Environmental Engineering Reference
In-Depth Information
Figure 13.15.
Some causes of piping failure around conduits: (a) Inadequate compaction due to the
presence of cutoff collars; (b) Inadequate compaction under pipe; (c) Cracking in soil or
extremely weathered rock in the sides of a trench; (d) Cracks due to differential settle-
ment; (e) Use of corrugations or other roughening of the surface of cutoff collars or con-
crete surround.
(1972) and Charles (1997). This can occur on the sides of culverts which are con-
structed in a trench. Melvill (1997) describes measurements on several South
African dams. Knight (1990) measured very low horizontal stresses at Hinze dam
in Queensland, Australia, sufficient to give virtually zero effective stress. Sherard et al.
(1972b) point out that it can also occur where the concrete culvert, or concrete sur-
round around a pipe, has a sharp corner. In this case piping can be expected above
the culvert. Drying of the soil during construction can also cause cracks which
allow initiation of piping. Sherard et al. (1972b) point out that, if the rate of filling
is sufficiently slow, the soil swells, providing increased compressive stresses and
preventing piping initiation.
- Making compaction of soil difficult, particularly if collars are provided at close
intervals or the concrete is formed with corrugated steel sheet or other non-smooth
formwork, preventing compaction of the soil adjacent to the conduit.
Figure 13.15 shows some of these features. There is also some evidence that excavat-
ing a trench through an existing dam to install a conduit can lead to conditions con-
ducive to initiation of piping.
