A masonry structure protects the ground below from drying out and a reserve of moisture will have built up after a few years. During dry months, or particularly extended periods of drought, the loss of moisture to the atmosphere will cause a clay subgrade to shrink around the structure, which evidences itself first at the corners. This relative movement causes the corners to appear to drop, creating diagonal cracks radiating upward and outward from the mound edges.
Trees will cause a similar but unsymmetrical effect if located close to a corner, or may cause a central panel to drop if central to a wall. Typical cracking patterns associated with these three conditions are shown in Figure 2.8.
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