فهرست مطالب

Civil Engineering - Volume:17 Issue: 2, 2019
  • Volume:17 Issue: 2, 2019
  • تاریخ انتشار: 1397/12/02
  • تعداد عناوین: 10
|
  • Determination of Hydraulic Conductivity Using a Modified Cylindrical-Half-Spherical Piezocone Model
    Mingfei Zhang, Li, yuan Tong * Pages 161-170
    To obtain more accurate values of in situ hydraulic conductivity, the present paper has outlined a new method based on the analysis and comparison of existing methods using piezocone data. Owing to results obtained from many numerical simulations and in situ tests, more substantial assumptions are proposed as being more suitable: (1) the flow surface of pore water is assumed to be cylindrical-half-spherical in shape, and (2) the negative exponential function rules the distribution of excess pore water pressure in the soil around the cone. A comparison is carried out between the proposed approach and existing methods based on the graphical and statistical analysis of test data obtained from Quaternary deposits in the Yangtze Delta region. According to the qualitative graphical analysis, the proposed method can evaluate the hydraulic conductivity of soil more accurately. Five different indices and a new graphical analysis using cumulative frequency can be utilized to assess the similar equations. In addition, the results revealed the accuracy and validity of the proposed method, with these methods. The reasonable assumptions, logical derivation, and mathematical analysis together indicate the academic value and application potential of the proposed method. This model and the graphical analysis using cumulative frequency have important guiding significance for the similar analysis.
    Keywords: Hydraulic conductivity, CPTU, Cylindrical-half-spherical flow model, Statistical assessment, Cumulative frequency
  • Non-associative Fractional-Order Bounding-Surface Model for Granular Soils Considering State Dependence
    Yifei Sun *, Yufeng Gao, Yang Shen Pages 171-179
    The constitutive behaviour of granular soil is usually non-associative and depends on the soil density and pressure. To simulate such dependence of the non-associative stress–strain response on material state, two distinct yielding and plastic potential surfaces were usually suggested in the traditional elastoplastic models, which, however, made the model to become complex. To solve this problem, a simple fractional-order plasticity model without using any plastic potential functions was proposed before. However, the model did not consider the dependence of deformation on the density and pressure of soil, which could make the model incompatible with the critical-state soil mechanics. In contrast to the previous study, a state-dependent non-associative bounding-surface model within the framework of critical-state soil mechanics is proposed in this study. The plastic flow direction is obtained using a state-dependent fractional-order differentiation of the bounding surface. To demonstrate the capability of the model, drained and undrained triaxial test results of different granular soils under a variety of initial states are simulated, from which good agreement between the model predictions and the test results is observed.
    Keywords: Factional calculus, Plastic flow, Granular soil, Constitutive model
  • Evaluation of Liquefaction Potential in Sand–Tire Crumb Mixtures Using the Energy Approach
    Nader Shariatmadari *, Mehran Karimpour, Fard, Alireza Shargh Pages 181-191
    This research explores performance of the energy approach for evaluating the liquefaction potential of sand–tire crumb mixtures. Thirty-six stress-controlled undrained cyclic torsional tests were performed under three different confining pressures on samples with different rubber contents. The effect of such parameters as the rubber content and confining pressure on liquefaction behavior of sand–rubber mixtures was studied using the energy approach. Test results indicated that, unlike the observations made on the pure sand sample, in the case of sand–rubber mixture with 25% rubber content, when the initial liquefaction is triggered, the dissipated shear energy increases with continued cyclic loading. The increasing trend is stopped when the pore water pressure reached the initial consolidation stress (Ru = 1). This observation indicates that for the case of sand–tire crumb mixture, the dissipated energy per volume is associated only with the progression of pore water pressure. Moreover, the results show that the required energy for liquefaction occurrence decreases with the increase in the rubber content. The minimum amount of required energy is determined for mixtures with 10% rubber content. As the result, the inclusion of crumb rubber decreases the liquefaction resistance of sand. However, when the rubber content increases from 10 to 25%, the resistance to liquefaction improves. The generation rate for mixture with 25% rubber content is somewhat faster than that of the clean sand and the mixture with 10% tire crumbs, as it is expected.
    Keywords: Scrap tire, Tire crumb, Liquefaction resistance, Energy approach, Hollow cylinder torsional apparatus, Pore water pressure
  • Undrained Shear Strength and Pore Pressure Changes Due to Prestress Concrete Pile Installation in Soft Clay
    Xuepeng Li, Guojun Cai *, Songyu Liu, Anand J. Puppala, Jinhuo Zheng, Tao Jiang Pages 193-203
    This study focuses on the changes of undrained shear strength (Su) and pore pressure (u2) due to the installation of prestress concrete pile (PCP) in Fujian marine soft clay in China. The changes of Su values due to PCP installation are measured by vane shear test (VST) investigation. The piezocone penetration testing (CPTU) is conducted for detecting the change of pore pressure when the PCPs are driven one by one according to the predesigned PCPs construction sequence. The results show that the Su-post values of Fuzhou soft clay are lost about 17% and the Su-post300 value is 1.36 times as much as Su-pre value. It is also shown that three fitting straight lines of Su-pre, Su-post, and Su-post300 are almost parallel which helps predicting the Su value with different depths. The relationship between the normalized excess pore pressure, ∆u/σ′v0, and the ratio of radial distance to pile radius, lg(r/R), is also proposed based on CPTU data, which can be used to predict the excess pore pressure from different distance.
    Keywords: PCP, CPTU, Undrained shear strength, Pore pressure, Soft clay
  • Seismic in-Soil Isolation of Solid Waste Landfill Using Geosynthetic Liners: Shaking Table Modeling of Tehran Landfill
    Vahid Mirhaji, Yaser Jafarian, Mohammad H. Baziar, Mohammad K. Jafari Pages 205-217
    Seismic loads may damage municipal solid waste (MSW) landfills through the relative movements within the landfill system. These movements can disrupt performance of drainage and gas collection systems, thereby resulting in environmental pollution. The smooth synthetic materials might be placed beneath the structures to provide seismic protection by absorbing the imparted energy of earthquakes through the sliding mechanism. It has been found that a high strength geomembrane placed over the other smooth and lubricated geomembrane sheets constitutes an efficient seismic liner. In the present study, experimental investigations were conducted to evaluate role of in-soil base isolation on seismic response of the Tehran MSW landfill. Results of geophysical and geotechnical investigations in the landfill site are presented in detail. Shaking table tests were conducted on the MSW embankment isolated by semi-elliptic shaped liners and subjected to harmonic sinusoidal base excitations. The results for the isolated and non-isolated cases are compared in terms of permanent displacement and seismic response. It has been observed that at all elevations the spectral accelerations within the waste decreased by base isolation, especially for the more intense excitations. Results of the present study demonstrate a suitable application of geosynthetic liners for seismic retrofitting of landfills.
    Keywords: Seismic response, Municipal solid waste landfill, Base isolation, Geosynthetic liner, Permanent displacement
  • An Investigation on the Formation of Cracks at the Corner Turns of the Modular Block Earth Walls
    Murat Hamderi *, Erol Guler, Ayman Raouf Pages 219-230
    The design manuals for Geosynthetic Reinforced Soil Retaining Walls include the methodology for various conditions, except the case where the wall has a curved corner turn. Lately, some problems were reportedly associated with these types of walls. One of the typical problems is cracking/separation of the modular blocks. The most common method for analysing the behaviour of reinforced soil walls is a 2-D plane-strain analysis, which is insufficient for the current problem. Therefore, in this study, a 3-D finite-element (FE) model, that is capable of modelling corner turns, has been established. The main elements of the model are modular blocks, interface elements, soil, and reinforcements. As a first step, the performance of the FE model was evaluated by comparing the stress–strain response of a laboratory-scale wall with its counterpart in the FE program. Later, a large-size modular block wall model was created and run with various input parameters. The modelling results revealed that the reinforcement stiffness and the soil modulus are effective in the separation and cracking of blocks. It is considered that the cracking of blocks is related to an excessive stress build-up. These stresses reduced when the reinforcement stiffness increased. It is foreseen that the crack occurrence is less likely to happen under reduced stress.
    Keywords: Segmental modular block walls, TNO DIANA, Cracks, Corner turns, Separation
  • Physical and Numerical Modeling of Stone Column Behavior in Loose Sand
    Reza Jamshidi Chenari *, Mehran Karimpour Fard, Masoud Jamshidi Chenari, Javad Shamsi Sosahab Pages 231-244
    Stone column technique has been successfully applied for the foundation improvement. A rigid box with dimensions of 1.5 × 1.5 × 1.2 m equipped with a jacking-pump system was used to loading the stone column reinforced bed. Laboratory tests are carried out on stone columns with 6.3 cm diameter and different patterns surrounded by loose sand. The parameters varied in this experimental and numerical investigation are length, diameter and number of stone columns. The number of stone columns in these tests are 4, 5 and 9 and lengths of stone columns are 30, 40 and 50. A 40 × 40 × 2 cm steel plate is used as the model foundation. In these tests, the variation of bearing capacity ratio (BCR) and settlement reduction factor (SRF) are reported for different length and number of stone columns. The results show that by increasing the number and length of stone columns, BCR value will increase and SRF value will decrease. Finite-element analyses have also been performed using the ABAQUS software. The numerical results from the FEM were first validated with the experimental results and then some parametric analyses were conducted to investigate the effect of stone column diameter.
    Keywords: Stone column, Loose sand, Settlement, Bearing capacity, Stiffness improvement factor
  • Analytical Plastic Solution Around Soil-Digging Holes for Inclined Building and its Application
    Yun, juan Chen, Xin Zhang * Pages 245-252
    Soil’s plastic zone around digging holes affects building’s incline-rectifying scheme and effect. In this paper, a new simplified method is utilized to analyze soil’s plastic zone around digging holes. It can take into account the influence of soil-digging dynamic disturbance and lateral earth pressure, while the previous method does not have the ability. First, based on Superposition Principle and Rubin’s Answer, analytical solution of soil’s plastic zone around circular digging holes is established. Then, combined with the Topology Theory, soil’s plastic zone scope around oval holes can be gotten with Affine Function, extruded from circular holes during building’s incline-rectifying process. Assume that soil will collapse when two digging holes’ plastic zones are connected, and by which soil’s maximum width between adjacent two digging holes can be determined. Finally, based on the above theoretical results, an incline-rectifying case of high residential building is simulated by the finite element method, taking into consideration the interaction of superstructure, foundation and soil. Simulations are compared with the field monitoring data. Results show that, the incline rate of this residential building is reduced to be less than 1.50‰, meeting the requirements, and numerical simulation is in good agreement with the field monitoring results, analytical plastic solution can be able to effectively guide building’s incline-rectifying construction.
    Keywords: Building, Soil-digging, Incline-rectifying, Plastic zone, Finite element method
  • Bearing Capacity of Group of Stone Columns with Granular Blankets
    Javad Nazariafshar *, Nima Mehrannia, Farzin Kalantary, Navid Ganjian Pages 253-263
    In this research, the bearing capacity of group of floating stone columns with granular blankets was studied using laboratory tests. To investigate the effect of geosynthetic reinforcement on the bearing capacity of these stone columns, geotextiles and geogrids were used for the reinforcement of the stone columns and blankets, respectively. In this paper, certain large body laboratory tests were performed on stone columns with a diameter of 60 mm and lengths of 200 and 350 mm. The main objective of this research is to evaluate the bearing capacity of group of stone columns with granular blankets, record the stress concentration ratio in the group of stone columns without blankets, and investigate the load ratio parameter. The results indicate that the simultaneous application of group of stone columns and granular blankets significantly increased the ultimate bearing capacity of soft soils. Using geosynthetics for the reinforcement of granular blankets or stone columns improved the efficiency of these granular blankets or stone columns. As length of stone columns increased or stone columns were encased in geotextiles, the stress concentration ratio and stiffness of such stone columns were increased. Increasing the length of the short stone columns is more efficient than reinforcing them. Bulging was decreased in reinforced and unreinforced stone columns that were placed under the granular blanket.
    Keywords: Group of stone columns, Granular blanket, Bearing capacity, Reinforcement, Ground improvement
  • Vibration Propagation of Diverse Footings on Saturated Sand
    Guangya Ding *, Fan Sun, Hongtao Fu Pages 265-279
    Dynamic behaviour of diverse footings resting on saturated sand filled in a large model groove was investigated. The vibration parameters include the frequency and waveform, which were related to the footing height, shape, stiffness, and embedment. A vertical-vibration attenuation equation for saturated sand was proposed. Experiments on the dynamic response of the footings resting on geogrid-reinforced saturated sand were carried out in terms of the geogrid layers, burial depth, and geogrid area. The results show that, the velocity amplitude increases with an increase in the frequency from 0 to 27 Hz and is maximum at the resonant frequency. Moreover, the vertical velocity for a rectangular footing is the highest among three different footing models. Increases in the footing stiffness and footing height lead to a beneficial reduction in the dynamic response, and the vibration velocity increases with an increase in the footing embedment in sandy soil. In addition, the vertical velocity reduces with the increase in the number of geogrid layers, and with the increases in the geogrid area and burial depth.
    Keywords: Saturated sand, Geogrid, Footing, Vibration, Propagation pattern