David Mašín

3D Modeling of an NATM Tunnel in High K0 Clay Using Two Different Constitutive Models

This paper studies the accuracy of the three-dimensional finite-element predictions of a displacement field induced by tunneling using new Austrian tunneling method (NATM) in stiff clays with high K0 conditions. The studies are applied to the Heathrow express trial tunnel. Two different constitutive models are used to represent London Clay, namely a hypoplastic model for clays and the modified Cam-clay (MCC) model. Good quality laboratory data are used for parameter calibration and accurate field measurements are used to initialize K0 and void ratio. The hypoplastic model gives better predictions than the MCC model with satisfactory estimate for the displacement magnitude and slightly overestimated width of the surface settlement trough. Parametric studies demonstrate the influence of variation of the predicted soil behavior in the very-small-strain to large-strain range and the influence of the time dependency of the shotcrete lining behavior.

Effects of Pillar Depth and Shielding on the Interaction of Crossing Multitunnels

AbstractAny new tunnel excavation may damage adjacent existing tunnels in congested cities. To evaluate the impact of new tunnel construction on nearby existing tunnels, a series of three-dimensional centrifuge model tests were conducted with numerical backanalyses using an advanced hypoplasticity constitutive model. The influences of the pillar depth-to-diameter ratio (P/D) on two-tunnel interaction and the effects of shielding on three-tunnel interaction were investigated. The maximum measured settlement of an existing tunnel caused by a new tunnel excavation at P/D of 0.5 underneath was approximately 50% larger than that when P/D was 2.0. This is attributed to a smaller mobilized shear modulus, resulting from a larger reduction in confining stress of soil acting on the invert of the existing tunnel in the former than in the latter. When the new tunnel was excavated underneath two perpendicularly crossing tunnels, the lower existing tunnel “shielded” the upper one from the influence of tunnel excavation...

Coupled Thermohydromechanical Double-Structure Model for Expansive Soils

AbstractIn this paper, development of a thermohydromechanical model for expansive soils including double structure is described. The model is based on the previously developed double-structure hypo...

Thermal Water Retention Characteristics of Compacted Bentonite

Compacted bentonites are popularly being considered as buffer or backfill material in high level nuclear waste repositories around the world. These bentonites may undergo various conditions including hydration from around geo-environmental water and heating by the radiation of the used fuel during its period of operation. Water retention properties of as-compacted Czech bentonite B75 with three initial dry densities (1.27 g/cm3, 1.60 g/cm3 and 1.90 g/cm3) and bentonite powders were investigated within temperature 20–80 °C at unconfined conditions. Vapor equilibrium method was used to control constant relative humidity. The influence of temperature on water retention properties was analyzed and discussed. Results show that the temperature decreased water retention capacity for all cases. The water retention capacity is lower at high temperature especially at lower suction. The temperature has more significant effect on drying path than wetting path. Furthermore, the volume swelling decreased with the increased temperature upon saturation. The hysteretic behavior decreased with the increase of temperature for all studied materials.

Observed and calculated gravity anomalies above a tunnel driven in clays – implication for errors in gravity interpretation

The aim of this work was to evaluate the gravity effect of a deformation zone, which forms around subsurface cavities and to assess which method of density determination of soils in the surveyed area is optimal. We measured and modelled a gravity anomaly above a tunnel excavated in stiff clays. Technical parameters of the tunnel were known. The shape of the deformation zone around the tunnel was determined by geotechnical modelling. Material parameters of clays necessary for the geotechnical model were obtained from a measurement of borehole samples in a soil mechanics laboratory. For calculation of the gravity anomaly of the tunnel we used densities of clays derived from laboratory measurements of borehole samples. However, the fit between observed and calculated gravity data was not accurate in this case. The reason was probably that the volume and water content in the borehole clay samples changed after their removal from the depth. A better fit was obtained when we used densities derived from a gamma-gamma well log. We found that the gravity effect of the deformation zone that forms around the tunnel in stiff clays is negligible (0.001 mGal). In our case, a concrete tunnel lining has a bigger influence on the amplitude of the anomaly than the deformation zone (0.008 mGal). When we neglect the lining in the interpretation, we obtain an error of 20% in the estimate of the tunnel depth.

Modelling of Deep Excavation Collapse Using Hypoplastic Model for Soft Clays

The extended abstract presents an application of the newly developed hypoplastic model for soft clays. The model is applied to simulation of the well-documented geotechnical failure of a deep excavation occurring at the Nicoll Highway in Singapore. The marine clays occurring at the site are typical example of natural normally consolidated soft soils, which are difficult to characterize and model as they typically suffer from large deformations under loading and their stress-strain behavior is complex. In the new hypoplastic model, anisotropic asymptotic state boundary surface (ASBS) has been implemented to account for soil strength anisotropy. The apparent rotation of the ASBS is implemented by skewing the stress space. Additionally, the tensor L was made bilinear in D to more realistically predict the stress path of natural K0 consolidated soils. The model has been implemented in the form of a user-defined subroutine for Abaqus and Plaxis finite element software. Firstly, it is demonstrated that the model predictions compare well with experimental data on element tests, demonstrating the effect of various features of the model on predictions. Secondly, numerical simulations of Nicoll highway excavation focusing on specific cross-section within the collapse area are presented, demanding good fit between measurement and simulations up to the final stages immediately preceding the excavation collapse.

Experimental Study on Highly Compacted Bentonite Aggregates Subjected to Wetting and Drying

Microstructure of compacted bentonite is an important aspect which controls its hydro-mechanical behavior, also is vital in constitutive modelling. In order to understand this mechanism, the wetting and drying behavior of compacted bentonite with initial dry densities of 1.9 g/cm3 was investigated by environmental scanning electron microscopy (ESEM). The attempt of quantitative interpretation was done by digital image analysis at aggregates level (in micro meters). The macro pores between aggregates were clearly sensitive to change of suction, while there were only small volume changes of the aggregates. Volumetric strain of the measured aggregates varied. After one wetting-drying cycle, some new macro pores were formed and the hysteretic behavior of inter-aggregate pores was observed.

Modelling of Spudcan Foundation Penetrations Using an Improved Hypoplastic Model for Soft Clays

In this paper, a new hypoplastic model for K0 consolidated soft clays is presented. The enhanced model is characterized by a small number of parameters, adding two parameters to the original model, whilst significantly improving model predictions of undrained stress paths of K0 consolidated soils. However, the calibration procedure remains simple – model can be calibrated using K0 consolidated triaxial tests and oedometer only. It is demonstrated that the model predictions compare well with element test experimental data. The validation of the model involves large scale boundary value problem simulations incorporating very large deformations, in this case retrospective simulations of centrifuge test of spudcan installation. The enhanced model shows better performance compared to a parent hypoplastic model.

Dynamical In Situ Study of Morphological Changes of Bentonite in ESEM

The bentonite has a variety of industrial applications. It is used as a stabilizing and suspending agent, an adsorbent and clarifying agent, a buffer material for barriers in geological disposal facilities for radioactive waste, liners for remediation of contaminated soils and ponds. Characterization of the morphological changes of the bentonite during the process of swelling and shrinkage is very important for these applications [1-3]. The environmental scanning electron microscope (ESEM) allows observation under the water vapour pressure from units to thousands of Pa and at a selected temperature from -20 ° C to 1500 ° C. Moreover, these samples do not have to be treated, namely dried and covered with the conductive metal layer [1, 4]. ESEM is also very effective for in-situ investigation of bentonites, which can be realised under dynamically changing relative humidity (RH).

SHEAR STRENGTH OF SOILS FROM THE DOBKOVICKY LANDSLIDE IN THE CENTRAL BOHEMIAN UPLANDS DETERMINATED BY LABORATORY TESTS

The aim of this paper is to evaluate soil shear strength from the Dobkovicky landslide. The landslide was activated on June 6, 2013 after heavy rains and the D8 motorway, which was under construction at the time, was damaged. The laboratory tests were carried out on two types of soils, clay and tuff, both from the surface of the rupture. Critical and residual friction angles were evaluated on both types of soils.