any civil / geotechnical engineers among us?? (1 Viewer)

[Benny G]

I'm a CE major and taking a geotech class this semester. Most of the class grade is a huge design project. This semester, we're designing a ski resort from the ground up.

A couple of things I'd like to include in the upcoming phase are...

1) What % of relative compaction should the soil be below footings??

I haven't been able to find much in many many books or online. What I've seen tends to point towards 95%, but nothing solid yet..

2) What size area has to be excavated for a footing whose base will be placed within ~1 foot of the surface? Say the footing will be 5'x5'. How much excess area has to be removed in order to place the footing? A foot or two on all sides? More? Less? I haven't been able to find anything about this.

If anybody can offer any advice, that'd be great. I'm not looking for exact answers, just general advice. Even if you could point me in the right direction, such as towards regulations similar to IBC, that'd be great.

Thanks very much in advance!!

 

[Troy Madlem]

Here are a couple thoughts on your project.

1. When footings are constructed on fill, engineered fill should be compacted to 97% modified maximum dry density in accordance with ASTM D-1557. When footings are constructed on undistributed native soils, the site is proof rolled with a medium weight roller (ie loaded dump truck or similar weight/size vehicle) to discover any pockets of soft or unstable soil. Any soft spots are excavated and replaced with engineered fill.

2. There are two ways to construct the footings. On smaller projects alot of contractors will prefer to over excavate the hole for the footing (by only a few inches), set the rebar in the hole and place the concrete without any side forms. If you require the footings to be formed quantities for excavation are typically based on a hole 1'6" wider on each side then with 1:1 sideslopes up to the surface. You can check your State's Department of Transportation specifications for how they calculate excavation quantities.

 

[Ron-P]

2) What size area has to be excavated for a footing whose base will be placed within ~1 foot of the surface? Say the footing will be 5'x5'. How much excess area has to be removed in order to place the footing? A foot or two on all sides? More? Less? I haven't been able to find anything about this.

Usually excavation of twice the footing depth. If your footing is 5x5x5 you over-excavate 5' below the footing and as a standard 5' around the perimeter of the footing. Again, it will depend on the soils report. Do you have one?


Peace Out~

 

[Alex-C]

I guess I am a civil engineer, I have this formal looking piece of paper framed on my wall that says I am so it must be official.

 

[Benny G]

Wow! Thank you very much Troy and Ron.

Troy:

(You have a PM)

For #1, the footings will be placed on natural soil. There is sufficient sand to satisfy bearing capacity requirements, but my group planned on compacting it with a "heavy, vibratory, steel-drum roller" (which according to one book compacts sand to depths of ~6 ft). I "REALLY" love the idea of checking for soft spots with a loaded dump truck. It's certainly very cost effective.

Would using a roller for compaction make the dump truck testing unnecessary?

For #2, I checked MDOT's (Michigan) website per your advice, and here is what I found:

"...The plan quantities will be determined by the space bounded by the existing ground surface or exposed portions of the existing substructure, the elevation of the bottom of the foundation, and 1:1 slopes extending outward from points 18 inches outside the bottom of the footing, unleess otherwise shown on the plans."

i.e. exactly what you said!!

Ron:

Our footings are going to be 5x5x1 (thickness just preliminary), which will be placed on or very near the surface. 5' of fill will be placed later which will make the footings underground.

We do have soil boring logs. Our design borehole is approximately: 6.5' medium dense sand (to be compacted), then about 6' of soft clay. Some excavation and borrowing will take place prior to footing construction.

My group does understand that the footings will not satisfy settlement requirements, but that's not until the next part of the project. The footings will have to be redesigned for that phase. Right now, we're doing bearing capacity, so the soft clay isn't an issue.