Analytical solutions for cavity contraction in strain-softening materials with linear or exponential strength decay.

This paper presents an analytical investigation into the contraction of spherical and cylindrical cavities excavated in strain-softening rock masses obeying the Mohr-Coulomb or Tresca yield criterion, with linear or exponential uniaxial compressive strength decay. The derivation of the ground response curves is based on the simplifying assumption that the strains inside the plastic zone are completely plastic. This significantly simplifies the mathematical formulation, enabling the derivation of closed-form solutions. An alternative simplifying approach which partially neglects the elastic strains inside the plastic zone and which is commonly adopted in the literature, is also examined. The accuracy of the simplified solutions is evaluated by comparing their predictions with rigorous solutions obtained by numerical finite-difference analyses. The investigation demonstrates that the proposed closed-form solutions represent a significant improvement on those based on the commonly-made simplifying assumption involving partial neglect of elastic strains.

Effects of Creep on Shield Tunnelling Through Squeezing Ground.

The present work aims to improve the reliability of shield jamming and lining damage risk assessment in squeezing ground by analysing the effects of creep on the evolution of rock pressure over time. The study is based on numerical simulations of typical mechanised tunnelling processes, generally consisting of shield advance phases alternating with shorter or longer standstills for lining installation, maintenance, etc. A linear elastic-viscous plastic constitutive model based upon Perzyna's overstress theory is employed, which considers the time-dependency of plastic deformations via a single viscosity parameter. The investigations demonstrate the following: (i) shield loading during advance increases with increasing viscosity under certain conditions, which contradicts the common perception in many existing works that creep is thoroughly favourable for shield jamming; (ii) creep is thoroughly unfavourable for shield loading during long standstills and long-term lining loading, due to the additional viscoplastic ground deformations manifested over time; (iii) the commonly adopted simplifying assumption of continuous excavation with the gross advance rate is adequate only where standstills are very short (e.g., for lining erection during the stop-and-go shield tunnelling process), but otherwise underestimates the shield loading, even in cases of regular inspection and maintenance standstills lasting only a few hours. Two application examples, the Fréjus safety gallery and the Gotthard Base tunnel, demonstrate the need to consider creep and the accuracy of modelling tunnel construction by a semi-discrete approach, where only the very short standstills for lining erection are considered via an average advance rate, but longer standstills are explicitly simulated.

Large strain correction for tunnel analyses considering hydromechanical coupling and ground anisotropy.

The deformations resulting from tunnel analyses for heavily squeezing ground may be very large, necessitating numerical formulations that consider geometric nonlinearity. Alternatively, for a certain class of problems, routine small strain analyses can be performed, and their results can be corrected to account for large strains by means of a simple hyperbolic expression proposed a few years ago. The present paper shows that this correction equation is sufficiently accurate for practical purposes even in the case of anisotropic material behaviour and for hydromechanically coupled, steady state or transient analyses of tunnels. The accuracy of the equation prediction varies amongst these cases but is satisfactory overall for the purpose of preliminary calculations, thus broadening its value and usefulness as a preliminary design tool.

Dabigatran plasma levels, aPTT and thromboelastography in patients with AF: implications for allowing early non-elective surgical procedures.

According to current recommendations, patients on dabigatran should stop the drug 24-96 h before scheduled surgery. This may seem too long for non-elective cases. The aim of our study was to assess the number of patients on dabigatran who could theoretically undergo surgery 12 h post last drug dosing. We measured dabigatran plasma trough concentration by Hemoclot assay in 75 consecutive patients receiving dabigatran. Coagulation was assessed by aPTT and thromboelastography (TEG). Plasma levels ≤30 ng/ml were considered low. TEG parameters measured were clot reaction time (R), clot growth index (k), angle (α), maximal amplitude (MA) and the percentage of clot lysed after 30 min (LY30). Twelve patients (16%) had low plasma dabigatran levels 11.6 ± 0.9 h post last dosing. These patients compared to those with higher levels had significantly different aPTT (37.7 ± 4.4 vs. 49.6 ± 9.2 s; p < 0.001) and TEG R (6.7 ± 1.3 vs. 8.4 ± 2.6 min; p = 0.002). Only three of the patients with low levels had an aPTT >40 s. Within those with levels >30 ng/ml, four patients (6.4%) had plasma dabigatran levels ≥200 ng/ml, all with aPTT >65 s and TEG R >11 min. When the analysis was restricted to patients with creatinine clearance >80 ml/min, six (27.3%) had low plasma dabigatran levels. In this theoretical study, with a low risk population, it is suggested that one-sixth of patients receiving dabigatran have low drug concentrations at 12 h. Further studies are needed to confirm that such patients with low trough levels can actually undergo safely early surgery if necessary.