GTS NX - New eXperience

Design Considerations
Stress relaxation
Excavation methods
Water level
Reinforcements
Ground status (rock quality)
Earth pressure coefficient (Ko)

Solution
GTS NX is fully equipped to model and analyze each phase of tunnel construction, including a simulation of the excavation itself and the installation of supports. Shield TBM analysis can be performed to obtain maximum axial forces, sheer forces, bending moments and radial displacements of the tunnel for each phase of the construction sequence.

GTS NX also enables you perform analyses of various types of tunnel linings such as unsupported rock, shotcrete, and unreinforced concrete systems. You can also take multiple load cases into account such as vertical stresses in soil and horizontal pressures on the tunnel surface. Afterwards you can run the analysis to calculate the resulting forces and moments for each element of the lining.

Linear Static Analysis

Linear static analysis assumes ground and rock materials to be linear-elastic materials and uses static analysis to determine the behavioral characteristics when loading is applied. Ground materials only display linear-elastic properties in the early stages of loading, where only a small strain is generated. However, because Linear-elastic analysis does not consider failure and idealizes the stress-strain relationship linearly,it is widely used for simple analysis, such as identifying the stress distribution or concentration in the in-situ ground.

In a broad sense, linear behavior can be viewed as a special form of nonlinear behavior but because linear analysis is convenient and intuitive, linear static analysis is categorized as a separate analysis type. Linear analysis, including Linear dynamic analysis, utilizes the elastic behavior of a defined material for analysis. Also, the non-linear behavior (tension only, compression only and non-linear elastic behavior) of elastic link elements or truss elements are ignored and the elements are used in the analysis as elastic bodies. Consequently, it is useful to use Linear static analysis to observe the approximate behavior of the ground, to organize the initial stage conditions of the construction stage or to conduct tunnel lining analysis.

Particularly, Linear static analysis does not require recursive calculations and hence, it has a short calculation time. GTS NX considers pore pressure in Linear-static analysis and drained and undrained conditions can be assigned to elastic materials for analysis.

Nonlinear Static Analysis

All physical phenomena includes non-linearity. Ground or structural behaviors are not an exception. Non-linear static analysis is used to simulate the behavior of ground considering such non-linearity, when the change with time is small and can be ignored. GTS NX considers the following non-linearity.
Non-linearity of material : This occurs when the stress-strain relationship is non-linear. Most ground materials have this non-linearity.
Geometric non-linearity: If the displacement-strain relationship is non-linear, the linear assumption is no longer applicable when the displacement is large, or the rotational deformation is large.
Non-linearity of load and boundary: Non-linearity that includes the non-linear behavior at an interface, or non-linearity caused by the direction change of a load due to strain, caused by forces such as the follower force.
GTS NX can consider all non-linearity mentioned above in analysis. Non-linear analysis can take a long time for complex non-linear systems because repeated calculations are conducted. Hence for the practicality, considering appropriate non-linearity can result in analysis results that simulate non-linear behavior, while maintaining the accuracy with little computational cost.

Construction Stage Analysis

Construction stage analysis can be used to simulate the construction process of the ground using numerical analysis. Construction stage analysis consists of multiple stages and loading/ boundary conditions, as well as elements, can be added or removed at each stage. This loading/ boundary or element change is applied at the start of each stage. GTS NX can use following types of analysis features to conduct Construction stage analysis.

Stress-Slope Analysis
Analysis of stress and slope stability during the construction process

Seepage Analysis
Stage by stage Steady state seepage analysis, Stage by stage Transient seepage analysis

Stress-Seepage-Slope coupled analysis
Sequential Seepage-stress analysis and Slope stability analysis during the construction process

Consolidation analysis
Consolidation analysis for environment change and construction process of embankment

Fully-coupled Stress-Seepage analysis
Stress analysis fully coupled with Transient seepage phenomenon

When conducting Construction stage analysis, the following should be considered.
Addition/Removal of element
Loading/Unloading of weight
Change in boundary condition
Change in rock material property
Definition of load distribution factor
Step by step underground water level
Drained-Undrained analysis
Initialization of displacement
Stress Analysis for initial construction stage (Consider Ko condition)
Restart

Fully-Coupled Stress-Seepage Analysis

Analysis that couples the seepage phenomenon and ground stress analysis can be classified in various ways, depending on the coupling.

The simplest way is to obtain the pore water pressure distribution by conducting seepage analysis beforehand, and reflecting it in the total stress/effective stress relationship equation of the stress analysis conducted in the following step. Such analysis is called sequential analysis. This method can be used to understand the static stress state of the given steady groundwater flow. However, since deformation due to stress analysis does not influence the seepage phenomenon inversely, there is no two-way coupling.

Fully-coupled Stress-Seepage analysis is the two-way coupled analysis between seepage analysis and stress analysis. Both analyses are used to solve the coupled equation. It can display the pore water pressure, stress or deformation changes with time.

The consolidation analysis begins with the assumption that steady state pore water pressure can be maintained, and is used to see the changes in excess pore water pressure. In other words, this analysis is used to simulate the phenomenon of how excess pore water pressure changes with the changes in load/boundary conditions.

Fully-coupled Stress-Seepage analysis does not follow assumption that steady state pore water pressure is maintained. Hence, it is suitable for simulating the transient seepage phenomenon, stress analysis and stability in abnormal condition in a fully coupled form. Unlike the consolidation analysis, it is possible to define the changes in seepage boundary conditions with time, boundary flow rate etc. In other words, for Fully-coupled tress-Seepage analysis, it is possible to use all the transient seepage boundary conditions, structural load and boundary conditions.

This analysis can be applied to the ground stability analysis for rainfall or the of large-scale dam stability analysis for water level change. The seepage boundary conditions (Head/Flux) can all be used to analyze not only the changes in excess pore water pressure, but also the consolidation analysis that considers the total change in pore water pressure.