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A new bridge
was constructed over the Maine Turnpike to provide better access to the Portland
Jetport. Approach embankments had to be built on the underlying soft clay
found in the area, and measures had to be taken to prevent slope failure.
The existing factor of safety was very low, and different alternatives were
considered to reduce the risk of failure. Tire shreds were chosen to be
used as a lightweight fill material in the embankments. This was the least
expensive alternative and didn't impact the surrounding wetlands as stabilizing
berms would have.
Two layers of tire shreds were
used. Each layer was completely surrounded by geotextiles, and the layers
were separated by fill material with low permeability. Guidelines specify
that a single layer of tire shred fill should be no more than 10 feet, so two
separate layers were necessary to achieve the desired fill thickness of up to 20
feet. The figure below shows the placement of the fill layers.
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Figure 1. Typical embankment section, Portland Jetport
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The tire shreds
that were used were large in size and fairly uniformly graded. This
resulted in a unit weight of about 49 pcf, which was lower than originally
expected. The tire shred fill experienced rapid compression during loading,
but didn't have much time-dependent settlement. After 5 months, settlement
varied from 5 to 21 inches in the upper layer of tire shreds, and from 5 to 8
inches in the lower layer. The foundation under the embankment settled 2
to 15 inches, accounting for much of the tire shreds' settlement as well.
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