جستجوی مقالات مرتبط با کلیدواژه "خاک نرم" در نشریات گروه "عمران"

The increasing construction of high-rise structures in cities and the placement of buildings on soils with different layers show the need for engineers to pay attention to the effect of different conditions of soil layers on the seismic response of structures. In this study, after validating the 3D numerical model using the previous shaking table test, the effect of soil layering on the seismic response of resistant concrete buildings has been investigated by considering the soil-structure interaction (SSI). Using Abaqus finite element software, a set of numerical modeling for a 15-story building placed on layered soil with different values of shear wave velocity has been simulated. Nonlinear dynamic analysis under seismic motion has been performed in a direct way and the results have been compared and discussed in terms of maximum lateral deflection, shear force, and inter-story drifts of floors. The results showed that the deeper the soil layer with the lower shear wave speed and the closer it is to the ground surface, the higher the values of lateral deflection and shear force and the inter-story drifts. Also, according to the results, soil layering has a major contribution to the seismic response of buildings by considering SSI, and considering the soil-structure interaction ensures the safe and economical design of structures.

 In this research, comprehensive studies have been carried out by laboratory and numerical modeling methods on the variables affecting soil improvement with soil-cement columns under different conditions of overhead and seismic loading. experimental studies are aimed at evaluating the performance of soil-cement columns in soft ground stabilization under two conditions of rigid footing and embankment fill, and based on the results, the numerical modeling was defined and validated (numerical modeling error less than 8%).
The results of experimental studies showed that the stabilization by using soil-cement column under embankment fill condition caused more stabilization of soft soil compared to the condition of rigid footing and the amount of settlement decreased by 75% and 55% respectively. The results of numerical modeling showed that the use of soil-cement column has improved the seismic behavior of soft soil and the average of settlement under the loading of 5 major earthquakes has decreased by 54.13% under rigid footing conditions and 71.63% under embankment fill.

Although cement stabilization is used extensively to modify the soft clays, it may show limited success in some applications. Hence, this paper presents a multiscale investigation on the viability of employing zeolite and fiber to enhance the durability of cement treated soil against the freezing-thawing (F-T) cycles. In so doing, a wide range (0 to 30%) of additives including sole cement and cement-zeolite mixture (CZ) with different cement replacement were separately added to a soft soil sample and then mixed with the optimal fiber content of 0.75% (by weight), which was determined by the indirect tensile strength test. A set of experiments at various curing days (up to 90 days) were performed to study the mechanical and microstructural changes of the stabilized soils. The results indicated that while a low level of cement can modify the geo-mechanical parameters of soil sample, the compressive strength of cemented soil could decay up to 60% when the specimens exposure to the successive F-T cycles. Such changes may be ascribe to the F-T-induced particles rearrangement and degradation of the cementation structure-bonding, forming many new voids and cracks subsequently decreasing the interlocking of matrix. As a result, to get the strength guidelines threshold and make the composite water proofing a high dosage of sole cement and a long time of curing (at least 28-day) are needed, which may be uneconomical and lade to the brittle behavior. Adding zeolite (≤ 25% proportion) to supplant part of cement could effectively enhance the engineering properties of cement-mixed soil, due to an increase in the cementitious products [e.g. Calcium-aluminate-hydrate (CAH) and Calcium-silicate-hydrate (CSH)] induced by the pozzolanic activity, subsequent reduction of the inter pore-spaces and eventually a more compacted microstructure, as confirmed by the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images. It should be emphasized that the zeolite/cement ratio is a very influential factor on the behavior of cement-zeolite mixture. Therefore, the cemented soil mixes with Zopt showed a further (up to 1.3 folds) resistance relative to the mere cemented soil as well as a greater tensile strength; however, the binary system was still vulnerable under the F-T action. In this case, the insertion of fiber could significantly enhance the soil durability (decrease the degree of damage by an average value of 50%), which was more evident at the small binder dosage and early stage of curing time. Incorporating fiber into the system also led to a higher tensile strength (nearly 1.5 times) than those deduced from the stabilization alone. Moreover, this strategy was effective to overcome the brittle nature of stabilized mixes, resulting an increase the post-strength up to 270%. These observations can be justified by the extended cementing gels formation and the enhanced interlocking of matrix through the CZ-fiber application. Overall, the combination of CZ blend and fiber can be considered as an effective technique for the soft soil modification with the fact that triggered a prominent reduction (~ 30%) in the needed amount of cement an time of curing (up to 3 folds) for the successful treatment against the F-T cycles.

Rail transport has unique advantages over other modes of transport, such as less environmental vulnerability, less pollution and more safety than other modes of transport. Due to the lack of lands with adequate load-bearing capacity, parts of the railway route are built on soft soils, which the possibility of creating instability and subsidence of railway lines in this type of soil is one of the main concerns of engineers. High-speed trains are now used in developed countries to reduce travel time, which can significantly increase the dynamic responses of components on railroad tracks, especially when the train is traveling at a critical speed. One of the ground improvement methods that has been widely used recently for the improvement of soft sediments and loose fine-grained soils is the rock column method. High-speed trains are two important factors in the operation and use of the railway complex. In this research, the performance of stone columns in the bed of high-speed trains to reduce soil subsidence and prevent the occurrence of asymmetric subsidence in different conditions and under the influence of various factors has been investigated. The results showed that with increasing column length Stone and its diameter decreases, asymmetric settling and subsidence due to high-speed train passage is reduced. Also, increasing the hardness of the geogrid reduces the subsidence.

Due to population growth in large cities and the growth of cities vertically, in order to facilitate serviceability and provide low-cost and easier access to the needed resources, engineers are forced to build high-rise buildings on less desirable soils. Given the seismic conditions of our dear country Iran and the variety in thickness of soil in different places, it is important to know the structure suffers the most damage under which conditions. In this study, a nonlinear three-dimensional model of soil-structure interaction in tall buildings with finite element method by Abacus software in the time domain is investigated to evaluate the effect of soil-structure interaction. the dynamic implicit used to calculate the problem. Thickness effect with considering soil-structure interaction for 4 different thicknesses by Drucker-Prager modeling method which also considers soil hardening properties under the effect of Lomaprita earthquake which is considered as an earthquake with low frequency content ( Earthquakes with low frequency content have more destructive effects), has been calculated. infinite boundaries used to absorb the reflect of waves. the boundaries The research results show that changes in thickness have significant effects on the analysis results. In this study, for each model, lateral displacement , acceleration, subsidence, acceleration spectrum and stress contours are investigated.

The compressibility behavior of clay soil has been a major concern in geotechnical engineering. When clay is subjected to a load, effective stress increases with time as a result of dissipation of induced excess pore water pressure, which is known as primary consolidation. After the complete dissipation of the excess pore water pressure, if the load is continuously maintained on the clay, further deformation can be observed over a long period of time, which is known as the secondary compression or creep. When buildings, embankments, and other structures are built on clay soils, large settlements can often be found. Especially, time-dependent deformation in structures built on soft soil may cause damage. Therefore, the study of creep behavior of clay soil for geotechnical application is necessary. Before construction of a structure, to provide a suitable bed, different ways, such as preloading, consolidation and appropriate additives, are applied. In recent years, improvement of soil characteristics using different additives is widely employed. In this study experimental programs are conducted to investigate the effects of low percent Nano silica on creep behavior. One-dimensional single stage and stepwise compression creep tests are conducted using standard oedometer apparatus under different stress levels on soft clay samples. Creep mechanisms are explained based on contacts and deformation of particle and relationship between coefficients of secondary compression and void ratio. Test results clearly indicate that the behavior of soft clay affects percentage of Nano silica and stress levels. With increasing the Nano silica to a specified level, the creep rate of samples decreases, and thus, the soil sample becomes denser. However by a more increase in ratio of Nano silica, porosity and coefficient of secondary compression decreases. On the other hand, it was observed that there is a nonlinear relation between the vertical stress level and coefficient of secondary compression. Also in stabilized samples with Nano silica, the values of compressibility index are smaller than the samples without Nano silica. Of course with increasing the Nano silica to 0.5 weight percentage, due to soil resistance increment against settlement the compressibility index decreases, and with more increase in the weight percentage of Nano silica the compressibility index increases.