A Review on Estimation of in Situ Rock Stress

Abstract

Successful estimation of in situ rock stress field (rock stress tensor) is of paramount importance for safe and economic design as well as execution of any project within the domains of Civil Engineering, Mining Engineering, Petroleum Engineering and Geological / Geophysical research. The regional rock stress field is primarily governed by the Earth’s tectonic movements, whereas the project specific in situ stress field is controlled by overburden pressure, topography, geological structures etc. While using a geotechnical software for designing an engineering structure, in situ rock stress shares equal weightage beside rock strength parameters. Several authors have proposed different empirical relationships between overburden depth, rock density, deformation modulus to calculate the magnitude of vertical stress and ratio of average horizontal stress to vertical stress (k). Use of geological structures like fault, fold, intrusive dike, joint & fracture have also been proposed to decipher directions of principal stresses. Existence of high horizontal stress relative to vertical stress is indicated by phenomenon like core disking, borehole break out during exploratory drilling. However, in situ rock mechanical tests (Overcoring, Hydraulic Fracturing, Hydraulic Testing in Pre-existing Fractures or HTPF etc) must be performed to ascertain the magnitudes and directions of all three principal stresses, prevalent to a project site. Owing to the uncertainties associated with absolute quantification of rock stress magnitude, use of the term ‘stress estimation’ is preferred. Although the term ‘stress measurement’ is used when the stress tensor is estimated by applying aforesaid in situ rock mechanical tests.

Introduction

The text discusses ground characterization for engineering projects, focusing on rock strength and rock stresses. Proper evaluation of these factors is crucial for the success of both surface and subsurface projects, as exceeding rock strength can lead to issues like ground settlement, slope failure, or rock bursts.

Key Concepts:

1. Types of Rock Stresses:

   • In situ stresses: Natural stresses in the rock before any engineering work begins, accumulated through geological processes.

   • Induced stresses: Stresses created by engineering activities such as drilling, excavation, or blasting.

2. Components of In Situ Stresses:

   • These stresses are divided into three principal components: σ1 (maximum), σ2 (intermediate), and σ3 (minimum), where σ1 > σ2 > σ3.

   • The vertical stress is calculated by multiplying overburden depth with the Earth's crust density, while horizontal stress magnitudes can be estimated using empirical relationships.

3. Methods for Measuring Rock Stresses:

   • Several techniques exist to measure in situ rock stresses, including Overcoring, Hydraulic Fracturing, and Hydraulic Testing in Pre-existing Fractures (HTPF). Some methods disturb the stress field, while others rely on indirect measurements.

4. Terminology and Classification:

   • In situ stresses are classified as gravitational, tectonic, and residual stresses. These stresses are divided into external (regional) and internal (residual) categories.

   • Induced stresses are those created by engineering activities, while thermal stresses result from temperature changes, and palaeo-stress refers to past stresses no longer acting.

5. Factors Influencing Rock Stresses:

   • Major factors include the weight of overlying strata, tectonic movements, and human activities such as drilling and blasting.

6. Applications in Engineering:

   • Knowledge of in situ rock stresses is crucial for various fields like Civil Engineering, Mining, Petroleum Engineering, and Geology. Applications include:

     • Stability of underground excavations (e.g., tunnels, mines).

     • Drilling, blasting, and reservoir management.

     • Prediction of rock bursts, fluid flow, and slope stability.

     • Geological research such as earthquake prediction and plate tectonics.

7. Strategy for Rock Stress Estimation:

   • A strategy for estimating in situ rock stresses depends on factors such as project size, budget, and site complexity. Critical questions to address include:

     • The type of information needed (e.g., stress directions, magnitudes).

     • Accuracy requirements and methods for assessing uncertainty.

     • Whether a complementary approach (including modeling) is necessary.

8. Preliminary Estimation:

   • During the initial phase of a project, broad estimates of in situ rock stress can be made using empirical formulas and geological surveys, which help guide further detailed investigation and design decisions. This includes estimating vertical stress and Earth Pressure Coefficient (k).

Conclusion

The foregoing sections have given a brief review of origin, mode of occurrences and significance of in situ rock stresses within the Earth’s crust. Also, the available methods to estimate / measure in situ rock stress magnitude and direction have been covered. Following conclusions can be drawn out of the preceding discussions. 1) Estimation of the in situ rock stress field is the most unpredictable and complex phenomenon related to any rock engineering project. 2) Various factors control the generation and distribution of rock stresses in regional as well as local scale. The dominant factors include Earth’s tectonic movement, weight of overburden rock, effect of topography, disposition of geological structures etc. 3) During preliminary investigation phases, magnitudes of vertical and horizontal stresses can be calculated from overburden depth, Rock density and deformation modulus of rock. The directions of horizontal stresses can be obtained from orientation of various geological structures in the field. 4) For final stage of design and construction of any rock engineering project, measurement of in situ rock stress tensor in form of magnitudes and directions of three principal stresses (i.e. ?1, ?2 and ?3) is required. This becomes obvious when the project is large sized and or situated in complex geological region. 5) There exist many methods for in situ rock stress measurement, but the widely used methods are Overcoring method, Hydraulic Fracturing method and Hydraulic testing in pre-existing fractures (HTPF) method 6) All three methods have limitations. But application of Hydraulic Fracturing (HF) in conjunction with Hydraulic testing in pre-existing fractures (HTPF) can reveal the complete three-dimensional scenario of in situ rock stresses, prevalent in any ground.

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Copyright © 2025 Saradindu Mukherjee, Dr. Amitava Patra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.