| A large number of loess flowslides have occurred in Heifangtai,Gansu Province and south Jingyang platform,Shaanxi Province in the Loess Plateau region of China,after human resettlements and engineering activites.Loess flowslide typically develops at the saturated base of the loess slope,which are induced by the decrease or loss of effective stress due to excess pore water pressure.The development of loess flowslides are characterized by loose soil particles,saturated base,rapid deformation and initiation of failure,and the high-speed and long runout with fluidized and liquified movements.Loess flowslide poses a great threat to the safety of the local community,and it accounts for 29%of the landslides in the Heifangtai Plateau and 42%in the South Jingyang Plateau.The study of deformation and influencing factors of static liquefaction in loess of great significance to the prevention and treatment of loess slopes in the Loess Plateau of China.This study focuses on the large-strain behaviours of the sandy loess(sampled from Shenmu),silty loess(from Heifangtai),and clayey loess(from Jingyang)in the Loess Plateau of China.On the basis of field investigation and testing of basic physical and mechanical parameters,mineral composition and natural structure of the sample,the liquefaction ability of the undisturbed loess in the three study areas was first studied by triaxial ICU test(macroscopic difference);Secondly,the effects of structure on the susecptability of static liquefaction of loess is studied by comparing the the results of the undisturbed and the reconstituted loess samples.The effects of natural pore ratio on the susecptability of static liquefaction of loess is studied by applying different confining pressures to the reconstituted samples in the same area.The article size distribution on the susecptability of static liquefaction of loess is studied by remodeling samples with different sand content(microscopic influencing factors).And then the experimental results have indicated that the modified state parametercan be used to quantitatively characterize the susecptability of the static liquefaction of loess.Finally,The post-liquefaction microstructure in loess is also studied by scanning electron microscopy(SEM).The main conclusions are as follows:(1)The physical parameters and mineral composition of loess vary depending on the sampling location in the Loess Plateau of China.Because of the high mineral composition,the fabric of the clayey loess is relatively close,and the skeletal morphology is mainly the aggregates form of fine clay particles with more pronounced cementation effects.On the other hand,the effects of cementation on sandy loess are relatively weaker due to the higher sand content,and the skeletal morphology is mainly the single particles.Thus the skeletal morphology in loess are predominately in the form of single particle or cementated aggregate.(2)The particle roundness of the reconstituted loess sample is greater as compared to the undisturbed samples,and the particle size as well as pores are more uniformly distributed.The clay mineral content and cementation are both reduced by disturbance.With the increase of sand content in remolded samples,the loess sample is mainly composed of large particles,with less uniformly distributed particle size and pores as well as reduced particle cementation.The skeletal morphology is dominated by a single granular particle.(3)The mechanical behaviours of undisturbed loess in different regions is vastly different in undrained shear tests.The gradient of critical state line(CSL),M of the undisturbed loess varies in q-p'plane depending on the sampling location.The location and shape of the CSLs of undisturbed loess in e-Inp plane is different,indicating that the potential liquefaction zone and non-liquefaction zone of loess in different regions are different.The CSL of the Heifangtai loess exhibits nonlinear characteristics.(4)The structure of loess plays an important role in the static liquefaction of loess.In the q-p'plane and e-Inp plane,the instability line(IL)and the critical state line(CSL)of undisturbed loess and reconstituted loess are different lines,segmenting the"instability zone"and"potential liquefaction zone"of the two soils in these two planes.The change of the modified state parameter,incidates that the structure of loess can either facilitate or impede the liquefaction at different states of the soil.The results shows that when the initial void ratio is similar,the liquefaction susecptibility of undisturbed loess and reconstituted loess under the same confining pressure are quite different.The liquefaction susecptibility of reconstituted loess is greater than that of undisturbed loess under the low stress condition of less than 100 kilopascal.With the increase of the average principal stress,the liquefaction susecptibility of undisturbed loess is larger than that of reconstituted loess.(5)The different natural void ratios of loess(i.e.different depth in the loess slope)will lead to great differences in the liquefaction susecptibility.(6)Particle size distribution affects the static liquefaction of loess.Under natural grading,the overall liquefaction susecptibility of loess in Heifangtai is the highest,while loess in Shenmu area has little or no liquefaction suspectibility.The authors observed that the liquefaction susceptibility of loess with different grading were different under similar initial void ratio,and the liquefaction susceptibility was non-linear with sand content.The reconstructed soil sample with the closest grading to the intact Heifangtai loess has the highest liquefaction suspectibility,and the particles of reconstructed soil sample are the loess partciles sampled from the Shenmu area,which was found with the lowest static liquefaction suspectibilty under natrual grading.Therefore,the high concentration of loess flowslide in Heifangtai is due to the saturation at the base of the loess layer caused by irrigation,and the contributing factor is that the particle size distribution of the Heifangtai loess is more suspectible to static liquefaction.(7)According to the experimental results,the static liquefaction characteristics of loess are classified into three categories by modifying the state parameter value?_m:?_m>0.035:Liquefaction or flow instability;-0.005??_m?0.035:Limited or temporary flow instability;?_m<-0.005:Nonflow behavior.where ?_m represents the modified state parameter that is used as a criterion to characterize the static liquefaction susecptability of the Chinese loess.The rationality of this criterion has been verified by multiple groups of loess with different properties and particle size distributions.(8)Liquefaction changes the microstructure of loess.Soil particles are disaggregated after liquefaction,and particles becomes more closely packed with decreasing void ratio.According to the PCAS results,the content of fine particles less than 100um~2 increases significantly after liquefaction,while the content of coarse particles decreases significantly.Through the comparative analysis of particle geometry parameters,it can be seen that the particle shape of liquefied soil has changed,and the sorting of particles has become less graded,and the complexity of morphology has increased. |
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