North Abutment Fill
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The north abutment
of the Merrymeeting bridge in Topsham was located on soft marine clay. To
increase the factor of safety for the stability of the slope, something had to
be done to stabilize the clay or reduce load. The least expensive option
was to excavate some of the abutment slope and replace it with lightweight tire
shreds. Figure 1 shows a cross section of the filled area.
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Figure 1. Cross section through North Abutment tire shred fill
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To achieve the 14
ft layer of tire shreds that was desired for the project, the layer was
overbuilt by 1.5 feet to account for the compressibility of the material.
During placement of the overlying soil, the tire shred fill compressed 14.6 in
(370 mm). In the next 60 days, the fill compressed another 5.3 in (135
mm), and over the following year, another 0.6 in (15 mm) of time dependent
settlement occurred. The total settlement of 10.5 in (520 mm) was 13%
greater than the expected settlement of 18 in. This was due to the fact
that time dependent settlement was not accounted for. The final compressed
unit weight of the tire shreds was about 56 pcf, a value very close to the
predicted value of 57 pcf.
Lateral pressures
were measured at the completion of tire shred placement (10/3/96) and the
completion of soil cover and surcharge placement (10/9/96). Table 1 gives
measured lateral pressures for these dates, as well as for 10/31/96.
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Table 1. Summary of lateral pressures on North Abutment Wall
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PC1-1
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PC2-1
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PC1-2
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PC2-2
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PC1-3
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PC2-3
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Date
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Cell elev. =
6.70 m
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Cell elev. =
7.77m
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Cell elev. =
8.84 m
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10/3/962
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7.841
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7.41
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6.04
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7.27
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2.62
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1.41
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10/9/963
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17.04
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20.04
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19.61
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30.22
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17.05
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10.91
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10/31/96
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18.27
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21.05
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20.98
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32.84
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20.24
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12.31
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1Horizontal pressure in kPa. (kPa x 20.885 = psf)
2Date tire shred placement completed.
3Date soil cover and surcharge placement completed
Rigid Frame Bridge
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A three foot wide
strip of tire shreds was used as backfill against a rigid frame bridge in
Topsham. The tire shreds act as a compressible layer that allows movement
of the granular borrow, and the development of active earth pressures.
Figure 2 shows the placement of the tire shreds.
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Figure 2. Cross section of rigid frame bridge showing location of tire
shred zone (Humphrey, et al., 1998)
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Figure 3 shows the
placement of instruments that were installed to measure pressures, strain, and
temperature.
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Figure 3. Cross section showing location of instrumentation at station
1004+50 (Humphrey, et al., 1998)
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Figure 4 gives
horizontal stress values measured by pressure cells. One of the cells
(PC1-1) was placed against the granular borrow material. The data shows
that the pressure for the tire shreds was less than half the pressure for the
soil.
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Figure 4. Horizontal stress as measured by the pressure cells and
approximate fill elevation versus date (Humphrey, et al., 1998)
Figure 5. Comparison of pressures measured on September 1, 1996 (Humphrey,
et al., 1998)
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