If soil is a main component of an SRW structure, then it is necessary for wall designers to know and understand the properties of these soils. Soils come in a “near infinite” number of types and compositions. In commercial projects, SRW wall designers often learn about the properties of the soils on a project site from a Soil Boring Log. Knowledgeable civil or geotechnical engineers evaluate this information in order to predict a completed SRW’s performance. In the absence of detailed soils information, wall designers must make some assumptions about the soil properties in order to proceed. When assumptions about soils must be made, they are usually, and should be, conservative in order to preserve the necessary safety factors for wall integrity. There are some soils that should never be used in a SRW. A detailed discussion of all soil types and properties is beyond the scope of this manual. The determination of particular soil suitability for use in a SRW rightfully belongs within the realm of a trained and experienced civil or geotechnical engineer.
The soils that are of critical interest to a wall designer are categorized into five basic “zones” with respect to their location in and around the finished wall.
The leveling pad is not really a zone, per se, but is an integral and necessary part of a well-designed, well-built SRW. It consists of a material similar to that of road gravel that allows for drainage, but also contains enough fines to allow it to compact well and “hold its shape”. Class 5, ¾” minus, crush & run and road base are some of the regional names given to this type of material. The dimensions for the leveling pad vary and are discussed elsewhere in this manual.
The drainage zone is typically an imported, well-draining crushed rock material that fills the voids in and around the facing units to a minimum depth of one foot behind the back of the facing. This zone functions as a “French drain” to transport water otherwise trapped behind the finished wall to a drainage collection or dispersal area. This material should be relatively free of fine-grained materials and also should be “self-compacting”. This eliminates the need to operate compaction equipment in close proximity to the back of the wall facing.
The foundation soil zone comprises the area immediately beneath the facing components and drainage zone and is responsible for providing adequate support for the weight of the retained wall above. If the wall is a reinforced SRW, the foundation zone also extends beneath and behind the wall to a distance roughly equal to the depth of the embedded soil reinforcement.
The reinforced soil zone only exists in MSE walls and extends from the back of the drainage zone to an embedded depth equal to the back of the geosynthetic soil reinforcement. This soil may have its origin on-site or it can be a “select fill” material brought on-site from elsewhere. The properties of this material strongly influence the performance characteristics of the reinforced soil mass and, as such, have a significant effect on the strength, length and quantity of soil reinforcement in the finished wall. Ultimately, the design of a finished soil-reinforced wall is greatly affected by the material confined within this soil zone.
The retained soil zone is the material either behind the reinforced soil zone, in the case of a soil-reinforced SRW, or behind the drainage zone in a gravity retaining wall. Soil characteristics within this zone also have a significant effect on the design of the finished wall in the same way that the reinforced soil zone does.