Deep Soil Mixing Ltd, a ground remediation and soil stabilisation specialist, has completed a slope stabilisation and ground remediation project in Mytholmroyd, West Yorkshire, for Calderdale Council, after an initial approach in early 2017 to put forward a scheme for the stabilisation of a hillside and asbestos fibre encapsulation.

"Taking the contamination out was a major issue so we worked to find a solution to lock it in. This was not simple though due to the terrain of the site - the slope angle of the failed material was 30 degrees but steepened up to 50 degrees in some areas and the area was 60m wide and extended up over 120m of the slope," said Robert McGall, managing director, Deep Soil Mixing.

Councillor Barry Collins, Calderdale Council's Cabinet Member for Regeneration and Economic Development, said: "Storm Eva caused unprecedented flooding across Calderdale in December 2015. One of the major impacts was a landslip at Scout Road in Mytholmroyd, which we are working on as part of our ongoing flood recovery work. Before this permanent solution was put in place, the Council ensured safety at the site through continued monitoring and protection measures."

Using its controlled soil mixing technique, Deep Soil Mixing has over six months transformed the challenging site through the use of mass mixing using its Allu bucket on a 20t excavator.

The soil stabilisation process Deep Soil Mixing undertook on the hillside which covers approximately 3,500m² has proven to be an economical and practical solution - the soil had to be stabilised and the asbestos fibres contained in the ground locked in before Calderdale Council landscapes the site.

Prior to commencement on site, Deep Soil Mixing undertook rigorous testing and assessment of the ground to assess its suitability for soil mixing. Laboratory tests were also undertaken using samples of the soil to be treated mixed with different proportions of a wide selection of binders to select the most appropriate mix. From these results, Deep Soil Mixing was then able to prepare a detailed specification.

"The target strength was 175kPa but we achieved up to 250kPa in places," says McGall.

Deep Soil Mixing also installed a King Post Wall at the bottom of the affected area to create an anchor point for the stability of the slope. This was to ensure that any slips during the soil stabilisation work on the upper slope would be contained and would not affect the surrounding buildings and services. It also created a stable working platform from which the mixing operation could commence.

The King Post Wall extended to 6m below ground and socketed into the underlying mudstone bedrock and had a retained height of 1.5m to 2m.

"The wall was formed using soil mixing with steel sections placed into the columns to form the king posts rather than coring to avoid the need for material to be removed from the site," said Deep Soil Mixing director Colin Critchlow.

The slope had to be mixed in very carefully designed cells because of the risk of further slippages and the stabilisation process was carried out in rows, the rows were 60m wide, 4 to 5m deep and each cell covered a 6m section of the row width. In total 15 rows were used to treat the slope. This meant that precision was key during the stabilisation process and detailed records and daily monitoring was required to ensure the health and safety of the workforce and the neighbours was ensured. 

The workforce had to be fully asbestos certified to work with and dispose of asbestos to commence work upon the site, and this ensured some tricky working conditions with full face masks and disposable clothes, as well as decontamination when exiting the working area. This meant that the project was extremely challenging both from a health and safety perspective and on a technical level.

"Soil mixing proved to be an ideal solution for this slope stabilisation project, but it can also be used in a wide variety of other applications," said Critchlow. "In addition to the environmental advantage, stabilisation of soft soils by adding binders to reduce settlements and/or improve the stability of the land can be both a quick and cost-effective solution compared to some traditional method of piling."

As well as being suitable for some contaminated ground, if developers have a site with soft ground being present including peat and alluvial deposits, glacial till, bogs, etc soil mixing will improve the soil using the soil mixing system.

By making use of the existing ground and improving it to achieve a specified bearing capacity and shear strength, there is no need to excavate, cart away or dispose of the existing material off-site and then bring in costly and bulky fill materials. It is not though, just about improving unsuitable ground but aims to turn poor quality soils into an acceptable foundation bearing strata.