Computation Mechanics & Numerical Modeling

(2) Plasticity-based multi-yield surfaces constitutive model of cohesive interfaces for modeling mortar joints

The main objective is to develop a refined constitutive model by addressing the limitations of the contact-based cohesive surface (e.g., underestimation of shear strength, no compressive-related failure criterion considered, no dilatancy effects considered).

Highlights:

• Two hyperbolic yield surfaces capable of capturing various failure modes (e.g., tensile cracking, shear sliding, compressive crushing)

• An unassociated flow rule to capture the 'dilatancy' phenomenon

• A fully implicit Euler backward integration algorithm combined with the local-global Newton-Raphson method guarantees accuracy

• An error-based auto-adaptive sub-stepping algorithm to achieve robustness and efficiency

(3) Damage-plasticity-based multi-yield surfaces constitutive model for cohesive interfaces

This model is developed by extending the previous plasticity-based model, to simulate the cyclic behavior of mortar joints (e.g., stiffness degradation, stiffness recovery, irreversible deformation)

Highlights:

• An extension from plasticity-based framework to damage-plasticity framework

• Consideration of stiffness degradation, stiffness recovery, and strength softening/hardening

• A robust semi-implicit algorithm for the damage-plasticity integration problem

(4) Finite element modeling of masonry walls under combined in-plane and out-of-plane loading

Current design guidelines and behavior assessments for masonry walls are limited to pure in-plane (IP) or pure out-of-plane (OOP) loading. This is a pioneering work to investigate the structural behavior of masonry walls under combined IP and OOP loading. Emphasis is put on both unreinforced and reinforced masonry (URM and RM) walls. Moreover, a series of capacity interaction curves are developed to provide practical guidance for designing masonry walls in such loading scenarios.

Highlights:

• Micro modeling strategies employed for the simulation URM and RM walls (shown below)

• User-friendly python tools developed for the modeling of masonry walls

• A large numerical database for masonry walls generated with various wall configurations (a total of 252 models for unreinforced masonry walls and 336 models for reinforced masonry walls)

• Comprehensive investigation of design parameters (i.e., aspect ratio, slenderness ratio, pre-compression load, reinforcement ratio) on IP and OOP capacity interactions of masonry walls

• Analytical model proposed for IP-OOP capacity interaction curves

(5) Discrete element modeling of tunnelling excavation in fractured rock zone

This project focuses on Nepal’s first TBM excavation tunnel, the Bheri Babai Diversion Multipurpose Project, which navigates through significantly fractured jointed rock. To assess the stability of the tunnel lining, the excavation process is simulated using discrete element modeling in the Itasca 3DEC. This method provides a detailed analysis of rock behavior and fracture patterns, ultimately improving the prediction of structural stability during excavation.

Highlights:

• Simulation of the TBM driving procedure, including the advancement of the rigid cutter-head, its interaction with the flexible rock mass, and the applied torque.

• Detailed modeling of the fractured rock mass, taking into account joint spacings, dips, and inclinations.

(6) Finite element modeling of bond-slip behavior of support lining structures in the railway tunnel

Understanding bond-slip behavior is crucial for the performance of reinforced concrete structures. This sub-topic focuses on simulating the mechanical response, such as crack distribution, of support lining structures following tunnel excavation. 

Highlights:

• Application of Winkler foundation beam theory to model the contact relationship between concrete and surrounding soil.

• Simulation of compressive crushing and tensile cracking behaviors with the Solid65 concrete element in the ANSYS finite element package.

• Explicit modeling of reinforcement bars with non-zero length spring elements, while bond-slip behavior is represented by zero-length spring elements and force-slip relationships.

(7) Numerical simulation of dynamic behaviors of support lining structure in heavy-haul

railway tunnel with voided base

This project aims to analyze the time-history behavior of support lining structures in railway tunnels, particularly under the voided base condition, which is common in heavy-haul railway tunnels.

Highlights:

• Dynamic load determination incorporating the axial load of the vehicle body and track irregularities.

• Development of three-dimensional finite difference models for the load-tunnel-soil system in Itasca FLAC 3D.

• Prediction of the fatigue life of concrete lining based on the S-N curve of concrete.

(8) Formation mechanism of voided bases in railway tunnels

Voided bases present a significant threat to the structural safety of tunnel support systems. This study investigates the formation mechanism of voided bases in railway tunnels, primarily attributed to the differing mechanical behaviors of concrete lining and surrounding soil. Concrete lining is typically more rigid than soil, leading to limited irreversible deformation under repeated loading. In contrast, soil, which is more flexible, undergoes permanent deformation that does not fully recover during the unloading phase. Over time, this discrepancy causes the formation of voids beneath the concrete lining. This study provides a mechanical explanation for this phenomenon, offering insights into the underlying causes. 

Highlights:

• Cyclic plasticity model of soil considering ‘Ratcheting Effect’ developed 

• Two-dimensional plain-strain finite element model developed for track-tunnel-soil system