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In-situ stress is the natural pre-existing stress confined in the rock before it is drilled, excavated or affected by outside influences. The in-situ stresses originate in the earth crust due to different factors, mainly the weight of the overlaying rock layers and tectonic movements (see [[:File:GeoWikiWriteOff2021-Tayyib-Figure1.png|Figure 1]]). The other factors are summarized in [[:File:GeoWikiWriteOff2021-Tayyib-Figure2.png|Figure 2]]. The in-situ stress can vary within one rock mass from one location to another due to varying rock properties. It is important to determine the magnitude and direction of in-situ stresses before doing underground work or designing underground structures, see [[:File:GeoWikiWriteOff2021-Tayyib-Table1.png|Table 1]] for their different applications. In-situ stress characterization is the science of estimating the stress magnitudes and determining the orientation of three principle stresses: maximum horizontal stress, minimum horizontal stress, and vertical stress.

In-situ stress is the natural pre-existing stress confined in the rock before it is drilled, excavated or affected by outside influences. The in-situ stresses originate in the earth crust due to different factors, mainly the weight of the overlaying rock layers and tectonic movements (see [[:File:GeoWikiWriteOff2021-Tayyib-Figure1.png|Figure 1]]). The other factors are summarized in [[:File:GeoWikiWriteOff2021-Tayyib-Figure2.png|Figure 2]]. The in-situ stress can vary within one rock mass from one location to another due to varying rock properties. It is important to determine the magnitude and direction of in-situ stresses before doing underground work or designing underground structures, see [[:File:GeoWikiWriteOff2021-Tayyib-Table1.png|Table 1]] for their different applications. In-situ stress characterization is the science of estimating the stress magnitudes and determining the orientation of three principle stresses: maximum horizontal stress, minimum horizontal stress, and vertical stress.

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File:GeoWikiWriteOff2021-Tayyib-Figure1.png|{{Figure number|1}}Movement of the tectonic plate (Earth’s outer shell: Crust & Lithospheric Mantle) generate in-situ stress. (from Duarte & Schellart, 2016) [1]

File:GeoWikiWriteOff2021-Tayyib-Figure1.png|{{Figure number|1}}Movement of the tectonic plate (Earth’s outer shell: Crust & Lithospheric Mantle) generate in-situ stress (from Duarte & Schellart<ref>Duarte, J., C., and W. P. Schellart, 2016, Introduction to plate boundaries and natural hazards, ''in'' J. C. Duarte, and W. P. Schellart, eds., Plate boundaries and natural hazards: AGU Geophysical Monograph Series 219, p. 1-10.</ref>).

File:GeoWikiWriteOff2021-Tayyib-Figure2.png|{{Figure number|2}}Summary of the different factors causing rock stress. (from Amadei & Stephanson, 1997) [2]

File:GeoWikiWriteOff2021-Tayyib-Figure2.png|{{Figure number|2}}Summary of the different factors causing rock stress. (from Amadei & Stephanson, 1997) [2]

File:GeoWikiWriteOff2021-Tayyib-Table1.png|'''Table 1''' Activities requiring knowledge of in-situ stresses. (from Amadei & Stephanson,1997 as cited in Zhang, L 2016) [3.0]

File:GeoWikiWriteOff2021-Tayyib-Table1.png|'''Table 1''' Activities requiring knowledge of in-situ stresses. (from Amadei & Stephanson,1997 as cited in Zhang, L 2016) [3.0]

==Notation of Stress==

==Notation of Stress==

Stress is often represented by the Greek letter sigma (σ) and can be defined as the force applied over an area. When the force acts perpendicular to a plane, the stress is called a Normal Stress (σn), whereas when the force acts parallel to a plane, the stress is called a Horizontal Stress (σs). Generally, the stress acting on a plane is oblique which means it is neither parallel nor at a right angle to that plane. Therefore, the stress vector is resolved into normal and shear components that are aligned with the three cartesian axes: x, y and z. Since the shear stress component is generally not aligned with these axes, it needs to be resolved further into two components (see [[:File:GeoWikiWriteOff2021-Tayyib-Figure3.png|Figure 3]]).  

Stress is often represented by the Greek letter sigma (σ) and can be defined as the force applied over an area. When the force acts perpendicular to a plane, the stress is called a Normal Stress (σn), whereas when the force acts parallel to a plane, the stress is called a Horizontal Stress (σs). Generally, the stress acting on a plane is oblique which means it is neither parallel nor at a right angle to that plane. Therefore, the stress vector is resolved into normal and shear components that are aligned with the three cartesian axes: x, y and z. Since the shear stress component is generally not aligned with these axes, it needs to be resolved further into two components (see [[:File:GeoWikiWriteOff2021-Tayyib-Figure3.png|Figure 3]]).  

These components act on each visible face of an infinitesimal cube used to represent a point within a rock mass. This results in a total of nine stress components that can be organized in a 3x3 matrix, called the stress tensor (see figure 4). Assuming the rock is at rest, the stresses of equal magnitudes and opposite directions will cancel out each other and prevent the cube from rotating. There is a special orientation in space where all shear stresses equal to zero and only three normal compressive components exist, called principle stresses (see Figure 5). The three principle stresses are the vertical stress (σV), the maximum horizontal stress (σH), and the minimum horizontal stress (σh).  

These components act on each visible face of an infinitesimal cube used to represent a point within a rock mass. This results in a total of nine stress components that can be organized in a 3x3 matrix, called the stress tensor (see figure 4). Assuming the rock is at rest, the stresses of equal magnitudes and opposite directions will cancel out each other and prevent the cube from rotating. There is a special orientation in space where all shear stresses equal to zero and only three normal compressive components exist, called principle stresses (see Figure 5). The three principle stresses are the vertical stress (σV), the maximum horizontal stress (σH), and the minimum horizontal stress (σh).  

==References==

==References==

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2 Amadei, B & Stephansson, O, 1997, Rock stress and its measurement. London: Chapman & Hall.

2 Amadei, B & Stephansson, O, 1997, Rock stress and its measurement. London: Chapman & Hall.