The relation between the state indices and the characteristic features of undrained behaviour of silty sand

The characteristic features of a series of isotropically and K₀-consolidated undrained triaxial compression behaviour of a silty sand were investigated for a range of initial void ratio ($e)$ and mean effective confining stress ($p^{\prime }$). The silty sand used in this study contained about 10% natural fines. The critical state line (CSL) of K₀-consolidated specimens, K₀U was slightly lower than the CSL for isotropically consolidated specimens, CIU. The respective CSLs for K₀U and CIU were used to define state indices, such as state parameter ($\psi )$, state index ($I_s$), state pressure index ($I_p$) and modified state parameter (${\psi }_m$), within critical state soil mechanics (CSSM) framework. It was found that each state index exhibit a unique relation with liquefaction potential, irrespective of consolidation type, however different relationships were observed between state indices and the stress ratio at the triggering of liquefaction, ${\eta }_{\mathit{IS}}$ or the liquefaction resistance, $q_{\mathit{IS}}$. The correlation of characteristic features of undrained shearing (i.e., liquefaction potential, ${\eta }_{\mathit{IS}}$ and $q_{\mathit{IS}}$) and drained shearing (maximum rate of volume change, ${\left(d{\epsilon }_v/d{\epsilon }_a\right)}_{\mathit{m}\mathit{a}\mathit{x}}$) with state indices were compared statistically in terms of root mean square deviations (RMSD). All characteristic features of undrained shearing generally showed the best correlation with $I_p$ in term of RMSDs, however $\psi$ showed comparatively wider scatter for the specimens showing flow and limited flow behaviour.     

On the influence of grain shape on the cumulative deformations in sand under drained high-cyclic loading

The cumulative response of three granular materials with significantly different grain shape and surface characteristics (glass beads, natural sand with subrounded grains and crushed sand with very angular particles) but identical grain size distribution curve has been studied in drained cyclic triaxial tests. For each material, several tests with 100,000 cycles and different amplitudes, densities, average mean pressures and average stress ratios have been performed. In case of glass beads and natural sand, an approximately square relationship between the residual strain accumulation rates and stress or strain amplitude was found (${\stackrel{?}{\epsilon }}^{\mathrm{a}\mathrm{c}\mathrm{c}}$ ～ ${\left({\epsilon }^{\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}}\right)}^2$), while an almost proportional dependence was measured for the crushed sand (${\stackrel{?}{\epsilon }}^{\mathrm{a}\mathrm{c}\mathrm{c}}$ ～ ${\epsilon }^{\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}}$). The largest differences in the cumulative response of the three tested materials were observed regarding the pressure-dependence of ${\stackrel{?}{\epsilon }}^{\mathrm{a}\mathrm{c}\mathrm{c}}$. For glass beads and (less pronounced) for natural sand, the residual strain accumulation rates decreased with average mean pressure, while the opposite tendency was obtained for the crushed sand. At small pressures, the residual strains were much larger for the glass beads than for the natural sand and particularly the crushed sand, while these differences in the accumulated strains almost diminished at larger pressures. Independent of the shape and the surface characteristics of the particles, it was confirmed that the average stress ratio is the governing parameter of the cyclic flow rule. Finally, the parameters of the high-cycle accumulation (HCA) model proposed by Niemunis et al. (2005) were analyzed considering the grain shape parameters (aspect ratio, circularity) obtained from an automated grain shape analysis.     

Characterization of large strain consolidation parameters of soft materials with time-variant compressibility at low effective stress

The one-dimensional (1D) large strain consolidation (LSC) of saturated soft materials that are deposited at very low-density usually exhibit time-variant compressibility (void ratio vs vertical effective stress ($e-{\sigma }_{\mathrm{v}}^{\prime }$)) relation. The 1D column-like model test serves as an effective approach for characterizing this consolidation characteristic if all the physical parameters (including the settlement rate, pore pressure and density) are measured. Unfortunately, the density measurement is not always realistic due to its high cost (e.g., with X-rays) and the time-effect must be roughly neglected by using an average compressibility relation. This can further lead to erroneous estimations of the materials’ permeability ($k$) relation (permeability vs void ratio ($k-e$)) in the LSC analysis. This paper presents two modifications on two conventional equations for compressibility and permeability, respectively. The first one describes the compressibility curve’s movement in the $\ln e-\ln {\sigma }_{\mathrm{v}}^{\prime }$ plane, and the other quantifies the ratio between the permeability calibrated by neglecting time-effect and its true value. These modifications originate from deep comparative analyses of several physical parameters between the column test and numerical prediction. Meanwhile, a simple hand-calculation procedure is proposed to estimate the new constants.     

Deformation and cyclic resistance of sand in large-strain undrained torsional shear tests with initial static shear stress

This paper presents the findings from an experimental study focusing on the undrained cyclic behavior of sand in the presence of initial static shear stress. A series of undrained cyclic torsional shear tests was performed on saturated air-pluviated Toyoura sand specimens up to single amplitude shear strain ($\gamma$_SA) exceeding 50%. Two types of cyclic loading conditions, namely, stress reversal (SR) and stress non-reversal (SNR), were employed by changing the amplitude of the combined initial static shear and cyclic shear stresses. The tests covered a broad range of initial states in terms of relative density (Dr = 20–74%) and the initial static shear stress ratio (α = 0–0.30). The following five distinct modes of deformation were identified from the tests based on the density state, the transient undrained peak shear stress, and the combined cyclic and static shear stresses: 1) static liquefaction, 2) cyclic liquefaction, 3) cyclic mobility, 4) shear deformation failure, and 5) limited deformation. Of these, cyclic liquefaction and static liquefaction are the most critical. They occur in very loose sand (D_r ≤ 24%) under SR and SNR, respectively, and are characterized by abrupt flow-type shear deformation. Cyclic mobility occurs under SR in loose to dense sand with D_r ≥ 24%. Contrarily, shear deformation failure typically occurs under SNR in sand with 24 < D_r < 65%, and limited deformation may take place in dense sand with D_r ≥ 65%. In this paper, a stress-void ratio-based predictive method is proposed to identify the likely mode of deformation/failure in sand under undrained shear loading with static shear. Furthermore, the cyclic resistance is evaluated at three different levels of $\gamma$_SA (i.e., small, $\gamma$_SA = 3%; moderate, $\gamma$_SA = 7.5%; and large, $\gamma$_SA = 20%). The results show that, independent of the density state, the cyclic resistance continuously decreases with an increase in α at the small $\gamma$_SA level, while it first decreases and then increases for both loose and dense sand at the moderate and large $\gamma$_SA levels.     

Window ejected flame width and depth evolution along facade from under-ventilated enclosure fires

This paper investigates window ejected flame width and depth evolutions along facade from under-ventilated enclosure fires. Experiments are carried out by 1:4 scale model. Two CCD cameras are employed to record the evolutions of flame depth and width. The flame base position (vertical height above the bottom of the opening), flame depth (width) along with their maximum values and corresponding positions (vertical height above the flame base position) are measured and analyzed by non-dimensional scaling. It is found that the flame base position is independent of fire heat release rate and its ratio to opening height is nearly the same. The flame depth for all heat release rates and the flame width for openings with aspect ratio in the range of $0.5\le W/H\le 1.5$ (the “(semi-) axi-symmetrical flame type”) first increases, reaching a maximum value and then decreases with height. However, for the opening with a relative larger aspect ratio ($W/H=2$) (“wall flame type”), the flame width decreases monotonously with height. The maximum flame width and its corresponding vertical position for “(semi-) axi-symmetrical flame type” are found to be well correlated by characteristic length scale ${\ell }_1$ [=${(A\sqrt{H})}^{2/5}$] non-dimensionally after normalized by the flame height. Meanwhile, the maximum flame depth for all conditions is found to be well correlated by characteristic length scale ${\ell }_2$ [=${(A{H}^2)}^{1/4}$] non-dimensionally after normalized by the flame height.