Hi Rakra,

When you say "...most refusing on cobbles/boulders at depth" I assume you are referring to your SPTs- you then go on to say the "size of cobbles/boulders is unknown".

If you cored into the cobbles and boulders (HQ coring), how is their size not known? Surely the size of the cored dimension was measured?

If you mean to say that your boreholes were terminated at the top of the cobble/boulder horizon, and given the documented activity in the area, I would strongly suggest you obtain a full profile before speculating on foundation types...you need to ensure your foundations are taken into something stable, as opposed to just considering bearing capacity etc. - go and core through the boulders down to bedrock in at least a few strategically-spaced borings!

All the best,
Mike

Thanks Mad Mike for the reply. Sorry yes should have clarified. Yes was referring to SPT refusal on cobbles/boulders. with regards to the size of boulders, how to determine size if boulder is larger than HQ size? Secondly, the boreholes were targeted to terminate at 30m depth. There is lack of past investigations in the area and little information on thickness of this unconsolidated unit, hence you are right, need further drilling to bedrock.

Depending on the size and loading of your foundation ( assuming a caisson type of structure), perhaps the 5 m thick fine-grained will control the design. If you use sheet piles, make sure that they can be installed thru the cobbles. You said that there are submarine landslides so a sheet pile type of structure may be work better. Just check the installation issues thru the cobbles.

Bottom line, define first your geotechnical constraints and then plan for the type of step.