20220109 - Geotech · OPW23/0182

                                PROPOSED RESIDENCE
                                21 MAHER TERRACE, SUNSHINE BEACH

hydrostatic pressure behind the chosen retention system, and to limit the need for extensive dewatering,
subsurface drainage may need to be implemented as follows:
        Install inclined subsurface drainage (e.g. 50 mm perforated PVC pipe) at 3 m spacings, starting as
         deep as possible behind the lower level retaining wall then drilled up into the slope. The inclined
         drains would need to extend into the natural ground slope towards the west far enough to ensure
         groundwater is intercepted at potentially higher elevations.
        Pin 300 mm wide Megaflo strip drains, or similar behind the wall panels and connect into a collection
         drain along the toe of the wall.
        All collected groundwater from subsurface drainage is to be piped to the legal point of downstream
         discharge (e.g. stormwater infrastructure along Maher Terrace).

It is anticipated that minimal groundwater seepage should migrate into the excavation area following
installation of the above subsurface drainage. Such seepage should be readily removable using a gravity
drainage or submersible sump-pump(s), at a low point in the excavation.

It is anticipated that a passive (permanent) dewatering system will be incorporated into the ground floor
system, thus enabling for design without hydrostatic pressure. This could comprise a series of drains behind
retaining walls and beneath the ground floor, connected by pipes to drain under gravity to the Council road
stormwater system (down slope).

Retaining structures greater than 1 m high, or those required to support surcharge loads or sloping backfill,
will require engineer design and certification of construction. Retaining wall design should be carried out in
accordance with AS4678-2002 Earth retaining structures.

Design for temporary and permanent retaining structures could be carried out using a trapezoidal (for
propped or anchored walls) pressure distribution applicable to the wall type and based on the parameters
listed in Table 4 overleaf. These values do not include allowance for hydrostatic pressures nor transient (e.g.
construction) or adjacent footing loadings.
Table 4: Retaining Structure Design Parameters
                                                                     Lateral earth pressure coefficients
                              Unit
                                         Friction
 Retained material           weight
                                         angle (°)            Ka                        Ko
                             (t/m3)                                                                        Kp
                                                       (Cantilever wall)        (Non-yielding wall)


 Loose Sands                   1.8           28               0.36                     0.53                2.8


 Stiff Clays                   1.8           25               0.41                     0.58                2.5


 Future Controlled Fill*        *            *                   *                       *                  *

Note:    * Depends on fill material type and level of compaction.

Retaining structures must be founded as per the recommendations in Section 6.4.2 above with the piled
footings, if required, also designed to resist lateral loads imposed by the retaining wall. Detailed design must
include an assessment of stability.

6.6        Drainage
Surface diversion drainage should be constructed above the crest of any new cut or fill embankment or
retaining walls (e.g. grassed or landscaped swales or diversion mounds). Adequate site drainage must be
installed to ensure that stormwater runoff is directed away from building walls, footings, batters and retaining
walls.

If groundwater seepage is encountered during excavations, Tectonic must be notified to assess the impact
on stability.



16 July 2021
Report No. 21236-001-Rev0                                   16