Prediction Of The Wetting-induced Collapse Behaviour Of Unsaturated Soils

Some natural soils and compacted soils under constant applied stresses show a sudden and dramatic volume decrease when there is an increase in natural water content,these socalled collapsible soils are widely distributed and used as construction material in geotechnical engineering.As the water sources to these soils are increasing,it becomes increasingly important to interpret the collapse mechanism and predict the collapse deformation induced by wetting using simple models.This study aims at addressing the following problems.First,understanding the mechanism to wetting-induced collapse behaviour,with special reference to collapsible loess soils.Second,developing simple models to predict the collapse behaviour.Third,investigating the important information required for reliably predicting the collapse deformation in the field,for example,the wetting front depth.Last,predicting the soil-water characteristic curve(i.e.SWCC)that is important in the proposed models from soil physical properties.In this study,various interpretations and discussions on the wetting-induced collapse behaviour are summarized and critically reviewed in the light of the experimental results reported in the literature,with special reference to loess soils.The microstructure is thought to primarily control the collapse behaviour on behalf of the soil itself.An open,potentially metastable and dual-porosity structure,with high suction or bonding agents at the contacts to stabilise the soil structure at dry condition,is prerequisite for collapse to occur upon wetting.This study introduces the collapsible soil microstructure,with special reference to loess soils,from four elements(including particle pattern,contact relation,pore form and bonding material).Collapse is essentially process of the failure of macro pores and transition from open to a closer structure.Clay and calcium carbonate bonding contributetoresisting structural failure at dry condition and initiating collapse during wetting.Based on the experimental evidence,collapsible soilsare found to experience three distinct phases during wetting(pre-collapse phase,collapse phase and post-collapse phase).In the pre-collapse phase,the soil may collapse slightly,maintain constant volume or swell slightly,the soil structure remains intact.In the collapse phase,the soil undergoes significant volumetric compression with decreasing matric suction;the soil structure alters as a result of bonding breakage and particle rearrangement.In the post-collapse phase,the soil may collapse at the same rate as that in the collapse phase,slow down collapsing or stop collapsing;this phase is in a low suction range.The relationship betweenvoid ratio and logarithmic suction could be linear in each phase,such that the stiffness parameter for changes in matric suction is constant in each phase.Although there are nine possible strain paths for completely describing the collapse behaviour,one with constant-volume pre-collapse and post-collapse phase is suggested for its high possibility to occur in model A.For such a scenario,three parameters are required: critical suction value,s0,stiffness parameter for changes in matric suction in the collapse phase,?s and final suction,sf.s0 could be estimated from initial collapse water content and yield suction.?scould be a linear function of logarithmic net stress,por exponential function of p.sf is assumed to correspond to 85% degree of saturation on the SWCC.Some of the methods for estimating the model parameters are validated using the measured data from the literature.Model B is based on foundation of an equation which was proposed to represent the relationship between percent collapse potential(i.e.PCP)and matric suction.PCP is defined as the ratio between collapse potential at any suction value and total collapse potential,which could be regarded as normalized void ratio.In the equation,fitting parameters m and n are independent of the applied stress,p,the other fitting parameter ? could be a power function of pat a given initial matric suction.In other words,constant m and n and varying ? can reasonably represent the PCP and suction relationships under different applied stresses.To use this model,two suction-controlled wetting tests under different applied stresses are required for the determination of three fitting parameters.The model B is validated using the results of several wetting tests reported in the literature.In order to reliably predict the collapse deformation in the field,the wetting front depth andvariation of water content/suction profile above the wetting front in collapsible soils are required.These information is significantly influenced by environmental factors.For this reason,the wetting front depth and variation of soil water contents are investigated at a site in the Loess Plateau of China,taking account of the influence of environmental factors.The maximum wetting front depth is determined as 2 m;the water contents in unsaturated loess are primarilycontrolled by precipitation and evaporation.Besides,the temperature effect contributes partially to the variation of water contents,especially at high suction values.A numerical model using the software VADOSE/W to simulate the flow behaviour in unsaturated loess taking account of environmental factorsis provided.The SWCC is an important property not only for the estimation of unsaturated soil properties,but also in the proposed models.The experimental methodsfor determining the SWCC arecumbersome and expensive.“One-point” methods are proposed for predicting the SWCCsof natural and remoulded Malan loess from physical properties.The predicted SWCC is expressed in the form of the Brutsaert(1967)equation.The model parameters are suggested to be determined through the best correlations obtained in the present study along with one measured data point.The measured data point should be within the transition zone.The proposed methods are validated using other measured data from the literature.Thisstudy aims at addressing fundamental problems associated with collapse due to wetting.The proposed models and methods are simple for both understanding and use in engineering practice applications.The results of the validationstudies are encouraging as these models and methods can provide reasonable predictions.The results achieved in this study contribute to promoting the application of unsaturated soil mechanics into practice applications associated with collapsible soils.