Browsing by Author "Roy, Nishant"

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Assessment of Vulnerability of Rock Slope Considering Material and Seismic Variability
(Springer, 2018-10) Roy, Nishant
Stability analysis of rock slopes has always been a critical and challenging task for the geotechnical engineering professionals. The complexities associated with the stability analysis arise due to the heterogeneous, anisotropic and variable nature of the rock mass. Assessment of slope stability becomes further challenging under earthquake motions which are random in nature. Thus, uncertainties in both material and loading parameters are required to be considered for a robust assessment of the vulnerability of slopes in geologically complex and seismically active regions.
Bearing capacity of circular footing supported on coir fiber-reinforced soil
(Taylor & Francis, 2017-04) Roy, Nishant
A series of triaxial compression tests on sandy soils reinforced with varying percentages of coir fibers have been numerically simulated to observe its effect on the elastic properties and friction angle of the sand. Test results indicated that with increase in fiber content, the elastic properties and friction angle of soil improved significantly. Three dimensional numerical investigation has also been carried out to investigate the load settlement characteristics of a circular footing resting on fiber-reinforced sand using the properties obtained from the triaxial simulations. This study provides an insight into the effectiveness of addition of coir fibers in the sand of different strengths on the bearing capacity of circular footing.
Discussion on “Ultimate Bearing Capacity of Rock Masses under Square and Rectangular Footings”
(Elsevier, 2021) Roy, Nishant
This discussion is based on the original paper by Mansouri et al. (2019) (hereafter the authors of the original paper). The original paper presents an interesting study dealing with the determination of the bearing capacity of rock masses under square and rectangular footing resting on the ground surface. The bearing capacity is evaluated using a commercial finite element code PLAXIS 3D with the built-in Hoek-Brown constitutive model. The authors use the load-settlement curve obtained from numerical analyses to report the bearing capacity coefficient Nσ (Table 5 of the original paper). The original paper states various methods such as tangent intersection, log-log, hyperbolic, and 0.1B for the determination of the bearing capacity from the load-settlement curve. The authors claim to employ each of the methods and report the least value as the bearing capacity. As examples, the authors provide two load-settlement curves (Fig. 2 and Fig. 3 of the original paper) and highlight the determination of the ultimate bearing capacity based on the 0.1B method and the tangent intersection method. In this discussion, the discusser presents the concerns related to the calculation and reporting of the bearing capacity using the 0.1B method where the bearing capacity should be reported corresponding to a settlement of 10 percent of the footing width. The concerns are discussed below.
Dynamic response of dry ashlar masonry arch using discrete element method
(Springer, 2021-01) Roy, Nishant
The nonlinear dynamic response of the ashlar masonry arch is obtained for harmonic support excitations. The UDEC software is used for performing the analysis which duly accounts for the non-linearity arising due to the friction existing between the two ashlar masonry block units. The analysis is performed for two frequencies and five peak ground acceleration (PGA) levels assuming the support excitation to be harmonic. Deformation of the arch is mapped with the help of the displacements of the centroids of the arch masonry block units. The dynamic behavior and stability of the arch are investigated under two parametric variations, namely, the PGA and height to span (H/L) ratio of the arch. The analysis shows that the failure of arch takes place with bulging near the end segments of the arch creating large gaps between the ashlar masonry units. Further, as the frequency of excitation is increased, the arch becomes more stable; it also appears that there exists an optimum value of the H/L ratio for which the dynamic stability of the arch is maximum.
Effect of Embedment Depth on the Seismic Bearing Capacity of Strip Footing in Rock Mass
(ASCE, 2022-07) Roy, Nishant
This study presents the numerical solution for the seismic bearing capacity of embedded strip footing in Hoek–Brown rock mass. The upper-bound solutions are presented by employing the finite-element limit analysis. The influence of various parameters namely, geological strength index (GSI), yield parameter of the rock (mi), rock strength ratio (σci/γB), embedment depth ratio (d/B), and the horizontal seismic coefficient (kh) on the seismic bearing capacity factor (N) of strip footing is assessed. The results are presented as design tables and charts for a wide range of parameters. A parametric study is performed and a discussion on the influence of each parameter on the seismic bearing capacity factor is made. The influence of the considered parameters on the failure pattern is highlighted. It is noted that the lateral extent of the failure surface increases with an increase in the depth of embedment which has a positive influence on the seismic bearing capacity of strip footing. Introduction
Pothole recognition using DeepCNN with layer permutation scheme
(SPIE, 2022-07) Lahoti, Mukund; Singh, Ajit Pratap; Roy, Nishant
Pavements need frequent maintenance and management, for which one needs detailed information related to the road condition. At present, much of the condition monitoring around the world is done manually in situ or using high-end equipment. This is quite impractical, especially on a national or global scale because it takes time and involves high cost and effort. Pothole recognition using deep-learning-inspired image classification is currently being researched. Convolutional neural networks (CNNs) have drastically enhanced the techniques of image classification. We discuss an improved model based on CNN for pothole recognition. It introduces a neural architecture, namely layer permutation scheme for CNN (LPS-CNN). There is always a trade-off between network complexity and processing time efficiency in neural network architectures. The specific importance of the proposed neural network is that it learns a sparse architecture of the receptive neurons, which turns out to be fruitful in attaining maximum efficiency (for pothole recognition) with a reasonable processing time. The practical applicability of the proposed model is tested on the publicly available DSH 2017 pothole dataset. The proposed LPS-CNN outperforms all other existing architectures for pothole recognition and achieves an exceptional average accuracy of 99.6% on the benchmark pothole dataset.
Prediction Model for Performance Evaluation of Tunnel Excavation in Blocky Rock Mass
(ASCE, 2018-06) Roy, Nishant
The response of tunnel excavations in the blocky rock mass is dominated by the presence of discontinuities. However, many of the studies reported in the literature consider these geological features in a simplistic manner by adopting the equivalent continuum approach, thereby neglecting the structurally controlled mechanism of response. To overcome the aforementioned limitation, the present study adopts the Voronoi tessellation scheme in a discrete-element-based framework to simulate the blocky rock mass. An attempt was made to frame the prediction model for convergence strains of the tunnel by considering the uncertainties of joint parameters and in situ stress ratio. The prediction model was framed following the identification of important parameters affecting the strains using the robust central composite design. Finally, the concept of critical strain was used to demonstrate the applicability of the framed model through the probabilistic assessment of various performance levels of tunnel excavation
Relative influence of strength and geometric parameters on the behavior of jointed rock slopes
(Springer, 2019-10) Roy, Nishant
The assessment of performance of jointed rock slopes poses significant challenges due to the variable nature of geological material. A number of past investigations have incorporated the inherent variability in prediction of slope behavior through the use of probabilistic-based approaches. However, such investigations have primarily attempted to determine the probability of failure and rarely focused on the assessment of relative contribution of individual parameters on the slope behavior. In the present study, an efficient and robust way of determining the relative influence of variable strength and geometric parameters on slope behavior has been discussed. The effects of each individual parameter and their combined effect or interaction on slope stability have been highlighted. In addition, mathematical model (response surface) for prediction of slope behavior has been framed using the efficient central composite design (CCD) model. Moreover, adequacy of the present approach in identifying the influencing parameters has been highlighted. It is believed that the identification of influencing parameters using the present approach may help practitioners in choosing the appropriate measures for improving the stability of jointed rock slopes.
A Review of Seismic Damage of Mountain Tunnels and Probable Failure Mechanisms
(Springer, 2016-09) Roy, Nishant
Severe cases of damages of mountain tunnels following 1995 Hyogoken-Nanbu (Japan), 1999 Chi-Chi (Taiwan), 2004 Mid-Niigata Prefecture (Japan) and 2008 Wenchuan (China) earthquakes have challenged the traditional belief of tunnel structures being seldom damaged in seismic events. These experiences are a reminder that seismic behaviour of mountain tunnels must be further studied in detail. Such investigations assume greater significance as more number of tunnels are being planned to be constructed to meet the infrastructural needs of mountainous regions all around the world. In this paper, seismic damages of mountain tunnels have been reviewed. Prominent failure patterns have been identified based on the case histories of damages. Damages in the form of cracking of tunnel lining, portal cracking, landslide induced failures, uplift of bottom pavement, failures of sidewalls, shearing failure of tunnel liner and spalling of concrete have been majorly observed. Based on the damage patterns and earthquake data, main factors leading to instabilities have been discussed. Probable failure mechanisms of mountain tunnels under seismic loading conditions have been explained. Seismic analyses of a circular lined tunnel in blocky rock mass have been carried out through discrete element based approach. The significant role of different seismic parameters like frequency, peak ground acceleration has been identified. Moreover, effect of tunnel depth on the seismic response of tunnels has been investigated. It is believed that the present study will help in advancing the present state of understanding with regard to the behavior of tunnels under seismic conditions.
Seismic Bearing Capacity of Embedded Strip Footing in Sloping Rock Mass Ground
(Springer, 2022-11) Roy, Nishant
The numerical solution for the seismic bearing capacity of embedded strip footing in a sloping ground composed of rock mass is presented. The ground is modeled using the non-linear Hoek–Brown criterion and the upper bound solutions are evaluated using the finite-element limit analysis. The results are presented in the non-dimensional form as seismic bearing capacity factor, which is the ratio of the ultimate seismic bearing capacity to the unconfined compressive strength of the rock. The effect of parameters such as Geological Strength Index, yield parameter of the rock, angle of inclination of the ground, horizontal seismic coefficient, and the embedment depth ratio is evaluated. The impact of each factor on the evolution of the failure surface is highlighted. It is noted that the inclination of the ground has a negative impact on the seismic bearing capacity of the embedded strip footing and should be accounted for in practice.
Seismic isolation of tunnels in blocky rock mass using expanded polystyrene (EPS) Geofoam
(Springer, 2019-06) Roy, Nishant
Damages to mountain tunnels reported following recent seismic events have challenged the long-held belief of buried structures to have adequate earthquake-resistant features. Field observations also highlight that the geological joints play a dominating role in governing the seismic response. However, most investigations idealize the geological medium as a continuous domain, which has an inherent limitation in their inability to capture the wave–joint interaction. Given the above, the present study attempts to assess the seismic response of a circular lined tunnel in a distinct element-based framework, which can capture the interaction between the wave and joints. First, the seismic response of a circular lined tunnel under the action of a recorded earthquake time history is performed for two different depths. Subsequently, the feasibility of using Expanded Polystyrene (EPS) Geofoam as an effective buffer material for seismic isolation of tunnels is investigated. The evolution of variation of axial force and bending moment in the tunnel liners are highlighted for both the cases. A marked reduction in the seismic demand is observed when EPS Geofoam is utilized as a coating material around the liner. The results of the numerical simulation highlight the promising capabilities of using EPS Geofoam as a protective material for underground tunnels.
Stability Analysis of Failed Slope in Tindharia Region, West Bengal, India — A Case Study
(Springer, 2021-08) Roy, Nishant
The present study provides a discussion on landslide event which occurred along Hill Cart road, in the year 2011. The concerned area, situated in the Tindharia region of West Bengal, is famous as a UNESCO world heritage site and draws tourists owing to the famous toy train operated by the Darjeeling Himalayan Railway. This paper presents a description of the events associated with the catastrophic landslide which resulted in the abrupt halting of the operations of the toy train network and disruption of road traffic. Observations made during the site visit are presented, summarizing the important geological findings. Data collected during the site visit is utilized to identify the probable failure surface. Back analysis is attempted using the limit equilibrium method, to assess the effect of geomechanical parameters on the stability of the slope while replicating the probable range of parameters leading to instability. The results of the back analysis are further utilized in a finite element numerical model to assess the stability of the new slope profile. Measures for improving the stability of the new slope by incorporation of rock bolts are discussed. It is believed that the findings of the present study will be beneficial for future stability analyses at the concerned site.
A Three Dimensional Comparative Study of Seismic Behaviour of Vertical and Batter Pile Groups
(Springer, 2017-08) Roy, Nishant
To a practicing foundation engineer, the performance of batter pile under seismic conditions still remains a questionable prospect. The contradictory findings reported by various investigators with regard to the performance of batter piles add to this dilemma. This calls for a rigorous three-dimensional investigation to evaluate seismic behavior of batter pile groups. In this study, a comparative assessment of three-dimensional seismic behavior of a 2 × 2 vertical and batter pile groups having batter angle of 15° was carried out using a full three-dimensional finite element code developed in MATLAB (Sarkar 2009). The effects of centre to centre spacing of piles and soil modulus values were investigated. Idealized soil profiles having constant and triangular variation of soil modulus were adopted for the study. Results of analyses for both the vertical and batter pile groups are presented in terms of dynamic stiffness and kinematic interaction factors. Results indicate better seismic performance of batter pile groups in comparison to that of vertical pile groups. To demonstrate the importance of the findings, a five-storied portal frame structure supported separately on vertical and batter pile groups were considered and analyzed for El-Centro Earthquake (1940) time history. The difference in structural response considering vertical and batter pile groups is highlighted.
Transverse Dynamic Response of Circular Tunnel Excavation in Blocky Rock Mass using Distinct Element Method
(ASCE, 2018-10) Roy, Nishant
Recent cases of damage to mountain tunnels have raised concerns about the seismic performance of underground structures. The damaged sections have been majorly observed in highly fractured and weathered geological regions. The literature suggests that the interaction of the stress waves with the discontinuous medium may considerably influence the behavior of tunnel excavations. However, most of the investigations toward the evaluation of the transverse dynamic response of tunnels idealize the geological medium as a continuum medium, failing to capture the interaction between the stress waves and joints. To overcome the limitation, the present study attempted to assess the dynamic behavior of a lined circular tunnel passing through a blocky discontinuous rock mass using the Voronoi tessellation scheme in a distinct element framework. The influence of frequency of input motion, tunnel depth, in situ stress ratio, and types of seismic waves was studied with regard to the dynamic forces that developed in the liner. Moreover, the results, presented in terms of incremental dynamic forces along the tunnel liner, were utilized to corroborate past damages reported in the literature.