- 30th November 2018
- Posted by: Sophie Smith
- Category: Projects
Torrential rain on Boxing Day 2015 in the aftermath of Storm Eva caused a landslip at Scout Road in Mytholmroyd in the Upper Calder Valley. It blocked the road and led to the temporary relocation of Scout Road Academy and the evacuation of nearly 20 homes for safety reasons.
The video below gives an insight into the disruption caused.
The woodland area affected by the landslip is known as Hathershelf Wood, where historically sandstone was quarried. More recently, the quarry basin formed a landfill site containing asbestos, which Calderdale Council took ownership of following its closure in the 1970s.
As part of its flood recovery work across the borough, Calderdale Council started a programme of repairs to help get life back to normal for local people as soon as possible, and to increase resilience against any future landslips. Safety was the priority, and the steep, unstable slopes and exposed asbestos received immediate attention.
Following inspections to assess the extent of the instability, tree felling was carried out to enable safe access for ground investigations and technical surveys. Options for the permanent remediation of the hillside were outlined. Due to many constraints on site, including asbestos contamination, slope instability, limited access, narrow country roads and working in close proximity to a school and the general public, the options were limited.
The permanent works option had the following criteria to meet:
- Provide long term stability to the slope and remove the groundwater and surface water issues that caused the mechanism of failure.
- Avoid the risk of disturbing the asbestos within the landfill area and minimise the risk of disturbing the asbestos within the landslip.
- Provide a capping to the slope and/or make the asbestos immobile.
- Carry out remediation work using small plant and machinery capable of working with limited space and access, or alternatively, by strengthening working platforms.
- Remove the scar left by the landslip quickly to re-establish the natural landscape.
Temporary works – to protect the public and residents
A ground investigation including soil nail suitability tests were completed in Spring 2016 to inform detailed design of both potential permanent and proposed temporary works. It also re-establishes an array of monitoring installations (inclinometers and standpipe piezometers) across the site.
In order to allow the school to re-open, the site needed to be geotechnically and environmentally stable. A landslide barrier needed to be installed to protect the public against further landslip material reaching the highway. The installation of a temporary environmental barrier was also required where asbestos had been exposed. Initially, a route had to be worked onto the site as none existed and not all the land was owned by the local authority.
Traditional engineering works to remediate landslides usually involve earthworks and substantial excavation to remove weak and unsupported soil. Due to the exposed asbestos within the landslip material, this form of treatment was not feasible. The risk of exacerbating the problem was high, and therefore a remedial treatment that provided encapsulation of the asbestos, as well as ground improvement of the weakened sub soils, was preferred to achieve the desired goals.
The cement stabilisation option, which involved mixing the weakened soil within the landslip zone with cement grout, achieved the required criteria and was the preferred option. It enabled the asbestos to be bound within the cement whilst also increasing the strength of the weakened soils within the slope.
Cement stabilisation was not feasible for the whole of the landslip area. King post and gabion retaining walls, drainage and soil nailing were also carried out during enabling, temporary and permanent work stages.
Site trials and laboratory testing
To achieve the best design mix for the cement stabilisation, and to ensure the working methods were suitable given the many constraints on and surrounding the site, it was necessary to carry out a site trial on a small section of the affected area. At the same time samples were taken of the material so that laboratory tests could determine the best mix design for the cement stabilisation.
The landslip occurred in several locations across the upper and mid slope, but the cement stabilisation methods were not feasible on the upper western slope where instability was localised. At this location a soil nailing solution was preferred using lightweight plant and equipment secured with harness support. Approximately 500 soil nails were installed, securing slope stabilisation netting.
The main mechanisms of slope failure were:
- Groundwater issuing from bedrock within the mid slope
- Surface water run-off discharging over the high wall of
- Failure of the in-situ drainage.
These issues formed a major part of the temporary and permanent works with over a kilometre of new and improved drainage being installed. Ninety percent of the drainage network from the site to the River Calder outfall will also be upgraded. The final phase of this drainage work will be completed in 2019.
An important part of the project has been the landscaping and planting on completion of the stabilisation works. The first phase of this work involved the hydra seeding of the lower and upper slopes which took place in early September 2018. The photos show the landscaping six weeks after the hydra seeding.
The benefits this project has provided are:
- Improved protection for over 80 nearby homes and around 30 non-residential properties.
- Temporary works allowed the school to re-open and residents to return to their homes at the earliest opportunity.
- Permanent works have provided long term stability to the hillside.
- New and improved drainage has removed or controlled the mechanisms of failure (surface water run-off and groundwater issuing from the hillside).
- A geotechnically and environmentally stable hillside.
- Landscaping designed to blend in with natural features. Where hard structures are being retained, suitable screening has been introduced.
- Landscaping and newly planted bushes have brought more greenery to the area and will absorb water to improve flood resilience.