Attachment 4 - Geotech Report · MCU23/0030

                              50 NOOSA PARADE, NOOSA HEADS
                              PROPOSED RESIDENCE

To minimise inflow rates and reduce the risk of external dewatering care must be taken to ensure the
temporary shoring wall is ‘water-tight’. Special attention would need given to the detailing such as ensuring
secant piles overlap or using interlocking sheet piles as opposed to overlapping sheets which leak.
Design for temporary and permanent retaining structures could be carried out using a trapezoidal (for
propped walls) pressure distribution applicable to the wall type and based on the parameters listed in Table 3
below. These values do not include allowance for hydrostatic pressures nor transient (e.g. construction) or
adjacent footing loadings. The values in Table 3 could also be utilised in assessing lateral earth pressures
for the design of the proposed in-ground pool to the north of the site.

Table 3: Retaining Structure Design Parameters

                                                            Lateral earth pressure coefficients
                             Unit     Friction
 Retained material          weight     angle
                            (t/m3)   (Degrees)            Ka                   Ko
                                                                                                  Kp
                                                   (Cantilever wall)   (Non-yielding wall)

 Loose Sands
                             1.7        30               0.33                  0.5                3.0
 (< 2 m BGL)

 Medium Dense Sands
                             1.8        33               0.29                 0.46                3.39
 (> 2 m)

The advice of a specialist contractor must be sought regarding feasibility, design and installation of the
preferred retention and dewatering system for this site. It will be necessary to conduct numerical analysis to
optimise the retention system.
The difficulty and risk associated with managing the basement excavation at this site should not be
underestimated. It is recommended that building, retention and dewatering contractors who are well
experienced and familiar with such construction in the local area be engaged to undertake the works.

6.3       Building Foundations
6.3.1        Footing Options

The feasibility of adopting a high level footing system will depend on the final building levels, earthworks and
structure loads. For anticipated working column loads in the order of 1500 kN, and considering the bearing
capacity of the foundation soils at basement level, the adoption of individual strip and pad footings for the
building is not considered to be practically feasible. Therefore, footing options that may be considered for the
residence include a well stiffened raft slab or pile. Discussion on a raft slab and piled footing system is given
in Sections 6.3.2 and 6.3.3 respectively.
Resistance to uplift from hydrostatic pressures must be addressed (refer Section 6.2 above) by the
structural RPEQ. Piles or tie down anchors to resist uplift may be required.
Where structures are proposed outside the basement footprint (e.g. outdoor entertaining area), the building
would need to be articulated along this junction(s), and the external supported on piles founded in medium
dense soils at least 2 m BGL. Screw piles would be a suitable pile type in this circumstance.

6.3.2        Raft Footing Design

It is anticipated that a well stiffened raft slab footing system could be adopted to support the proposed
building a basement level provided that total and differential settlements can be accommodated. The raft
would need to be designed to evenly distribute building loads throughout the slab, including concentrated
column loads. For the proposed two storey residence over a single level basement, we estimate a gross
distributed load of about 30 kPa (approximately 10 kPa per level) at the foundation level. With a basement
excavation depth of about 3 m BGL (below current ground levels), the nett distributed load at foundation



2 February 2023
Report No. 22500-001-Rev0                              10