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The world's second
largest retaining wall test facility was built at the University of Maine.
The structure, shown below in figure 1, is 16 feet tall with a plan area of 15
ft by 15 ft, and can be loaded with a concrete block surcharge of 750 psf. |
Figure 1. Cross section through University of Maine retaining wall test
facility (Tweedie, et al., 1998a)
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Tire shreds from
three different producers were tested using the facility. Two of the shred
types were 3-in maximum size and still contained steel belts. The third
tire shred sample was 1.5-in maximum size and most of the steel belts had been
removed. The tire shreds were placed in 8-in lifts and compacted with a
2300-lb walk-behind roller, and the 750 psf surcharge was applied.
At rest conditions
were measured for the compacted tire shreds. Figure 2 gives the earth
pressures for the tire shred samples, as well as for typical granular
fill. Earth pressure exerted at the base of the retaining wall was much
less for tire shreds than for granular fill.
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Figure 2. Measured horizontal stress distribution as determined from load
cell measurements (Tweedie, et al., 1998a)
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Coefficients of
lateral earth pressure are given in table 1. The value of the coefficient
decreases with increasing depth for all surcharge loadings on each type of tire
shreds.
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Table 1. Coefficient of lateral earth pressure at rest (Tweedie, et al.,
1998a)
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Surcharge
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Supplier
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Depth (m)
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0
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12.0 kPa
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23.9 kPa
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35.9 kPa
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Pine State Recycling
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0
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0.93a
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0.55
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0.46
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0.47
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2
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0.37
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0.32
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0.32
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0.32
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|
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4
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0.28
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0.26
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0.26
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0.25
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Palmer Shredding
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0
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0.94a
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0.58
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0.51
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0.51
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2
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0.37
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0.33
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0.27
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0.33
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4
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0.29
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0.27
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0.17
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0.24
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F&B Enterprises
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0
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0.99a
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0.51
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0.44
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0.45
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2
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0.39
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0.33
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0.32
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0.32
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4
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0.31
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0.28
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0.26
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0.25
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Average
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0
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0.95
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0.55
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0.47b
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4
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0.29
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0.27
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0.24b
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1 m = 3.28 ft, 1 kPa = 20.89 psf
a Value Found at a Depth of 0.5 m (1.64 ft)
b Average from 12.0 kPa (250 psf) and 35.9 kPa (750 psf) Surcharges
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For active
conditions, the wall of the test facility was rotated outward about its
base. Rotations of up to 0.4H were used, but pressures were still dropping
at this point, so true active pressures were not achieved. Rotations
greater than 0.01H (0.8 degrees) are not usually tolerable, so this rotation value is
used for discussion. Figure 3 gives the measured horizontal stress at
0.01H and a surcharge of 35.9 kPa (750 psf). Typical results for granular
fill are also included on the graph as a comparison.
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Figure 3. Comparison of at active earth pressures for tire shreds and
conventional soil backfill with a 750 psf surcharge (Tweedie et al., 1988a)
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The active earth
pressure coefficient at a rotation of 0.01H and a surcharge of 35.9 kPa (750 psf)
was approximately constant with depth and type of tire shreds. The values
ranged from 0.22 to 0.25.
Figure 4 gives a
plot of vertical force versus horizontal force on the front face of the
wall. This type of graph was used to determine the interface friction
between the tire shreds and the wall's concrete face. Calculated values
ranged from 30 to 32 degrees.
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Figure 4. Shear force versus horizontal force for Palmer Shredding tire
shreds; the failure envelope from direct shear tests on tire shreds is also
shown for comparison (Tweedie et al., 1998a)
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