Glass fiber-reinforced plastic mortar (GRPM) pipes are widely used in municipal and industrial engineering. However, their socket interfaces are prone to leakage failure under external loads. Existing studies lack targeted test methods for large-diameter GRPM interfaces. In this study, we design a four-point bending test device to study interface mechanical properties and water tightness and reveal its failure mechanism.

Based on simply supported beam stress theory, the test simplifies the GRPM pipe with a socket interface into a simply supported beam model and applies symmetrical vertical load to simulate the actual engineering load effect on the pipe interface. The test device comprises three core subsystems: support, loading, and measurement systems. The support system adopts a 500 mm-high steel base and arc-shaped pipe supports processed by a 50 mm thick steel plate, which can effectively transmit load and constrain the horizontal displacement of the pipe body while allowing the interface to rotate freely in the plane. The loading system uses a 500 kN three-channel electro-hydraulic servo testing machine matched with a self-developed arc-shaped loading beam, which realizes pure bending stress in the interface area and avoids stress concentration damage to the pipe wall. The measurement system adopts a DH3816N static strain testing system equipped with six displacement sensors, which are symmetrically arranged at the pipe ends, 600 mm away from the pipe ends and directly above the interface to collect real-time vertical displacement data. The test specimen is a DN800 GRPM pipe with a socket interface, with a total length of 3.0 m after assembly. The test process includes device installation, specimen alignment, water injection, 5 kN preloading debugging, and formal loading at a constant rate of 0.05 mm/s. The test terminates when rapid interface leakage occurs or the maximum vertical load reaches 300 kN, monitoring interface vertical displacement, relative rotation angle, axial pull-out amount, and water leakage state synchronously.

The following are the test results: (1) The socket interface maintains good water tightness under 0–64 kN load. The initial leakage occurred at 64 kN, marking the functional failure of water tightness, with an interface vertical displacement of 16.9 mm and a relative rotation angle of 1.90°. As the load increases to 90, 160, and 220 kN, the leakage rate and volume gradually increase, resulting in complete water tightness failure. (2) The interface vertical displacement increases rapidly before reaching 80 kN and slows down considerably from 80 to 300 kN. No structural damage occurs at 300 kN, and the interface still retains good mechanical bearing capacity. (3) The relative rotation angle increases linearly before leakage and grows slowly afterward, reaching 3.90° at the end of loading, which is only 2.0° higher than that at initial leakage. (4) Crisp sounds are heard at 120, 190, 250, and 280 kN due to sudden socket and spigot axial pull-out. The maximum axial pull-out displacements at the interface bottom and middle are 57 and 47 mm, respectively, accounting for 15% and 12.4% of the socket length.

Although conventional GRPM socket interfaces have excellent structural reliability, premature water tightness failure at 64 kN is a critical design flaw, mainly due to interface multidirectional deformation exceeding the rubber ring’s effective sealing range. Optimizing sealing ring height and socket length and adding spigot-thickened sections can effectively improve water tightness. The proposed test method provides a reliable basis for pipeline design optimization and can serve as an experimental teaching platform for civil engineering graduates.

The "black hole effect" at tunnel entrances, caused by abrupt luminance transitions between exterior and interior environments, often induces momentary driver blindness and significantly increases traffic accident risk. While sunshade structures can mitigate this sudden luminance drop, current design practices lack systematic, quantitative guidance on key parameters such as length and transmittance. This deficiency hinders the establishment of smooth luminance transitions and full resolution of the black hole effect. Moreover, optimizing sunshade parameters typically relies on costly, high-risk real-vehicle eye-tracking experiments unsuitable for educational purposes. To address these challenges, this study developed a safe, economical, and highly operable eye-tracking platform based on a scaled tunnel model. This platform systematically evaluates the influence of sunshade length and transmittance gradients on drivers’ visual adaptation, providing theoretical support for design optimization and serving as a practical tool for traffic engineering education.

A 1:50 scale tunnel visual behavior eye-tracking platform was constructed using illuminance data from full-scale real-vehicle tunnel experiments. The tunnel structure, built with foam panels, employed directional lighting techniques with controllable luminaires to reproduce typical tunnel entrance luminance transitions. To overcome the limitation of fixed sunshade parameters in real tunnels, blue shading films and a segmented multilayer arrangement method simulated different length and transmittance gradient combinations. Fifteen test scenarios were designed, covering sunshade lengths from 45 m to 105 m and various transmittance gradient configurations. A high-resolution imaging device mounted on a remote-controlled model vehicle simulated the driver’s perspective during tunnel passage. Subjects wore Tobii Pro Glasses 3 eye trackers to observe reconstructed driving scenes. Pupil diameter data were collected across three spatial segments—approach zone, sunshade zone, and tunnel entrance zone—to investigate the influence of sunshade design parameters on visual adaptation.

Results revealed that sunshades significantly mitigated the black hole effect by reducing pupil dilation during tunnel entry. Without a sunshade, the average pupil area change rate reached 51.4%. With sunshades, this rate decreased to 20–45%. Longer sunshades (90–105 m) provided stronger buffering, reducing pupil change rates to 20–31%. Crucially, optimized transmittance gradients achieved comparable mitigation at shorter lengths. For instance, a 75 m sunshade using a high–medium–low gradient combination reduced the pupil change rate to 34%, matching the performance of a 90 m sunshade with uniform transmittance. These results confirm that well-designed transmittance gradients can substitute for additional structural length, offering an efficient and economical solution for tunnel entrance safety.

By establishing a scaled-model-based eye-tracking platform, this study quantified the visual buffering effects of tunnel entrance sunshades, overcoming limitations of real-vehicle experiments such as uncontrollable parameters and safety risks. The findings demonstrate that appropriate combinations of sunshade length and transmittance gradient effectively reduce pupil fluctuation and alleviate the black hole effect. Furthermore, under structural constraints, refined transmittance gradient design achieves outcomes comparable to longer sunshades, validating the “light over length” optimization strategy. The proposed platform and methodology not only enhance tunnel visual safety but also provide a replicable, scalable solution for experimental teaching in transportation engineering.