مجله زمین شناسی مهندسی
سال سوم شماره 2 (پاییز و زمستان 1388)

Shear strength characteristics of sand-gravel mixtures in high gravel contents

Engineering characteristics of alluvium and cemented materials of the slopes around the Milad Tower, and the results of slopes stability analyses under static condition is presented in this paper. Also in the paper, the feasibility of developing and using artificial neural networks (ANNS) for slope stability prediction is investigated. According to the geometry of slopes and strength and deformation properties of alluviums, factor of safety is calculated in 2D and 3D by PLAXIS7.2 and PLAXIS 3D Tunnel codes, respectively, and the results are also compared. In addition, stability of slopes is investigated through the use of MLP artificial neural networks (ANNs), which developed in MATLAB environment. The database used for development of the model comprises a series of 252 factor of safety for different slopes conditions (2D, 3D, flatted and 18 inclined from horizon at top of cut). The optimal ANN architecture (hidden nodes, transfer functions and training) is obtained by a trial-and-error approach in accordance to error indexes and real data. The input data for slope stability estimation consist of values of geotechnical and geometrical input parameters. As an output, the network estimates the factor of safety (FoS). The results indicate that the ANN model is able to accurately predict the FoS of the slopes.

Engineered waste landfills separate the buried waste from the groundwater system and the above ground environment during and after the operational periods of the disposal facility. Liners of these landfills are of great import-anttce as they play a crucial role in minimizing pollutant migration and therefore preventing ground water contamination. Liners may consist of a locally available soil and bentonite mixtures. In this study effect of NaCl and MgCl2 solutions of various concentrations has been investigated.The mixtures were prepared with a local clay specimen from Varamin and a proportion of bentonite content of 10 % and 20 % of total dry weight. Laboratory tests including 1D consolidation for determining permeability and compressibility of the soil mixtures, liquid limit and free swell tests were carried out. Results indicated that both solution type and its concentration affect the properties of the mixtures. Comparison of hydraulic conductivity of different salt solutions shows that the divalent cations have more effect than monovalent cations. Comparison of different salt concentrations for a particular salt on a particular soil mixture shows that the hydraulic conductivity decreases with decreasing salt concentration. This decrease can be attributed to an increase in diffuse double layer thickness.

Silakhor Brojerd area is between high Zagros zone and Sanandaj-Sirjan zone and is one of the seismic areas, with high frequency of occurrence and medium magnitude and sometimes high magnitude, in Iran. Area of maximum destruction and fault plane solutions show a NW-SE trend. Observation of aftershock distribution, caused by earthquake of 1384/2/31, on the region delineated that most of them have occurred in the epicenter area of the main shock, on Ghale-Hatam fault. Also most of aftershocks, caused by earthquake of 1385/1/13 in Brojerd, have occurred in epicenter area of the main shock on Doroud fault. Moreover, focal mechanisms of earthquakes, are right lateral strike-slip with some normal or reverse dip-slip. These different solutions, next to each others indicate partitioning in this area of Zagros.

Studying the seismic behavior of earth dams is one of the most important problems in the geotechnical science. Using weak recorded responses on dam body is the most appropriate method in studying seismic behavior of dams. Available accelerograms on Masjed Soleiman dam body is a powerful tool to reach to this aim. Using recorded accelerograms in the gallery station as the input motion and also using different masses for foundation, 2D numerical analysis are performed and the dam-foundation interaction problem is studied. Recorded and calculated responses of dam body are compared with each other based on different approaches. It should be mentioned that the used earthquake records in this research are weak and linear behavior for dam body is assumed. The obtained results show that time-frequency distribution (TFD) is an appropriate method to process the recorded and calculated signals and also using below 50 per cent (20-30 per cent) for the mass of foundation is can lead to the nearest results to recorded responses.

Shear behavior of sand-gravel mixtures is an important issue in geotechnical engineering due to extensive use in earth dams, borrow materials and slope stability problems of natural or man made slopes. Experimental study of behavior of this type of material is usually difficult due to problems associated with use of large testing apparatus or large specimens. Several researchers have proposed methods for the test of these materials by removing larger fraction of them and testing finer matrix. In this research, a regular set of large direct shear tests with a 300*300*160 mm shear box performed to determine shear strength and dilation characteristics of sand-gravel mixtures in high gravel contents. Bad graded fine sand was used besides uniform gravel grains with maximum grain size of 12.5 mm as testing materials. The tests conducted on mixtures prepared in four gravel contents of 70, 80, 90 and 100 percents. Samples constructed in three relative densities of 35, 60 and 85% and tested by different surcharge pressures of 100, 200 and 300 kPa. Experiments conducted on completely dry samples in consolidated drained condition. Shear strength behavior of mixtures was investigated and it was concluded that both shear strength and friction angle increases with gravel content, relative density and surcharge pressure. However, dilation increases by increase in gravel content and relative density and reduces with increase in surcharge pressure. Results were compared with previous experiments on mixtures with less gravel contents and it was observed that in little gravel contents the behavior is controlled by the both sandy matrix and gravel grains. However, as gravel content increases from floating limit, the behavior is mainly controlled by gravelly part of mixture and sand fraction has little effects on its behavior.

Jet grouting is a method for improving of soil properties and its physical characteristics. However, in this method due to high pressure and velocity of cement slurry the soil structure has been damaged as some parts are moved from the borehole replacing with cement slurry. The grains, which are remained in the borehole, mixed with slurry (cement) and create an improved mass of soil. This mass is named “Soilcrete”. Soilcrete mass has special characteristics such as high strength, low deformability and very low permeability. In this paper, principles governing to jet grouting and effective parameters have been explained. Then the test results obtained from Soilcrete column have been analyzed and discussed. Based on the results, jet grouting has led to increase and improvement of physical and mechanical characteristics of soils, i.e. uniaxial compressive strength, cohesion and internal friction angle. Finally the values of jet grouting parameters are recommended in order to achieve larger diameters in the mentioned site based on trial grouting results.