Journal Articles
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Item A Critical Review of Construction, Analysis and Behaviour of Stone Columns(Kluwer Academic Publishers, 2013) Dheerendra Babu, M.R.; Nayak, S.; Shivashankar, R.Stone columns have been used as an effective technique for improving the engineering behaviour of soft clayey grounds and loose silt deposits. The soil improvement via stone columns are achieved from accelerating the consolidation of weak soil due to shortened drainage path, increasing the load carrying capacity and/or settlement reduction due to inclusion of stronger granular material. This paper discusses the techniques, methods of construction of stone columns, mechanisms of stone column behaviour under load and associated design philosophies along with some practical findings from recent research programs. © 2012 Springer Science+Business Media B.V.Item Effect of soil parameters on dynamic cone penetration indices of laterite sub-grade soils from India(2009) George, V.; Nageshwar Rao, Ch.; Shivashankar, R.The focus of this study was on correlating the effect of grain-size, maximum dry-density (MDD), field moisture content, and the void ratios on penetration measured using the dynamic cone penetrometer (DCP) for laterite soils blended with fines. Tests were performed on soil samples compacted to MDD for moulding water contents set to the optimum moisture content (OMC), dry of OMC, and wet of OMC un-soaked condition. The results indicated that an increase in the fines-content caused a decrease in the MDD, and an increase in the OMC and the DCP penetration. Regressions were developed correlating various parameters. © Springer Science+Business Media B.V. 2008.Item PFWD, DCP and CBR correlations for evaluation of lateritic subgrades(2009) George, V.; Rao, N.C.; Shivashankar, R.The evaluation of subgrade strength plays a major role in pavement design. An understanding of subgrade properties enables the selection of road materials for sub-base and base courses. In developing countries like India, apart from the use of well-established traditional pavement evaluation techniques such as the California bearing ratio (CBR) test and the dynamic cone penetrometer (DCP) test, the use of non-destructive testing devices such as the portable falling weight deflectometers (PFWDs) has gained popularity in recent years. This is mainly because of the inherent capability of PFWDs in obtaining quick estimates of the modulus of subgrade in addition to their simplicity in design and portability. Thus, there exists a need to correlate the results obtained using PFWDs with those obtained using traditional approaches such as the CBR and the DCP for the benefit of road engineers. This work focuses on exploring the correlations between these approaches for lateritic soils in Dakshina Kannada district, India.Item Stone Columns with Vertical Circumferential Nails: Laboratory Model Study(2010) Shivashankar, R.; Dheerendra Babu, M.R.D.; Nayak, S.; Manjunath, R.This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns. The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying capacity substantially. © 2010 Springer Science+Business Media B.V.Item Performance of stone columns with circumferential nails(2011) Nayak, S.; Shivashankar, R.; Dheerendra Babu, M.R.D.Stone columns are often used as an effective technique for improving the performance of soft ground. Stone columns derive their load-carrying capacity due to lateral confinement from the surrounding soil. Very soft soils offer very low lateral confinement, leading to large settlements and low load-carrying capacities. In this paper, an alternative method of enhancing the performance of stone columns in soft soils by reinforcing the stone columns with circumferential nails driven vertically is suggested. The method was developed in laboratory-scale model tests and a series of plate load tests were performed in unit cell tanks to investigate the performance of stone columns reinforced with circumferential nails. The investigation was carried out by varying the depth of nails below ground level, the number of nails and the diameter of nails with different diameter stone columns and area ratios (orspacing). It was found that the circumferentially reinforced stone columns have much higher load-carrying capacity with a significant reduction in settlement and less lateral bulging in comparison with plain stone columns.Item Experimental Studies on Behaviour of Stone Columns in Layered Soils(2011) Shivashankar, R.; Dheerendra Babu, M.R.D.; Nayak, S.; Rajathkumar, V.Stone columns are found to be effective and economical ground improvement technique in soft grounds. Understanding its behaviour when they are installed in stratified soils, in particular when the upper layer consists of weak soil, will be of great practical significance. This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the behaviour of stone columns in layered soils, consisting of weak soft clay overlying a relatively stronger silty soil, for various thicknesses of the top layer. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. Laboratory tests were carried out on a column of 90 mm diameter surrounded by layered soil, for an area ratio of 15%. It is found that the depth of top weak layer thickness has a significant influence on the stiffness, load bearing capacity and bulging behavior of stone columns. © 2011 Springer Science+Business Media B.V.Item Effects of soil parameters and moisture content on stiffness of unsaturated blended laterite using tri-axial tests for rural roads(2011) George, V.; Nageshwar Rao, C.; Shivashankar, R.Evaluation of subgrade strength and stiffness is essential in the design of pavements and construction of embankments for highways in order to ensure that roads perform their functions satisfactorily over a longer period. The grain size distribution and compaction moisture-content have an immense influence on the strength and stiffness of subgrades. Among the various laboratory tests for evaluation of shear strength parameters, and the modulus of elasticity of subgrades, the tri-axial test plays a major role in describing the geotechnical characteristics of the subgrade. This paper deals with an experimental investigation on unsaturated laterite soils blended with varying percentages of fines. It focuses on a study of the influence of various soil parameters on the stiffness and strength of soil samples compacted at optimum moisture content (OMC), at the drier side of OMC, and also at the wetter side of OMC, and the development of meaningful correlations. © 2011 CAFET-INNOVA TECHNICAL SOCIETY.Item Some studies of strip footing on granular bed overlying weak soil(2011) Srinath Shetty, K.; Shivashankar, R.; Shailesh, K.An increasing proportion of building development takes place on poor and difficult ground which presents the Geotechnical Engineer with the challenge of providing satisfactory foundation performance. Ground behaviour can be modified by ground treatment / improvement so that ground properties are improved. Ground improvement can be defined as the controlled alteration of the state, nature or mass behaviour of ground materials in order to achieve an intended satisfactory response to existing or projected environmental and engineering actions. Improving the load carrying capacity of a soft soil by providing a blanket of sand/gravel on it is a proven technique. A compacted granular bed laid over soft soil improves the bearing capacity and settlement response of footing placed over latter. Thus this technique, apart from being widely used beneath embankments, is also used to improve the bearing capacity of foundation soils for light to moderately loaded structures. This paper presents a study of load transfer mechanism and bearing capacity behaviour of a strip footing on granular bed overlying weak shedi soil. The experimental investigations were conducted in laboratory to study the behaviour of model strip footing of various widths on granular bed of varying thickness overlying weak soft soil at various water contents and densities. It is concluded that shedi soil behaves as a weak soil from shear strength tests conducted in laboratory and also from model load tests. Only for the case of model footing resting directly on soil, punching shear failure is most likely to occur. Providing a layer of granular bed (quarry dust) over the soft soil increases its bearing capacity by about 30 to 50%. As the thickness of granular bed increases, the influence width due to loading at soil-granular bed interface also increases as observed from the settlement profile. © Research India Publications.Item Behaviour of tunnel and surrounding strata using 3Dec numerical modelling: Case study of an U/G metro rail project(CAFET INNOVA Technical Society cafetinnova@gmail.com 1-2-18/103, Mohini Mansion, Gagan Mahal Road, Domalguda, Hyderabad 500029, 2012) Rebello, R.; Sastry, V.R.; Shivashankar, R.; Laksmi, S.Tunnel serves various purposes including transportation. A case study of an underground metro rail project involving considerable length of the tunnel was taken up for this study. This paper presents the analysis carried out for a stretch of 350m long tunnel being constructed for an underground metro. Modeling was carried out at seven different cross sections of tunnel, using 3DEC software, incorporating the strata characteristics. Study was carried out for the stability of tunnel with and without support. Observations were made about the behaviour of tunnel under given conditions for Vertical Stress, Vertical Displacement, at points like crown of tunnel and at the surface. Minute displacement of 0.05mm was observed particularly at the chainage where the tunnel is completely passing strong rock at Chainage 9345m. Tunnel is passing through Moderately Weathered Rock at Chainage 9295m, and the analysis resulted in a displacement of 4mm without support which has reduced to 0.5mm after placing the lining. Higher displacements were observed at chainages where the tunnel is passing through Silty sand, Highly Weathered Rock and Sandy Silt. Displacement at the tunnel crown increased to 18.6mm where the tunnel is passing through Silty sand (Chainage 9495m). © 2012 CAFET-INNOVA TECHNICAL SOCIETY.Item Seismic response analysis of reinforced concrete frames including soil flexibility(Techno-Press, 2013) Jayalekshmi, B.R.; Deepthi Poojary, V.G.; Venkataramana, K.; Shivashankar, R.The seismic response of RC space frame structures with isolated footing resting on a shallow soil stratum on rock is presented in this paper. Homogeneous soil stratum of different stiffness in the very soft to stiff range is considered. Soil, footing and super structure are considered to be the parts of an integral system. A finite element model of the integrated system is developed and subjected to scaled acceleration time histories recorded during two different real earthquakes. Dynamic analysis is performed using mode superposition method of transient analysis. A parametric study is conducted to investigate the effect of flexibility of soil in the dynamic behaviour of low-rise building frames. The time histories and Fourier spectra of roof displacement, base shear and structural response quantities of the space frame on compliant base are presented and compared with the fixed base condition. Results indicate that the incorporation of soil flexibility is required for the realistic estimate of structural seismic response especially for single storey structures resting on very soft soil. Copyright © 2013 Techno-Press, Ltd.