geotech report · 132003.220660.5

The piled wall retention system may comprise contiguous piled walls constructed using grout injected
continuous flight auger (CFA) methods. However, for a small site such as this their use is unlikely to be
economic. We also do not recommend the use of sheet piles due to the potential to generate vibrations that
could cause damage to the existing house.

Conventional bored piles would not be suitable as the side walls of the pile holes would collapse during
drilling, particularly if seepage is encountered, and could cause adjacent ground surface movements which
extend below nearby structures. Such collapse could occur at any stage of drilling or pouring and as such we
do not recommend their use. Bored piles drilled using hand auger methods may be used but sacrificial
temporary liners would be required to prevent the sands collapsing into the open pile holes.

Construction of contiguous pile walls must be of high quality taking care to prevent soil loss through gaps
that will most likely occur between the piles as this would add to the possibility of ground subsidence
occurring outside the excavation. Such gaps must be rectified progressively during excavation such as by dry
packing the gaps with non-shrink cementitious mortar. Alternatively, consideration may be given to providing
a shotcrete face to the contiguous pile walls. The shotcrete facing would need to be applied over the full
height of the excavation face (expected to be a maximum of 1.5m vertical height) and must be applied
without delay and progressively with the excavation in front of the contiguous pile wall, following the
installation of drainage measures between the piles.

The lateral restraint to the proposed shoring should be provided by a combination of socket below BEL and
internal props at the higher level, unless the structural engineer assesses that the deflections can be
maintained within limits that will not cause damage to the adjoining portion of the house without the use of
props. Props must be progressively installed as excavation proceeds and excavations must not extend below
the designed stage level until the props have been successfully installed. We assume that permanent lateral
restraint would be provided by the structure.



6.5     Retention Design Parameters
The following characteristic earth pressure coefficients and soil parameters may be adopted for the design
of shoring piled walls, conventional retaining walls that will be propped by the structure, any underpins
supporting a soil profile and landscape retaining walls:
•     For retaining walls that will be temporarily propped, backfilled and permanently restrained by the
      structure, any cantilever or internally propped piled walls and underpins supporting a soil profile, we
      recommend the use of a triangular lateral earth pressure distribution with an ‘at rest’ earth pressure
      coefficient (ko) of 0.6 for the soil profile, assuming a horizontal backfill surface.
•     For landscape retaining walls, where we assume minor movements will be accepted, we recommend the
      use of a triangular lateral earth pressure distribution with an ‘active’ earth pressure coefficient (ka) of 0.4
      for the soil profile, assuming a horizontal backfill surface.
•     A bulk unit weight of 20kN/m3 should be adopted for the retained profile.




32032URrpt                                            14