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The immersed tunnel of the Hong Kong-Zhuhai-Macao Bridge (HZMB) is the first of its kind constructed using semi-rigid elements and making a breakthrough in the structural theory. This paper explains the reason for applying the semi-rigid elements in an immersed tunnel and provides the design details using the example of the HZMB Tunnel. A theoretical analysis method based on the body-spring model is proposed for segmental joints of the semi-rigid elements. Potential challenges in the application of the semi-rigid elements are identified. Compared with the rigid (monolithic) elements and flexible (segmental) elements, the semi-rigid elements can improve the safety of segmental joints by increasing the flexural rigidity and joint friction. In addition, applying the semi-rigid elements can reduce the construction cost by retaining prestressed cables. Despite the advantages of the semi-rigid elements, some uncertainties arise due to the functional change of prestressed cables from a temporary structure to a permanent one. The location, number, and prestress level of the remaining prestressed cables need to be studied in accordance with the mechanical and waterproofing properties. The long-term prestress loss of prestressed cables should also be taken into consideration, especially the loss of asymmetric prestress in a marine environment. With the development of economy in costal regions, more immersed tunnels will face the similar engineering geological problems as the HZMB Tunnel. Discussing the technology of semi-rigid element has both engineering and theoretical significance to the development of immersed tunnels.

Although segmental elements have excellent deformation and anti-crack abilities, differential settlements of immersed tunnels caused by differential compression of soil (or foundation) and surcharge (or discharge) on the natural foundation section should be concerned in the service stage. Especially, when prestressed cables are cut off after immersing construction, shear keys on segmental joints are the only structural guarantee to resist deformation during the occurrence of differential settlements. Currently, uniform criteria are not available for the design of shear keys in segmental joints, and few physical model tests have been performed to examine the mechanical properties of segmental joints in immersed tunnels. This study investigates the mechanical properties of segmental joints in the immersed tunnel of the Hongkong-Zhuhai-Macao Bridge in China. A series of 1:4.69 large-scale segmental joint model tests were conducted on a settlement platform. Shear force distribution and transfer mechanisms on segmental joints were studied, and allowable differential settlements of segments were determined. In addition, failure features of shear keys and reinforcement measures were propounded. The results of this study are useful for structural safety assessment and further application of segmental type immersed tunnels.

Thermal residual stresses in polycrystalline diamond compact (PDC) cutter arising from the difference in thermal expansion between the polycrystalline diamond (PCD) and the supporting tungsten carbide substrate after sintering at high pressure and high temperature were investigated using finite element simulation, laboratory tests and theoretical analysis. The obtained results show that although compressive residual stresses exist both in the interface of PCD table and in the most region of PCD table surface, the tensile residual stress, which is a fatal shortage to PDC, can also occur near the outer diameter area of PCD table, and the maximum value is 690 MPa. Distribution of tensile stress in the PCD table is given through experimental results, which is well consistent with the numerical results. This finding may be significant in designing new PDC cutters with lower residual stress and high cutting behavior.