Installing a concrete barge on poor soil 1

[kay]

Gents,
I need to instal a concrete barge, plan dimensions 36m*28m in a tidal swamp area,inter tidal range 2.25m, barge will have a free board of 1.0m above high high water mark.
The soil beneath proposed barge location is very soft to organic clays with some organic content to about 15m depths below mud line, no good for any foundation support.
Presently water depth in the area is 4.0m
Barge weight is about 5000Tonnes, barge height is 5.0m
There was a soil survey for the area. Average Liquidity index values are 0.34 between 7 and 16m depths below proposed barge location.
NB Pressure due to barge is ~50KN/m2 ie 25% more than initial hydrostatic pressure - OCR considerations ??

My proposal is to do a soil substitution using a bucket dredger to about 10m depth (maybe lower) with structural sand within a cofferdam allowing for 5m buffer area around the barge outer foot prints - this provides a prepared sand bed for the barge.
The idea is to instal the sheetpiles to 12m depth, 2m beyond zone of soil substitution, cut off level will be at say 0.5m below mud line.
The consideration in this proposal is to avoid detrimental immediate and long term settlement of the barge, equipment mounted o the barge can not tolerate misalignment of parts.
Burland and Burbridge influence depth for B=30m is 12m depth below base of foundtion.

Stability considerations for the barge are ok, my worry is on the settlement and shear on the barge foundation.

Any suggestions and recommendations, sorry this is a long one.


Many thanks

 

[CarlB]

Your suggested method may well get the job done, if you can effectively compact the sand. Can't say if your proposed depth of soil replacement is sufficient, depends on soil properties. You may find considerable settlement may still occur within soils below the 10m depth - there are significant stress increases within a depth of twice the width of the load - or to a depth of 56 m.

Have you considered:

1. A surcharging and wick drain program, that is, to install wick drains or similar on a grid pattern (to accelerate settlement/consolidation), and placement of a surcharge, such as a pile of soil, causing subsoil to consolidate? If time is available, may well be more cost effective than the deep 'soil substitution' you describe.

2. Placement of stone columns or sand compaction piles (SCP). Could be cost effective if expertise/equipment locally available.

A few notes.
You can estimate the soil compression index from the liquid index, assume normally consolidated, and calulate expected settlement. See a soils text for the relationship.

You say a 25% increase above the 'hydrostatic pressure' but the effective pressure on the soil is not the hydrostatic pressure. As the water level rises above the mud line, the total pressure rises but the pore pressure rises equally, and the effective pressure remains constant.

Hope this gives you a few ideas,

Carl