ENGINEERING GEOLOGY
Year:2020 | Volume:13 | Issue:2|Papers Count:7

Determination of optimum distance between twin tunnels is very important. There are many empirical models to estimate width of the pillar, all of which have their own limitations. In this study an attempt was made to present a new criterion for design of the pillar between twin (parallel) tunnels. It is based on ground reaction curve and numerical modelling. Strain magnitude observed when normal stress (σ 1) in the pillar reaches the value of compressive strength of the rock mass (σ cm), denotes the allowable ratio of width to height (w/h) which shows the optimum distance between twin tunnels. Pooneh twin tunnels located in Arak-Khoramabad Expressway, was selected as a case study. This Tunnel is made of eight zones in layers of marly limestone and one zone in alluvial deposits. Results of the suggested method show a good agreement between the pillar dimensions seen in the case study obtained from instrumentation and the method (new criterion). In zone 3, as an example, the amounts of rock mass compressive strength and strain in the pillar are 3. 23 MPa and 0. 053, respectively. In this zone, 1. 8 was obtained for the w/h ratio, according to the analysis the minimum width for the stable pillar must be 17. 1 m. Taking all aspects of geomechanical and geometrical characteristics of the tunnels into consideration, this method is capable of determining the optimum width of the pillar when designing the twin tunnels in static conditions with a high level of precision in all Iranian tunnels.

Finite Element codes (FE) are increasingly used to simulate the propagation of elastic waves and dispersion, especially in surface-wave techniques. The common problem when using such methods is unwanted reflections from the boundaries of the model, which is still a challenging issue. In this paper, the effect of different parameters is evaluated to improve the performance of Absorbing Layers using Increasing Damping (ALID) in order to reduce the reflected waves from the boundaries and increase the resolution of the results. In this regard, after identifying the appropriate specifications of ALID layers in Abaqus software, the effect of thickness and number of ALID layers in different soils media with shear wave velocities of 200, 800 and 2000 m / s are investigated on the amplitude damping of incident waves at the boundaries. The results show that applying ALID with the gradual increasing damping significantly prevents the reflection of the waves into the media in comparison with the constant and double increasing damping. Furthermore, the hard soil requires more ALID layers and soft soil including high damping needs lower ALID layers. also increasing the number of ALID layers are more effective than increasing their thickness.

Presence of natural and man-made cavities can affected rock mass strength. This is more important especially when the cavities are located in shallow depth and have sensitive applications such as metro and military targets. Therefore, the aim of this study was to evaluate the effect of elliptical cavity on rock behavior under static and dynamic loads. In order to evaluate the effect of the cavity under static stress conditions, two groups of intact and hole-bearing granite cores were provided and tested by a hydraulic servo control device under uniaxial compressive loading test. During the test, in addition to the stress recording, damage and deformability of the samples were recorded by using the strain gauge, acoustic emission sensor and camera. Split Hopkinson pressure bar (SHPB) test apparatus was used for doing dynamic loading test. In addition, the damage process was recorded using a high-speed camera with 10 micro-seconds interval of frame capability. The obtained results showed that the presence of cavity reduced the rock resistance up to 50% of the initial strength under static stresses. Furthermore, the cavity causes an increase in strain, which significantly reduces the elastic modulus. Dynamic tests show that rock strength can increase up to more than 4. 3 times of the initial value, depending on the loading rate. Also, the obtained images show that in case of high applied stress, shear failure is dominated and if the applied stress is significantly reduced, shear mechanism becomes to tensile mechanism.

Hydraulic fracturing defined as a process in which a hydraulic loading caused by fluid injection in a part of wells result fracture propagation in the rock. The main purpose of this research is simulation of hydraulic fracturing based on the concepts of Finite Element Method (FEM) and parameters affecting it. For this goal, ABAQUS software has been used to examine hydraulic fracture initiation pressure in a simulated petroleum reservoir in which the cohesive elements theory with traction-separation law existed. To do so, a cohesive crack model has been introduced, govern equations has been discussed and then the way of building a poroelastic three-dimensional model with cohesive elements is described. The results show that the fracture pressure obtained from FEM model is in agreement with the analytical values. Fluid leak-off rate diagram shows three different time steps that the first two steps represent the expression of leak-off jump and third stage shows dynamic leak-off. Fluid pressure changes along the fracture shows a decreasing trend as the fluid pressure is lower than the initial pore pressure due to the phenomenon of fluid-lag at the fracture tip. Finally, a sensitivity analysis was conducted on a number of parameters on the fracture initiation pressure and results show increasing in all parameters except for Poisson's ratio, led to an increase in fracture initiation pressure. Also, the sensitivity function calculations for each of the parameters show that Young's modulus and leak-off coefficient have the highest and lowest effect on fracture initiation pressure, respectively.

The area of Charousa and Lendeh in the Kohgiluyeh and Boyer-Ahmad province in southwestern Iran is geologically part of the folded Zagros. In this area, the core of the Kuh-e-Sefid and Sartal anticline of the Ilam-Sarvak limestone are the main high altitudes of the region. At these altitudes the scar of large landslides and evidence of deep seated gravitational slope deformation are visible. The largest of these are the Almor landslide in the Sartal anticline with an approximate volume of over 600 million cubic meters. In this study, the evidence of deep seated gravitational slope deformation of these anticlines has been investigated and the causing factors for Almor landslide have been determined. To investigate of this issue, a combination of geological field survey, image processing studies, and laboratory studies of rock mechanics and soil mechanics were used to prepare preliminary data. This data are processed and analyzed by a combination of kinematic analysis, limit equilibrium sensitivity analysis, and finite element numerical modeling. Finally, a three-dimensional geological model is proposed to illustrate the proposed mechanism for this landslide. According to this study, a combination of deep seated gravitational slope deformation, along with wedge failure and three hinge buckling, was the causes of this avalanche.

The use of waste materials from various factories and industries in the soil to improve engineering properties is one of the new approaches in this field. Inflatable ash is also an important by-product of coal-fired power plants or from the waste heat of some plants, which has a good pozzolanic character and is used in construction projects for use in concrete and in geotechnical projects. Soil stabilization is used. In this study, a type of fly ash with different chemical properties was used in comparison with other studies aimed at improving the existing soil in central Iran. Density, permeability and CBR tests were performed on samples with different percentages of fly ash with or without lime at different processing times. According to the results of the study, it can be concluded that the addition of fly ash up to 20% in the lime-free sandy soil will increase the maximum specific gravity dry weight and this amount will decrease to 10% despite the 10% lime content. Find out. The results also show that 20% of fly ash is an optimum value for stabilizing soil with or without lime so that the highest California bearing ratio is achieved in this area. In addition, the use of fly ash has a direct relationship with the reduced permeability of the specimens. Finally, the results are interpreted on the basis of microscopic photographs of the samples.

Elastic modulus is one of the most important parameters in geotecal engineering that is usually measured through laboratory or in situ testing. Cyclic triaxial testing is a well-known laboratory test to determine the dynamic and static parameters of soils. In the current study, the dynamic elastic modulus of sand-gravel mixture at very small strain levels (about 10-5) has been studied using a triaxial cyclic apparatus along with local axial strain measurement. Influence of grading characteristics (curvature and uniformity coefficients), gravel content, mean effective confining stress and relative density on dynamic elastic modulus have been investigated. Based on the results of the tests, a comprehensive database including 120 records was established. Based on the database, two models have been developed to predict the dynamic elastic modulus using the group method of data handling (GMDH). In the first model the curvature coefficient (Cc) and in the second model the uniformity coefficient (Cu) is used as input parameter of soil grading charecteristics. Comparison of the RMSE in the two models indicates that the second model has better predictions than the first model. However, the coefficients of correlation (R) of models are satisfactory which indicates the GMDH has a good ability to predict the dynamic elastic modulus.