February 12, 2016

What Soils are Best for Building Retaining Walls? Working the With On-site Soils

 The soils used behind and below a wall are a critical part of the total wall structure.  A reinforced retaining wall contains four basic building materials – blocks, wall rock, geogrid reinforcement and soil.  Understanding the properties and characteristics of soils is the key to building better walls.  Different soil types will compact differently and require different amounts of reinforcement.  Knowing the type of soil you are working with is important to building a quality retaining wall.

In general, granular soils are better to build with than clay soils.  Sand and gravel will compact better, drain better and often will require less reinforcement.  Sands and gravels also have better design properties and this can greatly impact the pressure a retaining wall will deal with.  Soils are typically defined by an internal strength characteristic known as a friction angle.  This angle is roughly equivalent to the natural angle a pile of this soil would make if it were an oven dry pile of individual particles.  Dry clay (with particles not clumped together) will form a much flatter pile naturally than gravel would.  This means clay would also apply more pressure to your retaining wall.

Clay Soil
Sandy Soil

If the on-site soils are of a very low quality, you should remove them and replace them with better soils.  Using stronger soils will reduce reinforcement requirements, allow faster compaction, and have better long-term performance.  Heavy clays and organic soils are both unsuitable for use in the geogrid reinforced zone and should be removed and replaced.  If you are unsure about what soils you are working with and you have concerns that they are unsuitable you can always check with a qualified geotechnical engineer to obtain an accurate soil classification.