Page 223 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
222 CHAPTER SIX

1
tightly. Soil particles less than 480 in. are called fines. Laboratory
tests of soils containing 56% fines showed moisture constantly rising
to the surface and evaporating at an average rate of about 12 gallons
per 1000 sq. ft. per 24 hours with a water table as much as 30 in.
below the surface. Field tests have also shown that substantial
amounts of moisture migrate upward through fine soil even when the
water table is as much as 20 ft. below the surface. Figure 6-31 indi-
cates the height of capillary moisture rise which can be expected with
various soil types. Any basement or slab-on-grade built without pro-
tection on moist soil would be exposed to a continuous capillary
migration of moisture toward the structure. Since both concrete and

BUILDING SLAB

VAPOR ONLY
LIQUID AND VAPOR
CAPILLARY RISE

SATURATION
ZONE

CAPILLARY RISE OF MOISTURE
WATER TABLE

Soil Type Saturation Zone, ft Capillary Rise, ft

Clay 5 8
Silt 5 8
Fine Sand 1-5 3-8
Coarse Sand 0-1 1-3
Gravel 0 0

FIGURE 6-31
Capillary moisture rise. (from Harold B. Olin, Construction Principles, Materials and Methods, Van Nostrand
Reinhold).

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