Sighthill TRA Stabilisation

The site had been subject to upfilling with materials being deposited from the development of the M8 motorway, but more synonymously with Sighthill, waste from the former St Rollox Chemical works which produced nearly 1 million cubic metres of a by-product termed galligu. Galligu is manmade and does not conform to any classification (geo-technically or chemically) as it is variable in nature, colour, properties and composition as the process from which it came was developed over time and was not consistent.

The identification of galligu was fundamental to the successful remediation as this material could impact on ground water and potentially generate hydrogen sulphide gas. A major consideration throughout the project was the management and reduction of odours associated with H2S.

Due to varying periods of development, which included parkland and residential development, the conceptual model was extremely complex with material having been historically imported, relocated, mixed and exposed to different elements over time. The solution was to develop a strategy allowing different materials to be placed in different locations both on plan and in elevation. In some cases, these materials were treated to allow them to meet criteria. This required a significant pre-classification exercise with extensive planning, materials management, tracking and verification.

VHE introduced and employed combinations of techniques to maximise the reuse of materials on site through recovery and revival dealing with, above others, a material that is unique to the location and history of the site. This was undertaken around two primary schools and a recently completed housing development while accommodating other separate contracts through giving access, temporary possession and phased handover.

With historic odours being an issue and with the proximity of the schools, residents and adjacent businesses environmental management and monitoring were huge undertakings with over 10,000 readings taken for hydrogen sulphide in addition to dust, noise and vibration.

The bespoke nature of galligu resulted in huge risk to the project but VHE’s resourcefulness, experience in ground modeling and maximising material capability developed a solution to manage the materials in order of hierarchy. Managing the most critical material resulted in the generation of unsuitable material from a former tip which VHE put together and executed a process of material recovery maximising its value, suitability and benefit of reuse.

Stabilisation was undertaken on materials via two separate processes.

Contaminated material was stabilised by precise dosage using a tractor with front mounted spreader and rear rotovator in a purpose built bay enabling the treatment of 9,300m3 of soils.

For moisture content modification, VHE used a Stehr Soil Stabilisation SBF 24-6 dust free stabilser to maximize treatment of 64,000m3 of Galligu and other cohesive soil. This generated an engineered cohesive Class 2 sub-formation material as low as 10-10.

Complex segregation across the site including processing a former landfill using non-conventional plant, mechanical product selection, hand picking and sorting, rescreening crushing with reuse as biomass and recycled metals resulted in 99% recovery against a zero waste ideal.

The overall solution to encapsulate the galligu in-situ using a slurry wall, which was up to 20 metres in depth and created using a 135 tonne long reach excavator, with 120,000m2 geocomposite clay liner avoided the removal of 700,000m3 of material. The discovery of additional previously unidentified galligu was managed on site to avoid any disposal with the management of over 1,200,000m3 material movements.

VHE worked closely with regulators throughout the project to develop acceptable solutions to issues as they arose and with the client in terms of their expectations for future development. VHE had to consider cultural heritage management and mitigation in terms of archaeology and historical stone circles that would be reincorporated into the final development and ecology matters also required significant stakeholder co- ordination.