Soil stresses comprise of those internal forces which are responsible for keeping a mass of soil particles together. In-situ stresses are so called since they represent the stresses present in the soil as they exist in-place, at the site location.
Understanding the magnitude and nature of these stresses is essential to help determine soil behavior and characteristics including:
- Foundation bearing capacity.
- Slope stability.
- Lateral earth pressures on retaining structures.
In-situ stress analysis is widely recognized as a fundamental parameter in the engineering and construction of underground openings. It is of particular importance in boring, tunneling, and other trenchless construction activities, where failure modes, such as settlement, subsidence, heaving, and caving are a possibility.
What Is In-Situ Stress?
The in-situ stresses present within a soil body or rock formation are imposed primarily by overburden forces (the weight of overlying material) and tectonic forces. Soils are a three-phase system comprising of a combination of solids (soil particles), liquids (mostly water), and gas (air).
The liquid and gas phases occupy the voids between the particles to form the soil mass. The behavioral characteristics of this soil mass are governed mainly by the interaction of these three phases of matter. This three-phase composition also gives rise to complex states of stress within the soil body.
In-situ principal stresses exist as three mutually orthogonal stresses; these are termed:
- Vertical stress, also known as overburden stress (σv).
- Minimum horizontal stress (σh).
- Maximum horizontal stress (σH).
These principal stresses vary widely in terms of magnitude and orientation depending on geographic, geologic, and tectonic regions.
These stresses, and their corresponding strains, strongly influence the failure modes of soils and rock formations. In-situ stress tests are typically done to gain specific knowledge about soil or rock behavior and are usually the starting point for any geomechanical study. Information obtained from testing can be used to:
- Predict the response of structures constructed on, or near to, the soils under consideration.
- Determine the flow of groundwater.
- Determine the boundary conditions necessary for further stress analysis and computer modeling.
In-Situ Stress Measurement Techniques
Knowledge of in-field stresses is essential for engineers and contractors to identify high-risk areas, as well as design and implement appropriate control measures to minimize the probability of failures.
While gravitational and overburden stresses can be determined with a reasonable degree of accuracy, horizontal stress measurements require direct, in-field measurements.
Numerous in-situ stress measurement techniques exist which help to interpret the relevant stress parameters and geological conditions at a given site location. (Read Carrying Out In-Situ Stress Measurements: Hydraulic Fracturing Vs. Overcoring Methods.)
Hydraulic fracturing procedures have been in use since the early 1960s and are currently one of the most commonly used methods for determining in-situ stresses. This technique is especially prevalent in the oil and gas industry. Conventional hydraulic fracturing involves using hydraulic pressures created by injecting fluid into a borehole to estimate in-situ stress values.
Hydraulic fracturing is performed by first drilling a borehole at the site location and isolating a section of the borehole using straddle packers. With the top and bottom segments of the borehole sealed off, a fluid is slowly injected into the isolated portion, applying a hydrostatic pressure on the sidewalls of the borehole.
The pressure is increased until the applied tensile stress exceeds the tensile strength of the surrounding soil or rock, causing fractures to occur in opposite directions perpendicular to the plane of least resistance, i.e., the minimum principal stress direction.
The pressure at which this breakdown occurs is measured and recorded.
Following the initiation and propagation of the perpendicular fractures, the pressure required to hold the fractures open, also known as the shut-in pressure, is also recorded. The breakdown pressure (Pb) and the shut-in pressure (Ps) are used as inputs in various formulae to obtain the major and minor principal stresses respectively.
This method of in-situ analysis is based on the observation of the response of the wall of a borehole due to overcoring a section of the hole. Typically, the strain or deformation of an isolated borehole section is measured and recorded using appropriate measuring devices once the overcoring operations have commenced.
Overcoring is performed by first drilling a borehole to the desired measurement depth. A pilot hole, which is significantly smaller than the diameter of the borehole, is then created from the bottom center of the borehole.
Upon completion of drilling operations, the equipment is retrieved, along with the cored rock and soil samples, for further laboratory stress testing. (Read Understanding Key Laboratory Testing Methods for Site Investigation.)
Next, a strain gauge is inserted into the pilot hole, and overcoring is commenced at the diameter of the borehole through the length of the pilot hole. Radial displacements resulting from the ensuing stress relief is collected and used in the calculation of principal stresses.
Flat Jack Test
The flat jack test is a relatively simple and inexpensive method of determining the magnitude of in-situ stresses in a soil mass. During this method, two pins are inserted at a fixed distance (d) apart in an open excavation or borehole. A narrow slot is then cut into the soil or rock between the pins, causing the pins to move closer together due to compressive normal stresses.
A flat jack is then inserted into the slot and pressurized to exert a stress on the outer edges slotted hole; thus, causing the pins to move outward again. The jack applies this continuous pressure until the distance between the pins reaches the original ‘d’ value before the slot was cut.
It is assumed that the force required to return the pins to their default position is the same as the pre-existing normal stress.
What We've Learned
In-situ stresses are essential parameters which provide engineers with an understanding of the condition and properties of undisturbed soil and rock at a given site location.
Determination of these stresses allow for effective monitoring of existing ground conditions and help to facilitate crucial construction and engineering design decisions.
Each in-situ stress measurement technique has its own set of benefits and limitations; it is, therefore, essential to understand the applicability of each method to ensure the accuracy and validity of the output results.