Sharif Journal of Civil Engineering
Year:2015 | Volume:31-2 | Issue:3.1|Papers Count:15

Soil strength properties are inherently variable, due to the nature of their formation. Slope stability problems are among the most important issues in geotechnical en- gineering, and they are investigated in this study con- sidering the effect of heterogeneity and anisotropy of soil strength properties in long term behavior. To this aim, the random field theory combined with the finite differ- ence method has been adopted in the frame of Monte- Carlo simulations. The factor of safety of the slope has been calculated by the C - Æ reduction method and the probability of failure has then been investigated through Monte-Carlo simulations.Different stochastic parameters were considered in order to study their effect on the stability of natural slopes in long term schemes or under drained conditions. The co- efficient of variation of the cohesion and internal friction angle representing strength properties, scale of fiuctua- tion of the parameters, cross correlation between strength parameters and heterogeneity anisotropy, are among all stochastic parameters investigated in this study. Three different values of 10, 50 and 90 for the CoV of param- eters were considered, in order to cover both low and high variability. Different vertical correlation lengths, ranging from 0.1 to 50m, are considered to cover a wide range, from fully uncorrelated to fully correlated param- eters.The First Order Second Moment (FOSM), the First Or- der Reliability Method (FORM), and the Random Finite Difference Method (RFDM) were adopted to investigate the probability of failure. The latter was invoked by a combination of random field theory with the finite differ- ence method through Monte-Carlo simulations by FISH programming in FLAC software.The results show that considering heterogeneity causes variability in results. The coefficient of variation and the scale of fluctuation have been found to have the most ef- fect on the probability of failure. When the coefficient of variation of shear strength parameters increases, the mean factor of safety decreases, and the probability of failure increases. The vertical scale of fluctuation of pa- rameters induces an increase in variance function, due to its increase up to a critical value. Furthermore, it seems that cross correlation has less effect on variations of the factor of safety of natural slopes.

Over the last three decades, structural control systems have developed into three categories: Passive, Active and Semi active. The hysteretic metallic damper is among one of the most popular supplemental energy dissipation devices, as a type of passive control system. The accordion metallic damper (AMD) was suggested to be the best hysteretic metallic damper because of its high capacity to absorb energy and its desirable behav- ior characteristics, including convenient fabrication and installation. The efficiency of AMD to control and mit- igate the response of structures under seismic loading is established, both experimentally and analytically.In this research, for the purpose of improvement, the ef- fect of increasing the number of accordion tube layers on the damping behavior of AMD is investigated using experimental, analytical and parametric studies. Ex- perimental studies were conducted on single layer and two layer specimens under axial loading by a dynamic Roell-Amsler actuator. Analytical studies, based on the finite element method, and inelastic dynamic analysis have been carried out on a series of single and multi-layer AMD models, which have been verified by experimental results. Also, the effect of changing geometrical parame- ters on the damping behavior, by increasing the number of layers, was evaluated, and an ideal geometric model is made available. Finally, an analytical study has been performed to determine the effectiveness of a multilayer damper on the tolerability of low-cycle fatigue.The results show that increasing the number of layers has a great influence on the amount of dissipated en- ergy, loading capacity and elastic stiffness, due to greater stable behavior. This includes improving the buckling mode, preventing destructive modes of buckling and in- teraction effects between the layers. Parametric study shows that the combination of geometric parameters, such as thickness of layer, radius of tube, length of tube and radius of corrugation, will provide ideal model ge- ometry with optimal energy absorption. A fatigue study shows that increasing the number of layers has good ef- fects on increasing the number of cycles tolerated, and reduces the damage damper coefficient by decreasing the stress focus on critical areas of the damper wall.

A common reason for uneven settlement of existing struc- tures founded on pile foundations is negative skin fric- tion (NSF) developed along the piles.As a result of fill placement the soil surrounding the piles in soft ground settles more than the piles.In that case negative skin friction occurs producing pile settlement (down drag) and additional compressive force (down drag) due to the hanging effect around the pile.(NSF) is usually mobilized up to the shear strength of soil in most cases except at very close proximity to the neutral plane.The relative displacement between the settling subsoil and the pile shaft surface should not be more than a few millimeters to have even total settle- ment. Disregarding the effect of down drag forces on piles due to consolidation of the surrounding soil leads to damage in buildings and foundations. Many methods especially numerical simulation methods have been pro- posed to calculate the drag load on single piles. How- ever much less work has been done on drag load on a batter pile. Batter piles are commonly used to sup- port marine structures and deep foundations. Based on the experimental data from Hanna & Nguyen (1986) shaft resistance decreases by the increase of angle of in- clination for both compression and tension forces. On the other hand it was found from full scale results re- ported by Mayerhof (1973) that the shaft resistance ca- pacity for the pile increases due to an increase in the inclination angle. In this study the effect of negative skin friction on vertical piles and batter piles is investi- gated. The analysis was undertaken using Abacus soft- ware which is based on the Finite Element Method (FEM).A single vertical end-bearing and one skin friction pile under different earth surface loadings were analyzed and the drag load force was studied. Subsequently the per- formance of the single batter end-bearing and friction pile with different inclination angles between 0-300 was analyzed. After different combinations of the existed loadings in both surrounded areas of pile drag loading considering different cases. The vertical pile was mod- eled and its results were compared to those of previous studies for model validation. Then single batter piles with different angles were modeled using the results.

Rapid development and industrialization has produced large quantities of waste. Much of this waste does not have any effective use and its disposal has become more difficult and expensive because of the increasingly strin- gent environmental regulations and the shortage of suit- able nearby disposal sites. It has been observed that some of this waste has high potential and can be gain- fully utilized in other industries. To use industrial waste will not only help in solving environmental pollution problems associated with its disposal but also help in the conservation of natural resources.Blast furnace slag and steel making slag is produced dur- ing the iron and steel making process. These materials are widely utilized in construction industries all over the world. Use of these by-products depends on their chem- ical composition and physical properties. Despite some industrial uses of blast furnace slag in Iran it is still considered a by-product requiring storage. In this arti- cle the effect of basic oxygen slag (BOS) on the swelling and shrinkage characteristics of clay soil has been inves- tigated. Kaolinite samples have been treated with 1 3 and 5% lime and 10 15 and 20% BOS slag. Samples have been cured at35o C for periods of 1 7 28 and 90 days after which they were subjected to swelling unconfined compressive strength and liquid limit plastic limit and shrinkage limit tests. Results showed that the concur- rent addition of lime and BOS slag to kaolinite com- pared to lime treated kaolinite samples significantly re- duces their swelling potential. The unconfined compres- sive strength of lime/BOS slag treated kaolinite samples also showed a significant increase when compared to lime only treated samples. The amount of swelling reduction or the increase in unconfined compressive strength was found to be a function of the percentage of lime and BOS slag addition as well as the curing period.

Underground conduits voids or buried pipes have been since ancient times for carrying oil water gas sewage slurry and similar materials from one location to an- other. A conduit placed in a relatively narrow ditch is known as a ditch conduit and it is often covered with lo- cally available unreinforced earth fill. Designing a buried conduit covered with granular soil backfill requires that the vertical load acting on the conduit is estimated. A vertical load that develops above a buried pipe/ buried conduit often differs from free-field stress. The arching action of a granular soil mass overlying buried conduits can reduce the vertical pressure on them. Reduction in the load on the conduit occurs due to mobilization of the shear resistance along the walls of the ditch during settlement of the backfill within the ditch. However in some cases designers need more reduction in pressure acting over the conduit which is possible by placing a geosynthetic layer within the soil backfill. This paper represents an analytical method to evaluate stress re- duction on a buried conduit reinforced with a single re- inforcement layer. Vertical pressure acting on the buried conduit is estimated due to the simultaneous effects of soil arching action and the geosynthetic layer. A nu- merical example to evaluate vertical pressure over the conduit is explained. The effects of various parameters such as tensile stiffness of reinforcement elastic modu- lus of soft material burial depth of conduit on the pres- sure over the conduit and efficiency are investigated and discussed. Comparison results of reinforced and unreinforced backfill shows the significant effect of re- inforcement in decreasing stress acting on the conduit. This reduction is more highlighted when the depth of the buried conduit is increased. Comparisons of act- ing pressure on the conduit with the present approach shows satisfactory agreement with those obtained from other studies under the same conditions irrespective of unreinforced and reinforced backfill.

Geosynthetic materials such as geotextile geogride and geocell (as 3D-inclusion reinforcements) have been widely used in geotechnical engineering applications for e.g. road construction layers stable embankments over soft soil longer-lasting road construction layers and expe- dient access over soft ground. An additional possible use would be to improve the bearing capacity of foot- ings. Although analytical solutions on the bearing ca- pacity of planar reinforcements have often been stud- ied there is a major lack of study into the bearing ca- pacity of footings when supported on geocell-reinforced soil. Thus according to the widespread use of geocell- reinforced beds providing an analytical approach to the design of shallow foundations and to explain their stress- settlement behavior could be very useful. Due to the three-dimensional mechanism of a geocell the cell walls of geocell reinforcement keep the encapsulated soil from being displaced from the applied load by confining the material through the hoop action of a cell thereby in- creasing the shear strength of the composite system This paper presents a new analytical method based on the theory of pavement layers and the theory of multi- layered soils for estimating the pressure-settlement re- sponse of circular footings resting on a reinforced sand bed with a single layer of geocell The elastic modulus of the unreinforced layers and geocell reinforced soil layer (geocell and soil inside the geocell pockets) as a compos- ite material was evaluated by an equivalent composite material which was developed from triaxial compression tests on unreinforced and geocell-reinforced soil samples Comparisons of the present analytical results with plate load test results show favorable agreement and thus indicate the accuracy and appropriate performance of this method Also parametric studies are carried out to investigate the influence of the dimensionless soil stiff- ness modulus stiffness geotextile used in constructing the geocell the height of the geocell layer and diameter plate loading on the pressure settlement variations of the footing.

Bottled water is the fastest growing beverage industry in the world and recycling their plastic waste has be- come a major challenge worldwide The present study provides an approach for using waste p lastic strips as reinforcement material in soil backfill The experimental tests investigate the behaviour of soil alone and plastic waste-soil mixtures using static triaxial tests and CBR tests The triaxial tests were carried out on the unrein- forced soil samples and plastic-soil mixture samples at three confining pressures of 50 100 and 150 kPa The effect of small and large sizes of waste plastic was exam- ined by three waste plastic strip contents of 0 5% 1% and 1 5% (by total weight of the sample) The CBR tests were performed by standard CBR apparatus with a load rate of 1 27 mm/min on unreinforced and rein- forced samples (i e waste plastic-soil mixture) with a large size of waste plastic particles at four plastic con- tents of 0 5 1 1 5 and 2% The results of the triaxial tests show that the strength of reinforced samples in- creases with increasing the size and percentage of plastic particles For example for the plastic content of 1 5% at confining pressure of 50 kPa and strain level of 6% a maximum improvement in the strength of the plastic- reinforced sample was obtained as 1 25 times the unre- inforced sample The results of CBR tests depict that the provision of waste plastic particles reduces the pen- etration of shaft into the sample and increases its CBR value Overall the results of CBR and triaxial tests demonstrate that the inclusion of waste plastic strips in soil with appropriate amounts improves the strength and deformation behavior of foundation beds substantially The proposed technique can be used to advantage in bearing capacity improvement and settlement reduction in the design of shallow footings embankments and road construction.

A major issue facing structural engineers is assessing the effects of dynamic loads on structural systems includ- ing beams The importance of this matter arises from moving vehicles such as cars and trains on bridge struc- tures which are usually simulated by beam structures Hence in several studies in order to explore the dynamic response of beam structures under excitation of dynamic loads various analytical and numerical methods have been utilized In this study to examine easier and faster procedures for finding the dynamic response of Euler- Bernoulli Timoshenko and Higher-Order beams a sim- ple semi-analytical method based on characteristic or- thogonal polynomials and trigonometric functions com- patible with boundary conditions is presented To this end discrete equations of motion are derived for the three mentioned theories due to a moving mass ac- cording to the Hamilton principle Then the govern- ing equations are transformed into ordinary differential equations in the time domain and by applying an ap- proximate method the displacement field of the beam is achieved In order to consider the efficiency conver- gence rate and accuracy of this method two numerical examples are provided to compare the results of this pa- per with those presented by other researchers In this regard in the former the free vibration frequencies of the beam with various theories for different boundary conditions were obtained and it is shown that the re- sults of all three theories give a good convergence rate and high accuracy Furthermore in the latter example the dynamic response of the beams subjected to a mov- ing mass for different values of base beam slenderness was achieved and compared with other studies Analy- sis of the maximum dynamic response of the beam and the time history diagrams Showed that the obtained re- sults are in close agreement with those obtained from the numerical method despite using the lower number of shape functions

Among the most suitable methods to obtain building seismic data in order to specify the level of its seismic vulnerability and prioritize it for undertaking detailed seismic assessment studies is the use of a qualitative evaluation method Qualitative evaluation from this perspective also known as rapid assessment is usually scheduled for a large number of buildings Since a huge volume of information has to be collected in such a study filling special forms and checklists for the studied build- ings is necessary In this paper considering the great number of masonry buildings and their common weak- points this type of building is examined Two different methodologies including FEMA154 and Publication No 376 of the Management and Planning Organization are used for rapid and detailed evaluation of a large number of school masonry buildings with emphasis on Publi- cation No 376 and a detailed study is accomplished Actual cases of buildings located in Khuzestan province are considered The existing condition of these buildings is identified by field surveys and strength-of-material ex- periments In addition two dimensional models of roofs using the finite element method and theoretical models for the walls of masonry buildings are constructed The joists are modeled with beam elements For brick arch and joist-and-block roofs use is made of shell elements No considerable tensile stresses are observed in the roofs Then the lateral loads distributed from the roofs are ap- plied to the walls For the walls four failure modes are considered including mortar joint slippage overturning diagonal over-tension and toe over-compression It is shown that the rapid and comprehensive evaluation pro- cedures are consistent with Publication No 376 More- over suggestions are offered to modify the fundamental relationship of rapid assessment in Publication No 376 which leads to evaluation results similar to FEMA154.

This paper applies the finite difference method using FLAC to evaluate the undrained bearing capacity of rigid strip footings resting on two-layered clay deposits For modeling the soil elastic-perfect plastic constitutive some numerical analyses are carried out and the results are compared with theoretical solutions in order to de- termine the accuracy of the model In these analyses values for elastic modulus Poisson ratio undrained soil shear strength foundation width element dimensions in order to identify the effect of each parameter on the final result and to free the modeling from numerical in- accuracies Next the ratio of depth of the upper layer to the width of the foundation (H/B) are varied from 0 15 to 3 while the ratio of undrained soil shear strength of the upper to the lower layer (cUt /cUb) is varied from 0 2 to 5 in order to model and analyze the most probable cases that could be encountered in practice Results of analyses show that whilst in the case of soft over strong clay general failure usually occurs in the case of strong over soft clay as the top soil gets stronger the soil will most likely experience punching failure which is asso- ciated with a high amount of plastic settlement before reaching its ultimate bearing capacity.Afterwards the results of analyses are compared with the results obtained by other researchers using limit anal- ysis FEM empirical and semi-empirical solutions In the end an equation is presented for each case (strong over soft clay and soft over strong clay) which repre- sents the bearing capacity of two-layered clays and their precision when describing the numerical results is ex- amined A comparison between the results of analyses and those given by the presented formula in this paper shows strong agreement between numerical results and the formula provided for each case (R2 > 0.99) This proves how reliable these equations are in determining the undrained bearing capacity of strip footings placed over two-layered clays without needing to consult charts and figures

In earthquake-prone regions steel moment frames are a desirable choice for constructing structures due to their appropriate ductility and high energy dissipation capac- ity The main weakness of moment frames is the possi- bility of excessive displacement when subjected to forces resulting from powerful earthquakes which may lead to total destruction of a structure In recent years us- ing cable as a bracing system for moment frames has become an issue of interest to many researchers Ca- ble bracings unlike steel bracings are not subjected to complex phenomenon such as inelastic buckling and due to their high tensile strength and the possibility of high pre-tensioning create considerable stiffness The main weakness of this system is the occurrence of slack- ing phenomenon in the cables when redirecting the lat- eral force which has a dynamic impact on the system Researchers have proposed solutions including the use of special tools and connections and different arrange- ments of cables and dampers in the cable bracing system Using these methods it is possible for the system to use its total capacity at the same time Also by keeping all cables stretched slacking would be prevented when redirecting the lateral force.Studies on Y-shaped cable bracing systems show that all cables in this system participate in providing the lateral resistance of the frame without requiring any spe- cial tools and connections Moreover Y-shaped brac- ing systems are not active in small displacements; they do not affect the elastic behavior of the moment frame and do not reduce flexibility. However, when large dis- placements occur in the frame, this system is activated, which, hence, increases the ductility and stability of the moment frame.In this paper, for investigating the Y-shaped cable brac- ing system operation, in order to keep all cables un- der tension, and the effect of geometrical parameters on system behavior, a program was written in MAT- LAB software. Considering there was no possibil- ity of having limit points in the force-displacement curve of the system, this program was written based on the Newton-Raphson incremental-iterative method for non-linear analysis of structures, which can draw the pushover curve of the bracing system and apply its var- ious geometric parameters as a variable. The results of system modeling in this program indicate that the ar- rangement causes high flexibility, and changing system stiffness is easily possible (by changing the length of the connecting cable and cable diameter). It was also ob- served that with a specific amount of geometric param- eter, the system had maximum lateral stiffness and the most durability against any incidence of cable slacking.

Permanent deformation is an important factor in flexible pavement design. Through increasing traffic load and tire pressure most permanent deformation occurs in the upper layer rather than in subgrade Wheel tracking is a popular HMA test, since it simu- lates the permanent deformation resistance of mixtures. The basic similarity of these devices is in using a cyclic wheel passing through the surface of specimens, and the cumulative rut after the proposed cycles is used as a measure of rutting potential. The wheel tracking test is undertaken using the Iran University of Science and Technology, ABRC Wheel Track machine, by applying 8000 cycles of wheel load of 50kg that turns around an axis and applies the wheel load to specimens. The test was done at 60 oC.In this study, the rutting performance of hot mix as- phalt specimens made with bentonite as bitumen mod- ifiers were studied. 10%, 15%, 20%, 25% and 30% ben- tonite content by weight of asphalt cement were used as its modifier. A propeller mixer was used for blend- ing the bentonite and asphalt cement. In this study, the rutting performance of modified asphalt mixtures was evaluated through various laboratory tests, such as Marshall stability and permanent deformation The test methods used in this evaluation were the dynamic creep test and wheel tracking tests. The results demonstrated that the use of bentonite can enhance Marshall stabil- ity and resistance to permanent deformation of mix- tures. The mixtures also show good resistance to perma- nent deformation, evaluated by means of wheel tracking tests.

Low temperature cracking is the main distress in as- phalt pavements in winter. Use of modifiers or addi- tives in the asphalt mixture is a suitable and common method for improving its mechanical properties, espe- cially under high temperature service conditions. How- ever, for cold climates, where road pavement usually ex- perience subzero temperatures, the overall failure mech- anism of asphalt layers may occur mainly due to elas- tic brittle fracture and the growth of initiated cracks inside the pavements. Hot mix asphalt (HMA) con- crete is perhaps the most complicated material in flexible pavement systems since its properties depend upon tem- perature and loading conditions. Fracture toughness is the most important parameter for characterizing crack growth and the failure of cracked materials and struc- tures such as asphalt pavements. Cracked semi-circular specimens subjected to three-point bending have been recognized as appropriate test specimens for conduct- ing fracture tests in brittle materials. The manufactur- ing and pre-cracking of the specimen are simple. A non complicated loading fixture is also required for a fracture test. Hence, the main aim of this research is to study the effect of different additives, including Poly phospho- ric acid (PPA), Styrene butadiene styrene (SBS), Anti striping agent (ANTI), Crumb rubber (CR) and FT- paraffin wax (sasobit), on the low temperature mode I fracture resistance of asphalt mixtures. A series of asphalt samples with different percentages of the men- tioned additives were manufactured in the shape of semi circular specimens containing vertical edge cracks. The test samples were then loaded monotonically using a symmetric three-point bend fixture at a constant sub- zero temperature of -15oC. The value of mode I fracture toughness (KIC) was determined by recording the criti- cal fracture loads of tested specimens. It is shown that all the investigated additives can increase the low tem- perature fracture toughness of the asphalt mixture, and maximum increase in the value of KIC occurs when the sasobit and CR additives are used.

Tunnel face support during boring operations is gaining more interest as tunneling development expands towards geo-technically less appropriate zones with heavy traffic. Good tunnel face support is a pre-requisite for success- ful safe boring, and evaluating optimum pressure is an important factor in the performance of the Earth Pres- sure Balance (EPB) machine in urban tunneling. Ap- plying pressures lower than equilibrium results in face subsidence, while higher than equilibrium causes an up- lift in ground surface, leading in either case, to opera- tion disruption, floor subsidence and damage to nearby buildings. Optimum pressure for face support is evalu- ated here using analytical and numerical methods, and parameters such as soil physical and mechanical charac- teristics, height of over-burden, water table level, tunnel geometry, and traffic and structural loads. As the tun- nel for line 7 of the Tehran subway (Eastern-Western piece) was excavated in an urban zone, in this work, the Jancsecz analytical method was used to measure the re- quired pressure for the tunnel face support. The tunnel was bored using a 9/164m diameter ground pressure bal- ancing machine. The final tunnel cover was tetragonal in geometry, 35cm thick and 1.50m wide, making an out- side diameter of 8.85m for the tunnel. Data obtained for this study was obtained from 13 boreholes and 5 wells excavated deliberately along the tunnel. The flow di- rection of underground water about the tunneling zone was found to be north-east to south-west. Considering the average water level obtained from the boreholes, only 35% of the tunnel remains above water and 65% is below water level. To illustrate water pressure fluctuation, this was considered in the analytical method, but ignored in the numerical one, which employed PLAXIS3D soft- ware. When comparing the results, a pressure difference of 20-25 kPa was noted, indicating acceptable confor- mance of the data.