RFI - Attachment C - Slope Stability Assessment and Geotechnical report · MCU20/0026

Project No. 1-22543                                                                                                          Page | 23
June, 2020
Ref: 1-22543, 2020-05-20, BR SSA VER 1
Sanya Holdings – Slope Stability Assessment and Geotechnical Report - Proposed Residence, 40 Park Crescent, Sunshine Beach




    6.3 Basement Excavation and Retention Methods

           6.3.1       Introduction

The design and construction method adopted for basement retention depends upon a number
of factors. These include:-

          Strength and type of subsurface.
          Boundary conditions i.e. location and type (i.e. movement sensitivity) of nearby building,
           need for underpinning, etc.
          Final wall profile and need for waterproofing.

           6.3.2       Geotechnical Model

Based on the supplied contour plan, the existing ground elevation varies from approximately RL
27.0m at the western corner of the site down to approximately RL 16.0m at the eastern corner
of the site. Assuming a basement (ground floor) excavation level of approx. RL 19.0m, the
overall cuts on the site will vary from 0m up to approximately 5.0m (i.e. basement excavation is
in from the boundary lines on the high side of the site).

Drilling, DCP testing carried out and a site inspection indicate a consistent subsurface profile
with very loose to loose sands to approx. 2.5m depth, transitioning to medium dense to dense
sands extending to the base of the proposed cuts.

           6.3.3       Boundary Conditions

Boundary conditions for each face consist of:-

          South Western Face – Park Crescent with underground services (council services,
           water, comms, power).
          North Eastern Face – Adjoining council easement.
          North Western Face – Approx. 2m wide service easement then neighbouring two storey
           residential property approx. on the easement boundary.
          South Eastern Face – Neighbouring three storey residential property, approx. 2.0m off
           the boundary line, with block retaining walls along parts of the boundary line.

The effect of these boundary conditions must be taken into account in the design of a suitable
retention option for the south western, north western and south eastern boundary lines.

           6.3.4       Possible Retention Options

The following options could be considered for temporary basement retention along the three
site boundaries noted above:-

      1) Batter slope to stable batter angle - This method is only applicable where sufficient
         space is available. Batters could be utilised where space allows within the site,
         however given the adjoining properties on all sides and predominantly loose sand
         profile, this would not be possible or recommended.
      2) Structural Solution - This method includes the construction of retention systems
         (temporary or permanent) along the boundaries to support the material on the



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