Southwark HAPS C0478 Dulwich Upper Wood Retaining Wall
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C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall 000 Southwark HAPS C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall London Borough of Southwark Document Number: 60493385-C0478-REP-0003-A 01 November 2019 C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall Quality information Prepared by Checked by Verified By Approved by Robert Jackson Stephen Glenfield Douglas Prince Douglas Prince Associate Director Engineer Principal Engineer Principal Engineer Revision History Revision Revision date Details Authorised Name Position A 01/11/2019 First Issue DP Douglas Prince Principal Engineer Distribution List # Hard Copies PDF Required Association / Company Name N/A Yes London Borough of Southwark Prepared for: London Bourogh of Southwark CONWAYAECOM 60493385-C0478-REP-0003-A Offline Wall Options Report ii C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall Prepared for: London Borough of Southwark Prepared by: Robert Jackson Associate Director T: 020 3043 9368 E: [email protected] CONWAY AECOM Limited Sunley House 4 Bedford Park Surrey Croydon CRO 2AP UK T: +44 20 8639 3500 conwayaecom.com © 2019 CONWAY AECOM Limited. All Rights Reserved. This document has been prepared by CONWAY AECOM Limited for the sole use of our client (the “Client”) and in accordance with generally accepted consultancy principles, the budget for fees and the terms of reference agreed between CONWAY AECOM Limited and the Client. Any information provided by third parties and referred to herein has not been checked or verified by CONWAY AECOM Limited, unless otherwise expressly stated in the document. No third party may rely upon this document without the prior and express written agreement of CONWAY AECOM Limited. Prepared for: London Bourogh of Southwark CONWAYAECOM 60493385-C0478-REP-0003-A Offline Wall Options Report iii C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall Table of Contents Executive Summary ................................................................................................................................................ 1 1. Introduction .................................................................................................................................................. 3 1.1 Scheme ............................................................................................................................................ 3 1.2 Summary of Survey Information ....................................................................................................... 3 1.2.1 Topographical Survey ....................................................................................................................... 3 1.2.2 Summary of NRSWA C2 enquiries ................................................................................................... 3 1.2.3 Geotechnical Investigation ................................................................................................................ 4 1.2.4 Ecology ............................................................................................................................................. 4 2. Options ........................................................................................................................................................ 5 2.1 General ............................................................................................................................................. 5 2.1.1 Workshop ......................................................................................................................................... 5 2.1.2 Design life and maintenance ............................................................................................................ 5 2.2 Summary of on-line wall design ........................................................................................................ 5 2.3 Off Line Wall Option 1: Construct a new wall in front of the existing wall – raise the footway above existing wall ........................................................................................................................... 6 2.4 Offline Wall Option 2: Construct a new wall in front of the existing wall with new footway – reduced carriageway ........................................................................................................................ 7 2.5 Offline Wall Option 3: Construct a new wall in front of the existing wall reducing the footway width. ................................................................................................................................................ 7 2.6 Option 4: Construct Gabion baskets in front of the existing wall ....................................................... 8 2.7 Option 5: Construct new wall in front of the existing wall, divert footway through Woodland. ........... 8 3. Conclusion & Recommendations ................................................................................................................. 9 Appendix A General Arrangement Drawing for On-line Wall ................................................................................. 10 Appendix B Drawings for Offline Wall Options 1 & 2 ............................................................................................. 11 Appendix C Risk Register ..................................................................................................................................... 12 Prepared for: London Bourogh of Southwark CONWAYAECOM 60493385-C0478-REP-0003-A Offline Wall Options Report iv C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall Executive Summary The brick retaining wall on Farquhar Road in Crystal Palace SE19, bounding the Dulwich Upper Woodland is approximately 50 m long and is in a poor state of repair, with noticeable bulging of the wall and slippages of brick work. The wall is acting as a retaining wall, whereas the original design was almost certainly intended as free standing. It is assumed that the foundations are inadequate for the lateral earth pressure being applied to the wall. A Feasibility Study & Options Report (Document number 60493385-C0478-REP-0001) was prepared by AECOM on behalf of London Borough of Southwark (LBS) for an on-line replacement wall, such that there would be no encroachment of the new wall into the highway area and therefore a “stopping-up” order would not be required. A detailed design for the preferred option, a reinforced masonry cantilever wall, was prepared. This scheme required excavation behind the existing wall and the consequent removal of 34 trees within Dulwich Upper Woodland. Therefore, London Borough of Southwark have commissioned AECOM to investigate other possible designs for the scheme that do not require removal of the trees. These new designs would necessitate construction of the replacement wall off-line, in front of the existing wall. This report presents the findings of that investigation and is supplementary to the original Feasibility Study & Options Report. Five options for off-line construction of the replacement wall were presented to LBS at a workshop. Three options were dismissed: • Construction of an L-shaped stabilising wall in reinforced concrete in front of the existing wall – this would require piles to stabilise the wall and would therefore be uneconomical and access for the piling rig would require cutting-back of the tress canopy at the front of the woodland; • Construction of a stabilising wall in front of the existing wall with gabion baskets is not recommended due to the risk of vandalism or theft and the reduced design life of such a structure; and • Construction of a path diversion through the woods is not recommended due principally to the risks to personal safety introduced by separation of the path from the highway. The relative advantages and disadvantages of the two remaining technically feasible off-line wall options and on-line wall option designed previously are summarised in the table below: On-line Wall Option Off-line Wall Option 1 Off-line Wall Option 2 Description Wall reconstructed on the Existing wall remains in Existing wall remains in line of the original. place, but a new off-line wall place, but a new off-line wall is constructed in front of the is constructed in front of the existing. Footway will be existing. Footway will be diverted onto top of the new diverted in front of new wall wall with a new ramp and & the carriageway width will stairs at north end of wall. be reduced. Prepared for: London Bourogh of Southwark CONWAYAECOM 60493385-C0478-REP-0003-A Offline Wall Options Report 1 C0478 Dulwich Upper Wood Retaining Wall Supplementary Feasibility Study & Options Report for an Off-line Wall Status of Design Construction design The current design is at the The current design is at the completed feasibility stage. Detailed feasibility stage. Detailed design of the new ramps and design of new wall is adaption of existing wall with required because this wall design of new wall will be design will be lower than the required. An additional on-line option. topographical survey will be required for the design and construction of the new ramps. Footway 2.0 m minimum, as existing 2.0 m width. The ramps and 2.0