90% compaction

[davidj]

We've just set the pole for the solar system - I'll post some photos on our thread soon.  Spoiler alert - we blew out the forms!

However, during the footing inspection (which had to happen when I wasn't around), the inspector noted that we need a soil compaction report indicating 90% compaction for the final.  Anyone know how much one of these might cost?  And what do we need to do to get 90% compaction?  It's reasonably sandy soil and quite wet after the recent snow.  The hole is 4ft deep - the footing is a 4ft x 4ft x 4ft concrete block.  There's maybe a 1ft wide gap to be filled on a couple of sides of the footing and 5ft on the other two (we pulled out a bunch of tree roots whilst digging and also didn't quite get it in the right spot because of the mess with the roots).

Any suggestions welcomed...

[Tome]

I would try and request a visual inspection and avoid the engineering or percent compaction approach. Even if you have to overcut the excavation and install conditioning stone it will be less expensive than dealing with soils engineers and density techs. I assume a proof roll with on site equipment is not possible.

Do you think you may have irritated the inspector with the roots etc.  If so you  want to clean up the excavation a bit before he/she comes back out.

The 90 Percent compaction refers to how compacted the footing soil is compared to a sample of the same soil compacted at optimum moisture. In order to achieve th 90% you will have to make some compaction effort and have the moisture reasonably close to optimum. If you want to read more about this search the ASTM standards for either a standard or modified soil  proctor. All this work obviously becomes expensive and may require a licensed engineer. One good thing is the 90 % is not an overly aggressive hurdle to make.

Let me know if you need more information on the testing. Good luck.

Tom

[Dave Sparks]

I would ask the inspector how much for soil inspections in your area. Down here in the southern Sierra they are $500 or so.

Usually the inspector is there to inspect the rebar and look at your plan. They are all different but I do not get your post. The forms blew out? Does this mean the concrete is in?

[davidj]

Dave,

We had the forms/pole/rebar inspected before the concrete pour and, at that point, the inspector noted we needed a compaction report for the back fill around the sides of the footing (there was more backfill than is typical due to the need to remove tree stumps).  Underneath the footing was fine as it was undisturbed.

We did the pour last Friday and now we're back filling so we can put up scaffolding and set the panels.  We're doing 1ft-ish lifts and compacting with a rammer compactor.  The soil's a bit wet but hopefully we can get it to 90% compaction.  We also get 4' of snow on the grounds for 4 months + a bunch of snow melt to help settle everything down before the inspection in the Spring.

[Ndrmyr]

It would be easy to under estimate the importance of proper compaction.  I back-filled behind a seawall prior to pouring a slab with varying degrees of aggregate.  I would have thought it was fairly compact, but, I had a big track-hoe compact it by punching every square inch with the bucket.  I was shocked at the shrinkage.  I would have thought and inch or two at the most....it shrank by 6-8".  It was money well spent.  That slab is perfect, not a spider crack in the entire 47 yds.
Sometimes as DIYers, we are tempted to cut a corner, and that's fine and our choice. It is important to know what corners can not be cut without creating future misery.
I think most will agree, the foundation is essential to the longevity of our projects.  It is also not easily upgraded in the future.  My recommendation is, at least in this, to err on the side of caution.