Journal of Engineering Geology

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Introduction
Hydraulic fracturing is one of the most important stimulation methods for oil and gas reservoirs with low permeability. Various factors, such as in-situ stresses, joints and natural fractures of the formation, fluid rheology, mechanical properties of the formation, injection fluid flow rate and perforation operation, effect on the pressure and hydraulic fracture geometry. Many researchers have studied hydraulic fracturing behavior of rocks since decades ago. The researches have showed that hydraulic fracturing operations increase the production of oil wells by up to 30 percent and increase gas wells by 90 percent. Currently, this operation is performed on about 60% of all drilled wells.
Material and methods
In this research, for the experimental investigation of the hydraulic fracturing, considering the reservoir condition, 39 concrete cubic samples with 100 × 100 × 100 mm dimensions and 60 concrete cylindrical samples with a diameter of 54 mm and a height of 110 mm were constructed and the effect of the sample geometry and in-situ stress field on the fracture geometry, breakdown pressure, the pattern of crack propagation and finally the cross fractures in vertical wellbores were investigated.
Results and discussion
In cubic specimens under uniaxial stress, with increasing vertical stress to 8 MPa, first the breakdown pressure has been increased by about 132% and then with increasing vertical stress to 16 MPa, the breakdown pressure has been decreased by about 69%. In cylindrical specimens under uniaxial stress, with increasing vertical stress to 12 MPa, first the breakdown pressure increased by about 113% and then with increasing vertical stress to 16 MPa, the breakdown pressure decreased by about 6%. As the vertical stress increases to a certain limit, the pores and micro-cracks inside the sample close, thus the tensile strength and breakdown pressure increase. In the following, increasing vertical stress causes more small cracks to open and reduces the tensile strength of the rock. In cubic specimens under triaxial stresses, with increasing vertical stress, the breakdown pressure has been increased. Also, in cylindrical specimens under triaxial stresses as the maximum horizontal stress increased, the breakdown pressure increased.
Conclusion
The obtained results demonstrated that increasing the uniaxial stress in the vertical wellbore in both types of studied sample geometry, first the breakdown pressure increases and then from one boundary onwards, with increasing vertical stress, the breakdown pressure decreases. In cubic specimens under triaxial stress, with increasing vertical stress, the breakdown pressure increases. Additionally, in cylindrical specimens under triaxial stresses as the maximum horizontal stress increases, the breakdown pressure increases, so that, in the 8 MPa maximum horizontal stress, the breakdown pressure increases by about 81%../files/site1/files/151/3.pdf
 

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The soil of the Arvand free zone in the north of Khorramshahr is fine cohesive and cannot be used in earth works. On the other hand, suitable materials for this purpose (coarse-grained soils) are located at the farther distances which a considerable cost requires. In this regard, it is trying to improve the soil with lime and furnace steel slag. This study is focused on improvement of the fine-grained soil by adding various contents of lime and furnace steel slag. For this purpose, after sampling and performance of compaction tests, different amounts of slag (10, 20 and 30% by weight of dry soil) and lime (2, 4 and 6% by weight of dry soil) were added to the soil and after curing for 28 days, the effect of additives on the physical and mechanical properties of soil was investigated by using several tests such as Atterberg limits, compaction, uniaxial compressive strength (UCS) and CBR as soaked and unsoaked. Based on USCS classification the study soil is CL, its plasticity index is about 25% and sulphate ion content is more than 0.5%. Experimental results show that by adding slag and lime at different contents to soil, mechanical properties of soil improve dramatically, so plastic index of soil decreased and UCS and CBR has been increased. Also, the maximum dry unit weight of soil increases and the optimum moisture content decreases. The test results also indicate that the effect of lime on soil is higher than slag and the effect of slag for less than 35% is not considerable, however the test result of unsoaked CBR show that the bearing of soil increase in the more than slag content 20% is significant. According to the previous studies, due to the relatively high sulphate ion content in the soil, the use of lime alone is inappropriate and the slag can only physically improve soil conditions but also chemically prevent the formation of large volume minerals (like Ettringite) by the reaction of lime with soil sulphate ion../files/site1/files/152/%D8%AD%D8%B3%DB%8C%D9%86_%D8%B2%D8%A7%D8%AF%D9%87.pdf

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Expansive soils contain clay minerals such as compacted kaolin which are widespread in nature. Displacements of this type of soils are associated with matric suction and degree of saturation. To determine the in-situ characteristics, necessary measures may be required to deal with the possible failure related to this type of soil. Different constitutive models of unsaturated soils have been considered the subject of many recent researchers (Sheng et al. 2004; Wheeler et al. 2003; Nuth and Laloui 2008; Zhang and Lytton 2009 a, b 2012). However, those constitutive models are generally complicated that are not properly implemented in computer programs for practical applications. The Barcelona Basic Model (BBM) is one of the geomechanical constitutive models to capture the elastoplastic behavior of unsaturated soils../files/site1/files/152/%D8%B5%D8%A7%D8%AF%D9%82_%D8%A2%D8%A8%D8%A7%D8%AF%DB%8C.pdf

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An accurate study of the physical-chemical properties of soil has a significant effect on the determination of mechanical parameters, classification and evaluation of soils, and the civil project management aspects. The obtained accurate results of the chemical parameters of soil can determine the type and amount of cement or lime used in stabilizing the problematic soils, which leads to better estimation of sub-channel substrate consolidation behavior, accuracy in engineering designs, and so on. Previous researches showed that the chemical changes such as increasing soil solutes are one of the affecting factors on the change of the mechanical parameters of the soil [1-3]. In this study, the effect of the waves on the saturated paste and the saturation extract was presented as a novel method in order to determine the soil chemical parameters as well as the soil dispersivity potential more precisely. Also, the effect of the waves on the arid and semi-arid regions can be considered as one of the environmental parameters affecting the solubility of solutes in the saturated soil and pre estimated dispersive and semi-dispersive of the soils../files/site1/files/152/%D8%B4%D8%A7%D9%87%D8%B1%D8%AE%DB%8C.pdf

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In coastal industrial areas, in addition to the presence of loose soil, sulfate attack on soil improvement elements, such as soil-cement, is a double problem. Generally, the use of type V cement is recommended as one of the methods to reduce the detrimental effects. Considering the limited resources of this type of cement, firstly to determin the relationship between the cement content and the strength obtained in sulfated environments is one of the important engineering question in this field and secondly, as an alternative option, the use of type II cement which is more available, is suggested to use in combination with suitable additives. The present study pursues the above two goals by making cylindrical soil-cement specimens with sand, water and Portland sulfate resistant cements. Sodium sulfate is used as the sulfate in soil and water. In the research, first of all, the relation between type V cement content and unconfined compressive strength of soil-cement is obtained at 0% to 5% sulfate concentration, which results in a cement content of 400 kg/m³ completely limited the sulfate attack effects in a sulfate concentration of 2%. Secondly, the combination of type II cement with barium chloride and hydroxide was tested. The related results show that the combination of type II cement with barium chloride and hydroxide had higher strengths, about 2.7 to 3.3 times, respectively (in 362 days), than the soil-cement containing type V cement../files/site1/files/152/%D8%A2%D8%AA%D8%B4_%D8%A8%D9%86%D8%AF.pdf
 
 

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Existence of sodium elements in fine-grained of some soils causes dispersive phenomenon in them.  Failure to accurately identify dispersive clays leads to damage because dispersive clay soil particles disperse under certain conditions and wash away quickly. This research assesses dispersive degree of outcropped soils in southeast of Yazd. Finally, the modification of soil dispersive potential was investigated by using nanocellulose.  After performing a series of physical, chemical and mechanical tests, characterization/ specification of the studied soils were determined. Then dispersive degree was specified by conducting chemical, pin hole, crumb and double hydrometer tests. Finally, soil dispersivity stabilization was performed using sample preparation with 0.5, 1 and 1.5% nanocellulose. The results showed that the studied soils have moderate dispersive in borehole A and extreme to slightly dispersive in borehole B. Therefore, it can be concluded that the closer we get to the center of the plain, the greater the dispersibility. The results of the dispersibility stabilization soil tests indicate that the optimum moisture content and dry specific gravity increases and decreases. Uniaxial strength and CBR increases. Also, it shows that the increase in nanocellulose has a positive effect on the modified samples and improves the soil dispersibility in this area../files/site1/files/152/%D8%B2%D8%A7%D8%B1%D8%B9.pdf
 

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Clayey soils in terms of sharp reduction in strength and swelling ability as a results of water and moisture absorption, it is considered as one of the most problematic soils in civil engineering and construction works. Nowadays, Nano materials such as Nano clay are used to improve and stabilize of clay. On the other side, the increasing volume of municipal waste and residues materials especially debris of building destruction have caused many problems in mega cities such as environmental issues due to incorrect disposal of waste material. Main propose of this research is study of possibility in effecting Nano clay and limestone powder mixture for improve geotechnical properties of Kuye Nasr clayey soil in Tabriz City. In this study, Nano clay and limestone powder in both separate and combined conditions with 5 and 10 percentage are mixed with clay. Curing of stabilized specimens have been performed in 7, 14 and 28 days. For evaluating geotechnical behavior of mixture materials some tests were performed such as Atterberg limits, Compaction, uniaxial strength and direct shear (in 1, 2 and 3 kg/cm2 vertical stress). Results show that the simultaneous effects of 5% Nano clay with 10% limestone powder with 7 days curing period in ambient temperature conditions in clay reduced plasticity index by 72%, improved graining skeleton structure, reduced void ratio of inter grains and increased shear strength by 33%.

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In current paper the effects of surface unreinforced / reinforced sand layers coupled with and without single and group sand columns on the bearing capacity – settlement behavior of soft clays has been investigated. In this regard behavior of soft clay, clay + unreinforced / reinforced sand layer, clay + single / group sand piles and clay + unreinforced / reinforced sand layer + single / group piles samples has been assessed. Geogrid was adopted as the reinforcement, a circular plate 5cm in diameter as the loading surface and C.B.R. apparatus as the loading system. Results show that employing unreinforced / reinforced sand layers at a settlement ratio of 5% improves bearing capacity by 4 t0 7 times the soft clay. Coupling the surface unreinforced / reinforced sand layers with single / group sand piles further increases the bearing capacity by 7 to 9 times that of soft clay.

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In this study, due to the landslide in schist rocks, in the wall of Mouteh gold mines, including of the eastern wall of ChahKhatoon mine, it is important to identify the effective factors. Therefore, due to the diversity of schists in Chah Khatoon and Sanjadeh gold mines (two active mines in Mouteh Complex), to survey the mineralogy of schist rocks in Moteh gold mine has been done by identifying important factors in changes in rock strength. Cosequently, 10 schist samples from walls of these mines were considered for mineralogical, XRD studies. In the next step, these schists were subjected to uniaxial compressive strength (UCS) and Brazilian tests to estimate the mechanical properties and quality of rock mass in different zones of mineral walls. The results showed that the UCS and Brazilian index in these schists are directly and inversely related to the SiO₂ and Al₂O₃ contents of the rocks, respectively, as well as the secondary structures.Some factors such as the presence of secondary structures, continuous surface area, particle size, and mineralogical composition play an important role in the failure modes of these rocks. UCS and Brazilian strength of schists vary from 10 MPa to 72 MPa and 1.9 to 10.2 MPa, respectively. The lowest UCS occurs in strongly weathered rocks with low silica content. However, the type of clay minerals is effective in the stability of the mineral wall. Considering the presence of montmorillonite clay mineral in the eastern wall of Chahkhatoun mine, the rock resistance is moderate despite the high percentage of silica. UCS values of wet and dry rock samples containing muscovite and montmorillonite clay minerals were more different from those of other rocks. In this regard, the rocks with Illite clay minerals are more resistant than Smectite and montmorillonite minerals. In general, the resistance of schists depends on various factors such as mineralogy, which is of great importance because of its involvement in the formation of secondary structures.

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Soil stabilization and reinforcement has long played an important role in civil engineering, especially in geotechnics, and over time and the need for a more robust and stable ground to withstand gravity and higher shear forces, has become particularly important. Also, in recent years, with the entry of the environment into the construction industry, with the aim of reducing the adverse effects of industrial waste and construction waste on people's living environment and preserving the environment for the future, in many cases reduces the economic costs of projects. In this research, granular soil is reinforced in two loose and semi-dense states using a waste material called ethylene-vinyl acetate (EVA). The experiments were performed without adding moisture, by weight percentage method and using CBR device. The results show that soil resistance increases significantly with the use of these additives and its effect on soil increases with decreasing soil specific gravity. Also, the optimal amount of additives in loose and semi-dense state is 2% additive and 1% additive, respectively.


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In this study, it is attempted to analyze sensitivity and reliability in order to evaluate the liquefaction potential in soil layers in Tabriz. 62 boreholes that had possible conditions for liquefaction were selected. Seismic mapping was simulated using finite fault method and then the effect of soil layers on PGA was estimated. In continue, the liquefaction potential index was estimated and the zoning map of liquefaction risk was presented. In final, through sensitivity and reliability analysis of the Monte Carlo method, the rate of density function against safety factor of the soil layers versus to liquefaction was determined.

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The goal of this study was improvement of sandy soil using a combination of polystyrene foam container waste and Portland cement. For this purpose, Babolsar sand was used as the base soil. Strips of disposable polystyrene foam container waste in “chips” of 50 ´ 5 mm and 50 ´ 10 mm were added to the soil at 0.0%, 0.1%, 0.2% and 0.3% by weight along with 3% Portland cement at a relative density of 70%. All samples were cured for 7 days under saturated conditions and then tested using a large-scale direct shear apparatus. The results showed that, in both cemented and uncemented samples, the addition of foam chips increased the cohesion and internal friction angles, which increased the shear strength of the soil. At higher percentages and using larger-sized foam chips, the shear strength increased even more. In uncemented samples, the stiffness did not change with the addition of foam chips, yet the final dilation of the samples decreased. In cemented samples, both the stiffness and softening behavior after the peak strength point decreased. The final dilation of the cemented samples increased at higher foam chip contents and for the larger sized chips. The results of numerical analysis showed that the use of foam chips increased the safety factor of a slope improved in this manner. It also was found that the foam chips with a lower length-to-width ratio had a greater effect on increasing the safety factor of the tested slopes.