TECHNICAL NOTE Consolidation Behavior of Organic Soil in Normal Kolkata Deposit Bh. Koti Reddy • Ramendu Bikas Sahu • Sudipta Ghosh Received: 3 April 2013 / Accepted: 9 August 2013 Ó Indian Geotechnical Society 2013 Abstract Organic soils which pose significant problems to design engineers with its poor compressibility associated with significantly different properties compared to inorganic soils exist in Normal Kolkata Deposit at various depths. In this paper an attempt has been made to investigate the pri- mary as well as secondary consolidation behavior of organic clay by conducting one-dimensional incremental loading tests. Cubical un-disturbed block samples of 300 mm size were collected from Jadavpur, Kolkata and Howrah maidan with in situ water content ranging from 60 to 90 % and organic content (OC) up to 28 %. Test results indicate that the rate and magnitude of settlements increase with the increase in OC. The compression index, C c , was found to be in the range of 0.62–0.65 for Jadavpur soil and 0.85–0.91 for Howrah soil with pre-consolidation pressures of 95–103 and 110–120 kPa respectively. The observed values of C a /C c are 0.038 and 0.051 for Jadavpur and Howrah organic soils respectively. Long duration consolidation tests which were conducted to investigate the effect of increase in loading duration on consolidation parameters revealed no significant change in the behavior. As a whole, the consolidation behavior of Jadavpur soil (OC \ 15 %) was found closely related to that of soft soils at lower end and for Howrah soil (OC 25–28 %) the same behavior was noticed at higher end. Keywords Normal Kolkata Deposit Á Block samples Á Organic soil Á Consolidation Á Incremental load tests Á Creep Introduction Kolkata is situated on the alluvial plains of southern Ben- gal, India formed by Ganga–Brahmaputra river system. The soil formation in this region mainly consists of two distinct soil deposits, Normal Kolkata Deposit in and around Kolkata and river channel deposit along the existing Adiganga channel. Dastidar and Ghosh [5] presented a general and broad view with respect to stratification and engineering properties of Normal Kolkata Deposit for a large number of sites in Kolkata region. The study found that there is a thin fibrous soft and highly compressible peat layer at mean sea level followed by a soft and highly compressible grey silty clay/clayey silt layer with timber pieces in decomposed or semi-decomposed state. Benarjee and Sen [3] studied the depositional history of Bengal basin and its impact on geotechnical properties. Their study also revealed occurrence of organic layers at different depths in the deposit. Generally first organic layer follows the top desiccated brown silty clay/clayey silt layer and extends from few centimeters to 1.5 m in the form of horizontal bands and often referred as peat [3, 5] though a clear and complete characterization has not been attempted so far. Abdallah et al. [1] studied the effect of organic matter on the structure, plasticity, compaction and shear strength characteristics for illitic soil and observed that these properties are greatly affected by the organic content (OC). Mesri and Ajlouni [16] in their state of art paper referred fibrous peats as problematic soils on account of its poor stiffness characteristics associated with significantly Bh. K. Reddy (&) Á R. B. Sahu Á S. Ghosh Department of Civil Engineering, Jadavpur University, Kolkata, India e-mail: kotireddy68@gmail.com R. B. Sahu e-mail: rbsahu_1963@yahoo.co.in S. Ghosh e-mail: sghosh56@yahoo.com 123 Indian Geotech J DOI 10.1007/s40098-013-0076-0