test was conducted using the LWA only, and the second test was the LWA together with the designed grade of geogrid. The interaction factor was then calculated by dividing the tangent of the angle of friction from test 2, by the tangent of the angle of friction from test 1. The resultant interaction factor was 0.85 and greater than the initial interaction adopted for the design. The final design was then transferred onto construction drawings and submitted for approval to the local authority. The requirements of BD70/03 indicate that only soils classified as 6I or 6J could be used within the reinforced soil block. Therefore as part of the submission to the local authority, a departure from standard was included together with the test results and the detailed design of the reinforced soil structure. Both the local authority and the client’s consulting engineer approved the departure from standard and the detailed design.
4 CONSTRUCTION Construction of modular block walls is a relatively straightforward procedure and this project was no different. However, the nature of the LWA provided the contractor with new challenges in terms of handling, placing and compaction.
Figure 6. LWA being blown into place prior to grading and compaction
Figure 5. Geogrid fixed into the face with polymer connectors
bars of the geogrid and is then locked in to place between the blocks. The high efficiency connection is an important feature of the system. One of the limiting factors on the design strength of the geogrid is the connection efficiency at the face, which may be as low as 25% in systems using a frictional connection only. This is of particular concern where the vertical confining stresses are low, such as in walls up to 8 metres high.
Once the formation level had been prepared the in-situ concrete strip footing for the facing was cast to line and level. As a precaution the footing was designed wider than normal to assist with the load spreading over the weak foundation, particularly as the finished brick face of the structure was sharing the same base. In order to contain the LWA during construction it was decided to raise the modular block face and the general granular fill to the rear of the structure more or less simultaneously. This was devised to prevent the LWA blowing or rolling a way during the placing and compaction and it worked well. Delivery of the LWA was by wagon to one side of the canal where it could be used in the works there, or pneumatically blown directly in to place on the park side of the project through delivery pipes attached to the scaffolding alongside the bridge. The pneumatic placement of the LWA imparts a reasonable level of compaction; however, in all areas a vibrating plate was used to compact the material to a satisfactory density. The HDPE geogrids are simply cut from the delivered roll to the length dictated by the design. The vertical spacing between layers for this project was 450mm (every 3 courses of blockwork). The method of connection in to the face uses a moulded polymer connector that hooks around the transverse
Figure 7. Overview of the bridge construction
Once connected to the face it is important to take out any slack present in the geogrid and connection. If left in, this slack would in time manifest itself as a post constructional forward movement of the face. When using conventional granular fill materials this is not a problem as the geogrid may be tensioned lightly using a steel beam and bar mechanism. The dense granular fill provides the perfect reaction to this tensioning effort. However the LWA provides no such reaction, therefore the contractor paid spe-
cial attention to this tensioning, preferring to do it in small increments rather than using one singular effort.
5 CONCLUSIONS In order for the design and construction of this scheme to come in on budget and be completed in a timely fashion the whole team had to be innovative. The first use of LWA in a geogrid reinforced soil structure presented some new challenges and unknown parameters to the designers whilst the contractor needed to be inventive about the way plant and materials were transported and used on site. Modular block reinforced soil structures have once again shown themselves to be an attractive and economical alternative to so-called conventional construction methods such as reinforced concrete. The combination with LWA has opened up new opportunities for the construction of earth retaining structures over weak foundations
Figure 8. Close-up of the modular block facing unit and stainless steel brick-tie
The stainless steel ties used to attach the finished brick face to the modular block face, are simply placed during construction. They are inserted down a vertical groove in the side of the block and may then be adjusted to the height of the mortar bed between bricks as they are placed. Once constructed to full height the brick façade was completed to match in with the arch bridge. The construction phase of the project was completed without problems and within programme.
ACKNOWLEDGEMENTS The authors which to acknowledge the help and cooperation of the following: • The Client: English Partnerships, Central Milton Keynes, Buckinghamshire, UK • The Local Authority: Milton Keynes Council, Central Milton Keynes, Buckinghamshire, UK • The Consulting Engineers: Pell Frischmann Consultants Ltd, Milton Keynes, Buckinghamshire, UK • The Contractor: Jackson Civil Engineering Limited, Milton Keynes, Buckinghamshire, UK
REFERENCES BBA Certificate No. 00/R122, 2000, ‘Tensar TW1 Wall System for Retaining Walls and Bridge Abutments’, British Board of Agrément, Watford, UK BS 8006: 1995 “Code of practice for strengthened reinforced soils and other fills”, British Standards Institution, London, UK. BD70/03: Highways Agency Design Manual for Roads and Bridges – Strengthened/Reinforced Soils and other Fills for Retaining Walls and other Bridge Abutments
Figure 9. Showing the completed scheme