Dynamic Instability Mechanism And Quantitative Stability Evaluation Method Of Loess Building Slope

The loess area in northwest China is prone to strong earthquakes.Owing to the influence of the special topography of "Loess tableland,Loess ridge and Loess mound",there is a shortage of construction land,many buildings in loess areas are built on slopes in northwestern China.There are two fundamental reasons for the particularity of dynamic instability of loess building slope.One is that the elevation amplification effect of seismic wave leads to the enhanced dynamic response of the slope and the buildings on the top of the slope,and the other is that the deformation and failure of the slope are further aggravated due to the additional load effect of the building on the top of the slope and the interaction of "soil-structure".Based on the author’s previous experience of earthquake field investigation,the dynamic instability mechanism of loess building slope and the quantitative evaluation method of stability are systematically and deeply studied by using shaking table model test,three dimensional numerical analyses,the acceleration signal processing technology of Wavelet Packet transform and Hilbert Huang transform.The main contents and innovations are as follows:to clarify the amplification and attenuation mechanism of seismic wave along the vertical height and horizontal depth of the slope,to propose the instability index and critical threshold of the loess slope and building system,to quantify the amplification effect of loess slope,and to establish a quantitative evaluation method of stability that gives consideration to the safety of building deformation and slope sliding.The corresponding main research results are as follows:(1)The dynamic instability characteristics of loess building slope and the critical instability state discrimination method are summarized and proposed.Under the action of earthquake,the damage of loess building slope is mainly composed of caused by "the tensile failure at the top of slope,the shear failure at the lower slope and the interreaction area between soil and structure".The special feature of the damage of the cut-fill slope lies in the damage of cut-fill sections.The increase of dynamic soil pressure is the main reason that leads to the damage of cut-fill sections.The critical failure state identification method is given,which takes the peak acceleration,the peak earth pressure,the residual strain of the grid and the proportion of lowfrequency energy as the identification indexes.(2)The dynamic response law of the loess slope and the building on the top of the slope is revealed.Considering the influence of factors such as slope ratio,slope height,the distance between building and slope shoulder,seismic wave intensity and the angle of cut-fill surface,the seismic stability of loess slope and building is systematically evaluated by taking sliding range,safety coefficient,acceleration amplification factor of "soil-structure" interaction part and maximum horizontal deformation between layers of building on the top of slope as evaluation indexes respectively.The amplification and attenuation rules of seismic waves from bottom to top and from inside to outside are clarified.(3)The mechanism of the damage effect of seismic waves on loess slopes is clarified.The variation law of time domain,frequency domain and energy of seismic wave in the propagation process is analyzed.The amplification-deamplification process of seismic wave by loess slope is revealed from the perspective of energy distribution characteristics and time-frequency variation characteristics.The seismic damage analysis of loess building slope is realized.The mechanism of the damage effect of seismic waves in different frequency bands on different parts of the slope during the fluctuation of seismic waves from bottom to top and from inside to outside is revealed.The damage effect of pile foundation as a secondary wave source on the soil body is quantified,and the relationship between the damage degree of the slope and the time-frequency variation characteristics of the acceleration is proposed.The relationship between peak particle velocity and tensile strength is clarified,and the seismic damage discrimination model for loess slopes with PGV as the evaluation index is established.(4)The quantitative relationship of the site amplification effect of loess slop is established.The elevation amplification effect of loess slope is quantified by wavelength.The effective wavelength range and frequency range of elevation amplification effect of loess slope are proposed:0.5 λω<λ<4 λω and 0.25 fω<f<2fω(fω and λω are the natural frequency and natural wavelength of the model,respectively).The relationship between the risk of instability of buildings and the distance from building to slope surface is established,and the safety level of buildings on slopes is divided into three grades with the sliding range of slope as the reference value.