To determine the energy attenuation and energy reflection coefficients in layered foundation is the key factor to reveal the dynamic response characteristics of the high-speed railway foundation. Based on the foundation test model under the dynamic loading of the high-speed railway, the energy attenuation and energy reflection coefficients were introduced and the attenuation formulas of the vibration acceleration in the layered foundation were deduced. Five scale models of 1:1, 1:2, 1:5, 1:10 and 1:20 are established respectively by using Abaqus technique. The energy attenuation mechanism and interfacial energy reflection characteristics in the layered foundation were analyzed. Results show that it is appropriate to use vibration acceleration to characterize the propagation rule of the energy attenuation in the layered foundation. The size effect equations of the energy attenuation and energy reflection coefficients in the layered foundation are deduced and the size effect of the energy attenuation is revealed. Based on the nonlinear relationships among the energy attenuation coefficient, the model scale, the loading amplitude and the vibration frequency, the energy attenuation equation of the scale test model is constructed. The reliability of the theoretical and simulation results is verified by the scale test model. The conclusions obtained in this study can provide a reference for a similar engineering practice.
The ultra-small spacing tunnel is a new tunnel type under the urban engineering environment, which not only has high requirements for deformation and settlement control, but also has great difficulty in construction technology. Since the middle rock pillar is the key part in the design and construction of ultra-small spacing tunnels, to explore the stability of the middle rock pillar in the ultra-small spacing (0-200 mm) subway tunnels, this study examined the Gangding-Shipaiqiao ultra-small spacing tunnels on Guangzhou Metro Line 3 in China by using ABAQUS and theoretical analysis, which mainly involved the plastic zone of the surrounding rock, the stress distribution and skew displacement effect of the middle rock pillar by defining the skew coefficient. The tunnel cross-sections of three different types were optimized, and a simplified calculation method was proposed with the load and ultimate bearing capacity of the middle rock pillar. Results show that the theoretical analysis of the bearing capacity of the middle rock pillar is essentially consistent with the results of numerical calculation. The conclusions obtained in the study are of important theoretical value to provide on-site engineering construction guidance.
To reveal the effect of sand content on the mechanical performance and energy dissipation of cement improved subgrade soil, using universal testing machine and SHPB test device, unconfined compressive strength (UCS) and impact compression strength under different impact load (0.2, 0.3, 0.4, and 0.5 MPa) were carried out for the cement improved subgrade soil with different sand content (0%, 5%, 10%, 15%, and 20%). Results show that the dynamic and static stress-strain curves of the cement improved soil have similar variation trend. With the increase of the sand content, the UCS and impact compressive strength of the cement improved soil both increase first, then decrease later, showing the form of a quadratic function. The strength growth rate and the dynamic increase factor (DIF) reach the maximum values when the sand content is 10%, which is 64.7% and 18.6% larger than that of ordinary improved subgrade soil, respectively. In addition, when the sand content increases from 0% to 20%, the specific dissipation energy increases first, and decreases later. Mixing 10% natural sand is the optimal proportion to obtain better energy dissipation capacity of the sand-cement-improved soil.
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