PORT WORKS DESIGN MANUAL PART 3 Guide To - The University ...

3.4 WorksIf, despite all the precautions described previously, shear failure or significant mud wavesoccur during fill placement, remedial works will be necessary. This section describes waysto rectify the damage.Site investigation must be carried out first to identify the extent of the failure before decidingthe remedial works to be undertaken. The extent of failure may be assessed by acombination of the following methods:• Estimation of the scale and extent of mud waves from site observations andsounding surveys of seabed profile.• Back analysis of reclamation stability based on the loading conditions beforefailure and soil parameters from previous site investigations to estimate theprobable location and depth of failure zone.« Carrying out post-failure ground investigation and soil testing, and comparingsoil samples, profile and strength parameters obtained before and after failure.Since any disturbed material may have a lower remoulded strength, tests such as field vaneshear tests are required to provide appropriate strength parameters for the design of remedialworks. In addition, disturbed mud is generally slower to consolidate than undisturbed mudand therefore the effect of disturbance on the coefficient of consolidation should beinvestigated before starting the design.(1) Complete RemovalWhere mud waves are generated, complete removal of all disturbed mud and replacementwith fill is the quickest way of rectifying the situation. The method is often employed toremove small-scale mud waves generated in reclamation projects. The advantage of themethod is the relatively small settlement in the area of the replacement fill. Thedisadvantages are the relatively high cost of dredging the mud, the need for a dumpingground for the dredged mud and the need for additional fill. Subsequent differentialsettlement may occur between the non-disturbed area and the rectified area where muddisturbed at greater depth is also removed and replaced by fill. This should be checked inthe remedial proposal.Stability should be checked to determine the factor of safety against slip failure in theadjacent reclamation if the disturbed marine mud is removed.The settlement and lateral