Leeson Road landslip FAQs

It is necessary to close Leeson Road for up to 12 weeks to drill new boreholes to understand what is happening in the layers of ground below the road. New monitoring equipment will also be installed that will detect movement in those layers. The monitoring will be linked to an early warning system to ensure that if there are indications of another landslip, the Isle of Wight Council and Island Roads can shut the road for the safety of the travelling public if needed. The Isle of Wight Council and Island Roads remain committed to being able to re-opening Leeson Road as quickly as possible and keep it open in the long term.

Three boreholes are going to be drilled on Leeson Road to a depth of 130 metres. Two are going to be close to the site of the landslip in December 2023 and the third will be situated slightly further south towards Ventnor. The three boreholes will host two inclinometers (measuring ground movement) and a series of piezometers (measuring groundwater) and are going to serve two key purposes. The first is to sample, investigate and log the layers of ground below Leeson Road. The second purpose is to use the boreholes to house monitoring equipment throughout the layers and depths of the boreholes and to return continual data that indicates types of underground movement, groundwater levels and pore-water pressures. The boreholes will serve to provide additional geological information for other technical consultants and organisations that wish to use new data to inform studies on further work on the St Lawrence and Bonchurch landslide areas.

The closure is currently set for 10 - 12 weeks, this includes time for the drilling to take place and for the monitoring to be installed as well as allowing for any issues arising during the drilling operations. The drilling teams will be working weekends to try and ensure that the works can be completed earlier. When the first set of data has been analysed the council and Island Roads will be able to decide if it is possible to reopen the road. The movement monitoring and sampling are key to inform a decision to reopen the road.

Leeson Road is between 5.8 metres and 6.2 metres wide in the area where the drilling will take place, the size of the drilling machinery is 2.5 metres wide and needs 2 metres either side of the machine for it to be operated. This is 4.5 metres in total. There also needs to be a 0.5 metre safety zone around all of the working area taking the minimum amount of road needed to 5 metres. The minimum road width that is required for vehicles to travel safely is 3.0 metres. With the available road width, the amount of room required for the drilling machinery and the safety zone there is not enough room left on the road to allow any vehicles through the closure.

The work will be carried out 7 days per week whenever it is possible. The time it would take to set up and take down the site every night would shorten the available time to drill and lengthen the overall works time that the Isle of Wight Council and Island Roads have committed to keeping as short as possible so it is not efficient to try to reopen the road at night.

The plant and equipment required to complete the boreholes is substantial and therefore the site will be closed to all vehicles including the emergency services during these works. Island Roads have informed the emergency services of the closure and advised them of the diversion route during the closure period.

The regular monitoring of all the surface devices (inclinometers and extensometers) have been returning data that has continued to show various levels of movement of up to 20mm in the area. The varied movements recorded clearly demonstrate that the landslip area is still moving all of the time and requires the additional monitoring that the council and Island Roads are putting in place to ensure public safety.

At a meeting on Wednesday 4 September 2024 with councillors, geologists, and residents the Isle of Wight Council and Island Roads were asked to consider the possibility of making the diversion routes around the Leeson Road closure and in particular in Wroxall and Newport Road less congested and easier to travel along. Following on from this request, there will be a series of temporary on-street parking suspensions at various points along the route in Wroxall and a complete parking suspension will be introduced along Newport Road between Lowtherville Road and St Margarets Glade. Residents currently using the on-street parking will be asked to park in the surrounding streets while the suspensions are in place. The suspensions will provide more opportunities for traffic to pull in at narrow points of the road and improve the overall traffic flow along the route. These suspensions will be monitored to ensure they are achieving an improved traffic flow.

Our ongoing monitoring of various points along Leeson Road has observed some small vertical movement at isolated locations on Leeson Road, particularly those areas close to the rear scarp.

The safety of road users is – and always will be – the priority of Island Roads and the IW Council. Both Atkins and Jacobs have advised that further specialist monitoring is required to inform a decision on when - and indeed if - the road can be re-opened. Only this data will enable us to understand the nature and pace of ongoing movement and anything happening deep below the surface and therefore calculate – as best as we are able - the risks of further failures to the area including Leeson Road.

We are currently undertaking weekly level monitoring of Leeson Road which is supplemented by visual inspection. This level survey is concentrated on 84 points along the most vulnerable section, directly behind the landslip area. We have also made – and continue to make – many ancillary visits to study and assess the site. Furthermore, we have also installed survey points for use with ‘total station’ surveying equipment which uses an automated theodolite and a electronic distance measurement (EDM) to provide accurate measurements of horizontal and vertical movement.

While monitoring is already underway, we require more specialist information on which to base future decisions. Therefore, we have to be sure that the future monitoring regime gives us the very best, most accurate, information. This has involved developing a plan which we are confident deploys the right equipment at the right location. That plan is now in place and the necessary equipment is due to be installed in April subject to there being no further major movement, the weather being suitable and the necessary consents from private landowners being received.

All the advice from our consultants is that more information is needed before we can properly asses the risks around re-opening the road. We are not prepared to expose road users to real risk by re-opening the road without the necessary data being available to support such a decision. It is also important to remember that the risk of further collapse is down to the wider geological movement in the area – not necessarily the weight of any traffic using it. In that respect, it does not matter if the road was fully open or fully closed: the risk, and subsequent need for a wider review, remains the same. This failure area has acted differently to other landslips in that it happened very quickly and in a vertical direction. Landslips experienced previously in the Undercliff for instance are rotational failures that show a progressive slip, rather than the sudden downward drop experienced at Bonchurch.

As much as we’d like to, we are simply unable to put a timeframe on this. Any decision will be based on the data and expert guidance that we receive and even if data indicates that movement is very slow or steady, a future management plan will be required based on this data. Geological movement does not occur at pre -announced times and achieving trends or patterns in behaviour often takes many years and is influenced by rainfall levels. It is of note that February and into March, we have continued to see very high levels of rainfall, which needs to be taken into account given that groundwater is a trigger for continued movement.

We are monitoring the Graben (a known geological fault) at Newport Road very closely as we always do. We have intervened in this location several times since the beginning of the highways PFI project and will continue to do so as necessary. We also plan to take action to seal recent cracking but this is best done in dry conditions when the substructure has had the chance to de-water and the sealant itself is more effectively laid. At the same time, Southern Water also has drainage issues at Newport Road which need to be addressed. So far, we have liaised with Southern Water to limit work while Leeson Road is closed. However should the need for these works become urgent (in order to maintain their infrastructure) then we will have to consider how these are best managed given the wider traffic issues. We are also liaising with Southern Water regarding works required to rectify the problems in Pier Street that are currently being managed. Again, we are seeking the best (or least problematic) timing for such works. With regard to Island Roads works, if these can be rescheduled to later in the year this has been done and will continue to be done. Each piece of work has to be judged on its merits and urgency. For instance, due to the need to keep traffic moving through Wroxall. Isle of Wight Council and Island Roads have already postponed works on St John’s Road until later in the year.