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Backfilling Trench with GW

Backfilling Trench with GW

Backfilling Trench with GW

Can anyboby can give me some idea how to backfill a 10 feet deep 1000 feet long trench to 90% compaction with import but the trench is fulled with 5 feet groundwater(100 gal/min). I know that you will suggest flooding or 2-sack slurry them, but what if these solutions can NOT be the options. Anyone, please?

RE: Backfilling Trench with GW

In lieu of compacted soil backfill, which would require a dry excavation, you might consider Controlled Low Strength Material (CLSM), or flowable fill, which is esentially a fluid concrete mix with a strength of 100 to 1200 psi.  It can be placed under water by tremie methods.  There is an ACI Spec. for it.  You can also do an internet search for information on CLSM.  It is not cheap, but is easy to place, conforms to the shape of the excavation being filled, can be driven on 24 hours later, and does not require density testing every lift.

RE: Backfilling Trench with GW

Thank you for the suggestion but sorry no budget to do that. Excavated or adjacent drier soils are the only source we have.

RE: Backfilling Trench with GW

If you can't dewater the excavation such that backfill can be placed in controlled lifts and compacted, and the soil is relatively clean sand, you can dump it loose into the excavation and compact it later with 8 to 10 passes of a large vibratory drum roller from the surface (effective compaction to 70% relative density achievable to 5-7 feet, depending on depth to groundwater), or using the vibro-compaction process (contact Hayward-Baker for more info).  Impact densification or dynamic compaction(contact H-B again)could also be considered, depending on soil type (not effetive in clays), and vibration damage considerations. Have you contacted a geotechnical engineer for advice?

RE: Backfilling Trench with GW

Since you have a small budget and high groundwater, I would suggest using concrete aggregate or concrete aggregate spoils for the backfill.  Since you only need 90 percent compaction, you can get that will little effort.

Another option would be to put in coarse to medium sand and then temporarily drop the water table to the excavation bottom.  That should tighten the material fairly well.

RE: Backfilling Trench with GW

Ron: Your are the MAN, orI should say "person".

Your comment definitely can help my situation here and save me big bucks.  By the way, if I may ask, since that we all know that it is impossible to get a max on the pea gravel in the lab, also, the gravel seems compacted pretty good with few passes by the vibrate-drum. How can I convince the inspector and other authorities that my trench has been well compacted with pea gravel?  Thanks first!!

RE: Backfilling Trench with GW

Your argument is based on the "dry rodded unit weight" of the aggregate as compared to a Proctor as you would do for soil.  Show them that reasonable consolidation is achieved with just the manipulation of dry rodding.  Then take a cylinder mold and drop the aggregate into it and screed the top.   Don't do anything else to it, just weigh it in the mold.  Compare the weight of the material in the mold to the dry rodded weight done previously.

Now take the cylinder mold and hold it up against the vibratory drum for a couple of seconds.  Add aggregate to overfill then hold against drum again.  Screed the top and weigh.  You'll likely find your "vibrated" weight to meet or exceed your dry rodded weight (the "standard").

RE: Backfilling Trench with GW

Rodding? like we do on the specific gravity or concrete cylinder?
What more can I say? Ron, are you sure that you are a structurl engineer? Thanks again!!

RE: Backfilling Trench with GW

The only problem with using coarse aggregate for trench backfill is that if the surrounding native soils are cohesionless sands, they can ravel into the voids in the gravel over time, resulting in surface subsidence.  You may need to consider a geotextile envelope to prevent this.  If the native soils are cohesive, this may not be a problem.  In some areas, gravel is not cheap. The gravel filled trench will also become a direct conduit for surface water infiltration, unless covered with impervious soil, asphalt, etc.

RE: Backfilling Trench with GW

Your opinion are great, especially that an underground conduit is created if gravel are used. Never thought about that! Thank you!!

RE: Backfilling Trench with GW

KAM...good and valid points.

RE: Backfilling Trench with GW

The problem that we have notice from gravel is that it will damage the conduit. We alway just use an underwater backfill (coarse white sand) and go with it. It is very common to get over 100% Astm d 698 using this method. The gravel is great for under footing  areas. And the unit weight method for gravel is correct. Only no way to test due to the voids. Also Nukes are very sensitive in trenches due to wall moisture, request sandcone method or ballon.

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