Knowledge Compendium

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Tunnel Vision

Date: 27/10/2011

In order to min­imise the dis­rup­tion caused by road­works and to sup­port the High­ways Agency’s (HA) objec­tive of ensur­ing reli­able jour­neys, great efforts are made to carry out works when traf­fic flows are low.

Dart­ford Cross­ing forms a very strate­gic part of the road net­work on the M25 in the south east of Eng­land. The Cross­ing was designed to han­dle up to 135,000 vehi­cle move­ments each day.  But, it is not uncom­mon for more than 170,000 vehi­cle move­ments to take place in one day.

Any dis­rup­tion on the south­bound bridge or the north­bound twin tun­nels causes traf­fic to build up rapidly there­fore main­te­nance work is con­strained to a small work­ing win­dow which is at most between 20:30 and 06:00, but cer­tain times the hours of work­ing are more restricted

In light of this, there were con­cerns about the poten­tial dis­rup­tion dur­ing a planned resur­fac­ing and water­proof­ing of the tun­nels; the Agency there­fore worked with the Ser­vice Provider and their Con­trac­tor to look at what could be done to speed up the deliv­ery of the works.

A typ­i­cal sequence of works for such a project con­sists of set­ting up traf­fic man­age­ment and diver­sion routes, remov­ing the sur­fac­ing, sand asphalt, water­proof­ing, inspect­ing and repair­ing the deck slabs, seal­ing the joints, reap­ply­ing water­proof­ing, lay­ing the new sur­fac­ing, lane mark­ings and then remov­ing traf­fic man­age­ment. When this sequence of work was analysed the team realised that it would need a num­ber of road clo­sures that could not be car­ried out within the restricted work­ing hours, because the mate­ri­als the con­trac­tors planned to use needed a longer period of time to cure. A clo­sure of the tun­nel would require sig­nif­i­cant advance notice dur­ing which time dete­ri­o­ra­tion of the sur­face would con­tinue com­pro­mis­ing the integrity of the structure.

Advance dis­cus­sions to con­sider options for reduc­ing the time spent on the resur­fac­ing and water­proof­ing were held with the Con­trac­tor, Con­nect Plus and HA Net­work Ser­vices Divi­sion. The prob­lem was the phys­i­cal para­me­ters of the prod­ucts used. The mate­ri­als needed time to cure and this meant that it was not pos­si­ble to work within the allow­able timeframes.

The con­trac­tor pro­posed a num­ber of options, one of these was using a water­proof­ing prod­uct (known as Flex­i­plast) which is laid by tankers fit­ted with a spray bar, each pass cov­er­ing a full lane width and which had a cur­ing period of 60 min­utes; this was con­sid­er­ably less than the con­ven­tional sys­tems. As this prod­uct had never been used before as a water­proof­ing on a high­way, the Agency had to iden­tify and con­sider any asso­ci­ated risks, i.e. that the prod­uct may not be effec­tive, it may not be achiev­able to install and/or it may not be durable.

To address these issues, High­ways Agency’s Net­work Ser­vices Divi­sion agreed require­ments with the con­trac­tor to deter­mine per­for­mance require­ments and quan­tify those risks. The first stage was to estab­lish if it was phys­i­cally pos­si­ble to carry out the work. A series of tri­als were agreed to repli­cate the pro­posed work. These demon­strated that the sys­tem could achieve the contractor’s programme.

The sec­ond stage was to ensure the prod­uct was still effec­tive dur­ing cold weather after being treated with salt, but also that it was able to still per­form dur­ing extreme tem­per­a­tures. Salt can cause a struc­ture to dete­ri­o­rate there­fore it needs to be pro­tected by water­proof­ing, but the water­proof­ing agent needs to still be effec­tive once in con­tact with salt. A nor­mal assess­ment requires a series of tests and tri­als but it was not pos­si­ble to do these in the avail­able time. There­fore Net­work Ser­vices staff exam­ined test data from other sites in Europe and looked at the per­for­mance. These demon­strated that the prod­uct was durable but it had not been used on a road. After dis­cussing options it was iden­ti­fied that Potas­sium Acetate could be used as an alter­na­tive to salt. In addi­tion a com­pre­hen­sive cor­ro­sion mon­i­tor­ing sys­tem would be in place.  The con­trac­tor had a num­ber of sen­sors inside the tun­nel and was able to demon­strate that the tem­per­a­ture inside did not vary as much as that out­side and that the lim­ited test data from this prod­uct man­u­fac­turer cov­ered this range.

Both tun­nels were laid with this new prod­uct in sec­tions of up to 100m length. It was com­pleted on time and to bud­get and was fea­tured in “New Civil Engineer”.

The con­trac­tor agreed to an inspec­tion and report­ing regime to mon­i­tor per­for­mance and allow HA Net­work Ser­vices staff to assess its suitability.

With more test­ing, this prod­uct may be suit­able for gen­eral use but it does demon­strate that iden­ti­fy­ing and assess­ing risks allows more inno­v­a­tive prod­ucts to be considered.

For fur­ther infor­ma­tion, please con­tact Aqdas Malik, Senior Struc­tures Advi­sor High­ways Agency