Numerical analysis of reinforced concrete shear keys in immersion joints

Immersion joints are susceptible to be damaged first under various loadings due ito their smaller stiffhess compared to the adjacent tunnel elements. The induced Ideformations and the forces in the joint are transferred by shear keys. If failure occurs in the i shear keys, excessive defbrmations may lead to severe leakage. This paper presents threedimensional numerical analysis results for RC shear keys by using the concrete damaged plasticity (CDP) model. Two types of shear keys, HSK1 and HSK2 with different numbers 0f tenons respectively, are considered, as well as a cyclic loading protocol. The tensile damages of the shear keys are obtained. The numerical results indicate that the damages occur firstly at the bottom of the tenons, close to the clamped edge. Then, the damages develop along the direction to both the middle bottom of the tenon and the middle top of the shear keys. The bottom cracks are more diffxcult to be observed than the top cracks and cross the key. By comparing the experimental results, the obtained tensile damages are in accordance with the observed tensile cracks

Experimental assessment of the mechanical behavior of immersion joints and a seismic mitigation method in immersed tunnels

With the rapid progress of urbanization, an increasing number of infrastructure works have been constructed by engineers around the world since the past two centuries, among which many tunnels. Due to the advantages compared to other tunnel types, immersed tunnel techniques are widely adopted and nowadays there are more than 200 immersed tunnels worldwide. The immersion joints, which are between the adjacent tunnel elements, are normally regarded as the weakest parts in the tunnel due to their smaller stiffness than that of the elements. Moreover, the immersion joint is the key component in the water proof system. When an immersed tunnel experiences various loadings, i.e. earthquakes, differential settlement, sinking ships or anchorage impact, deformations occur in the joint and excessive deformations could cause possible damage to the joint, resulting in water leakage which jeopardizes the safety of the immersed tunnel. As known, the configuration of an immersion joint is complicated, mainly involving the primary rubber seal, the secondary rubber seal, the shear keys, the steel shell and the pre-stressing cables. Such a complex configuration leads to difficulties to investigate the behavior of the joint. To have a comprehensive understanding of that, an experimental investigation on the joint subjected to combined loadings is reported in the present thesis. In particular, the behavior of the joint subjected to excessive shear deformation is investigated and subsequently, the failure behavior is also included. It is widely recognized that the shear keys have an important contribution to the shear behavior of the joint. However, the failure behavior of the joint with both the shear keys and the rubber seal is largely unknown due to the lack of experimental investigations. Moreover, it is proved that the flexible immersion joint has a contribution in the seismic response reduction but the application of seismic mitigation devices in the joint has never been considered though such concept has been accepted for buildings for decades. Therefore, the main part of the thesis was divided into two parts, more specifically the mechanical behavior of the joint and the seismic mitigation for immersion joint respectively. Based on an extensive literature review, an experimental program has been elaborated in order to investigate the mechanical behavior of the joint subjected to axial, bending and shear loadings. A geometric scale of 1:10 was selected for technical reasons, such as manufacturing, testing and measurements. Based on that, two tunnel elements, between which an immersion joint was positioned, were designed as well as the rubber seal and the shear keys. The dimensions of a single element are 3800mm x 1150mm x 1250mm with the walls and slabs having a thickness of 150mm. The cross-sectional dimensions of the rubber seal are 37.5mm x 70mm (flange including) with a total length of 9.67m. Two types of shear keys have been investigated, namely steel shear keys and concrete shear keys and each type of the shear keys was divided into two groups depending on their position in the joint and the loading situation. The steel shear keys were connected to the element by bolts while the concrete shear keys were casted together with the element to increase the shear strength. For the model specimens, a unique test set-up has been developed allowing that one element is movable while the other one is fixed, resulting in an axial, bending and shear deformations in the joint respectively. Only horizontal loading was applied in this experiment. The axial load and the bending moment were provided by a set of four hydraulic jacks which are controlled independently while the shear force was applied by an additional jack. Further, a testing procedure was elaborated, consisting of three loading protocols, namely axial compression, compression-bending moment and compression-shear loading cases. For the axial loading case, the hydraulic jacks first provided the gradually increasing axial force then the jacks were unloaded. For the compression-bending case the axial force was applied to an specific value to simulate the initial water pressure in the joint and then a bending moment was imposed. The immersion joint was allowed to rotate, resulting in the occurrence of an opening of the joint. For the shear loading cases, an axial force was also applied at the beginning followed by a reciprocating shear force with increasing amplitude. The shear force was increased until all the shear keys failed. During the experiments, measurements were executed after each successive increase of the load or displacement, comprising axial and shear deformations. Regarding the axial and flexural performance of the joint, the compression-release curve and the bending moment-rotation curves with different levels of axial forces were obtained. Through the obtained load-deformation curves, both the axial stiffness and the flexural stiffness of the joint were derived for use in practice. During this testing cases, a hysteretic loop was observed in both axial and flexural behavior of the joint, indicating that the rubber seal is not perfectly hyper-elastic material as assumed and energy-dissipation did occur. Moreover, an asymmetric bending behavior was observed as the axial force increased. The static and dynamic behavior of the joint were investigated by imposing static and dynamic shear loading respectively. The static and dynamic load-displacement curves of the joint with different loading scenarios were obtained. Accordingly, the static and dynamic shear stiffness of the joint were derived as well. A comparison was made between the static and dynamic shear performance of the joint. The failure behavior of the joint with the steel shear keys and concrete shear keys were investigated by applying reciprocating shear loads with increasing amplitude under a constant axial force. The failure mode of the joint with these two types of shear keys as well as the shear capacity of the joint were obtained. Both series of test results show that the shear keys were not activated at the same time, resulting in a difference between the design shear capacity and experimental one. Finally, a significant contribution of the rubber seal in the shear direction was found, indicating that the shear behavior of the rubber seal should be taken into account in the design procedure. A general literature study with respect to seismic mitigation methods was performed serving as a starting point for the application to immersion joints. To achieve this, a buckling energydissipation device (BEDD) on the basis of the Buckling Restrained Brace (BRB) was introduced and a detailed design procedure for the seismic mitigation device in the joint was provided supposing that the device can work in coordination with the joint in such a way that the maximum energy dissipation is reached. In order to validate the design procedure, a largescale experiment was conducted on an immersion joint subjected to compression-bending moment cases. The bending moment-rotation curves of the joint with seismic mitigation devices as well as the hysteretic performance of the device itself were obtained through axial transducers and strain gauges on the device. It was experimentally proved that the hysteretic performance of the joint was enhanced by using the seismic mitigation device though the performance of the device itself did not meet the expectation. However, it indicated that such application of the seismic mitigation method has a high potential in energy dissipation in immersion joints. In the presented PhD thesis, the mechanical behavior of the joint subjected to axial, flexural and shear loadings and the seismic mitigation method for immersion joints have been studied comprehensively in an experimental way. As the first attempt ever on such issue, the results gained from these investigation give clear insights on the behavior of joint under different loading scenarios. The obtained stiffnesses can be used in further numerical analyses. The proposed seismic mitigation method for immersion joints is shown to be feasible not only to enhance the seismic performance of the joint but also to provide an additional way for energy dissipation of immersed tunnels. The material characteristics of the rubber seal are found to play a much more important role than what is expected from conventional design

Experimental investigation on the static shear stiffness of an immersion joint

The static shear stiffness of an immersion joint subjected to combined compression and shear forces is investigated by experiments in a test set-up. To explore the performance of the immersion joint, the compression-shear loads, which are applied on a specimen according to test protocol, are determined based on real design situation. In this test, the main focus is on the steel shear keys and the rubber sealing. For the applied loading schemes, different levels of axial force, corresponding to the water depth of the joint, are considered as well as the changing amplitudes of the shear force. The force-displacement curve is obtained and the hysteresis is observed during the whole test. The joint‘s static shear stiffness is calculated, showing to increase linearly with respect to the magnitude of the axial force. Moreover, it is found that the rubber sealing has a significant influence on the shear behavior of the joint

Static and dynamic experimental analysis of an immersion joint

As the weakest part of an immersed tunnel, the immersion joint is the key element in research in this field. Relatively large deformations and internal forces may be induced in the immersion joint subjected to various loading types. Based on a real project, the shear mechanical behaviour is investigated by large scale model test. To explore the performance of the immersion joint, compression-shear loads are applied on a tunnel segment in a specific test set-up. For the applied loading schemes, different levels of axial force, corresponding to the water depth of the joint, are considered as well as varying amplitudes of the shear force. Based on these results, both the static and dynamic shear stiffness of an immersion joint were analysed. The results of the test indicate that the static shear stiffness of the joint increases linearly with the axial force and the same trend is found for the dynamic one. Moreover, the dynamic stiffness is larger than the static one

Click-aware Structure Transfer with Sample Weight Assignment for Post-Click Conversion Rate Estimation

Post-click Conversion Rate (CVR) prediction task plays an essential role in industrial applications, such as recommendation and advertising. Conventional CVR methods typically suffer from the data sparsity problem as they rely only on samples where the user has clicked. To address this problem, researchers have introduced the method of multi-task learning, which utilizes non-clicked samples and shares feature representations of the Click-Through Rate (CTR) task with the CVR task. However, it should be noted that the CVR and CTR tasks are fundamentally different and may even be contradictory. Therefore, introducing a large amount of CTR information without distinction may drown out valuable information related to CVR. This phenomenon is called the curse of knowledge problem in this paper. To tackle this issue, we argue that a trade-off should be achieved between the introduction of large amounts of auxiliary information and the protection of valuable information related to CVR. Hence, we propose a Click-aware Structure Transfer model with sample Weight Assignment, abbreviated as CSTWA. It pays more attention to the latent structure information, which can filter the input information that is related to CVR, instead of directly sharing feature representations. Meanwhile, to capture the representation conflict between CTR and CVR, we calibrate the representation layer and reweight the discriminant layer to excavate the click bias information from the CTR tower. Moreover, it incorporates a sample weight assignment algorithm biased towards CVR modeling, to make the knowledge from CTR would not mislead the CVR. Extensive experiments on industrial and public datasets have demonstrated that CSTWA significantly outperforms widely used and competitive models

Human activities accelerated the degradation of saline seepweed red beaches by amplifying top‐down and bottom‐up forces

Salt marshes dominated by saline seepweed (Suaeda heteroptera) provide important ecosystem services such as sequestering carbon (blue carbon), maintaining healthy fisheries, and protecting shorelines. These salt marshes also constitute stunning red beach landscapes, and the resulting tourism significantly contributes to the local economy. However, land use change and degradation have led to a substantial loss of the red beach area. It remains unclear how human activities influence the top‐down and bottom‐up forces that regulate the distribution and succession of these salt marshes and lead to the degradation of the red beaches. We examined how bottom‐up forces influenced the germination, emergence, and colonization of saline seepweed with field measurements and a laboratory experiment. We also examined whether top‐down forces affected the red beach distribution by conducting a field survey for crab burrows and density, laboratory feeding trials, and waterbird investigations. The higher sediment accretion rate induced by human activities limited the establishment of new red beaches. The construction of tourism facilities and the frequent presence of tourists reduced the density of waterbirds, which in turn increased the density of crabs, intensifying the top‐down forces such as predators and herbivores that drive the degradation of the coastal red beaches. Our results show that sediment accretion and plant–herbivory changes induced by human activities were likely the two primary ecological processes leading to the degradation of the red beaches. Human activities significantly shaped the abundance and distribution of the red beaches by altering both top‐down and bottom‐up ecological processes. Our findings can help us better understand the dynamics of salt marshes and have implications for the management and restoration of coastal wetlands

Experimental investigation on the flexural mechanical behaviour of an immersion joint

The immersed tunnelling technique is commonly used for river or sea crossings worldwide. Seismic safety criteria of immersed tunnels involve the shear stiffness, axial stiffness, flexural stiffness, and opening deformations of the immersion joints. Therefore, it is necessary to conduct the mechanical analysis of the joint between the immersed tunnel elements. An experi-ment of an immersion joint is presented in this paper, mainly dealing with the experiment design, axial behaviour and flexural behaviour of the immersion joint. The geometric scale of this experi-ment is 1:10. The model joint in this paper includes two 3.8m x 1.15m x 1.2m segments with a rubber gasket and horizontal steel shear keys between them. Different levels of water pressure were considered due to the significant changes of water depth in real project. The displacements of an immersion joint under multi-level loads were measured and analysed considering the hyper-elastic property of a GINA gasket. It can be found that the mechanical behaviour of a GINA gasket is significantly affected by both flexure and axial loadings. Moreover, the flexural stiffness ratio of the joint with respect to that of the tunnel element in service states ranges from 1/27 to 1/272. The results are useful for the further numerical analysis of immersion joint and more related publi-cations are expected in the future