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

Chapter 10 SEEPAGE
10 SEEPAGE
10.1 SEEPAGE
When water flows through a porous medium such as soil, energy or head is lost through friction
similar to what happens in flow through pipes and open channels. For example, energy or head
losses occur when water seeps through an earth dam or under a sheet pile cofferdam (Figure 10.1
(a) and (b)). The flow through the soils also exert seepage forces on the individual soil grains,
which affect the intergranular or effective stresses in the soil masses. Seepage can create problems
especially in water control structures such as excessive seepage losses, uplift pressures and
potential detrimental piping and erosion.
This section discusses two of the many methods available which are simple and easy to use. They
are Lane’s weighted creep theory and flownets. Flownets, if properly constructed are more
accurate than the former and result in more realistic determinations of seepage pressure and piping
potential.
10.2 LANE’S WEIGHTED CREEP THEORY
Lane’s theory may be used for designing low concrete hydraulic structures on pervious foundations.
The concept is based on the following principles:-
a) The weighted-creep distance of a cross section of a hydraulic structure is the sum of the
vertical creep distances (steeper than 45°) plus one-third of the horizontal creep distances
(less than 45°).
b) The weighted-creep head ratio is the weighted-creep distance divided by the effective head.
c) Reverse filter drains, weep holes, and pipe drains are aids to security from underseepage,
and recommended safe weighted-creep head ratios may be reduced as much as 10 percent
if they are used.
d) Care must be exercised to ensure that cutoffs are properly tied in at the ends so that the
water will not outflank them.
e) The upward pressure to be used in the design may be estimated by assuming that the drop
in pressure from headwater to tailwater along the contact line of the hydraulic struicture and
the foundation is proportional to the weighted-creep distance.
The Lane’s weighted-creep ratios are as shown in Table 10.1.
Table 10.1 Lane’s Weighted-Creep Ratios
Materials
Ratio
Very fiine sand or silt 8.5
Fine sand 7.0
Medium sand 6.0
Coarse sand 5.0
Fine gravel 4.0
Medium gravel 3.5
Coarse gravel including cobbles 3.0
Boulders with some gravels and conbbles 2.5
Soft clay 3.0
Medium clay 2.0
Hard clay 1.8
Very hard clay or hardpan 1.6
March 2009 10-1