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Working With Soils

Document imprimable.pdf
The soils used below and behind the wall are a critical part of the total wall structure.

A reinforced retaining wall is a structure containing three basic building materials - the block facing, the synthetic geogrid reinforcing materials, and the infill materials surrounding the geogrid layers.


Understanding the properties 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.
Check the on-site soils carefully before beginning, and get a written identification of the soil type. A soils report from a local engineer will be required before a design and/or permit is issued for most walls above 4 ft. (1.2 m). The table below provides a general classification of soils.

Soil Selection

If the on-site soils are of a very low quality, you should remove and replace them with better backfill material in the reinforced zone and the foundation area. The cost of removal will be offset by reduced reinforcement, faster compaction, and better long-term performance.

In the reinforced zone, the type of soil used will determine the amount of grid reinforcement needed. Heavy clays and organic soils are both unsuitable in the reinforced zone. Generally, any soil with a friction angle lower than 27° or a plasticity index (PI) of greater than 20 should be removed and replaced. Soils with friction angles between 27° and 31° will require additional care, and attention to water management when placed and compacted. This will include extra inspections by an on-site engineer.

You must use infill soils that meet or exceed those specified in the engineered specifications and drawings. Have the soils tested before placing and compacting.

Typical Friction Angle and
Soil Unit Weights Compacted to 95% Standard Proctor
Soil Type Soil Friction
Soil Unit
Crushed stone, gravel 34° + 110 - 135
Clean sands 32 - 34° 100 - 130
Silty sands/sandy silt 28 - 30° 110 - 125
Sandy clay 26 - 28° 100 - 120
Other soils Determined by testing
All soil friction angles and unit weights are provided as reference only and are subject to change based on geographic area and site conditions.

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