The Constructions of Deduction

Many of the important hydraulic structures, such as weirs and barrage, were designed on the basis of Bligh’s theory between the periods 1910 to 1925. In 1926 – 27, the upper Chenab canal siphons, designed on Bligh’s theory, started posing undermining troubles. Investigations started, which ultimately lead to Khosla’s theory . The main principles of this theory are summarized below:          (a) The seepage water does not creep along the bottom contour of pucca flood as started by Bligh, but on the other hand, this water moves along a set of stream-lines. This steady seepage in a vertical plane for a homogeneous soil can be expressed by Laplacian equation: Where, φ = Flow potential = Kh; K = the co-efficient of permeability of soil as defined by Darcy’s law, and h is the residual head at any point within the soil. The above equation represents two sets of curves intersecting each other orthogonally. The resultant flow diagram showing both of the curves is called a

Bligh, in his theory, had calculated the length of the creep, by simply adding the horizontal creep length and the vertical creep length, thereby making no distinction between the two creeps. However, Lane, on the basis of his analysis carried out on about 200 dams all over the world, stipulated that the horizontal creep is less effective in reducing uplift (or in causing loss of head) than the vertical creep. He, therefore, suggested a weightage factor of 1/3 for the horizontal creep, as against 1.0 for the vertical creep. Thus in Fig–1,  the total Lane’s creep length (L l ) is given by L l = (d 1 + d 1 ) + (1/3) L 1 + (d 2 + d 2 ) + (1/3) L 2 + (d 3 + d 3 ) = (1/3) (L 1 + L 2 ) + 2(d 1 + d 2 + d 3 ) = (1/3) b + 2(d 1 + d 2 + d 3 ) To ensure safety against piping, according to this theory, the creep length Ll must no be less than C1H L , where H L is the head causing flow, and C 1 is Lane’s creep coefficient given in table –2 Table – 2: V

According to Bligh’s Theory , the percolating water follows the outline of the base of the foundation of the hydraulic structure. In other words, water creeps along the bottom contour of the structure. The length of the path thus traversed by water is called the length of the creep. Further, it is assumed in this theory, that the loss of head is proportional to the length of the creep. If HL is the total head loss between the upstream and the downstream, and L is the length of creep, then the loss of head per unit of creep length (i.e. H L /L) is called the hydraulic gradient. Further, Bligh makes no distinction between horizontal and vertical creep. Consider a section a shown in Fig above. Let H L be the difference of water levels between upstream and downstream ends. Water will seep along the bottom contour as shown by arrows. It starts percolating at A and emerges at B. The total length of creep is given by L = d 1 + d 1 + L 1 + d 2 + d 2 + L 2 + d 3 + d 3 = (L 1 +

1. Failure due to Subsurface Flow:-      (a) Failure by Piping or undermining                                                               The water from the upstream side continuously percolates through the bottom of the foundation and emerges at the downstream end of the weir or barrage floor. The force of percolating water removes the soil particles by scouring at the point of emergence. As the process of removal of soil particles goes on continuously, a depression is formed which extends backwards towards the upstream through the bottom of the foundation. A hollow pipe like formation thus develops under the foundation due to which the weir or barrage may fail by subsiding. This phenomenon is known as failure by piping or undermining. (b) Failure by Direct uplift                                          The percolating water exerts an upward pressure on the foundation of the weir or barrage. If this uplift pressure is not counterbalanced by the self weight of the structure,

Define Gauge in Railway Track:-                                                    The Gauge of a railway track is defined as the clear distance between the inner or running faces of two track rails. The distance between the inner faces of a pair of wheels is called the ‘ wheel gauge ’. Different Gauges in India & Abroad :-                                                                                       In 18 th century, the British Railway were using the flanges on the outside of rails and the gauge was defined as the distance between the outer faces of the rails. The gauge then maintained was 5’ (1.524 m). Subsequently, the adoption of flanges inside the wheel on rails changes the definition of gauge. The position of rails of track was not changed in view of economy and clear distance between inner faces was defined by gauge. So Present gauge = Past gauge – 2 × rail width at top.                                  = 1.435 m. A gauge of 1.435 m, is the

Head Work:-                              Any hydraulic structure which supplies water to the off taking canal known as headwork . Headwork divided into two parts:- Storage Headwork :-                                            Dam is constructed across a river valley to form storage reservoir, known as storage head works. Water is supplied to the canal form this reservoir through canal regulator . The storage headwork is generally used to store the water.   Diversion Headwork :-                                             The works, which are constructed at the head of the canal, in order to divert the river water towards the canal, so as to ensure a regulated continuous supply of silt-free water with a certain minimum head into the canal, are known as Diversion Head Works. Objectives of Diversion Head Works :- To rise the water level at head of the canal. To form a storage by constructing dykes (embankments) on both  the bank of riv