CURRENT PROJECTSCanford Cliffs Stabilisation Works
A 9-month programme of stabilisation works to address the 2017 landslip below Cliff Drive will start in September 2020, following a period of extensive planning and monitoring.
24th April 2017
28th July 2020
Click to enlarge image
In February 2017 a section of cliff approximately 10m wide x 0.5m deep eroded and slipped at Canford Cliffs. There was concern the material might travel down to beach level, especially in the event of bad weather. Three blocks of beach huts were temporarily closed for safety reasons, along with a short stretch of undermined footpath on Cliff Drive. The programme of planned investment in new seafront infrastructure and beach huts at Canford Cliffs, due to start in 2017, was postponed. Since then, extensive planning and monitoring of the cliff face has taken place to understand how mobile it is and the best methods to secure it.
Specialist contractor, CAN Geotechnical has been appointed to deliver the £2.5m cliff stabilisation scheme which will start work in September 2020. It is anticipated to take around nine months, completing in Spring / Summer 2021. This is subject to weather conditions and government guidelines for working arrangements related to Covid-19.
Soil nails (up to 15m in length), will be used to secure the cliff. This method was previously used during the 1990s in this same area and regular monitoring has demonstrated the cliff has remained stable.
Cliff Drive temporary traffic & other arrangements
- A section of Cliff Drive will be closed to vehicles and on-road parking suspended for the duration of the works. Temporary traffic arrangements will be advertised in August; these arrangements will help ensure public and contractor safety and protect the special designation afforded to this area, a Site of Nature Conservation Interest.
- Access will be maintained for residents, emergency services and construction vehicles only.
- Pedestrian access to Cliff Drive will also be maintained, although the zig-zag path to the beach will be closed (Flaghead Chine and Canford Cliffs Chine pathways will remain open).
Beach hut tenants will be advised about the arrangements for accessing and using their beach huts during the period of works.
Project Progress 2019/20
- July 2020 Specialist contractor CAN Geotechnical has been appointed to deliver the cliff stabilisation scheme which will involve using soil nails (up to 15m in length) to stabilise the cliff face. Where possible, the works are planned to reduce the impact on beach users.
- January 2020 Vegetation was cut back to enable structural engineers to assess the condition of the cliff face and monitor its stability, helping with the final preparations for engineering works to stabilise the cliff. Cutting the vegetation close to ground level allowed us to complete wildlife surveys to ensure all protected reptiles and nesting birds are excluded from the proposed working area. Reptiles were moved to a pre-prepared ‘friendly’ habitat outside of the exclusion zone; once the stabilisation works are complete they will be returned to re-colonise the area.
- December 2019 / January 2020 Another section of cliff moved significantly to the east of the 2017 slip. This increased the risk of material piling up at the back of the beach huts which could be of sufficient force to cause a partial collapse of beach huts. Block number 6 was closed (along with Blocks 4 and 5) for safety reasons.
- April 2019 Borehole drilling on the promenade and beach hut terraces around block numbers 1–11 (borehole location map, pdf). This further ground investigation was required to provide the necessary information to complete the designs for cliff stabilisation, the new beach hut foundations and the retaining wall located behind the beach huts.
- March 2019 Cliff stabilisation works to go ahead. Geology reports having been reviewed and options considered, the Council has agreed a recommendation for programme to stabilise the whole cliff face at a cost of approximately £2.5 million.
Project Preparations 2017/2018
- December 2018 We cleared more vegetation extending eastwards towards the zig-zag footway and Holm Oak (which remains), enabling structural engineers to assess the condition of the cliff face and monitor its stability. This work reveals the full extent of the required cliff stabilisation work. Spraying and vegetation clearance continued throughout the winter months to allow wildlife mitigation. By cutting the vegetation close to ground level we provide light for the reptiles, areas for nesting birds and give the native heathers a chance to re-establish.
- During July 2018 Reptile fencing was used to create an exclusion zone, preventing reptiles from moving into the potential cliff stabilisation working area; those already in the area were moved to pre-prepared ‘reptile-friendly’ habitats outside the exclusion zone. Surveys continued until winter hibernation to ensure they had all been removed.
- 29th May 2018 Spraying cliff vegetation to allow reptile surveys; a reduction in thick vegetation will encourage protected reptiles to gradually move away from sprayed areas in preparation for the stabilisation works later in the year. Further spraying & cutting throughout the growing season as required.
- 19th March 2018 Drilling for soil samples up to a depth of 30m, over a period of 3-4 weeks. This work is part of the continuing ground investigation to help understand the geology of the site; the information collected will help inform what cliff stabilisation works will be needed in the future.
- 8th January 2018 Further cliff movement requires a safety zone to be set up on the promenade at the base of the area.
- 18th September 2017 Work began to remove vegetation cover from the site; bare ground allowed engineers to gain a better understanding of the condition of the cliff face during the monitoring period. This work was carried out in two stages to give the opportunity to move any emerging protected reptiles to the reptile-friendly landscape prepared for them on either side of the cleared area.
- 12th June 2017 We assessed the cliff stabilisation schemes that were implemented in the 1970s either side of the current slip before deciding what scheme is needed (and if we need to re-do the 70s work); photos below.
- 4th May 2017 Channel Coastal Observatory started their monitoring; photos below.
- 2nd May 2017 All the targets (45 in total, 3 rows of 15) are now in place.
- 24th April 2017 Work commenced on the installation of steel targets at regular intervals along and down a 300m section of cliff at Canford Cliffs. The monitoring of these targets will identify whether the cliff is moving or stable, and help establish what future cliff stabilisation works may be needed; photos below. Once a fortnight, over an initial three month monitoring period, a laser scanner read these targets to accurately measure their position, within a +/-2mm tolerance, to see if there was any movement. The targets were needed because the laser cannot ‘see’ through the vegetation to read the cliff face itself. The regional Channel Coastal Observatory conducted the monitoring using the same laser scanner they use to measure the level of Poole’s beaches for coast protection purposes.
June 2017: Assessing previous cliff stabilisation work
Exposing the previous stabilising system either side of the current cliff slip ready for close observation.
May 2017: Monitoring methods
1. Small markers (pins) are inserted into areas of hardstanding, such as the pavements / promenade to act as reference points
2. Scanner targets are set on the markers each time a new scan is undertaken, enabling the scan data to be coordinated to Ordnance Survey grid
3. During a survey, the scanner emits laser light which reflects off hard objects back to the scanner; enabling it to record data points in relation to itself up do a distance of 300m.
5. The data from each survey will then be compared to see if there is any movement on the box targets in the cliff, and hence the cliff itself
6. The laser scanning equipment used to do the survey can record information to within a few millimetres of accuracy
7. The scanner also has an in-built camera which takes photographic images of the cliff to add colour to the 3D dataset
April 2017: The slip and installing targets to monitor cliff movement
The area of cliff affected by the landslip is part of the Poole Cliffs SNCI (Site of Nature Conservation Interest) which is managed by the Council for it’s nature conservation interest. It provides an ideal habitat for the nationally rare Sand Lizard and Dartford Warbler which are both protected species, and other birds that might be nesting in the gorse bushes.
During the period of cliff stability monitoring it was necessary to clear cliff vegetation to enable structural engineers to assess the condition of the cliff face. At the same time reptiles were encouraged to gradually move away from the ‘exclusion zone’ to pre-prepared reptile-friendly habitats beyond it. Surveys were carried out until winter hibernation to ensure the zone was clear of reptiles in anticipation of cliff stabilisation works.
The Council’s biodiversity officer has been working, and continues to work throughout this project to protect both habitats and wildlife.
Historical cliff slips at Canford Cliffs
Over the past 40 years there have been three significant cliff slips at Canford Cliffs Chine. The first two happened during the 1970s and one of these resulted in a block of beach huts being demolished (these were only reinstated in 2014). The third significant slip took place in 1993.
Each of the slips at Canford Cliffs required stabilisation schemes including a system of transverse steel beams positioned down the cliff face and a series of soil nails to pin and hold a top section of the cliff.