Foundation systems for Weak & Expansive soil
Hi,
I've been studying the foundation part of the building and I came across a couple of questions if anyone can help me with:
1. Form my understanding, weak soil is not the same as expansive soil, the soil only considered as expansive when there's water presents or the soil has high clay content? So if it's only mensioning the soil is "silty clay", it's considered as more of a weak soil rather than expansive soil?
2. For expansive soil, we consider using Post tensioned slab, or grade beams on piers, or only drilled piers support slab, correct?
3. For weak soil, we use raft/ mat slab, or end-bearing pier/ pile, correct?
4. Belled Cassion bears on high capacity soil below, so if there's weak soil or expansive soil through out, we can not use it, correct?
5. Can friction piles be used in expansive soil or weak soil?
Thank you!!
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Jessie, your question inspired me.
Weak soil is not necessarily the same as expansive soil. We'll cover this topic in our weekly 40 Minutes of Competence session Thursday July 3 at 6pm Eastern Time at this Zoom link. We'll address the topic with the following practice test item:
An owner has hired an architect to design a large building on a site with 11 feet deep seam of weak expansive soil. Which of the following offers the most appropriate foundation system? Select the 3 best answers.
Belled caisson
Conventional spread footing
Grade beam on shallow piers
Mat foundation
Post-tensioned slab foundation
Unanchored floating foundation
(We meet every week at that same time and place, though this week we won't for the Juneteenth holiday)
--Michael Ermann, Amber Book creator
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Hi Michael,
Thank you for your answer! I've been attending 40 minutes competence every week and it inspired me so much. I have my exam next week so I'm going to give my answer here before the July 3rd meeting...
My answer is: Belled caisson, Post-tensioned slab foundation, Mat foundation.
Although Belled caisson can't be used in weak soil or expansive soil through out, given it's a large building, I assume the caisson could reach pass 11' weak soil and bear on stable soil below.
Post-tensioned slab foundation is an obvious answer.
Grade beam on shallow piers, if it's deep piers I think it would be a correct answer, but the option is "shallow piers" so I think it doesn't have a stable soil or rock within the 11' range. I had a thought of "what if it's friction piers", but I think friction piers also needs not expansive soil, stiff sand or clay is ok.
Mat foundation, I selected it only because it asked me to select 3 answers, I think given careful design it could be a possible answer, but if the question have other options I wouldn't select it.
Thank you so much for your great question! Please correct me if I'm wrong!
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