Analytical Chem Soil HW Solutions 42

Analytical Chem Soil HW Solutions 42 -...

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42 The flowrate q is calculated from q ( m 3 /s per metre length) = k.H.N F /N H where k, the permeability of the embankment = 10 -6 m/s; H, the overall head drop = 8 m ie the level of the top surface of the canal above datum, NOT 6 m which is the level of the base of the canal and a common mistake); N F , the number of flowtubes = 2 × 2 for symmetry = 4; and N H , the number of equipotential drops, = 4 Hence m s m m s m q / / 10 8 4 4 ) ( 8 ) / ( 10 3 6 6 × = × × = or q = 28.8 litre/hour per metre run If the drain became blocked, the top flowline would rise and emerge on the downstream face of the embankment leading to erosion and failure. . Flownets in anisotropic soils Q3.10 Figure 3.44 shows a true cross-section through a long cofferdam. It is proposed to dewater the cofferdam by lowering the water level inside it to the floor of the excavation. Investigate the suitability of this proposal by means of a carefully-sketched flownet on an
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Unformatted text preview: appropriately-transformed cross-section (horizontal scale factor α = √ k v /k h ). How might the stability of the base be ensured? [University of London 2nd year BEng (Civil Engineering) examination, Queen Mary and Westfield College] Q3.10 Solution The flownet must be sketched on a transformed section, with the horizontal distances reduced by a transformation factor α = √ (k v /k h ) to account for the relatively higher horizontal permeability (see main text Section 3.14). = (k v /k h ) = (2.5 10-5 ÷ 10-4 ) = (0.25) = 0.5 The cross section is re-drawn with the horizontal dimensions reduced by the transformation factor 0.5, and the flownet is sketched according to the rules and procedures set out in main text Sections 3.8 and 3.9, in Figure Q3.10....
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