Background

The M25 J19 off slip is located on the clockwise London Orbital Motorway and serves the northern part of Watford, Hertfordshire. Following inspections, the road had been identified as having longitudinal cracking and depressions in Lane 1 and Lane 2. The road section was widened in 2009 to provide additional lanes for the mainline All Lane Running Smart Motorway project.

Connect Plus Services, who manage the M25 and surrounding link roads, contacted Geobear to discuss options for providing a solution to their problem.

The existing pavement was a flexible composite pavement, and the new widened hard shoulder was a rigid pavement with an asphalt overlay. The interface between the two pavements fell under the left wheel track of lane 1 of the slip road. Cracking reflected in the wheel track, and after a few years, depressions started to broaden in Lane 1 and 2. As the cracking opened up, more surface water could enter the pavement foundations, exacerbating the problem. After multiple resurfacing interventions, a solution was required to treat the cause of the problem.

Pavement and geotechnical investigations identified a weakness in the sub-base, and micro voiding was likely present at the interface between the two pavement foundations; the pavement had also settled by up to 26mm in places. So a solution was required that stabilised the pavement foundations to stop any further movement and also relevel the pavement back to its original levels.

 

Customer Alternatives

The client had previously undertaken numerous resurfacing interventions which hadn’t resolved the issue; therefore, a full pavement reconstruction would have been their only alternative. This would involve excavating and replacing hundreds of tons of pavement foundation and asphalt, resulting in a slip road closure for several weeks. Due to the high frequency of traffic at this location, the closure was not an option for the client, an alternative solution that minimised disruption had to be found.

 

Geobear solution

Geobear proposed two processes for the remediation of the slip road. Firstly, stabilisation of the pavement foundation, followed by a releveling treatment. Geobear contracted a surveyor to mark out the site per the client’s design level data; A 1.0m x 1.0m grid was marked, and existing levels were taken to ensure the required levels would be met during the releveling phase.

The site technicians drilled each point on the 1.0m grid to a depth of 0.9m using rig mounted vertical drills to eliminate HAVS. A 12mm steel tube was then inserted into each hole to depth. The technician then injects a structural geopolymer to permeate into the sub-base. Predetermined trigger times and monitoring is utilised to control the process. Laser-levelling equipment with an accuracy of +/- 0.5mm was utilised to detect ground movement, thus ensuring the material had consolidated the sub-base without causing lift to the pavement surface at this stage.

Following the stabilisation, the releveling phase of the project could begin. This involved injecting a geopolymer at the interface of the sub-base and underlying clay layer. This provided a platform for the expanding geopolymer to lift the pavement which was undertaken in lifts of up to 10mm per injection phase. Releveling injections would start in the areas requiring the most lift and then work outwards to complete the lifting process. Some areas would require multiple injections to meet the design levels. The laser monitoring equipment, coupled with the on-site surveyors meant the releveling process was monitored in real-time.

Following the completion of the works, the surveyors compiled the releveling report which was issued to the client.

Due to the busy location, the working windows meant that only 5 hours were available per shift. However, Geobear managed to complete the works in 5 shifts and the client resurfaced the road following our works.

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