Identified Area of Land Slip, South Calder Water, Wishaw
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AGENDAITEMW 13 NORTH LANARKSHIRE COUNCIL REPORT To: COMMUNITY SERVICES COMMITTEE Subject: IDENTIFIED AREA OF LANDSLIP SOUTH CALDER WATER, WISHAW From: DIRECTOR OF COMMUNITY SE RVIC ES Date: 20 APRIL 2005 Ref: KW/DM I. PURPOSE OF REPORT 1.I Produced in consultation with the Director of Administration and Director of Planning and Environment, this report seeks to update Committee of the ground investigations into the identified area of landslip at the south bank of the Calder Water between the residential properties of Hillfoot GardenslHillfoot Avenue and Swinstie Sewage Treatment Works, Coltness, Wishaw. Further, this report recommends to Committee a response to the terms of the Engineering Consultants Report and also makes further recommendations. 2. BACKGROUND 2.1 In August 1999, the Committee considered an interim report on the above which advised that the opinion of Senior Counsel was that a consultant engineers report should be commissioned to ascertain the up-to-date position in respect of the landslip and monitoring works undertaken since 1978 by Motherwell District Council and continued by NLC. 2.2 Committee agreed to the commissioning of a specialist engineering consultants report on the current state of the landslip, present and future risk to neighbouring land and property interests and possible stabilisation works with estimated costs. The Halcrow Group Limited were duly appointed and submitted a report dated October 2000. Their report recommended that further investigation should be undertaken and monitoring of instrumentation carried out to determine the rate of ground movement at depth and on the surface at the locus of the western and eastern slip areas for a period of not less than a year. 2.3 Tenders for the recommended supplementary ground investigation were obtained and reported to Committee in October 2001. The Halcrow Group Limited has submitted their supplementary report on the ground investigation carried out in 2002 to install instrumentation together with its subsequent monitoring. 3. SUSTAl NAB1LlTY IMPLICATIONS 3.1 There are no sustainability implications arising from the report from Halcrow Group Limited. H:\HILLFOOT GARDENS COMMITTEE REPORT.DOC 4.1 The ground area studied by the consultants comprises two separate locations situated immediately north and west of Hillfoot Gardens, Site A, and east of Hillfoot Gardens/Hillfoot Avenue, Site B. Site A and Site B are shown on Figure 1 attached. 4.2 The monitoring studies have recorded movement within the ground at both site A and site B. There are differences in the rate of movement observed. The greatest rate of movement is located within the mass of soil that slipped in 1978 at Site A. The maximum rate of movement observed was 59 mm per year. The rate of movement behind the crest of the slope at Site A is an order of magnitude smaller at less than 10mm per year. The trend of movement at Site B is also small in comparison to the slipped mass at Site A. The report concludes the rate of soil mass movement at the west slope (Site A) appears to be reducing with time. 4.3 Part of the investigation involved undertaking current laboratory testing techniques to establish up-to-date geotechnical properties of the soil. Based on the recent testing additional slope stability analysis has been undertaken by the Halcrow Group Limited to reassess the factor of safety against potential deep seated rotational slope failure. The latest analyses confirm that the resisting shear strength is only marginally greater than the disturbing gravitational forces resulting in the theoretical factor of safety approaching unity. 4.4 In general the latest report confirms the findings of the previous reports that in engineering terms the low factors of safety indicate the likelihood of further ground movement in the long term. The report concludes that on the basis of the current rates of movement there would appear to be no immediate threat to either the housing or the roads and associated footways. In the long term through the processes of natural erosion and degradation ground displacement is likely to continue and the property considered by the consultants to be at long-term risk is shown on Figure 2. 4.5 The full conclusions and recommendations of the Halcrow Group Limited's report dated February 2004 form Appendix 1 to this report. 5. CORPORATE CONSIDERATIONS 5.1 The landslip was first identified a number of years ago and the former authority, Motherwell District Council, then commissioned a consultants' report. That confirmed the instability of the slopes and indicated that, in the event of no remedial or preventative measures being taken, the degradation of the landform would be expected to affect a number of adjoining properties. 5.2 Following receipt of the previous consultants' report, Motherwell District Council obtained legal advice indicating that there was no basis for liability on the part of the Council in respect of risk to adjoining land and property due to the occurrence of the landslip. On the basis of that advice, Motherwell District Council determined to undertake no remedial works. 5.3 The Head of Legal Services, following further consultation with Senior Counsel on the matter, similarly has previously advised that there is no basis for liability on the part of North Lanarkshire Council. Indeed, there could be a serious doubt about the legal ability of the Council to undertake preventative works which would substantially be for the sole benefit of private landowners. He has, however, recommended that (1) the Council keep itself reasonably well informed as to the state of its own property and in particular H:\HILLFOOT GARDENS COMMITTEE REPORT.DOC the unstable slopes (2) subject to the agreement of the consultants Halcrow Group Limited, that the Council share the contents of the report with neighbouring proprietors whose property, in the consultants opinion, could, potentially, be affected by landslip in the long term, and with their insurers, if requested (3) that if such proprietors were to decide to undertake remedial steps, such steps should be at the cost of the proprietors themselves but the Council would co-operate in so far as reasonably possible to allow such remedial works to be carried out by the proprietors, and (4) given the Council's obligations in respect of the issuing of property enquiry certificates, the Council has a duty to update its current response to property enquirers in light of the new information contained in the report prepared by Halcrow Group Limited. 6. RECOMMENDATIONS It is recommended that the Committee (i) Note the contents of the Halcrow Crouch Consultants report of February 2004 relating to the current state of the landslip at the south bank of the Calder Water between the residential properties of Hillfoot Gardens/Hillfoot Avenue and Swinstie Sewage Treatment Works, Coltness, Wishaw. (ii) Further that the Committee notes that the Director of Administration in consultation with the Director of Planning and Environment will amend and update as appropriate the Council's response in respect of property enquiry certificates for the residential areas of Hillfoot Gardens/Hillfoot Avenue. (iii) That the Committee approves the retention of the Halcrow Group Limited to undertake further monitoring of the inclinometers and piezometers installed on land in Council control . List of Background Papers available from Members Library Halcrow Group Limited. North Branchal Wood, Wishaw Interpretative Geotechnical Report on Ground Investigation, Carried out in 2002. February 2004-03-04 Report to Community Services Committee 31 August 1999 Report to Community Services Committee 31 October 2000 Local Government Access to Information Act: for further information about this report, please contact David Miller on 01236 618112 H:\HILLFOOT GARDENS COMMITTEE REPORT.DOC Appendix 1 Halcrow February 2004 H:\HILLFOOT GARDENS COMMITTEE REPORT.DOC Appendix 1 Cont'd - Halcrow Febriuar e+w4%w&- Y 23 H:\HILLFOOT GARDENS COMMITTEE REPORT.DOC Figure 2 Properties potentially affected in the long term c 1:1000 Based upon the Ordnance Survey mapping with ule Permissin of Her Majesty’s Stataneq ortke. 0 Crown Copyright. Unauthorised mpmducbion .hihges Cmm Copyright and may lead to prosecution or civil proceedis. Nath Lanarkshire Council OS Licence LA09041 L .