One of the most common complaints we hear is about soil eroding, steep hills untouched by lawnmowers and general yard chaos that could be fixed by a gorgeous retaining wall. In the bad old days, most retaining walls were made of "railroad ties", really landscape timbers. Since it's not the old days, there are now rules regarding the chemicals that are used to treat those landscape timbers. That equals more expensive. The best and most solid (and time-tested) solution to retaining walls is the modular block retaining wall.
Modular block, or segmental, retaining walls employ interlocking concrete units that tie-back into the earth to efficiently resist loads. These pre-engineered modular systems are an attractive, economical, and durable alternative to stone or poured concrete retaining walls. The inherent design flexibility can accommodate a wide variety of site constraints, project sizes, and aesthetic preferences.
Individual, usually identical, precast concrete units either interlock, offset stack, or are placed structurally independent of each other and anchored into the backfill. The components of a complete system can include foundation soil; leveling pad; precast concrete units of high-strength concrete; shear pins if units don't interlock; multiple-depth walls or additional soil reinforcement such as geotextile, welded wire fabric, or dead-man anchors if the wall is over a certain height; retained soil; and drainage fill. The soil reinforcement consists of horizontal layers that extend into the backfill.
Being gravity structures, these systems rely on their own weight and mass to resist overturn and sliding forces. The segmental nature affords the wall a permeability to relieve pressure, so less material is required for resistance. Because they are considered flexible structures, the footings usually need not reach the frost line. Some systems allow for landscaping of the wall between tiers (depending on site conditions), while others are designed as structural frames to be covered with landscaping.
Installation
These systems usually require less excavation than poured-in-place walls, and on-site soils can often be used. They require no heavy equipment, mortar or formwork, and usually can be constructed by hand. This affords flexibility in construction scheduling. Proper backfilling and grading for base tier is fairly critical. These systems can also be overlaid onto existing older retaining walls.
Benefits/Costs
These systems allow for some design flexibility, such as curved walls, and construction is generally faster than poured-in-place concrete or stone walls. Site conditions have a major impact on costs. Installed costs are usually less than poured-in-place and especially stone walls. Controlled manufacturing conditions ensure a durable, damage-resistant product. Maintenance is negligible as there is no mortar to repoint, and the flexible systems generally eliminate cracking.
Information from NAHB Research Center at TOOLBASE.ORG
