1. Ph.D Theses
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Item Numerical Analysis of Pile-Supported Geogrid-Reinforced Embankments on Soft Grounds(National Institute of Technology Karnataka, Surathkal, 2022) Patel, Radhika M.; Jayalekshmi, B.R.; Shivashankar, R.In recent decades, the column supported embankments are often constructed at places where soft clay exists within a considerable depth and the construction of roads or rail roads or bridge approach roads is of great demand due to rapid increase of industrialization and urbanization. The column supported embankment has many advantages over the other conventional consolidation based techniques. Such as, these embankments can be constructed at a stretch without prolonged time delay and the embankment loads are directly transferred to the hard strata through piles. Geosynthetics has several advantages for improving the soft grounds, among them providing geogrids as basal-reinforcement below the embankments constructed over soft subsoils of shallow depth is one of the well-known technique. The basal- reinforcements can also be provided above the piles instead of pile caps or raft above piles. The geosynthetics can also be provided in the embankment body to steepen the embankment side slopes. The response of these basal or body-reinforced embankments with or without pile supports under static loading conditions is well-addressed in literature. Most of the studies on dynamic response of these embankments considered cyclic loads or sinusoidal loads to represent traffic loading. Though there are studies available on the seismic response of these embankments, the response of these embankments considering full 3-Dimensional finite element model subjected to time- history loading of different earthquakes is not yet addressed. Hence, in the present study both static and seismic response of basal or body reinforced embankments with and without pile supports are studied using 3-dimensional finite element analysis. In the first part of the study, the response of basal geogrid-reinforced pile- supported embankments subjected to self-weight and traffic load are studied using 3- dimensional finite element models. The influence of various parameters such as, embankment height, geogrid tensile modulus, pile length, pile type and pile spacing are studied. Based on the results of numerical analysis, the modifications to the soil arching coefficient (Cc) including the effect of pile length and pile spacing are proposed and compared with the existing analytical equations. Crest settlements, toe lateral displacements, differential settlements at crest, stress distribution ratio, lateral stress viii distribution ratio and coefficient of lateral pressure along embankment height were considered to analyse the response of these embankments. The analysis of results indicates that, the end-bearing pile supported embankments performs better than floating pile supported embankments in terms of settlements, differential settlements and lateral displacements even at larger pile spacing. The addition of basal geogrid could further reduce the settlements and lateral displacements in the embankment. The analytical equation for Cc proposed based on the 3-dimensional finite element analysis results considered the effect of pile spacing, which the earlier methods did not consider. Hence the proposed analytical equation could able to give the more accurate results of pile loads than the existing methods. The crest centre settlements were further increased by the addition of traffic load. These basal geogrid-reinforced pile-supported embankments should stand safe during disastrous situations like earthquakes. Hence the second part of the study analyses the seismic response of basal geogrid-reinforced pile-supported embankments subjected to seismic excitations. Time-history analysis was performed on the 3- Dimensional finite element models of basal geogrid-reinforced pile-supported embankments. The seismic response of embankment in terms of vertical and lateral displacements, differential settlements, vertical and lateral stress distribution on pile and the foundation soil between piles, amplification coefficient, lateral earth pressure along the embankment height and the pore water pressure are studied by considering the height of embankment, side slope of embankment, basal geogrid tensile modulus, length of pile, spacing of pile and type of pile. The analysis of results shows that the embankment height is an important parameter to consider in the seismic design of basal geogrid reinforcement. 4 m high embankment experiences very less differential settlements caused by seismic excitations among the different embankment heights considered. About 8 % reduction of toe lateral displacements are observed by the addition of basal geogrid. But the embankment with pile supports shows a reduction of 40.8 % and the combination of both pile supports and basal geogrid could reduce 46 % of toe lateral displacements. Addition of basal geogrid increases both vertical and lateral stresses on piles due to seismic excitations. The variation of coefficient of lateral pressure along the embankment elevation is random for the considered parameters, this indicates that the formation of soil arching in a geogrid reinforced pile supported ix embankment subjected to seismic loading is not uniform like in the case of self-weight analysis. Basal geosynthetic-reinforcements are the most commonly used ground improvement technique for the embankments constructed over shallow depth soft clays. The width of the basal-reinforcement provided should be adequate to withstand lateral sliding, rotational failure and excessive settlements under both static and seismic loading conditions. In the third part of the study, an attempt has been made to study the optimum width of basal geosynthetic-reinforcement subjected to both static and seismic loading conditions. Finite element models of basal geosynthetic-reinforced embankments including the effect of embankment height, embankment side slope, tensile modulus of geosynthetic, number of layers of geosynthetic, stiffness of embankment fill, stiffness of foundation soil and intensity of seismic loading were studied. Based on the results of crest settlements, toe lateral displacements and lateral displacements at the crest, the required width and tensile modulus of basal geogrid were identified. Basal geogrid having a minimum tensile modulus of 500 kN/m with a width equal to the base width (B) of embankment is found to be sufficient to reduce settlements at places where static loading is predominant or in low seismic regions. Basal geogrid of width equal to ‘B+H’ having tensile modulus of 4000 kN/m is recommended to reduce the lateral displacements in embankments at active seismic regions. Further reduction of about 6 % in lateral displacements are seen by providing 4 layers of basal geogrid with a total tensile modulus equal to 4000 kN/m. The geosynthetics are also used as embankment body-reinforcements to steepen the embankment side slopes. These slopes are stable under static loading conditions but, under seismic loading conditions, repairable damages or sometimes complete failure of slopes may occur. Hence the present study is also extended to analyse the seismic response of body-reinforced embankments considering the effect of embankment side slope and foundation soil stiffness using finite element analysis. From the analysis it is observed that, in unreinforced embankments the face lateral displacements increase as the steepness of slope increases and the embankment above soft soil displaces more than the embankment on stiff soil.Item Buckling and Dynamic Behavior of Non-Uniformly Heated Cylindrical Panels(National Institute of Technology Karnataka, Surathkal, 2018) Bhagat, Vinod; Jeyaraj, P.; Murigendrappa, S. M.Today, curved panels especially cylindrical and conical are considered as a backbone of numerous engineering structures. Knowledge of buckling and dynamic behavior of structures over a range of temperature is essential for their better design. Most of the studies carried out on heated panels are based on uniform temperature distribution assumption. However, in real life application, the cylindrical panels employed in structures are exposed to non-uniform temperature variation due to the location of the heating source and thermal boundary conditions. In the present study, the thermal buckling strength of the non-uniformly heated metallic panel predicted numerically is validated experimentally using in-house developed experimental set-up. Further studies are extended to investigate the effect of non-uniform temperature variation on buckling strength and free vibration characteristics of metallic, laminated composite, and functionally graded carbon nanotube (FGCNT) reinforced polymer composite, cylindrical panels using the finite element method. Finally, the optimization of a non-uniformly heated laminated cylindrical panel against thermal buckling strength and fundamental natural frequency is also carried out. Typical variation of temperature-deflection plot for different temperature fields is obtained experimentally and further, inflection point method is used to predict the critical buckling temperature from temperature-deflection plot. Experimental studies are further extended to analyze the influence of geometrical parameters and structural boundary constraints on critical buckling temperature. Experimentation results reveal that the location of the heat source and resulting non-uniform ivtemperature field influences the thermal buckling strength significantly. Among three cases examined in experimentation for the position of heat source, minimal buckling strength is observed when the heater is located at the center of the panel while maximum buckling strength is observed when the heater is located at the forefront curved edge. It is also found that aspect ratio and structural boundary constraints play a major role in deciding the buckling strength of the panel. From the numerical studies carried out on non-uniformly heated panels, a relation known as magnification factor is established to evaluate the buckling strength of non-uniformly heated cylindrical panels knowing the buckling strength of uniformly heated panels. Among five cases investigated for the position of heat source, the highest magnification factor is observed for a panel with the heat source located at the forefront curved edge. It is observed that the free vibration mode shapes of the panel change significantly with increase in elevated temperature. The changes are observed in terms switching of modes with a significant change in modal indices. With the rise in temperature, nodal and anti-nodal positions of a particular free vibration mode shape are shifting towards the location where the intensity of the heat source is high and structural stiffness is low. It is found that for a stiffer panel, the buckling strength of the laminated and FG-CNT composite panels with temperature-dependent elastic properties is significantly lesser than that of the panels with temperature independent elastic properties. Panel with maximum area exposed to a peak temperature of particular non-uniform temperature fields shows lowest buckling strength. Functional grading of CNTs with more amount of CNTs located close to top and bottom of the panel (FG-X) results in higher buckling strength and free vibration frequencies compared to those panel with maximum CNTs distribution near the mid-plane. Free vibration frequencies of non-uniformly heated FG-CNT panel with temperature dependent properties is observed to decrease drastically with elevated temperature compared to the panel with temperature independent properties. Variation vin frequencies observed in a pre-stressed panel with temperature dependent and independent properties is more significant in stiffer panels. Irrespective of temperature dependent and independent properties, shifting of nodal and anti-nodal lines and change of modal indices are also observed at elevated temperature. Well-known and generally acknowledged optimization technique, particle swarm optimization is employed for the optimization of thermal buckling strength of laminated composite panels exposed to five different temperature fields. Two different optimization approach like single objective optimization approach and multiobjective optimization approach are employed. In single objective optimization, the panel is exposed known temperature field whereas, in multi-objective optimization, the panel is exposed to unknown temperature fields when in-service. It is found from the analysis that the variation in the optimum buckling strength of non-uniformly heated panels is more significant at lower curvature ratio. Whereas, variation in the optimum fiber orientation under different temperature fields is significant at higher curvature ratio. Multi-objective optimization approach has proved to be superior to that of single objective optimization approach when panels are exposed to the unpredictable thermal environment. Further, studies are carried out on optimization of both thermal buckling strength and fundamental free vibration frequency of heated panels using particle swarm optimization in conjunction with the artificial neural network. Multiobjective design index (MODI) has been derived for the panel considering buckling strength and fundamental frequency as objectives for optimization. It is found that MODI of the cylindrical panels under thermal load is complex and significantly influenced by the temperature fields, lamination scheme, in-plane boundary constraints, elevated temperature and geometric parameters. It is also observed that the MODI of the panel can be maximized by optimizing laminate orientations. Further, it is observed that panel with lamination scheme of (θ°/–θ°/θ°/–θ°)S gives higher value of MODI compared to other lamination schemes considered.