Journal of Engineering Geology

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./files/site1/files/7Extended_Abstract.pdfExtended Abstract
 (Paper pages 135-156)
Introduction
Many civil structures (e.g. tunnel walls, bridge pillars, dam abutments and road foundations) are subjected to both static and dynamic loads. Cyclic loading leads to occurring fatigue phenomenon. Fatigue is the tendency of materials to break, or the process of damage accumulation, under cyclic loading. It was found that the dynamic fatigue strength can be reduced by 30-70 percent on average compared to uniaxial compression strength. Different materials show different response when they are subjected to cyclic loading. Some materials become stronger and more ductile, while others become weaker and more brittle. Although it is clear that the mechanical properties of rock under dynamic loads varied dramatically from those under static loads, the nature of dynamic failure in rock remains unclear, especially in cyclic loading condition. Fatigue behavior of rocks was rarely studied in respect to other materials such as steel and soil. The performed researches on fatigue behavior of rocks indicated that fatigue life will be decreased by increasing load amplitude in logarithmic and exponentially pattern. Also, strain softening is the dominated behavior of rocks against cyclic loading. Furthermore, some parameters such as maximum load level, confining pressures, amplitude, and loading frequency have considerable effects on fatigue behavior of rocks. However, available data on fatigue behavior remain insufficient for solving the practical tasks of predicting rock bursts and earthquakes. Obtained results are inconclusive and sometimes discordant. The aim of the current work was to assess tonalite rock fatigue behaviour under different loading conditions to describe the fatigue damage process of the granitic rock.
Material and methods
Several core samples were prepared to perform this research. The core samples were prepared with a L/D ratio of 2.5 with an average diameter of 54 mm. Before the fatigue tests, the physical and mechanical properties of the rocks were measured. Uniaxial compressive strength test (UCS) has been done on 5 core samples. The tests were performed in the load-control mode with a 1.6 kN/s loading rate. The tests were conducted to obtain the physico-mechanical parameters of the rocks in static loading condition, and provided a reference for subsequent dynamic tests. The cyclic tests were performed in both load and displacement control modes. To record axial and lateral strains during the fatigue tests, four strain gauges have been employed with arrangement of two axial and two laterals. Also, three acoustic emission sensors were installed on top, mean and bottom of the core samples to record cracking sound. In order to doing the tests a servocontrol Instron machine with 500 kN capacity was employed. The fatigue tests were conducted with three different maximum loads, 1 Hz frequency, and constant amplitude (0.82 of uniaxial compressive strength). The maximum stress level (the ratio of maximum cyclic stress to static strength) was varied 0.80, 0.85, and 0.90. The amplitude level (the ratio of amplitude stress to static strength) ranged from 0.50 to 0.70 and 0.90. Finally, Multi stages loading with increasing amplitude were applied for the displacement control tests. The results of fatigue tests have been evaluated by fatigue damage parameters including maximum and minimum axial strain, maximum and minimum lateral strain, tangent and secant modulus, toughness and hysteresis energy.
Results and discussion
The obtained results indicated that during fatigue process failure occurs below the maximum strength loading condition as a result of accumulative damage. Analysis of the fatigue test results showed that the fatigue failure consisted of three stages: fatigue crack formation (initiation phase I), stable crack propagation (uniform velocity phase II), and unstable crack propagation resulting in a sudden breakdown (accelerated phase III). By comparing the axial and lateral deformation, it was found that lateral deformation is more sensitive to fatigue. At higher stress levels, considerable part of fatigue life is response to crake development, whereas at lower stress levels, crack acceleration phase of fatigue life is distinguishable. Descending trend of loading and unloading tangent modulus shows a scatter pattern. This behavior may be related to the calculation method and loading condition, as well as microstructure and behavior of the rock mass. In spite of tangent modulus results, the three-stages of damage process (especially phase I and II) for secant modulus in both loading and unloading conditions are clear. The result is due to the method of calculation and increase in axial strain with increasing number of cycles. Brittle behavior of this type of rock leads acceleration phase to be hidden and unclear in most of fatigue damage parameters. A dramatic decrease of toughness and hysteresis energy in the first few cycles is due to the closing of pre-existing micro fractures. In fact, during the initial cycle, the rock behaves in a more ductile fashion than in the next few cycles. Thereafter, toughness begins to increase slowly, then steadily, and finally rapidly. A similar behavior was found for hysteresis energy as well. This fact indicated that cracks generated in parallel to loading direction. Fatigue displacement control tests show a strain softening behavior for the granitic rocks. This behavior is highlighted in variation of maximum stress during the tests. This parameter, especially in final step of loading, shows distinguishable decreasing trend.
Conclusion
The tonalite rocks were subjected to uniaxial cyclic loading in both load and displacement control mode. The following conclusions were drawn from this research.
-Accumulated fatigue damage occurs in an obvious three-stage process. This is the result of the micro-fracturing mechanism in the fatigue process.
-By comparing axial and lateral strain damages, it was found that crack propagation occurred in the loading direction and crack opening occurred in the lateral direction. So, among fatigue damage parameters, lateral strain shows the best three-stage fatigue damage behavior.
- Strain softening was found as rock response to cyclic displacement control loading.

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Introduction
Loess soil is one of the problematic soils that should be improved its geotechnical properties before the project is implemented. Lack of attention to this issue has caused in many problems for civil projects in Golestan province. This has been more evident in some of the rural areas built on this type of soil. Moreover there are many reports regarding different geological hazard such as subsidence, divergence, erosion and landslide in Golestan loess soil. Among the different types of loess soils found in Golestan province, silty loess should be given more attention due to their large extent and being the bed soil of many villages, and many reports of its hazards.
One of the methods for improving soil mechanical behavior and its geotechnical properties is to use additives to reduce geological hazards. Due to the fine-grained structure of loess soils, the application of nanoparticles is more efficient and could result in solving many of the related problems. Nanotechnology is new scientific field which affects many aspects of engineering and in recent years, many efforts have been made to use this new technology in various geotechnical branches.
So far, research has been carried out on the improvement of various soil types with additives such as cement, bitumen, ash, lime and various types of nanoparticles. Nowadays, the use of nanoparticle additives due to reduction of environmental pollution than other additives has a wider application in improving the physical and chemical properties of problematic soils.
In the present study, the effect of nano-kaolinite on strength properties including uniaxial compressive strength, elasticity modulus, cohesion, and internal friction angle of silty Loess in Kalaleh city of Golestan province have been investigated.
Material and methods
In order to carry out the present research, sample of the silty loess soil from Kaleh city of Golestan province was collected and prepared. Then, 0.5, 1, 1.5, 2, 3 and 4 weight percent of nano-kaolinite were added to soil samples. The soil samples were prepared in a natural state (without additives) and with the additive for uniaxial compressive strength and direct shear tests. Strength properties of soil specimens including uniaxial compressive strength, elastic modulus (based on uniaxial compressive strength test), cohesion and internal friction angle (based on direct shear testing) were determined for native soil and its mixture with different percentage of nano-kaolinite. The data were analyzed and the effect of nano-kaolinite on the strength properties of the silty loess soil sample was investigated.
Results and discussion
Uniaxial compressive strength and modulus of elasticity have been increased with increasing amount of nano-kaolinite, and after 2% nano-kaolinite, increase in nano-kaolinite did not have any significant effect on uniaxial compressive strength and modulus of elasticity. The uniaxial compressive strength and the modulus of soil elasticity in the natural state (without nano-kaolinite) are 1.12 and 15.89 kg/cm² respectively, and when 2% of the nano-kaolinite is added to the soil, the values ​​of these properties are maximal and reached to 1.19 and 18.10 kg/cm², respectively.
For native soil (without nano-kaolinite), the cohesion value is equal to 0.09 kg/cm², and with increasing nano-kaolinite from 0.5 to 2%, the cohesion shows an incremental trend and reached to 0.16 kg/cm². With increasing the additive percent from 2 to 4% the amount of cohesion were constant and equal to 0.16 kg/cm². The increasing of cohesion can be attributed to the fact that nanoparticles enhanced water absorption of soil particles which caused in better cohesion and also they affected chemical actions and surface electrical charge of soil particles.
Conclusion
The results of the uniaxial compressive strength tests show that adding up to 2 weight percent Nano-kaolinite to the dry soil increases the uniaxial compressive strength and modulus of elasticity of silty loess soil in the Golestan province, which can be due to proper locking between the nanoparticles and soil particles and increased cohesion.
The results of direct shear tests showed that adding up to 2% nano-kaolinite to dry soil increased the cohesion of the soil and consequently increased the shear strength of the soil.
On the other hand, adding the different amount of nano-kaolinite has not changed much in the internal friction angle of the silty loess soil in the Golestan province.
 
 

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Groundwater salinization in semiarid regions is a limiting factor of use with strategic importance. In this study, the sources of salinity, chemistry, and quality of groundwater in Robat (Khorramabad plain, Iran) were identified through the geochemical methods. Using data analysis, the concentration of cations and anions were recognized with the order of Ca²⁺>Na⁺ >Mg²⁺>K⁺ and HCO₃^-> Cl^-> SO₄²⁺> NO₃^-> F^-, respectively. The high concentration of Na⁺, Cl^-, and EC in some places is attributed to the gypsum and salty formations. In the study area, the salinization processes are identified by natural and artificial activities. The salinization mechanisms are identified by the natural dissolution of gypsum and salt from Gachsaran formation and man-made sources including boreholes drilled through Gachsaran Formation, salt mining, and agricultural activity. Also, the high concentration of nitrate is related to agricultural fertilizers and karstification effects. It is seen that the atmospheric NO₃^-. HCO₃^-, Ca²⁺, and Mg²⁺ concentration exceeded the standard limit in a few samples probably due to the calcareous formation. Besides, hydrochemical facies of the groundwater are Ca- HCO₃ and Na-K-HCO₃. Due to the presence of calcareous and salt bearing formations, 46%, 26%, and 20% of all samples show a higher concentration of Ca²⁺, Na+, and Mg²⁺, respectively, which exceed the permissible limits. Sulfate and fluoride concentrations are less than the permissible limits. However, due to the presence of calcareous formation, salt bearing formation, and use of agricultural fertilizers, 100%, 26%, and 20% of all samples show a higher concentration of HCO₃^-, Cl^-, and NO₃^- than the permissible limits.

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