Dry Dock Foundation - Sub Base Coarse Layers 2

[farhanjnawab]

Dear Friends,

We have Dry Dock Ship Repair Foundation (100 mtr x 11mtr x 0.5 mtr thick) below which there lies 200mm thick subbase compacted to 100 % MDD. We are supposed to demolish the foundation and remove the sub base layer, re-lay, compact, PCC and then lay concrete foundation a new. While excavating 200mm sub base, due to a long excavator teeth, the depth increased to 300-400mm. After 200mm subbase, we have dune sand ( very fine sand)

I need your valuable feedbacks on the following,

1. Can we restore back the extra depth (i, e 100-200mm) with the same dune sand and then put back 200mm subbase? or to re-lay 200mm x 2 layers and compact?

2.How much min built-up depth with sub base should be there below a ship repiar dock foundation?

3. How to go about executing the job, i mean dressing, levelling etc?

Pls help.

 

[Ron]

1. Yes, if the sand is compactable and confined, you should be able to restore with same material. If sand is saturated, you might consider a gravel backfill to bring to level.

2. This depends on the loads and use conditions. Can't say without more info.

3. Compact in thin lifts (layers) to achieve required compaction. Grading can be done with a variety of devices, including a small front loader, a box blade, and other methods.

 

[cvg]

in order to achieve maximum density with clean sand you need good moisture control and confinement. Without both it will be difficult to compact. You would need to use a relative density method rather than proctor. 70% RD is usually acceptable.

Rather than bring in gravel, bring in a good, well graded crushed aggregate. This would be the same material typically used for road base. Much easier to compact and relatively inexpensive.

 

[Ron]

cvg...the gravel is only if the water table is an issue. Base material will pump if water table is at that level...gravel won't.

 

[cvg]

Ron - missed that, you are right - base material is not as free draining as gravel. If water table is that high, further improvements might also be necessary - depending on what type of material the subbase is.

If no water than aggregate base is the way to go.

 

[Ron]

cvg..I agree...base material is better way if not a water issue.