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The recent surge of interest towards the mechanical response of rock mass produced by tunnel-type anchorage (TTA) has generated a handful of theories and an array of empirical explorations on the topic. However, none of these have attempted to arrange the existing achievements in a systematic way. The present work puts forward an integrative framew...
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A suspension bridge that uses suspension cables as the major bearing structure has the most robust spanning capacities. There are two types of anchorage: gravity-type anchorage and tunnel-type anchorage (TTAs). The TTAs structure exhibits complex mechanical and deformation characteristics under tens of thousands of tons of load transferred from bri...
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... Therefore, the anchorage is one of the most important structures of a suspension bridge. The anchorage can be basically categorized into the tunnel type anchorage [5] and the gravity anchorage [6], in which the gravity anchorage is the most used type. The gravity anchorage resists the vertical component of the cable tension force with its own weight and the horizontal component of the cable tension force with the friction between the anchorage concrete and the underlying rock mass [6][7][8]. ...
... The interaction between anchorage concrete and foundation rock mass, in general, is hardly considered because all the elements are assumed to be continuous in the FEM. Although certain effects have been made on the contact problems of gravity anchorage with theoretical solutions [5,15,16], numerical simulation [7,14] and model testing [17,18], systematic investigation work is not yet available. ...
... Therefore, these measures should not be ignored in the anchorage design work. (5) The main limitation of the current paper is that the design and construction work of the bridge project were abandoned due to local policy reasons. Therefore, no DEM simulation was performed for the real prototype anchorage. ...
Traditionally, the numerical simulation work of a bridge gravity anchorage structure is performed with a continuous method, such as the finite element method (FEM). However, since the rock mass and gravity anchorage structure are assumed to be continuous in the FEM, the interaction between the rock mass foundation and the concrete of the anchorage is not frequently considered. This paper aims to investigate the problem of the interaction between the rock mass foundation and the concrete of the anchorage. The discrete element method (DEM), which has been verified to be suitable for the modelling of contact problems of discrete blocks, is introduced in this paper to simulate the mechanical behavior of the rock-concrete system of the gravity anchorage structure and its rock mass foundation. Based on the in-situ scale model test for a bridge, the mechanical behavior of the rock-concrete interface was discussed with the DEM method. With the calibrated DEM model, the displacement of the foundation rock mass, contact stresses, and yield state on the rock-concrete interface were numerically investigated. The anti-sliding effect of the keyway and the step at the bottom of the gravity anchorage structure was analyzed. The results show that the anchorage deformation under the design conditions is basically characterized by the rigid rotation around the keyway of platform #2, and that such rotation subsequently affects the anti-shear capacity of the entire gravity anchorage to a large extent. The anchorage scale model could remain stable under the design lateral load such that the rock-concrete interface would remain intact and sufficient shear resistance could be provided by the keyway and steps.
... Cable-supported bridges are classified into cable-stayed and suspension bridges. Suspension bridges are one of the main types of long-span bridges [1,2] and possess significant benefits in terms of material properties and height-span ratio of the stiffening girders [3]. Suspension bridges are comprised of main beams, tower piers, cables, and anchorages, with the anchorages playing the major role in anchoring the suspension bridge's main cables [4]. ...
... Seo et al. [24] analyzed the failure mode from the initial stage to the failure stage of the tunneltype anchorage through a two-dimensional small-scaled model test and image processing, and confirmed that the failure mode of the tunnel-type anchorage exhibited a wedgeshape. Additionally, the presence of discontinuities, such as joints and artificial and naturally occurring faults, influence the failure mode of tunnel-type anchorages [3]. However, there has been little study examining tunnel-type anchorages in relation to rock joint properties and geometry. ...
... Tunnel-type anchorages are rarely used in design, and their behavior when subjected to a pull-out load has still not been determined. Tunnel-type anchorages have mechanical characteristics similar to uplift piles and rock anchors [3]. Several model experiments and numerical simulations have been performed to investigate the mechanical behavior of uplift piles [14][15][16][17][18][19][20][21][22][23]. ...
In this study, the pull-out behavior of a tunnel-type anchorage was examined by considering both geometric and rock joint characteristics. Three-dimensional finite element analyses were performed with reference to the tunnel-type anchorage cases designed and constructed in Korea. The factors influencing the anchorage response were analyzed: the enlarged part, anchorage spacing, joint orientation, spacing, and the shear strength of the rock joints. According to the numerical studies, the size of the enlarged part influenced the failure shape of the tunnel-type anchorage. It was found that the anchorage spacing, the relationship between the tunnel-type anchorage, and the joint orientation and spacing greatly influenced the pull-out behavior of the anchorage. Additionally, the friction angle had a larger impact on the anchorage’s pull-out resistance than the cohesion between the rock joints.
... Due to the poor geological conditions, it is easy to cause safety accidents in the construction process so that the safety of the open caisson excavation is a critical issue for the project [15,16]. Cases on excavation performance have been reported by many researchers around the world [17][18][19][20]. According to the previous research, field monitoring of soil pressure and internal force of structures are essential for engineers to verify the behaviors of the caisson foundation by analytical or numerical approaches [21]. ...
Oujiang River North Estuary Bridge in Wenzhou is the world’s first double-deck suspension bridge under construction with three-tower and four-span. It is the first time to build large open caisson foundation in the deep marine soft clay in estuary with strong tide, extending the application scope of caisson. To study the deformation and stress characteristics of large open caisson during excavation and ensure the safety of anchorage excavation, a large number of sensors are arranged in the caisson. By analyzing the change of tip resistance, lateral soil pressure, and posture parameters during caisson excavation, the stress characteristics and deformation of caisson are described. The result shows the following. (1) Because of the thixotropy of soft clay, the reaction force of partition wall in deep soft soil area of caisson is similar to that of blade foot, and the reaction force of blade foot can be effectively reduced through the layering construction of caisson. (2) The height of caisson construction and the sand-bearing stratum will obviously affect the plane torsion angle of caisson. When the caisson enters the sand-bearing stratum, the lateral soil pressure increases significantly, which leads to the increase of the plane torsion angle. (3) The inclination and central deviation of caisson are sensitive to the caisson construction and stratum property. It can be found that the lateral soil pressure, plane torsion angle, inclination, and central deviation of caisson are sensitive to stratum property, and inhomogeneity of stratum easily leads to inclination of caisson. Based on the field monitoring data, the stress characteristics and geometric posture of caisson during sinking are studied, which provide technical guidance for scheme design and subsidence prediction analysis of caisson in deep marine soft clay. It can provide a good opportunity to study the behaviors of large caisson foundation constructed in deep marine soft clay and has great significance and reference value for construction optimization of anchorage structure.
1. Introduction
Caisson foundation is a structure that uses the gravity of the caisson to overcome the side friction and end resistance to sink to the predetermined elevation. It has the characteristics of large depth, high stiffness, good integrity, and large bearing capacity [1, 2]. Therefore, it is widely used in the port terminals, breakwaters, and other structures and is widely used in the main tower or anchorage foundation of large span suspension bridges worldwide [3, 4].
Open caisson needs to pass through multiple heterogeneous soil layers in the process of sinking. The uncertainty and complexity of construction lead to frequent accidents such as deflection, sudden sinking, and sand gushing in construction [5]. In recent years, many studies have investigated the stress characteristics of caisson during excavation [6]. GeuGuwen [7] studied the force of the blade foot during the sinking process through field experiments. Chiou [8] analyzed a laterally loaded bridge caisson foundation in gravel by situ lateral load test. Lai et al. [9] studied the installation mechanism and soil deformation characteristics of GDCO caissons using the 3D large deformation finite-element (LDFE) method performed by the Coupled Eulerian-Lagrangian (CEL) approach.
With the rapid development of caisson foundation to a larger plane and deeper sinking depth, large caissons show different mechanical properties from small- and medium-sized caissons, and there are significant differences in construction technology control and spatial mechanical properties [10–12]. At present, the research results and theories are mostly based on large-diameter pile foundation, which are more suitable for small- and medium-sized caissons. The structure design and construction large open caisson foundation will encounter more and more complex problems, such as the stress of caisson structure, side wall friction, caisson deformation, and abnormal conditions [13, 14].
Due to the poor geological conditions, it is easy to cause safety accidents in the construction process so that the safety of the open caisson excavation is a critical issue for the project [15, 16]. Cases on excavation performance have been reported by many researchers around the world [17–20]. According to the previous research, field monitoring of soil pressure and internal force of structures are essential for engineers to verify the behaviors of the caisson foundation by analytical or numerical approaches [21]. Moreover, it is helpful for construction organizations to reduce the risk, which is vital for the safe and orderly proceed of project.
Most of the existing research studies focus on small and medium size caissons, but few on the resistance of foundation end of large and super large caissons [22]. The side friction plays a leading role during construction in small- and medium-sized caisson foundations, but the stress characteristics of large and super large caisson foundations are much different. Qin et al. [23] pointed out that, with the increase of the plane size of the caisson, the end resistance has gradually become a controlling factor compared with the sidewall friction resistance. At present, the construction of caisson foundation is mainly large and super large caissons. However, the mechanical characteristics of small and medium caissons cannot be applied to large and super large caisson foundations. Zhang et al. [24] proposed an improved soil-water-caisson interaction algorithm with the method of smoothed-particle hydrodynamics (SPH) to realize the simulation of the whole sinking and excavation process of the open caisson. Therefore, combined with the sinking process of large caisson, it is of great engineering significance to study the change law of the end resistance, sidewall friction resistance, and geometric posture of the large and super large caisson in the process of soil extraction [25, 26].
In this study, the south anchorage of Oujiang River North Estuary Bridge is selected as the research object. It is the first time to build large open caisson foundation in the deep marine soft clay in estuary with strong tide so that the study focuses on the deformation and stress characteristics of caisson during excavation; the items are monitored, including the reaction force of partition wall and blade foot, lateral soil pressure, and geometric posture of caisson. This research ensures the safety of excavation project and provides a good opportunity to study the behaviors of large open caisson foundation constructed in deep marine soft clay, and it has important significance and reference value for construction optimization of anchorage structure.
2. Project Descriptions
2.1. Overview
The Oujiang River North Estuary Bridge, located in Wenzhou of Zhejiang, Southeastern China, is a key part of the Ningbo-Dongguan Expressway and the national highway G228, the total length is 2090 m, and the maximum span is 800 m. The main bridge is designed to be a double-deck suspension bridge with three-tower and four-span; the elevation layout of main bridge is shown in Figure 1.
... The tunnel anchor is one of the anchorage structures of suspension bridges built in the mountains. Relying on its special inverted wedge shape, the anchor body can maximize the strength of the surrounding rock to bear tens of thousands of tons of load [1] . Given the discontinuity, heterogeneity, and anisotropy of the surrounding rock, accurate evaluations of the bearing capacity of the tunnel anchor system are difficult [2] . ...
Tunnel anchor is a relatively new type of anchor for suspension bridges. Loading tests on a scale model in site are generally considered to be the most direct way to evaluate the bearing capacity of the tunnel anchor system. The response of the anchor body has received minimal attention and involves only a few or no measuring points, because the anchor plug does not undergo elastoplastic failure in a model test. This article intends to explore the relationship between the strain response of the anchor and the state of the surrounding rock. Optical fiber strain sensing technology based on adjustable wavelength optical time domain reflectometry (i.e., TW–COTDR) was applied to the scale model test of Baotaping Bridge tunnel anchor. The strain distribution and evolution of the entire anchor body were determined by optical measurements during overload. In the elastic and plastic stages of the anchor system, the strain around the anchor body linearly decreased from back to front, except for the top arch and the bottom plate at the end. The anchor strain–load curve was nonlinear, similar to the displacement–load curve, but unrelated to concrete damage, which signaled that plasticity began to appear in the anchor system. Therefore, the strain response of the anchor body can be used as an alternative to determine bearing capacity of the anchor system, especially when the displacement is too small to observe.
... In a ground-anchored suspension bridge, the anchorage performs the function of supporting the tension force acting on the main cable. As one of the construction methods of anchorage, gravity-type anchorage (GTA) (see Figure 1) is widely used due to a variety of advantages such as simple mechanical configuration, ease of construction, and its vast applications [1]. The GTA resists the cable load via the weight of the anchorage structure itself, and this depends on the frictional resistance between the anchorages (concrete) and the foundation (rock). ...
A three-dimensional combined finite-discrete element element method (FDEM), parallelized by a general-purpose graphic-processing-unit (GPGPU), was applied to identify the fracture process of rough concrete–rock joints under direct shearing. The development process of shear resistance under the complex interaction between the rough concrete–rock joint surfaces, i.e., asperity dilatation, sliding, and degradation, was numerically simulated in terms of various asperity roughness under constant normal confinement. It was found that joint roughness significantly affects the development of overall joint shear resistance. The main mechanism for the joint shear resistance was identified as asperity sliding in the case of smoother joint roughness and asperity degradation in the case of rougher joint asperity. Moreover, it was established that the bulk internal friction angle increased with asperity angle increments in the Mohr–Coulomb criterion, and these results follow Patton’s theoretical model. Finally, the friction coefficient in FDEM appears to be an important parameter for simulating the direct shear test because the friction coefficient affects the bulk shear strength as well as the bulk internal friction angle. In addition, the friction coefficient of the rock–concrete joints contributes to the variation of the internal friction angle at the smooth joint than the rough joint.
... [1][2][3][4][5] Therefore, the surrounding rock may become unstable or even fall down and the initial support may crack and be destroyed when the construction method of the underground excavation tunnel is not properly selected. [6][7][8][9][10][11] It is key to ensure tunnel construction safety to simulate the construction process of a small interval tunnel in the turn line of metro, [12][13][14][15][16] analyze the rationality of the proposed construction method, and predict the surrounding rock and initial support stress and deformation and possible damage parts. ...
The surrounding rock may become unstable or even fall down and the initial support may crack and be destroyed when the construction method of the underground excavation tunnel is not properly selected in the turn line of metro. . A section of the Santunbei turn line of Urumqi Metro Line 1# was taken as the engineering background. The proposed construction method was analyzed by numerical simulation. Numerical analysis shows that the final surface settlement caused by the proposed construction method is 3.0 mm and the horizontal convergence is 3.2 mm. It also turns out that the proposed construction method causes less deformation, and the method can be applied to the construction of the small interval tunnel in the Santunbei turn line of metro. The rationality of the method and numerical model was further verified by comparison between the monitored data of surface settlement, horizontal convergence and vault sinking, and numerical simulation results. Finally, the deformation and stress of the six construction methods were compared. The deformation and stress caused by the six construction methods are almost the same. It indicates that the construction spacing between the left and right tunnels does not affect the safety of tunnel construction. Therefore, the appropriate construction spacing could be selected according to the resource configuration, instead of deformation and stress.
To explore the impact of soft rock tunnel-type anchor construction and main cable tensioning on surrounding rock and surface deformation, relying on the tunnel-type anchorages project of BDC Yangtze River Bridge, Numerical modeling was employed to analyze the mechanical effects under the conditions of anchor hole excavation, anchorage plug body casting, and anchor bearing using FLAC3D. The results show that: (1) After the anchor hole excavation, part of the rock body yielded, resulting in the appearance of a tensile stress zone in the vault and bottom plate at the catwalk. Surface deformation primarily occurred during the catwalk excavation stage. (2) The casting of the anchor plug body caused the surrounding rock to have clamping effect on the anchor plug body, and the anchor plug body had little effect on the surface buildings. (3) The displacement and surface deformation of the anchor plug body increase with the increase of the tension in the primary cable; under the same tension, the displacement of the measuring point which is farther away from the horizontal distance of the anchorage plug unit is smaller, and the displacement between the two anchors by the superimposed stress is the largest. (4) Under the main cable design load (1P), the adjacent rock is in a condition of compression. The tensile stress is mainly distributed in the structure of the anchor body, and the nearby rock is almost not damaged; as the load progressively increases, the adjacent rock in contact with the tunnel anchor arch and sidewall gradually appears in the area of shear damage zone. Until the 11P load, the shear damage zone of surrounding rock between the two anchors is penetrated.
Tunnel-type anchorages (TTAs) installed in human gathering areas are characterized by a shallow burial depth, and in many instances, they utilize soft rock as the bearing stratum. However, the stability control measures and the principle of shallow TTAs in soft rock have not been fully studied. Hence, a structure suitable for improving the stability of shallow TTAs in soft rock strata, named the anti-pull tie (APT), was added to the floor of the back face. Physical tests and numerical models were established to study the influence of the APT on the load transfer of TTAs, the mechanical response of the surrounding rock, the stress distribution of the interface, and the failure model. The mechanical characteristics of APTs were also studied. The results show that the ultimate bearing capacity of TTAs with an APT is increased by approximately 11.8%, as compared to the TTAs without an APT. Also, the bearing capacity of TTAs increases approximately linearly with increasing height, width, length, and quantity of APTs, and decreases approximately linearly with increasing distance from the back face and slope angle of the tie slope. The normal squeezing between the tie slope and the surrounding rock increases the shear resistance of the interface and expands the range of the surrounding rock participating in bearing sharing. Both tension and compression zones exist in the APT during loading. The tension zone extends from the tie toe to the tie bottom along the tie slope. The range of the tie body tension zone constantly expands to the deep part of the APT with an increasing load. The peak tensile stress value is located at the tie toe. The distribution of compressive stress in the tie body is the largest at the tie top, followed by the tie slope, and then the tie bottom.
To investigate the effects of interface roughness on the pull-out performance and stability of a tunnel-type anchorage (TTA) installed in soft rock, tensile load tests are performed with model plug bodies with different surface roughnesses installed in soft rock model materials in a rectangle chamber with digital image correlation (DIC) capability. From the model tests, it is observed that the ultimate bearing capacity of a TTA decreases gradually with decreasing interface roughness. A TTA with rough interface successively goes through four states during loading, including linear elastic deformation, slow nonlinear deformation, plastic deformation and failure. Nevertheless, a TTA with low interface roughness directly enters a hardened state after undergoing linear elastic deformation. Additionally, the load when a TTA enters the hardened state decreases as the interface roughness decreases. The failure mode of TTA installed in soft rock is not a single rock mass failure or interface slipping failure, but gradually evolves from a single rock mass failure (or interface slipping and debonding) to a dual failure of rock mass failure and interface slipping and debonding. Nevertheless, whether rock mass failure or interface failure occurs first depends on the interface roughness. With a decrease in the interface roughness, a TTA installed in soft rock changes from a failure order of shear (or tensile‒shear) failure of the surrounding rock near the back end of the plug body crown → interface slipping and debonding → the tensile‒shear and tensile failure of the surrounding rock near the middle front end of the crown to the failure order of interface slipping and debonding → tensile‒shear failure of the surrounding rock near the front end of the crown → the tensile failure of the surrounding rock near the back end crown. Additionally, an interface with higher surface roughness mainly produces mixed failure characterized by the shear of the convex body of the surface and the soil particles near the convex body during slipping, while an interface with lower surface roughness produces a single failure characterized by the shear of soil particles.
