جستجوی مقالات مرتبط با کلیدواژه « نشست زمین » در نشریات گروه « زمین شناسی »

The purpose of this study is to determine the characteristics of sediments laid down in South Golbaf Playa and their influence on the formation of pseudokarstic bodlands in this area, as well as to investigate development of these badlands. For this purpose, textural, mineralogical and chemical (pH, EC, ESP and SAR) characteristics of the sediments were determined. These data revealed that the deposits are mainly comprised of erodible loams (mudrocks). According to field evidence, filling of the tectonic depression with sediments led to overflowing of surficial runoff as well as creation of gullies on the surface of playa. These events caused drainage and contraction of sediments and the appearance of fissures. The entry of runoff into these fissures and the consequent erosion led to creation of pseudokarstic badland.

In the last decade, the phenomenon of subsidence has occurred in a large part of the plains of Iran due to excessive harvesting of groundwater and climatic drought. This has caused a great deal of environmental, economic and social damage. In this study, the effect of fine-grained layer thickness and groundwater level drop on the Tehran-Karaj-Shahriar aquifer is investigated. In the first step, fine-layer thickness maps, groundwater level changes, bedrock level, permeability and hydrodynamic coefficients (storage coefficient and transferability) were prepared in GIS. Subsequently, the extent and location of the subsidence map was prepared using differential interferometry and ENVISAT ASAR satellite imagery from 2004 to 2009. The results of this study show a maximum subsidence rate of about 17 cm in the central part of the aquifer and an average groundwater level reduction of 0.42 m / year. Comparison of the subsidence map with the groundwater change map shows that the amount of subsidence in the northwestern aquifer with the greatest decrease in water level (27 m) is low (less than 0.5 cm / year) in contrast to the maximum subsidence (17 Cm/ year) occurred in central areas showing a 5 to 10 m drop in groundwater level. To interpret these results, fine-grained layer thickness maps, permeability, hydrodynamic coefficients and bed scales were used. The results confirm that the areas with maximum subsidence are in accordance with the thickness map of the fine-grained layers.

Ground settlement due to tunneling and the effects of the engineering geological factors on its dimensions and extensions, is a very important problem in shallow tunnel excavation projects in urban areas. Empirical method is one of the usual methods to study this subject. The empirical and dimension-less parameters of VL and k are the most important parameters in relation to this method that are estimated according to engineering geological factors. In this research, the values of these parameters were initially estimated based on preceding studies and the ground settlement was predicted using these estimated values of VL and k. In next stage, the results of predictions were compared with the real (measured) settlements happened due to Abuzar tunnel excavation. As the real settlements are less than the predicted ones, it was concluded that the real VL must be lower than the predicted values or the real k must be higher than the predicted values. With regard to the high dependency of these parameters to the soil cohesion, it seems natural cementation of Tehran alluvia has acted as a factor to increase the soil cohesion and has caused to decrease ground settlement due to excavation of Abuzar tunnel. For validation of this hypothesis, preceding findings about alluvia cementation were reviewed and the results of in-situ and laboratory shear and triaxial tests were compared with together. Then it is concluded that the higher cohesions of in-situ shear tests are occurred due to natural cementation of materials existing in Abuzar tunnel route.

It is over two decades that groundwater flow models are routinely implemented for better management of groundwater resources. Modeling groundwater flow with the help of the ground water modeling system (GMS) in the Damghan plain aquifer in northern Iran، which experiences declining levels and numerous environmental hazards، has demonstrated that، (a) in the worst case scenario the aquifer will face 320 cm of drawdown by year 2019 and (b) land subsidence is observed mainly in areas that are subjected to an accelerated water level drawdown rate، such as، the southern part of the aquifer. Four different rainfall scenarios that have been modeled demonstrate that some areas of the aquifer are slightly impacted by climatic change in contrast to some other areas that are being influenced substantially. Together with GMS، Genetic Expression Programming (GEP) and Multiple Linear Regression (MLR) models were used to forecast land subsidence by applying developing functional relations to the long-term groundwater drawdown data. This segment of the study shows that a 35. 4 cm and 39. 45 cm settlement will occur if the groundwater level drops by 295 cm and 343 cm، respectively. This research shows that the water level in the Damghan aquifer continues to decline and the land subsidence will intensify. It is، therefore، needed to reduce groundwater pumping in high-risk areas.

Nowadays urban transportation is one of the most important problem in the large cities. The best way to reduce this problem is to use the underground tunnels. The excavation of tunnel and other underground structures lead to remove a part of insitu rock and soil masses and the considerable changes in stress conditions around them and surface settlement has been occurred, consequently. Therefore, estimation of surface settlement due to the tunnel excavation, especially in urban areas, is of great importance. The Niayesh subway tunnel was built in the northern part of Tehran and it includes north and south tunnels. From a geological point of view, the Niayesh highway tunnels are mostly placed in the Hezar Dareh Formation (A) and also some parts especially the eastern and middle parts of north tunnel are placed in Kahrizak Formation (Bn). The tunnels have been excavated by the New Austrian Tunnelling Method (NATM). In this method, primary shotcrete applied as a short-term support and the final lining is used to satisfy final support. In this paper, surface settlement has been studied by empirical methods, numerical analysis and real settlements. For the empirical and numerical methods, O’Reilly and New (1982) method and finite element method by PLAXIS2D software have been used, respectively. The five sections (CS-1 to CS-5) have been selected in order to study the surface settlement during Niayesh tunnel excavation. On the basis of the achieved results, the numerical method in all sections (except section 3) is in agreement with the real settlements. However, empirical methods have estimated the settlements more than real settlements in these sections. Also, the achieved results from real settlements, empirical methods and numerical analysis show that the maximum settlement due to tunnel excavation is more than allowable settlement and it places in the warning range.

The purpose of this paper is to assess the engineering geological characteristics of soil and rock، and to predict ground surface settlement due to tunneling in main part of Tabriz subway line 2 which has a length of 12 km. The total length of the excavation line is 20 km between International fair and Kara malek. Tunnels will be excavated by employing EPB Shields 9. 2 m in diameter. Geology in this section is mainly composed of sand (SM)، silt (ML) and Sedimentary rocks (marl، siltstone، claystone and sandstone). Laboratory experiments and field studies were conducted for the study area. Field studies consisted of geological mapping، in situ testing، core drilling and sampling for laboratory studies. Uniaxial compressive tests، triaxial compressive strength test and deformability tests were conducted in the laboratory. For more Precise investigations، Subway path was divided into three geotechnical zones. These zones mainly consist of fine grained soils، coarse grained soils and Sedimentary rocks، respectively. Settlement predictions were performed using PLAXIS 3D Tunnel، a 3D finite element computer software. Excavation، ground support and face support steps in FEM analyses were simulated. Predictions were performed for typical geological parts in each zone، which is considered as critical in terms of ground surface settlement. Based on the results acquired from this method، ground surface settlement in some parts of Tabriz urban railway line 2 can be a problem during tunneling. Further، the ground condition improvement such as grouting and dewatering must be performed before tunneling.

In this paper reasons of land subsidence in Andisheh street area in Northwest of Mashad city are investigated. The subsidence in this area began simultaneous with the using of constructed building in 2008. This phenomenon results to 1.5 meter ground subsidence and 4 residential building completely destroyed and 15 other buildings became out of service. Aerial photograph indicate that this area is a part of Chelbazeh Fan and the streams of area were used for depot of demolition wastes in the past. For evaluation of thickness and properties of man-made soils in this area 46 handy borehole excavated and electrical resistivity are measured in 9 points. Finally the isothickness map of man-made soil is produced based on the results of Geotechnical and geoelectrical studies. The results of insitu tests showed that clear difference between the density of man-made and natural soil. The average density of man-made and insitu soil were 1.55 g/cm3 and more than 1.8 g/cm3, respectively. The compressibility tests show that the man-made soil are high potential of compressibility and there is a inverse linear relationship between settlement percentage and soil density.