Report on the safety of the Sandown/Shanklin Cliff Path.

Hi Susan,

Could you please leave an annotation regarding your views when you have digested the information you have been provided, as the members of the wightdogs website http://www.wightdogs.com are very interested in the councils reply to your request.

Regards

I will attempt to digitise and upload the pertinent parts of the document once I have received it.

Susan

Excerpt from the Report on the Inspection of the Cliff Path (Survey date 13th April 2010)

7.0 RECOMMENDATIONS

7.1 The recommended works assume that in general a natural erosion of the cliff face by small and even larger rock falls is acceptable in areas where the paths route can be easily redirected inland. Therefore all remedial works are to be centred around drop 6 whilst the cliff either side is left to regress naturally.

7.2 It is fairly clear that failure in the vicinity of drop 6 (where the gap underneath the fencing is) was caused by a large volume of water being directed by the kerbstones into a corner Therefore the kerb stones and fence should be re-aligned to cut straight across the dog leg corner such that future heavy water flow is directed further downhill. Thought must be given to the drainage paths of re-directed water to avoid creating a problem elsewhere. The sodden ground by the belay bars suggests a significant drainage problem at least the near surface. If possible ground drainage should be improved.

7.3 The inspection suggests that the cliff immediately below the cliff at drop 6 is reasonably sound. However there is no room available for losing more of the crest at this point and therefore remedial works to the cliff face are recommended at this point. The remedial works are summarised as follows:

A] Re- align fence as shown by the drawing showing remedial works contained in Appendix 1 .

B] Resin bond kerb stones to direct water safely downhill away from
the area covered by drops 1-7.

C] Scale loose rock flakes, fallen kerb stones and the like away from
the failure gully and 2.0m either side.

D] Trim back the overhang under the rock netting.

E] Install a grid of 4.0m nails in a 1.5m x 1.5m grid.

F] Install an 8.0 x 8.0m area of rock netting with Enka Mat underlay over the 4.0m nails [note cable around perimeter required]

G] Install say 10 nr 2.0m nails to secure the new netted area into the old netting.

8 BUDGET COSTS

8.1 The following is an itemised Bill of Quantities for the works. Please note that quantities are provisional and may be increased or reduced in each area lo achieve the final result. However the quantities in total are considered adequate to accommodate any variations. An estimate of the cost for these works is £13,500.00

(The covering letter sent by the council states that the estimated cost of the works will be approximately £24,750)

Full text of the report.

REPORT ON THE INSPECTION OF THE CLIFF LOCATED APROXIMATELY 1OOM NORTH OF [SHANKLIN, ISLE OF WIGHT] WHERE A RECENT SLOPE FAILURE HAS CLOSED THE CLIFF TOP PATH

SURVEY DATE: 1 3™ APRIL 2010
REPORT DATE: 22ND APRIL 2010

1.0 INTRODUCTION

1.1 This report covers the findings of the inspection of a 30m section of the cliff face located approximately 100m North of Shanklin Sailing Club, Shanklin, Isle of Wight. The inspection was undertaken by an Engineering geologist from Vertical Technology Ltd on 13"' April 2010.

1.2 The requirement for a survey was prompted by a failure of the cliff immediately adjacent to the cliff top path which has resulted in a closure of the path for pedestrian access. The failure apparently occurred following some very heavy rain where strong water flow was directed downhill to the point fl failure. At a second location just to, the South, a section of crest has dropped 0.5M. The purpose of the survey was to assess the geotechnical condition, probable short term and long term stability of the cliff face immediately below the path and make comment on any potential remedial measures required to enable the path to be re-opened .

1.3 The inspection was undertaken on behalf of the Isle of Wight County
Council by an Engineering Geologist from Vertical Technology Ltd using roped access techniques. During the inspection, the rope access team also cleared some loose rock and small trees from the rock face.

2.0 SURVEY METHOD

2.1 A total of 7 vertical abseil drops were made to cover the survey area. A site layout and drop plan is contained in Appendix I. Anchors for the rope system comprised substantial trees beyond the rock face crest or anchor bars securely driven into the ground. All roped access was undertaken to IRATA [Industrial Roped Access Trade Association] Guidelines 2000 and BS7985 2002.

2.2 Annotated photo-elevations showing the section of cliff inspected, survey information, drop and photo locations are contained in Appendix 2.

2.3 Photographs of specific features are included in Appendix 3

2.4 Site plans and drawings ate contained in Appendix I.

2.5 The orientations are given as an angle of dip and dip direction.

2.6 At the time of the survey it had been dry for 2 weeks and the weather was fine.

3.0 SITE LAYOUT

3.1 The survey area comprises a 30.0m long section of cliff approximately 20.0m high. The cliff is located immediately below the cliff top path and behind the beach promenade. Between the promenade and the ciff is a 30degree talus slope formed of wet sand [photo 8]. A timber rock fall barrier has been constructed to protect the promenade from falling debris.

3.2 The cliff top path rises steadily uphill to the North from behind the sailing club to a steeper section and dog leg in the location of the recent cliff failure [photo 1). At the point of cliff collapse there is a bottle neck where the cliff immediately abuts the seaward edge of the path with a property boundary immediately inland [photo 2]. Elsewhere
there is generally a 2.0m wide crest between the path and cliff edge with iron railings.

3.3 The site layout is shown by the plan in Appendix I.

4.0 GEOLOGY

4.1 The rocks forming the cliff are Cretaceous in age of the Lower Greensand group.

4.2 On this site, these rocks comprise a thickly bedded weak to very weak orange and green/grey silty fine sandstone. The structure of the rock mass is such that rock falls usually turn to sand on impact with the ground. The bedding in this location has a slight dip to the North. Joints are often smooth and planar but likely to undulate moderately in orientation.

5.0 FINDINGS

5.1 Erosion of this soft and friable sandstone cliff is an ongoing problem affecting the cliff tlop path. Over the years, slope failure [generally small but occasionally larger] has brought the cliff edge ever closer to the path whereby occasionally it has been necessary to move the path back inland. At critical locations erosion protection has been installed usually comprising anchored timber boarding and occasionally rock netting revetments. It is clear from the dog leg on the paths route at this location, that previous failures have affected this section of cliff.

5.2 There are 2 obvious points in the survey area where there are recent stability issues.
The buttress covered by drops 1-4 has a 0.5m subsidence of the crest. There has been a significant loss of cliff material in the location of drop 6 which has undermined the path edge.

5.3 Drops 1-4 [Photo elevation 1]

5.3.1 This covers the buttress with the subsided crest. On the South edge of this buttress [drops 1-2] is an obvious corner with an open joint at the back sub-vertical striking 140degree [photos 4 & 5]. In 2007 a 5.5m high rock pillar was removed from the top of this corner. The dilation and condition of this joint looks very similar to 3 years ago, but since no measurements were made at the time this is merely conjecture. There are however, no obvious signs of recent movement along this joint.

5.3.2 The front face of the buttress is vertical becoming slightly overhanging below 5.0m and orientated 170degrees. It is likely that this face has formed along a major joint plane. There is a sub-vertical joint sinking 050degrees which runs down the full height of the cliff (photos 10 & 11]. No other joints were seen to daylight into the front of the buttress.

5.3.3 A simple- explanation for the subsidence would be that the buttress has simply dropped down (photos 14-17], but there are no signs of movement in the buttress close to the magnitude to cause this. Probing with a 1.2m bar along the top found a line of very soft ground orientated 180degrees and it was possible to probe to 1.2m with ease into damp silt sand. Extending South from the subsidence is a vague line of depression suggesting this feature extends beyond this immediate point [photo 13]. The top of drop 4 gives a clear section through the crest and it shows a 1.0m deep x 0.5m wide soil and sand filled trench containing the end of an earthen pipe [photos 19 & 21]. The rear face of the trench appears to be a joint face corresponding to the joint sub-vertical striking 185degree shown in photo 18. The outer side of the trench meets the feather edge of the rock buttress [photo 19]. It's not known whether this is a natural feature or manmade and no obvious indication as to how this subsidence occurred.

5.3.4 It's worth noting that the belay bars used for drops 1 and 2 close to a park bench [photo 1] penetrated very wet and saturated silty sand [photo 12].

5.3.5 Below the trench it appears that the back of this buttress is delineated by an open [sand filled] sub-vertical striking 180degree joint [photo 21]. This joint appears to close down around 9.0m where it passes behind the cliff face just above the point where a small rock pillar was removed [photos 22 &23].

5.3.6 On the North edge of thebutress a tree was removed and below this are a number of small unstable slabs and flakes largely weakened by root action [photos20, 21 &22].

5.4 Drops 4-5 [Photo elevation 2]

5.4.1 At the top between drops 4 and 5 there is a planar joint face sub-vertical striking 185degrees [photo 18 & 21]. At the crest is a section of concrete slab left overhanging the cliff following a previous failure. This joint appears to close down around 9.0m where it passes behind the cliff face just above the point where a small rock pillar was removed [photos 22 & 23).

5.4.2 Around 5.0m down from the crest a tree was removed with an extremely unstable looking lock pillar immediately below which was also removed [photo 22]. Removal of the rock pillar exposed a clean cut open corner formed on two sub-vertical joints strinking 135degrees and 030degrees respectively [photo 23].

5.5 Drops 5-6 [photo elevations 2 & 3]

5.5.1 This section of cliff projects slightly more and contains a number of potentially unstable vertical slabs and flakes formed on sub-vertical joints striking 180degrees [photo 24|.

5.6 Drop 6 - 7 [Photo elevation 3]

5.6.1 Drop 6 covers the recent cliff failure immediately under the path which has formed a gully like feature [photo 26]. There is clear evidence that there has been strong water flow down the gully although the area is now dry. The gully has formed by the failure of several small vertical flakes and slabs on a joint set orientated sub-vertical striking 040degrees [photos 27, 30, 31 & 32). The area now appears fairly solid and stable with a clean planar face orientated 170degrees immediately under the path [photo 27].

5.6.2 The cliff face right of drop 6 is covered by a rock netting revetment. The top of the failure has removed material from under the top corner of the netting leaving an overhang of roots and the netting suspended in mid air [photo 28 & 29]. The netting dates to the 90's and has generally done a good job of erosion control with the steepest sections loosing around 200mm in that time and other areas encouraging vegetation growth [photo 33].

6.0 STABILITY ASSESSMENT

6.1 Appendix 1 contains a drawing showing the layout of major joints and potential planes of weakness. There are 3 clear joint sets all sub-vertical with minor changes of dip and dip direction either way.

Joint set 1: sub-vertical striking 175 degrees - 185 degrees
Joint set 2: sub-vertical striking 030 degrees - 040 degrees
Joint set 3: sub-vertical striking 130 degrees - 140 degrees

6.2 The bedding is effectively sub-horizontal and unlikely to be a factor effecting stability.

6.3 The inspection has shown that the joint set orientated sub-vertical striking around 180 degrees creates many unstable vertical flakes and slabs in the upper 9.0m and defines much of the upper cliff face at this point. Failure of these is relatively small producing a gradual regression of the cliff edge. Although fairly planar it is possible that one of these
joints may undulate such that a joint say 1,0m back from the face dips steeply to 090 degrees and will daylight further down the face producing the potential for a much larger failure [for example the buttress covered by drops 1 - 4].

6.4 The unstable pillar removed on drop 4 - 5 has failed on a very steep wedge formed on joint sets 2 &'4. This demonstrates another potential failure mechanism on this cliff [albeit localised].

6.5 Although the gully formed on drop 6 is from the failure of blocks on joints striking040 degrees it is felt likely that this weakness was exploited by the heavy water flow and is restricted to this immediate area.

6.6 Appendix 1 contains a drawing showing the most likely areas of failure. Essentially it is anticipated that the most likely failures will be along the line orientated 180 degrees as highlighted in red. This is roughly in line with the current path and will bring the cliff edge, closer over time as the 2.0m of crest is lost. A possible wedge failure has been identified at the Southern end highlighted in blue.

6.7 It is felt that failure of the red zone is the most likely and further failure around the bottle neck at drop 6 is less likely providing appropriate remedial measures are undertaken.

7.0 RECOMMENDATIONS

7.1 The recommended works assume that in general a natural erosion of the cliff face by small and even larger rock falls is acceptable in areas where the paths route can be easily redirected inland. Therefore all remedial works are to be centred around drop 6 whilst the cliff either side is left to regress naturally.

7.2 It is fairly clear that failure in the vicinity of drop 6 (where the gap underneath the fencing is) was caused by a large volume of water being directed by the kerbstones into a corner Therefore the kerb stones and fence should be re-aligned to cut straight across the dog leg corner such that future heavy water flow is directed further downhill. Thought must be given to the drainage paths of re-directed water to avoid creating a problem elsewhere. The sodden ground by the belay bars suggests a significant drainage problem at least the near surface. If possible ground drainage should be improved.

7.3 The inspection suggests that the cliff immediately below the cliff at drop 6 is reasonably sound. However there is no room available for losing more of the crest at this point and therefore remedial works to the cliff face are recommended at this point. The remedial works are summarised as follows:

A] Re- align fence as shown by the drawing showing remedial works contained in Appendix 1 .

B] Resin bond kerb stones to direct water safely downhill away from
the area covered by drops 1-7.

C] Scale loose rock flakes, fallen kerb stones and the like away from
the failure gully and 2.0m either side.

D] Trim back the overhang under the rock netting.

E] Install a grid of 4.0m nails in a 1.5m x 1.5m grid.

F] Install an 8.0 x 8.0m area of rock netting with Enka Mat underlay over the 4.0m nails [note cable around perimeter required]

G] Install say 10 nr 2.0m nails to secure the new netted area into the old netting.

8 BUDGET COSTS

8.1 The following is an itemised Bill of Quantities for the works. Please note that quantities are provisional and may be increased or reduced in each area lo achieve the final result. However the quantities in total are considered adequate to accommodate any variations. An estimate of the cost for these works is £13,500.00
(The covering letter sent by the council states that the estimated cost of the works will be approximately £24,750)

After reading the report if full and carefully looking at the diagrams (which unfortunately I can't add as this site only allows text)it appears clear to me that the "expert" only expects gradual minor cliff falls to continue here and mainly to take place after heavy rain. He advises realigning the fence and stopping rainwater run-off going over the edge of the cliff here. There is nothing in the report that says it is unsafe for people to use the path or that says that any larger falls are imminent. It is clear that, provided the gap under the existing fencing is fenced off, the path could be reopened at least until the late autumn when it would need to be kept an eye on for further falls if the advised remedial works were not done.
I note with interest that the report quotes £13,500 for the work whereas the covering letter sent to me estimates the cost of the works at £24,750 ie over £11,000 extra!

Thank you for your trouble Susan Mills,
the "experts" appear to be keeping their fingers crossed that we don't have heavy rain.

Very reassuring!

Regards,

Mr Figg