grade beam construction - earth form vs formed

[archeng59]

for grade beam (conc beam spanning between piers) construction, I prefer not to allow the contractor to use earth forms because of the inability to control the vertical sides of the trench, cleanliness of the bottom of the trench due to sloughing of the soil, inability of the contractor to control beam depth and width. I have a statement in the general notes and formwork specs that grade beams shall not be earth formed. however, I get pressure on every job to allow the contractor to use earth forms for economic reasons.

any thoughts from other engineers would be appreciated.

I do allow earth forms for footings.

 

[GeoPaveTraffic]

As a geotech I prefer earth formed for one reason. No backfill is required. This pretty much eliminates problems with the slab adjacent to the grade beams. Cleanliness of the bottom should not be a big deal since the beam doesn't "bear" on the bottom, assuming of course that the required cover is provided.

Maybe you could require that the dimensions of the beam be 2 or 3 inches larger if it is bank poured thereby making cleanliness less of problem and providing the contractor the option right in the specification.

 

[archeng59]

one major problem with previous projects where I allowed earth formed grade beams is that the soil at the top of the trench sloughs into the trench while the carton forms and reinforcing are installed. By the time the concrete is poured, the cross-section is a trapezoid. On a couple of projects, the trench width was as much as 2 inches wider at the top than the bottom. In my opinion, the grade beam bears on the soil to a certain extent because the sides of the grade beam are not vertical. not that a formed face is truly vertical, but more vertical than the typical trench. In my area of the country, there are expansive clays. swelling soils could create upheaving on the sides of the grade beam with that much variation in the width from bottom to top.

 

[GeoPaveTraffic]

It is very unlikely that swelling soils could push a grade beam up. Most are heavly reinforced and could resist significant uplift forces. Also, even with the shape that you indicate, 2" in say 24", that is still a pretty vertical face. Any upward force by swelling soil would seek the path of least resistance which is more likely to the the soil next to the grade beam.

I just don't see much of an issue, but then again its not my grade beam.

 

[stewbaby]

I like the use of earthen forms when feasible, the clear distance to rebar and such can be designed with some "fat" if roughness or sloughing is a concern. We have alot of sandy soil though and find in handy to incorporate a form that can stay in place such as Stay Form

http://amico-stayform.com