14169-001-Rev0 17 Webb Rd -Revised Draft · 132002.220527.1

                                   17 WEBB ROAD, SUNSHINE BEACH
                                   PROPOSED RESIDENCE

7.3            Retaining Structures
Design for temporary or permanent retaining structures could be carried out using a triangular pressure
distribution applicable to the wall type and based on the parameters listed in Table 3. The values in Table 3
assume that the walls are fully drained and no allowance is included for hydrostatic pressures, construction
loading, up-slope footing loadings, the effects of sloping ground and backfill compaction stresses.


Table 3: Retaining Structure Design Parameters
                                                                       Lateral earth pressure coefficients
                             Unit       Friction
 Retained material          weight       angle
                                                              Ka                          Ko
                            (t/m3)     (Degrees)                                                             Kp
                                                       (Cantilever wall)          (Non-yielding wall)
 Loose to Medium
                             1.8           30                     0.3                     0.5                3.0
 Dense Sand
 Future Fill                  *             *                      *                       *                  *

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



7.4            Permanent Drainage
                                               T
Adequate site drainage must be installed to ensure that natural runoff is directed away from building walls
and footings.
                                            AF
Subsurface drainage must be installed behind retaining walls in order to prevent the development of
hydrostatic pressure. This normally comprises slotted ‘aggi’ pipe wrapped in filter ‘sock’ and placed in gravel
backfill.

All excess stormwater collected around the site (including overflow from rainwater tanks and the swimming
pool) must be directed by pipes / channels / swales to an infiltration system located at least 5 m down slope
                                     R

of the proposed building, and not closer than 10 m from the steep dune slope.

Design of the stormwater infiltration system requires an estimate of soil permeability.
                         D


Using Hazen’s equation, where soil permeability (k) =              10-2 x D10² m/s

                                                       For D10 = 0.16 mm (Ref. Section 5.3)

                                                       k=          10-2 x 0.16²

                                                       k=          0.000256 m/s (2.56 x 10-4 m/s)
                                                         =         22 m/day

This order of permeability is in line with expectations for a fine grained sand (typically 10-3 to 10-4 m/s).



8.0       LIMITATIONS
Your attention is drawn to the document “Report Limitations”, which is included in Appendix D of this report.
The statements presented in this document are intended to advise you of what your realistic expectations of
this report should be, and to present you with recommendations on how to minimise the risks associated with
the services provided by Tectonic for this project.




26 February 2015
Report No. 14169-001-Rev0                                    13