Mat Foundation Vs. Grade Beam on Piers



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    Stephen Starks



    The soils report for the subject question informs you that there is both sandy clay and bentonite. Bentonite is a type of highly absoprptive clay, resulting in great potnetial for heaving due to the action of frost.

    The chapter 9 reading indicates that foundations on this type of "expansive" soil need to be isolated.

    Mat foundations and spread footings all require direct contact with the soil in order to spread the structural load out, which is often a good approach unless you are dealing with expansive soils. If expansive soils are present direct soil contact, without prior treatment, is highly unadvisable and could result in failures.

    Belled piers are best used for soils with poor bearing capacity (weak), but are less suited to expansive conditions due to the issues related above seing as they also rely on load distribution rather than direct bearing

    Grade beams on piers are the correct solution in this scenario, because they do not rely on load distribution, but rather a direct bearing approach. The piers typically bypass the clay and bear on suitable material below it. They also allow for a seperation between the grade beam and the expansive conditions below (also in the reading). This space allows the soil to heave without tearing the foundation apart (sort of a "flow with the river, not against it" approach).

    As always, do your own research, but I hope this helps a little.

    Good luck.

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    Ashley Biren

    Thank you Stephen for the in-depth answer! Very helpful

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