رفتار سازه و مطالعه پارامتریک از خاکریزهای تقویت شده در رس نرم
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|28552||2001||25 صفحه PDF||سفارش دهید||4250 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Computers and Geotechnics, Volume 28, Issue 3, April 2001, Pages 209–233
Geotechnical behaviour of a reinforced embankment on soft ground is studied by a numerical model based on the finite element method. Special emphasis is given to the stress states (stress levels and pore pressures), displacements, tensile forces in the geosynthetic and overall stability, during and after construction. The influence of some parameters, namely geosynthetic stiffness and viscosity, embankment width and construction sequence, is also analysed. Several conclusions are obtained.
The increasing occupation of the ground over the last decades, due to economical and social development of the populations, has led to the necessity of using soils with bad geotechnical characteristics as foundation of multiple engineering works. Particularly, the construction of embankments on soft soils, characterised by their low strength, high deformability and low permeability, has become nowadays an increasing reality, despite the difficulties associated to these works, generally related to overall stability deficiency and to high settlements that develop slowly. Geotechnical engineers have developed several alternatives to solve these problems and, in recent years, geosynthetic reinforcement has been added to the list of possible solutions when embankments must be constructed on very soft foundations. In many cases, the use of a geotextile or geogrid can significantly increase the safety factor, improve performance in terms of displacements and reduce costs in comparison with more conventional solutions. Over the two last decades, with the availability of faster computers with larger data storage, it has been possible to perform finite element analyses with increasingly complex formulations. The earliest analyses used elastic models for the materials but quickly moved to non-linear elastic and elasto-plastic models. A critical state model was used by Wroth and Simpson  on a trial embankment and coupled analyses have been capable of predicting the pore pressure response in the field and in centrifuge tests , ,  and . Using reinforcement and interface elements, Kwok  performed a parametric study to investigate the effect of the reinforcement stiffness, the subsoil conditions, the geometry and the embankment constitutive model. Hird and Kwok  used interface elements and showed that useful information regarding the transmission of shear stresses from the soil to the reinforcement could be extracted. Hird and Kwok  carried out a parametric study and concluded that sufficiently stiff and strong reinforcement may significantly reduce subsoil deformations and, for a subsoil of constant strength, the effect of reinforcement reduces with increasing depth. Undrained plane strain finite element analyses have been compared with limit equilibrium solutions  and  and with plasticity solutions for strip footings ,  and . Rowe and Soderman  found that the finite element results were within 7% of the predicted plasticity failure heights for highly reinforced embankments. In Jewell  and , the mechanisms of reinforced embankments on soft foundation soils were examined and design formulations were provided. Jewell  concluded that limit equilibrium methods can be applied to reinforced embankments on soft soil and also presented approximate analytical solutions for foundation of uniform strength and limited depth and foundation with strength increasing with depth. In this paper, the geotechnical behaviour of a reinforced embankment on soft soil is simulated by a numerical model, developed by Borges , based on the finite element method. Basically, the model uses the following theoretical hypotheses: (a) plane strain conditions; (b) coupled formulation of the flow and equilibrium equations, considering soil constitutive relations formulated in effective stresses (extension of Biot's consolidation theory)  and ; this formulation is applied to all phase of the problem, both during the embankment construction and in the post-construction period; (c) utilisation of the critical states model [p,q,θ] , ,  and  to simulate constitutive behaviour of the foundation and embankment soils; (d) utilisation of a hardening elasto-plastic model to simulate ‘instantaneous’ constitutive behaviour of the reinforcement; (e) simulation of viscous behaviour of the geosynthetics (time-dependent constitutive relations) using a rheological model based on a series of Kelvin's models ; (f) simulation of constitutive behaviour of the soil–geosynthetic interfaces using a hardening elasto-plastic model. The study includes analyses of a reference embankment. The effect of some parameters, like geosynthetic stiffness and viscosity, embankment width and construction sequence, is also studied. In order to verify accuracy of the numerical model used in this paper, Borges  compared numerical and field results of two reinforced embankments on soft soils, one constructed up to failure  and other observed until the end of consolidation processes . In both cases, numerical and field results are similar, namely in terms of settlements, pore pressures and strains or tensile forces in the reinforcement. Regarding horizontal displacements, some quantitative differences were observed, despite an overall qualitative similarity.
نتیجه گیری انگلیسی
In this paper, the structural behaviour and a parametric study of a reinforced embankment on soft soil are presented, allowing to formulate the following conclusions. In the foundation, with exception of a loaded zone near the drainage surface, effective volumetric stresses suffer low variations during construction. On the other hand, shear stresses suffer significant variations, which implies the rotation of the principal stresses. These increases of shear stresses, associated with low variations of volumetric stresses, determinate significant increases of the stress levels. During the post-construction period, characterised by the consolidation processes, principal stresses significantly increase (increases of volumetric stresses) and their directions do not have significant alterations (low variations of shear stresses). During construction, there are important upwards vertical displacements, near the embankment toe, and horizontal displacements are outwards. After construction, there is a generalised settlement and, globally, horizontal displacements are outwards too. Reinforcement stiffness increases the tensile forces in the geosynthetic and decreases the horizontal displacements and long time settlements, specially differential ones. As reinforcement stiffness decreases with time due to geosynthetic viscosity, high viscosity implies opposite variations to the ones determined by the stiffness, i.e. decreases reinforcement tensile loads and increases horizontal displacements and settlements. In terms of overall stability, embankment width influences the type of failure surface that can occur. The probability of occurring a failure surface without cutting the embankment and the reinforcement is more effective for smaller values of the embankment width. In this case, the geosynthetic does not directly contribute with its strength to the overall stability. Finally, the most important alterations due to the construction sequence (constructing each embankment layer starting from its extremities) concern the long time settlements, which are significantly reduced in terms of maximum and differential values.