Note regarding certain characters in the Subsoil of Calcutta.

BY
SURGEON-MAJOR D. D. CUNNINGHAM, M.B.,

SPECIAL ASSISTANT TO THE SANITARY COMMISSIONER WITH THE GOVERNMENT OF INDIA.

      The scheme for the construction of a dock at Kidderpore, one of the
suburbs of Calcutta, has afforded special facilities for acquiring information,
both in regard to the nature of the subsoil and the extraordinarily incorrect
ideas which are commonly entertained regarding it. When the question of
the construction of the dock was still under consideration, it was authoritatively
announced that the subsoil in which it was intended to lay the foundations
consisted of "blue clay," that it contained no subsoil water, and that its im-
permeability almost equalled that of cast-iron, so that no accumulation of water
would ever occur in the excavations necessary for the construction of the
dock beyond that due to direct rainfall so long as means were taken to exclude
surface drainage. Having for many years had special occasion to consider the
nature of the subsoil in Calcutta, I entertained great doubts as to the correct-
ness of these statements, and accordingly tried a series of experiments with
the view of obtaining actual data in place of "speculative opinions," as the
advocates of the dock-scheme styled all views which tended to cast any doubt
on the impermeable nature of the soil. The present note contains an outline of
these experiments, the result of which was to show clearly that the so-called
"blue clay" is not clay at all, and that, in place of being impermeable, it
contains a very high proportion of air or water content according to the
conditions to which it is exposed.

      The materials experimented with consisted of samples of the "blue clay,"
one obtained from a member of the Committee which was appointed to enquire
into the sanitary aspects of the dock-scheme, the other procured by myself
from the bottom of the initial excavations for the dock-walls. Both were
precisely alike, consisting when first obtained of a slate-coloured, rather evil-
odoured, dense, moist substance, permeated in many places by channels partly or
wholly filled with soft brownish matter and corresponding with the sites at
one time occupied by the smaller roots of trees, which formerly grew on the
present subsoil, and whose remains occur in abundance in it. On keeping
the material, it rapidly parted with its moisture, becoming, as it did so, of a pale
grey colour and hard consistence. Specimens were submitted to analysis

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