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The Stability of Coastal Slopes in the Culm of S 8 W H

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The Stability of Coastal Slopes in the Culm of S 8 W H THE STABILITY OF COASTAL SLOPES IN THE CULM OF S8WH W&S^ ENGLAND Michael Henry de Freitas A thesis submitted for the degree of Doctor of Philosophy of the University of London Department of Geology Imperial College London, SWT DECEMBER 1981 ABSTRACT cn This work describes the forms of coastal instability dev- eloped in the Culm and uses this instability to examine the nature of the discrepancy separating analytical notions of failure from the failure which exists in reality. The work concludes that failure criteria expressed in terms of principal stress, cohesion and friction do not include a sufficient measure of likely rock response, viz. .the release of residual strain and its effect upon failure. Culm consists of sandstones, siltstones, mudstones and shales, intimately interbedded and deformed by folding and faulting. The physiography of the area has developed since the mid- Oligocene (approx. 30 my BP) and is dominated by erosion platforms into which has been cut the coastline, the most recent period of retreat starting 7000 years ago when rising sea levels approached present sea levels. The instability seen at present ranges in age from 3000 years to present day and the erosion cycles of which this instability is part range in duration from 100 years to 6000 years. Average rates of erosion vary between 0.18 to 0.29 ft/yr (0.05 to 0.09 m/yr) along Bideford Bay, and 0.57 ft/yr (0,17 m/yr) along the Atlantic coast. The basic forms of instability are wedge translation, toppl- ing and their combination, which only develops to any extent in Bideford Bay. Back analyses at 26 sites has revealed that the Culm exhibits a consistent non^linear relationship between the angle of sliding friction and the percentage of sandstone, a general lack of cohesion and an ability to generate new joints when stressed. The new joints follow geological directions and their repeated occurrence in field failures implies that failure criteria should include a measure of their cause. It is suggested that the measure of maximum stress should be augmented by that of the total work done during failure. Th'ts thesis hag been combleW in mmory of Eileen Cecilia Labfhorn 51^1" October 1916 - 2 August 1977 Go forlh ubon fhy journey Christian soul (ao frotvi this world in the name of 6od The omniboteni' Father who creaW thee. In the rwme of Jesos Christ" our Lord who died Ar thee. Arifi in the name of the Holy 3birif Who halh been |)oureJ out on +hee. Go in the ncime of Angels I Archtin^els; In tk name of Thncedoms ^ of Powers; ^l^d in the name of holy Virqins ^ all the Saints of God. Go on thy ck)urg^ | May thy bidce today be toond in jeace, ^ tvifly ttiy dwelling be the Mount of Sion. Throoqn Christ our LoiW. hmm. Cardinal Lhn Henry NeWman (ii) TIATE 1 Amonq ihe shinqle ai CloVedy C.NL.N. Henemm y Exhibira a\ the Royal f\cadmy 1865 by\ permission Lai'na Art Gallery Newcastle A view from Skitterinq Kock ^hoioinq [iie targe diTft oFTne Hobby with ^isartoh Wood beyond. CONTENTS . Oil') Abstract 1 Frontise: Plate 1 (ii) List of Contents (iii' List of Line Drawings (vi) List of Tables (ix) List of Plates (x) Units (xi) Acknowledgements (xiii) PART I: Chapters 1, 2, 3 and 4 Chapter 1. Introduction 1.1 Choice of area 3 1.2 Choice of studies 7 1.3 Choice of approach 11 Chapter 2. Coastal Geology 2.1 Introduction 15 2.2 Stratigraphy 18 2.3 Stratigraphic history 25 2.4 Structure 31 2.5 Structural history 45 Chapter 3. Culm Strength 3.1 Introduction 51 3.2 Joints and folding 53 3.3 Deterioration of shales 38 3.4 Small scale assessments of strength 67 3.5 Hydrogeology 73 3.6 Pore water pressures 81 3.7 Creep 94 3.8 Present seismicity 96 3.9 Strength of folded Culm 98 CONTENTS Chapter 4. Coastal Evolution 4.1 Introduction 106 4.2 Cliff height 111 4.3 Cliff shape II7 4.4 Cliff age 128 4.5 Erosion rates 142 PART II. Chapters 5, 6, 7 and 8 Chapter 5. Translation 5.1 Introduction 149 5.2 Simple examples 150 5.2.1 (GR. 3975-2610) Classical translation 150 5.2.2 (GR. 4006-2680) Classical wedge 152 5.2.3 (GR. 3785-2423) Sliding surface; details 152 5.2.4 (GR. 3020-2630) Initiation of wedge failure 155 5.2.5 (GR. 2945-2645) Repeated failures 159 5.2.6 (GR. 3220-2450 to 3225-2438) Failure combinations 163 5.3 Influence of folds 167 5.3.1 (GR. 3765-2415) Influence of axial surface 167 5.3.2 (GR. 3740-2405) Failure across axial surface 167 5.3.3 (GR. 2460-2725) Failure along fold trough 171 5.3.4 (GR.,2090-1720) Failure of fold nose 175 5.4 Associated deformation 179 5.4.1 (GR. 3160-2530) Buckling failure 180 5.4.2 (GR. 2998-2655) Details of buckling 182 5.4.3 (GR. 3535-2370) Deformation on impact 187 5.4.4 (GR. 3340-2386) Failure on impact 189 5.4.5 (GR. 3615-2385) Lateral failure 192 5.5 Resume 195 Chapter 6. Toppling 6.1 Introduction CONTENTS / \ 6.2 Elements of toppling 6.2.1 (OR. 3552-23 68) Classical relationships 201 6.2.2 (OR. 3508-2372) Toppling to trans- lation 20 6 6.2.3 (GR. 2995-2 660) Multiple rotation 208 6.2.4 (GR. 2448-2405) Development of distress joints 212 6.3 Inclined toppling 6.3.1 (GR. 2O6O-I715) Simple inclined toppling 218 6.3.2 (GR. 3970-2 600 to 4238-2607) In- clined toppling-vs-translation 221 6. 4 Resume 22 4 Chapter 7- Large Failures 7.1 Introduction 230 7.2. (GR. 3020-2630 to 30 60-2615) Age restriction on size 231 7.3 (GR. 3710-2397 to 3770-2430) Age relation- ship with slope 237 7.4 (GR. 3315-2400) Boundaries of large scale instability 241 7.5 (GR. 3356-23 68 to 3420-23 68) Internal structure of large scale instability 249 7.6 (GR. 3464-2385 to 3430-2387) Failure in the sea cliffs. 258 7.7 Resume 270 Chapter 8. Discussion 8.1 Reflections 279 8.2 Critique 289 8.3 Conclusion 292 References 300 Appendix A1-A54 LIST OF LINE DRAWINGS Lvi) CHAPTER 1: In troduc tion Page Fig. 1. 1 Area of study- 4 Fig. 1. 2 General geology (1974) 6 Fi g. 1. 3 Primary location map of sites 8 CHAPTER 2: Coastal Geology Fig. 2. 1 Maps of the Culm (1901), (1971) 17 Fig. 2. 2 Distribution of Culm lithologies 24 Fig. 2. 3 Palaeogeography; Middle-Upper Devonian 27 Fig. 2. 4 General structure of folds in cliffs 32 Fig. 2. 5 Structure near Westward Ho'. 34 Fig. 2. 6 Structure from Hartland Qy. to Widemouth Bay 37 Fig. 2. 7 Structure from Widemouth Bay to Rusey Beach 38 Fig. 2. 8 . Structure from Millook to Boscastle 40 Fig. 2. 9 General faulting of the cliffs 42 Fig. 2. 10 General structural history 47 CHAPTER 3: Culm Strength Fig. 3 .1 {a-c) Joints and folding 54 Fig. 3 .2 Joints, stress and porewater pressure 57 Fig. 3 .3 Strain energy 59 Fig. 3 .4 Jointing and bedding in Culm 60 Fig. 3. 5 Degradation of shear strength 72 Fig. 3 . 6 Gouge characters 74 Fig. 3 .7 Hydrogeological boundaries 75 Fig. 3. 8 Water levels in the Culm 77 Fig. 3 .9 Springs in the Culm 78 Fig. 3. 10 Head with depth 80 Fig. 3. 11 Pump test site 82 Summary logs for pump test site 83 Fig. 3 .1 2 Fig. 3 .1 3 Bideford Bay simplified section 87 88 Fi g. 3. 14 Planar fissure pattern Fig. 3. 15 Folded fissure pattern 89 Fig. 3 .1 6 Fold and planar networks (A) 91 93 Fig. 3 .1 7 Fold and planar networks (B) LIST OF LINE DRAWINGS LHu) Page Fig. 3 .1 8 Arctan profile for Culm 99 Fig. 3 .1 9 Slide at Collumpton 100 Fig. 3. 20 Water levels at Collumpton 102 Fig. 3 .2 1 Joint spacing in Culm sandstones 104 CHAPTER 4: Coastal Evolution Fig. 4. 1 (a-b) Slope development 108 Fig. 4. 2(a- c) Normal cycle of erosion 112 Fig. 4. 3 Coastal development with emergence 118 Fig. 4. 4 The shape of Bideford Bay 120 Fig. 4. 5 North Devon cliffs 122 Fig. 4. 6 The distribution of cliff types 124 Fig. 4. 7 The retreat of a.cliff 127 Fig. 4. 8(a- c) River profiles for Bideford Bay 134 Fig. 4. 9 Cliff history for Bideford Bay 141 Fig. 4. 10 Historical notes: Gore and Clovelly view 143 CHAPTER 5: Translation Fig. 5. 1 GR. 3975-2610: Classical translation 151 Fig. 5. 2 GR. 4006-2680: Classical wedge 153 Fig. 5. 3 GR . 3785-2423: Sliding surface; de tails 154 Fig. 5. 4 (a-b) GR. 3020-2630: Initiation of wedge failure 15 6-8 Fig. 5. 5 (a —b ) GR. 2945-2645: Repeated failures 160- 2 Fig. 5. 6( a-c) GR. 3220-2450 to 3225-2438: Failure combinations 164- 5 Fig. 5. 7 GR. 3765-2415: Influence of axial surface 168 Fig. 5. 8(a- b) GR. 3740-2405: Failure across axial surf ace 169- 70 Fig. 5. 9(a- c) GR. 2460-2725: Failure along fold trough 172- 4 Fig. 5. 10 (a -b)GR. 2090-1720: Failure of fold nose 175a -b Fig.
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