What is the most common contributing factor to masonry retaining wall movement?

Hydrostatic pressure is considered the most common contributing factor to masonry retaining wall movement. This pressure builds up in the soil behind the wall, especially in areas where there is poor drainage or heavy rainfall. When water saturates the soil, it increases the weight of the retained material, leading to increased lateral forces on the wall. If the wall is not designed to withstand these forces or if proper drainage systems are not in place to alleviate the pressure, it can cause the wall to move, bow, tilt, or even fail entirely.

Inadequate drainage can exacerbate the problem, as water pooling behind the wall increases hydrostatic pressure significantly. This principle is crucial in the design and maintenance of retaining walls, as engineers must consider soil saturation and proper drainage solutions to prevent movement.

While the other factors mentioned may contribute to wall stability in certain contexts, hydrostatic pressure is the primary reason for movement in masonry retaining walls.