Backfill compaction

[karma134]

Dear fellow Engineers

Our designer has proposed a backfill on top on concrete and confined by concrete on 3 sides by concrete. The designer has proposed to use a Cohesion less/ free draining material for this purpose and relative density of 80%. The backfill will be carrying the weight of the whole RCC intake structure along with intake gates and trash rack.
Now my query lies :
1. in the settlement of the backfill and how can I determine its expected settlement before the job.Or more specifically should I be worried about the settlement in this case.
2. Also since the backfill is exposed on the abutment on one side(as said earlier the backfill is confined on 3 sides by concrete)should I be worried about the ingress of water into the backfill and fear the possibility of bulking of the cohesion less backfill.However there is drainage pipe provided at the bottom.
3. Also am I wrong to assume that the RD of 80 % which is minimum which could be achieved without compaction and by the virtue of its own weight. I am asking this because the designers has asked to compact in layers of not more 300 mm. Since a CL material needs a vibration for compaction and I am reluctant to use a roller in a concrete structure. Will the manual hand compactors work in this case?

PS: I have uploaded a a rough drawing of the structure for your reference.

KARMA

 

[karma134]

@Hoaokapohaku: The gradations specs are correct. Yes the designer has specifically mentioned that it should be a free draining fill. We are using a manual potable vibration machine and then excavator to compact the fill. The results were good the test showed an average of 95 % compaction.
@fattdad is right for cohesionless soil at least.
@to all: The term Relative Density as far as I know is only used for Cohesion less soil and which is equivalent compaction factor.
@ Regarding the lab test. We got OMC at 12 % and the corresponding MDD at 1.7 g/cc.


KARMA

 

[Hoaokapohaku]

This thread has been confusing because the participants are understanding the terms used in different ways. As it has turned out, sounds like what was really wanted was just properly compacted granular structural fill. The "cohesionless" and "free draining" requirements are just saying you want granular material, not a material with a bunch of clay content that could swell, and you want a material easily placed and compacted to a degree that will prevent settlement.

What you have seems to be working so I'm glad your concerns are addressed.

 

[karma134]

hahaha right. Maybe that is why it is called an Engineering process of coming to a conclusion.Yes i learned a lot here. THANK YOU ALL.

KARMA

 

[Riggly]

I am not sure of the nature of the intake structure, but I have a few comments. I am concerned with the drains at the bottom of the confinement. Hopefully, there are proper and durable measures in place to prevent the sand from migrating into the drains and causing significant settlement. I have seen that happened on a roadway abutment - material falling through weep holes due to broken pipes and resulting in settlement. Can the drains be affected by flood conditions around the weir? Is the fill supporting a slab? If there is any significant settlement, most of that load may be transfered to the side walls, effectively causing the slab to span longer over the width of the structure, before it cracks or deflected, and rest on the settled fill- result may be cracking in middle of the slab. For the dimension of the fill area I would recommend a walk-behind smooth drum compactor, not a jumping jack which typically would take forever, and would tend to make a mess in granular soils in such a location that needs more precise compaction.