numerical method

Nowadays, according to the increase in fuel demand, the oil wells drilling process has increased to exploit the reservoirs. The most important issue in oil and gas wells drilling topic, is the appropriate drilling rate and reducing the cost. Studies and the usage of geomechanical methods can play an important role in this matter. One of the studies regarded to the topic of reducing drilling costs that has received a lot of attention today is the well wall stability and determining the appropriate mud weight to avoid instability in the well wall. In recent years, a lot of research has been done in this field. In the present study, the well wall stability status of one of the excavated wells in Iran’s southwest oil fields, in 88.3 inches, in the upper section of Sarvak Formation was analyzed by using analytical and numerical methods through FLAC3D software. The input parameters in this research are obtained by interpreting the data from Dipole Shear Sonic Imager (DSI), the logs from Formation Micro Imager (FMI), density, caliper and gamma, MDT tools and also from the daily excavation and geology reports. Also, the window for the required mud weight to meet the dissipation situations, fracture opening, well wall symmetric downfalls, and well eruption was determined. The beneficial results of this study can be used to reduce the time spent on excavating the wells in the investigated field in the future, which results in saving drilling costs.

The study of variations of damage intensity for different earthquakes indicates the importance of the site effects on earthquake induced damage and ground motion parameters such as peak ground acceleration and induced amplification. In the occurrence of an earthquake, local site conditions such as soil characteristics, dimension of topographic irregularities, seismic bedrock depth, etc. as well as characteristics of incident wave have important e ects on seismic ground responseLocal site effects play an important role in earthquake-resistant designs and should be assessed carefully. By assessment of induced damages to structures and major infrastructures, seismic geotechnical researchers have concluded that the site conditions significantly influence on the failure distribution in urban and rural areas. Consequently, to determine the characteristics of seismic motions of the ground, it is essential to study the effective geotechnical factors. Consideration of the effects of the site response in the design of civil structures systems is of important to mitigate the damages to a certain extent on structures and the environment. Hence, it is relatively crucial to reliably attain the dynamic soil parameters of an earthquake-prone city/state. The nature of local site effects can be explained by using different methods such as simple theoretical analysis of ground response, measuring of surface and subsurface actual motion at the same site, and measuring the movements of the ground in sites with different conditions in comparison with the proposed site. In this paper, the effective factors on ground motion and site response in the soil classes II and III for Tehran city under the influence of Tabas and Bam earthquake were reviewed with numerical method and by using the finite difference software (FLAC 2D). Some of the effective parameters of the site dynamic characteristics on the seismic response of the ground surface have been studied and the response of peak ground acceleration and the resonance function has been compared. The results of this study show that at both sites, the horizontal maximum acceleration of the ground surface increases with respect to the bedrock shear velocity, reflecting the influence of the site on ground movement. At both sites, with increasing soil internal friction angle, the maximum acceleration at ground level increases and the acceleration response decreases. Also, as the shear wave velocity increases, the horizontal maximum acceleration at ground level for both structures decreases. Increasing the shear wave velocity in the surface layer, assuming the thickness of the layer is constant, results in a decrease in the natural frequency, which is far from the normal frequency range of conventional structures in Tehran. This indicates a decrease in the earthquake's dynamic force on surface structures. The natural frequency of five to seven story buildings in Tehran is about 0.5 to 0.7 Hz. As the surface layer thickness and bedrock depth increase, the horizontal maximum acceleration of the ground surface also decreases, meaning that the structural response can also be reduced.

In the urban tunneling project, one of the most important parameters to be considered is the subsidence level. Metro tunnels are one of the most important urban transportation infrastructures. Due to shallow depth of drilling, the most important factors to prevent the maximum subsidence and subsequent damage of surface and subsurface structures are deformation control and shear forces. The various methods of estimating the subsidence are divided in three categories, namely; Empirical, analytical and numerical methods. In this research, according to the geotechnical conditions of the study area using the data collected from the monitoring during the excavation of twin tunnels of metro by TBM, the empirical methods for the subsidence ( s=A_2+s_max/2 ), the maximum subsidence (s_max=A_1-A_2 ) and the inflection point (ⅈ=x_0 ) are presented by using Boltzmann function for Isfahan twin metro tunnel.

Free vibration of soil often occurs during earthquakes. Since the vibration caused by earthquake does not have (steady state harmonic vibration) continuity, the alluvium vibrates with its natural frequency between two natural seismic waves. This study evaluates the effect of piles on the period of free vibration of a soil layer using numerical method. In the first stage, using analytical equations for calculation of vibration period of a soil layer and a column with continuous mass, the results were analyzed by the software. In the second step, piles with the same dimensions and distance were added step by step, and the vibration period for the soil layer with piles was calculated. The friction or floating effects of the piles on alluvial soil vibration period was also examined. The results show that as the number of piles increases, the differences between the results of one dimensional analysis of alluvium soil and the results of the software become different, and this creates the need for specific arrangements for seismic analysis of this kind of alluvium (with inserted piles). The results also suggest that end-bearing piles have a greater effect on alluvial soil vibration period, and with increased amount of the floating of these piles, these effects decline.

Lack of proper drinking water is a major problem in the Central and Eastern cities of the country. Therefore, the necessity of water transmission projects are posed and since meeting these needs using traditional methods are very difficult, time-consuming and costly; so the mechanized excavation of tunnels and segmental lining are used. The water transferred through the pipes has significant surface destruction. That would result in serious environmental problems. Also, transferring water through the pipes is more expensive than the tunnel. In this paper, after introducing all procedures of segments design, segmental lining of Golab water conveyance tunnel using finite element was modeled. Firstly, loads on segmental lining was determined by UDEC software after installation in tunnel. Then, these amounts of load on the segments used for modeling was applied in ABAQUS software. Results obtained by 3D modeling software showed that the greatest amount of displacement of segments under different stress conditions concerns to the bottom segment. Based on the performed modeling, due to the high compressive stress, the crack is only seen in segment concrete and these results are totally in compliance with Golab tunnel segments observations.

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.