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Intended for
English Heritage
Document type
Report
Date
April 2013
TINTAGEL CASTLE
BRIDGE GEOTECHNICAL
EVALUATION
DESK STUDY AND SITE
VISIT REPORT
DESK STUDY AND SITE VISIT REPORT
61031755/GT/R01
CONTENTS
1. INTRODUCTION 1 1.1 Limitations of Report 1 1.2 Proposed Development – Bridge Conceptual Designs 1 2. SCOPE AND OBJECTIVES 2 3. INFORMATION SOURCES 3 4. SITE LOCATION, DESCRIPTION AND SURROUNDING
LAND USE 3 4.1 Study Site 3 4.2 Surrounding Land Use 7 4.3 Brief Monument Description and History 7 4.4 Literature Survey - Geomorphology and Geology of the
Study Site 7 4.5 Literature Survey – Stability of Site 9 5. SITE VISIT - GEOMORPHOLOGICAL AND GEOLOGICAL
OBSERVATIONS AND INTERPRETATION 10 5.1 Erosion 10 5.2 Rock 10 5.3 Structural Controls 11 5.4 Hydrology 14 6. DESK STUDY AND SITE VISIT - GEOTECHNICAL
RECOMMENDATIONS 14 6.1 Mainland Rock Slope and Landing 14 6.2 Island Rock Slope and Landing 15 7. SITE VISIT - ASSESSMENT OF FURTHER
INVESTIGATION AND ACCESS FOR PLANT AND
EQUIPMENT 15
TABLE OF FIGURES
Figure 1 Location/alignment of the proposed high level bridge between Tintagel
Island and the adjoining mainland ............................................................ 2 Figure 2 Tintagel Island and the neck of land joining the Island to the mainland
............................................................................................................ 4 Figure 3 Mainland Face ............................................................................ 5 Figure 4 Mainland Face. The ticket booth is at the bottom of the mainland
steps with the two rest landings visible on the steps ................................... 5 Figure 5 The Island face .......................................................................... 6 Figure 6 Island face, wide angle................................................................ 6 Figure 7 Coastline development controlled by major faults - one crosses the
line of the bridge (after Wilson, 1952) ....................................................... 9 Figure 8 Faults and the major joint set ..................................................... 12 Figure 9 Normal faulting/fault zone visible on the western cliff below the upper
ward. ................................................................................................... 13 Figure 10 Major joint set. ........................................................................ 14
APPENDICES
Appendix 1 Appendix 2 -
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1. INTRODUCTION
Ramboll UK Limited has been appointed by English Heritage to undertake conceptual design for
improved access between Tintagel Island and the adjoining mainland via a new high level bridge.
This report has been prepared to provide preliminary geotechnical information to inform the
development of options for the conceptual designs of the bridge. It reports the findings of a
geotechnical desk study of the site along with a walkover and a preliminary survey of rock
discontinuities and rock mass strength.
This report has been prepared by Ramboll UK Limited solely for the benefit of English Heritage.
It shall not be relied upon or transferred to any third party without the prior written authorisation
of Ramboll UK Limited.
The site visit occurred on 20 March 2013, on a bright sunny day during an unusually cold and wet
early spring.
1.1 Limitations of Report
This report has been prepared on the basis of the proposed end use as defined by the Client. If
this end use is altered or changed, then it would be necessary to review the findings of this
report.
The conclusions and recommendations within this report are based upon information derived
from a variety of sources. Ramboll UK Limited cannot accept any liability for the accuracy or
otherwise of any information derived from third party sources as these are outside the control of
Ramboll UK Limited.
1.2 Proposed Development – Bridge Conceptual Designs
A high level bridge is proposed between Tintagel Island and the adjoining mainland; Figure 1.
Locations of the proposed bridge landings are the south east of the Island Courtyard and to the
north east of the stairway leading from the Lower Courtyard on the mainland.
Proposed high level bridge
Existing bridge
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Figure 1 Location/alignment of the proposed high level bridge between Tintagel Island and the adjoining
mainland
2. SCOPE AND OBJECTIVES
The scope of work consisted of a desk based assessment of information obtained from a variety
of sources along with a site walkover by two experienced engineering geologists from Ramboll
UK. Preliminary rock mass strength and discontinuity assessments were carried out from site
observations to derive the geology of the site and the geotechnical constraints in the context of
the design and construction of a high level footbridge.
The objectives of the desk study were as follows,
To present factual data with regard to the geological setting of the site and its immediate
surroundings.
To conduct a site walkover (including a preliminary rock mass strength and discontinuity
survey) in order to gain further information on the geology and on likely construction
problems, and to assess access for investigation plant and equipment.
To interpret this historical and geological information in terms of its implications for further
investigation work, and developing options for the new high level bridge.
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3. INFORMATION SOURCES
The site is part of a national monument in an SSSI – there are many sources of information
available. The following have more complete reference lists:
http://www.bgs.ac.uk/services - used to access the 1:50000 geology information
http://www.english-heritage.org.uk/T - The Tintagel phase map has been used as a base
map
Google – Map Data Imaging.
Tintagel SSSI Citation Sheet CITATION COUNTY: CORNWALL, SITE NAME: TINTAGEL CLIFFS,
DISTRICT: NORTH CORNWALL. Date Notified (Under 1981 Act): 1988
Geological Conservation Review Volume 28: Coastal Geomorphology of Great Britain Chapter 3:
Hard-rock cliffs – GCR site reports Site: TINTAGEL (GCR ID: 1846) Extracted from the Geological
Conservation Review. An introduction to this volume is available at:
http://www.jncc.gov.uk/page-2731
The following reports were concerned with rock stability of the monument cliff faces and the
Island and Mainland faces above the existing footpaths and bridge:
Tintagel Castle, Cornwall remedial Works 2000 Parts 1 and 2. Babtie and Vertical Technology.
Commissioned by English Heritage.
Tintagel Castle, Cornwall Slope Stability/Safety Inspection, June 2003. Babtie and Vertical
Technology. Commissioned by English Heritage.
Gifford (now Ramboll UK Limited) Report: GEOTECHNICAL AND GEOLOGICAL CONSIDERATIONS.
The record of geotechnical conditions presented in Babtie Group’s Geotechnical Survey, Section 3
of Report B was reviewed, and then extended by a site reconnaissance undertaken by John H
Chairman, Consulting Engineering Geologist. On completion of this site reconnaissance Gifford
and Partners’ geotechnical team leader, Dr Mike Cooper, reviewed the findings.
4. SITE LOCATION, DESCRIPTION AND SURROUNDING
LAND USE
National Grid Reference: SX 051890.
4.1 Study Site
Figure 2 shows the neck between the mainland and Tintagel Island.
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Figure 2 Tintagel Island and the neck of land joining the Island to the mainland
(from Google maps)
The Tintagel castle area is dominated by its very steep topography, including the eroded neck of
land dividing the island from the mainland. The castle lies on both sides of the neck. Erosion of
the sea cliffs has caused loss of many parts of the castle and earlier buildings.
This study focuses on the rock slopes between the Lower courtyard (or ward) on the mainland
and the Island Courtyard (or ward), in this report these are named the mainland face and the
island face; Figure 3 to Figure 6.
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Figure 3 Mainland Face
Figure 4 Mainland Face. The ticket booth is at the bottom of the mainland steps with the two rest
landings visible on the steps
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Figure 5 The Island face
Figure 6 Island face, wide angle
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4.2 Surrounding Land Use
Tintagel Castle is an important national monument with a large number of visitors. The area is
designated as an SSSI. The costal path passes through the mainland portion of the Tintagel site.
The open ground on the island is used for sheep grazing. Several buildings (offices, toilets and a
café) are present on the mainland to the west of the proposed bridge site.
4.3 Brief Monument Description and History
Tintagel Castle is an important national monument and the site area has a well-documented
history. The following description is taken mainly from the English Nature webpage.
The mainland section of the castle is in two parts, the lower and upper wards. The lower ward is
the courtyard forming the entrance to the whole castle, and enclosed on the south-east and
north-east sides by a curtain wall. On top of the crag is the upper ward, with a further curtain
wall and various small buildings belonging to the medieval castle. At the south end of the lower
courtyard is the medieval gateway forming the entrance to the castle. Outside this is the great
ditch (Dark Age), making the headland into a promontory fort. Inland and uphill, several more
banks and mounds are visible of unknown age.
On the island the inner ward contained the castle’s Great Hall, built on a sheltered, man-made
terrace. By 1337, only a century after being built, the hall was in decay, and a few years later it
was rebuilt as smaller buildings on the same site. This area was also a main focus of the Dark
Age occupation, although any Dark Age remains are now buried underneath the medieval castle.
4.4 Literature Survey - Geomorphology and Geology of the Study Site
Tintagel castle lies within an SSSI: the cliffs exhibit considerable geomorphological and
geological interest in addition to supporting an outstanding flora and fauna. The SSSI site
contains hanging valleys, waterfalls, hogs-back and bevelled cliffs. It demonstrates very clearly
the relationship of geological structure to cliff development. There are several academic studies
available covering the coastal exposures.
The adjoining area displays strong relationships between coastal forms and bedrock structures.
These relationships have been studied in detail esp. Wilson (1952). Between West Cove, Tintagel
and Bossiney Haven (SX 065 896), the coastline is complex. Three promontories, Tintagel
Island, Barras Nose and Willapark, each with a narrow neck, are in different stages of separation
from the mainland. The cliffs are mainly slope-over-wall forms bevelled at about +80 mOD, but
at Willapark there is an excellent example of a hogback cliff.
The coast is cut into Upper Devonian slates, siliceous sandstones, pillow lavas and tuffs and
phyllites, which have been overthrust towards the NNW (Wilson, 1951). The overthrust strata
were affected by approximately parallel normal faulting. The beds dip generally to the west and
the normal faulting throws the thrust-slices down to the west or north-west (Wilson 1952).
The normal faults have had considerable influence on cliff forms. Figure 2 shows the Island form
– a square with the corners at the cardinal compass points - controlled by the main orthogonal
discontinuities. South of Tintagel Island, some short stretches of cliffline are true fault-line cliffs.
Elsewhere, erosion has cut cliffs back from their original fault-controlled position.
The Tintagel castle cliffs and platforms consist of lower Carboniferous and upper Devonian rocks.
Some of the north-ward thrust faulting has been "low angle" and has resulted in the stacking of
successive layers of Delabole Slates (this term is not currently used by the BGS) and Tintagel
Group rocks above each other out of their original depositional sequence. Thus on the island the
uppermost rocks are Delabole Slates which are older than the rocks of the Tintagel Group that lie
beneath them, (and which are clearly visible as black slates both below Iron Gate and on the
access path on the headland). Below this layer of Tintagel Group rocks lie more of the older
Delabole Slates as seen in Tintagel Haven (Gifford Stability Report).
The Tintagel Group can also be seen as black slates, above Delabole Slates, on the west cliffs
immediately below the Lower and Upper Castle Wards, but here the group is also visible lower
down, out of sequence, as volcanic rocks at the base of the cliff. It is in this area that the
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dramatic effect of fissuring and faulting is best demonstrated. The stability of the west face of
the rock mass supporting the Upper and Lower Wards is almost completely controlled by the
orientation of major fractures threatening incipient toppling failures of the rock that would cause
loss of significant parts of the monument. The grassy sloping surface of the clifftop to the south
of the Monument lies on the plane of a fault sloping steeply to the north, evidencing an obvious
past failure by sliding on this fault plane. This is typical of the way in which the complex and
disrupted geology of Tintagel has both brought about its current form, and threatens its future
stability (Gifford Stability Report).
The three main groups of rock originated from soft materials deposited in a shallow sea. The
oldest rocks present are the Upper Delabole Slates of the Upper Devonian Period, these are
generally light coloured, and are followed by younger, darker, Lower Carboniferous slates and
siltstones. The youngest rocks present are volcanic agglomerates, originally a mixture of lavas
and ash ejected from volcanoes into the shallow sea on top of the previous deposits, still during
the Lower Carboniferous period. Together the dark slates, siltstones and volcanics make up the
Tintagel Group.
The main groups of rock have been assigned the following names and general rock descriptions
by the British Geological Survey (BGS) on the 1:50,000 scale mapping information available on
the BGS web information service.
Bedrock geology outcropping at the top of the Island and the mainland is described by the BGS
as the Tredorn Slate Formation - Slate. This is a Sedimentary Bedrock formed approximately
354 million to 364 million years ago in the Devonian Period. The local depositional environment
was dominated by open seas with pelagite deposits. This formation is generally found to be a
greenish grey quartz-chlorite-mica slate, locally interbedded with thinly bedded, commonly
lenticular bioclastic limestone and dolomite beds, up to 0.15 metres thick, and with sandstone,
siltstone and rare tuff beds.
This formation is older than the rock units encountered below it on this site, this is believed to be
due to a large-throw thrust fault between the Tredorn Slate Formation and the underlying
younger rocks.
The BGS describes the Tintagel Volcanic Formation (shown on the 1:50000 map as mainly
outcropping in the cliffs) as a Tuff and Agglomerate. The Tintagel Volcanic Formation is an
igneous bedrock formed approximately 327 million to 354 million years ago during the
Carboniferous Period. The depositional environment for this formation was dominated by
explosive eruptions of magma. The lower boundary of this formation is taken at the sharp
contact of tuffs, lavas and agglomerates of the Tintagel Volcanic Formation with the siltstone-
striped mudstones of the underlying Barras Nose Formation. The upper boundary is taken at the
sharp contact of the finely banded dark to pale grey mudstones with thin siltstones of the
overlying Trambley Cove Formation with the tuffs, lavas and agglomerates of the Tintagel
Volcanic Formation. The Trambley Cove formation was not apparent on the site visit.
The BGS 1:50000 information describes the Barras Nose Formation as mainly outcropping in the
cliffs as Slate. This formation is described as a Sedimentary Bedrock formed approximately 327
million to 354 million years ago during the Carboniferous Period. The local depositional
environment for this deposit was dominated by open seas with pelagite deposits. The formation
consists of dark grey and black mudstones with variably abundant very thin beds and laminae of
cross laminated and graded siltstone and sandstone. The mudstones contain scattered sideritic,
carbonaceous silty sandstone and argillaceous limestone nodules, containing goniatites. A thin
fossiliferous limestone is present at or very near the top of the Formation. Volcanic material
locally occurs within this formation as lenses of sheared vesicular lava and tuff. The upper
boundary is taken at the sharp contact of tuffs, lavas and agglomerates of the overlying Tintagel
Volcanic Formation with the mudstones and thin siltstones of the Barras Nose Formation. The
depositional lower boundary is not seen.
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Figure 7 Coastline development controlled by major faults - one crosses the line of the bridge (after Wilson, 1952)
The faulting at Tintagel is dominated by two important fault zones: the Castle Fault between
West Cove and Smith's Cliff, and the Caves Fault Zone, which cuts through the Island across
Tintagel Haven to Barras Gug. The thrust planes lie at low angles to the horizontal, but the
normal faults form steeply sloping shear zones, which Wilson noted are easily worked on by
marine erosion where exposed. Joints, particularly with a general alignment towards 325° –
330° and north–south joints also play an important part in the coastal morphology of this site.
Erosion has taken place along structurally controlled weak zones and preferred locations
depending upon the location of sea level relative to exposed features. Where structural
weaknesses were flat or gently dipping, they have only influenced the process of marine erosion
if they occurred close to sea level. In contrast, steeply inclined lines or zones of weakness could
control the direction of marine erosion over a large range of sea levels, for if the line of weakness
continues through the cliffs both above and below sea level, any features associated with the
particular line of weakness can continue to develop whether sea level falls or rises. (Wilson
1952).
Most of the faults on this coastline strike in a direction more or less parallel to the direction of
maximum fetch. The normal faults appear to have been most important as they trend at an
acute angle to the present-day coastline. Once the sea had penetrated into these parallel fault-
zones it began to cut back the cliffline by undercutting the harder rock bands between the
inclined shatter zones. Since many of the faults dip seawards at about 45°, cliffs develop by
removal of the shatter zone material and the development of a structurally controlled sloping
surface. The sea would subsequently cut a vertical wall in the lower part of the slope to produce
the slope-over-wall form (Wilson 1952).
4.5 Literature Survey – Stability of Site
Rock fall is an on-going risk that is addressed by regular inspection with scaling and netting (see
the Babtie/Vertical Technology reports). Large scale failures have occurred in the past and
potential large scale failures are discussed in reports.
The Gifford Stability Report states:
In Area 7 (the South West facing cliffs under the mainland castle - this area is not directly on the
line of the new bridge) the most significant risk on the whole site threatens the west side of the
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Upper and Lower Wards. Further development of the flexural toppling failure on this face would
irrevocably destroy a large proportion of this part of the monument…….
Mitigation was suggested:
on site review points to the possibility of stabilising this face by rock anchoring using through-
drilling techniques from readily accessible locations at the east of the Lower and Upper Wards.
5. SITE VISIT - GEOMORPHOLOGICAL AND GEOLOGICAL
OBSERVATIONS AND INTERPRETATION
Detailed rock descriptions of observed exposures are given in the Appendix. These descriptions
include structural information on the exposures where possible quoting dip angle followed by a
dip direction.
5.1 Erosion
Collapsed stairways and walls were observed along the strip of rock linking the mainland and the
Island, these observations show that the neck is eroding with over one metre vertical loss in 40
years (this was verified by conversation with the Site Manager, Matt Ward). The eroded material
on the west side consists of very slabby boulders. Large more spherical boulders are present on
the east side of the neck.
The fault zones created by the normal faults are obviously weaker and being preferentially
eroded at sea level to form features described on this site as caves. In most cases these caves
do not penetrate more than a few metres into the exposed cliff face but what is known as Merlin's
cave runs right through the island from the Haven to the Atlantic (90 metres long, 6 metres high,
6 metres wide).
5.2 Rock
See Appendix for detailed rock descriptions and photographs.
The rock at all locations was observed as strong. No joints were open except the extremely close
lineations (possibly bedding) within the Barras Nose Formation mudstone these were found to be
tight to partly open and smooth with a silvery lustre. Partly open joints were filled with greenish
brown silt and a white (possibly calcareous) hard deposit.
The Tredorn Slate Formation was observed as a strong, prismatic, bluish green to bluish grey
slate containing frequent veins of orange and white - translucent quartz. The quartz veins are
predominantly 10 mm to 100 mm thick, however some have been observed up to 700 mm thick.
The outcrop is dark grey where it has been weathered. Fresh surfaces along slatey cleavage are
striated. Lineation within the slate could be the remains of bedding. Dip of the 'bedding' is
observed as 16°/114° and 24°/188°. Joints are very close to closely spaced and tight. There
are two major discontinuity sets within this formation running roughly perpendicular to each
other with one set dipping 54°/320° and 56°/354°, and the other dipping 78°/262°.
Barras Nose Formation was found to be a strong, black mudstone interbedded with green mottled
orange foliated siltstones. Frequent orange to white - translucent quartz veins predominantly
10 mm to 100 mm but were also observed up to 650 mm, veins of black to dark grey calcite up
to 20 mm thick are also present. The dip of the 650 mm thick quartz vein is 24°/336°. The
650 mm thick quartz vein is likely to be the surface representation of Merlin's Cave Fault.
Lineations (possibly bedding) within the mudstone are extremely close, tight to partly open and
smooth with a silvery lustre. Partly open joints are filled with greenish brown silt and white
(possibly calcareous) hard deposit. The 'bedding' of this formation dips at 28°/178°.
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5.3 Structural Controls
There are two major structural controls evident in the area of the neck, these are shown on
Figure 8.
Faulting – the site is dominated by normal faults with a dip of between 25° and 45° and a
dip direction of 320° (North-West). The faults (and the fault zones) are controlling the
formation of the caves and the shape of the coastline. The turfed slopes are located on the
mainland are parallel to the dip of the faults. The proposed landing on the Island will be
directly above the fault zone that comes up from Merlin's Cave to outcrop on the island
face at the thick quartz band just below the netting. Merlin's Cave follows the fault zone at
sea level and goes right through to the Atlantic side of the Island from The Haven.
Major joint set - persistent over many 10s of metres in the Tredorn Slate Formation, dip
80° dip direction 050° (North East). This discontinuity set is controlling the near vertical
faces to the east of the Island, the small fin to the east of the bridge on the Island face,
the eastern faces of rocks at sea level on the Atlantic/island side of the bridge, the fin of
rock above the ticket booth, and also the western faces of the Upper and Lower Courtyards
on the mainland. Toppling failure of the latter is a concern with respect to the integrity of
the monument – however this failure mechanism, if it occurs, is located on the other side
of the rock to the proposed bridge landing.
Another major, widely spaced, persistent over several metres, joint set, dipping North West
(parallel to the fault below), is visible on the western side of the mainland castle in the cliffs
below the Upper and Lower courtyard (Figure 10). This joint set is considered to be the
structural control of the angle of the turf slope above (south west) the ticket booth.
Discontinuities at this angle will not daylight on this slope and will produce a stable condition for
a compressive foundation load – especially as foundation loads at this location on the face are
likely to have a component acting into the slope.
Closely spaced discontinuities were observed in the Tredorn Slate Formation: observed joints are
very close to closely spaced and tight. There are two major sets running roughly perpendicular
to each other with one set dipping 54°/320° and 56°/354°, and the other dipping 78°/082°.
Fresh surfaces along slatey cleavage are striated. These closely spaced discontinuities are
approximately following the directions of the normal faulting and the major joint set.
Extremely closely spaced discontinuities were observed in the Barras Nose Formation: Lineations
(possibly bedding) within the mudstone are extremely close, tight to partly open and smooth with
a silvery lustre. Partly open joints filled with greenish brown silt and white (possibly calcareous)
deposit. The 'bedding' dips at 28°/178°.
Above the ticket booth is a forty degree turfed slope. This follows the North West dipping joint
set visible in the western cliffs. It is not obvious that this follows a fault. No fault is visible in the
fin of rock above the ticket booth.
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Figure 8 Faults and the major joint set
Fault at sea level dip 40°dip
direction 320°, 350°
Fault at high level surface
Joint set dip 80° dip
direction 050° to
090°
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Figure 9 Normal faulting/fault zone visible on the western cliff below the upper ward.
The face contains thick quartz veins and quartz filled tension gashes.
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Figure 10 Major joint set.
Slightly over-vertical South West facing cliff faces under the Lower Ward caused by the major joint set
dip 80° dip direction 050° (North East). Also visible is the joint set parallel to the normal faulting dip
45° dip direction North West (this has created the major overhanging sloping lip and several obvious
smaller ones – the joints are non-persistent over the length of the exposure).
5.4 Hydrology
No seepages of water were visible on either face or on the adjacent cliffs after an extensive
period of wet weather lasting several months. The only seepages visible were from the interface
between the long turf slope west of the mainland lower courtyard and the rock just above the
beach.
6. DESK STUDY AND SITE VISIT - GEOTECHNICAL
RECOMMENDATIONS
The following recommendations have been developed to inform the assessment of high level
bridge conceptual designs and the selection of preferred concepts. No particular geological
feature or characteristic of this site is considered to prevent the construction of a high level
bridge however certain risks and issues need to be considered in the conceptual design of the
bridge.
6.1 Mainland Rock Slope and Landing
The pace of erosion at the neck strongly argues against any construction below the ticket booth
on the mainland.
Joint set parallel
to normal fault
Overhanging lip
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Construction on the land side above the ticket booth will have to take into account the possibility
of persistent joints (over several metres) lying parallel to the slope. Discontinuities at this angle
will not daylight onto the slope and will produce a stable condition for a foundation load with a
component into the slope. There is no evidence of these joints having any infill that may be
compressible resulting in a reduction in rock mass strength or greater than anticipated
deformation when subject to a normal load.
The report produced by Babtie and Vertical Technology (2000 Part 2) gives details of three probe
holes drilled adjacent to each of the upper and lower rest platforms present in the current steps.
These gave depths to intact bedrock of between 0.5 metres and 1.5 metres.
The bridge landing on the mainland is located at the beginning of the existing steps of the Lower
Court, at the top of the steep turf covered slope. Based on the probe holes and the surrounding
exposures it is probable that rock will be present at a shallow depth at this location.
The front, North West, steep turf covered slope is controlled by the North West dipping joint set.
There is no evidence that the less steep North East slope has any controlling joint set or adverse
discontinuities in relation to the stability of the bridge landing.
The landing is remote from the overhanging part of the south west face.
6.2 Island Rock Slope and Landing
The pace of erosion at the neck strongly argues against any construction on the Island face until
the change in slope above the landing of the current wooden bridge is passed. This places any
foundation on the island slope into the fault zone unless it terminates above the thick quartz
band. The fault zone is dipping into the island giving less concern with global stability but the
rock is obviously weaker than the surrounding rock – it is being preferentially eroded at sea level
to form the cave. This will not be an issue at the landing level – construction can occur on this
weaker rock.
7. SITE VISIT - ASSESSMENT OF FURTHER
INVESTIGATION AND ACCESS FOR PLANT AND
EQUIPMENT
The preliminary rock mapping during the site visit was restricted by the difficulties in accessing
the large areas of steep (to vertical) exposed rock mass. Detailed rock discontinuity mapping will
be required to locate any defects which result in the rock mass being weaker than the intact rock.
Specialist contractors will be needed to access the rock faces.
Also further investigation will be required to establish:
The rock profile under the mainland turfed slope at potential foundation locations
The strength and general integrity of the rock at potential foundation locations. This is
especially important on the island face where – depending on the bridge design - the
founding rock is likely to be composed of fault zone material.
Road access exists to within 100 metres of the ticket booth – a wide walkway has been
constructed from the road to the ticket booth. This is in part a wooden structure with stairs
which may be passable (with a minimum of temporary works) by a small rubber tracked rig. This
would allow investigation for foundation elements in the lower part of the mainland face above
the ticket booth.
Obtaining cores from the areas likely to be loaded by the bridge foundations will be safely
accomplished by angled drilling from a rig sited on the top of the faces. To get cores from the
DESK STUDY AND SITE VISIT REPORT
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Desk Study and Site Visit Page 16 Report Ref: 61031755/GT/R01
loaded zone of the mainland, the borehole will be at an acute angle to the joint set in the face.
At this angle the borehole will likely miss any joint and not give information on spacing or form of
the discontinuities. A separate borehole will be needed; drilled at an angle of opposite dip, to
cross the discontinuities, the findings of this borehole can then be correlated with the
discontinuity observations made of the exposures.
The mainland rock face investigation will require a rig to operate from within the lower courtyard.
The island rock face investigation will require a rig to operate from East of the island courtyard -
these investigation positions match the proposed bridge landings.
Access to the island will be difficult as the only current route is across the low level bridge via the
steps cut into the rock slopes. Hand-held rigs could be transported across to the island in
sections however the capacity of these types of rig is limited in terms of depth and the strength
of rock that can be cored/recovered. A hand held rig should be considered for probing for the
rock head under the turf and soil on the island. In strong slate this will only give short
retrievable cores but will allow several metres of probe hole.
The requirement for long cores at depth (the length and depth will be a function of the foundation
location and loads) in the slate will govern the rig specification and any additional temporary
works that may be needed to get equipment across to the island.
Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited
Desk Study and Site Visit Report Ref: 61031755/GT/R01
APPENDIX 1
STRATA OBSERVED DURING SITE VISIT
Tredorn Slate Formation
BGS website description; 'Greenish grey quartz-chlorite-mica slate, locally interbedded with thinly
bedded, commonly lenticular bioclastic limestone and dolomite beds, up to 0.15 metres thick,
and with sandstone, siltstone and rare tuff beds.'
Field description; (Strata description location 1) Fresh outcrops observed as a strong, prismatic,
bluish green to bluish grey slate containing frequent veins of orange and white - translucent
quartz. The quartz veins are predominantly 10 mm to 100 mm thick however they are also
observed up to 700 mm thick.
The outcrop is dark grey where it has been weathered.
Fresh surfaces along slaty cleavage are striated. Lineation within the slate could be remains of
bedding. Dip of the 'bedding' is observed as 16/114 and 24/188. Joints are very close to closely
spaced and tight. There are two major sets running roughly perpendicular to each other with one
set dipping 54/320 and 56/354, and the other dipping 78/262.
Tredorn Slate Formation 700 mm quartz vein Quartz crystals within vein
Barras Nose Formation
BGS website description; 'Dark grey and black mudstones with variably abundant very thin beds
and laminae of cross laminated and graded siltstone and sandstone. The mudstones contain
scattered sideritic, carbonaceous silty sandstone and argillaceous limestone nodules, containing
goniatites. A thin fossiliferous limestone is present at or very near the top of the Formation.
Volcanic material locally occurs as lenses of sheared vesicular lava and tuff.'
Field description; (Strata description location 2) Observed as a strong, black mudstone
interbedded with green mottled orange foliated siltstones. Frequent orange to white -
translucent quartz veins predominantly 10 mm to 100 mm but also observed up to 650 mm and
black to dark grey calcite up to 20 mm thick are present. The dip of the 650 mm thick quartz
vein is 24/336. The 650 mm thick quartz vein could possibly be the surface representation of
Merlin's Cave Fault.
Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited
Desk Study and Site Visit Report Ref: 61031755/GT/R01
Lineations within the mudstone thought to be bedding are extremely close, tight to partly open
and smooth with a silvery lustre. Partly open joints filled with greenish brown silt and white
(possibly calcareous) deposit. The 'bedding' dips at 28/178.
Dark Barras Nose Formation mudstone Overview of the Formation
(Strata description location 3) A lens of possible vesicular lava is present within the Barras Nose
Formation directly above the thick quartz vein in the form of a hard, black, orange iron stained
rock. Vesicles are up to 30 mm diameter. The deposit contains frequent white quartz veins and
black calcite veins.
Possible lava within Barras Nose
Formation
Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited
Desk Study and Site Visit Report Ref: 61031755/GT/R01
Strata description locations