The steep angle of the slopes within Dinah’s Hollow and the nature of the landslides has led to soil nailing being deemed the best solution to prevent further damage to the hollow and safeguard vehicles passing through it.
The majority of the angle of the slopes ranges from 60 degrees to 70 degrees, with the upper parts slackening off to between 45 degrees and 30 degrees. A small section of the eastern slopes towards Cann Common are known to have been previously regraded to an angle of around 30 degrees.
This steep angle, combined with poor drainage, has led to landslides generated from the back of the slope.
The real possibility of prolonged and intense rainfall resulting from climate change could cause a major collapse.
Soil nailing technique
Soil nailing is a ground stabilisation technique where steel nails are inserted into the slope face and then mesh attached to the steel nails to hold the soil in place. This method has been used successfully in other locations in Dorset:
- to stabilise the slopes surrounding the A3066 road tunnel in Beaminster following a landslip in July 2012
- as part of the East Cliff Stabilisation Programme in Lyme Regis in 2013, and
- used in 2023/24 for the Sturminster Newton Embankment Stabilisation.
The outline Dinah’s Hollow scheme design includes:
- soils nails arranged in a diamond pattern (typical spacing of 2.4m horizontal and 1.2m vertical)
- soil nails ranging from 5m to 9m in length
- a high tensile flexible facing system comprising steel wire mesh with associated nail plates, with steel wire anchor ropes and fixings as required
- high containment kerb and a hard slope facia system with a combined typical height of 1.5m, running along the toe of the slopes
Openings will be created within the mesh for identified existing trees and for additional planting. Sections of the slope will also be re-profiled, where appropriate.
When stabilisation is complete, it is planned to keep single-lane running for traffic within the hollow, with carriageway edge lining keeping vehicles in the centre of the road.
Bioengineering
Bioengineering techniques have been assessed for their suitability as a solution for stabilisation of Dinah’s Hollow.
These techniques address surface stability issues and are typically anti-soil erosion solution, where plants are established to protect the surface soil particles from being removed by impacting rain drops, sheet flow, river flows and waves. This planting slows the water and thereby reduces the energy down to a level that does not erode the soil.
Although there are many techniques to suit the many different site conditions and style of result desired, when considering bioengineering solutions, the geotechnical stability of the soil must be established.
Plant shoots benefit the soil surface very quickly, but their roots take time to grow to an architecture and depth where they might be perceived to have a benefit. This means that their effect can only be assumed to be in the first 200mm to 400mm (depth).
At Dinah’s Hollow, ‘failures’ are not surface slippages but have been generated from the back of the Hollow’s slope (slope plane). This means that any bioengineering solution would still need to be accompanied by civil engineering to address the deep slope failures.
Electrokinetic engineering
Electrokinetic techniques have been assessed for their suitability as a solution for stabilisation of Dinah’s Hollow.
Electrokinetic strengthening of slopes uses electro-osmosis to transport water through fine grained soils with low hydraulic permeability, such as clays or cohesive soils (fine-grained, low-strength, and easily deformable soils that have a tendency for particles to adhere).
The process does not work in coarse grained soils, which generally have a relatively high hydraulic permeability, such as sands.
The geology of Dinah’s Hollow has been mapped as the Shaftsbury Sandstone Member passing into the Cann Sandstone Member at the lower, southern end of the hollow, both of Cretaceous Age. These geological materials of Dinah’s Hollow comprise weak to medium-strong medium grained sandstone, weathered to a silty sand.
As such, this material is predominantly a coarse-grained soil and is therefore not suitable to be treated using electrokinetic techniques.
It should be noted that strengthening suitable slopes using electrokinetic techniques requires the insertion of anodes and cathodes, which to operate at safe operating voltages need to be installed at close spacings.
These spacings may be closer than those required for a soil nail and flexible facing system and would still require the removal of trees and vegetation to insert anodes and cathodes.
There is also limited evidence on the long-term performance of electrokinetic systems and no examples where this system has been used on slopes with gradients similar to those found at Dinah’s Hollow.