Geoscience Reference
In-Depth Information
1 INTRODUCTION
The utilization of clayey soils as the backfill of a reinforced retaining wall is
of environmental and economic significance. A pioneer study on the behavior
of 5-m-high reinforced walls using volcanic ash clay (Kanto loam) as the
backfill has been performed in Japan since 1982 (Tatsuoka and Yamauchi,
1986). In this study, a nonwoven, low-stiffness geotextile was used as the
reinforcing material. A relatively low degree of compaction was obtained
because the compaction was conducted under natural water content
(v
100%). Large deformations were measured for the reinforced clay
walls during the long-term monitoring. In this study, Tatsuoka and Yamauchi
(1986) found that the nonwoven geotextile may effect degree of compaction
and provide drainage function during the rainfall. In the subsequent studies
on the reinforced clay wall for railway test embankment (Tatsuoka et al.,
1992), relatively stiff geogrid and geosynthetic composite for soil
reinforcement were used. In addition, layers of sand filter and cast-in-place
rigid RC facing (namely, RRR method) were used to provide drainage and
lateral confinement of the soil mass. Consequently, the deformation of the
reinforced wall under intensive rainfalls was significantly reduced. Wu (1992)
reported a 3-m-high timber-faced steep wall using a clayey sand (classified as
SC based on USCS) reinforced with a heat-bonded nonwoven geotextile. In this
study, a clayey soil was air-dried, crushed, sieved through a No. 4 sieve
(4.76-mm opening), and mixed with silt and sand under carefully controlled
conditions. The soil was subsequently mixed with water to achieve a 2% wet
optimum water content and was cured in a constant moisture room. The
compaction effort was provided by a vibration plate compactor weighted
700 N under 76-mm soil lift to achieve 95% relative compaction of the
Standard Proctor test. A reinforced wall backfilled with medium-dense sand
(relative density
<
67%) was also built for comparison purposes. The result
of loading tests showed that the ultimate load of the clay wall was larger
than the sandy wall. However, a conclusion regarding the feasibility of using
clay as the backfill has not been reported. Itoh et al. (1994) also reported a
7.5-m-high steep reinforced wall using a high-plasticity clay (classified as CH
based on USCS, approximately on the A-line) formed by weathered
mudstone. The compaction was provided by a vibrating roller weighted 70 kN
under 0.25-m lift and 4 to 8 passes. In this study, the control of the clod size
of clay before compaction, the water content of soil during compaction, and
the relative compaction achieved were not reported. Large horizontal and
vertical deformations (
<
0.4m and 0.6m, respectively) of the wall face were
measured in the five months since completion. Inadequate compaction
conditions might account for the large deformation of the wall face. So far,
clayey soils have not yet been widely accepted for permanent soil structures
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