Current situation and development trend of design methods for subgrade structure of high speed railways

Purpose–This method will become a new development trend in subgrade structure design for high speed railways.Design/methodology/approach–This paper summarizes the structural types and design methods of subgrade bed for high speed railways in China,Japan,France,Germany,the United States and other countries based on the study and analysis of existing literature and combined with the research results and practices of high speed railway subgrade engineering at home and abroad.Findings–It is found that in foreign countries,the layered reinforced structure is generally adopted for the subgrade bed of high speed railways,and the unified double-layer or multi-layer structure is adopted for the surface layer of subgrade bed,while the simple structure is adopted in China;in foreign countries,different inspection parameters are adopted to evaluate the compaction state of fillers according to their respective understanding and practice,while in China,compaction coefficient,subsoil coefficient and dynamic deformation modulus are adopted for such evaluation;in foreign countries,the subgrade top deformation control method,the subgrade bottom deformation control method,the subsurface fill strength control method are mainly adopted in subgrade bed structure design of high speed railways,while in China,dynamic deformation control of subgrade surface and dynamic strain control of subgrade bed bottom layer is adopted in the design.However,the cumulative deformation of subgrade caused by train cyclic vibration load is not considered in the existing design methods.Originality/value–This paper introduces a new subgrade structure design method based on whole-process dynamics analysis that meets subgrade functional requirements and is established on the basis of the existing research at home and abroad on prediction methods for cumulative deformation of subgrade soil.

Earth Pressure of Retaining Structure Induced by Subgrade under Rainfall

This article selects the retaining wall as the research object, introducing the rainfall infiltration model, considering the infiltration of rainwater into the groundwater recharge, analysizing the variation of earth pressure in the subgrade retaining wall. On this occasion, the back of retaining wall produces stable seepage water and compares with the non drainage water body. The results show that, with the infiltration of rainwater into the groundwater recharge, the greater the active earth pressure under the condition of rainfall appears, more quickly the active earth pressure of the retaining wall with the drainage body increases. The matrix suction of unsaturated soils, which is infiltrated into soil of subgrade, has a positive effect on the shear strength of the earth pressure.

Analysis of Earthquake-Triggered Failure Mechanisms of Slopes and Sliding Surfaces

Earthquake-induced landslides along the Dujiangyan-Yingxiu highway after the Ms 8.0 Wenchuan earthquake in 2008 were investigated. It was found that： （1） slopes were shattered and damaged during the earthquake and open tension cracks formed on the tops of the slopes; （2） the upper parts of slopes collapsed and slid, while the lower parts remained basically intact, indicating that the upper parts of slopes would be damaged more heavily than the lower parts during an earthquake. Large-scale shaking table model tests were conducted to study failure behavior of slopes under the Wenchuan seismic wave, which reproduced the process of deformation and failure of slopes. Tension cracks emerged at the top and upper part of model, while the bottom of the model remained intact, consistent with field investigations. Depth of the tension crack at the top of model is 32 cm, i.e., 3.2 m compared to the prototype natural slope with a height of 14 m when the length scale ratio （proto/model） is lo. Acceleration at the top of the slope was almost twice as large as that at the toe when the measured accelerations on shaking table are 4.85 m/s2 and 6.49 m/s2, which means that seismic force at the top of the slope is twice the magnitude of that at the toe. By use of the dynamic-strength-reduction method, numerical simulation was conducted to explore the process and mechanism of formation of the sliding surface, with other quantified information. The earthquake-induced failure surfaces commonly consist of tension cracks and shear zones. Within 5 mfrom the top of the slope, the dynamic sliding surface will be about 1 m shallower than the pseudo-static sliding surface in a horizontal direction when the peak ground acceleration （PGA） is 1 m/s2; the dynamic sliding surface will be about 2 m deeper than the pseudo-static sliding surface in a horizontal direction when the PGA is lo m/sL and the depths of the dynamic sliding surface and the pseudo-static sliding surface will be almost the same when the PGA is 2 m/s2. Based on these findings, it is suggested that the key point of anti-seismic design, as well as for mitigation of post-earthquake, secondary mountain hazards, is to prevent tension cracks from forming in the upper part of the slope. Therefore, the depth of tension cracks in slope surfaces is the key to reinforcement of slopes. The depth of the sliding surface from the pseudo-static method can be a reference for slope reinforcement mitigation.

In-situ experiment investigations of hydrothermal process of highway in deep seasonal frozen soil regions of Inner Mongolia, China

To reveal the influencing factors and changing rules for the hydrothermal interaction process of highway subgrade, the field measurements of Shiwei-Labudalin Highway in Inner Mongolia, China was conducted for 3 years, based on which the freezing-thawing rules and water content changing characteristics were analyzed. The main results show the subgrade presents a frequent freezing-thawing alternation, and the water content of subgrade exhibits an obvious seasonal alternation. The subbase has the maximum water content, while the base has the minimum water content. The change of water flux is concentrated in the thawing period and consistent with the change of temperature gradient. The subbase layer has the most active water flux due to the heat absorption and impermeability of pavement that easily causes the water accumulation in this layer. Therefore, the prevention and treatment for the freezing-thawing disease should be started from heat insulation and water resistance.