Geo-environmental Investigation · MCU25/0020

6.4.3        Raft Footing Design
It is anticipated that a well stiffened raft slab footing system could be adopted to support the proposed
building provided that total and differential settlements, as estimated and given in Table 5, can be
accommodated. The raft would need to be designed to evenly distribute building loads throughout the
slab, including concentrated column loads. SCG has indicated that gross pressures beneath the
basement floor slab would be in the order of about 30 kPa to 40 kPa.
Analysis indicates that settlements should generally within the feasible limits for a well stiffened raft slab
footing system. Further analysis should be carried out once structural design details are finalised to
confirm the above estimates remain valid. This should also include finite element analysis to resolve
soil/structure interaction with input from the structural designer to accurately calculate
stresses/displacements and subgrade/slab reactions. Resistance to uplift from hydrostatic pressures
should also be considered by the design RPEQ. Piles or tie down anchors to resist uplift may be required.
Should the anticipated settlements preclude the use of a well stiffened raft slab, piled footings would be
required (Ref. Section 6.4.4). As outlined above, concentrated wall/column loads or peak perimeter
pressures exceeding 40 kPa would also need to be transferred using piled footings.
Excavations for the basement are expected to expose mostly loose to medium dense sands, however
some firm clay layers may be present. The raft subgrade must be inspected by a geotechnical RPEQ to
confirm the foundation materials and allowable bearing pressure, and to advise on treatment of any
unsuitable zones (if encountered).

6.4.4        Piled Footing Design
Where anticipated settlement cannot be tolerated in a stiffened raft solution or in areas with
concentrated/elevated peak pressures, a piled footing system would be required. Considering the likely
order of column loads to be supported, piles would need to be socketed into the dense to very dense
sands at about 7 m BGL. Piles may be designed using an allowable end bearing pressure of 1 MPa in
dense or very dense sands.
It is envisaged that steel screw piles could be adopted for the development. The use of bored piles,
without the use of steel liners and dewatering equipment, would be precluded by the presence of
shallow groundwater and loose sands near surface. Driven piles would likely not be suitable given the
suburban environment (e.g. noise disturbance) and potential vibratory effects on adjacent structures
(residences and underground services). Grout injected CFA piles could also be considered, as we
anticipate would also be used for the perimeter secant shoring system that is proposed.
If steel screw piles are adopted, it will be important to ensure close supervision of installation, as piling
may not encounter sufficient drilling resistance (from torque measurement) in dense sands at the
suggested founding depth, and piles could extend to much greater depth than necessary.
It is therefore recommended that site specific dynamic load pile testing should be conducted. Where
tension or compression capacity developed in the footings is critical for the future structure (e.g.
resistance to uplift forces etc.), then cyclic load tests (including measurement of deflections under
tension and compression cycles) should also be undertaken prior to construction. Kentledge for static
testing, or simulated cyclic loads, should prove the piles to a minimum of two times the expected
working load within tolerable deflections.
The advice of a specialist piling contractor must be sought regarding an assessment of suitable pile
types, pile design, and associated constructability considerations. The most cost-effective piling solution
depends on a combination of pile type, diameter and length. Piling companies are best equipped to

Geo-environmental Investigation Proposed Residence
11 March 2025 | 25040-001-Rev0                                                                                  16