Scarborough Spa Coast Protection & Slope Stabilisation Scheme NOVEMBER 2016
Contents • Context • Slope Stabilisation, Geology and Engineering • Landscape Management • Ecology • Trees • Cultural Heritage • Noise, Vibration, Dust • Access • Next Steps
Slope Stability The South Cliff upon which the proposed works area lies is inherently unstable. As a result the area has witnessed major landslides, including in 1737, when the cliff collapsed destroying the spa buildings. In 1993, approximately 1km to the south of the proposed works area, the Holbeck Hotel was destroyed with a substantial part of the Holbeck Gardens removed also. The cliff retreated 70m inland, whilst the mud/debris flow overran the sea wall to project 135m onto the beach. Numerous smaller scale landslides have occurred within the Spa Gardens, resulting in the closure of footpaths. Furthermore, investigations in the 1970s and 1980s found that several locations in The Spa Complex were being adversely affected by earth pressure and soil movements, probably caused by water pressure. Wells were sunk to manage water pressure, but these were found to be no longer effective in the 1990s.
History of slope failure at Scarborough Spa A A The north east coastline of England has a history of slips. Many people will remember the famous landslip that caused Holbeck Hall to fall into the sea. However, the particular mechanism of how the coast slips depends on numerous factors. This includes: • If the area has previously slipped • The local ground conditions (or geology). • The depth to rock head or competent material • The local ground water conditions At Scarborough Spa, deep seated slips occur along the top of the bedrock (sandstone) through a thin layer of weak sandstone. The engraving above , is of a large slip that destroyed the original spa building in 1737. By considering the history, geology and groundwater conditions at the site, we can develop an engineering solution to reduce the risk of future slope failure. Deep seated multiple rotational landslide mainly developed in till Coastal slopes developed in glacial till and underlying bedrock. Currently affected by small-scale shallow landslides
Risk of Slope failure Esplanade This poster shows a section through the Spa Building, running down from the esplanade to the Spa Building. We have carried out ground investigations Spa Building and conducted engineering analysis that has helped us to quantify the risk of slope failure behind the Spa Building. Our analysis has shown there is risk of two types of slope failure in this particular section: The first is a deep slip occurring from the Esplanade, passing beneath the Spa building and surfacing in front of the building but behind the sea-wall (See the red line on the Image to the right). The second is shallow slips occurring from the mid-way down the slope and surfacing in the bottom third of the slope above the lower paths (See the blue line on the image above the deep slip line to the right ) In addition there is also the risk of lots of very shallow slips occurring at the surface.
Proposed Solution: Deep Slips To resist the deep seated slip (shown in Red) it is proposed to install a number of steel piles embedded into the bed rock below (Sandstone). The piles terminate in the Sandstone as it is a strong competent layer that slips cannot easily pass through. The steel piles prevent slips occurring above the Pile to resist deep Sandstone by reinforcing the soil in that area. slips The piles will have a diameter of approximately 0.4m (or just over a foot). The piles will be installed by widening the existing path to make room for a piling platform. To ensure the path is stable during construction the piling platform will be constructed using soil nails above and below the path, and a shotcrete (sprayed Soil nails in lower concrete) face. After construction, the slope to allow path will be returned to its original width. This will be achieved by placing installation of piles engineered fill in front of the shotcrete face and then finishing off with a traditional stone wall.
Proposed Solution for Shallow Slips The steel piles are installed deep into the sandstone to arrest the risk of deep slips, however on their own they do not prevent all the possible slips. There is still the risk of shallow slips occurring between the top of the slope and the piles (as shown by the blue line). To resist the shallow slips above the piles it is necessary to install soil nails above the pile and re-grade the soil profile. The soil (temporary nails penetrate potential slip circles, by acting like reinforcement elements. construction For the soil nails to work they must be installed in conjunction with a facing system. The facing system platform) consists of steel head plates and a steel mesh, which help to transfer the weight of the soil between the nails back into the soil nails. Soil nail heads and mesh can look unsightly. Therefore they have been designed with a covering (geo-cell) that will allow topsoil to be replaced and planted with seeds – see the later soil nail head detail. As a result, a short time after construction the topsoil will ‘green’ up and the soil nails and topsoil containers (geo-cell) will not be visible. Finally, between the ground and the steel mesh an erosion protection mat and a drainage layer is installed. These will help to prevent surface ground water eroding the soil and reduce the water within the slope, thus improving the stability.
Creation of temporary construction platform and soil nail installation Precise placement of nails along the length of the construction zones using anchored long arm reach excavator plant
General Arrangement The Proposed Development for which planning permission is sought comprises: “ Slope stabilisation scheme including clearance works to remove existing trees, regrading of existing slope and use of soil nailing and soil piling measures, retention and reinstatement of masonry walls, installation of new drainage measures and subsequent landscaping works.” • Soil nails – to sufficient length (between 2.5m to 25m) to extend past the active slip plane and provide shear resistance to increase the factor of safety against shallow failures. The spacing of the soil nails will vary from 1.5m to 2.5m horizontally and 1m to 1.5m vertically. • Nail Heads – to fix the head of the nail in place and hence reduce local surface instability between nails. Nail heads will be of sufficient size and thickness to ensure adequate load transfer to the nails. • Mesh throughout area – laid beneath the nail heads to help with erosion control and establishment of vegetation on the slope. • Top soil retention – where required, a geocell system will be used over the top of the mesh to retain topsoil in place and hide the soil nails and head plates.
Zone 1
Zone 2A
Zone 2B
Zone 3
Retained Path Network Following the completion of the engineering activities a scheme of landscape reinstatement works would be implemented to ensure that the intrinsic character of the public space is retained and enhanced in the long term. Dense woodland would be removed to create an open parkland space from which views of the Scarborough coastline can be enjoyed. The network of footpaths would be rationalised and reduced in overall extent. Any perceived adverse effect that would arise due, primarily, to the removal of a large number of mature trees, would be offset by the creation of a more diverse and attractive garden space that would have a strong relationship with the Spa buildings and the wider public spaces of the South Cliff and Valley Gardens. The site would lie at the heart of possible future Heritage Lottery funded South Cliff and Valley Gardens scheme and could complement the future aspirations for the comprehensive restoration and enhancement of the whole of the public gardens.
Soil Nailed Wall Retention of existing walls Existing masonry walls will be retained if at all possible. In locations where they are not damaged and there is not considered to be a long term risk associated with keeping them it is considered that they should be preserved. Where new masonry facing is required we will design replacement masonry walls that will be sympathetic with the appearance of the existing walls. Soil Nailing Soil nailing will be adopted in areas within the gardens where the slope angles are considered significant and in line with recently completed study and recommendations made on slope stability.
Nail Head and Vegetated Facing
Masonry Walls and Soil Nails
Soil Nail and Re-grading The Finished Look Slopes LESS THAN 45 o Slopes GREATER THAN 45 o
Arboricultural Survey As proposed Survey as existing Detailed tree and vegetation survey undertaken to establish: • Vegetation structure • Species composition • Quality of trees Overall the survey was able to demonstrate the poor quality of the tree and vegetation coverage. We will clear the site of this poor quality vegetation – with the exception of a handful of higher quality trees Phased site clearance is planned in January 2017 and is expected to be completed before the beginning of the nesting season in March 2017.
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