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June 6-7, 2019

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TRENCH 2012 SLURRY WALLS & TRENCH STABILITY BACKGROUND

Slurry wallsThe 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.

Trench Pricing and free trial

You can order Trench software program (Single User licence) by clicking on the buy button. Your order will be processed as soon as possible. All orders are shipped with federal express at no additional charge. Trench incorporated German DIN standards to analyze the stability in a modern software.

Single User licence: $950.00
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