Soils

Soil Identification

Clay soils put more pressure on a wall than sandy soils because they hold moisture. Identify the soils at your site.

To identify the soils, a good test is to pick up a small handful of the soil in the palm of your hand and squeeze it to form a ball. Take a sample from at least 12 in. (300 mm) below the surface.

Clay soil will stick together to form a ball. Clay soils retain moisture which will add pressure behind the walls. Typically most soils will be classified as clay and can be used in your project. However, they may require additional reinforcement.

Sandy soil, will not stick together because they are granular with no silty fine particles. These soils allow for good drainage and are ideal for building walls.

Organic soils will stick together but will not hold once the pressure is released. They should only be used to finish off the top 8 in. (200 mm) of a wall. NEVER use organic soils to build the wall.

Working with Soils

The soils used below and behind the wall are a critical part of the total wall structure. A reinforced retaining wall contains three basic building materials - the retaining wall blocks, the geogrid reinforcement, and the infill soils surrounding the geogrid layers.

Understanding the property and characteristics of soils is key to building better walls. Different soil types will dictate the amount of time needed for compaction, the amount of reinforcement required, and potentially the cost of the wall.

Granular soils are better to build with than clay soils. Sand and gravel will compact better, drain better, and often will need less reinforcement. Soils are typically defined by a friction angle or measurement of the internal strength of the soil. This angle is approximately the natural angle of repose. As soil falls off a conveyor to make a pile, the angle it creates represents the natural angle of repose. Check with a qualified geotechnical engineer to obtain an accurate soil classification.

Soil Selection

If the on-site soils are of a very low quality under or behind the wall, you should remove and replace them with stronger soils. Using stronger soils will reduce reinforcement, allow faster compaction and have better long-term performance. Heavy clays and organic soils are both unsuitable in the reinforced zone and should be removed and replaced. Silty sands and sand with clay will require additional care, and attention to water management when placed and compacted.

Compaction

Proper placement and compaction of the infill soils is critical.

The most important step in getting proper compaction is the placement of the soil in "lifts". Compacting in lifts, or layers, of less than 8 in. (200 mm) will facilitate quality compaction. Placement and compaction in lifts that exceed 8 in. (200 mm) will result in less than adequate soil strength. Compaction equipment must be sized according to the type of material being compacted. Always backfill and compact after each course of block is placed. Consult with a local equipment supplier to ensure that proper compaction equipment is used.

The consolidation zone runs from the back of the block back 3 ft. (0.9 m) into the infill soil. Only walk behind plate compaction equipment is allowed within the consolidation zone. A minimum of two passes with a walk behind plate compactor is required, starting on top of the block and compacting in paths that run parallel with the wall to the back of the excavated area.