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Structural Engineering & Soils Question

Structural Engineering & Soils Question

(OP)
Hello, apologies if this is the wrong forum, but I was hoping to get a little insight as to what you all expect/need when designing a foundation system from a soils aspect? The area of Colorado in which I live (Littleton) is primarily a sandy-clay soil and what I'm trying to create is a small addition (80 SF) on the front of the house for a mud-room (the entire development was constructed between 1968-1974). The local building dept. does not require a soils report, but my structural engineer says he needs one to determine the type of foundation he'll use, which I understand.

He wants to match the existing foundation type, which we assume to be piers, (no original construction set exists) based upon exploratory digging down to 4'-0" and no sign of footings, etc.

My issue is that I can't seem to get a straight answer from him as to what he needs from a soils investigation? I happen to have a friend in geo-tech and she suggested a standard proctor of the top 2'-0" for swell potential, PI & soil makeup. When I suggested that to our structural engineer, he said that wouldn't give him sufficient information, but can't/won't tell me exactly what he needs. My geo-tech friend seemed surprised, but is hesitant to suggest anything else without more information.

My question is, what do you all look for when designing for concrete piers/helicals or shallow spread footings? Is the top 2'-0" useless? Do we need bores down to bedrock?

I'm trying to keep a happy wife and project moving forward, but seem to be going in a circle and was hoping for some insight/suggestions as to what questions to be asking in order to keep going.

thanks.

RE: Structural Engineering & Soils Question

I'm not sure why a Proctor test tells about shrink-swell, but there are regular tests in addition to the Atterberg tests to help. Your geotech should be able to do all that and make recommendations as to foundations to use.

Before getting too far into it I'd check with the local building inspection folks to see what they typically see and find is needed.

Shrink-swell problems can affect soil to deep depths, usually dependent on weather and any trees nearby that draw water.

I had a friend in your town many years ago who found that with the seasons he had to change the elevation of posts in his basement two times a year to take into effect of those changes. If you are not having to see such effects chances re you have very deep foundations.

RE: Structural Engineering & Soils Question

Denver area has troublesome expansive soils so piers are common. The structural engineer does not want to take responsibility for anything regarding this which can be appreciated. Most Denver geotechnical engineers would put a boring down, take samples and check for expansion properties then give you a canned paragraph of what type of piers and how deep is best for your addition unless a spread footing will work. Some local geo engineers know the area well and can tell you what they always recommend. The structural will want a letter of recommendation or report either way.

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