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CONTENTS
Volume 9, Number 5, November 2015
 

Abstract
This study investigates the behavior of group of tapered and cylindrical piles. The bearing capacities of groups of tapered and cylindrical piles are computed and compared. Modeling of group of piles in this study is conducted in sand using three-dimensional finite element software. For this purpose, total bearing capacity of each group is firstly calculated using the load-displacement curve under specific load and common techniques. Then, the model of group of piles is reloaded under this calculated capacity to find group settlements, stress states on the lateral surfaces of group block, efficiency of group and etc. In order to calculate the efficiency of each group, single tapered and cylindrical piles are modeled separately. Comparison for both tapered and cylindrical group of piles with same volume is conducted and a relation to predict tapered pile group efficiency is developed. A parametric study is also performed by changing parameters such as tapered angle, angle of internal friction of sand, dilatancy angle of soil and coefficient of lateral earth pressure to find their influences on single pile and pile group behavior.

Key Words
group of pile; tapered pile; bearing capacity; 3D modeling; finite element

Address
Department of Civil and Environmental Engineering, Shiraz University, Shiraz, Iran.


Abstract
Two fundamental issues exist in the damage theory of geo-material based on the concept of thermodynamics: existence or nonexistence of the dissipation potential, and whether the dissipation potential could be decoupled into a damage potential and a plastic one or not. Thermodynamics theory of elastoplastic damage assumes the existence of dissipation potential, but the presence of dissipation potential is conditional. Based on the dissipation inequality in accord with the second law of thermodynamics, the sufficient and necessary conditions are given for the existence of the dissipation potential separately in total and incremental forms firstly, and proved strictly in theory. With taking advantage of the basic mechanical properties of geo-materials, the nonexistence of the dissipative potential is verified. The sufficient and necessary conditions are also given and proved for the decoupling of the dissipation potential of geo-materials in total and incremental forms. Similarly, the non-decoupling of the dissipation potential has also been proved, which indicates the dissipation potential of geo-materials in total or incremental forms could not be decoupled into a dissipative potential for plasticity and that for damage respectively. The research results for the fundamental issues in the thermodynamics theory of damage will help establish and improve the theoretic basis of elastoplastic damage constitutive model for geo-materials.

Key Words
geo-material; constitutive relation; thermodynamics; damage; dissipative potential; decoupling

Address
Chongqing Key Laboratory of Geomechanics & Geoenvironmental Protection, Department of Civil Engineering, Logistic Engineering University, Chongqing 401311, China.


Abstract
Laboratory model tests were conducted to investigate the effect of surcharge loading rate on the magnitude of lateral displacement of prefabricated vertical drains (PVDs) improved deposit. The test results indicate that under the condition that the system had sufficient factor of safety (FS) (FS ≥ 1.2), for the similar model ground under the same total applied surcharge load, the lateral displacement increases with the increase of loading rate. The test results have been used to check the validity of a previously proposed method for predicting the maximum lateral displacement, and it shows that the data points are around the middle line of the predicted range, which supports the usefulness of the proposed method. The basic idea of the prediction method is an empirical relationship between the normalized lateral displacement (NLD) and a ration of load to the undrained shear strength of the deposit (RLS). The model test results offer some modifications of the NLD-RLS relationship: (1) instead of a bilinear relationship, NLD-RLS relationship may be entirely nonlinear; (2) the upper bound value of RLS for the proposed method can be used may be limited to 2.1 instead of the originally proposed value of 3.0.

Key Words
lateral displacement; prefabricated vertical drain; model test; embankment; soft deposit

Address
Department of Civil Engineering and Architecture, Saga University, Saga, Japan.

Abstract
Soil-reinforcement interaction mechanism is an important issue in the design of geosynthetic reinforced soil structures. This mechanism depends on the soil properties, reinforcement characteristics and interaction between these two elements (soil and reinforcement). In this work the shear strength of sand/geotextile interfaces were characterized through direct and simple shear tests. The direct shear tests were performed on a conventional direct shear device and on a large scale direct shear apparatus. Unreinforced sand and one layer reinforced sand specimens were characterized trough simple shear tests. The interfaces shear strength achieved with the large scale direct shear device were slightly larger than those obtained with the conventional direct shear apparatus. Notwithstanding the differences between the shear strength characterization through simple shear and direct shear tests, it was concluded that the shear strength of one layer reinforced sand is similar to the sand/geotextile interface direct shear strength.

Key Words
soil reinforcement; soil/geosynthetic interfaces; direct shear tests; simple shear tests

Address
(1) Castorina Silva Vieira, Maria de Lurdes Lopes:
Department of Civil Engineering, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias, 4200 465 Porto, Portugal;
(2) Laura Caldeira:
National Laboratory of Civil Engineering, Av. do Brasil, 1700-066 Lisbon, Portugal.

Abstract
Influenced by various mining activities, fractures in rock masses have different densities, set numbers and lengths, which induce different mechanical properties and failure modes of rock masses. Therefore, precisely expressing the failure criterion of the fractured rock influenced by coal mining is significant for the support design, safety assessment and disaster prevention of underground mining engineering subjected to multiple mining activities. By adopting PFC2D particle flow simulation software, this study investigated the propagation and fractal evolution laws of the micro cracks occurring in two typical kinds of rocks under uniaxial compressive condition. Furthermore, it calculated compressive strengths of the rocks with different confining pressures and box-counting dimensions. Moreover, the quantitative relation between the box-counting dimension of the rocks and the empirical parameters m and s in Hoek-Brown strength criterion was established. Results showed that with the increase of the strain, the box-counting dimension of the rocks first increased slowly at the beginning and then exhibited an exponential increase approximately. In the case of small strains of same value, the box-counting dimensions of hard rocks were smaller than those of weak rocks, while the former increased rapidly and were larger than the latter under large strain. The results also presented that there was a negative correlation between the parameters m and s in Hoek-Brown strength criterion and the box-counting dimension of the rocks suffering from variable mining activities. In other words, as the box-counting dimensions increased, the parameters m and s decreased linearly, and their relationship could be described using first order polynomial function.

Key Words
fractured rock mass; Hoek-Brown strength criterion; rock mass parameters; box-counting dimension; numerical simulation

Address
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, 266590 Qingdao, China.


Abstract
This article considers the problems of cylindrical bending of functionally graded plates in which material properties vary through the thickness. The variation of the material properties follows two power-law distributions in terms of the volume fractions of constituents. In addition, this paper considers orthotropic materials rather than isotropic materials. The traction-free condition on the top surface is replaced with the condition of uniform load applied on the top surface. Numerical results are presented to show the effect of the material distribution on the deflections and stresses. Results show that, all other parameters remaining the same, the studied quantities (stress, deflection) of P-FGM and E-FGM plates are always proportional to those of homogeneous isotropic plates. Therefore, one can predict the behaviour of P-FGM and E-FGM plates knowing that of similar homogeneous plates.

Key Words
functionally graded plates; cylindrical bending; stress; elasticity solutions

Address
Université Ibn Khaldoun, BP 78 Zaaroura, 14000 Tiaret, Algérie.

Abstract
Single treatment and staged treatments in vertical wells are widely applied in sandstone and mudstone thin interbedded (SMTI) reservoir to stimulate the reservoir. The keys and difficulties of stimulating this category of formations are to avoid hydraulic fracture propagating through the interface between shale and sand as well as control the fracture height. In this paper, the cohesive zone method was utilized to build the 3-dimensional fracture dynamic propagation model in shale and sand interbedded formation based on the cohesive damage element. Staged treatments and single treatment were simulated by single fracture propagation model and double fractures propagation model respectively. Study on the changes of fracture vicinity stress field during propagation is to compare and analyze the parameters which influence the interfacial induced stresses between two different fracturing methods. As a result, we can prejudge how difficult it is that the fracture propagates along its height direction. The induced stress increases as the pumping rate increasing and it changes as a parabolic function of the fluid viscosity. The optimized pump rate is 4.8 m3/min and fluid viscosity is 0.1 Pa⋅s to avoid the over extending of hydraulic fracture in height direction. The simulation outcomes were applied in the field to optimize the treatment parameters and the staged treatments was suggested to get a better production than single treatment.

Key Words
multilayer stress interference; sandstone and mudstone interbedded reservoir; CZM; induced stress; fracture height control

Address
(1) Haiyan Zhu, Xudong Zhang, Jianchun Guo, Jingya Huang:
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Sichuan, Chengdu, China;
(2) Yaqin Xu:
Safety Environment Quality Surveillance & Inspection Institude, CNPC Chuanqing Drilling Engineering Company Limited, Sichuan Guanghan, China;
(3) Li Chen:
Luming company, Shengli Oil Field, Sinopec, Dongying, China;
(4) Shuhang Yuan:
Research Institute of Engineering and Technology, PetroChina Southwest Oil and Gas Fields Company, Sichuan Guanghan, China;
(5) Yonghui Wang:
Research Institute of Petroleum Exploration and Development-Langfang, Petrochina, Langfang, China.

Abstract
A three-dimensional elastoplastic constitutive model, also known as a UH model (Yao et al. 2009), was developed to describe the stress-strain relationship for normally consolidated and overconsolidated soils. In this paper, an acoustic tensor and discriminator of bifurcation for the UH model are derived for the strain localization of saturated clays under undrained and fully and partially drained conditions. Analytical analysis is performed to illustrate the points of bifurcation for the UH model with different three-dimensional stress paths. Numerical analyses of cubic specimens for the bifurcation of saturated clays under undrained and fully and partially drained conditions are conducted using ABAQUS with the UH model. Analytical and numerical analyses show the similar bifurcation behaviour of overconsolidated clays in three-dimensional stress states and various drainage conditions. The results of analytical and numerical analyses show that (1) the occurrence of bifurcation is dependent on the stress path and drainage condition; and (2) bifurcation can appear in either a strain-hardening or strain-softening regime.

Key Words
bifurcation; over-consolidated; constitutive model; undrained condition; partially drained condition; numerical simulation

Address
(1) De\'an Sun, Liwen Chen:
Department of Civil Engineering, Shanghai University, Shanghai, 200072, China;
(2) Junran Zhang:
Henan Province Key Laboratory of Geotechnical and Structural Engineering, North China University of Water Resources and Electric Power, Zhengzhou, 450045, China;
(3) Annan Zhou:
School of Civil, Environmental & Chemical Engineering, RMIT University, Melbourne, 3001, Australia.


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