The application of asphalt concrete waterproofing layer (ACWL) for the subgrade has been a trend in Chinese high-speed railway. The purpose of this research is to discuss the dynamic characteristics of full cross-section ACWL in the ballastless track structure under the train loads. The laboratory tests were conducted to evaluate the performance of the asphalt mixtures for the ACWL and a test section of ACWL was constructed on the high-speed railway in north China. The linear viscoelastic behavior of the asphalt concrete obtained from the test section was characterized by the generalized Maxwell model according to the results of dynamic modulus test. Then a 3D finite element model for the interaction system of vehicle and ballastless track structure was presented and validated by field measured data. The results indicated that the tensile strain at the bottom of the ACWL was at a relatively low level and the vertical dynamic responses of each structural layer are obviously reduced due to the application of ACWL. Therefore, the full cross-section ACWL helps to reduce the vibration of the track structure and maintain the long-term stability of the subgrade.

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In urban construction, more and more foundation pit excavations are adjacent to existing buildings with composite foundations. Therefore, it is necessary to study the interaction between foundation piles and supporting structures. In this article, a centrifuge model test has been carried out to study the vertical load transfer behavior of composite foundation with rigid piles and its responses to adjacent excavation. The test model was designed based on a real excavation project adjacent to an existing composite foundation. The sand pluviation method was used to build the foundation model. An in-flight excavation method and a loading device for multilevel uniform surface load were used in the test. Finally, the variations of induced axial force, pile skin friction, and pile-soil stress ratio along with the depth were investigated. Some useful conclusions can be drawn as follows: (1) the negative skin friction in the upper part of piles, induced by the overlaying load, increases first and then decreases along with the depth; (2) pile-soil stress ratio decreases along with the depth under initial loads, while it increases first and then decreases when the load is larger; (3) pile-soil stress ratio always increases with loads in the same depth, indicating that mobilized skin friction of piles is continuously developed; and (4) more significant changes of the axial force, skin friction, and pile-soil stress ratio of composite foundation piles are observed in the shallow soil and for the later excavation stage. Therefore, the deformation of the foundation pit should be strictly controlled during the later excavation stages.

Asphalt concrete paved on the surface of a roadbed as a ballastless track substructure has an excellent waterproofing and vibration attenuation performance. However, the mechanical characteristics and the failure mode of this structure under the actions of a cyclic train load and ambient air temperature changes are still unclear. Therefore, a test section of an asphalt concrete substructure was constructed based on a high-speed railway ballastless track project in north China. In situ forced vibration tests and temperature-induced deformation monitoring tests were performed to investigate the mechanical responses of the asphalt concrete, respectively. Test results show that the bottom of the asphalt concrete layer is in the tensile state under the action of the cyclic train load. The surface of the asphalt concrete in contact with the base plate is subjected to tensile stress near the expansion joint under the action of the negative temperature gradient. Changes in the ambient temperature lead to more significant mechanical responses of the asphalt concrete substructure than the cyclic train load, especially near the expansion joint of the base plate. Therefore, the passive tensile failure mode may occur near the expansion joint of the base plate. However, it has also proved that setting isolation layers under the base plate near the expansion joint is an effective method to significantly reduce responses near the expansion joint in this research.