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Volume 7, Number 3, September 2014

Estimation of fracture initiation pressure is one of the most difficult technical challenges in hydraulic fracturing treatment of vertical or horizontal oil wells. In this study, the influence of in-situ stresses and pore pressure values on fracture initiation pressure and its profile in vertical and horizontal oil wells in a normal stress regime have been investigated. Cohesive elements with traction-separation law (XFEM-based cohesive law) are used for simulating the fracturing process in a fluid-solid coupling finite element model. The maximum nominal stress criterion is selected for initiation of damage in the cohesive elements. The stress intensity factors are verified for both XFEM-based cohesive law and analytical solution to show the validation of the cohesive law in fracture modeling where the compared results are in a very good agreement with less than 1% error. The results showed that, generally by increasing the difference between the maximum and minimum horizontal stress, the fracture pressure and its profile has been strongly changed in the vertical wells. Also, it\'s been clearly observed that in a horizontal well drilled in the direction of minimum horizontal stress, the values of fracture pressure have been significantly affected by the difference between overburden pressure and maximum horizontal stress. Additionally, increasing pore pressure from under-pressure regime to over-pressure state has made a considerable fall on fracture pressure in both vertical and horizontal oil wells.

Key Words
hydraulic fracturing; in-situ stress; pore pressure; fracture pressure profile; cohesive elements; finite element; XFEM-based cohesive law

(1) Seyed Erfan Saberhosseini:
Department of Petroleum Engineering, Science and research branch, Islamic Azad University, Tehran, Iran;
(2) Reza Keshavarzi:
Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran;
(3) Kaveh Ahangari:
Department of Mining Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran.

Analytical and numerical modeling of soft or problematic soils stabilized with lime and cement require a number of soil parameters which are usually obtained from expensive and time-consuming laboratory experiments. The high shear strength of lime and cement stabilized soils make it extremely difficult to obtain high quality laboratory data in some cases. In this study, an alternative method is proposed, which uses the unconfined compressive strength and estimating functions available in literature to evaluate the shear strength parameters of the treated materials. The estimated properties were applied in finite element model to determine which estimating function is more appropriate for lime and cement treated granular soils. The results show that at the mid-range strength of the stabilized soils, most of applied functions have a good compatibility with laboratory conditions. However, application of some functions at lower or higher strengths would lead to underestimation or overestimation of the unconfined compressive strength.

Key Words
lime and cement stabilization; finite element modeling; compressive strength; failure criterion; cohesion; internal friction angel

(1) Omid Azadegan:
Department of Civil Engineering, Shahid Bahonar University of Kerman, Iran;
(2) Jie Li:
School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Australia;
(3) S. Hadi Jafari:
Young Researchers Club, Yasuj Branch, Islamic Azad University, Yasuj, Iran.

In the conventional design of retaining structures in a seismic zone, seismic inertia forces are commonly assumed to act upwards and towards the wall facing to cause a maximum active thrust or act upwards and towards the backfill to cause a minimum passive resistance. However, under certain circumstances this design approach might underestimate the dynamic active thrust or overestimate the dynamic passive resistance acting on a rigid retaining structure. In this study, a new analytical method for dynamic active and passive forces in c-φ soils with an infinite slope was proposed based on the Rankine earth pressure theory and the Mohr-Coulomb yield criterion, to investigate the influence of seismic inertia force directions on the total active and passive forces. Four combinations of seismic acceleration with both vertical (upwards or downwards) and horizontal (towards the wall or backfill) directions, were considered. A series of dimensionless dynamic active and passive force charts were developed to evaluate the key influence factors, such as backfill inclination β, dimensionless cohesion c/γH, friction angle φ, horizontal and vertical seismic coefficients, kh and kv. A comparative study shows that a combination of downward and towards-the-wall seismic inertia forces causes a maximum active thrust while a combination of upward and towards-the-wall seismic inertia forces causes a minimum passive resistance. This understanding is recommended for use in the design of retaining structures in a seismic zone.

Key Words
earth pressure; retaining structures; analytical solution; horizontal and vertical seismic coefficients

(1) Ting-Kai Nian, Bo Liu:
School of Civil Engineering & State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China;
(2) Ting-Kai Nian, Run-Qiu Huang:
State Key Laboratory of Geohazard Prevention and Geoenvironmental Protection, Chengdu University of Technology, Chengdu 610059, China;
(3) Jie Han:
Department of Civil, Environmental and Architectural Engineering, the University of Kansas, Lawrence, KS 66045, USA.

A series of one-dimensional consolidation tests and triaxial creep tests were performed on Nansha clays, which are interactive marine and terrestrial deposits, to investigate their time-dependent behaviour. Based on experimental observations of oedometer tests, normally consolidated soils exhibit larger secondary compression than overconsolidated soils; the secondary consolidation coefficient () generally gets the maximum value as load approaches the preconsolidation pressure. The postsurcharge secondary consolidation coefficient (Cα') is significantly less than . The observed secondary compression behaviour is consistent with the /Cc concept, regardless of surcharging. The /Cc ratio is a constant that is applicable to the recompression and compression ranges. Compared with the stage-loading test, the single-loading oedometer test can evaluate the entire process of secondary compression; varies significantly with time and is larger than the obtained from the stage-loading test. Based on experimental observations of triaxial creep tests, the creep for the drained state differs from the creep for the undrained state. The behaviour can be predicted by a characteristic relationship among axial strain rate, deviator stress level and time.

Key Words
secondary consolidation coefficient; creep; time-dependent; interactive marine and terrestrial deposit clay; laboratory

(1) Xiaoping Chen, Qingzi Luo:
MOE Key Laboratory of Disaster Forecast and Control in Engineering, College of Science and Engineering, Jinan University, Guangzhou, 510632, PR China;
(2) Qiujuan Zhou:
Guangdong Technical College of Water Resources and Electric Engineering, Guangzhou, 510635, PR China.

This paper presents a detailed study focused on investigating the effects of silt content on the static and dynamic properties of sand-silt mixtures. Specimens with a low-plastic silt content of 0, 15, 30 and 50% by weight were tested in static triaxial, cyclic triaxial, and resonant columns in addition to consolidation tests to determine such parameters as compression index, internal friction angle, cohesion, cyclic stress ratio, maximum shear modulus, normalized shear modulus and damping ratio. The test procedures were performed on specimens of three cases: constant void ratio index, e = 0.582; same peak deviator stress of 290 kPa; and constant relative density, Dr = 30%. The test results obtained for both the constant-void-ratio- index and constant-relative-density specimens showed that as silt content increased, the internal friction angle, cyclic stress ratio and maximum shear modulus decreased, but cohesion increased. In testing of the same deviator stress specimens, both cohesion and internal friction angle were insignificantly altered with the increase in silt content. In addition, as silt content increased, the maximum shear modulus increased. The cyclic stress ratio first decreased as silt content increased to reach the threshold silt content and increased thereafter with further increases in silt content. Furthermore, the damping ratio was investigated based on different silt contents in three types of specimens.

Key Words
sand-silt mixture; critical state parameter; cyclic stress ratio; shear modulus; damping ratio

Department of Civil Engineering, National Kaohsiung University of Applied Sciences, No. 415 Chien-Kung Road, Kaohsiung 80778, Taiwan R.O.C.

The study of the mechanical behavior of rockfill materials under three-dimensional loading conditions is a current research focus area. This paper presents a microscale numerical study of rockfill deformation and strength characteristics using the Combined Finite-Discrete Element Method (FDEM). Two features unique to this study are the consideration of irregular particle shapes and particle crushability. A polydisperse assembly of irregular polyhedra was prepared to reproduce the mechanical behavior of rockfill materials subjected to axial compression at a constant mean stress for a range of intermediate principal stress ratios in the interval [0, 1]. The simulation results, including the stress-strain characteristics, relationship between principal strains, and principal deviator strains are discussed. The stress-dilatancy behavior is described using a linear dilatancy equation with its material constants varying with the intermediate principal stress ratio. The failure surface in the principal stress space and its traces in the deviatoric and meridian plane are also presented. The modified Lade-Duncan criterion most closely describes the stress points at failure.

Key Words
rockfill; microscale modeling; combined FDEM; stress anisotropy; dilatancy; failure envelope

(1) Gang Ma, Xiao-Lin Chang, Wei Zhou, Tang-Tat Ng:
State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, 430072, China;
(2) Tang-Tat Ng:
Civil Engineering Department, University of New Mexico, Albuquerque, NM 87131, USA.

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