© 2008 by Raymond Alexander Kukkee
Geogrid Reinforcing: With or without a Retaining Wall
How to Build a Geogrid -Reinforced Retaining Wall"What is that stuff you have there, snow fence?" Your curious neighbour asks, eying the big rolls of black mesh on the back of the truck. "Don't ask me" your wife says. She turns to you. "Don't tell me we're going to tear up the yard again?" "It's geogrid reinforcing web, we're going to build a geogrid reinforced retaining wall at the bottom of the hill." you reply. "It will make the yard stop sliding down hill and keep the new retaining wall from getting pushed out like the old one, you'll see. In fact, I might not even rebuild the old wall, we don't really need it with this stuff". "That ground has been slumping for years, your broken wall wasn't heavy enough!" the neighbour grins, "You think you can make it stop it sliding? It's not likely, I'll believe it when I see it." He leaves, shaking his head in amusement. " Never mind, he's a doubting Thomas. I believe you...how does it work?" The wife asks. She's a smart girl. She always asks the right questions. How DOES geogrid mesh work? How do you build a geogrid reinforced retaining wall, or better yet, not have to build a vertical wall at all? The key to geogrid reinforcing is a specially fabricated ground mesh, usually made of polyester, polyethylene or other cords coated with a tough, protective coating for durability and strength. It is incorporated into the construction of a retaining wall to make the uphill soil itself become an integral part of the reinforcing wall to help prevent downhill movement. Technically it is a simple process of mechanically stabilizing the earth. Geogrid has high tensile strength and the mesh, when incorporated into layers of compacted fill, imparts it's tensile strength to the compacted soil, similar to the way reinforcing steel does when placed in poured concrete structures. The mesh may be tied directly into the retaining wall structure itself. Always installed perpendicular to the slope to maximize the tensile strength, the mesh is held in place by the weight of the compacted fill, providing an anchor for the structural components of any retaining wall it is attached to. The MSE (mechanically stabilized earth) or geogrid method of construction is applicable in highly variable situations and must be carefully planned with the unique characteristics of the site in mind. Geogrid may be used in two distinctive ways: a) As a significant and extended deadmen' anchor in the construction of a masonry retaining wall, where the weight of the soil itself imparts weight, stability, and maintains the masonry wall in position, or b) Without any vertical wall structure at all. Incorporated directly into the sloped landscape, geogrid mesh makes the compacted soil and the landscape surface become it's own retaining wall, complete with grass, trees and bushes. No additional vertical or sub-vertical wall structure is required. 1. To incorporate geogrid into a masonry retaining wall, it is typically installed between courses of block or stone:
- Excavate the footing area for the wall to level and construct footings for the wall. In areas with plastic clay, severe frost, or wet conditions, a suitable reinforced concrete footing may be is essential for stability of any masonry structure. A layer of coarse crushed rock 12" thick may provide adequate support in the same conditions for dry block wall construction that may include ballast.
- Excavate and remove the topsoil on the slope above and stockpile it for later use.
- Remove 8" or more of subsoil, depending on how many layers of geogrid are to to be established. Individual geogrid layers should not be more than 8" thick. Stockpile the subsoil, and smooth the remaining grade to the desired slope
- Establish the first course of masonry blocks on the footing or crushed rock base.
- Place perforated weeping tile on the uphill side of the blocks with the holes facing down, and cover with crushed rock. Fill the blocks if necessary with crushed rock as per specifications. The geogrid mesh may be installed on top of the first row, or as in this example , on top of the second row.
- Establish the second row of blocks, alternating the joints as is standard practice.
- Backfill the second row level with free-draining granular material, such as gravel, or use crushed rock. On the uphill side there should be a foot of free-draining material horizontally adjacent to the block. Smooth the transition area from the granular to the subsoil grade and compact all material using a plate packer or equivalent.
- Roll out the geogrid mesh perpendicular to the wall, placing the mesh on top of the masonry course. Extend the mesh for the full length up the slope if possible, especially if the slope is steep. Overlap the grid sheets laterally 6" or to the manufacturer's specifications.
- Spread the subsoil layer on top of the mesh to a depth of 8" and compact the entire slope using the plate packer.
- Install the next masonry row on top of the geogrid mesh . Backfill the course with granular fill. Roll out another layer of mesh, again, incorporating it into the masonry wall.
- Complete the layer by covering it with 8" of fill and compacting it.
- For more extreme conditions and a higher wall, repeat the process , using several layers of mesh as necessary, compacting each layer of subsoil fill over the grid. Import additional fill or soil if required, correcting the final grade to the slope desired.
- Landscape the area, spread the topsoil , grade, smooth and establish grass or sod and bushes as soon as possible to minimize any surface erosion.
- Excavate the topsoil and stockpile it for future use.Remove the depth of subsoil required, allowing for 8" for each layer of mesh required.
- Smooth and establish the excavated grade.
- Excavate the topsoil from a "footing" area and install a layer of crushed rock for drainage. Install a 4 " perforated tile on the uphill side of the crushed rock "footing" and cover it with crushed rock.
- Start the roll of geogrid mesh on top of the layer of crushed rock, keeping in mind the correct orientation of the mesh specified by the manufacturer.
- Cover the end with a layer of crushed rock . Roll out the geogrid mesh perpendicular to the slope of the hill and cover and compact it with 8" of subsoil. Overlap as required by the manufacturer, usually 6-8".
- Apply the second layer of mesh, repeating the process, covering the lower end of the mesh with free-draining granular material and compacting each layer.
- Do subsequent layers as required, tapering the final layer at the bottom end to the original grade. Compact the final layer as required.
- Spread topsoil, rake smooth, roll, and proceed with landscaping. Early establishment of grass using seed or sod and the placement of bushes and trees is desirable to avoid unnecessary erosion of the surface.