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When they are dry, swell ing salls often can be recognized by their "popcorn" appearance. 38 Characteristics Swell ing 50115 contain a high percentage of certain kinds of clay particles which are capable of absorbing large quantities of water. Soi I volume may expand 10 percent or more as the clay becomes wet. The power f u 1 torce of expansion Is capable of exerting pressures of 20,000 pst or greater on foundations_ slabs or other confining structures. Subsurface Colorado swelling salls tend to remain at a constant moisture content In their natural state and are usually relatively dry at the outset of disturbance tor construction on them. Exposure to natural or man-caused water sources during or atter development results In swelling. In many instances the 501ls do not regain their original dryness after construction, but remain somewhat moist and expanded due to the changed environment. Consequences Swell ing soi Is are one of the nation's most prevalent causes of damage to buildings and construction. Annual losses are est i mated I n the range of $2 b i III on. The I asses I nc I ude severe structural damage, cracked driveways, sidewalks and basement floors, heaving of roads and highway structures, condemnation of bui Idin~s, and disruption of pipel ines and sewer lines. The destructive forces may be upward, horizontal, or both. Aggravating Circumstances Des' gn and construct i on of structures wh i I e unaware of the existence and behavior of swelling soi Is can worsen a readily manageable situation. Where swell ing soi Is are not recognized, Improper bui Iding or structure design, faulty construction, inappropriate landscaping and long term maintenance practices unsuited to the specific soil conditions can become a continuing, costly problem. Design problems might Include Improper foundation loading, improper depth or diameter of dri lied piers. Insufficient reinforcing