The Dudhganga project, prasently under construction in Kolhapur district, Maharashtra, consists of a 75 m high and 445 m long masonry dam in the gorge with earthen flanks across Dudhganga river in the Krishna valley. This will create e reservoir of capacity 9.72 × 106 m3 to be utilised for irrigation of 53,520 ha of land on either banks. The dam site is located, in the Deccan basalt province of indian peninsule, near Asangaon on the eastern slops of the continental divide in a terrain representing dissected up lend with steep sided young valleys showing structural rock benches.
Presence of thin horizontal layers of weak pyritiferous shale, with near zero cohesion and ф values ranging between 20° and 28°, below the dam seat has resulted in modification of design of the structure. A shear key is provided within the foundation to ensure stability against sliding thereby increasing the cost of the project by 20 percent.
An intercalation of gray and pink quartzite with thin micaceous and pyritiferous shale, of the lower Kaladgi formation of Cuddapah age (upr. Prscambrian), occuring as inliers within the Deccan traps (upr. Cretaceous to lr. Tertiary), predominantly constitutes the foundation of the dam (fig 1). For a short stretch of 280 m on the extreme end of left flank the structure will rest on Deccan basalts which are exposed at a place 200 m upstream of the dam axis elso.
(Figure in full paper)
Subhorizontal quartzite interlayered with thin shele bands (5cm to few meters are well exposed in the narrow river gorge section, extending for considerable distance both upstream and downstream of dam axis. Drilling indicates that the traps on the left flank are composed of 37 m thick porphyritic basalts with an underlying tuffaceous zone. On the surface there is a veneer of red-brown lataritic soil with reduced A-horizon, minor humic and clay content and a thicker B-horizond of hydrous iron oxides. Quartzites exposed in the river section are fresh and hard but traversed by open vertical joints and horizontal bedding planes. This is underlain by quartzite horizon highly intercalated with shale partings (quartzite/shale ratio 3:1)few centimeters in thickness to bands as thick as 3 m.
(Figure in full paper)
Very high dissemination of specks and spherules of pyrites, some of which are reniform in shape, are present in these shales and quartzite bands. Spherules are formed of several fine concentric growths of pyrites resembling pisolitic texture. On oxidation these pyritiferous shales emit sulphurous odour and on alteration forms a greyish white fibrous and granular encrustation (Fig 3), This associated with large increase in volume causes easy crumpling and disintegration of the inherently weak shales present in the foundation.
The pyrite spherules present inside the rock body on a freshly broken surface are generally unaltered, on the other hand alteration of pyrites are common along the joint and bedding planes coming in contact with circulating water.
