Few slope failures call for the emergency response required for one in West Yorkshire where the debris contained asbestos fibres.
The 2015 Boxing Day floods resulting from heavy rainfall from Storm Eva caused devastation across Yorkshire and Lancashire and for most it is now just a memory. However, for residents of Mytholmroyd in West Yorkshire, the final repairs to a landslip are only just being completed more than two years later.
The failed material that covered Scout Road after the slope failure in December 2015 was waste dumped illegally – probably in the 1960s or earlier – in a former quarry and the ground movement exposed blue asbestos contamination. Remediating the slope and managing the contamination called for a novel solution and careful environmental management from ground improvement specialist Deep Soil Mixing (DSM).
scout road stills 1
Immediately after the landslide, Calderdale Council, which owned most of the land, covered the site while plans to manage the problem and clear the road were developed.
While the road was reopened in autumn 2016, the work to stabilise the slope and contain the contamination has only just being completed.
“Taking the contamination out was a major issue so we worked to find a solution to lock it in,” says Deep Soil Mixing managing director Robert McGall.
This was not simple though due to the terrain of the site – the slope angle of the failed material was 30° but steepened up to 50° in some areas and the area was 60m wide and extended up over 120m of the slope.
DSM was first approached by consultant JBA, which was working for the council, in the spring following the incident and asked to look at potential remediation options.
The council tendered the solution put forward by DSM to its framework contractors, which all then turned to DSM as a subcontractor. DSM has just delivered the scheme for Hall Construction with design support from Byrne Looby.
The solution involved construction of a king post wall using soil mixed columns at the bottom of the council-owned land to create a shear key that would provide a platform to sequentially soil mix the slope and lock in the asbestos material.
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 the removed from site,” says Deep Soil Mixing director Colin Critchlow.
Mass mixing, wet mix techniques were used for the slope itself. “The wet mix avoids creating airborne fibres,” adds Critchlow.
The slope angle and access were not the only challenges – the contamination meant that all staff working on the scheme had to undergo a full medical and Cat 3 asbestos training before starting on site and use full protective equipment the whole time they were working.
“Staff could only work for four hours wearing protective equipment before they had to come off to be decontaminated, have a break and eat, before re-dressing and working another four hour shift,” says Critchlow.
“All the plant had to be decontaminated at the end of the project too. Filters had to be changed and the machine had to be steam cleaned three times before it could be moved off site.”
DSM undertook some trials on site to test both the mix design and to demonstrate that the solution was viable.
“We brought in a smaller machine, pumping equipment and grout to demonstrate the mix and understand how quickly it would go off,” says McGall. “We undertook laboratory tests before moving the trial onto site.”
The final mix used a cement base powder with additives to improve workability but achieve a high early strength.
“The target strength was 175kPa but we achieved up to 250kPa in places,” says McGall.
In total 5,200m3 of material was treated during the soil mixing process.
The stabilisation process was carried out in rows, with each row split into cells.
The rows were 60m wide, 4 to 5m deep and each cell covered a 6m section of the row width.
“The cell size was based on the reach of the excavator arm,” says Critchlow.
The mixing was undertaken using a four shaft mixing bucket from Allu mounted on the arm of a 25t excavator.
“The slope and size of the benches limited the size of machine we could us,” adds Critchlow.
Each cell did not treat all the waste material but created an arch shape that locked the contaminated material in by keying to the cell below and the bedrock.
In total 15 rows were used to treat the slope.
Some of the concern during the concept stage was that the work might trigger further movement on the failed slope but only 3mm of movement was recorded during monitoring. The other issue was the high forces placed on the king post wall during the early stages of the work but monitoring of the structure showed movements of less than 5mm.
With the main stabilisation works now complete, Hall Construction’s work on soil nailing, installation of drainage and tree planting got underway at the start of the year and was expected to be completed in mid-April.
McGall says that the work was challenging from an environmental perspective, but less so from a technical standpoint as he says that DSM has undertaken slope stabilisation work using the techniques before. Nonethless, both Critchlow and McGall believe that the knowledge gained on the Mytholmroyd site could be used in a benefit other projects in the future.