Prediction and interpretation of the performance of a deep excavation in Berlin sand

This paper describes the application of a generalized effective stress soil model, MIT‐S1, within a commercial finite element program, for simulating the performance of the support system for the 20m deep excavation of the M1 pit adjacent to the main station “Hauptbahnhof” in Berlin. The M1 pit was excavated underwater and supported by a perimeter diaphragm wall with a single row of prestressed anchors. Parameters for the soil model were based on an extensive program of laboratory tests on the local Berlin Sands. This calibration process highlights the practical difficulties in both measurements of critical state soil properties and in model parameter selection. The predictions of excavation performance are strongly affected by vertical profiles of two key state parameters, the initial earth pressure ratio, K0, and the in‐situ void ratio, e0. These are estimated from field dynamic penetration test data and geological history. The results show good agreement between computed and measured wall deflections and tie‐back forces for three instrumented sections. Much larger wall deflections were measured at a fourth section and may be due to spatial variability in sand properties that has not been considered in the current analyses. The results of this study highlight the importance of basic state parameter information for successful application of advanced soil models.National Science Foundation (U.S.) (Wester Europe program grant INT-0089508)German Academic Exchange Service (DAAD

Microstructural analysis of sands with varying degrees of internal stability

Internal erosion involves the migration of particles through a geotechnical structure. Internal erosion poses a significant hazard to embankment dams and flood embankments. The fundamental mechanisms operate at the particle scale and a thorough understanding of these mechanisms can inform the filter design and specification process and reduce the hazard that internal erosion is known to pose to many engineered embankment structures. Engineers have long acknowledged the importance of the grain scale interactions, but until recently, explanations of the mechanisms have been purely hypothetical, as direct observation of the internal structure of filters was not possible. Recent research has used the discrete-element method to establish a particle-scale basis for Ke´zdi’s filter internal stability criterion. The discrete-element method can provide significant useful data on soil microstructure, so a discrete-element method model is inherently ideal. This study therefore examines a number of real sand samples with varying degrees of internal stability at the particle scale using high-resolution microcomputed tomography. The correlation between coordination number and internal stability is confirmed, with the coordination number values being significantly higher for the real material

Centrifuge Modeling Studies of Site Response in Soft Clay over Wide Strain Range

Centrifuge models of soft-clay deposits were shaken with suites of earthquake ground motions to study site response over a wide strain range. The models were constructed in an innovative hinged-plate container to effectively reproduce one-dimensional ground-response boundary conditions. Dense sensor arrays facilitate back-calculation of modulus-reduction and damping values that show modest misfits from empirical models. Low-amplitude base motions produced nearly elastic response in which ground motions were amplified through the soil column, and the fundamental site period was approximately 1.0 s. High-intensity base motions produced shear strains higher than 10%, mobilizing shear failure in clay at stresses larger than the undrained monotonic shear strength. The authors attribute these high mobilized stresses to rate effects, which should be considered in strength-parameter selection for nonlinear analysis. This nonlinear response deamplified short-period spectral accelerations and lengthened the site period to 3.0 s. The nonlinearity in spectral amplification is parameterized in a form used for site terms in ground-motion prediction equations to provide empirical constraint unavailable from ground-motion databases. © 2013 American Society of Civil Engineers

Uplift resistance of horizontal strip anchors in sand: a cavity expansion approach

This letter presents an analytical cavity expansion theory-based method for predicting peak uplift resistance of shallow horizontal strip anchors buried in sand. Based on an analytical two-dimensional stress solution for loading analysis around a cylindrical cavity, the method was developed by assuming that the peak anchor uplift resistance can be approximated by the cavity breakout pressure. In the new cavity expansion model, the ultimate failure is reached once the plastic zone develops to the ground surface, and the biaxial state of in-situ ground stresses is taken into account. A database consisting of 75 model tests on shallow strip anchors in sands was compiled to valid the new method. The predicted results and measured data are in reasonable agreement, with a mean over-prediction of the peak uplift resistance by 1.6%. The reliability of the new solution was also checked by comparing with other commonly used analytical solutions. It is shown that the present solution can provide a simple analytical tool for predictions of the peak uplift resistance of strip anchors in sand while a sliding-block failure mechanism dominates

The performance of stochastic designs in wellbore drilling operations

© 2018, The Author(s). Wellbore drilling operations frequently entail the combination of a wide range of variables. This is underpinned by the numerous factors that must be considered in order to ensure safety and productivity. The heterogeneity and sometimes unpredictable behaviour of underground systems increases the sensitivity of drilling activities. Quite often the operating parameters are set to certify effective and efficient working processes. However, failings in the management of drilling and operating conditions sometimes result in catastrophes such as well collapse or fluid loss. This study investigates the hypothesis that optimising drilling parameters, for instance mud pressure, is crucial if the margin of safe operating conditions is to be properly defined. This was conducted via two main stages: first a deterministic analysis—where the operating conditions are predicted by conventional modelling procedures—and then a probabilistic analysis via stochastic simulations—where a window of optimised operation conditions can be obtained. The outcome of additional stochastic analyses can be used to improve results derived from deterministic models. The incorporation of stochastic techniques in the evaluation of wellbore instability indicates that margins of the safe mud weight window are adjustable and can be extended considerably beyond the limits of deterministic predictions. The safe mud window is influenced and hence can also be amended based on the degree of uncertainty and the permissible level of confidence. The refinement of results from deterministic analyses by additional stochastic simulations is vital if a more accurate and reliable representation of safe in situ and operating conditions is to be obtained during wellbore operations.Published versio

Oxidations with permanganate in a strong alkaline medium: Calculation of deprotonation constant

417-422The process 2MnO + substrate -> + 2MnO + product, has been studied employing the substrates methanol, ethanol, n-butanol and formaldehyde in aqueous solutions of [alkali] in the region of 0.1-2.0 mol dm-3. The reactive species is the alkoxy anion and the deprotonation constant (KB) of these substrates can be calculated from the kinetic data, which are in good agreement with literature data obtained by other methods. The procedure seems to be applicable for the determination of the deprotonation constant of other organic substrates containing a hydroxyl group. A mechanism based on electron abstraction from the alkoxy anion and simultaneous nucleophilic attack by OH- has been proposed