As a result of the dry conditions that have persisted in California for the past six years, consulting engineers have discovered that a number of older pools have under gone differential movement and cracking. This is mostly found in areas with thick clay soil and mature trees near the pool.
These changes in the pool all start with a "perfect storm" that all starts with the mineral solids, water, and air that comprise soil (which if small enough becomes "clay-sized"):
Due to the chemistry of clay particles, a majority of the surface carries a slight negative electrical charge. This attracts the positively charged side of water molecules. The closer the water molecule, the stronger the attraction. Consequently, clay soil attracts and holds water with various levels of force. Think of it as a dirt sponge. This force causes clay soil to undergo volume changes with variations in moisture content. When water is available, it is attracted to the particles and pushes them further apart, causing the soil to swell. The particles hold on to this water until something comes along that can pull it away, resulting in shrinkage. Plant roots, particularly trees, have the ability to pull away this “stored” water in times of need, as they feed through capillary action.