TINTAGEL CASTLE BRIDGE GEOTECHNICAL EVALUATION DESK STUDY ... · Desk Study and Site Visit Page 7...

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 -


Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited

Desk Study and Site Visit Page 1 Report Ref: 61031755/GT/R01

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




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.




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.

http://www.bgs.ac.uk/services

http://www.jncc.gov.uk/page-2731




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.




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




Figure 5 The Island face

Figure 6 Island face, wide angle




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




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.




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




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°.




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.




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°




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.




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




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




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.


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.



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



Strata description locations

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