Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 10 SEEPAGE
Factor of safety against piping = i crit
i
= 3
The above example assumed the permeability of the soil to be isotropic. Generally, the horizontal
(K h ) and vertical coefficients of permeability (K v ) of a soil differ, usually the former is greater than
the latter. In such instances, the method of drawing the flownet need to be modified. Use of a
transformed section is an easily applied method which accounts for the different rates of
permeability.
Vertical dimensions are selected in accord with the scale desired for the drawing. Horizontal
dimensions, however, are modified by multiplying all horizontal lengths by the factor √(k v /k h ). The
conventional flownet is then drawn on the transformed section. For flow through the anisotropic
soil, the seepage, q is
q=H w
H w
N f
N d
N f
N d
K v K h (10.1)
= head difference
= number of flow channels
= number of pressure drops
In addition to the flow net and weighted-creep methods of estimating the distribution of uplift
pressure are Khosla’s method of independent variables and Rao’s relaxation method which can be
used for making computations of uplift at critical points along the base of the structure. Because
these theories are highly mathematical they are not discussed in this text.
10.4 CONTROL OF SEEPAGE
Piping can occur any place in the system, but usaully it occurs where the flow is concentrated e.g.
at the downstream toe of the dam or at any place where seepage water exits. Once seepage forces
are large enough to move particles, piping and erosion can start, and usually continues until either
all the soils in the vicinity are carried away or the structure collapses. Cohesionless soils, especially
silty soils, are highly susceptible to piping
Uplift and seepage problems may be alleviated or controlled by several methods. Among which are:
a) Construction of cut-off wall or trench to completely block the seeping water
b) Installation of an impervious blanket e.g an apron to lengthen the drainage path so that
more of the head is lost and thus the hydraulic gradient in the critical region is reduced.
c) Installation of relief wells and other kinds of drains can be used to relief high uplift
pressures at the base of hydraulic structures
d) Installation of protective filter, which consists of one or more layers of free-draining
granular materials placed in less pervious foundation or base materials to prevent the
movement of soil particles that are susceptible to piping while at the same allowing the
seepage water ro escape with relatively little head loss. The requirements for a protective
filter are discussed in Item 10.5 below
10.5 PROTECTIVE FILTER REQUIREMENTS
In generaal, the four basic requirements of the protective filter layer for controlling the seepage
problems such as piping and uplift pressures are as follows:
March 2009 10-5