Highways Agency - Safe roads, Reliable journeys, Informed travellers

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