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MAKING THE MOST OF THE MOTORWAY
00:00 19 Jan 1995
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The widening of the M25 seems to go on forever, but in the process
it is making widespread use of some of the most up-to-date
construction systems, designed to lower costs and reduce the
environmental impact.
The 8.5km stretch between the M3 and the A3 now under construction by Mowlem Civil Engineering not only incorporates one of the most elaborate noise barrier systems so far installed in the UK, it also makes greater use of reinforced soil structures than in any previous project.
The carriageway is being widened on each side by converting the existing hard shoulder into a traffic lane and constructing a new hard shoulder alongside. To create the space required without extending beyond the present fence line, particularly between junctions 10 and 11, the contractors have had to cut into the existing slopes in some places and build up the embankments in others.
The much steeper slopes would normally be retained using conventional means such as concrete retaining walls, sheet piling, etc, and in fact the original design by the Department of Transport proposed sheet piling and gabions for the support structures. The suggestion that reinforced soil could be used as an alternative came from Mowlem, using an in-house design. After discussions with consultants WS Atkins and the DoT, the design was accepted and has proved very successful.
While sheet piling has been required on a few locations, by far the greater length of the new slopes between junctions 10 and 11 are composed of reinforced soil. By the time Mowlem has completed its œ35 million contract for the widening operation, a total length of some 6km will have been constructed on both sides of the carriageway. Some sections are as long as 800m and rise to a height of 6m, the total surface area is about 20,000 m2.
Mowlem's engineers are somewhat cagey about the exact details of the in-house design system which they have devised, beyond saying that it conforms to the recent guidelines issued by the DoT, and is designed for a life of 125 years. It is essentially a wrap-around system using geogrids as reinforcement, wrapped around a selected compacted fill, and with the topsoil retained by a geotextile fabric.
Where the earth wall is acting to retain a cutting, no further protection is needed. Where the new structure is being used to build up an embankment to support the new hard shoulder, the face is given an additional overall mesh cover to help retain the topsoil. As a final touch, the whole surface is hydroseeded with a variety of different mixes, carefully selected according to the aspect of the slopes and their location. Other plants such as clovers are included in addition to grasses, but are primarily chosen for heavy root growth and short stems.
By using the system, Mowlem has been able to reduce the slopes from the 1:2 of the originals to 1.5:1 or 60¼, creating sufficient level ground to accommodate the new hard shoulder without the expense and delays of additional land acquisition or heavy support systems. It therefore offers considerable advantages in terms of cost, and provides a more attractive and environmentally-friendly final appearance.
Mowlem admits that it is labour intensive, which is something which has to be taken into account when the contract has to be completed to a very tight deadline and no night working is permitted. The entire contract, covering reconstruction of the central barrier, installation of lighting, all the earthworks, construction of the new hard shoulder, erection of the noise barrier, installation of new gantries and all the reinstatement work, covers only 44 weeks.
Construction started in February 1994 with the rebuilding of the central barrier and installation of the lighting. By the end of October the reinforced soil structures on the clockwise side were virtually complete and the construction of the new hard shoulder, much of it using recycled material in the lower layers, was under construction. The anti-clockwise side will follow on through the winter.
Over the contract there are 28 reinforced earth structures, either cuttings or embankments. Because completion of these is a crucial factor in the overall programme not all are constructed using the Mowlem system. Some of the work has been subcontracted to Comtec (UK) Ltd, which uses the Swiss designed Textomur system.
Although basically similar in form, this system uses a synthetic geotextile to retain the topsoil on the face, which is further strengthened by a steel mesh grid. The slope this produces is much smoother, and, in the project manager Steve Lowder's view, results in a much less natural appearance than with the Mowlem system.
'I think that by the time the vegetation has grown anyone driving past won't be able to tell they are man-made slopes,' said Lowder.
While construction has progressed well, there could be delays ahead. The process is weather sensitive: 'If the moisture content of the fill is too high you can't compact it,' points out Lowder. With much of the work continuing until Easter next year, the weather over the next few months could be an important factor in finishing on time.
In addition to the extensive use of reinforced soil structures, this section of the M25 has another claim to fame. The 13km of sound barrier along either side represent one of the most sophisticated road noise repression systems so far constructed in this country.
Depending on the location, the barrier is either inclined at 10¼ to reflect sound upwards or vertical to reflect or absorb noise. All types are supported on steel posts, fixed in individual piled foundations up to 5m deep.
The barriers themselves, which reach a maximum height of 6.5m, make use of four different materials - precast concrete panels, solid timber panels, a proprietary absorbent panel system from Germany, and transparent tinted acrylic. These are used in different combinations according to the degree of protection from noise required at specific locations.
The barrier installation, at an estimated cost of 15% of the overall contract sum, is being carried out by Mowlem Civil Engineering, in conjunction with Radian Systems.
The 8.5km stretch between the M3 and the A3 now under construction by Mowlem Civil Engineering not only incorporates one of the most elaborate noise barrier systems so far installed in the UK, it also makes greater use of reinforced soil structures than in any previous project.
The carriageway is being widened on each side by converting the existing hard shoulder into a traffic lane and constructing a new hard shoulder alongside. To create the space required without extending beyond the present fence line, particularly between junctions 10 and 11, the contractors have had to cut into the existing slopes in some places and build up the embankments in others.
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The much steeper slopes would normally be retained using conventional means such as concrete retaining walls, sheet piling, etc, and in fact the original design by the Department of Transport proposed sheet piling and gabions for the support structures. The suggestion that reinforced soil could be used as an alternative came from Mowlem, using an in-house design. After discussions with consultants WS Atkins and the DoT, the design was accepted and has proved very successful.
While sheet piling has been required on a few locations, by far the greater length of the new slopes between junctions 10 and 11 are composed of reinforced soil. By the time Mowlem has completed its œ35 million contract for the widening operation, a total length of some 6km will have been constructed on both sides of the carriageway. Some sections are as long as 800m and rise to a height of 6m, the total surface area is about 20,000 m2.
Mowlem's engineers are somewhat cagey about the exact details of the in-house design system which they have devised, beyond saying that it conforms to the recent guidelines issued by the DoT, and is designed for a life of 125 years. It is essentially a wrap-around system using geogrids as reinforcement, wrapped around a selected compacted fill, and with the topsoil retained by a geotextile fabric.
Where the earth wall is acting to retain a cutting, no further protection is needed. Where the new structure is being used to build up an embankment to support the new hard shoulder, the face is given an additional overall mesh cover to help retain the topsoil. As a final touch, the whole surface is hydroseeded with a variety of different mixes, carefully selected according to the aspect of the slopes and their location. Other plants such as clovers are included in addition to grasses, but are primarily chosen for heavy root growth and short stems.
By using the system, Mowlem has been able to reduce the slopes from the 1:2 of the originals to 1.5:1 or 60¼, creating sufficient level ground to accommodate the new hard shoulder without the expense and delays of additional land acquisition or heavy support systems. It therefore offers considerable advantages in terms of cost, and provides a more attractive and environmentally-friendly final appearance.
Mowlem admits that it is labour intensive, which is something which has to be taken into account when the contract has to be completed to a very tight deadline and no night working is permitted. The entire contract, covering reconstruction of the central barrier, installation of lighting, all the earthworks, construction of the new hard shoulder, erection of the noise barrier, installation of new gantries and all the reinstatement work, covers only 44 weeks.
Construction started in February 1994 with the rebuilding of the central barrier and installation of the lighting. By the end of October the reinforced soil structures on the clockwise side were virtually complete and the construction of the new hard shoulder, much of it using recycled material in the lower layers, was under construction. The anti-clockwise side will follow on through the winter.
Over the contract there are 28 reinforced earth structures, either cuttings or embankments. Because completion of these is a crucial factor in the overall programme not all are constructed using the Mowlem system. Some of the work has been subcontracted to Comtec (UK) Ltd, which uses the Swiss designed Textomur system.
Although basically similar in form, this system uses a synthetic geotextile to retain the topsoil on the face, which is further strengthened by a steel mesh grid. The slope this produces is much smoother, and, in the project manager Steve Lowder's view, results in a much less natural appearance than with the Mowlem system.
'I think that by the time the vegetation has grown anyone driving past won't be able to tell they are man-made slopes,' said Lowder.
While construction has progressed well, there could be delays ahead. The process is weather sensitive: 'If the moisture content of the fill is too high you can't compact it,' points out Lowder. With much of the work continuing until Easter next year, the weather over the next few months could be an important factor in finishing on time.
In addition to the extensive use of reinforced soil structures, this section of the M25 has another claim to fame. The 13km of sound barrier along either side represent one of the most sophisticated road noise repression systems so far constructed in this country.
Depending on the location, the barrier is either inclined at 10¼ to reflect sound upwards or vertical to reflect or absorb noise. All types are supported on steel posts, fixed in individual piled foundations up to 5m deep.
The barriers themselves, which reach a maximum height of 6.5m, make use of four different materials - precast concrete panels, solid timber panels, a proprietary absorbent panel system from Germany, and transparent tinted acrylic. These are used in different combinations according to the degree of protection from noise required at specific locations.
The barrier installation, at an estimated cost of 15% of the overall contract sum, is being carried out by Mowlem Civil Engineering, in conjunction with Radian Systems.
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