APPENDIX 2
STABILITY OF CLIFFS
ADJACENT TO CLIFF PATH
LAKE
ISLE OF WIGHT
GRAHAM DAWS
ASSOCIATES
ROCK MECHANICS SPECIALISTS
1
Graham Daws Associates Ltd
Vertical Technology Ltd
STABILITY OF CLIFFS ADJACENT TO CLIFF PATH, LAKE
ISLE OF WIGHT
CONTENTS
Page
1.
INTRODUCTION
1
2.
SITE INVESTIGATION
1
Abseil Drop 1
Abseil Drop 2
Abseil Drop 3
Cliff Path
1
2
2
2
3.
STABILITY ASSESSMENT
2
4.
CONCLUSIONS AND RECOMMENDATIONS
3
5.
REFERENCES
4
2.1
2.2
2.3
2.4
DRAWINGS AND FIGURES
APPENDIX 1 – Stability Assessment
(i)
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Graham Daws Associates Ltd
Vertical Technology Ltd
STABILITY OF CLIFFS ADJACENT TO CLIFF PATH, LAKE
ISLE OF WIGHT
1.
INTRODUCTION
In preparation for re painting, some steel railings along the cliff side of Cliff Path,
Lake, part of a hedge was removed which had grown through these railings. The
removal of a section of hedge caused part of the upper cliff to fall away up to the
edge of the path.
The location of this fall is shown on Drawing No. CPL/10/Plan and Fig. 1 shows
more details of the area.
The path was closed to vehicular traffic after this fall and a barrier was erected to
prevent pedestrian access to the cliff edge as shown on Fig. 1. It is planned to reopen the path and restrict the width so that a maximum vehicle weight of about 3.5
tonnes can be accommodated. Before this can happen an overview of the stability
of this area was considered necessary.
Consequently, Graham Daws Associates Ltd (GDA), a firm of Rock Mechanics
Specialists was asked to inspect this area in conjunction with Vertical Technology
Ltd (VT) and comment on any stability issues.
2.
SITE INVESTIGATION
A thorough examination of cliffs can only be carried out by intimate close up
inspection and consequently three abseil drops were carried out along the area of
concern. These locations are shown on Drawing No. CPL/10/Plan. The cliffs in
this area are in the Upper Greensand formation (a weak Sandstone).
2.1
Abseil Drop 1
This drop was over the area where the upper section of cliff had fallen due to the
hedge removal.
The upper portion of the cliff lies at an angle of about 600 for about 4.5m below
road level, below this the cliff lies at an inclination of 800. The upper section was
weak and prone to wind and rain erosion. The lower section was more competent.
Immediately to the West of the drop position the hedge had been severely undercut
as shown in Fig. 2. It is considered that this was the situation in the area where
the portion of hedge had been removed. A similar situation existed to the East of
the drop location with a maximum stand off distance from the base of the hedge to
the crest of the cliff of 0.3m.
There was a distinct jointing system within the upper part of the cliff and these
joints had isolated slabs and blocks, a typical example being shown on Fig. 3.
When erosion became more prominent, pinnacles were formed which eventually
detached. However, this was a natural weathering process and will only affect
stability in the very long term.
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Graham Daws Associates Ltd
Vertical Technology Ltd
Numerous “caves” were present along the cliff most likely caused by wind
erosion. Even though some of these were deep (up to about 1.5m) they will only
affect stability in the very long term.
2.2
Abseil Drop 2
This was located about 14m to the East of Drop 1 and was carried out in order to
assess the extent of undercutting of the hedge. The railings were located adjacent
to the roadway between Drops 1 and 2.
The crest of the cliff was located about 1m from the base of the hedge at this
location. However, this soon narrowed in the direction of Drop 1 to a distance of
about 0.3m.
Similar features on the cliff were noted as for Drop 1.
2.3
Abseil Drop 3
This was located about 30m to the West of Drop 1 and was carried out in order to
assess the extent of undercutting of the hedge. The railings were located within
the hedge between Drops 1 and 3. There was a small oak tree at the location of
this drop and the roots appeared to be confining the crest material and holding it in
place.
The crest of the cliff was located about 1m away from the base of the railings at
this location. However, after about 18m towards Drop 1 this distance narrowed to
a maximum of about 0.3m.
Similar features on the cliff were noted as for Drop 1.
2.4
Cliff Path
Drawing No. CPL/10/Plan shows a sketch plan of the area concerned. From the
abseil drops it was found that the crest of the cliff is very close to the edge of the
roadway for about 12m on either side of the location of Drop 1. This raises
concerns about stability in this region and the possibility of allowing vehicular
traffic as there is a wall to the West of Drop 1 location which will channel vehicles
close to the cliff edge as the roadway narrows to the East as shown on Fig. 4.
To the East of Drop 1 location a hedge is located on the opposite side of the road
to the cliff and it is understood that part of this could be removed and the roadway
set back a short distance from the cliff crest.
3.
STABILITY ASSESSMENT
No samples were collected from site and consequently this stability assessment
was carried out using published properties of Upper Greensand. Where
appropriate the most conservative values were used.
The assessment was made using the Stability Number method as developed by
Taylor[1] and applied to the upper 4.5m of cliff which had a slope angle of 600 to
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Vertical Technology Ltd
the horizontal. The lower main part of the cliff was more competent and
considered to be stable.
The Factor of Safety of the upper part of the cliff was found to be 1.9. Details of
how this was determined are given in Appendix 1. This does not take into
account deterioration caused by erosion.
The slope stability was further considered when a surcharge load equivalent to a
3.5 tonne vehicle was applied at road level. The Factor of Safety reduced to 1.85
which is considered to be acceptable.
It should be stressed that these analyses were carried out using conservative values
and consequently the actual Factor of Safety will be higher.
4.
CONCLUSIONS AND RECOMMENDATIONS
i)
The cliff crest has regressed to be extremely close to the edge of Cliff
Path for about 12m on either side of the location of abseil Drop 1. In
this area, there is a maximum distance of about 0.3m from the edge of
Cliff Path to the crest. The hedge bordering Cliff Path on the cliff side
is severely undercut in this area.
ii)
The roots of the hedge are helping to knit the upper section of cliff
together and prevent slippage of this material. They should be left intact
and not removed unless reinforcement works are carried out.
iii)
The overall stability of the slope is acceptable and remains so when a
surcharge load equivalent to a 3.5 tonne vehicle load is applied.
iv)
The main area of concern is that the crest of the cliff is extremely close
to the edge of Cliff Path as noted in i). The geometry of the road will
channel vehicles travelling in an Easterly direction towards the section of
cliff edge which is closest to the edge of Cliff Path. Consequently, it is
recommended that a width restricting device be erected at the road
junction before the road is re-opened and that a similar device is also
installed mid way along the walled section shown in Fig. 4 in order to
keep vehicles as far away as possible from the cliff crest. This width
restriction should automatically keep vehicle weight to a minimum.
Past the end of the wall it would be possible to set the roadway back
slightly from the crest of the cliff and this is recommended.
v)
Cliffs are transitional geomorphologic features and erosion will
continue. Consequently, in the longer term it is recommended that the
upper section of cliff which is closest to Cliff Path be reinforced. The
traditional method of achieving this locally has been to installed
anchored boarding as shown on Fig. 5
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Graham Daws Associates Ltd
Vertical Technology Ltd
5.
REFERENCES
1
Taylor, D W
“Fundamentals of Soil Mechanics” in “Smiths
Elements of Soil Mechanics”, 8th Edition, Blackwell
Publishing, 2006 pp 173 - 176
Eur. Ing. G. Daws BSc (Hons), PhD, CEng, FIMMM, FGS
3rd June 2010
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6
DRAWINGS
and
FIGURES
7
3m
Drop 3
~ 30m
Hedge
Railings in Hedge
Wall
~ 14m
Hedge
CLIFF
PATH
Railings on roadside
Railings, no Hedge
of Hedge
2m
Drop 1
Undercut area
~12m each side of Drop 1
Drop 2
Hedge
Client
VERTICAL TECHNOLOGY LTD
Project
CLIFF PATH - LAKE
Drawing Title
SKETCH PLAN
OF CLIFF PATH
Drg No
Scale
Drawn by
Checked by
CPL/10/Plan
G.D.
NTS
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
8
Abseil Drop 1
Client
Drawing Title
VERTICAL TECHNOLOGY LTD
AREA OF CLIFF FALL
ADJACENT TO PATH
Project
CLIFF PATH - LAKE
Drg No
Fig. 1
Drawn by
GD
Scale
NTS
Checked by
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
9
Undercut Hedge to West of Drop 1
Client
Drawing Title
VERTICAL TECHNOLOGY LTD
For location see Drawing No CPL/10/Plan
UNDERCUT HEDGE
ADJACENT TO PATH
Project
CLIFF PATH - LAKE
Drg No
Fig. 2
Drawn by
GD
Scale
NTS
Checked by
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
10
Slabs isolated by jointing
Client
Drawing Title
VERTICAL TECHNOLOGY LTD
EFFECTS OF JOINTING
IN THE GREENSAND
Project
CLIFF PATH - LAKE
Drg No
Fig. 3
Drawn by
GD
Scale
NTS
Checked by
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
11
Abseil Drop 1
2m
Abseil Drop 3
3m
Client
Drawing Title
VERTICAL TECHNOLOGY LTD
For details see Drawing CPL/10/Plan
GENERAL VIEW
OF CLIFF PATH
Project
CLIFF PATH - LAKE
Drg No
Fig. 4
Drawn by
GD
Scale
NTS
Checked by
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
12
Treated boarding anchored to cliff
to prevent erosion of cliff top
Client
Drawing Title
VERTICAL TECHNOLOGY LTD
TRADITIONAL CLIFF
TOP PROTECTION
Project
CLIFF PATH - LAKE
Drg No
Fig. 5
Drawn by
GD
Scale
NTS
Checked by
Date
June 10
Approved by
Graham Daws Associates Limited
Unit 3, Block14, Amber Business Centre, Riddings,Derbyshire DE55 4BR
Telephone 01773 603459, Fax 01773 540667
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APPENDIX 1
Stability Assessment
14
Graham Daws Associates Ltd
APPENDIX 1
STABILITY ASSESSMENT
Parameters used.
Cohesion, c
Angle of Friction, Φ
Rock Density, γ
Upper Slope Height
25 kPa
300
25 kN / m3
4.45m
Slope Factor of Safety, F = Shear Strength / Disturbing Shear
This F applies equally to cohesion and friction.
Use Taylor’s curves as below.
Try F = 2
Tan Φ / 2 = 0.577 / 2 = 0.288 (=Tan angle 160)
Use this angle to establish a new N value from the above chart
N = 0.13
So,
Cw = 0.13 x 25 x 4,45 = 14.46
And
F = 25 / 14.46 = 1.72
Try F = 1.75
Using procedure as above,
F = 2.24
Try F = 1.9
Using procedure as above,
F = 1.95, so F = 1.9 is acceptable
With surcharge equivalent to additional 0.7m in height and
Using procedure as above,
F = 1.85
1
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Graham Daws Associates Ltd
Unit 3 Block 14
Amber Business Centre
Riddings
Derbyshire
DE55 4BR
Tel: 01773 603459
Fax: 01773 540667
E Mail [email protected]
Web Site www.grahamdaws.co.uk
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