Insitu Stabilisation – HBM (AWCCT)

  • Client: KierWSP & Lincolnshire CC
  • Site: Green Lane
  • Road Classification: Class 4 (0.5msa)
  • Works Type:  In Situ Recycling
  • Material:  CBGM HBM – H2 Class
  • Depth: 240mm
  • Specification: TRL611


Green Lane was selected for a deep in situ reconstruction due to the significant amount of structural defects and the presence of coal tar bound material. The client wanted a long term and quick solution to the structural defects throughout the site.


A site investigation, laboratory testing and pavement design service was provided in accordance with TRL611. Through site investigation an area of clay bearing granular material was identified. Essentially two mix designs were established for the site. 1 Ordinary Portland Cement (OPC) design for the good quality granular materials and 1 Lime/OPC mixture to ensure performance and quality of the clay bearing material.

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A complication to the site was tar bound material in the upper layers. Due to the need to adhere to fixed levels on the site due to kerbing and drainage the site could not be built up to take account of the additional bulking of the material and new surfacing. To resolve this issue Allasso Recycling removed the upper layers containing coal tar and set aside. Then removed the inert material below and utilised this material for improvements to an adjacent bridleway. The coal tar material was then deposited back on the site with the levels reduced to take into account the new surfacing and bulking. This allowed the site to be returned to original level once the recycling and surfacing were complete

Process Overview

  1. Initial Planing to remove coal tar
  2. Planing to remove underlying inert material
  3. Reinstatement of coal tar material
  4. Lime Addition
  5. OPC addition
  6. Grading and Compaction
  7. Application of Curing membrane and Sealing grit
  8. Surfacing

Quality was monitored by an onsite technician to ensure compliance with the mix design. Works were completed under a road closure, with a running surface maintained at the end of every shift to allow access to the adjacent properties. Throughout the works access was maintained to properties through careful planning of the closure system.

Key Benefits

  • Sustainable construction
  • Coal tar encapsulation
  • Reduced vehicle movements
  • Significantly reduced cost
  • Improved solution for the use of Coal tar in the upper layers