فهرست مطالب

  • Volume:17 Issue:12, 2019
  • تاریخ انتشار: 1398/09/27
  • تعداد عناوین: 13
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  • A Study on Failure Surface of Helical Anchors in Sand by PIV/DIC Technique
    Hassan Motamedinia, Nader Hataf*, Ghassem Habibagahi Pages 1813-1827

    Helical anchors have been used to carry tension loads in different applications including transmission tower foundation, pipeline anchors, foundation repair elements, and excavation bracing. There have been numerous changes in the shape and size of helical anchors and piles since their first usage. Several researchers have studied their failure mechanism to find their pullout capacity. In this paper, the uplift capacity of helical screw anchors has been investigated through laboratory testing. Half-cut double-helix anchors were tested in a sand tank by varying helix size, helix spacing, and relative density of sand. A series of images were captured during the process of anchor pullout. The images were used to obtain displacement and strain fields by Particle Image Velocimetry (PIV). Load–displacement curves have been presented and compared to the earlier works. Afterwards, pullout capacity factors were calculated from peak load values. PIV analysis results were used to study the effects of helix spacing, helix size, and relative density of sand on the displacement fields. The results showed that the effect of helix spacing and soil density in increasing pullout load is more than that of helix size. Moreover, failure surfaces were discussed through displacement and strain fields. The findings indicated that failure surface above the top helix of deeply embedded anchors is a truncated cone with the angle of approximately ϕ/3, with the vertical and the failure surface shape between the two helices dependent on helix spacing.

    Keywords: Helical screw anchors, Experimental testing, Pullout capacity, Failure surface, Particle image velocimetry
  • Safety Standard for Slopes of Ultra-high Earth and Rock-Fill Dams in China Based on Reliability Analysis
    Yanlong Li*, Kangping Li, Lifeng Wen, Bin LiYunhe Liu Pages 1829-1844

    A specific standard for stability analysis of earth and rock-fill dams with a height of more than 200 m is lacking in China. Thus, the safety standard for slopes of these dams is explored on the basis of reliability analysis. The dams with a height over 200 m are divided into special Classes 1 and 2 depending on the risk levels and acceptable risk standards. The concept of reliability theory-based relative ratio of safety margin is utilized to establish the relationship between annual failure probability and safety factor, thereby obtaining the reasonable safety factors for different dams. The reliability indexes and safety factors of dams under various heights and slope ratios are calculated. Results show that the values of safety factors for special Classes 1 and 2 are 1.70 and 1.60 under normal conditions and 1.40 and 1.35 under seismic conditions. The validity of the proposed values of safety factors are verified by carrying out stability analysis for several practical projects in China. These results can provide a reference for exploring the safety standards of dams with a height of more than 200 m.

    Keywords: Earth, rock-fill dams, Stability analysis, Safety factor, Reliability index
  • Comparison of Shear Wave Velocity Prediction Models to Yangtze River Deltaic Sediments Based on Piezocone Test Data
    Qiwen Guo, Hongbo Che, Huangsong Pan, Mingfei Zhang, Liyuan Tong* Pages 1845-1858

    Seismic piezocone (SCPTu) data compiled from 37 sites in the southern Yangtze delta, China, are used to evaluate several existing empirical correlations in predicting the shear wave velocity of deltaic sediments. Four indices were employed in the assessment: the ratio of the estimated values to the measured Vs values, the root-mean-square error, the ranking index, and the ranking distance. It is shown that all the considered prediction models are biased, generally over-estimating the observed shear wave velocity of the predominant intermediate soils in the Yangtze delta. The results also reveal that the overall best correlation for the mixed deltaic soils is given by Robertson [Can Geotech J 46(11):1337–1355, 2009]. It is hypothesized that the unique depositional environment of the considered soils is the primary source of the observed prediction bias. Further statistical analysis indicates that partial drainage effects during cone penetration in deltaic intermediate sediments should be taken into careful consideration to derive reliable Vs values from CPTu data when applying correlations from the literature. More suitable site-specific shear wave velocity prediction correlations should be introduced, according to the recent findings on the Yangtze delta SCPTu database.

    Keywords: Shear wave velocity, Seismic piezocone test, Yangtze delta, Empirical correlations, Partial drainage effect
  • Reliability-Based Calibration of Resistance Factors in LRFD Method for Driven Pile Foundations on Inshore Regions of Iran
    Seyed Ali Asghari Pari, Ghassem Habibagahi*, Arsalan Ghahramani, Kazem Fakharian Pages 1859-1870

    This paper presents the improvement of load and resistance factor design (LRFD) method for axially loaded driven piles in Iran. The LRFD method has been well developed and successfully implemented in geotechnical engineering, especially in the design of pile foundations in different parts of the world. To extend the use of this method in Iran, it is necessary to use the results of reliable local pile load tests and construction records to calibrate LRFD resistance factors regionally. To this end, we first collected a comprehensive database of static and dynamic load tests which have been performed on driven piles in fine-grained soils across Iran. Based on this database, we calculated the resistance factors for different design methods using three different methods of reliability analysis (FORM, FOSM, and MCS) and for two levels of reliability (βT = 2.33 and 3). Finally, we used these calculations together with experience and engineering judgment to propose resistance factors for the National Building Code of Iran.

    Keywords: LRFD, Code calibration, Reliability analysis, Pile foundation design
  • Direct Shear Test for Coarse Granular Soil
    Katarzyna Dołżyk Szypcio* Pages 1871-1878

    A large direct shear apparatus was used to test coarse granular soil. In shear tests with a small shear box, deformation in shear band is homogeneous and stress–dilatancy relationships at pre-peak and post-peak phases of shearing are similar. However, in tests with a large shear box, the pre-peak and post-peak stress–dilatancy relationships are different. Using frictional state theory, it has been shown that the stress–dilatancy relationship for the large shear box is different from that for the small one conventionally used in direct shear tests, which suggests higher non-homogeneity deformation in the shear band in a large shear box in comparison to that in a small one. Parameter α of frictional state theory equals zero for the small and large shear boxes at failure. Parameter β = 1.4 for the small shear box and is a function of initial moisture and compaction for the large shear box. Average values of β range between 2.27 and 2.52 for dry coarse granular soil and between 2.17 and 2.30 for wet one. The value of critical frictional state angle Φ°= 41.2° of tested coarse granular soil is independent of soil moisture and compaction. The influence of the box’s size on stress–dilatancy relationship has also been observed in the previous studies.

    Keywords: Large direct shear, Dilatancy, Frictional state, Coarse granular soil, Stress–dilatancy
  • Load–Displacement Behavior of Driven Piles in Sand Using CPT-Based Stress and Strain Fields
    Mehdi Veiskarami, Fatemeh Valikhah, Abolfazl Eslami* Pages 1879-1893

    The bearing capacity of piles is often estimated by a variety of methods such as the limit equilibrium or the limit analysis. In contrast, the load–displacement behavior, which should not be disregarded in common practices, cannot be obtained as simply as the bearing capacity. The reason is its dependency on the stress and the strain (or the displacement) fields around the pile. In the current work, attempt has been made to predict the load–displacement behavior of driven piles in sand by direct and indirect implementation of the cone penetration test (CPT) data into the displacement field. CPT often serves as a very successful in situ test which provides a close link between the soil resistance and the bearing capacity, although it brings no direct information. A rather simple procedure is presented to indirectly use the CPT data to find the stress and strain fields. While the pattern of the failure mechanism has been obtained by the method of stress characteristics, the displacement (and strain) field has been found by the kinematics of the failure mechanism. The proposed procedure has been calibrated and verified by 98 case histories including pile load test results in conjunction with CPT data. Comparisons made by this new method show that the CPT-based method of stress characteristics can be successfully used in load–displacement prediction of driven piles.

    Keywords: Pile, CPT, Load–displacement, Stress characteristics method, Displacement field
  • Experimental and Numerical Study on Failure Modes and Shear Strength Parameters of Rock-Like Specimens Containing Two Infilled Flaws
    Huilin Le, Yang Shu, Feng Zhu, Haotian Fan, Yong Liu, Wuchao Wang, Shaorui Sun* Pages 1895-1908

    To investigate the influence of infilled flaws on mechanical properties and failure modes of rock masses, seven types of pre-existing infilled two-flaw specimens, which have different flaw inclination angle (α), rock bridge length (L2) and rock bridge inclination angle (β), were made from concrete. The crack coalescence process, failure modes and mechanical parameters of the specimens under triaxial or biaxial compression were studied by lab test and numerical tests, respectively. According to test results, two failure modes of specimen (shear failure, tensile–shear failure) and three rock bridge coalescence modes (tensile crack coalescence, shear crack coalescence, no coalescence) were identified. As the rock bridge length and inclination angle increase, the peak strengths of specimens also increase gradually, while the peak strengths of specimens decrease with flaw inclination angle being increase. The shear strength parameters (cohesion c and internal friction angle φ) of samples show nonlinear changes with various factors (flaw angle, rock bridge length, rock bridge angle). The particle flow code (PFC) was used to simulate the propagation process of microcracks and porosities, stress–strain curves for loading process were also obtained, numerical results are in good agreement with experimental results. The number of cracks and porosities increase rapidly in the post-peak stage, and a significant shear fracture zone was caused by cracks. This study provides a better understanding of peak strength and cracking behaviour of rock mass containing infilled flaws.

    Keywords: Two infilled flaws, Triaxial compression test, Numerical simulation, Failure mode, Shear strength parameter
  • Investigating the Effect of Soil Type on Non-linear Response Spectrum Using Wavelet Theory
    Ali Heidari*, Shirin Pahlavan, sadeghJalil Raeisi Pages 1909-1918

    In this paper, using wavelet denoising theory (WDT), the non-linear response spectrum (NRS) of single degree of freedom (SDOF) was plotted for different soil types, as well as different ductility for earthquake record. Employing the WDT, the ground wave frequencies were filtered in three stages. The wavelet denoising theory was used to smoothen the diffraction in the acceleration and noise spectrum. The NRS of the SDOF was computed for the main earthquake record (MER) and the wave was obtained in the third stage of WDT. In the third wavelet stage, the high frequencies of the MER were separated three times; in the first phase, the largest wave frequencies were eliminated. The results revealed that the third-phase spectrum had an average difference of less than 10% when compared with the original earthquake spectrum. The highest difference was observed in soil type 4 and the lowest difference was seen in soil type 1. In addition, shearing base force of SDOF structure was calculated for each of the four soil types employing the NRS of acceleration.

    Keywords: Earthquake record, Non-linear response spectrum, Single degree of freedom, Wavelet transforms
  • Analysis of Rockfall Hazards Based on the Effect of Rock Size and Shape
    Sharan Kumar Nagendran, Mohd Ashraf Mohammad Ismail* Pages 1919-1929

    Rock sizes and shapes influence the trajectories of rockfall. Thus, this study examined the bounce height and runout distance of falling rocks on the basis of different rock sizes, rock shapes, and ground surfaces. A laboratory experiment of rocks with various sizes falling from 35°, 45°, and 60° slope angles and vertically on different ground surfaces was conducted in this study to understand the mechanism of falling rocks. RocFall 5.0 (Rocscience), a 2D rockfall numerical simulation program, was used to perform the probable bounce height and runout distance for various rock shapes on different ground surfaces. The laboratory experiment and a numerical simulation were compared to validate the applicability of laboratory testing in rockfall assessment and calibrate the coefficient of restitution, which is a critical parameter in bouncing blocks. Results indicated that steep slopes and hard ground surfaces cause a high bounce height of falling rocks. Moreover, light rocks bounce higher than heavy rocks, and rocks with round shapes bounce high initially and then roll further away from the falling slope. Therefore, the influence of rock sizes and shapes and impact surface material must not be omitted in investigating rockfall protective measures.

    Keywords: Rockfall, Shapes, Sizes, Numerical analysis, Laboratory experiment
  • An Investigation into the Compressibility and Lateral Stresses of Sand–Carpet Mixtures Using a Large Oedometer Apparatus
    Mehran Karimpour, FardEmail author*, Habib Shahnazari, Ghazal Rezaie Soufi, Amirreza Saremi Pages 1931-1940

    In this paper, the deformation properties of sand, reinforced with carpet waste, are investigated by carrying out a set of large-scale oedometer tests under different overburden pressures. The oedometer apparatus employed in this study has been equipped with vertical and lateral pressure cells, which allow the value of the soil’s coefficient of lateral pressure at rest, to be determined as well as its compressibility. Carpet waste was added to sand at weight percentages of 0, 5, 10 and 15%. Results were indicative of a reduction in the coefficient of lateral pressure at rest, K0 with the increase in carpet content of the mixtures. The reduction in the value of K0 amounted to 10% at a carpet content of 15%. The compressibility properties of the mixtures were evaluated as well, by recording one-dimensional displacement in the samples. Results demonstrated an increase in the coefficient of volume change with the increase in carpet content, particularly for lower levels of overburden pressure. Predictive models were developed based on the multi-linear regression (MLR) procedure to predict the coefficient of lateral pressure at rest and coefficient of volume change for the sand–carpet mixtures normalized to the values of sand.

    Keywords: Carpet waste, Compressibility, Oedometer, Soil reinforcement
  • Upper Bound Solution of the Safety Factor for a Shield Tunnel Face Subjected to the Hoek–Brown Failure Criterion
    Fu Huang*, Yuan Feng, Zhiqi Zhang, Xiaoli Yang, Tonghua Ling Pages 1941-1950

    Assessment of the stability of a tunnel face under a setting support pressure is a key challenge in shield tunneling engineering, especially when a tunnel is excavated in a heavily fractured rock mass. In this paper, a factor of safety is introduced for investigating the face stability of a tunnel using the upper bound theorem in combination with the Hoek–Brown nonlinear failure criterion. The factor of safety for the tunnel face is defined as the ratio of the rate of the energy dissipation and the external rate of work in the kinematically admissible velocity field. The upper bound solution of the factor of safety is obtained from the optimal computation. The solutions provided by the presented approach are compared with those derived from a numerical simulation. The values of the factor of safety derived from these two methods are found to be in agreement, indicating that the proposed method is valid. Furthermore, based on the measured data of the chamber pressure and the parameters provided by the geological survey report, the proposed method is used to evaluate the stability of the tunnel face for an actual project. The effects of different parameters on the factor of safety are also discussed.

    Keywords: Upper bound theorem, Shield tunnel face, Factor of safety, Hoek–Brown failure criterion, Stability analysis
  • Experimental–Numerical Investigation of Embedment Effect on Foundation Behavior Under Vertical Loading
    Mohammad Arabpanahan, Seyed Rasoul Mirghaderi, Abdollah Hosseini, Amir Parsa Sharif Pages 1951-1969

    Embedded foundations have different behavioral characteristics in comparison to surface ones. In present study, to evaluate the behavioral aspects of five strip foundations with constant width and different embedment depths in sandy soil under monotonic vertical loading, experimental and numerical finite element (FE) modelings were implemented. Based on experimental results, the growth of foundation depth-to-width ratio from 0 to 1.2 caused the increase in ultimate bearing capacity and vertical stiffness for 33% and 23%, respectively. Moreover, regarding to the observed damages, an attempt was made to present an innovative categorization for dominant failure mode as a function of foundation embedment depth. The result of numerical framework designed in accordance with real material and interfaces properties showed a good agreement with experimental behavioral curves as well as ultimate bearing capacity and vertical stiffness. Achieved from FE modeling, the adequate dimensions for underlying soil model were suggested to eliminate any effect of boundary conditions on foundation vertical loading results. Finally, the validity of a common theoretical expression in literature for anticipating vertical stiffness of embedded foundation was explored. It was concluded that for considered domain of foundation embedment depth, the increase rate of vertical stiffness vs. foundation embedment curve achieved through theoretical expression was more than experimental and numerical results.

    Keywords: Embedded foundation, Ultimate bearing capacity, Vertical stiffness, Settlement, Soil swelling
  • Correction to: Load–Displacement Behavior of Driven Piles in Sand Using CPT-Based Stress and Strain Fields
    Mehdi Veiskarami, Fatemeh Valikhah, Abolfazl Eslami* Page 1971