Gridded Continuous Footings

[howardoark]

I was just reading a report by one of my competitors for a job where they underbid me by 1/3 and must have essentially given the reporting away for free. It's a fine report.

But they only did Atterberg Limits testing for expansive clays (I assume because they did them in house). The PI results came back average of 12, high of 18 with their one shallow result at 14. Based on that, they considered the site soils to be low to moderately expansive and indicated that the building should be supported on a grid of continuous footings with no isolated interior column footings.

In the fill section of the report, they indicated all imported fill should be non-expansive with a PI less than 15. I find that kind of annoying.

I realize people are tired of being sued for every crack in a sidewalk, but where should you draw the line on spread footings versus grids of continuous footings? For this particular job, I would have done EI testing and at the low end of moderate expansion potential, I would have moisture conditioned the soil and recommended spread footings (one and two story buildings). That would have saved the client a lot of money, but if their floors eventually warped, it wouldn't have been money well saved.

 

[BigH]

Haven't dealt with expansive soils, per se, but they also could have used geofoam, I think.

http://www.softoria.com/institute/geofoam/applications.html#610

 

[hokie66]

The recommendation to use a stiffened raft slab may have been due more to experience of local performance of different systems than to the strict interpretation of the test results.

 

[FixedEarth]

Even if there is medium expansion potential, it is the soils under the slab not the footings that would cause problems. So in their recommendation, they reduced their differential footing settlement to almost nothing but slabs and any partitions on them would be subject to movements.

Ther should have been at least on EI test. It is inadequate report. You always get what you pay for.

On the "no isolated footing" clause-it just doesn't make sense. The dead load pressure on a column can easily be adjusted to counter the swelling pressures. I would understand if there is a limitation on avoiding overlapping streeses from isolated footings but not completely avoiding them.

On the "PI < 15" clause, is also prescriptive. You can get high expansion potential with a soil of PI = 12 and medium expansion on another soil with a PI = 10. It would be better, more performance based, to say-all fill shall have EI < 30 and should be approved prior to importation.

 

[hokie66]

FixedEarth,
My interpretation of the recommendation for the "grid of continuous footings" was a grid of slab thickenings, which is a typical residential and small building solution. The slab and the footings move together, so differential movement is minimised. As to countering the swelling pressure with dead load, that is not so easy with a lightweight building. You may be correct about the requirements for a thorough investigation, but perhaps the geotech already knew the answer. I'll take local experience over testing every time.

 

[FixedEarth]

hokie66;
I agree balancing soil testing with experience.

However, I have never heard nor agree with eliminating column footings in a building just because we have expansive soils. We can increase allowable bearing and remove/replace non expansive material to a depth of few feet below the bottom of all isolated footings.

 

[hokie66]

We do that quite frequently. The stiffened sections of the raft slab are designed like grade beams, thus distributing the column loads

 

[apsix]

Supporting Hokie, our residential footings code (AS2870) mandates (or at least strongly recommends) not using isolated footings for highly reactive soils.
It's an effective solution.

 

[oldrunner]

In regards to aspix, the Australians seem to be way ahead of the US in addressing differential settlement. Where our prescriptive continuous footings will call out two bars, one top and the other bottom, I’ve seen their continuous footings with three bars on top and three below and the footing itself deeper. Isolated footings may pump depending on the moisture in the soil at a particular time of year. Sometimes it not noticed until the roof starts to leak because the differential vertical movement will telegragh through walls.

 

[csd72]

agreed, the Australian AS2870 is the only national code I know of that bases the design more on stiffness rather than just strength.

Depth of footing depends on how fragile the walls are and how poor the soil is then the reinforcing is usually just the minimum required to exceed the concrete cracking moment.

Back to the original comment. I am very dubious about using increased stress under footings to reduce expansion - what happens to the elements between the footings? You end up having to have everything else suspended over void formers to effectively prevent the floors from raising.

I cannot see how this would be cheaper than digging a continuous ditch with an excavator and filling it up with concrete and a bit of reinforcement.