Definition - What does In-Situ Stress mean?
In-situ stress is defined as the stress confined in a rock formation before it is disturbed through excavation or other outside influences. In-situ stress is one of the main parameters in the design of underground structures. In general, in-situ stresses in undisturbed rock formations consist of three mutually orthogonal stresses; two components that act horizontally, and one vertically.
Maximum Horizontal Stress
Minimum Horizontal Stress
Figure 5: Principal Stresses of the In-Situ Stress Field (source)
The three in-situ stresses, V, h and H correlate to three principal stresses:
- 1 is the greatest stress.
- 2 is intermediate stress.
- 3 is the least stress.
Due to tectonic activity and erosion, rock masses are not homogenous. Therefore in-situ stress can vary throughout one rock mass. At some points in the same rock mass, in-situ stress can be negligible, whereas, at other locations, it can be so high that it approaches failure stress.
If the in-situ stress is close to zero, joints are formed, making the excavation process simpler. In cases where the in-situ stress is high, any disturbance to the rock can release stored energy in the rock and fail. In-situ stresses must be adequately analyzed before any tunneling or underground excavation works.
In-situ stress is also called far-field stress.
Trenchlesspedia explains In-Situ Stress
Although the exact measurement of in-situ stress is difficult due to the changing state of rock masses, in-situ stresses are evaluated by several means. Observation of rock behavior or more invasive methods that disturb the existing rock formation are two methods used to examine in-situ stress.
Measuring In-situ Stress from Historical Data and Rock Observation
Three methods in which in-situ stresses from existing information & observation of rock behavior are understood include:
Using databases to gather and evaluate geological, topographical and tectonic information to determine which direction is the principal stress direction.
Finding the extent of the vertical stress component using rock density and overburden depth.
Analyzing borehole breakouts formed from stress concentrations that result in deformation and causes a hole to develop which is oval or elliptical. This gives an idea of the direction of the intermediate stress, 2 and the maximum stress, 1 that orthogonally acts. If the axes of the breakout do not pass through the center of the borehole, it indicates that the borehole does not correspond to the principal stress direction.
Determination of In-situ Stress from Field Investigations
Hydraulic fracturing uses pressurized water to create a crack in the borehole wall. This method is suitable for finding in-situ stresses that are deep within the rock formation. The continued pumping of water results in the hole, extending further down until the pressure falls to a steady value. Hydraulic fracturing allows for the least stress in the plane perpendicular to the borehole axis, 3 to be directly measured.
In flat jacking, an extensometer gauge is installed between two points in the rock surface, and a slot cut into the rock. So that the stresses acting across the two ends are relieved, the slot is cut wide for the flat jack's accommodation. The flat jack is pressurized until the distance between the two points restores to the value before the slot was cut. These measurements are used to calculate the maximum vertical stress and minimum horizontal stress.