Some investigations to quantify hysteresis associated with water retention behaviour of fine-grained soils

The soil water retention characteristics curve (SWRC) has been reported to be quite useful for estimation of unsaturated soil properties. However, the uniqueness of SWRC is questionable due to hysteresis associated with the drying- and wetting-path SWRCs and this poses great challenge in utilising the SWRC for reliable estimation of unsaturated soil properties. Although hysteresis associated with SWRCs has been extensively studied for coarse-grained soils, due to limited studies on wetting-path SWRC for fine-grained soils, the hysteresis for fine-grained soils is not well understood. The present work attempts to address this gap, by studying the drying- and wetting-path SWRCs for eight different fine-grained soils by employing Dew point Potentiameter (WP4C®), Environmental Chamber and Controlled Water Sprinkling method. The study employs the concept of ‘Suction Hysteresis’, ψ_h, for quantification of hysteresis. Further, the influence of various soil-specific properties on the variation of ψ_h-water content relationship (viz., slope of variation of suction hysteresis, S_ψh) has also been studied and demonstrated. The findings of the study are quite encouraging and it has been realised that extensive studies on soils of different characteristics would be quite useful in quantifying the variation of SWRC during drying and wetting cycles.     

Cyclic shearing and liquefaction of soil under irregular loading: an incremental model for the dynamic earthquake-induced deformation

The paper presents a mathematical model for the deformation of soil under irregular cyclic loading in the simple-shear conditions. The model includes the possible change in the effective pressure in saturated soil due to the cyclic shearing, the reciprocal influence of the effective pressure on the response of the soil to the shear loading, and the pore pressure dissipation due to the seepage of the pore fluid. The hysteresis curves for the strain–stress relationship are constructed in such a way that they produce both the required backbone curve and the required damping ratio as functions of the strain amplitude. At the same time, the approach enables the constitutive functions involved in the model to be specified in various ways depending on the soil under study. The constitutive functions can be calibrated independently of each other from the conventional cyclic shear tests. The constitutive model is incorporated in the boundary value problem for the dynamic site response analysis of level ground. A numerical solution is presented for the dynamic deformation and liquefaction of soil at the Port Island site during the 1995 Hyogoken-Nambu earthquake.     

Listen to the sound of moving sediment in a small gravel-bed river

In order to assess the dynamics of rivers, a reliable characterization of bedload transport particularly during unsteady flow regimes is required. In contrast to highly energetic cases in hillslope areas, we aim to answer the question whether the usage of acoustic measurements can improve the characterization of bedload in small rivers draining low land mountains with comparatively low water discharge and bedload. In addition to the investigation of natural flood events, controlled floods were generated by releasing water from a reservoir into a small gravel-bed stream. The controlled releases allow for an evaluation of bedload solely from channel storage or bank erosion. For acoustical in-situ characterization of bedload transport, hydrophones were mounted onto the bottom side of steel plates, thus recording the impacts of sediments via the acoustic vibrations on the surface of the plates while at the same time minimizing the disturbing noise resulting from water turbulence. Corresponding bedload traps are removable boxes with open lids fixed in the riverbed so that bedload material registered by the hydrophone is trapped. The acoustic signals correlate well with the quantity of the transported material. During summer flood events the highest transport rates occur at the beginning of the rising limb fea-turing clockwise hysteresis. This is due to the rising transport energy of the flow and the presence of loose, unconsolidated material. During typical winter flood events bedload shows anticlockwise loops. The intensification of bedload conveyance after the runoff peak can be explained by a decreasing stability of the bed material from the beginning to the end of a transport event. Anticlockwise behavior also results from a combination of bedload exhaustion in the vicinity of the monitoring station with a delayed arrival of new material from distal sources later in the hydrograph.     

Reinforced concrete scaled columns under cyclic actions

In the area of reduced-scale model testing, increasing the size of model structures as a means of improving the model performance is often inhibited by the capacity limitation of the available seismic testing facilities. This was the case in a recent experimental programme conducted at NTUA in which several 1:5.5-scale model frames were tested on an earthquake simulator. In order to improve the model reliability, the following measures were taken: strictly obeying the geometric scaling in almost all aspects; using deformed bars as model reinforcement; and using stone-aggregate microconcrete for constructing the models, etc. To evaluate the effectiveness of these measures, a parallel column test programme was conducted in which duplicates of typical columns in the test model frames were prepared, along with those of large sizes following similitude laws. The column specimens were tested under reversals of lateral load. Besides the scale effects, general rules regarding the influences of transverse confinement and axial force on the ductility and strength of columns were also confirmed on the small-scale model columns.     

Earthquake parameters affecting the performance of an RC frame with friction damper

In this paper, correlation between the ground motion parameters and the peak displacement demand of a reinforced concrete single degree of freedom system retrofitted by friction dampers is investigated. The aim is to reveal the best parameters that are used in selecting earthquake records for nonlinear dynamic analysis of the system. Performing numerous nonlinear time history analyses, it is observed that different earthquake parameters play role in peak displacement demand related to the ranges formed by the different periods of a reinforced concrete frame, the strength ratio of total system at slip displacement and the stiff/soft soil profiles. For the both soil types, out of sixteen ground motion parameters, the eight, Spectral Response Acceleration, Peak Ground Acceleration, Effective Peak Acceleration, Maximum Incremental Velocity, Peak Ground Velocity, Arias Intensity, Arias Intensity-based Parameter and Effective Peak Velocity correlate better with peak displacement demand.     

Impact of capillary hysteresis and trapping on vertically integrated models for CO2 storage

Vertically integrated models are frequently applied to study subsurface flow related to CO₂ storage scenarios in saline aquifers. In this paper, we study the impact of capillary-pressure hysteresis and CO₂ trapping on the integrated constitutive parameter functions. Our results show that for the initial drainage and a subsequent imbibition, trapping is the dominant contributor to hysteresis in integrated models. We also find that for advective processes like injection and plume migration in a sloped aquifer the correct treatment of the hysteretic nature of the capillary fringe is likely of secondary importance. However, for diffusive/dispersive processes such as a redistribution of the CO₂ plume due to buoyancy and capillary forces, the hysteretic nature of the capillary fringe may significantly impact the final distribution of the fluids and the timescale of the redistribution.     

A novel approach for modelling soil–water characteristic curves with hysteresis

Soil–water characteristic curves can be defined as the relationship between the degree of saturation and suction of an unsaturated soil. Geomaterials, such as clays, sands, and geotextiles, usually exhibit hysteresis between drying and wetting curves. In addition, each drying and wetting curve is nonlinear in shape, which may be approximated by sigmoid curves. In geotechnical engineering, it is common to adopt analytical expressions for these curves that must be calibrated iteratively by trying different values for the constitutive parameters. In this paper, a novel approach for modelling the nonlinear saturation–suction response with hysteresis is presented, where a simple differential equation is introduced to describe the shapes of the curves. The great advantage of this new technique is the ease with which the parameters can be determined. In addition, the implementation of the resulting equations into fully hydro-mechanical models for numerical analyses is straightforward. Some features of the behaviour predicted with the new representation are studied and validations against real laboratory curves for soils are presented. The technique is simple, yet versatile due to the rational basis used in the deduction of the equations, which allows for future extensions to soils displaying more complex unsaturated behaviour.     

Analysis of nonlinear site response using the LSST downhole accelerometer array data

Both linear and nonlinear behaviors of soil deposits were evaluated by strong and weak motion data observed on the surface and at depths of 6, 11, 17, 47 m at the Large Scale Seismic Test (LSST) array in Lotung, Taiwan. The soil properties measured by well logging and by the shear wave velocity profile measured by uphole and cross-hole methods are available. Both one-dimensional equivalent-linear method and nonlinear method are used for the evaluation have been used. The synthetic records at various depths are obtained by using the records at the bottom as input motion. These synthetic records are then compared with actual records at corresponding depths. Records of 13 earthquakes are used. We find that the synthetic records obtained from a linear model match well with actual records for small input motions, but the results obtained from a nonlinear model match poorly. On the other hand, the synthetic records using both the nonlinear model and equivalent-linear model are in good agreement with the observed records for large input motions. In these cases, the predicted response spectra using the linear model consistently overestimate the observed records. The threshold distinguishing the large and small input motions is 0.04 g at depth of 47 m for the LSST data. Thus, the nonlinearity started at 0.04 g and occurred unequivocally at 0.075 g. Furthermore, the dominant frequencies shift toward lower values when input motions become large. Clearly, the observed records at the LSST site manifest nonlinearity of soil response. The hysteresis loops evaluated by the nonlinear method show a permanent strain of about 0.01% in soil layers at higher ground motion input levels in this case.