Question regarding compaction of backfill on native soil

[Naggud]

Hello,

I have received an RFV requesting that we change the backfill material that we've specified underneath footings from granular fill to stockpiled native material (pit run sand).

The contractor has asked for the change because they are concerned that cross contamination between the native material an the granular backfill we wish to add on top of it will lead to compaction issues. I haven't come across this being an issue before.

I'm concerned that the native backfill will be susceptible to frost action. Do you think they're looking to use the native material as a cost saving move?

Thanks for your help in advance.

 

[cvg]

generally all requests are to reduce cost or increase constructability which results in cost savings. the cross contamination problem sounds fishy. have they actually tried it? if not, then just speculation.

Have them prove that the sand is not frost susceptible. or make them use clean sand.

 

[CWEngineer]

I am sure you are aware that you have to confirm the properties of the native material and ensure the design is adequate for this differnt type of soil.

 

[oldestguy]

The cross contamination thing probably relates to the Proctor lab test standard(s) to use for comparing to the in-place density achieved. Different mixes of these two materials will require a different laboratory Proctor test for each distinct mix. If this was a Corps of Engineers job that would be no problem, since that is their method of work. However, most jobs require only occasional Proctor tests, since the material gradation doesn't usually change much as the job goes along. Lab testing costs will go up every time the mix gradation changes, if this job is done right and it is likely to delay progress in filling.

As to degree of frost susecptibility, I have yet to see plan approving agencies allow lesser depth of foundation just to recognze that the soil is not frost susceptible. Usually a building code sets that depth regardless of soil type. An exceptiin is placing founationon bedrick. Better check that out before establising footing grades.

 

[BigH]

oldestguy - we put footings on clean sand at about 3 ft with frost depth of about 10ft and water table >12 ft. Never heard that the building inspectors for the city had any problem - was for a industrial plant in Northern Canada.