Geotechnical Desk Study - Islington Council

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.
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