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Educational Basic Civil Engineering

Discussion in 'Education' started by Tazul Islam, Jun 15, 2016. Replies: 62 | Views: 3808

  1. Tazul Islam
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    Tazul Islam Kazirhut Lover Member

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    [​IMG]

    ii) A straight horizontal floor depressed below the bed, but without any vertical cut-off.

    1. A straight horizontal floor of negligible thickness with a sheet pile at some
    Intermediate point
    The mathematical solution of the flow-nets of the above profiles have been given in the form of curves. From the curves, percentage pressures at various key points E, C, E1, C1 etc) be determined. The important points to note are:

    1. Junctions of pile with the floor on either side{E, C (bottom), E1, C1 (top) }
    2. Bottom point of the pile (D), and
    3. Junction of the bottom corners (D, D’) in case of depressed floor
    The percentage pressures at the key points of a simple forms will become valid for any complex profile, provided the following corrections are effected:

    [​IMG]

    • correction for mutual interference of piles
    • correction for the thickness of floor
    • correction for slope of the floor.
    Correction for Mutual Interference of Piles
    Let b1 = distance between the two piles 1 and 2, and

    D = the depth of the pile line (2), the influence of which on the neighbouring pile (1) of depth d must be determined

    b = total length of the impervious floor

    c = correction due to interference.

    The correction is applied as a percentage of the head

    [​IMG]

    This correction is positive when the point is considered to be at the rear of the interfering pile and negative for points considered in the forward or flow direction with the interfering pile.
     
  2. Tazul Islam
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    Tazul Islam Kazirhut Lover Member

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    Correction for Floor Thickness
    Standard profiles assuming the floors as having negligible thickness. Hence the values of the percentage pressures computed from the curves corresponds to the top levels (E1*, C1*) of the floor. However, the junction points of the floor and pile are at the bottom of the floor (E1, C1)

    [​IMG]

    The pressures at the actual points E1 and C1 are interpolated by assuming a straight line variation in pressures from the points E1* to D1 and from D1 to C1

    [​IMG]

    The corrected pressures at E1 should be less than the computed pressure t E1*. Therefore the correction for the pressure at E1 will be negative. And so also is for pressure at C1.

    Correction for Slope of Floor
    A correction for a sloping impervious floor is positive for the down slope in the flow direction and negative for the up slope in the direction of flow.

    No.

    Slope = Ver:Horiz


    The correction factor must be multiplied by the horizontal length of the slope and divided by the distance between the two poles between which the sloping floor exists.

    In the diagram above, correction for slope can be applied only to point E2. As the point E2 is terminating at the descending slope in the direction of flow, the correction will be positive. The value of correction will be:
    C.F. x bs/b1

    Where C.F. =correction factor

    bs = horizontal length of sloping floor

    b1 = horizontal distance between the pile lines
     
  3. Tazul Islam
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    Exit & Critical Gradient
    [​IMG]

    Every particle of water while seeping through the sub-soil, at any position will exert a force f, which will be tangential to the streamline at any point. As the streamlines bend upward, the tangential force f will be having a vertical component f1. Also at that point, there will be a downward force W due to the submerged weight of the soil particle. Thus at that point there will be two forces on the particle; one upward vertical component of f, and the other, the submerged weight. It is evident that if the soil particle is not to be dislodged, then the submerged weight must be greater than the upward vertical component of f. The upward vertical component force at any point is proportional to the water pressure gradient dp/dx.

    Hence for stability of the soil and for the prevention of erosion and piping, the seeping water when it emerges at the downstream side, at the exit position, the force f1 should be less than the submerged weight W. In other words the exit gradient at the downstream end must be safe.

    If at the exit point at the downstream side, the exit gradient is such that the force f1is just equal to the submerged weight of the soil particle, then that gradient is called critical gradient. Safe exit gradients = 0.2 to 0.25 of the critical exit gradient.

    Values of safe exit gradient may be taken as:

    • 0.14 to 0.17 for fine sand
    • 0.17 to 0.20 for coarse sand
    • 0.20 to 0.25 for shingle
    For the standard form consisting of a floor of a length b, and a vertical cut-off of depth d, the exit gradient at its downstream end is given by:

    Exit gradient GE = (H/d) x [​IMG]
     

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