جستجوی مقالات مرتبط با کلیدواژه "منحنی تنش" در نشریات گروه "عمران"

The prediction of tunnel convergence at great depth is the important factor in tunneling design. A large number of analytical solutions have been presented for circular tunnels. However; it is not an easy task to use them in practice due to their either complexity or difficulty of stepwise computations, particularly when the stress-strain behavior of rock mass is the strain softening.
Therefore; in this study, a simple analytical approach is presented to estimate circular tunnel convergence using the stress- strain curve of rock mass located on tunnel wall. This curve is obtained based on values of the principle stresses as well as their variation on tunnel wall due to tunnel face advancement.

Cemented sandy soils can be found in different parts of nature. Slopes and natural cuts are observed to be stable for long periods of time. Indeed the stability is related to the cementation and bonding between soil grains which induces an equivalent cohesion for coarse grained soil. Several experimental and theoretical studies have been performed to investigate the mechanical behavior of this category of geomaterials. In present research، a constitutive model is proposed for simulating the mechanical behavior of cemented sandy soils. The model is based on separating the mechanical behavior of cemented soil to two different parts; firstly the uncemented soil matrix and secondly the cemented bonds. The generalized plasticity model developed by Manzanal et al. (2011) is used for predicting the mechanical behavior of the uncemented soil matrix. The model is based on critical state concepts and is able to simulate the behavior of sandy soil in a wide range of confining pressures. It contains sixteen parameters which can be determined using ordinary geotechnical tests for the base soil. Also the elastic-plastic damage bond model proposed by Haeri and Hamidi (2009) is used for cemented bonds. The model has two additional parameters and is able to predict the brittle behavior of cemented bonds besides their degradation with increase in shear strain. Peak shear resistance of cemented bonds increases with confining pressure، however، the axial strain associated to the peak shear strength decreases with enhancement of confining stress. Also cement content is considered in this bond model which is its advantage in comparison with similar ones. Both components have been combined together based on deformation consistency and energy equilibrium equations. Deviatoric stress-shear strain curves besides volumetric strain-shear strain ones have been compared with the results of consolidated drained triaxial tests on a gypsum cemented sand to verify the proposed model. Also deviatoric stress-axial strain besides deviatoric stress-mean effective stress curves of model are compared with results of tests in consolidated undrained state. Results of verification indicate good performance of developed model in a wide range of cement contents and confining pressures. The proposed model has two distinct advantages. At first it considers the effect of cement content as a model parameter and shows variation of the results with this parameter. Secondly، it simulates the soil behavior in a wide range of confining pressures which enables using it in the boundary value problem in geotechnical engineering. However، it should be noted that the model predicts the mechanical behavior of cemented sand in cement contents less than floating limit. Increasing the cement content from the floating threshold changes its role from effective bonding between soil grains to a filler of voids. In this condition، the model can not predict the behavior of cemented soil due to the limitations in the elastic-plastic damage bond model applied in present constitutive model.

The potential use of leca and scoria aggregates available in Iran to produce high strength lightweight concrete has been studied. As a result of various concrete mixes with different lightweight aggregate amount, and using normal techniques, it was possible to obtain high quality lightweight concrete which is suitable for application in reinforced and prestressed concrete structures.In order to investigate the mechanical short term properties of this type of high strength concrete, different tests had done. The properties obtained include unit weight, compressive strength, static modulus of elasticity and poisson's ratio.Among the 28 day aged tested specimens, we could produce HSLWC containing scoria with 55 MPa compressive strength and 2070 kg/m3 weight.In LWC containing leca, a cube compressive strength of about 41 MPa with a unit weight of 1865 kg/m3 was reported.Static modulus of elasticity in concretes containingleca was between 15 to 19 GPa. It was between 17 to 19.5 GPa forconcretescontainingscoria. Some equations were offered to estimate static modulus of elasticity of these concretes based on their compressive strength and unit weight. The mechanical properties were improved by reducing the amount of lightweight aggregate. The results show that both scoria and leca were effective in improving the mechanical properties, but scoria could reach superior limits.Stress- strain curves of investigated concretes were studied and some recommendations are presented for estimating the curves.Stress-strain curves of investigated concretes are almost linear in ascending and descending branch.According to the observed coefficients, withincrease in concrete strength, the slope of the curve is reducedin comparison with modulus of elasticity. For scoria, the slope of curve is lower than modulus in all three mix designs, because of the higher modulus of elasticity of scoria aggregates, in comparison with leca.

Prediction of stress-strain behavior of geotechnical material is one of the major efforts of engineers and researchers in the field of geomechanics. Experimental tests like tri-axial shear strength tests are the most effective apparatus to prepare the mechanical characteristics of gravelly material; but due to difficulties in preparing test samples and costs of the tests, only several tests will be donein a new project. Artificial neural network is a kind of method, in which engineer could judge the results based on numerous data from other similar projects, which enable the engineer to have a good judgment on the material properties. In this research, the behavior of gravelly material was simulated by use of multi-layer perceptron neural network, which is the most useful kind of artificial neural networks in the field ofgeotechnical engineering. For instance, first exact information was provided from laboratory tests of various barrow areas of embankment dams in the country and effective parameters on shear strength of coarse-grained material were studied. After omitting incorrect or weak data, 95, 20 and 23 sets of data were used for learning, testing and evaluating data, respectively. Input parameters for the model were as follows: particle-size distribution curve, dry density, relative density, Los-angles abrasion percent, confining pressure, axial strain; and outputs were selected as deviator stress. In order to reach a steady state in the model and force the model to behave homogenous to the all inputs, data wasnormalized to the value between. 05 and 0.95. In the simulation, back-propagation algorithm was used for learning or error reduction. The aim of the simulations was defined to reduce error between realdata and predicted values; for instance root mean square error (RMS) was used to be minimized through simulation and predicted versus real graphs were used to observe the global error of the model. After modeling the data based on some criteria, it was shown that curves of stress-strain from simulation tests were in good agreement with those from laboratory. These close coherencies were observed in all training, testing and evaluation data, in which the RMS errors were 0.038, 0.037 and 0.026, respectively. To reach this ultimate step, a 10*19*1 multilayer perceptron was used via trial and error. In order to determine quality and quantity of the effect of inputs on outputs, and prove that the results were in good agreement with soil mechanic principles, sensitivity analyses were done on the average data of the inputs. Results show that confine pressure, uniformity coefficient and relative density of the material were the most effective parameters on the stress-strain curves; thus the model has enough capability to predict the stress-strain behavior of gravelly soils.

Stress distribution in lightweight aggregates concrete (LWAC) has been investigated in this research. Many studies have been already done on structural behavior of LWAC، however its properties are not well known as those of NWC. There are many provisions in different codes as equations or recommendations for designing reinforced concrete structures which are applicable for NWC. Because of different roles of aggregates in LWAC and NWC structural behavior of the LWAC needs more investigation. One of the characteristics which are applied as designing parameters is characteristic curve (stress-strain) of the concrete. In this research the stress-strain curve of the LWAC made with Scoria and Pumice has been studied. A total of 27 concrete mixes of LWAC made of scoria and pumice were tested and their stress – strain relationships have been studied. The experimental results show that rectangular stress block required to designing of LWAC structures are need more attention. The estimated flexural capacity with existing equations in codes comparison with experimental flexural capacity in LWAC is greater which lean lead to unsafety in structures. In this study، new equations for stress- strain behavior of LWAC are presented which can lead to prediction of more safe and accurate ultimate flexural strength.

In this study, the experimental investigation of confinement in concrete compressive elements using Glass Fiber Reinforced Polymer (GFRP) sheets has been conducted. Regarding the effects of concrete and fibers properties, the studied parameters include the strength of unconfined concrete core, thickness of confining layer and orientation of fibers in GFRP confining layer. The effects of these parameters on the behavior of compressive stress-strain curve, compressive strength, ultimate strain and energy absorption of confined concrete have been assessed to be used in design purposes. The final goal of this study is to compare the two methods of confining concrete columns using GFRP materials and using Slurry Infiltrated Fiber Concrete (SIFCON). Consequently, the results of compressive tests on concrete elements confined by SIFCON which were conducted by other researchers have been compared with the results of tests on GFRP confined concrete specimens. The results of this comparison show that the general behavior of stress-strain curves of concrete confined by SIFCON is different from that of GFRP confined concrete due to the significant post peak behavior of SIFCON confined concrete. SIFCON confinement method provides comparable results with GFRP confinement, considering the improvement of compressive strength and energy absorption capacity.