Martin Feinendegen

Undrained shear strength of clays as modified by pH variations

The undrained shear strength of clays is an important geotechnical parameter used during construction processes. Several laboratory tests were performed on kaolinite and smectite mixed with pore fluids with different pH values. Vane shear tests were carried out and it was found that the undrained shear resistance for clays increased considerably if the pore fluid had a high or a low pH. A possible explanation could be the dissolution of Al3+ which acts as a coagulant, increasing the internal shear resistance. Geochemical computations, Al measurements and ζ-potential experiments were performed to confirm this theory. The research suggests varying the pH may make a useful contribution to soil improvement techniques.RésuméLa résistance au cisaillement non drainé des argiles est un paramètre géotechnique important utilisé en phase de construction d’ouvrages. Plusieurs essais de laboratoire ont été mis en œuvre sur des kaolinites et des smectites préparées avec des fluides de différents pH. Des essais de cisaillement au scissomètre ont été réalisés et l’on a trouvé que la résistance au cisaillement non drainé des argiles augmente considérablement si le fluide interstitiel a un pH faible ou fort. Pour un pH faible, une explication possible pourrait venir de la dissolution des Al3+ qui agissent comme des coagulants, augmentant la résistance au cisaillement. Des calculs géochimiques, des mesures de teneur en Al et de potentiel ζ ont été réalisés pour confirmer cette théorie. Les résultats de cette recherche suggèrent que le fait de faire varier le pH de l’eau interstitielle peut contribuer utilement à l’amélioration de la résistance des sols.

The XFEM with an Explicit-Implicit Crack Description for Hydraulic Fracture Problems

The Extended Finite Element Method (XFEM) approach is applied to the coupled problem of fluid flow, solid deformation, and fracture propagation. The XFEM model description of hydraulic fracture propagation is part of a joint project in which the developed numerical model will be verified against large-scale laboratory experiments. XFEM forms an important basis towards future combination with heat and mass transport simulators and extension to more complex fracture systems. The crack is described implicitly using three level-sets to evaluate enrichment functions. Additionally, an explicit crack representation is used to up‐ date the crack during propagation. The level-set functions are computed exactly from the ex‐ plicit representation. This explicit/implicit representation is applied to a fluid-filled crack in an impermeable, elastic solid and compared to the early-time solution of a plane-strain hy‐ draulic fracture problem with a fluid lag.

Relationship between measured plastic limit and plastic limit estimated from undrained shear strength, water content ratio and liquidity index

Abstract The detection of the plastic limit of clays is subject to human error. Several attempts have been made to correlate across studies the geotechnical properties of fine-grained soils (water content, liquidity index, shear strength, etc.). Based on the premise that the liquidity index and water content ratio can be correlated directly, an alternative method to obtain indirectly the plastic limit is suggested here. The present study investigated 40 natural clayey samples of various mineralogies and origins and other publicly available data, where Atterberg limits and undrained shear strength values obtained with the vane shear tests were given. The liquidity index and water-content ratio correlate very well for defined undrained shear strength values of the clays. Solving the liquidity index equation for the plastic limit, estimated plastic limit values obtained by the liquidity index/water-content ratio relationship were compared with laboratory plastic-limit values. Preliminary results based on 62 values show an exponential trend with a multiple regression coefficient of 0.79. The data need to be confirmed on a larger database, however.

Geologische CO2-Speicherung: Vergleich unterschiedlicher Kopplungsansätze für die hydraulische Reaktivierung von Störzonen

Die numerische Simulation einer geologischen CO2-Speicherung stellt aufgrund ihrer Formulierung als Mehrphasenflussproblem einen erheblichen Aufwand in der Modellierung dar. Bei der Kopplung von Ergebnissen aus geomechanischen Verformungsberechnungen und hydraulischen Simulationen kommen hydro-mechanische Einweg- und Zweiwegkopplungen zum Einsatz. Einwegkopplungen liefern haufig zu ungenaue Ergebnisse. Zweiwegkopplungen hingegen sind aufgrund ihres iterativen Charakters fur Risikoanalysen, in denen mehrere hundert Szenarien betrachtet werden, sehr aufwandig. In diesem Beitrag wird basierend auf einer breit angelegten Parameterstudie ein Kriterium fur die Notwendigkeit der Anwendung einer Zweiwegkopplung dargestellt und ein neuer Ansatz fur eine hydro-mechanische quasi-Zweiwegkopplung von Storzonen erlautert.

“CO2RINA”—CO2 Storage Risk Integrated Analysis

While risk assessment for CO2-storage often has been conducted by using a lot of simplifications and conservatisms, our approach developed in the CO2RINA-research project is based on the integration of all models, existing at a moment in time. These models will be coupled by the so called transfer function approach which has been proven to be very powerful in the risk assessment for low level radioactive waste. This concept ensures, that the risk assessment is always consistent with the state of the models existing for a specific site. It can be immediately improved if the existing models will be improved over time. The approach was verified by comparison of the direct coupling of different process models in an overall model with a coupling by transfer functions by conducting a wide range of test calculations showing very good accuracy of the approach. The new coupling approach allows the incorporation of a variety of additional effects which are difficult to handle in an overall model. Examples for such processes are complex chemical and microbiological interactions, geomechanical feedback loops and migration of CO2 in the atmosphere. In the project there have been developed specified models for a generic site with parameters similar to the Ketzin site. These models include a reservoir model, a model for the alteration of the cementation of a mature well, a fault model, a model describing advection and diffusion through the cap rock, a complex model for the migration in a Quaternary aquifer including complex chemical interactions and a geomechanical model. Additionally there was shown the way of integration of microbiological processes which have been modelled in detail in the CO2BIOPERM project. The new approach is ready to be adapted to a specific CO2-storage site.

Mineralogical, Mechanical, and Electrical Properties of Clays and Their Relation to Clogging during Mechanical Tunnel Driving

One of the problems that can occur while boring through fine-grained soil is the adherence of clay to the EPB cutting wheel, inside the excavation chamber or in the transportation line, which might lead to long delays. Since the economic success of a tunneling project depends highly on the clogging potential of the encountered clay formations, it is of vital importance to understand this phenomenon and find possible solutions. Currently, there is no effective soil conditioning technique based on scientific data. The aim of this paper is to show how the mineralogical properties of clays play an important role for the mechanical and adhesive response. Mechanical laboratory tests, zeta (ζ) potential measurements, calculations, and clogging tests were performed to link the adhesive properties with the clay mineralogical composition.