Maximum Positive Bending Moments Research Articles

Numerical Model for Rectangular Pedestrian Underpass Excavations with Pipe-Roof Preconstruction Method: A Case Study

Under weak geological conditions, soil deformation and surface settlement are the key factors affecting the success of shallow-buried rectangular excavation. To investigate this issue, an underpass of Zhongxiao East Road in Taipei City was used as a case study. The surface settlement and lateral deformation of an underground diaphragm wall caused by the excavation of a rectangular pedestrian underpass using the pipe-roof preconstruction method (PPM) were investigated by 3D finite element analysis. The numerical analysis results showed that the constructed numerical analysis model had considerable accuracy. The use of PPM combined with a box culvert structure to form a pedestrian underpass could effectively control the surface displacement above the box culvert. Under the condition of the same sectional area, the smaller the width of the pipe-roof structure, the more the impact on the ground surface was reduced. The maximum positive bending moment and maximum negative bending moment on the pipe roof produced by excavation at each stage were roughly inversely related to the height per the width of the cross-section of the pipe diaphragm structure. The results showed that the pipe-roof structure was suitable for underground excavation with shallow-buried depth in the soft soil of the Taipei Basin. Moreover, the shallow-buried box culvert was more sensitive to the subsidence caused by construction than the deep-buried box culvert.

Numerical Analysis of the Influence of Deep Foundation Pit Construction on Adjacent Subway Stations in Soft Soil Areas

Deep foundation pit construction adjacent to a subway station in a soft soil area was numerically simulated with Midas GTS NX calculation software. The influence of the deep foundation pit construction on the deformation and stress of the subway station structure was studied, and the influence of the foundation pit retaining structure on the station was analyzed. The results show that during the foundation pit excavation process, the subway station slab rose as a whole and was greatly affected by the deformation of the common ground connecting wall, with the most unfavorable position changing as the excavation area changed. The excavation of foundation pits in different zones had a considerable influence on the east‐to‐west displacement of the common diaphragm wall outside the foundation pit. The maximum positive bending moment of the common diaphragm wall changed little, while the negative bending moment increased greatly during construction. Overall, the foundation pit excavation had a great impact on the negative moment of the common diaphragm wall. During the foundation pit excavation process, the subway station column lifted upward, and the maximum displacement, which was located at the west end of the station near the foundation pit, gradually weakened from west to east. As the foundation pit excavation process continued, the maximum axial force of the station column increased by 10.38%, and the pressure was the largest in the middle column. As the thickness of the diaphragm wall increased, the stiffness of the foundation pit retaining structure increased. After earthwork excavation and unloading, the locations in the retaining structure with high stiffnesses could resist deformations. The whole foundation pit was offset due to the high stiffness of the foundation pit retaining structure, which increased the horizontal deformation of the existing station structure. With increasing thickness, the relative horizontal deformation of the station slab gradually increased, mainly because the difference between the depths of the old and new diaphragm walls caused the embedded soil of the two same deep foundation pits to differ. Furthermore, there were great differences in the Earth pressure behind the wall. As the depth of the diaphragm wall increased, the active Earth pressure behind the diaphragm wall increased.

Frame structure analysis model for the effect of typical tunnel diseases on lining structure

The frame structure analysis method has been generally applied for the tunnel structure design, which was widely utilized due to its fast calculation speed and convenient modeling. In this paper, the frame analysis model was established to explore the effect of typical tunnel diseases of voids and lining insufficient thickness on the mechanical and deformation behavior of concrete linings. At first, the lining model with the diseases of void and lining insufficient thickness was established in the frame structure analysis platform. Subsequently, the lining inner force (taking the bending moment as an example) and the deformation behavior were explored. At final, the differences in the deformation behavior were investigated with different vertical loading conditions. The main results in this paper revealed that when the vertical loosening pressure can be transferred to the lining structure within the void zone, the maximum positive bending moment of the lining is located near the crown, while the maximum negative is at the shoulder, and the phenomenon of internal bending mainly occurs in the void area, and the phenomenon of external bending occurs at the shoulder. The lining has a significant downward displacement at the crown, while the deformation direction of the structure is outward at the shoulder. However, when the vertical loading cannot be transferred to the lining within the void zone, the obvious phenomenon of deformation outwards will occur within the void zone.

Investigation of piled behaviour due to an adjacent excavation: 3D numerical modelling

Zameer Ahmed Channaret al.,InternationalJournal of Emerging Trends in Engineering Research, 9(6), June 2021, 683–689683ABSTRACTIn congested cities, excavations are unavoidably constructed adjacent to high rising building supported by piled raft foundations which reduces differential settlements in the buildings. Since the excavations inevitably induce soil movement and stress changes in the ground, it may cause differential settlements to nearby piled raft foundation. In this numerical study, a 3D coupled consolidation numerical analysis (using a hypoplastic model, which considers strain dependent and path-dependent soil stiffness) was conducted to investigate a (2×2) piled raft responses to an adjacent 25-m deep excavation in saturated clay. The computed results have revealedthat the rate of piled raft settlement increased significantly beyond excavation stage h/He=0.5. This is because of the degradation of stiffness of clay with strain due to excavation-induced stress release. Differential settlement (i.e. tilting) was induced in the piled raft due to non-uniform stress release.Owing to separation of the raft from the ground due excavation, some of the working load was transferred to the four piles. The maximum positive bending moment was 200 kNm at Z/Lp=0.67. However, no any bending moment was induced in both the piles at the toes.

Experimental Analysis of Shield TBM Tunnel Lining Mechanical Behaviour in an Anisotropically-Jointed Rock Mass

Jointed rock masses represent a challenging geological environment for tunnel boring machine (TBM) tunneling at great depth. Especially in the case of rock masses that have anisotropic strength and deformation characteristics, the segmental lining is susceptible to asymmetrical loading and local instabilities during tunneling. This paper presents the results of experimental tests and numerical simulations of the interaction between an anisotropic rock mass and the segmental lining of a tunnel. In the tests, we considered different lateral pressure coefficients (σh/σv), joint dip angles, and joint spacings. In the numerical simulations, different joint cohesions, friction angles, and tensile strengths were considered in order to study the effects of joint mechanical parameters on the behaviour of the liner and to evaluate quantitative trends of this effect on the liner. We studied the internal force, deformation, and fracture of the segmental lining. It was shown that, under isotropic in situ stress (σh/σv, = 1), the anisotropy of the rock masses was a major control on the deformation and damage of the liner, with the maximum positive bending moment and tensile cracks on the liner developing mainly at the direction normal to the stratification. When loaded by increasingly anisotropic in situ stress states (σh/σv > 1), the deformation of the liner and the characteristics of the damage were observed to become increasingly influenced by the major principal stress and, correspondingly, less controlled by the anisotropy of the rock structure. The smaller joint spacing tended to induce larger rock mass pressure on the liner, thereby resulting in greater internal force and deformation of the liner. According to the results of the numerical simulations, the internal force on the liner and its deformation increased markedly with decreasing joint friction angle and cohesion values. The response of the internal force and deformation of the liner to the change in the joint tensile strength was relatively small.

Analysis of the behaviour of a novel support system in an anisotropically jointed rock mass

When constructing an excavation, the segmental lining installed in an anisotropic rock mass is susceptible to asymmetrical pressure and substantial local instabilities. To promote liner stability in such cases, a support system that combines a segmental lining with a highly deformable expanded clay and rock bolts is proposed. A series of physical model tests were performed to study the effect of this support system. In the tests, different dip angles of the primary rock mass jointing, different thicknesses of the expanded clay and different lengths and spacings of rock bolts were considered. Variations in the internal forces and deformation of the segmental lining were recorded in each case. The distribution of the internal forces and deformation of the segmental lining were anisotropic due to the rock mass anisotropy; the maximum positive bending moment and positive deformation on the liner mainly developed in the direction normal to the stratification. By using a sufficiently thick deformable clay layer, the anisotropy of the load transferred to the segmental liner can be minimized. By adding rock bolts, the combined support system can take full advantage of the reinforcement effect of the rock bolts and the yielding effect of the deformable clay layer, and the effect of the rock bolts plus the yielding layer is greater than the sum of the two individual effects. Such a system can change the internal force and deformation distribution of the segmental lining to improve liner stability.

Mechanical behaviour of segmental lining of a sub-rectangular shield tunnel under self-weight

In this paper, a novel loading setup was developed for performing full-scale loading tests on ‘standing’ segmental lining of a sub-rectangular shield tunnel, which for the first time allows the mechanical behaviour of segmental lining subject to self-weight to be assessed in a full-scale loading test. Granular rubber bearings with an equivalent coefficient of subgrade reaction close to hard plastic clay were used to simulate subgrade soils. The experimental results showed that the internal force distribution and deformation pattern of lining ring subject to self-weight loading differed significantly from that observed in a ‘lying’ loading test considering soil-and-water pressures due to the absence of lateral confinement and different boundary conditions. Specifically, maximum positive bending moments were observed at the crown and invert regions, and the largest negative bending moments occurred at the two waists. Stress concentration was not observed at the corners. The axial force was not universally compressive with tensile axial forces present at the crown region. Furthermore, the convergence deformation under self-weight loading was non-negligible. The overall lateral expansion deformation was close to the vertical convergence deformation, but the deformation was smaller at the invert than that at the crown. It is suggested that the influence of self-weight be considered in future full-scale loading tests, especially for shallowly-buried tunnels.

A New Simple Design Method for the Plate Foundation of a Transmission Tower in Subsidence Area

In this article, we present a simple design method for the plate foundation of a transmission tower in a mining area, which is based on the theory of beam rested on an elastic foundation. The corresponding theoretical model has been developed with the synergistic reaction of a composite protection plate by considering the mining subsidence of ground. On the basis of this model, the function of flexural deformation and the distribution function of bending moment of the protection plate and their corresponding base counterforce have been deduced. The analysis of selected example shows that the control bending moment of the protection plate is the maximum positive bending moment at the center of the cross section during the moving process of pelvic floor. The tilt deformation is minimum, when the tower is either at the bottom or at the verge of pelvic floor. The tilt deformation is maximum at one-fourth times of the length of pelvic floor and is away from the bottom and the verge of pelvic floor. The contact pressure between the plate foundation and the soil is similar to a U-shaped distribution, when the protection plate is in pelvic floor area of a positive (curvature) deformation, and it is a M-shaped distribution, when the protection plate is in pelvic floor area of a negative (curvature) deformation. We do not observe any change of the contact pressure at the midpoint of the protection plate during the moving process of the basin.

Long Term Field Test on Structure Forces of Sheet-Pile Wharf with Barrier Piles

Tan, H.; Jiao, Z., and Cai, Z., 2015. Long term field test on structure forces of sheet-pile wharf with barrier piles.Based on the first engineering application of new-type sheet-pile wharf with barrier piles, the field test has been conducted for 7 years. According to the long term field testing results, the structure forces of the wharf front wall and barrier piles have been obtained in different periods. The results have showed that the new wharf structure was effective for a large harbor basin, the maximum positive bending moments for both front wall and barrier piles appeared in dredging period, and the maximum negative bending moment appeared in operation period. The rebar stress of front wall was about twice larger than that of barrier piles. In order to make full use of the potential bearing capacity of front wall and barrier piles, the optimal design was to make them in the same stress level. The modified bending moment calculation method has been proposed based on the field testing data.

Analysis on Influence of Shield Tunnel Crossing under Intercity High-Speed Railway Station

Based on an engineering example of a certain shield tunnel passing through an intercity high-speed railway station which has been reinforced by pile-slab structure, this paper adopts finite element numerical simulation to study the influence of shield tunneling construction on the pile-slab reinforced structure and railway subgrade. The research result shows that the horizontal displacement and additional bending moment on both sides of bored piles are increasing in the process of shield tunneling, while those of the middle bored pile firstly increase and then decrease. The vertical displacement and axial force variation of the middle bored pile are more obvious than that on both sides of the bored piles. The settlement and additional bending moment of the reinforced concrete slab are relatively large in the center and midspan of both sides, the position of maximum settlement and negative bending moment is near the concrete slab just above the right tunnel, while the maximum positive bending moment is located in the very middle of the concrete slab. The pile-slab reinforced structure can effectively control the settlement and differential settlement of the intercity railway subgrade caused by shield tunneling, thus the intercity railway can keep normal operation.