The Use of Expansive Clays

Expansive Soils

• Expansive soil gets its characteristics from clay particles in the soil. Certain types of clays are made up of extremely small particles, which form sandwiches with water molecules when wet. This makes the clay expand greatly in size when wet -- and contract back to its original size as it dries.

Foundation Dangers

• This expansion and contraction can be deadly for foundations. Rain causes the soil to expand greatly, putting intense pressure on the foundation from every direction. When the soil dries, it contracts, pulling away from the foundations and depriving them of their support. The drying soil often becomes fretted with deep cracks, which will convey the next rain shower even deeper into the soil. This deep expansion causing exponentially more pressure, as the confining force of the surrounding soil becomes greater as depth increases, and more shrinkage when the clay dries.

Mitigation

• Early detection is the key to mitigating damage from expansive clay. When building a new house on expansive soil, the top few feet of the soil may be removed and replaced with a nonexpansive soil. Before laying the foundation, the soil may be pre-wet, so the new foundation rests on the greatest mass of soil, to prevent cracking and humping of the foundation slab when the soil expands in the future. In existing buildings, foundations may be supported with underpinnings, deeper footings or thicker slabs.

Positive Uses for Expansive Clays

• Expansive clays are almost always seen in a negative light, but the chemical reactions that cause expansion in these soils can be harnessed for positive ends. In demolition and construction, expansive clays can be used in place of dynamite for splitting large rocks. A hole is drilled in the rock and filled with a silicate with oxides of calcium, silicon and aluminum -- the same ingredients that give expansive clay its properties. When water is added, the silicate expands with great force, splitting the rock without the use of explosives.