QC6 How deep is the soft mud and quicksand ?

The map shows the  approximate key sources for sand and sealing type basaltic clays
Preliminary surveys surveys done in 2008 by the government suggest a slightly consolidated average depth of mud of around 800mm .The high organic content of this material  can be expected to limit its usefulness near causeway foundations .  It may be that soft materials extend deeper but such softness can be dealt with, in many cases by the use of engineering fabrics and water proofing.  The actual viable foundation depth is a question is best left to Vicroads to continue to clarify.

There are old sandy beach ridges underneath the basalt type clays laid down in recent centuries, but they are assumed to be small in overall extent ( eastern banks and near creeks  for eg )  .
The sound water holding capabilities of surface clays is a positive sign for both designing  to preventing sands from becoming unstable and to prevent surface water leaking away easily .
None of the above speculation is sufficient to make clear the difficult task of finding foundations. Readers will note  that the recent design options for a causeway ( 41 million ) do not suggest high cost of foundations is an issue.