International Journal of Civil Engineering
Volume:17 Issue: 1, Jan 2019

Production of energy from nuclear power plants generates high-level radioactive nuclear waste, harmful during dozens of 1000 years. Deep geological disposal of nuclear waste represents a reliable solutions for its safe isolation. Confinement of radioactive wastes relies on the multi-barrier concept in which isolation is provided by a series of engineered (canister, backfill) and natural (host rock) barriers. Few underground research laboratories have been built all over the world to test and validate storage solutions. The drilling of disposal drifts may generate cracks, fractures/strain localisation in shear bands within the rock surrounding the gallery especially in argillaceous rocks. These degradations affect the hydro-mechanical properties of the material, such as permeability, e.g., creating a preferential flow path for radionuclide migration. Hydraulic conductivity increase within this zone must remain limited to preserve the natural barrier. In addition, galleries are currently reinforced by different types of concrete supports such as shotcrete and/or prefab elements. Their purpose is twofold: avoiding partial collapse of the tunnel during drilling operations and limiting convergence of the surrounding rock. Properties of both concrete and rock mass are time dependent, due to shotcrete hydration and hydro-mechanical couplings within the host rock. By the use of a hydro-mechanical coupled finite-element code with a second-gradient regularization, this paper aims at investigating and predicting support and rock interactions (convergence and stress field). The effect of shotcrete hydration evolution, spraying time, and use of compressible wedges is studied to determine their relative influence.

We present a new approach for exploration of small- and large-scale geological structures in mechanized tunneling environments by analyzing actively excited seismic waves illuminating the soil in front of the tunnel-boring machine. Applying the concept of full waveform inversion, we exploit all information contained in the seismic recordings to image unknown disturbances of soil properties along the tunnel track. Owing to the lack of field data, we explore two-dimensional synthetic test models containing exemplary disturbances of the soil, where real recordings are replaced by synthetic ones obtained by solving the (an)elastic wave equation with a Nodal Discontinuous Galerkin method. An extension to three dimensions is straight forward but computationally expensive. Disturbances in these test models are reconstructed with an iterative procedure guided by minimization of the misfit between measured and predicted records. Our method is validated by means of a simple test case and different minimization procedures including a quasi-Newton method are compared. The presented method is able to reconstruct the spatial distribution of seismic wave velocities (P and S waves) and can detect large and small-scale objects ahead of the tunnel face characterized by wave velocities differing from that of the surrounding bedrock. The imaging process is stabilized by a multiscale approach, where the frequency content of the recordings is successively extended with the number of iterations. Incorporating viscoelastic attenuation of the surrounding bedrock does not compromise the capability of detecting large-scale objects as long as low-frequency waves are used.

Two contradictory requirements are basically demanded on single-component grouting mortars. On the one hand, a sufficient workability lasting for several hours; on the other hand, a rapid development of shear strength immediately after grouting. The latter is normally achieved through dewatering of the mortar into the surrounding soil. During dewatering also particles are transported to the soil and lead to a clogging in the interface, which can have a significant influence on the further dewatering process and thus on the consolidation behavior of the grout. In this research study, the dewatering behavior and redistribution of the particles of single-component grouting mortars have been systematically investigated under variation of the relevant material-specific parameters, such as the granulometry of the fines and aggregates. For the determination of the filtrate water, a filter press test was developed, simulating the conditions within the annular gap. For the evaluation of the redistribution of the particles, the particle size distribution, density and water content of individual layers of the dewatered grout were determined over the specimen height. The investigations revealed a significant influence of the granulometry of the particles on the grout properties. At early mortar ages, the amounts of filtrate water were at the same level independent of the granulometry and specific surface of the binder. However, the shear strengths increased steadily with increasing specific surface of the binder and age. With regard to the granulometry broken particles with a rough surface provided an essential contribution to the development of shear strength.

The interaction of underground structures with groundwater is an essential issue for all the phases of the construction and life of an underground work. This issue is essential for its potential effects on the environment, on the excavation technique and on the preservation of the hydraulic resource. A complete analysis of all the problems related to this interaction from the very beginning of the design is of crucial importance for the good execution of the construction and for the minimization of the maintenance cost during the life of the structure. A wrong waterproofing approach or an incorrect installation of the waterproofing system may lead to severe damage to the structure, and to high maintenance and refurbishment costs. In the technical literature, water inflow in tunnels has been recognized as one of the most important sources both of damage to the structures and of claims. Several case histories have highlighted the importance of water preservation and the potential danger of underground excavation for the environment if insufficient controls of underground water are applied. In the paper, a global overview of the techniques used is presented, some relevant case histories are discussed, and a risk analysis procedure is proposed.

This study investigates the evolution of the microstructure and mechanical properties of mortar. Mortar samples consisting of Portland cement CEM I42.5 R (~ 60 vol% of quartz sand 0/2 mm, w/c-ratio of 0.5) were prepared and stored according to EN 1015. After 1, 2, 7, 14 and 28 days, the samples were oven-dried until constant weight as well as vacuum-dried. The microstructure of the mortar samples was investigated using scanning electron microscopy. Phase analysis was performed using X-ray diffraction, allowing the description of the crystalline phase evolution during hardening. Mechanical properties were evaluated using nanoindentation. Based on the nanoindentation results, the effective Young’s modulus was calculated using the model by Hashin and Shtrikman. The moduli calculated based on the values of the nanoindentation experiments were compared to the Young’s modulus determined in compression experiments. The results show that the Young’s modulus determined by the nanoindentation and compression test describes a degressive curve progression. The studies show a correlation between the results from nanoindentation tests and the mechanical properties obtained from the compression tests. Therefore, the microstructural evolution of mortar, including the influence of pores on Young’s modulus, must be taken into account to estimate the macroproperties from the nanoindentation tests.

This research proposes a novel methodology of applying submodeling technique in the numerical simulation of mechanized tunnel excavation. A submodel is a smaller scale cut out of the full scale model (global model) in which the more in detail simulations along with higher degree of precision are conducted. Apparently, the submodel should include the near field around the TBM that is significantly affected by tunneling process and the region of interest such as ground surface where the model responses have to be surveyed. The appropriate size of the submodel is found by evaluation of the strain energy distribution in the domain while the energy gradient has to fulfill the predefined criterion. To analyze the submodel, the nodal displacements are derived from the global model and applied to the boundaries of the submodel. Using the proposed submodeling technique in the tunneling, the computational costs are reduced, while the submodel provides realistic prediction of the deformations in the system and lining forces. Finally, the proposed submodeling method is applied in both 2D and 3D tunneling simulations while two types of submodeling strategy (“fixed submodel” and “moving submodel”) are adopted for the 3D simulations. The results indicate that both approaches are adequate for predicting the lining forces and ground movements.

In this paper, a new procedure for the prediction of soil-tool abrasive wear is presented which drastically reduces the duration and, therefore, the cost of simulations in comparison to conventional 3D wear modeling. The goal is to extend the experimental data from a single scratch test to the wear of mixtures by means of equations obtained from discrete element method (DEM) simulations and geometric relations. We are predicting abrasive wear with a combination of numerical and experimental approaches taking two shapes of particles into account. Single wear is quantified by measuring the width of scratch induced by a single quartz particle. Geometrical relations together with the particle’s microscopic picture are used to find the depth of scratch. DEM mixture simulations result in equations for the number of contacts and normal contact forces. Finally, the wear rate is calculated for a specific soil sample as an example to clarify the developed prediction procedure. The DEM simulations are performed using PFC3D code for both a homogeneous soil sample and a mixture of two different soils. We are specially investigating a relation to predict the abrasive wear caused by a mixture of particles.

Within mechanized tunneling, slurry shields are used for excavations in soils with unstable tunnel face due to the possibility to support the tunnel face with pressurized slurry (bentonite suspension). Two key conditions have to be fulfilled to stabilize a tunnel face. These two conditions are sufficient face support pressure in the excavation chamber and the pressure transfer of slurry excess pressure, exceeding the pore pressure, onto the soil skeleton. In practice, the German standard DIN 4126 [Nachweis der Standsicherheit von Schlitzwänden (Stability analysis of diaphragm walls), Deutsche Institut für Normung, 2013] is usually used to predict this transfer. However, DIN 4126 (2013) cannot explain increased pore water pressures measured in practice close to the slurry supported tunnel face during excavation. The increased pore water pressures reduce the efficiency of slurry face support. These pressures are explained by on-going disturbance of the pressure transfer mechanism by periodic rotating cutting tools. The characteristics of disturbance are designated as excavation scale. Another factor of influence is the timespan during which the pressure transfer mechanism can achieve a significant decrease in its own permeability, and thereby to decrease considerably the flow through the tunnel face. By scale comparison of these two processes, a prognosis about occurrence of increased pore water pressures in saturated sands can be derived. It turns out that the different combinations of penetration rate and revolutions per minute of the cutting wheel, while keeping the advance rate constant, would result in different chance for causing increased pore pressures. Consequently, an excavation strategy for reducing the chance for increased pore pressures is suggested with respect to three reference slurry shields.

The large number of input factors involved in a sophisticated geotechnical computational model is a challenge in the concept of probabilistic analysis. In the context of model calibration and validation, conducting a sensitivity analysis is substantial as a first step. Sensitivity analysis techniques can determine the key factors which govern the system responses. In this paper, three commonly used sensitivity analysis methods are implemented on a sophisticated geotechnical problem. The computational model of a compressed air energy storage, mined in a rock salt formation, includes many input parameters, each with large amount of uncertainties. Sensitivity measures of different variables involved in the mechanical response of the cavern are computed by different global sensitivity methods, namely, Sobol/Saltelli, Random Balance Design, and Elementary Effect method. Since performing sensitivity analysis requires a large number of model evaluations, the concept of surrogate modelling is utilised to decrease the computational burden. In the following, the accuracy levels of various surrogate techniques are compared. In addition, a comparative study on the applied sensitivity analysis methods shows that the applied sensitivity analysis techniques provide identical parameter importance rankings, although some may also give more information about the system behaviour.

The flow behaviour of soil–foam mixtures, used as support medium in closed-mode tunnelling with Earth pressure balance shield machines, is an essential factor for the operation of the tunnel boring machine (TBM). On the one hand, a rather soft consistency is required providing a homogeneous face support pressure transfer to the tunnel face. High accuracy in face support regulation is crucial for settlement control, especially in sensitive environments, such as urban areas. On the other hand, a rather stiff consistency is preferable concerning transportation and disposal of the excavated ground to avoid additional treatments for landfilling, tipping or sewage management. So far, the flow behaviour of soil–foam mixtures has been investigated by index tests. Most notably, the slump test, known from concrete technology, is widely applied on soil–foam mixtures. However, flow is actually a non-static phenomenon and cannot be expressed by a single parameter, which is derived from an equilibrium-state condition at rest. This contribution focuses on the rheology of soil–foam mixtures aiming at a better understanding of the flow behaviour and of the influences the individual components have on it during the excavation process. Investigating the interaction between soil, water and foam provides optimisation strategies for the TBM performance.