Cyclone and Storm Surge - Iczmpwb.org

6.6
6.2.3. Construction Material
The embankments in the Sundarban are built without using any heavy earth moving equipment.
Transport from the borrow areas where the soils are excavated to the locations where the
embankment is built or repaired is executed by labourers carrying buckets on their heads.
Considering this system of transport and the expectation that the system will remain unchanged in
the coming decade, the embankments have to be constructed with material found close by; at least
the major part of their cross-section.
At some locations excellent Clays or Silty Clays have been found, but at the majority of the soils are
ranging from Fine Sandy Loams to Silty Clay Loams; materials with less resistance against wavedash
and scouring by current. Nevertheless these materials are suitable to be used for the construction
of the embankments provided that they are taken from locations where these soils are
sufficiently matured and properly consolidated. In practice however material to build or to repair the
embankment is taken from the top layers of the berm between the river and the embankment or from
the top soils of the adjacent rice fields, which cohesive qualities are strongly reduced by annually wet
ploughing and puddling. These soils have to be qualified as mud or dried mud.
Mud is a material with almost no cohesion and is less suitable to be used successfully for the
construction of sustainable embankments. (The difference in quality of dried mud and matured loams
can easily be demonstrated by putting a lump of each of the materials in separate buckets of water.
After a couple of hours, even without stirring, there is nothing left from the lump of mud and on the
bottom of the bucket is a layer of sediment. The lump of matured loam will show only small
deformation.)
The quality of an embankment largely depends on the proper choice of the material used for its
construction. The difference in costs per unit length between an earthen embankment and an
embankment of which the outer slope is protected by dry brick pitching is so large that the extra costs
required to build the unprotected earthen bank of proper material are mostly more than justified.
These better soils can often be found below the ploughing zone of the rice fields and in the
embankments (or their remains) stability of which is no longer secured. The soils of the berms need
to be examined carefully. Recent deposits should be avoided. If the soils of the outer berms are good
enough to be used as construction material, the quantities should be taken from the zone closest to
the edge of the river.