geotech report · 132003.220660.5

The outline of the proposed alterations and additions are indicated on the attached Figure 2. To achieve
design surface levels excavations are expected to extend to a maximum depth of about 1.5m below existing
levels. The excavations may extend below the footings of the adjacent portion of the northern side of the
existing house and the footings of any adjacent sections of existing landscape retaining walls that may
remain.

The bulk excavations for the proposed lift must be carefully completed and not extend below any potentially
impacted adjacent footings prior to any stabilisation/shoring works being installed. Prior to bulk excavation,
test pits must be excavated to expose the existing footings and assess the foundation materials. Based on
inspection of the test pits by the geotechnical and structural engineers, the need and extent of underpinning
and/or propping can then be determined and detailed. It would also be beneficial to source any ‘as built’
drawings of the house footings from SCG. Any existing landscape retaining walls within the site, particularly
those close to, or within the zone of influence (ZoI) of the excavations (defined as a horizontal distance equal
to twice the excavation depth, in metres) that are to remain must also be assessed by the structural engineer
to determine the need for, and detail, any wall strengthening measures that may be required. The house and
landscape retaining wall footings are expected to be founded in the sands and the test pits must not extend
significantly below the base of the footings as this may cause settlement of the footing and damage to the
structure above. Any underpins that will be supporting the soil profile will need to be designed in accordance
with the advice provided in Section 6.5 below, to resist lateral loading.

The house and landscape retaining wall footings will be founded in the sandy soils, and even where shoring
systems are appropriately designed and constructed, some movements are inevitable. Therefore, it is likely
underpinning would be required, particularly the house footings, depending on their founding depth. We
forewarn that even when completed to an appropriate design by an experienced contractor, underpinning is
inherently risky, and damage to structures being underpinned may occur. We recommend an allowance for
rectification to such structures be made in the project budget. Noting our concerns above and the possibility
of damage to the existing house, we strongly recommend a shoring system be installed prior to excavation
as outlined in Section 6.5 below.

On the basis of the investigation results, following partial demolition, the proposed excavations will
encounter the sandy soil profile. Any topsoil or root affected soils should be stripped and separately
stockpiled for re-use in landscape areas or disposed of.

Due to the small size of the site and restricted access, we expect the excavations will be completed using a
bucket fitted to a small tracked excavator, or hand tools. Due to the presence of poorly compacted sandy fill
and very loose natural sands, we do not recommend the use of rock breakers during demolition due to the
potential for transmission of vibrations which could cause damage to adjoining buildings and structures
founded in the sandy soils. We recommend that the removal of any sections of concrete floor slabs and
footings be completed using a diamond saw followed by removal of the concrete pieces using a bucket
attachment to the tracked excavator. We also note that ‘dropping’ of large sections of existing structure
during demolition should also be avoided in order to prevent the generation of potentially damaging
vibrations


32032URrpt                                         12