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Item Slope stability studies of excavated slopes in lateritic formations(Springer, 2019) Shivashankar, R.; Thomas, B.C.; Krishnanunni, K.T.; Venkat Reddy, D.V.The study area for this paper is coastal Karnataka in India, which has laterites and lateritic soils. The soil stratification in this area mainly consists of lithomargic clay, which is a product of laterization, sandwiched between the hard and porous weathered laterite crust at the top and the hard granite or granitic gneiss underneath. This lithomargic clay, locally called as ‘shedi soil’ behaves as dispersive soil and is also highly erosive. In the first stage of this study, laboratory erosion studies are conducted by using the hole erosion test apparatus on controlled shedi soil samples. Erosion observed in the HET is accelerated due to slaking irrespective of dispersive nature of the soil. Erosion problems were also dealt with using a stabilizer, calcium lignosulfonate and resulted in high increase in its erosion resistance. In the second stage of this study, slope stability studies of excavated slopes in lateritic formations are conducted considering intensity of rainfall, ponding and seepage, apart from the usual geotechnical parameters. The slopes steeper than 60° are not stable in the case of shedi soil considered here. © Springer Nature Singapore Pte Ltd 2019.Item A Study on the Seismic Behaviour of Embankments with Pile Supports and Basal Geogrid(Springer, 2020) Patel, R.M.; Jayalekshmi, B.R.; Shivashankar, R.For constructing the roads on soft grounds, basal geogrid-reinforced pile-supported embankments are a suitable solution over other conventional ground improvement techniques like preloading, embankment slope flattening, removing and replacing the soft soil, etc. Many studies are available on these basal geogrid-reinforced piled embankments to understand their behaviour under static loading conditions. But it is necessary to understand the behaviour of these geogrid-reinforced piled embankments under seismic excitations. Hence, finite element analysis of three-dimensional models of embankment having crest width of 20 m, height above ground of 6 m, with side slopes of 1V:1.5H consisting of pulverized fuel ash, overlying soft marine clay of 28 m thickness is carried out under seismic excitations corresponding to Zone III (IS:1893). Soft marine clay layer is improved by the addition of piles arranged in square grid pattern with 5.75% area replacement ratio. Geogrid with a tensile modulus of 4600 kN/m is used as the basal reinforcement. Initially, the embankment is analyzed without geogrid reinforcement and pile supports. Then, it is analyzed with (i) Basal geogrid (ii) With pile supports (iii) With basal geogrid and pile supports. The influence of various parameters of the embankment on maximum crest displacements, differential settlements at crest, toe horizontal displacements, stresses at pile head and foundation soil between piles and pile bending moment along the depth at peak acceleration are studied. Analysis of results shows that the embankment supported over piles with basal geogrid reinforcement will experience less crest settlements, differential settlements at crest and toe horizontal displacements due to earthquake load. © 2020, Springer Nature Singapore Pte Ltd.Item Stability analysis of stratified soil slopes by optimization technique using Janbu's generalized procedure of slices(2008) Arunkumar Bhat, K.; Shivashankar, R.; Yaji, R.K.The networks to facilitate communications have created large number of cuttings and embankments all along the west coast of India which is subjected to high intensity of rainfall. The heavy rainfall has converted the stable slope to unstable slopes over the years. Hence there is a need to study the effect of rainfall and other factors on the stability of slopes. Typical stratified slope is considered for the stability analysis and the general computer program is developed in C language which optimizes the factor of safety. The factor of safety is calculated using Janbu's GPS and Davidon-Fletcher-Powel (DFP) technique is used for optimization. The slope with berm or without berm and with or without tension crack are analyzed for stability. The factor of safety decreases with the increase in unit weight, pore pressure ratio, height of slope and slope angle and improves with shear strength parameters and berm width. The program is flexible in varying the input parameters to suit to the field conditions and their effects are presented in this paper.Item Experimental investigation on dynamic characteristics of structures founded on a dispersive soil(2008) Jayalekshmi, B.R.; Lohith, K.; Shivashankar, R.; Venkataramana, K.The objective of this paper is to evaluate the Soil-Structure-Interaction (SSI) effects on the seismic response of structures founded on Shedi soil of Dakshina Kannada. Shedi soil, which is a dispersive type of soil is highly vulnerable to dynamic loading in the saturated condition. Experimental investigations have been carried out on 1:10 scaled single bay three dimensional multistorey building models made of aluminium with its foundation resting on locally available Shedi soil (classifying as sandy silt) and sand in the saturated and dry conditions. The combined system of Soil-Foundation-Structure models is subjected to dynamic loading. The response of the model is measured at each floor level. This structural response is compared with that of a fixed base model to isolate the effect of soil structure interaction. The variations in natural frequency with various parameters such as different types of soil, degree of saturation of soil, number of storeys and the stiffening effect of walls are evaluated. The experimental results are presented and the modifications in dynamic characteristics due to the incorporation of soil flexibility are studied. Free vibration analysis of the three dimensional finite element model of the soil foundation structure system is carried out and the results are compared with the experimentally obtained values.Item PFWD, CBR and DCP evaluation of lateritic subgrades of Dakshina Kannada, India(2008) Nageshwar Rao, Ch.; George, G.; Shivashankar, R.The performance of pavements depends to a large extent on the strength and stiffness of the subgrades. Among the various methods of evaluating the subgrade strength, the use of portable falling weight deflectometers (PFWD) is gaining popularity in the recent years. This is due to its simplicity in design, portability, and the added advantages of providing quick and reliable estimates of the Young's modulus of elasticity of pavement subgrades. Hence it was felt that there is a need to study the correlation between results obtained using the PFWD and those obtained using the traditional approaches such as the California bearing ratio (CBR) test, the dynamic cone penetrometer (DCP) test. The work described herein focuses on exploring the correlations between the results obtained using the PFWD, and the results obtained using the CBR method and DCP for lateritic soils at various locations of Dakshina Kannada district of the State of Karnataka, India. Regression models were developed as part of this study to enable the prediction of CBR values based on the average of observed values of the Young's modulus obtained using the PFWD (Epfwd), and prediction of Epfwd from the average penetration-rates of DCPs performed for field density, and field-moisture content.Item Shake table tests to investigate the effi cacy of geomembranes for soil isolation in a space frame with isolated footing(2011) Jayalekshmi, B.R.; Shivashankar, R.; Venkataramana, K.; Ramesh Babu, R.; Reddy, G.R.; Parulekar, Y.M.; Patil, S.J.; Gundlapalli, P.Generally a base isolator shifts the natural period of the building away from that of the predominant period of the most probable earthquakes and provides additional damping to absorb the energy. The present study focuses on the effi cacy of soil, geofi bre reinforced soil and a layer of smooth geosynthetic membrane placed in soil in reducing the seismic response of a structure. Shake table tests are carried out in a tri-axial shaker system on a 1/3rd scaled model of a single storey, single bay RC space frame. A steel tank fi xed to the shake table is used as a container for soil and reinforced soil. The structure with different base conditions is subjected to sine sweep tests and the motion corresponding to the response spectrum of Zone III as per IS 1893(Part1):2002. Analysis of results shows that smooth geomembrane in sand can be effectively used to reduce the seismic response of the structure.Item Evaluation of the effect of soil-structure interaction on the raft of tall reinforced concrete chimneys under across wind load(Research Publishing Services editorial@rpsonline.com.sg, 2013) Jisha, S.V.; Jayalekshmi, B.R.; Shivashankar, R.A three dimensional soil-structure interaction (SSI) analysis of tall reinforced concrete chimneys with piled annular raft and un-piled annular raft subjected to across wind load is carried out in the present study. Effects of SSI were evaluated using four different soil types and three different ratios of external diameter to thickness of the annular raft. The across wind load was computed according to IS:4998 (Part 1)-1992. The integrated chimney-foundation-soil system was analysed by finite element software ANSYS based on direct method of SSI assuming linear elastic behaviour. FE analyses were carried out for two cases of SSI; (I) chimney with un-piled raft and (II) chimney with piled raft. The responses in raft were evaluated for both the cases and compared to that from conventional method of analysis. It is found that due to the addition of piles, there is considerable reduction in the responses in the raft when compared to that in the un-piled raft especially for loose sand and medium sand. © © 2013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore.Item Behaviour of prestressed reinforced foundation beds overlying weak soil(American Society of Civil Engineers (ASCE) onlinejls@asce.org, 2014) Shivashankar, R.; Jayamohan, J.Geosynthetics in foundation beds demonstrate their beneficial effects only after considerable settlements, since the strains occurring during initial settlements are insufficient to mobilize significant tensile load in the geosynthetic. Prestressing the reinforcement is a promising technique to overcome this shortcoming. This paper presents results from a series of laboratory scale bearing capacity tests carried out on model square footings to investigate the improvement in bearing capacity of geosynthetic reinforced foundation bed overlying weak ground due to prestressing the reinforcement. Prestressing the reinforcement in foundation bed results in significant improvement of bearing capacity of weak soil. A numerical model is suggested to predict the improvement in bearing capacity of footings with all the three cases - i.e., Granular Bed (GB), Reinforced Granular Bed (RGB), or Prestressed RGB on weak soil - by assuming a punching shear mechanism. These predictions are compared with laboratory scale experimental results. The proposed model is found to give good prediction of bearing capacity improvement. Finite element analyses are carried out using the FE program PLAXIS to study the effect of prestressing the reinforcement. Results obtained from finite element analyses are found to be in reasonably good agreement with the experimental results. © ASCE 2014.Item Dynamic soil-structure interaction analysis of 300m tall industrial reinforced concrete chimneys on piled raft foundations(Earthquake Engineering Research Institute, 2014) Jisha, S.V.; Jayalekshmi, B.R.; Shivashankar, R.Soil-structure interaction (SSI) analysis of 300m tall slender industrial reinforced concrete chimneys with piled raft foundation subjected to Elcentro (1940) ground motion is carried out in the present study. The transient analysis of three dimensional chimney-piled raft-soil system was conducted based on direct method of SSI using finite element method. Linear elastic material behaviour was assumed for the chimney, piled raft and soil. Parametric studies were conducted by considering different thickness of raft of piled raft foundation and different soil types to understand the significance of SSI. The time history analysis of the integrated chimney-foundation-soil was carried out with ground motion corresponding to the Imperial Valley earthquake at Elcentro (1940) with a magnitude of 7.0 and peak ground acceleration of 0.319g. The time history of acceleration was applied in the global X direction of the entire soil-structure model. The responses in terms of tangential and radial bending moments in raft, settlement of raft and tip deflection of chimney were investigated. The bending moments in raft of piled raft foundation obtained from SSI analysis were compared with that obtained from conventional analysis. It is found that the response in chimney and raft is considerably high for chimney-piled raft system resting on loose sand and it depends on the characteristics of ground motion also.Item Analysis of foundation of tall RC chimney with 3D finite element method(American Society of Civil Engineers (ASCE) onlinejls@asce.org, 2014) Jisha, S.V.; Jayalekshmi, B.R.; Shivashankar, R.3D finite element (FE) analyses were carried out for 100-m and 400-m-high RC chimneys having piled annular raft and annular raft foundations considering the flexibility of soil subjected to across-wind load. Stiffness of supporting soil and foundation were varied to evaluate the significance of SSI. The integrated chimney-foundation-soil system was analysed by finite element software ANSYS based on direct method of SSI assuming linear elastic material behaviour. FE analyses were carried out for two cases of SSI, namely: (1) chimney with annular raft foundation, and (2) chimney with piled annular raft foundation. The responses in raft, such as bending moments and settlements, were evaluated for both cases and compared to those obtained from the conventional method of analysis of annular raft foundation. It is found that the responses in raft depend on the stiffness of the underlying soil and the stiffness of foundation. © ASCE 2014.