The A14, a two-lane concrete constructed dual carriageway near Stowmarket was at the end of its design life. The east bound carriageway was re-constructed in 2023 but issues with failing subgrade hindered the project, therefore National Highways, John Sisk & Son Ltd and NORSE Highways (A local authority trading company covering Norfolk and Suffolk) decided to look at alternative options for the west bound carriageway reconstruction.
Combined Soil Stabilisation were contacted by NORSE Highways to assist with obtaining a departure from standard to undertake an alternative subgrade improvement technique currently not included in MCHW series 600. This would be to stabilise the subgrade to prevent deterioration due to construction activities. Following initial discussion on the feasibility and material types, a design mix was agreed in line with a CL840 Soil Cement HBM, a much higher requirement than normally required for similar works but as this was a major highway scheme looking at new techniques, we had to ensure all based were covered. Tight timescales were required due to programmed start dates and due to a relationship formed when CSSL delivered the 22km Norwich Northern Distributor Road, Norse recommended that John Sisk & Son Ltd the main contractor for the scheme engage with CSSL to ensure successful delivery of the scheme due to our extensive knowledge and expertise in stabilised road foundations.
Once the design results were received and assessed CSSL proposed the cement addition and upper and moisture contents for the soils and worked with NORSE, John Sisk & Son Ltd and National Highways to obtain the departure from standard and agree the specification for the works. The subgrade required a 51MPa surface stiffness, that had to be retained following site traffic, construction activities and adverse weather to ensure the minimum stiffness was achieved at time of the following layer being placed. Consideration for the sequencing of work was critical, due to it being next to a live carriageway and that the two-lane bypass had to be constructed so that access for construction, delivery vehicles and site operatives could always be retained. Therefore, one lane for each visit was required ensuring that no joints would fall under the wheel paths.
Due to a very tight programme two teams were mobilised for visit one which was successfully undertaken in January 2024. Rhino / Steumaster (extremely accurate self-propelled spreaders), WR240i Wirtgen state of the art mixers CCC 20t rollers and total station-controlled dozers were utilised to deliver the works. Due to the proximity to a live carriageway a dust free option was required this was covered by the Wirtgen mixer units (which CSSL operate 12) which all have the capability to operate integrated should dust become an issue. Part way through the works a meeting to discuss the works and site visit was arranged with National Highways to inspect the works. Feedback from John Sisk & Son Ltd and National Highways was excellent and the hope this technique will form part of the MCHW or DMBR in future editions, which will remove the requirements for a departure going forward.
For any HBM, Stabilisation or just Soil modification improvement works please contact us on sales@combinedssl.co.uk or call on 01204 674104.
