I have a client who wants to build a one storey small cottage near to a lake. I did geo investigation and encountered loose/very loose saturated sand up to more than 4 m. The sand is silty and the frost depth is 1.2 m.
What would be the most cost effective foundation for this light structure? I was thinking of helical piles but wondering if there is any cheaper option.
I assume this sand is relatively clean. If there is freezing weather it won’t heave then. So, I’d go for a floating slab. Reinforce with welded wire grids. Thickened edge helps for keeping wood members up from wet conditions. Grade can be built up if needed. Compaction of saturated sand can be done, but vibrators may not work well due to temporary loss of support from vibrating rollers, etc. Placing on a closed cell foam insulation will make for a more comfortable interior. Likely no settlement of consequence will occur.
Also what does one see in winter for heaved roads, uneven walkways, etc. This will provide clues. Also, SPT readings of saturated sands can be very misleading. If you have serious frost heaving, maybe still use a slab with 3" Styrofoam under it, but also extend that insulation out all the way around at a depth of 0.2 M. I’d go out a full width of the insulation board (one meter+?). By the way, for saturated soil, my bet is your frozen depth is much less than the 1.2M. Heat of fusion helps a lot. No lake there has 1.2 M of ice.
In my experience I have seen blow count drop as the boring enters saturated clean sand, yet actual density (dry) has not changed.
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