Geotechnical Desk Study - Islington Council

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Geotechnical Desk Study 8. Geotechnical Considerations Based on the local boreholes, the ground conditions are indicated to comprise London Clay below a thin layer of Made Ground which is consistent with site observations of the cutting slopes. The railway cutting and terrace forming the Highbury Roundhouse site area appear to have been constructed before 1895. Based on the local borehole evidence from the site, the terrace was formed by cutting only, ie it was not formed by cut and fill operations. Therefore London Clay can be anticipated to lie beneath the tarmac or concrete covered area. However, this should be confirmed by a site specific ground investigation. Movement of the retaining wall along the western side has occurred and the telegraph pole is leaning significantly. Longitudinal cracking of the tarmac/concrete surfacing was observed. However, signs of instability of the slope below this area were not seen. The historical maps show a building along the top of the cutting perpendicular to the Youth and Community Centre in the area. Therefore it is unclear whether the cracking and structure movements have been caused by movement of the slope or as a result of previous structural loading. As stated above, the wall only retains a low height of material which does not seem to be responsible for the significant wall movements. As the wall and telegraph pole are likely to have shallow foundations they may have been affected by swelling and shrinking of the highly plastic London Clay rather than slope movement. Although, if the foundations are very shallow they may have been affected by soil creep within the cutting. This wall has suffered extensive structural damage and requires removal. This wall could be replaced by another wall or a suitably founded fence in combination with some local regrading of the crest of the slope. The eastern retaining wall has undergone significant movement and requires either remediation or replacement as discussed in Section 7.4. The walls of a number of properties have had to be tied back as a result of structural movement probably resulting from swelling and shrinkage of the clay. There is a risk of contamination due to previous uses of the terraced area. However, no evidence of contamination was observed in front of the west side retaining wall at the top of the cutting and in addition no fly tipping or other piles of material were observed on the cutting slopes. It is recommended that an assessment is made of the likely contamination that may have been caused by the glass bottle works and latterly by the packing case factory. Any new construction on the terraced area would need to be founded sufficiently back from the top of the cutting so that the loading would not cause instability of the railway cutting. Slope stability analysis should be carried out to asses this risk using the results of a site specific ground investigation. However, typically any shallow pad foundations are likely to be required to be located 3m back from the top of a cutting. The type and size of the footings would be dependent on the loading of the structure. For a relatively lightweight 2 storey structure, pad foundations are likely to be acceptable founded at least 1m below ground level in the London Clay or deeper if trees or other mature vegetation is present. Based on the existing geotechnical data, the allowable bearing pressure for pad foundations in the London Clay is likely to be 100kPa for a settlement not exceeding 25mm. For more heavier structures, piled foundations are likely to be required. As the load is taken on the piles to deeper levels, piled foundations can be located closer to the top of the cutting. The piled foundations would typically be 15 to 20m in length depending on the loads that they would be required to carry. The area of any new building would also be constrained by the eastern retaining wall and the replacement of this wall using piles in front of the existing wall is recommended (see Section 10.3 and the sketch in Appendix F.2) as this would avoid disruption to the existing wall. 5044006/002 Geotechnical Desk Study (2).doc 18
Geotechnical Desk Study 9. Geotechnical Risks The main geotechnical risks are: • Instability of the west and east side retaining walls that would affect the scope or layout of any proposed development. • Presence of trees or other mature vegetation close to any proposed new development that would necessitate the structure to be founded on deeper pad foundations or piles taken below the zone of desiccation. • Presence of contamination below the tarmac/concrete area that would necessitate removal to a licensed tip and replacement with imported clean material. • Cracking of the walls of the Youth and Community Centre requiring stitching or underpinning if the building is to remain. • Presence of existing foundations from demolished buildings that would require removal prior to development of the site. • Instability of the railway cutting due to loading from any proposed development. • Unauthorised excavations into the side of the cutting which may cause slope instability. One embayment has been excavated into the cutting and it is recommended that no further excavations are permitted. • Effects of any additional loading on the tunnel should any proposed development extend into this area. 5044006/002 Geotechnical Desk Study (2).doc 19
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