11-12-2012, 06:46 PM
DAMAGE TO FOUNDATIONS FROM EXPANSIVE SOILS
DAMAGE TO FOUNDATIONS FROM EXPANSIVE SOILS.pdf (Size: 1.34 MB / Downloads: 164)
ABSTRACT
Expansive soils in many parts of the United States pose a significant hazard to foundations for light buildings. Swelling clays derived from residual soils can exert uplift pressures of as much as 5,500 PSF, which can do considerable damage to lightly-loaded wood-frame structures. Insurance companies pay out millions of dollars yearly to repair homes distressed by expansive soils.
Expansive soils owe their characteristics to the presence of swelling clay minerals. As they get wet, the clay minerals absorb water molecules and expand; conversely, as they dry they shrink, leaving large voids in the soil. Swelling clays can control the behavior of virtually any type of soil if the percentage of clay is more than about 5 percent by weight. Soils with smectite clay minerals, such as montmorillonite, exhibit the most profound swelling properties.
Potentially expansive soils can typically be recognized in the lab by their plastic properties. Inorganic clays of high plasticity, generally those with liquid limits exceeding 50 percent and plasticity index over 30, usually have high inherent swelling capacity. Expansion of soils can also be measured in the lab directly, by immersing a remolded soil sample and measuring its volume change.
In the field, expansive clay soils can be easily recognized in the dry season by the deep cracks, in roughly polygonal patterns, in the ground surface (see Fig. 1). The zone of seasonal moisture content fluctuation can extend from three to forty feet deep (see Fig. 2). This creates cyclic shrink/swell behavior in the upper portion of the soil column, and cracks can extend to much greater depths than imagined by most engineers.
Foundation Damage
The most obvious way in which expansive soils can damage foundations is by uplift as they swell with moisture increases. Swelling soils lift up and crack lightly-loaded, continuous strip footings, and frequently cause distress in floor slabs.
Because of the different building loads on different portions of a structure's foundation, the resultant uplift will vary in different areas. As shown in Fig. 3, the exterior corners of a uniformly-loaded rectangular slab foundation will only exert about one-fourth of the normal pressure on a swelling soil of that exerted at the central portion of the slab. As a result, the corners tend to be lifted up relative to the central portion. This phenomenon can be exacerbated by moisture differentials within soils at the edge of the slab. Such differential movement of the foundation can also cause distress to the framing of a structure.
Mitigation Measures
The best way to avoid damage from expansive soils is to extend building foundations beneath the zone of water content fluctuation. The reason is twofold: first, to provide for sufficient skin friction adhesion below the zone of drying; and, second, to resist upward movement when the surface soils become wet and begin to swell.
Successive drought years have demonstrated that the zone of seasonal fluctuation can extend much deeper than previously believed. Piers extending to depths of six feet can withstand normal annual fluctuations, but do not appear adequate when taken over the long haul, such as a two-year drought followed by an extremely wet year.
Another way of mitigating expansive soil problems is to collect surface runoff and to limit surface infiltration during the rainy winter months. In Orange County, California, considerable experience in subdivisions has shown that it is important to provide for positive surface drainage away from all points around building foundations. Much of this experience was incorporated into the Appendix Ch. 18 Excavation and Grading statutes of the Uniform Building Code (UBC), which were not adopted in the 2000 IBC.
Case Study - Swimming Pools
Swimming pools constructed near home foundations in expansive clay soils are frequently sources of damaging moisture. Such swimming pool shells are typically constructed of airblown mortar (gunite). The gunite skin of a swimming pool is rarely designed for swell pressures generated by expansive clays. Furthermore, in all swimming pools the water percolates through the shell and adds significant amounts of water to adjacent soils. Since pools are generally in arid climates, this creates a moisture differential which can be severe enough to differentially lift adjacent pool decks and house foundations, as shown in Fig. 6. It can lead to destruction of the pool itself.
A design for a swimming pool shell in a highly expansive soil environment is presented in Fig 7. Pools of this design have been constructed at several locations in California with great success. Construction begins with overexcavation for the pool. The excavation is lined with a single sheet of Hypalon R or similar HDPE membrane material to provide a barrier for the collection of moisture that seeps naturally through the pool shell. A second line of defense consists of crushable Enkamat K to retard potential swelling pressures against the pool sides. This also serves as a vertical subdrain material to guide water to a central collection drain beneath the pool. This drain between the liner and gunite shell then feeds into a drain pipe to convey water off the site. This system works best in sloping areas that can provide positive drainage from below the pool. The same design, however, could be used in flat terrain through the use of an impervious sump and automatic pump beneath the pool.