Centrifuge study of seismic response of soil-nailed walls supporting a footing on the ground surface

Seismic design of soil-nailed walls requires demonstrations of tolerable ranges of wall movements, especially when a surcharge load exists near the wall. In this study, the effect of surcharge location on seismically induced wall movements was investigated using four centrifuge tests. The axial tensile forces, developed along the soil nails during the seismic loadings, were also measured during the tests. At 50g centrifugal acceleration, model tests represented a 12-m-high prototype wall reinforced with five rows of soil nails. To apply a surcharge stress of 30 kPa at the specified location relative to the wall for each model test, a rigid footing was placed on the soil surface. The model soil-nailed walls were subjected to three successive earthquake motions. Surprisingly, it was found that the model wall with the footing located behind the soil-nailed region experienced the largest seismic movements, even more than when the footing was directly behind the wall. Further, the tests showed that the lower soil nails played a key role in the wall stability during earthquake shaking, acting as a pivot for the pre-collapse cases tested, whereas the upper soil nails needed to be sufficiently extended to properly contribute to the seismic stability of the wall.

DEM Simulation of the Load Transfer Mechanism of a GRPS Embankment with a Fixed Geogrid Technique

As a new technique, a fixed geogrid in a geogrid-reinforced and pile-supported (GRPS) embankments has been used to reduce the total and differential settlement. To investigate the load transfer mechanism of the fixed geogrid technique of a GRPS embankment, three discrete element method (DEM) models of pile-supported embankments were established, including an unreinforced embankment, a geogrid reinforced embankment, and a fixed geogrid reinforced embankment. The efficacy of the pile, the evolution law of the contact force chain and the axial force of the reinforcement, and the microscopic load-bearing structure of the soil were investigated. Numerical simulation results showed that the embankment self-weight and surcharge load were transferred to the pile through the soil arching and tensile membrane effect. The settlement could be effectively reduced via the addition of the reinforcement, and the fixed geogrid technique was more conducive to improving the load-bearing ratio of the pile than the traditional reinforcement technique. Compared with the traditional technique of a GRPS embankment, the fixed geogrid technique had a better effect on reducing the total and differential settlement. With the increase in the surcharge load and the settlement of the soft subsoil, the reinforcement transferred a greater load to the pile. The results also showed that the stress of the embankment fill was concentrated at the pile top in all three models. The GRPS embankment with a fixed geogrid technique had a lower soil stress concentration than the other two cases. The contact force chain and stress in the embankment also showed that the reformation of the microscopic load-bearing system of the embankment fill was the internal mechanism that caused the development of the soil arching and the redistribution of stress. Furthermore, the evolution of the fabric parameters in the arching area could reflect the evolution of the soil arching structure. In the fixed geogrid case, the proportion of the load transferred to the pile from the soil arching effect was reduced, and the vertical load transferred to the pile top by the tensile membrane effect accounted for 22–28% in this study. Under the combined effect of the tensile membrane and the soil arching, the efficacy of the pile could increase by 10%.

The Analysis of Anchoring Mechanism of Rock Slope in Two Layers Based on the Nonlinear Twin-Shear Strength Criterion

The nonlinear strength criterion is introduced into the paper firstly, and then the geometric anchoring characteristic model of rock slope separated from two layers according to rock type with the fracture surface of log-spiral curve considering nonlinear twin-shear criterion subjected to seismic loads is established. Finally, some correlated influential parameters are analyzed, and the conclusions are drawn, showing that the intermediate principal stress has great influence on the total anchoring force of bolt P, so when different criteria are selected, the influence of intermediate principal stress should not be omitted. The effects of thickness ratio h1/h2 on total anchoring force increase gradually with the increase of inclination angle ϕ . The total anchoring force P decreases gradually with the increase of inclination angle ϕ and surcharge load q. The total anchoring force increases gradually with the increase of horizontal seismic acceleration coefficients kh and geometric coefficients mi and GSI.

A 3D numerical analysis of the compaction effects on the behavior of panel-type MSE walls

Soil is weak in tension but strong in compression. The resistance to tensile deformation of soil is given by the tensile force of the reinforcement in the reinforced soil, and the tensile force of the reinforcement is generated by the frictional force at the soil-reinforcement interface. When the soil-reinforcement is effectively interacted by the compaction, the deformation of the soil becomes equal to the tensile deformation of the reinforcement material, which means that the soil is bound to the tensile force of the reinforcement material and thus has a great resistance to the tensile deformation. Therefore, compaction is one of the major parameters affecting the behavior of the mechanically stabilized earth (MSE) wall. In this study, a series of numerical analyses was performed to investigate the compaction effect on the behavior of the MSE walls. The results showed that the horizontal displacement of the MSE wall significantly increased during the construction and decreased because of surcharge load application after the construction. In addition, the strains of reinforcement increased significantly during the construction and decreased slightly because of surcharge load application after the construction. Therefore, it is important to consider the compaction loads when modeling the MSE walls, so that the lateral displacement at wall facing will not be underestimated during construction and will not be overestimated because of surcharge load application after the construction.

STUDI PARAMETRIK JARAK PENGARUH PENURUNAN DAN PERGERAKAN LATERAL AKIBAT VACUUM PRE-LOADING PADA DAMAGE AREA SEKITAR

ABSTRACTSoil-fill is type of soil with low bearing capacity, therefore it’s need soil improvement to resolve the settlement. Soil improvement divided into 2 categories, namely methods that use new material and reinforcement. Commonly used method is PVD combined with vacuum pre-loading. Pre-loading is an application to increase surcharge load which aims to reduce the primary settlement occurs. Pre-loading not only causes settlement, but also cause lateral displacement which cause damage to the outside area around the improvement area. Thus, an analysis of distance effect between the improvement boundary and outside of improvement area in needed to prevent damage to utility around the site. Deformation analysis will be assisted by 2-Dimensional finite element program. Width of the improvement area is 80 meters with a depth of PVD is 14.5 meters to verify parameters. With the parameters that have been verified, an analysis is carried out on PVD with depth of 5m to 30m to determined distance effect of settlement and lateral displacement from boundary of the improvement area to until the value of the settlement and lateral displacement reaches <2cm. Result of studies on general is to find distance effect caused by vacuum pre-loading in areas outside the improvement boundary.ABSTRAKTanah hasil urugan merupakan jenis tanah lunak dengan daya dukung yang rendah, sehingga terjadi penurunan konsolidasi dan membutuhkan perbaikan. Perbaikan tanah dibagi menjadi 2 kategori, yaitu metode yang menggunakan material baru dan menggunakan pemanfaatan perkuatan. Metode yang umum digunakan adalah PVD yang dikombinasikan dengan vacuum pre-loading. Pre-loading adalah aplikasi penambahan beban surcharge yang bertujuan agar terjadinya penurunan primer. Pre-loading tidak hanya menyebabkan penurunan, tetapi juga menyebabkan terjadinya perpindahan secara lateral kearah luar yang dapat menyebabkan kerusakan pada area luar disekitar daerah perbaikan. Sehingga, dibutuhkan analisis jarak pengaruh antara batas lahan perbaikan dengan daerah luar perbaikan, untuk mencegah kerusakan pada struktur atau utilitas disekitar lokasi perbaiki. Analisis deformasi menggunakan program elemen hingga 2D. Lebar area perbaikan 80 meter dengan kedalaman PVD 14.5 meter untuk melakukan verifikasi parameter. Dengan parameter yang telah diverifikasi, dilakukan analisis pada PVD dengan kedalaman 5m hingga 30m untuk mengetahui jarak pengaruh penurunan dan pergerakan lateral dari batas lahan perbaikan hingga nilai penurunan dan pergerakan lateral <2 cm. Hasil studi secara umum menunjukkan seberapa besar jarak pengaruh yang diakibatkan oleh vacuum pre-loading pada daerah diluar batas perbaikan.

Dynamic Stability Analysis of Slope Subjected to Surcharge Load considering Tensile Strength Cut-Off

Surcharge slopes are more vulnerable to instability under the effects of earthquake ground shaking, especially considering the tensile stress. In order to account for the adverse factors of seismic forces and tensile stress, the theory of soil with tensile strength cut-off is deduced and analyzed using the upper bound limit analysis method in this paper. Combined with the quasistatic analysis, the equation of critical acceleration expression for surcharge slope subjected to the dynamic conditions has been evaluated. By using the improved Newmark method, permanent displacements have been analyzed in the case of the classical earthquake ground motions. In addition, optimization algorithm has been undertaken, in which several influencing factors such as slope inclination, internal friction angle, surcharge factor, seismic load, and tension cut-off coefficient have been taken into account, and some results are verified with the classical solutions and FEM results. The results concluded the following: (1) The outcomes of verification results are accurate. (2) The critical acceleration of the slope is significantly affected by tension cut-off with the increasing of surcharge factor and seismic effects. (3) The permanent displacements of surcharge slope considering the tensile strength cut-off can be even 2 times of the traditional analysis; meanwhile, with more reduction of tensile strength, the cumulative displacements increase rapidly. Therefore, considering the influence of tensile strength cut-off is fundamental to the dynamic stability design of surcharge conditions.

Study on Dynamic Behavior of Bridge Pier by Impact Load Test Considering Scour

In this study, for the establishment of a safety evaluation method, non-destructive tests were performed by developing a full-scale model pier and simulating scour on the ground adjacent to a field pier. The surcharge load (0–250 kN) was applied to the full-scale model pier to analyze the load’s effect on the stability. For analyzing the pier’s behavior according to the impact direction, an impact was applied in the bridge axis direction, pier length direction, and pier’s outside direction. The impact height corresponded to the top of the pier. A 1-m deep scour was simulated along one side of the ground, which was adjacent to the pier foundation. The acceleration was measured using accelerometers when an impact was applied. The natural frequency, according to the impact direction and surcharge load, was calculated using a fast Fourier transform (FFT). In addition, the first mode (vibratory), second mode (vibratory), and third modes (torsion) were analyzed according to the pier behavior using the phase difference, and the effect of the scour occurrence on the natural frequency was analyzed. The first mode was most affected by the surcharge load and scour. The stability of the pier can be determined using the second mode, and the direction of the scour can be determined using the third mode.