In this study, a fiber Bragg grating (FBG) pressure sensor was designed and fabricated using 3D printing method for the measurement of vertical pressure. Raw material used for the fabrication of this new pressure sensor was Polylactic Acid (PLA). The effectiveness of embedding FBG sensors inside printed PLA material was first examined taking into account different infill densities. It is found that FBG sensor can be successfully embedded into PLA material without sacrificing its sensing performance. The change of infill density has no influence on both peak and residual wavelength change of FBG sensors during and after printing process. All FBG sensors printed inside PLA prototype with different infill density values exhibit almost the same peak and residual wavelength rise values in printing process. In addition, the higher the density value, the longer the time required to finish printing process. A simple FBG pressure sensor was designed and fabricated according to the successful experience of embedding FBG sensors into printed PLA. Measurement performance of this FBG pressure sensor was examined in laboratory cyclic loading tests. Calibration tests indicate that the maximum measured vertical pressure of this new pressure sensor was 2000 kPa with a measurement sensitivity of 13.22 kPa/pm.

The ballastless track system (BTS) is widely adopted in the construction of high-speed railways (HSRs), in which the vehicle-induced cyclic stresses are evenly transferred to subgrade at a smaller amplitude compared with the conventional ballast track system. In this study, both static and cyclic triaxial tests were performed on a subgrade fill procured from a HSR construction site in Mainland China. The subgrade fill contained high fine content (d < 0.075 mm). The stress-strain-strength relationship from static tests provided a reference for the cyclic tests. The permanent axial strain and resilient modulus under cyclic HSR loading were studied based on the stress levels in the field. Based on the test data, a new empirical equation is proposed to predict the accumulated permanent axial strain with cycle number and low cyclic deviator stress. A new method for determining all parameters in this equation is proposed and verified by fitting test data. The results show that the predictions from the newly proposed relationship are in good agreement with the experimental results. The resilient behavior of the fill under cyclic loading is also investigated. It is found that under low-stress levels the resilient modulus slightly decreases with the increase of stresses, which is not well described by existing models.

A soil-structure interface is defined as the contact surface between a soil and a structure through which stresses are transferred from the soil to the structure or vice versa. The ultimate shear strength at the interface is one of the key parameters required for the design and safety assessment of a structure in the soil, such as pile foundations, retaining walls, buried pipelines, and soil nails. In this paper, the shearing behavior of completely decomposed granite soil and steel interfaces is examined using a modified suction-controlled direct shear apparatus. A series of direct shear tests was performed on two different types of soil-steel interfaces under different stress state variables. The experimental results were used to investigate the influence of counterface roughness on the failure envelopes of soil-steel interfaces. Test results show that matric suction has a significant influence on the shear behavior and shear strength of the interfaces. It has been observed that the critical inte...