Many masonry projects, including stone arch bridges, require the use of retaining walls. Retaining walls simply are walls that are designed to hold back soil. This can be done either with or without mortar. In stone bridges, the spandrel walls and approaches are usually a pair of retaining walls holding soil between them.
As it happens, mortarless masonry seems to lend itself rather well to retaining walls, provided some basic conditions are met. The key to all mortarless stonework is to recognize how the forces at play behave and interact, and then use this information to advantage. When determining how to effectively build a mortarless retaining wall, then, it helps to understand how the soil it is retaining behaves.
Generally, soil wants to level itself out to a gentle slope. This means that the soil is always exerting thrust against the retaining wall. Therefore, the retaining wall must be heavy enough to resist this thrust. How heavy, of course, depends on the type of soil being contained. Another factor at play is freeze/thaw cycles. When wet soil freezes, it expands. This expansion force will push the retaining wall out slightly, unless the wall is very heavy compared to the soil, in which case the soil will tend to expand upwards. What needs to happen, then, is that when the soil thaws and then contracts, the retaining wall, if it has been shifted by the freeze and expansion, needs to naturally settle back into its old position.
To resist freeze/thaw cycles, the retaining walls must either be very thick relative to the height of soil being contained or able to self-correct. Large stones can be used successfully to resist the soil, but most builders do not have the means to move massive stones. In the end, most builders will resort to building self-correcting walls. The way this self-correction is achieved is by leaning the walls into the soil. This can be done in several ways. The walls can be built leaning or they can be stepped into the soil.
Stepped walls can be built something like stairs, or they can be built flush on the outside but stepped further into the soil as they go up on the inside. Both methods have sections of the walls actually using the soil as a support. In the case of the walls that are level on the outside but not on the inside, the overall wall thickness is greater at the top than the bottom. This, of course, results in a wall that tends to want to topple over; however, the soil is holding it up. Thus, when the soil expands, it tips the wall out slightly, but, when the soil contracts, the wall tips back into the soil again.
The stair-stepped wall works the same way, only the outside of the wall is not flush. It is worth mentioning that a dramatic lean into the soil is neither necessary nor, usually, desirable. In the case of the wall flush on the outside but widening on the inside as it goes up, the top probably shouldn’t be more than twice as thick as the base.