Information on the importance of retaining walls
Retaining walls are used to create separate patio levels on sloping ground or support an earthen bank. Common retaining wall materials include railroad ties, pressure treated beams, redwood or cedar posts, pressure treated posts and boards, fieldstone, flagstone, river rocks, or even broken pieces of concrete.
Because sloping earth, especially when wet, can exert significant pressure, retaining walls should not be higher than four feet. If you do need a higher wall, either a structural engineer or a qualified landscape architect should engineer it. The risk of a wall collapsing and injuring someone is not worth taking.
All retaining walls should have a firm foundation to minimize problems associated with earth settling beneath a wall, which can destabilize it and crack the mortar. Large rocks set in the ground make good footings for a stone wall, but brick or concrete block walls should have a concrete footing under them. If you live in areas where the ground freezes, the wall foundation should be placed below the frost line. Your local building inspector's office can tell you what that depth is.
Retaining walls can be made from timber, interlocking blocks, stoned, brick or concrete. Timber will cost the least, but it can suffer cracking and rot if it doesn’t have the proper drainage. Interlocking blocks will be stronger but can also crack or bulge if they aren’t tall enough or drained well to hold against what they protect. Stone or masonry retaining walls can be stacked or mortared in place, which is more stable. If a retaining wall is formed by rocks are stacked against an earthen bank, the bank should be angled back several degrees so the wall leans against it. The same will be true for brick or concrete.
When planning a retaining wall, let it flow with the natural curves of the land when possible. Following the topography not only breaks up the boredom of a straight wall, but results in little promontories and niches that make natural and inviting seating areas.
Groundwater should not be permitted to build up behind a retaining wall. Mortared brick or stone walls should have weep holes along the base at regular intervals to allow groundwater a way to pass through. Loose stone walls do not need specific weep holes because water will pass through openings between the rocks.
In particularly wet areas, a drain line behind the retaining wall near the base is advisable. This ditch, with a gravel base and perforated pipe sloped to carry the water away, will help prevent water and saturated soil from pushing a wall out of line or collapsing it.