geotech report · 132003.220660.5

•     Any surcharge affecting the walls (e.g. nearby footings, compaction stresses, sloping retained surfaces,
      construction loads etc) must be allowed for in the design using the appropriate above earth pressure
      coefficient.
•     Retaining walls should be designed as drained and provision made for permanent and effective drainage
      of the ground behind the walls. Subsurface drains should incorporate a non-woven geotextile fabric,
      such as Bidim A34, to act as a filter against subsoil erosion. The subsoil drains should discharge into the
      stormwater system.
•     Piled walls and any underpins supporting a soil profile must be designed as permanently drained and PVC
      spitter pipes should be installed at nominal 1.2m horizontal spacing just above the adjacent floor level.
      Holes will need to be drilled to allow installation of the pipes and/or use of gaps between contiguous
      piles. The end of the pipe penetrating the retained soils behind the retention system must be wrapped
      in a non-woven geotextile fabric, such as Bidim A34, to act as a filter against subsoil erosion. The pipes
      should discharge into the stormwater system.
•     We assume the lift pit walls will comprise the lower section of the retaining walls and will support the
      excavation and as such will therefore be drained as described above. If this is not the case then the lift
      pit walls should be designed to withstand full hydrostatic pressures (i.e. tanked) with a design
      groundwater level equivalent to the adjacent lower floor level. If a drained structure is designed, then
      the potential groundwater pressures may be alleviated by providing the lift pits with drainage and a
      pump-out system.
•     Lateral restraint of piled shoring walls and retaining walls founded in the soil profile below BEL or
      adjacent surface levels may be provided by the passive pressure of the soil below these levels. A ‘passive’
      earth pressure coefficient, Kp, of 3 may be adopted, provided a Factor of Safety of at least 2 is used in
      order to reduce the high deflections that are associated with achieving a full passive case. Localised
      excavations in front of the walls e.g. for buried services etc must also be taken into account when
      checking the embedment depth design.



6.6     Retaining Walls Supporting Engineered Fill
Over the eastern side of the lower ground level, the rear yard will be provided with a grass covered area
requiring the raising of site surface levels and will be supported by a maximum 1m high retaining wall. Advice
on footings is presented in Section 6.7 below.

The construction sequencing required for the retaining wall supporting the fill may involve:
•     Fill initially placed and then cut back to allow retaining wall construction; this would only be feasible
      where there was sufficient space to place the fill outside the area of the works within the site.
•     The retaining wall may be constructed first and then backfilled.

In either case, good compaction close to the retaining wall may not be feasible and some post-construction
settlement of the backfill surface may occur. In addition, care would be required not to transfer compaction
stresses to the retaining wall, hence the use of a free draining backfill such as ‘blue metal’ gravel or recycled
concrete which does not require significant compaction. Further guidance on engineered fill placement and
compaction is provided in Section 6.9 below.


32032URrpt                                          15