Backfill

[Allseeingeye]

I have specified sand backfill around water lines and conduit. The contractor is insistent on using CA-7. Has anyone experienced a problem with CA-7 used in this manner?

 

[JAE]

You might want to better describe CA-7 - so others who aren't familiar with that designation can respond.

 

[jdonville]

Allseeingeye,

If I remember correctly, CA-7 is 1-1/2-inch minus coarse aggregate (hence "CA") with no sizes smaller than 1mm. I would imagine that this could potentially leave voids or be difficult to compact well around the water lines/conduit. I would say the samallest diameter pipe I would use CA-7 with would probably be 4 inches. I would recommend that you stick to your guns (figuratively) and tell the contractor that they need to use sand.

I would think there shouldn't be a problem with a spec of 6 to 12 inches of sand above and below the conduit with CA-7 above and below the sand zone. This helps also if excavating in future for repair as the sand indicates that the utility is close and hand excavation becomes appropriate to avoid unnecessary damage.

Good luck,

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[Zambo]

Which materials are the waterlines and conduit made from?

 

[bushel3]

CA-7 is a uniform 3/4 inch stone. It is used often as a backfill material. Compaction can be achieved using CA-7 but not measured. Compaction is achieved by tamping the stone with the bucket of the backhoe or using a vibratory plate. However, due to the lack of fines the degree of compaction can not be measured using a nuclear density gauge. We often specify CA-7 when we are not worried about the degree of compaction or as a granular drainage fill for 1 foot behind a retaining wall. It is very easy to install.

 

[1969grad]

I agree with jdonville in that compacted sand should be installed until you have 12" of cover and then it does not matter if he uses CA-7.

BTW, my field experience is that it is very difficult to get a contractor to fully compact the soil around pipe to meet the drawing specs unless you continually watch him.

 

[Reindeer]

If you decide to let them use gravel, ask for a filter fabric wrap like Mirafi 140N. This is not an uncommon request (necessary if large excavation), but the cost of the labor will encourage him to change to sand.