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SLURRY WALLS & TRENCH STABILITY BACKGROUND
The continuous diaphragm wall also known as slurry wall is a structure formed
and cast in a slurry trench (Xanthakos, 1994). Slurry walls are constructed by
excavating a panel of given dimensions under slurry, usually a bentonite mix.
Continuous slurry wall trenches are typically used for cut-off barrier walls.
Structural slurry walls (for deep excavations) are typically formed by joining
panels of finite length in an alternating sequence. Typical panel lengths range
from 10 ft to 25 ft while most known slurry wall panels have thicknesses from 2
ft to 5 ft. For most applications panel depths are in the order of 60 ft to 80
ft as dictated by the project needs but panels have been constructed to depths
in excess of 200 ft for special applications.
Very often the issue of adequate panel or trench stability during trenching
arises. The basic factors affecting panel stability are: a) in-situ soil
strengths and weights, b) hydrostatic slurry head, c) water table elevation, d)
soil arching, e) filter cake formation, f) duration of excavation, g) panel
length and f) construction surcharge conditions. Positive hydrostatic slurry
head alone is usually not sufficient to ensure stability, soil arching and
filter cake formation contribute significantly to increased panel stability.
Xanthakos (1994) and others (Ng and Yan, 1998) provide more information on soil
arching and filter cake formation.
Many US. technical manuals such as AREMA approach the matter by a two
dimensional wedge stability analysis method, which is applicable only for
panels of infinite length. Applying 2D analyses methods in panels of finite
length can yield very conservative designs. For such cases, 3D effects can be
included in order to obtain a more economical design. For example, German
standards require that a specific three dimensional wedge analysis be carried
out when panel stability is in question. The effect of side friction and
cohesion is very significant in panels that have limited width. The German DIN
code treats side friction based on at-rest soil pressures plus surcharge. Side
friction can not exceed a maximum value calculated at a depth equal to the
panel length. The German DIN code penalizes side cohesion significantly by
reducing c by 1/3.
Panel stability is affected mostly by the depth to the water table and by
increased construction surcharges. Cohesion is directly related to panel
stability as doubling the cohesion results in a double factor of safety. Soil
friction variation within normal limits (28° to 34°) does not have as
significant impact on panel stability as cohesion, water, and surcharge. For
typical panel lengths that are smaller than 30 ft, safety factors calculated
for three-dimensional wedges are considerably larger than those give by
two-dimensional wedges. Concluding, three dimensional panel stability analyses
result in larger factors of safety compared to simpler two-dimensional wedge
methods. Hence, the effects of side friction and cohesion are too important to
be ignored and should always be incorporated when panel stability calculations
are required.
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