Study On The Water-sensitive Characteristics And Disasters Of Loess

Loess Plateau is located in the arid and semi-arid climatic region of the Northern Hemisphere,and agriculture is the main econimic basis for the living people in this region.The extensive irrigation is inevitable here,for the conflict between poor climatic conditions and the growing socio-economy.Geohazards triggered by irrigation are frequent,extensive,complex and disastrous,thus endangering the people's lives and property.The mechanism of the water-induced geo-hazard is closely related to the water sensitivity properties of its mechanical response.Supported by the national basic research program of China: Occurrence and evolution mechanism and control theory of major disaster and disaster chain in loess,this paper selects South Jingyang tableland of Shaanxi province as the main research region,and Lishi loess taken from this area as the research object.The technical measures including field surveys,basic soil tests,and microscopic detection tests were used to study the water-sensitive mechanical and catastrophic mechanism of loess from micro,meso,and macro scales,then the quantitative parameters of water sensitivity as well as evaluation system of catastrophe evaluation system were put forward.Combined with the geo-mechanical model of the loess slope in South Jingyang tableland,the catastrophic effects of loess water sensitivity on slope slipping were revealed.What we had done provides an effective method for the quantified of water sensitivity properties,and the comprehensive evaluation of its catastrophe.The main research results of this paper are as follows.(1)The unique engineering geological environment of South Jingyang tableland provided good conditions for the development of loess landslides.First of all,unreasonable pumping irrigation activities were triggers for landslides.Secondly,the high and steep slopes with a free surface were the main body for the occurrence of landslides.And finally,the deep loess layer with fragile structure and its interaction with water became the main reason for slope instability.(2)The sensitivity of soil structure to water is the main reason for the water sensitivity of loess.For the mechanical curves of water-sensitive loess can be normalizedby structural yield stesses.Based on the micro-structure analysis of natural loess,the geological model of loess can be summarized as a parallel connection of a rigid body(r)and a friction body(f).The r-f model stores a certain comprehensive structural potential,and the water-sensitive loess can be described as a series connection of r-f models.The greater the comprehensive structural potential of a soil,the stronger its water sensitivity.(3)The sensitivity of the loess mechanical response to water can be quantified by the water sensitivity index.The water sensitivity index is defined as the degree of decrease in the strength of loess when it is humidified to saturation,and the value is affected by the clay content and its distribution in the loess.The engineering catastrophe caused by the water sensitivity of the loess includes loess collapse,water immersion shear,and liquefaction.Combining the water sensitivity index and the collapsibility coefficient,water immersion shear deformation coefficient,and pore pressure ratio,a comprehensive evaluation system for assessing the disaster sensitivity to water under compression and shear conditions can be obtained.(4)According to the geo-mechanical model of slope and the stress path of homogeneous loess,the slip mechanism of loess slope induced by irrigation can be summarized as,cracking effect at the top of the tableland caused by collapsibility,softening effect at the toe of the slope triggered by water immersion shear,and high speed long-runout effect initiated by liquefaction.And the field investigations on slope geological structures,landslides development features and movement characteristics of the south Jingyang tableland confirmed these.(5)The loess landslides induced by irrigation in south Jingyang tableland include loess slides and loess flows.Loess slides developed here possessed the characteristics of low-speed and short-ranout.And the disaster evolution can be listed as water immersion shear deformation of loess-shear deformation in unsaturated loess-strain hardening-stop sliding.Meanwhile,loess flows developed here are featured by high-speed and long-runout,and the evolution process of it can be described as static liquefaction-slope sliding-shear liquefaction-high speed and long runout loess flow.