Journal of Engineering Geology

Studying the Effect of Roughness on Soil-Geotextile Interaction in Direct Shear Test

Mahmood Reza Abdi

One of the methods of increasing soil resistance against failure is soil reinforcement using geosynthetics. Soil-geosynthetic interactions are of great importance and are affected by friction and adhesion at their interface. Soil gradation, contact surface roughness and geotextile density are among the factors affecting soil-geotextiles interaction this study, to investigate the effects of these factors, large-scale direct shear tests have been conducted using a well and a poorly graded sand at a relative density of 80% reinforced with two geotextiles having different tensile strengths and mass per unit area. Samples were subjected to normal pressures of 12.5, 25 and 50kPa and sheared at a rate of 1 mm/min. Geotextile surface roughness was achieved by gluing two different single sized sand particles. Results show that increasing geotextile surface roughness increases shear strength at soil-geotextile interface. Geotextile tensile strength mobilization is shown to depend on soil grain size at the interface. The coarser and more angular the soil particles, the more effective the soil-reinforcement interactions. Geotextile tensile strength and its mass per unit area are shown to less important factors.

Study of Loose Soil Layer Effects on Excavations Supported by Steel Sheet Pile Walls-A Numerical Study

Masoud Amel Sakhi

Steel sheet pile walls are being widely used as earth retaining systems. Sometimes loose or soft soil layers are located in various depths in an excavation. This issue causes different effects on ground surface displacements, forces and moments acting on sheet piles and struts during excavation procedure, compared with a status that soil is totally uniform. These differences are not exactly considered in conventional design methods of sheet pile walls. In this paper, a deep excavation using finite element method is analyzed. Excavation’s depth is divided into three different layers. One of three layers is a loose soil layer and its position is modeled in three different situations, top, middle and bottom of the model. Obtained results are compared with results of excavation without the loose layer. The pseudo-static analysis is performed by applying 0.3g horizontal acceleration. The results indicate that when a loose layer is located beneath stiffer layers, bending moments acting on sheet pile wall and shear forces increase about (50~100)% and (15~50)%, respectively. Also, the middle loose layer changes the location of maximum lateral deformation of steel sheet pile wall.

Forecasting Surface Settlement Caused by Shield Tunneling Using ANN-BBO Model and ANFIS Based on Clustering Methods

Hadi Fattahi

Maximum surface settlement (MSS) is an important parameter for the design and operation of earth pressure balance (EPB) shields that should determine before operate tunneling. Artificial intelligence (AI) methods are accepted as a technology that offers an alternative way to tackle highly complex problems that can’t be modeled in mathematics. They can learn from examples and they are able to handle incomplete data and noisy. The adaptive network–based fuzzy inference system (ANFIS) and hybrid artificial neural network (ANN) with biogeography-based optimization algorithm (ANN-BBO) are kinds of AI systems that were used in this study to build a prediction model for the MSS caused by EPB shield tunneling. Two ANFIS models were implemented, ANFIS-subtractive clustering method (ANFIS-SCM) and ANFIS-fuzzy c–means clustering method (ANFIS-FCM). The estimation abilities offered using three models were presented by using field data of achieved from Bangkok Subway Project in Thailand. In these models, depth, distance from shaft, ground water level from tunnel invert, average face pressure, average penetrate rate, pitching angle, tail void grouting pressure and percent tail void grout filling were utilized as the input parameters, while the MSS was the output parameter. To compare the performance of models for MSS prediction, the coefficient of correlation (R²) and mean square error (MSE) of the models were calculated, indicating the good performance of the ANFIS-SCM model.

A Numerical Approach on Bearing Capacity of Drilled Shafts Embedded in Clay

Mohammad Mehdi Ahmadi

This study numerically investigates the bearing capacity of drilled shafts (bored piles) in clay using FLAC^2D. The results obtained in this study are compared with centrifuge test results. The results of the empirical relationships available in the literature are compared with the results of the present numerical study. A series of analyses is also conducted to assess the effects of various soil and pile parameters on the magnitude of tip and side resistance of bored piles embedded in clay. These parameters include the soil elastic modulus, pile length and diameter, undrained shear strength, unit weight, and Poisson’s ratio of soil. Furthermore, the coupling effect of soil undrained shear strength and elastic modulus of soil on tip resistance are investigated. The results show that the lower value of soil elastic modulus results to lower effect of soil undrained shear strength. The effect of soil undrained shear strength on tip resistance is approximately constant (about 83% for a change of soil undrained shear strength between 25 to 200 kPa) for the range of elastic modulus between 20 and 180 MPa. Also, a new equation is proposed to estimate the bearing capacity factor of N^*_c.

Investigating the Effect of Face Pressure on Ground Settlement in Tunneling with Earth Pressure Balance (EPB) Shield (Case Study: Qom Metro Line A)

Mehdi Hosseini

In the mechanized boring method, the factors affecting ground surface settlement can be mainly divided into five categories: geometric, geomechanic, boring machines working, operating and management parameters. In urban tunnels bored mainly in shallow soil bed, face pressure can be one of the factors preventing ground settlement. The Line A tunnel in Qom metro project is bored with an EPB (Earth Balance Pressure) mechanized boring machine. The effect of face pressure on ground surface settlement was analyzed in the present study according to five sections of the tunnel. These five sections were selected in different kilometers of the tunnel where settlement gauges were installed and the results could be validated. To investigate the effect of face pressure on maximum ground surface settlement, four pressure levels of 100 kPa, 150 kPa, 200 kPa, and 400 kPa were taken into consideration. These were 1, 1.5, 2, and 4 times of the initial face pressure level, respectively. The ground surface settlement was assessed at four pressure levels using the finite element software, PLAXIS 3D TUNNEL. The results were validated using ground-level instrumentation (settlement gauges) on all sections. The validation showed that the modeling results are in good agreement with the results obtained from settlement gauges. Comparison of the results indicated that a 4-fold increase in the face pressure led to a maximum decrease of 4.45 mm in the maximum settlement. Therefore, an increase in the face pressure can reduce settlement, although quite minimally. It was also found that an over-increased face pressure (face pressure over 200kPa) not only did not reduce the maximum ground surface settlement but also may lead to passive failure or uplift of ground surface ahead of the shield.

Prediction of Tunnelling-Induced Surface Settlement with Artificial Neural Networks, Case Study: Mashhad Subway Tunnel

In urban areas, it is essential to protect the existing adjacent structures and underground facilities from the damage due to tunneling. In order to minimize the risk, a tunnel engineer needs to be able to make reliable prediction of ground deformations induced by tunneling. Numerous investigations have been conducted in recent years to predict the settlement associated with tunneling; the selection of appropriate method depends on the complexity of the problems. This research intends to develop a method based on Artificial Neural Network (ANN) for the prediction of tunnelling-induced surface settlement. Surface settlements above a tunnel due to tunnel construction are predicted with the help of input variables that have direct physical significance. The data used in running the network models have been obtained from line 2 of Mashhad subway tunnel project. In order to predict the tunnelling-induced surface settlement, a Multi-Layer Perceptron (MLP) analysis is used. A three-layer, feed-forward, back-propagation neural network, with a topology of 7-24-1 was found to be optimum. For optimum ANN architecture, the correlation factor and the minimum of Mean Squared Error are 0.963 and 2.41E-04, respectively. The results showed that an appropriately trained neural network could reliably predict tunnelling-induced surface settlement.

A Finite Element Model for Simulating Flow around a Well with Helically Symmetric Perforations

Seyed Amirodin Sadrnejad

In a perforated well, fluids enter the wellbore through arrays of perforation tunnels. These perforations are typically distributed in a helical pattern around the wellbore. Available numerical models to simulate production flow into cased-and-perforated vertical wells have complicated boundary conditions or suffer from high computational costs. This paper presents a simple and at the same time efficient finite element model to simulate flow around a well with helically symmetric perforations. In the proposed model, by taking advantage of the symmetry, only a thickness of perforated interval containing a single perforation tunnel needs to be meshed. Angular phasing between adjacent perforations is considered by applying periodic boundary conditions on the upper and lower boundaries of the representative reservoir thickness. These boundary conditions involve periodic-pressure and periodic-velocity parts. Unlike the periodic-pressure part, the method of imposing the periodic-velocity condition within a single-variable flow problem is rather vague. In this regard, it is proved that in the proposed model, periodic-velocity condition is automatically satisfied in a weak sense. The accuracy and the computational efficiency of the proposed model are verified through comparison with available models. The model results, in terms of skin factor, are compared with the common semi-analytical model as well, and good agreement is obtained. The proposed model can readily be used as a numerical tool to study inflow of wells with helically symmetric perforations.

Influence of waste tire chips on steady state behavior of sand

Seyed Abolhasan Naeini

Materials such as waste tire chips were widely used to improve the strength of soil. The objective of this study is to discuss the residual strength or steady-state behavior of sand-waste tire chip mixtures. A series of undrained monotonic triaxial compression tests were conducted on reconstituted saturated specimens of sand and sand-tire chip mixtures with variation in the tire-chip contents from 0 to 4 percentages by dry weight of soil. The specimens are prepared using dry deposition method of preparation. The influence on residual resistance of varying confining pressure (100, 200, and 300 kPa) and sand mixture relative density (40, 65, and 80%) were evaluated. Tests results showed that by increasing the tire chip contents, the residual strength increased and steady-state lines move to the right of log S_us-e diagram. Also, the residual resistance improvement induced by tire chip inclusions was found to be sensitive to the relative density of samples and applied confining pressure.