Water normally reaches the
sub-surface due to capillary action, seepage from adjacent areas, or
percolation of rainwater. When water reaches the sub-surface, it immediately
affects the stability of the formation. The situation becomes worse
particularly if the soil is bad. A variety of problems arise due to the
proximity of the water to the subgrade. The nature of the problems faced and
the remedial measures for the same have been discussed in detail in Chapter 9.
The various sub-drainage systems
used under different conditions are briefly summarized here.
Provision of inverted filter Some
sub-drainage systems consist of a blanket of a non-cohesive graded
material, which acts as a capillary break (Fig. 9.3 of Previous few Page).
catch water drains The entrance of water into the subgrade can be checked
by constructing effective catch water drains that are duly paved.
Provision of sand piling Some
drainage systems are provided with an adequate number of sand piles of a
diameter of about 20 cm. This improves sub-surface drainage (Fig. 9.4 of
Previous few Page).
Drainage of water pockets by
perforated pipe Perforated pipes with a diameter of about
30 cm are provided at appropriate places to drain off the water pockets (Fig.
9.5 of Previous few Page).
Cement grouting Cement
can be used to grout the water pockets so as to ease the situation.
Drainage of water
pockets by puncturing holes In special situations where water is
trapped in ballast pockets because of an impervious layer that lies over a good
pervious layer, puncturing holes into the impervious layer can help drain away
the water (Fig. 9.6 of Previous few Page ).