High-Point Rendel Borough of Telford & Wrekin Council Ironbridge Gorge Instability The Interpretation of Ground Investigations at Jackfield and the Lloyds JANUARY 2005 R/2088/01 CONTROL SHEET Client: Borough of Telford & Wrekin Council Project: Ironbridge Gorge Landslides Topic: The Interpretation of Ground Investigations at Jackfield and The Lloyds Document Number: R/2088/01 Record of Status and Approval 6 Final 5 Final For Clients Approval DGC DGC DGC Draft Dec 04 Dec 04 Dec 04 4 Final For Clients Approval DGC DGC DGC Draft Apr 04 Apr 04 Apr 04 Sent by Email Draft For Clients Approval incorporating Client’s DGC DGC DGC 3 and EP’s comments Dec 03 Dec 03 Dec 03 Sent by Email DGC DGC DGC 2 Draft For Clients Approval 6/10/03 6/10/03 6/10/03 1 Draft For Internal Review and Client’s Comments DGC DGC DGC 15 /08/03 15 /08/03 15 /08/03 Issue Status Description of Amendments Prepared Ass. Mgr. Ass. Dir. No. by. Sign & Sign and Sign and Date Date Date High-Point Rendel Ltd The McLaren Building Telephone: (0121) 236 1993 35 Dale End Birmingham B4 7NN Fax: (0121) 236 1994 H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study TABLE OF CONTENTS Section Page EXECUTIVE SUMMARY NON TECHNICAL SUMMARY GLOSSARY 1 INTRODUCTION 1 1.1 Terms of Reference 1 1.2 Objectives 2 1.3 This Report 3 2 DESK STUDY 4 2.1 Background and Previous Studies 4 2.2 Geology and Geological Structure 7 2.3 Geomorphology and Landsliding 10 2.4 Frequency of Landslides 20 2.5 Hydrogeology, Hydrology and The River Severn 21 2.6 Ground Investigation and Geotechnical Properties 27 2.7 Ground Investigations and Published Geology 51 2.8 Monitoring Pins 55 2.9 Mining 58 2.10 Remedial Measures at Lloyds Head & Lloyds Cottage 68 2.11 Climate Change 69 2.12 Do Nothing Scenario 70 3 SLOPE INSTABILITY 73 3.1 Introduction 73 3.2 The Causes of Slope Instability 73 3.3 The Causes of Current Slope Instability: Jackfield 76 3.4 The Causes of Current Slope Instability: Lloyds Coppice 79 3.5 The Causes of Current Slope Instability: Lloyds Head 81 3.6 The Ground Model 82 H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study 3.7 Slope Stability Analysis 86 3.8 Environmental Considerations 93 3.9 Summary Conclusions of Ground Stability 94 4 IRONBRIDGE GORGE SLOPE STRATEGY PLAN 101 4.1 Introduction 101 4.2 The Risk Assessment 102 4.3 The Slope Management Strategy 108 4.4 Jackfield Strategy 111 4.5 Lloyds Coppice Strategy 113 4.6 Lloyds Head Strategy 114 4.7 Options for Remedial Works 115 4.8 Summary of Remedial Recommendations 118 4.9 Telford & Wrekin Management Plan 121 4.10 Programme 121 4.11 Health & Safety 122 4.12 Study Uncertainties 123 5 SUMMARY AND CONCLUSIONS 126 6 REFERENCES AND BIBLIOGRAPHY 132 Appendices Appendix A Information provided by Telford & Wrekin Council Appendix B1 Inclinometer Plots for Study Area Appendix B2 Piezometer Readings Appendix C Landslide Events Appendix D Lloyds Cottage Letter (HPR to TWC, 2 June 2003) Appendix E Slope Stability Analysis H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study Figures Figure 2.1 Study Area Location Plan Figure 2.2 Study Area Management Sub-units Figure 2.3 Geology and Geological Structure Figure 2.4 Geomorphological Units Figure 2.5 Areas of Recent and Historic Slippage Figure 2.6 The Shallow Translational Landslide at Jackfield Figure 2.7 River Severn Level v. Rainfall Figure 2.8 River Severn Level: Buildwas v. Welshbridge Figure 2.9 Relationship Between Small Landslide Events, Rainfall and River Levels Figure 2.10 Boreholes Positions, Cross Sections and Former River Course Figure 2.11 Shear Stress v. Mean of Principle Stress Figure 2.12 Superficial Geology & Made Ground Deposits Figure 2.13a Geological Cross Sections Jackfield; BB. EE and GG Figure 2.13b Geological Cross Sections Lloyds Coppice and Lloyds Head; 3 and 4 Figure 2.14 Monitoring Data Figure 2.15 Extent of Recorded Mining in the Study Area and Pit Locations Figure 2.16 The Effects of Shallow Mining on a Slope Figure 3.1 Summary Plan of Significant Data Figure 3.2 Schematic Cross Section of Jackfield Ground Model Figure 3.3 Schematic Cross Section of Lloyds Coppice Ground Model Figure 4.1 Possible Route for Road Diversion Figure 4.2 Recommended Remedial Works and Actions H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study EXECUTIVE SUMMARY Slope movement within the Ironbridge Gorge is primarily the result of the continued development of a valley system resulting from the incision of the River Severn into a geological sequence of relatively weak materials. River processes have eroded the riverbanks removing toe support of the valley sides and combined with high groundwaters following heavy rainfall ground movement occurs. In addition, the effects of the industrial revolution and continued industry into the twentieth century has resulted in the loading of the slopes with tile and mine waste and the overlying strata subsiding into the voids created by mining. Within the area of study mineral extraction has included coal and, clay for brick and tile manufacture. The study area has been divided into three main sites: Lloyds Head, Lloyds Coppice and Jackfield. Within these areas three sites have been identified to be at high risk from landslide and subsidence. Both the slopes on Lloyds Coppice and Jackfield have a history of landsliding and two areas in particular at these sites are currently showing active ground movement. Subsidence and bank erosion by river action is currently affecting the Lloyds Head area. Ground movement at all three sites is threatening the integrity and socio- economic structure of the Ironbridge area and the Ironbridge Gorge World Heritage Site. The sites of immediate concern are Lloyds Head Road, Lloyds Cottage area at The Lloyds and Salthouses Road at Jackfield. Ground movement and subsidence at these sites has been observed and confirmed by survey and the recent ground investigation. The remaining slopes of the study area have all been subject to ground movement in the past and there is no reason why these slopes will not be reactivated in the future. Although the extraction of mineral in parts of the Gorge cannot be substantiated as being a major contributory factor to slope failure, it is considered that the mining of clay, in particular, at Jackfield is likely to have had more effect on the stability of this slope due to thicker horizons of extraction compared to the coal. Mining records do not show the presence of mineral extraction in the areas of Lloyds Cottage and Lloyds Head, however, borehole data suggests that coal mining probably associated with the extraction of other minerals was undertaken at these sites. H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study The areas of current movement have been identified as sites that require remedial works ranging from urgent (The Lloyds in front of Lloyds Cottage) to within 2 years (at Lloyds Head and Jackfield) based on a relative risk assessment undertaken as part of the study. Further deterioration and neglect of all these sites will result in loss of the road network at Lloyds Head, The Lloyds and Salthouse Road, and the potential loss of property. The severance of any these roads would result in re-routing traffic and possibly threaten public safety as emergency vehicle access is restricted. In the case of the Tuckies there is no alternate vehicle access other than Salthouse Road. In addition, this would significantly affect the local socio-economic structure and potentially damage World Heritage Site status. Stabilisation works would maintain communication and emergency vehicle access between Coalport and Ironbridge and, the Tuckies and Coalford. Prior to the implementation of remedial works an automated telemetry early warning system should be installed comprising slope movement instrumentation, which would be able to inform the Council of rapid movement likely to threaten public safety and assets. The recommended remedial works for each of the three sites of concern are substantial, comprising riverbank protection, mechanical stabilisation of the slopes and drainage. All three sites require riverbank protection which is considered the primary trigger of landslides. Both the Lloyds Cottage and Jackfield sites require drainage to reduce the level of the groundwater below the sliding ground mass. Mechanical stabilisation by, for example piling, is required to prevent mass movement of the ground and loss of the road at Lloyds Cottage. Further stabilisation studies for Lloyds Head and Jackfield are required given the potential further deterioration of the ground until remediation measures are employed. In addition to the slope stabilisation works it will be necessary to continue monitoring the borehole instrumentation already installed in the area and undertake addition ground investigations and inspections on a periodic basis. Recommendations have been made for a strategic approach to manage the slope instability. This is discussed in the main report along with other recommendations for slope stabilisation at various sites, further ground investigations and action plans should road access be severed along The Lloyds, Salthouse Road and Lloyds Head. H:\GroundInvestigationsatJackfieldandTheLloyds.doc Borough Of Telford & Wrekin Council Ironbridge Gorge: Lloyds And Jackfield Landslide Study It is recommended that the Borough of Telford & Wrekin proceed with immediate effect to pursue funding for the stabilisation of The Lloyds at Lloyds Cottage and install real time monitoring on this and the Salthouse Road slope. Further studies have been recommended for the Lloyds Head area.