Appendix B - Geotechnical Assessment · OPW23/0179

 Project No. PG-10375                                                                                   Page | 8
 June 2023
 Ref: PG-10375, 2023-06-09, LTR VER 1
 Michael Cullen – Geotechnical Investigation - Proposed Residence, 128 Gympie Terrace, Noosaville




Pressures for unsurcharged propped walls could be designed based on a uniform
pressure distribution with depth, where the pressure P = 0.65ka  H (kPa) where H is the
depth of the excavation. The shoring wall should also be checked for a triangular
distribution calculated using the lateral earth pressure coefficients present in Table 3
for a propped wall.

The temporary support system should also take into consideration the possible needs
to over-excavate the basement for ground improvement works.

It should be noted that lateral deflection of the retention system will occur, irrespective
of the retention system adopted. In additional to lateral movements, vertical
movement of the soils behind the retention system will result. The effects of the vertical
and horizontal movements may be seen at a distance equal to 2 or 3 times the height
of the excavation.

Subject to detailed design, movements as much as 1% to 2% of the depth of the
excavation could occur behind a cantilevered pile wall.

11.0 PERMANENT RETAINING WALLS

Basement walls should be designed using a pressure coefficient for the at rest case of
K0 = 0.5.

Unit wall force P is given by:-

                       P           =           0.5 K0  H12 (kN/m run of wall)

Passive resistance at the base of the wall can be calculated from:-

                       P           =           0.5 Kp  Hw2

           where Kp                =           3.00 (ignores wall friction effects) for loose sands
                                               3.25 for medium dense sands
                                               3.85 for very dense sands

                       Hw          =           front face of wall contributing to passive resistance (m)

For the sliding resistance calculation, the angle of friction for sand () compacted to
not less than 75% density index can be taken as 32° for walls with a key. For walls
                                                                                                    2
without a key, the angle of friction for sliding resistance should be taken as 3  or 21°.

12.0 BUILDING FOUNDATIONS

Given the encountered subsurface profile, a high-level footing system would
generally be suitable for the support of the proposed structure, with appropriate
ground improvement works and due consideration of the potential hydrostatic uplift
forces and the potential for weaker layers above the indurated sand.