ADG1290.21 Dewatering Management Plan_Rev1_02.02.22 · OPW21/0315

                                                                                         ADG Consulting Pty Ltd


5    Dewatering methodology
5.1 Basement excavation and shoring
     It is understood that the excavation/basement perimeter will be stabilised along the perimeter of the
     basement excavations using secant piles (i.e. soldier pile wall with grout injection between piles socketed
     into dense indurated sands and deeper underlying strata such as low permeability marine clays; 'cut off’
     wall). Excavations to facilitate the construction of the basement will extend to approximately 3.1 m below
     ground level, which corresponds to a maximum elevation of ‐0.8 m AHD. Localised deeper excavations
     may be required for construction of other structural elements (i.e. footings & pile caps).
     All excavated spoil from within the excavation footprint will be appropriately neutralised with lime and
     validated in accordance with recommendations by DP in the Preliminary and Additional Geotechnical
     Investigation Reports (DP, 2020 & 2022).
5.2 Groundwater extraction
     Dewatering will be undertaken using a series of 6.0 m length, 40 mm diameter PVC spears installed to
     below maximum excavation depth at 1.5 m intervals around only the internal perimeter of the basement
     excavation. Dewatering only on the internal face of the secant pile (cut off) shoring wall will minimise the
     extent of dewatering to predominantly or completely within the site boundaries, and minimise
     dewatering flow rates and thereby also minimise the extent of the estimated cone of depression of the
     groundwater table outside of the basement excavation footprint. Potential or actual acid sulfate soils
     outside of the excavation footprint are therefore not anticipated to be impacted by dewatering on the
     internal face of the secant pile (cut off) shoring system. Alternatively, or in addition to the spear system,
     a sump and pump system may be employed where seepage rates are lower than estimated.

     The basement excavation must be dewatered to a minimum of approximately 1.0 m below the deepest
     point of excavation to allow construction to be undertaken in dry conditions. The water table is therefore
     expected to be lowered by minimum of approximately 1.8 m to around ‐1.85 m AHD. This limited depth
     of groundwater abstraction on the internal face only of the secant pile (cut off) shoring wall will minimise
     total groundwater extraction and minimise the estimated cone of depression in the standing
     groundwater table, generally to within the secant pile (cut off) wall only.

     It is proposed that the development will include a ‘tanked’ basement to prevent the requirement for a
     permanent dewatering facility at the site. Similarly, no permanent groundwater dewatering is proposed
     or likely to occur as a result of the development, with all services and pipework proposed to be situated
     above standing groundwater elevations (that is, at elevations above 0.25 mAHD) and subject to the
     nominated hold points at Section 5.7. Where any deviation is proposed from this arrangement, all
     pipework within the saturated zone, that outlets direct to waterways must be fitted with buried
     headstocks at suitable intervals to prevent ongoing dewatering of groundwater along pipework bedding
     in accordance with Development Approval RAP2020/0020 Condition 29.
5.3 Extraction/discharge rates and volume
5.3.1 Dewatering extraction calculation based on subsurface soil profile
     The dewatering extraction rate, area of influence and cone of depression has been estimated to be
     minimal due to the proposed dewatering only on the internal face of the secant pile (cut off) shoring wall,
     which will minimise the extent of dewatering to predominantly or completely within the site boundaries,
     and minimise dewatering flow rates and thereby also avoid and minimise the extent of the estimated
     cone of depression of the groundwater table outside of the basement excavation footprint.

     Based on the dewatering methodology proposed (internal face of secant shoring wall only) and extensive
     previous experience in similar ground conditions, a groundwater flow rate of approximately 3 to 5 L/sec
     or 180 to 300 L/min or 10,800 to 18,000 L/hr has been predicted for the development, with the estimated
     cone of depression radius to be limited to within 1 m of the secant pile shoring wall. Flow rates are
     predicted to be highest during initial drawdown of the groundwater table within the basement excavation
     footprint, and reducing to approximately 3 L/sec following establishment of the cone of depression within



ADG1290.21 Dewatering Management Plan_Rev1_02.02.22                                                        Page 10