Estimating in situ stress based on hydraulic fracturing data typically depends on interpretation of the breakdown, secondary breakdown (‘reopening’) and shut-in pressure. While it has been recognized that the near-wellbore stress field should be taken into account and that the compressibility of the injection system and the viscous flow of the fluid can diminish the accuracy of stress estimates, these issues have not been well quantified. A coupled numerical model that includes the compressibility of the injection system and the flow of a viscous fluid in a plane-strain hydraulic fracture extending from a wellbore, in an impermeable rock, and in the presence of a non-isotropic in situ stress field provides a basic tool for estimating the order of the error associated with hydraulic fracturing stress measurements under non-ideal conditions. The main findings of this work are model-based guidelines on the values of relevant dimensionless parameter groups to ensure sufficient accuracy of stress estimates that use idealized models. When these guidelines cannot be met under field conditions, the model can be further applied to obtain first-order corrections that account for compressibility, viscosity and near-wellbore effects.

The purpose of this paper is to investigate the strength, physical and engineering index parameters of selected dolomitic rocks with emphasis on grain size. For this purpose, three groups of dolomite from north western Iran, with the same mineral composition but different grain size, were selected; fine grain, medium grain and coarse grain. Three sets of laboratory experiments are performed on 32 samples: first; petrography tests for determining mineral composition and their percentage, and microstructure of rock containing grain size and grain size distribution, second; experiments to determine the physical properties of the rocks included density, compressional and shear wave velocity, and the third category of experiments included uniaxial compressive strength test, Brazilian tensile strength and point load strength. According to the results; there are significant positive correlation between grain size and uniaxial compressive strength (r = 0.89), point load strength (r = 0.58), Brazilian strength (r = 0.69), and average Young’s modulus (r = 0.64). Also, with increasing grain size, density decreases (r = –0.77). There is strong correlation between compressional wave velocity and shear velocity (r = 0.88). There are also a strong correlation among the uniaxial compressive strength, Brazilian tensile strength and point load strength.

After drilling a borehole in the ground and in a rocky environment, the materials around the borehole are crushed and separated in layers from the borehole wall; this causes the borehole cross section to lose its original circular shape, which redistributes stresses and further failure. This type of episodic failure, which occurs symmetrically and V‐shaped on both sides of the borehole and along the minor principal stress, is called breakout. The dimensions of breakout, i.e., its depth and width, are two important indicators that have recently been used in estimating in situ stresses; however, the dimensions of the breakout area depend on the in situ stresses and mechanical properties of the rock, which have not been well addressed so far. This paper presents a comprehensive investigation of breakout dimensions using finite element numerical analysis. The proposed numerical model is based on the equations governing the two‐dimensional breakout phenomenon under nonisotropic in situ stresses and plane strain condition. According to the results, increasing the failure function of the area around the breakout tip causes the breakout to expand, until the failure function is less than 1 for all points around the breakout tip, at which point the breakout expansion is stopped and breakout reaches stability. In the other part of the article, using 121 datasets obtained from numerical analysis, an artificial neural network is trained to predict breakout dimensions based on the input parameters of the problem. The main finding of this section is a model that shows that among the parameters affecting the borehole breakout, the internal friction angle of the rock has the greatest impact on the dimensions of the breakout.

Connected piled raft foundations (CPRFs) have proven to support heavy structures under vertical load. However, when subjected to seismic loading, significant load transfer from the raft beneath to the pile head may cause huge shear stress and bending moments in the piles. Thus, this paper investigates the behaviour of non-connected piled raft foundations (NCPRFs) subjected to earthquake loading. For this purpose, a three-dimensional finite element model was presented and validated against centrifuge test results. This numerical model was then employed to perform a series of numerical analysis. According to the results, pile slenderness ratio and raft thickness can be considered as essential factors in evaluating the dynamic response of NCPRF. The comparison of CPRF and NCPRF has also indicated that using NCPR system leads to significantly less shear force and bending moments along the piles, which results in less required reinforcement and more economical design in comparison with CPR systems.