Tintagel Castle Bridge Geotechnical Evaluation Desk Study and Site Visit Report
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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 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 - 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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. Existing bridge Proposed high level bridge Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 1 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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. Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 2 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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. Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 3 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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. Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 4 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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 Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 5 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT Figure 5 The Island face Figure 6 Island face, wide angle Tintagel Castle Bridge Geotechnical Evaluation Ramboll UK Limited Desk Study and Site Visit Page 6 Report Ref: 61031755/GT/R01 DESK STUDY AND SITE VISIT REPORT 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