Experiment Research On Strength Character Of Loess Under Bi-directional Cyclic Loading

Loess and loess-like soil is widely distributed in the central and western region of China,in the history many times high-intnsity earthquake triggered a large number of landslides,vibration depression and othe geological disasters, it make people’s life and property safetylysuffered huge losses. The dynamic load like earthquake has a complex stress state, the sxistingsimplified dynamics research has some limitations, so experimental research on the dynamicstrength characteristics of loess under the complex cyclic stress has the necessity.At present, the domestic dynamic characteristics research under complex bi-directionalcyclic loadings mainly concertrated in sandly soil and species, the related study of loess is less.In this case, the paper based on SDT-20computer control electro-hydraulic servo soildynamic triaxial apparatus, carried out experimental study on dynamic strength characteristicsof loess under different dry density, moisture content, consolidation confining pressure, axialand lateral cyclic stress amplitude, phase difference and so on, attemped to explore thedynamic strength of loess under different stress conditions, provide certain theoretical help forseismic design of engineering.By analyzing test results, we obtained the following conclusions:(1) For simplifying axial cyclic stress dynamic triaxial test, the main factors affectingdynamic strength are consolidation confining pressure, moisture content and dry density ofsoli sample. When the damage critera and failure times are constant, the dynamic strength ofloess increased with the increase of dry density and with the decrease of moisture content;(2)When the lateral cyclic stress amplitude on the remoulded sample we don’t changeconsolidatin confuning pressure, moisture content, dry density, axial cyclic stree, the damagetimes first increases and then decreases with the lateral cyclic stress increases, there isregularity peak intensity inflection point, turning pointe appeared in the lateral cyclic stressamplitude near to80kPa to100kPa.When the lateral dynamic strength amplitude is less thaninflection point, the loess strength value increases with the increase of lateral cyclic stress, thelateral cyclic stress constraint effect; when the lateral cyclic amplitude is larder than turningpoint, the loess strength calue decreases with the increase of lateral cyclic stress amplitude.(3) The dynamic triaxial test contains the phase difference can get some results, thedynamic strength value of loess samples will change when we only change the phasedifference;when the ohase difference changed from0degrees to360degrees, the dynamic strength of loess first reduce and then rising again,when the phase difference is180degrees,the dynamic strength values of loss meet minimum, when the phase fifference is360degrees,the dynamic strength values is basically same with the0degress’.When the axial cyclic stressamplitude is same and lateral cyclic stress amplitude is small, the influence of the phasedegree difference change is not clear, but with the increase of lateral cyclic stress smplitude,the influence of he phase degree difference change will be very significant when the phasedegree change every45degress.(4) Under the same damage standard, the internal friction angle and cohesive force ofloess increase along with the increase of dry density, but they decrease along with the increaseof moisture content; In bi-directional cyclic loadings triaxial test, when the axial and lateralcyclic stress amplitude ratio（σ1d/σ3d） is small, the internal friction angle and cohesive forceof loess decrease along with the increase of damage times; when the axial and lateral cyclicstress amplitude ratio（σ1d/σ3d） is larger, the cohesive force of loess will increase with theincrease of damage times, but the internal friction angle will decrease on the contrary.