Masonry Magazine July 2002 Page. 21
One of the biggest construction pitfalls that an inexperienced contractor will have is not taking the time and effort to properly place the block and backfill. Often to speed construction, the contractor backfills behind the wall in 12-, 14- and 18-inch lifts. Either you don't get full compaction, or to achieve full compaction you move the alignment of the wall so it starts getting a bulge or lacks the proposed batter. In the worst case scenario, improper placement of wall units and backfill can result in unacceptable performance.
Success is in the details
ACCORDING TO NCMA, the success of any segmental retaining wall installation depends on complete and accurate field information, careful planning and scheduling, the use of specified materials, proper construction procedures, and inspection. As the mason contractor knows, it is good practice to have the retaining wall location verified by the owner's representative. Existing and proposed finish grades shown on the drawings should be verified to ensure the planned design heights are in agreement with the topographic information from the grading plan.
The contractor should coordinate the delivery and storage of materials at the site to ensure unobstructed access to the work area and availability of materials. Materials delivered to the site should be accompanied by the manufacturer's certification that the materials meet or exceed the specified minimum requirements.
Construction occurs in the following sequence:
1. Excavation and construction of the leveling pad.
2. Setting, leveling and backfilling base course.
3. Placement and backfilling of units in succeeding courses.
4. Placement, tensioning, and backfilling of soil reinforcement (when required).
5. Compaction of backfill to the specified density.
6. Capping and finish grading.
As with any structure used to retain soil, careful attention should be paid to the compaction equipment and procedures used during construction. When compacting soil within 3 ft (0.9 m) of the front face of a wall, compaction tools should be limited to hand operated equipment, preferably a vibrating plate compactor. Reinforced soil can be compacted with walk-behind or self-propelled riding compaction equipment.
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 sixinches 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 set-back/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.