Attachment C - Soil Survey Report · MCU18/0013

Project No. 1-20119                                                                                                        Page | 25
March, 2018
Ref: 1-20119, 2018-03-02, BR VER 2
Sam Hopper – Slope Stability Assessment and Geotechnical Report - Proposed Residence, 24 Arakon Crescent, Sunshine Beach




    7.3 Basement Excavation and Retention Methods

           7.3.1       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.

           7.3.2       Geotechnical Model

Based on the supplied contour plan, the existing ground elevation varies from approximately RL
27.5m at the western side of the site down to approximately RL 18.0m at the eastern side of the
site. Assuming a basement (ground floor) excavation level of RL 20.5.0m, the overall cuts on
the site will vary from 0m up to approximately 7.0m, understood to be stepped as two smaller
cuts of up to approx. 5.0m.

DCP testing carried out, a site inspection as well as inspections of the adjacent site in 2007
indicate a consistent subsurface profile with very loose to loose sands to approx. 3.5m depth,
transitioning to dense to very dense sands extending to the base of the proposed cuts.

           7.3.3       Boundary Conditions

Boundary conditions for each face consist of:-

          Western Face – Arakon Crescent (single lane) with underground services (council
           services, water) and crib retaining wall on western side of road.
          Eastern Face – Internal portion of the site, adjoining nearby dune area.
          Northern Face – Neighbouring three storey residential property with block retaining wall
           approx. 1.0m to 2.0m high on boundary. Based upon our knowledge of the
           development in 2006/2007, we understand that a basement exists on the boundary line
           for the majority of its length at approx. RL 20m.
          Southern Face – Council easement approx. 3.5m wide. South of the easement is a
           neighbouring two to three storey residential property approx. 1.5m off the easement
           boundary.

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.

           7.3.4       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, this would not be possible or
         recommended.