Contents preface (VII) introduction 1—37

9.3. SEEPAGE FORCE
All the canal structures constructed in India up to the 19th century were mainly designed on the basis of the experience and intuition of the designer. The canal structures, not founded on solid rock, have the problem of seepage. Henry Darcy (11) was the first to propose an experimental relationship [Eq. (4.1)] for flow of water through porous medium. In 1895, following the damage to the Khanki weir on Chenab river a test program was initiated (12) . These experiments proved the validity of Darcy’s law. The experiments and observations on the Narora weir confirmed that the piping effect and uplift pressure due to seepage endanger the stability of structure founded on a permeable foundation.
Bligh (13) gave a simple method to calculate uplift pressure below a masonry or concrete structure. He stated that the length of the path of flow had the same effectiveness in reducing uplift pressures irrespective of the direction of flow. The length of the flow path along the bottom profile of the structure, i.e. , length of the uppermost flow line, was termed creep length. Thus, Bligh made no distinction between horizontal creep and vertical creep. According to Bligh’s theory, the creep length (i.e., the length of flow path) L in an idealised weir profile (Fig. 9.18) is equal to 2d₁ + b₁ + 2d₂ + b₂ + 2d₃. Thus, the loss of head per unit creep length is H/L which is the average hydraulic gradient. Here, H is the seepage head. Bligh termed the average hydraulic gradient as the percolation coefficient C and assigned safe values to it for different types of soil. For coarse-grained soil, the value of C is taken as 1/12 while for sand mixed with boulder and gravel, and for loam soil, the value of C may vary from 1/5 to 1/9. For fine micaceous sand of north Indian rivers, the value of C is 1/15. According to Bligh, if the average hydraulic gradient is less than the assigned safe value of C, there will be no danger of piping.

SURFACE AND SUBSURFACE FLOW CONSIDERATIONS FOR DESIGN OF CANAL STRUCTURES 335

	6.0
	5.5
							2	/s
							.0 m
	5.0				q	=		12
								.0
	4.5						10
							.0
							9
	4.0						.0
							8
							.0
							7
(Metres)	3.5						.0
						6
						5.0
2
	E	3.0						4.5
							4.0
	2.5						3.5
							3.0
							2.5
	2.0						2.0
							1.8
							1.6
	1.5						1.4
							1.2
							1.0
	1.0
	0.5	0.5	1.0	1.5	2.0		2.5	3.0	3.5	4.0
	0

E (Metres)