Layered Compaction overlying hydraulic fill

[charnott]

Have a query/accusation from one of the contractors on my scheme that I'd apprecaite some opinion on.

Site is an offshore reclaimed island with approx. 20m of hydraulic placed fill (rainbowed by dredger). Ground improvement by vibrocompaction brings that fill up to ~65% relative density. That fill is then topped with 900mm of the same material (dredged carbonate sand, minimal fines content encountered to date)placed in layers to 95% compaction.

second contractor is required to excavate trenches from this reclaimed level and place precast structures amongst other things. he excavates ~2m (so through the layered fill and into hydraulic fill), compacts the trench base to 95% and begins his construction.

second contractor says he is unable to compact the trench base to 95% and is asserting that this is because the hydraulic fill has not been compacted appropriately (~65%RD).

I dont agree with that assertion on two principles:

1. applied compactive effort by second contractor will compact the fill directly underlying even if it means it has only compacted a crust.
2. this work was undertaken without issue on the previous island which is essentially identical (terms of scale and material properties).

My belief is that the contractor is likley to be guilty of one or both of the following:
1. Inappropriate/insufficient compactive effort
2. Using unrepresentative lab MDD value to assess results of in-situ density (sand replacement).

Bit long-winded and i know there will be some further queries but I'm keen to hear others opinions/thoughts.

Cheers in advance

 

[BigInch]

I doubt you got what you thought you paid for on the previous island.

Independent events are seldomly independent.

 

[dicksewerrat]

Building on 1 meter of compacted sugar sand on top of 20 meters of uncompacted sugar sand is not very wise. I would inject a neat cement grout down about 10 meters and grout to grade. Then make sure you are 5-10 meters outside the footprint. don't buy any of those buildings on the first island.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[charnott]

BigInch - the underlying fill is ~65RD as stated, there is an extensive verification data set which confirms this.

dicksewerrat - unsure the basis of your statements! 'sugar sand', 'not very wise'?! there was me thinking this page was a forum for engineers to help one another, my mistake i didnt realise id walked into the 'i know more than you pi**ing contest'. I await your guidance textbooks on offshore reclamation schemes, oh wise sage!

 

[cvg]

well first of all, you have all the data or can obtain it. why are you asking for speculation on a forum? get out there and take some compaction tests at trench subgrade level and confirm or not confirm what contractor 2 is claiming! If I was to speculate based on absolutely no supporting data, then I would come up with the same conclusions as you.

 

[charnott]

Cvg- testings been commissioned and I now have the results however the letter response to the contractor couldn't wait so I had to word my supposition accordingly. I was hoping for a sanity check of my opinion based upon what I knew at the time.

 

[cvg]

just a thought - 65% RD seems low, I typically would specify 70% minimum for clean, granular material. And if there are layers of silt or clay intermixed in the dredged layers, than there may be some areas in the fill where the fill is not clean sand and RD may not be the most appropriate method for the compaction spec. this can be determined by sampling in the contractor no 2's trench and frankly the contractor should be backing up his claims with material testing showing that a) the fill is not clean or b)not compacted well by contractor no. 1

 

[oldestguy]

You could help those that know about different compaction method by stating specifically what you mean by "vibro compaction". There are many ways and many different results. Where did that 65% come from?

 

[BigHarvey]

65% relative density is a poor post treatment result if it is really sand. Now if the fines content was more than 5%, that could explain it and in this case your contractor N°2 might be right.

 

[Ron]

Second contractor is probably right. It is a constructability issue. It is like compacting on top of a foam mattress...nothing to resist the compactive effort so it doesn't work.

Hydraulic fill doesn't "crust". It has no cementation and will not bridge...it is elastic and just "bounces".

 

[BigInch]

I know what he's working with. There's a 99% probability it's got fines plenty more than 5% and the rest is wind blown, well rounded, sand. Only a camel can walk on the stuff ... when it's dry. Wet, no chance. Vibration packs the balls tight, but still doesn't fill the voids. No interlock between grains.

Independent events are seldomly independent.

 

[charnott]

CVG/BigHarvey - 65%RD (dependant upon how you calculate RD!!) is similar to many of the similar schemes around the region including Palm Jumeirah in Dubai and plenty other far less famous sites. Admittedly most of these projects require additional ground improvement and review for specific structures but its in the right ball-park for the overall scheme without making the work exorbitantly expensive. Interestingly there is a growing school of thought that RD as a performance criteria for fill causes far more problems than it solves, a recent paper in ICE Ground Improvement Vol 166 by Babak Hamidi et al 'Relative Density concept is not a reliable criterion' basically says its as good as guess work! anyways i digress, our 65% is pretty standard and has been developed with extensive review of predicted settlements and risks associated with liquefaction, induced lateral loadings, excessive settlement etc.

oldestguy - apologies, i thought that would have been clear, hopefuly this will clear up what im referring to:

http://www.menard-vibro.ae/index.php/pages?path=4_25&id=25

BigInch - perhaps you're following my post history or perhaps your guessing but im afraid your wrong regardless as I have moved projects and am dealing with different material than before. This is carbonate predominantly bioclastic sand which is in fact quite the opposite from dune sand being composed of skeletal remains, with weak particles that are prone to crushing, have high inistial void ratio, and highly variable particle size and type. Its a hugely under-analysed material when you consider the amount of construction that has been done on it and with it across the middle east region in the past 15 years. again there are plenty of papers that are slowly closing that gap.I've digressed again!

Ron- I think youve come closest to actually giving me an opinion on the actual issue i raised. My in-situ testing comparison of sand-cone and nuc gauge values should be in later this week. if i can satisfy myself the plant theyre using is appropriate and they are doign the number of passes they say they are and the results come in <95% then I might agree with you! So far, having asked around a few other colleagues the consensus is split but knowing both contractors as i do my suspicion remains with them until proved otherwise!