A thousand years ago, all retaining walls were built from small dry-stacked segments of masonry (rocks). These walls relied on gravity and friction to hold soils in a near-vertical position. Modern segmental concrete retaining walls differ very little from historical walls, except that segmental walls incorporate an interlock system between the units and often use geosynthetic materials as soil reinforcement to improve upon what can be achieved with gravity alone.
This apparently low-tech system, however, has several distinct advantages over the more "modern" approach of a cantilevered concrete retaining wall:
SRWs have an attractive and natural appearance in a wide variety of looks.
- There is a lot of flexibility in design.
- They are easy and fast to build.
- SRWs are very economical.
- SRWs have proven to be durable.
Freeze-thaw damage problems appear to be isolated and no cause for concern. Admixture manufacturers, such as W.R. Grace with its Trysta admixture, provide materials to improve unit durability and reduce efflorescence potential.
Some basic points:
SRW units are manufactured on a block machine, resulting in very uniform concrete masonry products that interlock.
- SRW systems are mostly proprietary.
- There are two types of SRWs: conventional gravity walls and reinforced soil walls.
There are several design factors:
A conventional SRW can fail in five ways. The designer must take into account all of these possible failure mechanisms and minimize the risks.
- Good drainage is essential to the performance of an SRW.
- The maximum height of an unreinforced SRW is directly proportional to the weight of the units, the setback per course, and the type of soil.
- The embedment depth of the SRW should be 1/10 the wall height and at least 6 inches.
- SRWs can be vertical but usually have a setback at each course, creating a batter.
- Shear capacity between units is created by friction between unit faces, by integral concrete shear keys that are part of the unit, by fiberglass-composite pins or clips installed between units, or by interlock created by fill installed within and between box-shaped (crib) units.
Each SRW system has unique construction techniques. The basic steps, however, are the same:
- Build the leveling pad.
- Lay the first course of SRW units.
- Backfill the first course.
- Install subsequent courses and backfill.
- For reinforced walls, pull the geosynthetic taut at each level, then hold or stake it during backfilling. Compact the infill soil prior to placing the next course.
- Set the wall caps using concrete adhesive.
- Grade the areas above and below the wall to a finished grade that drains water away from the wall.
The article includes 11 tips, many from VERSA-LOK's technical bulletins, to overcome some of the challenges of installing SRW systems.
Segmental retaining walls are a modern approach to an ancient system for converting sloped earth into vertical surfaces. The article also lists six steps to a successful SRW bid, and provides a list of SRW system developers.