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Water in the Excavation

When you're working in an area with a perched water table during the wet season, you sometimes find groundwater moving into your trench. If the flow is slow enough so you can pump the water out without it flowing right back in, then that's the best solution. You can place concrete in up to 1 inch of water—concrete is 2½ times heavier than water, and it will displace the water. You might want to thicken the footings in that case, because the bottom of the concrete may absorb some water and be a little weaker than normal.

But if the soil is loose and porous, and water and soil keep coming back into the trench as you pump the water out, use large aggregate to build up the trench. For this, large stone or cobbles 2-inch- or 3-inch-diameter rock are best.

When water is pooled in the trench, the author recommends placing large cobbles in the form bottom and compacting them down into the mud. Muck and water may fill the spaces between stones, but contact between the stones will pro-vide bearing. Be sure to use a stiff concrete mix when you cast the footings.

When you form the footings, place enough large stone into the wet, mucky zone to get up above the water table. Compact the stone down into the mud, then pour your footing. The large aggregate allows the muck to fill into the pore space, but as long as all the pieces of stone are in contact with each other, the stone can still transfer the load.

If the stone is piled so high in the forms that your footing becomes too thin (less than 4 inches thick), place transverse rebar to reinforce it, as shown (be sure that the footings are thick enough to cover the steel by at least 3 inches).

Fixes for Misplaced Footings

Spanning over a Soft Spot

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