Stress measurement techniques

Stress measurement techniques: 

Instrumentation and in-situ stress measurement techniques are used in various fields, including geotechnical engineering, rock mechanics, and petroleum engineering, to understand the stress conditions in the subsurface. These measurements are essential for designing and analyzing the stability of structures, such as tunnels, dams, and oil wells. Here are some commonly used techniques:

1. Conventional Borehole Overcoring: In this method, a cylindrical core sample is extracted from the subsurface using a drilling rig. The core is then analyzed in a laboratory to determine the stress state by measuring the deformation characteristics of the core. The measurement is based on the assumption that the core retains the in-situ stress state at the time of extraction.

2. Hydraulic Fracturing: This technique involves the creation of controlled fractures in the rock formation using pressurized fluid injection. By monitoring the pressure required to initiate and propagate fractures, as well as the orientation of the resulting fractures, engineers can estimate the in-situ stress conditions.

3. Borehole Deformation Measurements: Instruments such as strain gauges, extensometers, and stress cells can be installed in boreholes to directly measure the deformation of the surrounding rock mass. By analyzing the strain and displacement data, the in-situ stress state can be inferred.

4. Hydraulic Testing: By injecting fluid into a borehole and monitoring the resulting pressure response, engineers can estimate the stress conditions. Tests such as hydraulic testing, injection testing, and packer testing provide valuable data for stress estimation.

5. Flat Jack Testing: In this method, a flat, expandable jack is inserted into a borehole or a thin slot cut into the rock. The jack is then inflated with a fluid, exerting pressure on the surrounding rock mass. The resulting deformation is measured, allowing estimation of the in-situ stress state.

6. Overcoring with Borehole Jacks: Similar to conventional overcoring, this technique involves extracting a cylindrical core sample. However, specialized borehole jacks are used to apply known stresses to the core sample during extraction. By measuring the deformation response, the in-situ stress state can be determined.

7. Stress Relief Methods: By relieving the stress in a controlled manner using techniques like stress relief drilling or slotting, engineers can indirectly measure the in-situ stress state by monitoring the relaxation of the rock mass.

It's worth noting that each technique has its limitations and uncertainties, and multiple methods are often employed to obtain a more accurate estimation of in-situ stress conditions. Additionally, advances in technology and research continue to expand the range of available instrumentation and measurement techniques.