Research On The Dynamic Stability Of Yardang Impacted By The Construction Of Railway

Located in the northwest of the Qaidam Basin, Qinghai Province, the new local railway of Yuqia(Hongliu) to Yiliping has a main track of 98.053 km long, with 23.47 km passing through regions with a concentration of yardangs. As the Yardang area is a provincial-level scenic spot, the railway construction will inevitably impact the stability of the Yardangs in this area, which will in turn affect the safety during construction.The yardangs in the area will unavoidably obstruct the railway construction. Bridges and tunnels will be constructed to pass through the region where a large range of yardangs are distributed in a continuous pattern. However, massive earth excavation、construction vibration, and other operations during construction in the yardang area will pose a potential threat to the stability of the landform. Therefore, the research on the engineering condition of yardangs during the railway construction through a safety and stability assessment is the key to ensuring the sustainable, safe, and reliable operation of the railway line.In this paper, the influences of the characteristics of the Yardang area along the railway line and the railway construction on the dynamic stability of the Yardang area were analyzed by means of data collection, site investigation, strong vibration test and surface wave exploration on site, laboratory experiment and finite element method. Based on the investigation of the distribution and pattern of the yardangs along the railway line, the characteristics of ground acceleration attenuation and the shallow(0~30m) underground structure distribution in the Yardang area were obtained through an engineering geological survey. Then the soil dynamics of the yardangs were analyzed through a static/dynamic triaxial test. Based on this, the stability of typical yardangs was calculated before and after the massive earth excavation and construction vibration with 3D finite element numerical analyses. The main conclusions and innovation points of this papers can be summarized as follows.(1)The Yardang area where the railway passes is a typical aeolian landform. These yardangs, primarily small and medium-sized, are composed of lake sediments in the middle and lower Pleistocene Series, and part of the Pliocene and upper Pleistocene Series. The sediments include saliferous mudstone containing argillaceous siltstone, fine sand, and eluvial silty soil. According to their shapes, the yardangs in this area are divided into five categories: prismoid-shaped yardang(quadrangular platform), cone-shaped yardang, long-ridge yardang, multi-peak yardang, and unique shape yardang. According to their sizes, the yardangs are divided into three categories: mega-yardangs, medium-yardangs, and micro-yardangs.(2)There was a strong positive correlation between two morphological parameters, l and w(l: length; w:heigth), of the yardangs along the railway line. Then the least square method was used to fit 40 groups of data, yielding the result of R(l/w)=5.68.(3)Different construction techniques had varying impacts on the yardang landform. The vibration during roller compaction construction had the greatest influence on the landform. The 25 t road roller had an impact on the region within 33 m of the vibration source during construction. In the region more than 33 mm away from the vibration source, where the peak ground acceleration was 16.56cm·s-2, the vibration was weaker than an earthquake of intensity V and thus its influence on the yardangs on the construction site was ignorable.(4)The underground structure in the Yardang area has four horizontal layers and is distributed evenly. The thickness of soft overburden above it was small. The strata deeper than 1.5m showed a rapid increase in Vs and comprised salt deposits and soil, whose strength were relatively great as they were deposited long ago. The construction site in this area was classified as Class II site according to the results of the shear wave velocity and overburden thickness distribution obtained in the survey.(5)The water content of the soil samples collected from the yardangs was measured. It was found that the average water content was only 1.02%, which was consistent with the desert climate of the inland basin featuring low precipitation and high evaporation rate. The static/dynamic triaxial test demonstrated that the yardang soil had high cohesion(Cmax=131.6kPa) and large internal friction angle(φmax=72°). The soil’s maximum dynamic modulus of elasticity was 266 MPa.(6)The numerical calculation indicated that the minimum safety factor of the yardang without soft interlayer is 1.62, the yardang with soft interlayer is 1.48 Under the natural state. Under the construction vibration influence, Yadang’s excavation surface appeared obvious vibration amplification effect and this effect gradually increased along the bottom to the top, the maximum value is 1.78; the minimum dynamic safety factor of the yardang without soft interlayer is 1, the yardang with soft interlayer is 0.8. Horizontal loads is a major influence factor on yardang, the plastic zone appeared at two sides of weathered layer in yardang without soft interlayer; the yardang with soft interlayer concentrate on lower portion of soft interlayer.(7)The yardangs composed of deep-water deposits had relatively high strength and stability and thus were less susceptible to excavation, while the yardangs formed in shallow water showed low stability due to the occurrence of soft interlayers in the deposits. Moreover, the wind erosion, vehicles, and other external factors also affected the overall stability of the landform, especially in the region where the uncemented sand layers were exposed by massive excavation. Therefore, effective measures should be taken to protect the exposed unconsolidated sand layers during excavation.