The stabilizing action of bentonite was first exploited by the oil industry for drilling and stabilizing wells without the use of casing. Today sodium, or western, bentonite is used regularly to aid diaphragm wall placing, pile driving, caisson sinking, and soil grouting. In addition to speeding construction, bentonite avoids the noise and vibration problems that influence conventional foundation operations in urban areas. Bentonite also allows closer working to boundary lines. Bentonite is derived from montmorillonite clay and is usually supplied in paper bags as a finely ground, free flowing powder. Like portland cement, it must be stored under reasonably dry conditions. When the powder is dispersed in water it breaks down into a very fine colloidal clay. This clay continues to absorb free water so that it gels on standing. Bentonite can repeatedly become fluid on agitation. Yet it will regain its gel structure when left undisturbed. In foundation work, the gel penetrates around individual soil particles and maintains them in position by adhesion. The quality of bentonite needed to make a slurry with water varies from about 4 percent by weight for use in stiff clays to about 10 percent by weight for open gravels. Chemical additives can be included to increase or decrease the penetrating and gelling properties of a slurry. The preferred method of mixing is by high-speed stirrer for at least two minutes.