Designing Balance Into Your Retaining Wall Project |
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Printable Document (.pdf) |
| Engineers have the responsibility of designing cost effective structures that
are safe and reliable. On the surface this task seems to be relatively straight
forward and one that can easily be quantified. The questions that must be answered
to achieve this design standard will determine how complicated this process will be.
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What forces will be applied to the structure? What materials will be used
to build the structure? Are there other elements that may affect the performance
of the structure? During the construction process what safeguards will be in place
to ensure that plans and specifications are followed? What will be required after
completion of the project for the continued safe reliable performance of the structure?
What has our experience told us about what can go wrong in real life?
These questions have led to a series of changes over the last fifteen years in
the design of segmental retaining walls. Allan Block has helped to drive the
industry to ensure cost effectiveness with safety and reliability. During
this time frame many things have evolved, and design refinements are producing
a better final product that suits the needs of our customers.
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From our field experience and full scale testing we have arrived at conclusions
that change how we approach designs. This does not imply that the structures
built over the last fifteen years are not safe, but rather we have determined
that with a few simple changes we can build safer yet still efficient retaining
wall structures. For design guidelines that should be implemented to provide for
a safer more reliable structure continue to the next page.
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Project Compaction, Geogrid Spacing, and Geogrid Length
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Compaction:
Geogrid-reinforced structures are designed to perform as a composite
structure. In order for them to perform in this manner, consistent
compaction is mandatory. Actual installations are plagued with improper
compaction due to soil lifts in excess of the maximum 8-inch (200 mm)
lifts. Tighter specifications should be used on compaction and field testing
requirements.
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Geogrid Spacing:
Compound failure planes may develop when the reinforced mass is constructed
with geogrids that are not spaced closely enough together. Allan Block
recommends geogrid spacing of sixteen inches or less using light weight
geogrid-reinforcement. This is a more efficient way to distribute the
reinforcement throughout the mass, which develops a more coherent structure.
Lower strength geogrid is typically less costly therefore the project budget
is usually not affected by the increased total amount of geogrid.
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Geogrid Length:
We have concluded that grid lengths between 50 and 60 percent of the wall
height will provide a safe and efficient structure, but for simplicity we
are recommending 60 percent as the typical grid length for a starting point.
The exception is the top layer of grid which should be extended to intertwine
the reinforced mass with the retained soil mass. This eliminates potential
for soil cracks at the intersection of these two masses. Extending the grid
by approximately 3-feet (1 m), or to 90 percent of the wall height in seismic zones,
should accomplish the goal of tying the reinforced mass with the retained mass.
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