Large-scale Model Tests Research Articles

Reinforcement mechanism analysis of lattice beam and prestressed anchor rod system for loess slope

Lattice beam and prestressed anchor rod are used to enhance stability and prevent failure of soil or rock slopes. In this study, a model of Lattice beam and prestressed anchor rod (LBPAR) system was designed with reinforcement mechanisms and a model test was constructed with a circular slip surface of a loess slope. First, interaction between the loess slope and the LBPAR system was investigated by an LBPAR system analysis model. Stability of sliding mass from the sliding bed with an arc-shaped sliding surface was then studied by an experimental model designed. Finally, internal force distribution of lattice beams in the LBPAR system was investigated by using a large-scale physical model test. The results were compared to those calculated using the reverse beam method, indicating that the LBPAR system strengthened the sliding mass in space and improved the overall stability of the loess slope. With vertical loading, the axial tensile stress of the main anchor rod increases continuously. The bending area of the anchor rod was concentrated within 2 m of the sliding surface. And the maximum bending moment reaches 70 N·m. The sliding mass was subject to vertical load pressure, lattice beams’ pressure, and dead weight in the meantime and the maximum earth pressure value is near the node of the lattice beams. It is proved that such a method excels in the engineering design of loess landslides, which has promising applications in the future.

Mechanical behavior of soft rock roadway reinforced with NPR cables: A physical model test and case study

Deep soft rock roadways are prone to large deformations and failure when reinforced with traditional rockbolts owing to the stiffness mismatch between soft rocks and rockbolts. Thus, a novel rockbolt named “negative Poisson’s ratio (NPR) bolt/cable” that maintains a constant support force during loading is used to reinforce deep soft rock roadways to prevent large deformations. In this study, a large-scale physical model test was conducted to reveal the mechanical performance of NPR cables in soft rocks and obtain the mechanical properties of reinforced rock masses. The test results show that the rock mass reinforced with the NPR cable underwent progressive failure and exhibited the characteristics of a constant-resistance yielding-pressure under external loading, suggesting that NPR cables operate well with soft rock masses. Moreover, on-site investigations confirmed that the effective control of NPR cable supports slow and large deformations of soft rock roadways. An analytical model was developed to verify the physical model test results. The study findings can serve as a reference for theoretical model studies and field applications of NPR cables to prevent large deformations.

Mechanical characteristics of a tunnel boring machine cutterhead during rock breaking: Physical model tests and transient dynamic analysis

During tunnel excavation using a tunnel boring machine (TBM), the interaction between the cutter and the rock causes the cutterhead to generate abrupt vibrations, which could lead to engineering problems, such as bearing seal failure and even cutterhead cracking. In this study, large-scale physical model tests with various cutter spacings were performed to study the vibration and load characteristics of the cutterhead during the rock-breaking process. Through a TBM modal comprehensive test bench, the real-time acceleration of the cutterhead was obtained using a monitoring system. The test results showed that the complete time-domain curves were stepped and periodic. When the cutterhead was stuck or rock peeled off during the excavation, the monitoring curves showed abrupt vibrations, which were mainly concentrated at 0–3 Hz and were typical low-frequency vibrations. Taking measured data as the driving parameter, the real-time mechanical characteristics of the cutterhead were determined through transient dynamic analysis. Numerical simulation results showed that the vibration trend during excavation was the superposition of random vibrations generated by rock breaking and tunneling without rock cutting. During the tunneling process, the wear of the edge cutters should be highlighted in actual construction. The related methods and results can provide an essential reference for the selection of cutterheads.

Thermal-seepage coupled numerical simulation methodology for the artificial ground freezing process

This paper proposes a thermal-seepage coupled numerical simulation methodology for the artificial ground freezing (AGF) process using measured soil freezing characteristic curve (SFCC). Firstly, the relation equation between pore ice content and temperature is established based on the measured SFCC, and a thermal-seepage coupled simulation method of AGF is proposed. After that, the performance of the proposed method was investigated based on soil column and large-scale model tests. Finally, the proposed method is compared with the traditional apparent heat capacity method. The results show that: (1) By introducing SFCC, the relation equation between pore ice content and temperature can be established with a clear physical meaning and a direct parameter determination process; (2) The proposed method can reasonably simulate the thermal and hydraulic field of AGF with or without seepage; (3) Compared with the traditional apparent heat capacity method, the proposed method has a conform to physical essence of phase transition and potential heat release and shows comprehensive improvement in theoretical basis, calculation accuracy, robustness and efficiency. Overall, the research results can provide a further understanding and improvement for numerical simulation in AGF.

Performances of the Soil–Bentonite Cutoff Wall Composited with Geosynthetic Clay Liners: Large-Scale Model Tests and Numerical Simulations

The geosynthetic clay liner (GCL) overlap plays a key role in maintaining hydraulic performance of the barrier systems, e.g., the bottom liner and cover systems. However, its influences on the behavior of the vertical barrier have been rarely investigated. This paper aims to address this issue using the large-scale model test and 3-dimensional finite element (FE) modeling. In the model test, the GCL overlap at the width of 500 mm was tested under a constant hydraulic head of 1 m and confining stress ranging from 10 to 150 kPa using a newly developed large-scale apparatus. Compared with the flexible wall permeameter, this apparatus could guarantee full field-scale GCL overlap to be tested. Results showed that the effective hydraulic conductivity of the GCL overlap decreased from 10−8 to 10−9 cm/s as the confining stress increased from 10 to 150 kPa. The addition of supplemental bentonite paste in between the overlap with a water-to-bentonite ratio of 19:1 contributed to reducing the effective hydraulic conductivity by 60% compared with that for a GCL overlap without bentonite paste. The breakthrough time for the soil-bentonite (SB) cutoff wall composited with GCLs was 64% longer in comparison with that for the single SB wall. Additionally, the breakthrough after 50 years is made for the entire depth of the single SB wall while at the depth no more than 0.9 m for the composite wall with bentonite paste at the GCL overlap. With consideration that the depth of the groundwater table is generally greater than 1 m, the GCL–SB composite cutoff wall will exhibit a good performance in containing groundwater contaminants in the field, especially when applying bentonite paste at the GCL overlap.

Quasi-static test study of tunnel with resistance-limiting shock absorption layer in high-intensity seismic

The Sichuan-Tibet Railway is faced with risks of active faults, unfavorable geological conditions, high in-situ stress levels, and a high potential for Traditional tunnel seismic measures that can no longer guarantee the safety of tunnel structures in high-intensity earthquake areas. The new shock absorption capacity layer with resistance-limiting is proposed in this paper and the resistance-limiting shock absorption principle. To study the safety of the new anti-shock lining structure, a large-scale quasi-static model test was carried out based on the improved static-dynamic coupling shear force. The test analyzes the response rules of strain, structural internal force, and lining failure form of tunnel lining, taking into consideration three influencing factors including layer layout position, layout density, and tunnel buried depth. The results showed that: 1) Under the action of low-cycle reciprocating shear strain, the failure mode of the shallow buried tunnel is mainly the tensile failure of the arch waist and the failure of the arch waist and the side wall, and the failure mode of the deeply buried tunnel is mainly the compression failure of the arch waist; 2) Compared with the case of no shock absorption layer, the lining tensile stress on the inner side of the arch waist position is significantly reduced. Under the high shear strain (0.20%), the increased tensile stress trend was slowed down on the inner side walls; 3) The main working area of the resistance-limiting layer can effectively improve the lining stress distribution underground vibration and protects the tensile damage on the arch waist to the side wall. The width and depth of the crack are weakened and the seismic bearing capacity of the width and depth of the crack is weakened and the seismic bearing capacity of the lining structure is greatly strengthened; 4) The lining cracks without the resistance limiting layer develop violently, and the crack at the arch waist runs The lining structure failure mode of the resistance limiting layer arranged at the arch waist is similar to The lining structure failure mode of the resistance limiting layer.

Large-Scale Model Test on Water Pressure Resistance of Lining Structure of Water-Rich Tunnel

In order to solve the problem of testing the water pressure resistance of lining structures of water-rich tunnels and the difficulty of implementing the existing model tests, a large-scale model test method was proposed relying on the New Yuanliangshan Tunnel threatened by high pressure and rich water. This method creatively transformed the external water pressure of the lining structure into internal water pressure, and the conversion coefficient of water resistance of lining under different sizes and loading modes was obtained by numerical calculation. Results showed that the ultimate water pressure resistance of the lining structure under an external uniformly distributed water pressure and local water pressure was 1.44 and 0.67 times of that obtained from the large-scale model tests, respectively. By conducting the large-scale model tests and combining with the conversion coefficient, the water pressure resistance of the actual tunnel lining could be obtained. Research indicated that water pressure resistance of K2.0 (bearing water pressure of 2.0 MPa) type lining at the transition section of karst caves and K3.0 (bearing water pressure of 3.0 MPa) type lining at the section of karst caves of the New Yuanliangshan Tunnel was 3.33 MPa and not less than 4.36 MPa, respectively, and the high reliability of the large-scale model tests was verified by numerical calculation, implying that the model test method could be extended to similar tunnel projects.

Calculation model of heaving amount on embankments using multi-factor coupling of low swell potential mudstone as foundation for high-speed railways

With the swelling behaviour of red-bed low swell potential mudstone in the foundation of railway tracks, ballastless tracks for Lanzhou-Xinjiang high-speed railway (LXRH) may encounter transfinite heaving deformation on embankments. These are difficult to repair, can easily relapse and may persist for long periods of time. In this study, a theoretical calculation model for estimating the heaving amount on ballastless track embankments, caused by the low swell potential mudstone foundations, was established using the principle of viscoelasticity, Mindlin's elastic solution and the superposition method. The model comprehensively considered the expansibility, water content, thickness, density and overlying load of mudstone, and was built by quantifying the free and constrained heaving amounts. Two in-situ expansive rock foundation tests and a large-scale model test were conducted to validate the proposed theoretical model. The results of Case 2 indicated that 89.3% of the samples exhibited a relative error less than 20%; Cases 1 and 3 exhibited relative errors of less than 20% between the theoretical and experimental values. The theoretical calculation model can compensate for the deficiencies of existing theoretical analysis and estimate the heaving amount on embankments. It can also be used as a reference for the design and construction of ballastless track subgrades in low swell potential mudstone areas.

Calculation method and model tests of pile frost jacking for railway overhead contact systems in permafrost regions

One of the major challenges encountered in electrification reconstruction project of Golmud-Lhasa section of Qinghai-Tibet railway of China is ensuring the frost jacking stability of the pile foundation of overhead contact system mast (OCSM). To evaluate the resisting frost jacking performance of OCSM pile in permafrost regions, a theoretical model of OCSM pile in permafrost soils under the effect of frost heave was constructed based on superposition principle and deformation coordination relationship between the pile and soil. The model was solved by using the finite difference method combined with MATLAB. Then, large scale model tests were performed to investigate the thermodynamic behaviors of OCSM piles with different sections (equal section circular pile, straight cone cylindrical pile and curved cone cylindrical pile) in the permafrost soils under freeze-thaw actions, after which the theoretical model were validated. It was found that distribution curves of pile axial force obtained by the calculation method is basically consistent with model test results in the overall trend, implying that the calculation model is reasonable and feasible. During soil freezing, the pile is under tension as a whole, and the axial force is greatest near the depth of frost penetration. The maximum value of tangential frost-heave stress occurs near the ground surface. The total tangential frost heaving force of the curved cone cylindrical pile is the smallest, and the effect of resisting frost jacking is the best. These research results can provide a reference for resisting frost jacking design of OCSM pile in permafrost regions.

Large-scale testing of the mortise and tenon joint performance of the tunnel lining of prefabricated frame tunnels

Although prefabricated frame tunnels are widely used, research on their structural performance is limited, especially research on the weakest joints in prefabricated tunnels. In this study, a large-scale physical model test is used to study the mechanical properties of joints with bolts and without bolts under static loading, mainly considering the joint concrete and steel strain, opening amount, deflection, rotation angle, toughness, and ductility. The failure of the two joint types is quantitatively analysed using the virtual hinge angle and plasticity index. In addition, a finite element model is established, the model test results are compared, and the effects of the axial force and bolt preload on the joint performance are evaluated. The results show that the concrete strain and steel strain of the two joint types do not exceed the limit values under the ultimate load, providing sufficient ductility. The mechanical properties of the bolted joint are improved by the anchoring and crack arrest mechanism of the bolt. The deflection produced in each stage is lower and the deformation resistance is stronger for the bolted joint than for the boltless joint. Due to its larger damage area and greater number of cracks, the bolted joint exhibits a smaller rotation angle and stronger load-bearing capacity, and the material performance of the joint concrete is fully utilized. Increasing the axial force of a joint can significantly improve its bearing performance, but increasing the bolt preload has no significant effect. In the design of prefabricated frame tunnel joints, bolts can be added to improve the bearing capacity and deformation resistance.

Dynamic responses of a novel floating tapered trash blocking net system due to wave-current combination

To ensure the safe operation of coastal power plants in nearshore regions, an innovative tapered net system has been widely applied to block the floating bio-fouling and debris. Exposed to the combination of coastal waves and currents, these flexible nets are prone to significant deformation and excessive drawing force, which arises high risk of the system failure. To explore the hydrodynamic performance of the tapered fouling interception system, large-scale model tests have been conducted in wave-current flume under various pure current and wave-current combined conditions. Specifically, a dual-scale similarity is applied in the tapered net model to overcome the dilemma of Reynolds Similarity in Froude scaling tests. The results show that the existence of wave action leads to significant dynamic loading on the connector between the supporting frame and tapered nets, subsequently provoking frequent slack-taut on the connector. This nonlinear characteristics of the drawing force is easier to be triggered by the increased wave height and the enlarged current velocity. Whereas for the effect of wave period, the changing law deviates from the previous studies for a net panel, which demonstrates the importance of the overall shape consideration in the hydrodynamic study of the novel tapered trash blocking system.

Large-scale model test studies on a double-layer rubber dam

A new rubber dam with two layers of dam bodies anchored together into a rigid concrete base was proposed to improve the water-retaining capacity of a traditional rubber dam. A series of large-scale model tests was conducted to evaluate the static behaviour of a double-layer rubber dam under conditions of different internal and external water heads, anchoring distances and cross-sectional perimeters. It was found that the maximum tensile force of the Layer-1 dam is located at the anchoring point but that of the Layer-2 dam is located at the extruded free section. The optimal cross-sectional perimeter ratio was concluded as 0.8 with an optimal anchoring distance of 0.06 L1 and an internal water head in the upstream dam and in the downstream dam of 0.40 L1 and 0.36 L1, respectively, where L1 is the cross-sectional perimeter of the upstream dam.

Study on Interaction Mechanism of Natural Gas Pipe-Landslide System Reinforced by Micropile Groups Based on Model Test

Natural gas pipeline projects in mountainous areas are inevitably affected by geological disasters such as landslides, which pose a serious threat to the safe operation of pipelines along the routes crossing landslide areas. In this paper, based on a pipe-landslide project in a mountainous area in southwest China, the interaction mechanism and failure evolution process of the landslide-pipeline system reinforced by two kinds of micropiles are studied through indoor large-scale physical model tests, and some suggestions on the support work of the pipe-landslide project are put forward according to the test results. It was found that the deformation process of the engineering system composed of landslide, micropile, and pipeline presents a high degree of synergy under the external force and mainly experiences four stages: initial deformation period, uniform deformation period, accelerated deformation period, and residual deformation period. The bending deformation of the perforated pipe micropile is large at the 1/4 position of the pile top from the pile bottom, and the deformation of the screw micropile near the sliding surface is serious. The pipeline welding port is the weak position of the pipeline; after the failure of the pile, the pipeline interface is first cracked, along the interface position along the two ends of the tear, and finally completely broken. The screw micropile cannot effectively resist the landslide thrust at a large load level, so the risk of pipeline damage is greater. The yield strength and ultimate strength of the perforated pipe micropile are greater than those of the screw micropile, and the perforated pipe micropile can still exert a certain residual resistance after reaching the ultimate bearing capacity, which has a beneficial effect on the reinforcement of the pipeline crossing the landslide system. The research results provide important reference value for landslide-pipeline treatment engineering.

Land reclamation using the horizontal drainage enhanced geotextile sheet method

Increasingly waste materials or soft soil dredged from sea or river have to be used as fill materials for land reclamation. Although preloading using prefabricated vertical drains (PVDs) has been commonly used as the treatment method for soft soil, this method is time consuming as it can only be applied after all the fill materials have been placed. In this paper, a conceptual design for land reclamation using a horizontal drainage enhanced geotextile sheet (HDeGs) method combined with vacuum preloading is proposed. Large-scale model tests are carried out to verify the effectiveness of the HDeGs method. The proposed method is also compared with the existing prefabricated horizontal drain (PHD) method and the advantages and disadvantages of the HDeGs with vacuum preloading method are discussed. This study has demonstrated that the proposed HDeGs method is not only effective, but also more efficient compared with the PVD or PHD methods, as it can reduce substantially the construction time required for land reclamation.

Research of jacked pile penetration test based on airbag loading

In view of the problem that the model test of pile jacking cannot reproduce the soil's self-gravity and thus cannot truly reflect the pile jacking force, this paper developed an airbag loading system. The airbag is used to pressurize and simulate the overburden soil weight. The method of subsection pile jacking and coupling is ingenuously adopted to achieve long pile penetration. Because the airbag and pressure can be controlled well, it is convenient to simulate the cohesive soil foundation. The size of the model pile used in the test is larger than that of the usual centrifugal model test pile, so it belongs to large scale model test. The experimental results using this system show that the pile jacking force increases with the increase of penetration depth. The degradation of pile lateral resistance has little impact on pile jacking force. After modification based on the degradation theory of pile lateral resistance, the overall variation law of pile lateral resistance is good, and the difference value generated at the joint of pile segments is basically 0. With the increasing of pile depth, the pile lateral resistance is gradually exerted. The pile end resistance fluctuates with the depth, and accounts for a relatively high proportion of pile jacked force, which is maintained above 50%. The method of pile jacking with airbag loading in this paper provides a new idea for the study of the penetration characteristics of jacked pile.

Model Test of Micro-Pile Group Reinforcing High Steep Landslide

High steep landslides are a major concern for infrastructure construction in the mountainous areas of Western China. The micro-pile technique has been gradually used to prevent landslides, due to convenient construction and good performance. However, the application of the micro-pile technique on landslide prevention was generally implemented on the front edge of landslides, which is not applicable for the high steep landslides due to the limited operation space. In this study, a large-scale model test on the performance of a micro steep pile group on the prevention of high steep landsides was conducted in order to implement the micro-pile on the top of landslides. The force-deformation characteristics and failure modes of the steel pipe micro-pile group reinforcing high steep landslides were investigated. The test results showed that the landslide thrusts acting on the micro-pile group showed a triangle distribution. The maximum soil earth pressure was observed near the slip surface during landsides. The resistance of the micro pole group was distributed in an inverted triangle, mainly in the upper half of the loaded section. The sliding bed resistance is unevenly distributed along the height direction, and is larger near the slip surface. Once the landslide occurred, the force distribution of each row of steel pipe micro-piles was basically the same. The bending moment of the loaded section of the steel pipe micro-pile was mostly negative, with a larger bending moment in the range of eight times the pile diameter above the slip surface. The largest bending moment value is located at two times the pile diameter on the slip surface. On the other hand, the bending moment of the embedded section of the steel pipe micro-pile is mostly positive, showing a tension state with a maximum value at four times the pile diameter under the slip surface. This implies that the role of loaded and embedded sections of the micro-pile group on the landsides is different. The failure mode of the micro-pile group was mainly attributable to the bending failure within eight times the pile diameter above and below the slip surface.

Dynamic Response and Geogrid Strain Analysis of GRS Retaining Wall

Modular Geogrid Reinforced Soil (GRS) retaining walls, as flexible structures, usually have a certain deformation capacity. However, the deformation damage of the facing panels will directly affect the durability performance of the retaining wall and pose a threat to the safety and operation of the road and related facilities. In order to study the influence of different load factors on the deformation mode and failure characteristics of the retaining wall, an indoor large-scale model test was carried out. The test load considers the average load, peak value, amplitude and frequency of load under traffic load. The changes in settlement and horizontal deformation, geogrid strain and acceleration response of the GRS retaining wall are compared and analyzed. The results show that in the dynamic test, the two wall damage modes are “wall facing outward tilt” and “wall facing outward curved”. The maximum strain of the geogrid was 4.5% and 3.6%, respectively, which did not reach the damage strain. The peak load is the largest mechanical response of all load factors, followed by the load magnitude and average value, and finally the load frequency. In addition, combining the existing GRS retaining wall deformation and earth pressure calculation theory, a set of calculation methods for the strain of tendons under external load is proposed.

Large Scale Model Test Study of Foundation Pit Supported by Pile Anchors

Due to the special time–space and environmental effects of the foundation pit, there are many unstable factors in the construction process of the field test. The indoor model test can avoid many uncertainties in the construction process due to its operability, which can reduce the interference with the test results and improve the accuracy of the test. In order to further discuss the force-bearing characteristics and deformation laws of loess pits’ support structure in Northwest China, a large model test of foundation pit supported by a pile anchor with a geometric similarity ratio of 1:10 was designed and completed. The force and deformation characteristics of the support structure were systematically studied by simulating the conditions of additional load at the pit edge, soil layered excavated, and anchors tensioned. The test results show that: for the pile-anchor support structure, the anchors have significant limiting effects on the displacement of the piles. Especially, when the position of the first row of anchors is closer to the pile top, the displacement of the pile is smaller. The stress state of the piles was changed by the prestressed anchor. The passive stress state of piles is changed from one side of tension and the other side of compression to the active stress state of “S” shape, which makes the distribution of the bending moment of piles more reasonable. The measured earth pressure in the process of soil unloading has a nonlinear distribution, which is different from the classical Rankine earth pressure distribution; specifically, the passive earth pressure in front of the pile is more obvious. In addition, the prestress applied to the anchors has a more significant effect on the internal forces of the other anchors. Compared with sequential tensioning, the prestress loss caused by interval hole tensioning is significantly reduced. The greater the number of spaced holes, the smaller the prestress loss and the better the anchoring effect of the anchor. The results of the study can provide reference for similar model tests, and also for related engineering applications.

Influence of vertical grouting steel-tube connection mode on anti-sliding performance

Steel-tube grouting technology has been widely applied in the slope control engineering. While the anti-sliding performances of different connection modes of grouted steel tubes still needs further investigation. In this paper, a large-scale model test was used to study the influence of three types of pile connections, namely a whole pile without connection, with a reinforcement welded connection, and with a casing connection, on the anti-sliding performance of multi-stage grouted steel-tube pile. The results showed that the slope could be reinforced by all multi-stage grouted steel-tube pile structures controlled by different pile connection modes, and the composite anti-sliding body of the “root-shape” cement column could be formed around the pile, which effectively improved the perimeter of the pile and enhanced the anti-sliding performance of steel-tube pile structure. The steel-tube piles with different pile connection modes all suffered from bending failure near the sliding surface. The horizontal anti-sliding force of the welded steel tube 6 m in length was 1.4 t per root, which was higher than that of the whole pile. The horizontal anti-sliding force of steel casing connected steel-tube with 6 m in length was 8.8 t per root, which was higher than that of the whole pile. The casing connected steel tube had stronger anti-sliding ability due to the increased diameter of the steel tube near the sliding surface. Therefore, it is recommended that the casing connection mode be used in the connection design of steel pipes, and the reinforcement of a pile body near the sliding surface should be strengthened to improve the flexural failure capacity.

Soil disturbance induced by EPB shield tunnelling in multilayered ground with soft sand lying on hard rock: A model test and DEM study

A large scale model test and numerical simulation using three dimensional discrete element method (3D DEM) were carried out in parallel to study the soil disturbance induced by earth pressure balanced (EPB) shield tunnelling in multilayered ground with soft sand lying on hard rock. The model test used a miniature EPB shield machine which could fully reproduce the real tunnel construction of excavation and support. The DEM model simulating the model test was developed to capture the microscopic mechanisms of ground deformation. The surface and subsurface settlement were closely recorded in model test and compared with those of homogeneous ground to clarify the influence of underlying rock on the movement of overlying sand. The volume ratio of rock to sand in excavated muck was closely recorded in DEM simulation to quantitatively analyse the over excavation of soft sand. Results show that both the surface and subsurface settlement trough in multilayered ground are narrower than those in homogeneous ground, while the subsurface settlement in multilayered ground is larger than that in homogeneous ground. The width parameter of settlement trough decreases nonlinearly along the gravitational direction. The over excavation of overlying soft sand is influenced by the ratio of hard rock to tunnel cross section and the rotating speed of shield cutterhead.