Cover Story: Sidebar

Step-by-step SRW

The procedures and sequences used in building a segmented retaining wall are fairly universal. The following steps come from Keystone Retaining Wall Systems, Minneapolis, Minn. and are illustrated using their block system.

Step 1 — Prepare the Pad
Excavate the base trench per specs and start the leveling pad at the lowest elevation of the wall. Level the pad with six-inches of well-compacted granular fill. Keystone recommends additional trench depth for below grade placement at a ratio of one inch for each eight inches of wall above grade to a maximum of three courses below grade. This locks the wall in place and helps prevent erosion and scouring of the base.

Step 2 — Base course installation
Place the first course with the units side-by-side. With Keystone units, the paired pinholes should be facing up. Each unit should be leveled, side-to-side and front-to-back. The first course is critical for accurate results.

Step 3 — Placing pins
The Keystone units offer paired holes into which fiberglass pins are inserted. Once placed, the pins create an automatic setback/batter for following courses. They are also used to retain the geosynthetic material if used on tall walls.

Step 4 — Backfill
Fill in all voids and the areas between and within the units using crushed stone or clean granular fill, but not pea gravel. Place drain zones behind the units to achieve a total depth of two feet from the unit face. Compact the material in unit cavities to eliminate settling.

Use existing soil for backfill but not if it is high in clay or organic material as these hold water. In some situations, poor site soil will cause higher costs for reinforcement so consider using imported soil of a known quality. Place soil in eight-inch lifts on a course-by-course basis. Use hand operated not walk-behind mechanical compacting equipment within three feet of the back of the units to avoid localized overstress. Be sure the tops of all units are clean before laying the next course.

Step 5 — Geosynthetic installation
Tall walls will require geogrids or other geosynthetic material. This material is best whenever the wall height is greater than four feet — or even three feet in some situations. Keystone recommends geogrid material but engineering or architectural considerations may require other forms.

Excavate the soil area behind the units to the maximum embedment length of the geogrids. Level the soil visually. Cut the geogrids sections to the specified embedment length being careful to watch for orientation. Check manufacturer's criteria for biaxial or uniaxial geogrids — the correct orientation is usually to unroll the material perpendicular to the wall face.

Hook the geogrids over the pins in the units or, if the brand of SRW block is lacking pins, place another unit course on top of the anchoring block. Pull the geogrids taut to eliminate loose folds and stake or secure the back edge before and during backfilling and compaction. Once sufficient weight of backfill is in place to retain the geosynthetic material, remove the stakes. Place remaining sections of material abutting each other for continuous coverage at each layer. Place compacted backfill over geogrids in 8-inch lifts and avoid driving over the geogrids with tired equipment to prevent damage.

Step 6 — Install remaining courses
Complete the wall by repeating the previous steps. If using the Keystone system, place the next course of units over the fiberglass pins, fitting the pins into the kidney-shaped recesses. Center the unit over the two underlying units. Visually sight down in the kidney-shaped recess for pin positioning. Pull the Keystone unit toward the face of the wall until it makes full contact with both pins. This operation will create an automatic setback/batter for the course.

The top of the wall may call for cap blocks that may be installed using mortar or special adhesives designed for this application (Keystone recommends its KapSeal adhesive). Backfill the remaining area.

Step 7 — Finished
Step back and view a job well done.

Return to the Cover Story