Tushar Bhandari | Indian Institute of Technology Delhi (original) (raw)

Uploads

Papers by Tushar Bhandari

This paper presents the results of numerical analysis carried out using PLAXIS 3D on behaviour of... more This paper presents the results of numerical analysis carried out using PLAXIS 3D on behaviour of rock-socketed pile subjected to independent and combined loading. The numerical procedure adopted in the analysis was validated by comparing the load test results reported in the literature. After validation of the numerical analyses, the parametric analysis were performed on the rock-socketed pile subjected to independent loading and combined loading for different rock conditions, soil cover depths and socketing lengths. From the results of parametric study, it is found that the vertical and lateral load capacities under combined loading are not significantly affected, when the soil cover depth is high. However, if the soil cover depth is low, then the behaviour of rock-socketed pile under combined loading is found to be significantly different, compared to its behaviour under independent loading. It is also seen that the rock conditions and socketing length have profound effect on pile behaviour under combined loading.

Underground excavations usually consist of different shapes, varying from circular shapes in tunn... more Underground excavations usually consist of different shapes, varying from circular shapes in tunnels to complex shapes in hydroelectric projects. The stress distribution around an underground opening and resulting deformations are important to analyze in order to assess the stability of the excavation and design a suitable support system. While circular shapes can be well analyzed through analytical solutions, complex shapes may need a three-dimensional numerical analysis. As the arrangement of different components of excavation becomes complex, the analysis of excavation becomes more demanding. For the design of shotcrete lining for tunnels and cavern arches, development of shotcrete strength and stiffness with time is a very crucial aspect which is seldom paid enough attention to. This paper presents an approach to address the age-dependent shotcrete behavior in convergence confinement method for circular tunnels and discusses its impact in the design of shotcrete lining. Further, this behavior is incorporated in a FLAC3D analysis carried out to assess the stability of the excavations for a hydroelectric project that includes two caverns, eight tunnels and four shafts with different sizes and arrangement. The geology in the area was classified in two groups with different behavioral types. Two different material models namely the classical Mohr-Coulomb model and strain softening ubiquitous model were used to simulate isotropic behavior of a relatively weak rock mass and anisotropic behavior of a schistose rock mass characterized by strongly pronounced discontinuities. 1. Introduction Shotcrete is an important support element for tunnels excavated in accordance with the New Austrian Tunneling Method (NATM). The design of primary shotcrete lining can be done using various approaches including analytical and numerical solutions. The state of stresses and strains prevailing in the shotcrete lining is a major factor in all approaches for assessing its stability and eventually the stability of the tunnel or other underground openings. While design of shotcrete lining is also based on structural mechanics, it differs from the design of structural members of a building. For example, unlike building foundations, the loads that the tunnel support should carry are not easy to determine and furthermore the ground-support interaction plays a critical role in determining the stress state in the support as well as in the ground. The interaction near the tunnel face is governed by the non-linear time-dependent material behavior of shotcrete and the time-dependent excavation process. Modeling the excavation advance and support installation near the face is essentially a 3D problem which can be analyzed through complex numerical models which may not always be economic in terms of computational time. In such cases, simple analytical methods can be used to understand how a rock mass surrounding a tunnel deforms and how the support systems act to control this deformation. This paper is an attempt to analyze the influence of age-dependent behavior of shotcrete on the rock-shotcrete interaction. This is done by introducing a pseudo-dynamic approach to the commonly used " Convergence confinement method ". Finally, this behavior is also incorporated in a three-dimensional numerical analysis for a complex underground excavation involving two caverns and many intersecting tunnels.

The Finite Element Method (FEM) is widely used in the simulation of geotechnical applications. Ow... more The Finite Element Method (FEM) is widely used in the simulation of geotechnical applications. Owing to the limitations of FEM to model problems involving large deformations, many efforts have been made to develop methods free of mesh entanglement. One of these methods is the Material Point Method (MPM) which models the material as Lagrangian particles capable of moving through a background computational mesh in Eulerian manner. Although MPM represents the continuum by material points, solution is performed on the computational mesh. Thus, imposing boundary conditions is not aligned with the material representation. In this paper, a non-zero kinematic condition is introduced where an additional set of particles is incorporated to track the moving boundary. This approach is then applied to simulate the seismic motion resulting in failure of slopes. To validate this simulation procedure, two geotechnical applications are modelled using MPM. The first is to reproduce a shaking table experiment where the results of another numerical method are available. After validating the present numerical scheme for relatively large deformation problem, it is applied to simulate progression of a large-scale landslide during the Chi-Chi earthquake of Taiwan in which excessive material deformation and transportation is taking place.

Tehri Hydroelectric Project is located on the bank of river Bhagirathi in the state of Uttarakhan... more Tehri Hydroelectric Project is located on the bank of river Bhagirathi in the state of Uttarakhand, India. The 2400-MW project is comprised of Tehri dam and Tehri hydro power project (HPP - 1000-MW) Stage-I, Koteshwar HEP (400-MW) and Tehri Pumped Storage Project (PSP - 1000- MW). As a part of the Tehri PSP, two surge shafts are proposed to be constructed downstream of the power house along the tail race tunnels (TRT). The arrangement consists of surge shafts with surge chambers at the top of each shaft and is located in poor geology with rock cover of 390-m. The chambers were resized from the originally planned 28-m span to 18.5-m span due to poor geological conditions which called for a transition in excavation from an 18.5-m chamber invert to a 19.44-m diameter circular surge shaft.
This paper presents a three-dimensional FDM analysis carried out to assess the stability of the excavations that include two chambers (18.5-m wide, 16.4-m high, 27-m long), a link tunnel (12.5-m x 13.5-m x 33-m) to connect the chambers, and two circular shafts of 19.44-m diameter. The geology in the chambers was classified in two groups: “Sheared Phyllite (SP)” and “Thinly Bedded Phyllite Quartzitic (PQT)”. To account for the different behavior of the two types of rock mass, two different material models were selected for the analysis. The weaker rock “SP” was modelled with the classical Mohr-Coulomb model while “PQT” was modelled with a strain softening ubiquitous model (also based on Mohr-Coulomb) that considers strength anisotropy resulting from the presence of a family of strongly pronounced discontinuities (bedding / schistosity) and a strain-dependent post-peak softening.
The sequence of excavation was also modelled in the analysis considering heading and benching and support installation after each round of excavation with a round length of 2-3 meters. A system of rock bolts and shotcrete was designed as the support system and further pre-reinforcement in form of fore-poling was recommended in zones of poor quality rock mass as additional support measure. Results of the analysis are presented in the form of stress and displacement plots and displacement histories for nine monitoring sections each with five monitoring points.

Ahmedabad is an important city in the Gujarat state of India. It has experienced one of the larg... more Ahmedabad is an important city in the Gujarat state of India. It has experienced one of the largest earthquakes in India on 26 Jan 2001 with magnitude of Mw 7.7 that caused many destructions and human casualties. With respect to historical earthquakes in the region of Ahmedabad and existing active faults like Marginal Fault (East Cambay) , Marginal Fault(West Cambay), Son Narmada Fault (SNF), Island Belt Fault (IBF), etc., this region has high seismic potential. It is needed to provide horizontal Peak Ground Acceleration (PGA) maps with different risk levels for different parts of the region. The probabilistic hazard analysis of earthquakes has been carried out for occurrence probability of 2% and 10% in 50 years along with 2% and 10% in 100 years.

Deterministic seismic hazard analysis of Ahmedabad region has been carried out considering past e... more Deterministic seismic hazard analysis of Ahmedabad region has been carried out considering past earthquake data and available seismotectonic information. Earthquake catalogue of the region covering 350 km radius around the Ahmedabad city has been generated separately after processing of collected earthquake data since 1668 to 2010. Declustering of entire catalogue has been carried out to remove the dependent events. Shortest distances from each seismic source causing tectonic activity have been calculated using simple mathematical equations. The Peak Ground Acceleration (PGA) values at rock level have been estimated using predictive relationships for the region. Our analysis shows that peak ground acceleration from Ahmedabad region has been varied from 0.14 to 0.44 g with maximum credible earthquake (MCE) of magnitude 6.1 generated from East Cambay Fault. The PGA model presented in this article provides basic design parameters for the Ahmedabad region.