Development Application Part 2 of 2 - Geotech Report and Plans - OPW13/0816

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                                                                                  Job No. 213-15522


     5.0    FAILURE MECHANISMS

     Review of historic aerial  photographs of the Weyba Creek at the rear of the property
     indicates continual changing of the creek bed profile with significant
                                                                            scouring and sediment
     deposition along the creek frontage of No. 25 Dolphin Crescent and the adjacent properties
     to the immediate north.


     Deep holes appear to have been scoured immediately behind several of the properties,
     including No. 25 Dolphin Crescent.

     Based on the results of the field investigation, historic records and site
                                                                               photographs provided
     to aid this investigation, we advise that the failure mechanism
                                                                       responsible for the collapse of
     the revetment and retaining walls. jetty and outside edge of the deck is considered to be loss
     of foundation support.


     This loss of foundation support has been brought about by
                                                                   scouring of the Weyba Creek bed
     immediately adjacent to the revetment wall.

     The failure mechanism has also been       exacerbated. to varying degrees, by the addition
    surcharge of a retaining wall constructed on top of the revetment wall. backfilling behind the
    retaining wall, surcharging the backfill with high level footings supporting the timber deck,
    and possible build up of hydrostatic pressure behind the walls due to lack or absence of a
    subsurface drainage system.


    6.0    REMEDIAL CONSIDERATIONS

    We advise that unless significant and adequate measures to inhibit further
                                                                               scouring of the
    immediate creek bed and revetment batter are actioned. erosion and           of the rear of
                                                                            slumping
    the property would be expected to continue and would        place the residence in peril, ie. the
    revetment batter is currently about 2.Om to 3.4m off set from the rear of the residence.


    It is considered that a number of options to stabilise the revetment wall        area   available.
    including the reconstruction of the revetment wall (which would involve the reclamation of
    the scoured zone) or the mass rock stabilisation of the site (which could result in a loss of
    some yard area).



           6.1.1     Reinstatement of Revetment and Retaining Walls          (6P It AI i’)
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    If replacement of the revetment/retaining wall is adopted, it is recommended a
                                                                                     piled footing
    system be adopted for the support of the wall, with piles founded not less than 1.5m into the
    dense or better sand strata indicated beyond about 4.4m to 4.7m
                                                                           depth below existing
    ground levels immediately behind the remaining revetment batter. Deeper piles may be
    required for load capacities depending on the final wall design.

    Cased. jetted. cast insitu concrete piles or steel screw piles would be considered the most
    appropriate piling systems for the site conditions encountered and access restrictions
    imposed. However. it is recommended piling contractors be consulted for further advice as
    to the most appropriate system for this site. Reinstatement of the
                                                                       jetty structure, if proposed,
    could also be considered using a similar piling system with appropriate
                                                                                 bracing to ensure
    adequate lateral stability is maintained.




                                                   "#-SOIL SURVEYS ENGINEERING PTY LIMITED
    1.15522,2013-09.20, LR VER 0