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Marine Clays

Marine Clays

(OP)
Hi peoples,
I have a problem im trying to solve.
We are the structural engineer for a residential building (3 storey timber framed slab on ground) which has been built on a fill platform which is for flood protection and is average 1.5m deep.
It's still in construction and the builder noticed that the slab has settled 60mm. Up to 8m of marine clay (normally consolidated) wasn't identified as underlying the fill platform.
The settlement is occurring level across the slab (a stiff waffle slab) so that's good and the builder is willing to allow it to settle. Preliminary discussions suggest it could end up being 200mm.
Im interested to hear from anyone who is aware of any systems or processes that can speed up the consolidation process, conventional wick drains are difficult due to site constraints, additional pre-loading can be placed in some areas but not everywhere.
Has anyone used the vacuum consolidation method on a small scale project?
And one last question has anyone used injection grouting or any other similar method to stabilise/arrest the consolidation process in a soil mass. Bearing in mind we need to get under the slab which is 11m wide x 35m long.
Its a tricky problem but that's what we as Engineers are good at. I'm hoping someone out there might have an idea that we haven't thought of.

Cheers

RE: Marine Clays

You will be in luck if in its design life only 200mm settlement occurs. First you need to hire a geotechnical engineer who has local experience on embankment over soft marine clay, do a proper SI and assess how much the foundation is expected to settle and what differential settlement is expected. The latter could be more damaging. Goog luck!

RE: Marine Clays

Vacuum consolidation is associated with both horizontal and vertical drains. It's a different way of loading the soil.
Marine clays can't be grouted (too many fines). Yous vould think of a rigid inclusions array using driven ductile iron piles, micropiles or compaction grouting columns (with cement in the mortar) or jet grouting columns. This way you will transfer the loads to a deeper layer.
A proper site invistigation might prove useful to go further (this is generally how to avoid tricky problems !)

RE: Marine Clays

Some marine clays are unstable and sensitive to loading

RE: Marine Clays

(OP)
We are getting some CPT's to confirm the Marine Clay depth then we will be able to establish the expected settlements. I still want to look into options for accelerating the settlement through the primary range and then stabilising the soil mass from future creep settlement if possible.
Underpinning with piling is a last option at this stage.
Thanks for any ideas guys.

RE: Marine Clays

thechud,

CPTu is helpful to estimate the yield stress and coefficient of horizontal consolidation, but not enough to get the compression ratio (CR) and strain rate of creep (C_alfa/1+e0) for consolidation settlement and creep estimate. You need to run oedometer tests on high quality undisturbed samples to get CR and C_alfa/1+e0 if you want more accurate estimate of settlement.

For rigid inclusions, suggest you talk to Menard for CMC advice.

RE: Marine Clays

(OP)
Thanks Henryzau,
Can you please explain what CMC stands for

RE: Marine Clays

Controlled Modulus Column

RE: Marine Clays

Hi thechud, the fastest way is using driven piles seating in the hard layer. A pile loading test may be required to estimate the pile capacity. So, you may distribute the pile evenly within the area of 11m x 35m.
If it is possible to remove a couple meter of soft marine clays to reduce the negative skin friction and replaced by compacted cohesionless soils. Otherwise the pile capacity will be reduced by the negative skin friction.

RE: Marine Clays

I agree that you need a good geotechnical investigation to evaluate any approach.

Perhaps you could preload the foundation with an earth surcharge. If you extend the surcharge well past the edges of the slab, you may be able to keep the settlement reasonably uniform, especially if you increase the surcharge height near the perimeter to achieve uniform pressure increase across the slab.

Preloading with anchors might be feasible if the underlying hard layer is strong enough. You could lay some beams across the slab to distribute the anchor loads. Keep adjusting the anchors to force the settlements to be equal. Calculate the moments in the slab to avoid overstressing it.

You could core holes in the slab and install micropiles to support it, but the soil will settle away, so you need to address lateral stability and corrosion protection, not to mention utility settlement.

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