FREDERICK SHERRELL LTD
66 WEST STREET
TAVISTOCK
DEVON PL19 8AJ
Your Ref:
Telephone:
01822 612915
Facsimile:
01822 617394
info@fredericksherrell.co.uk
www.fredericksherrell.co.uk
CONSULTING ENGINEERING GEOLOGISTS AND GEOTECHNICAL ENGINEERS
JR.JM.635
Our Ref:
22nd May 2008
Environment Services Directorate,
Torbay Council,
Ground Floor, Roebuck House,
Abbey Road,
TORQUAY. TQ2 5TF
For the attention of Mr. N. Garswood
Dear Nick
Rockface Assessment – Rock Walk Gardens, Torquay
Preliminary Report
In response to your instructions in March 2008 we carried out an engineering geological inspection of
the cliffs and slopes at Rock Walk Gardens, Torquay following the removal of trees and vegetation by
Devon Tree Services.
The purpose of the inspection was to identify areas of instability and
recommend stabilisation measures where necessary. The inspection was carried out on foot by
viewing from the footpaths that traverse the slopes and run along the top and bottom.
The inspection followed an earlier engineering geological inspection and rockface assessment of the
cliffs and slopes at Rock Walk Gardens that was carried out by Frederick Sherrell Limited in 2007 and
presented in letter reports dated 12th February 2007 and 3rd May 2007. The reports recommended
extensive stabilisation works to the steep cliffs at the western end of the site and some localised
stabilisation measures to the slopes in the eastern part of the site. At the time of these reports the
site was covered in dense vegetation with numerous large trees; most of the slopes in the eastern
part of the site were hidden and/or inaccessible.
Our report of 3rd May 2007 included a
recommendation that, following a detailed arboricultural assessment, selective felling of large trees in
poor condition be carried out in order to minimise the risks of tree fall and associated soil slippage.
We understand that a report on the condition of trees at the site was subsequently prepared by Devon
Tree Services. In January 2008 Devon Tree Services commenced tree felling and clearance from the
site. The works comprised the felling of the vast majority of the trees from the cliffs and slopes,
together with clearance of most of the lower growing shrubs and scrub vegetation from the eastern
part of the site. The vegetation clearance revealed a very irregular rockhead profile with numerous
Directors:
R.D.J. Gould, B.Sc., C.GeoI., F.G.S.
J. Rickeard, B.Sc., A.R.S.M., C.GeoI., F.G.S.
Registered Office : 66 West Street, Tavistock, Devon PL19 8AJ. Registered in England No. 2797097 V.A.T. Registration No. 591 3193 33
unstable pinnacles and areas of loose, fractured bedrock that were not visible during our 2007
inspections. We understand that a number of rockfalls occurred during the works, one of which
resulted in damage to the railings alongside the easternmost footpath. In order to allow the safe reopening of Torbay Road to traffic, a scaffold rockfall barrier was erected within the eastbound
carriageway.
The footpaths within the site remain closed to the public pending stabilisatioon
measures.
The results of our inspection and assessment are summarised in the paragraphs below. Rockfall
hazards and proposed stabilisation measures are summarised in Table 1 and indicated in Figures 2A
and 2B.
1.
THE SITE
1 – 1.
Rock Walk Gardens are located on the northern side of Torbay Road between Torre
Abbey Sands and Torquay Harbour, as indicated in Figure 1. On the southern side of Torbay Road
there is a pedestrian promenade with flower beds, cafes and a theatre. The Gardens extend for a
length of approximately 400m and rise from approximately 6m above Ordnance Datum (AOD) at the
base, adjacent to Torbay Road, to a maximum height of approximately 38m AOD on Rock Walk at the
top of the cliff.
1 – 2.
Prior to the construction of Torbay Road the cliffs and slopes at Rock Walk Gardens
were probably exposed to sea erosion and wave action. Old Ordnance Survey maps from the second
half of the 19th Century show Torbay Road at the toe of the cliffs; a sea wall is shown along the
southern side of the road. At the end of the 19th Century the promenade was built on the south side
of Torbay Road and the Royal Terrace Gardens created within the steep slopes on the north side of
the road.
1 – 3.
In the western third of the Gardens limestone bedrock is exposed in a steep sub-
vertical cliff face approximately 25m high. The limestone bedrock is fractured and solution weathered
and rock bolts and anchors are visible in places. Vegetation obscures part of the cliff, notably in the
upper levels.
1 – 4.
In the eastern two thirds of the Gardens the ground slope generally varies between 30o
and 60o to the horizontal. Limestone bedrock crops out in places, notably at the eastern end, but, for
the most part, the slope is mantled by topsoil and gravelly overburden soils. A series of footpaths zigzag up the slope to Rock Walk. The footpaths are supported by drystone walls that are in places
loose and potentially unstable. Three timber footbridges link sections of the footpaths in the upper
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part of the slope, as shown in Figure 2B.
2.
GEOLOGY OF THE SITE
2 – 1.
Slightly weathered pale grey strong limestone bedrock is exposed in the cliffs in the
western part of the site and in outcrops in the steep slopes in the eastern part. The limestone
bedrock is dissected by rough, slightly wavy bedding planes that typically dip towards the southwest
at between 30o and 60o to the horizontal, locally increasing to 80o. The rockmass is also broken by
three sets of wavy, rough, slightly dilated joints :
i.
dip 50 – 90o towards the northeast (i.e. opposite to bedding)
ii.
dip 38 – 85o towards the west
iii.
dip 52 – 90o towards the east/southeast
A number of irregular, non-systematic fractures are also visible.
2 – 2.
The bedding and joint fractures vary considerably in spacing and persistence. At some
locations, notably in the upper levels of the cliffs, the bedrock is thinly bedded and close to medium
jointed, whilst in the lower levels the limestone typically becomes thickly bedded and widely jointed or
massive. At some locations the fractures are enlarged by solution weathering and are partially infilled
with clay and gravelly sand.
2 – 3.
In the upper parts of the steep cliff, and in much of the eastern part of the site, the
limestone bedrock is mantled by a thin layer of humic topsoil and loose silty sandy gravel, cobble and
boulder overburden.
3.
STABILITY ASSESSMENT
3 - 1.
Western Part of the Site (chainage 0 – 1301)
3 - 1.1.
The cliff-face in the western part of the site slopes steeply and there is evidence of
small-scale rockfall debris at the toe of the cliff. Rockbolts and areas of netting are visible in places,
although parts of the face are still obscured by dense vegetation. The vegetation has a binding effect
1
Chainage figures relate to distance from the western end of the former toll house, as shown in the
topographical survey drawings prepared by Centre Line Surveys (April/May 2008)
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on the soil and also provides protection from rainfall erosion and run-off. However roots also have a
destabilising effect on the rock, prising apart discontinuities in the bedrock.
3 – 1.2.
The inspection revealed the presence of chain-link netting over parts of the cliff. The
netting is secured to the face by steel dowels and face plates, some of which are badly corroded. The
chain-link netting is helping to retain loose soil and broken rock. However the netting is at least 18
years old and it is likely to be severely corroded and weakened under the plastic coating. Modern
rockfall protection works utilise “double-twist” plastic-coated galvanised netting that is much stronger
than chain-link and less likely to fail catastrophically if one or two strands break. There is in our
opinion a significant risk of failure of the existing dowels and/or netting which would result in soil
and/or broken rock falling onto the path below, potentially injuring path users and garden workers.
3 – 1.3.
Gravelly head soils are exposed in the upper levels of the cliff in slopes that are much
steeper than the interpreted internal angle of friction of the gravelly soils. The soils are probably held
in place by a combination of soil suction2 and the binding action of roots, although the reserve of
stability is likely to be low. If the soil slope were to become saturated (eg as a result of a burst pipe)
the soil suction forces would be destroyed and the soils in the steep slope would probably collapse,
potentially injuring people on the path below and possibly undermining or causing subsidence of Rock
Walk and the retaining walls further upslope. The probability of such an event is assessed to be low
in the near future, but the potential consequences are severe, resulting in a medium risk designation.
3 – 1.4.
The limestone bedrock exposed in the cliff-face is generally strong. Discontinuities are
generally rough and slightly wavy and a relatively high angle of friction is mobilised on discontinuity
surfaces at low confining stresses. Bedding planes dip out of the face at angles generally less than
the interpreted angle of friction. Consequently there is a very low probability of large scale planar
failure affecting the face as a whole.
3 - 1.5.
A prominent joint set dips steeply towards the northeast (ie into the cliff-face), giving
rise to potential medium-scale topple failure locally. Extensive rock bolting has been carried out in the
past to stabilise areas of potential topple failure, but our 2007 inspection revealed evidence of
additional areas of potential topple failure that were hidden by vegetation at the time of the previous
stabilisation works. Further areas of potential topple failure may exist, hidden by the curtain of dense
vegetation that is still present. Such failures could potentially cause serious injury to people working
in the gardens or using the footpath. The probability of topple failure and resulting medium-scale
2
Soil suction or “apparent cohesion” can result in significant cohesive strength and is caused by surface tension
of tiny menisci of water linking grains in the damp but not saturated soil.
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rockfalls is assessed to be moderate in the near to medium future and the potential consequences are
severe. The combination of moderate probability and severe consequences results in a medium to
high risk designation.
3 - 1.6.
A number of irregular joints and fractures also occur that could cause small to medium
scale blockfalls in places.
3 – 1.7.
The potential hazards are summarised in Table 1 for different areas within the site,
together with an assessed risk rating for each hazard. The extent of each area is indicated in Figures
2A and 2B.
3 – 2.
Eastern Part of the Site (chainage 130 – 400)
3 – 2.1.
The slopes in the eastern part of the site generally slope at between 35o and 50o to the
vertical, increasing to 60o in some places.
Until recently the dense vegetation cover obscured
potential rockfall hazards and also served to trap any small falls of rock, reducing the likelihood of
them impacting the footpaths. Removal of vegetation from the slopes has revealed a number of
potential rockfall hazards and areas of instability within the eastern part of the site as follows :
i.
unravelling of highly fractured limestone bedrock giving rise to potential small to medium scale
rockfalls
ii.
unstable promontories of limestone showing signs of potential medium to large scale planar
and/or topple failure with potentially severe consequences for people using the footpaths and
pavement below and traffic using Torbay Road
iii.
loose boulders resting on the soil slopes that could potentially roll down onto the paths below
iv.
settlement of the outer edge of Rock Walk where the slope is steeper than the interpreted
internal angle of friction of the soils giving rise to potential slippage of the upper part of the
slope locally. Soil slippage could potentially injure people on the paths below and possibly
undermine Rock Walk and the retaining walls further upslope
v.
loose and unstable sections of drystone walling that could unravel or collapse with potentially
serious consequences for people using the footpaths below.
3 – 2.2.
The potential hazards are summarised in Table 1 for different areas within the site,
together with an assessed risk rating for each hazard. The extent of each area is indicated in Figures
2A and 2B.
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4.
RECOMMENDATIONS
4 - 1.
The inspection has revealed evidence of potential rockfalls and soil slips within the
cliffs and slopes at Rock Walk Gardens. Before the Gardens can be safely reopened to the public
stabilisation measures will be necessary in order to reduce the risks associated with these hazards to
an acceptable level. The proposed stabilisation measures are summarised in Table 1 and Figures 2A
and 2B and briefly described in the paragraphs below.
4 – 2.
In the western part of the site (Areas 1 and 2 in Figure 2A) the measures
recommended in our report of 3rd May 2007 should be implemented, namely :
i.
removal of vegetation from the cliff,
ii.
scaling of soil and loose rock from the face using hand tools,
iii.
installation of a revetment of rockfall netting over the upper part of the face (Area 2), secured
by galvanised steel dowels, galvanised hollow bars and galvanised steel wire cables
iv.
installation of galvanised steel dowels up to 4m in length
v.
concrete dentition of open joints
In addition to these measures a small area of “shotcrete” (sprayed concrete) is proposed to secure an
area of broken rock at the top of the cliff where Rock Walk is at risk of being undermined (chainage
75).
4 – 3.
In the eastern part of the site (Areas 3 – 10 in Figures 2A and 2B) extensive
stabilisation measures will be required to ensure the safety of both people using the footpaths and
traffic on Torbay Road. The stabilisation measures will include the following :
i.
removal of remaining vegetation from the slopes,
ii.
through scaling of soil and loose rock from the face using hand tools,
iii.
removal of potentially unstable blocks of rock using percussive tools (jack-hammers) and/or
expanding bentonite
iv.
installation of rockfall netting over large areas of the slope, secured by galvanised steel
dowels, galvanised hollow bars and galvanised steel wire cables
v.
installation of galvanised steel dowels up to 4m in length
vi.
installation of double-corrosion protected rock anchors 6m in length
vii.
construction of a concrete buttress to support an unstable rockface
viii.
dismantling unstable sections of drystone wall
ix.
removal of potentially unstable timber footbridges
-6-
4 – 4.
Where there is a potential risk of slippage undermining Rock Walk it may be necessary
to strengthen the soil slope with long soil nails extending beneath Rock Walk and beyond the
boundary with the properties upslope. In order to design the soil nail stabilisation we recommend that
6 rotary cored boreholes are drilled in the locations indicated in Figures 2A and 2B. (We are awaiting
a price for the cost of the borehole investigation from specialist contractors Geotechnical Engineering
Ltd.)
4 – 5.
The proposed stabilisation measures assume that only the central and easternmost
footpaths will be retained. The stabilisation measures include dismantling unstable sections of wall
that present a risk but do not include strengthening or rebuilding of the existing walls adjacent to the
footpaths.
4 – 6.
The proposed stabilisation measures will need to be reviewed and amended as the
work progresses. In particular removal of vegetation and scaling of loose rock from the slopes is
likely to reveal hazards that are not visible at present and may result in an increase in the amount of
netting and/or bolting that is required.
A relatively high contingency should be allowed to
accommodate any changes in the extent of the works that result.
4 – 7.
The proposed stabilisation measures will need to be reviewed in the light of proposals
for landscaping the gardens. In some areas it may be possible to install short sections of rockfall
catchfencing instead of rockfall netting. Public benches should not be installed at the base of any unnetted slopes.
4 – 8.
The precise depth and position of the sewer in Rock Walk should be ascertained
before stabilisation measures commence in order to avoid the possibility of top-fixing bars/dowels
striking the pipe.
--------------------------
We are currently preparing a detailed Specification, Bill of Quantities and Drawings for the works and
will obtain an estimate of likely costs for the works from specialist contractors Celtic Rock Services
Ltd. In the meantime if you have any queries please do not hesitate to contact us.
Yours sincerely
for Frederick Sherrell Ltd
Jeremy Rickeard
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