Masonry Magazine July 2002 Page. 20
Cover Story
Drainage wale (optional)
Cap unit (optional)
SRW units
Setback batter
Finished grade
Leveling pad
Slope for positive drainage
Low permeability soil
Geosynthetic reinforcement
Retained Soil Zone
Limit of excavation
Compacted common backfill
Drainage fill
Compacted reinforced (infill) soil zone
Drainage collection pipe
thereby increasing the stability of the wall. These materials are usually high-tensile strength sheet material that comes in rolls of varying widths and can be geogrids or geotextiles in form. However, most in use today are geogrids.
The graphic illustrates how a soil-reinforced wall is engineered with the geosynthetic material. The geosynthetic material is placed between the units along a course and extended back into the retained soil to create a composite gravity mass structure. The weight of the block, various pins and indentations, or a combination of these features retains the geosynthetic material to the wall structure. By integrating the soil with the weight of the blocks themselves, the tensions between wall and soil are used to the advantage of the builder, instead of the wall fighting against the pressure of the soil.
The mechanical wall system comprised of the SRW units and reinforced soil mass offers the required resistance to external forces associated with taller walls, surcharged structures, or difficult soil conditions. In fact, these systems are often referred to as MSE-mechanically stabilized earth-walls.
Designs and definitions
SOIL-REINFORCED SRWS are engineered to control external stability by adjusting the geosynthetic material length-the distance it projects into the retained soil. Increasing the length increases the resistance to overturning, base sliding, and bearing failures. Sometimes the length of the uppermost layer is locally extended in order to provide adequate anchorage or pullout capacity for all the geosynthetic layers. Quality and strength of the material, along with its interaction with the soil, may affect the length of the material specified.
Typically, the spacing of the material (vertically) decreases with depth below the top of the wall because earth pressures increase linearly with depth. The spacing should be limited to prevent bulging of the wall face between geosynthetic connection points and to prevent exceeding the shear capacity between SRW units.
Image courtesy of National Concrete Masonry Association
Quality and strength of the material, along with its interaction with the soil, may affect the length of the material specified.
Although the engineer or architect will normally do the calculations and incorporate them in the specifications, it's a good idea for the mason contractor to understand the concepts and design factors of these walls. As the wall goes up, the contractor will be involved in inspections and will handle the adjustments that are necessary to ensure the stability and durability of the end product. Soil conditions can also change as the site is cleared or the depth plumbed, meaning onsite decisions will need to be made to correct for abnormalities.
Drainage is essential. Normally, drainage is provided by well-graded aggregates between the wall and the retained soil. A properly designed drainage system relieves hydrostatic pressure in the soil, prevents the soil from washing through the face of the wall, provides a stiff leveling pad to support a column of stacked facing units, and provides a working surface during the construction phase. Surface water drainage should be designed to minimize erosion of the topsoil in front of the wall toe and to direct surface water away from the structure.
Wall embedment is the depth of the wall face that is below grade. The primary benefit of wall embedment is to ensure the SRW is not undermined by erosion of the soil in front of the wall. Increasing the depth of embedment provides greater stability when site conditions include weak bearing capacity of the underlying soils, steep slopes near the toe of the wall, potential scour at the toe-particularly in waterfront or submerged applications-and seasonal soil volume changes or seismic loads.
Vertical surcharge loadings are often imposed behind the top of the wall in addition to the load due to the retained soil. These loads increase lateral pressure on the structure and can be caused by a sloped backfill; a uniform surcharge due to buildings and/or parking lots; or line or point loads from heavy footings or continuous footings close to the wall.
According to Carter, "It's highly recommended that the backfill be placed in thin lifts of 6 to 8 inches. If you look from the back of the block toward the fill, the first three feet should be compacted with a lightweight, walk behind compactor. One
18 Masonry
July 2002
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