Rock Stress Measurements Research Articles

A novel indirect optical method for rock stress measurement using micro-deformation field analysis

Stress measurement plays a crucial role in geomechanics and rock engineering, especially for the design and construction of large-scale rock projects. This paper presents a novel method, based on the traditional stress relief approach, for indirectly measuring rock stress using optical techniques. The proposed method allows for the acquisition of full-field strain evolution on the borehole's inner wall before and after disturbance, facilitating the determination of three-dimensional (3D) stress information at multiple points within a single borehole. The study focuses on presenting the method's theoretical framework, laboratory validation results, and equipment design conception. The theoretical framework comprises three key components: the optical imaging method of the borehole wall, the digital image correlation (DIC) method, and the stress calculation procedure. Laboratory validation tests investigate strain field distribution on the borehole wall under varying stress conditions, with stress results derived from DIC strain data. Remarkably, the optical method demonstrates better measurement accuracy during the unloading stage compared to conventional strain gauge methods. At relatively high stress levels, the optical method demonstrates a relative error of less than 7% and an absolute error within 0.5 MPa. Furthermore, a comparative analysis between the optical method and the conventional contact resistance strain gauge method highlights the optical method's enhanced accuracy and stability, particularly during the unloading stage. The proposed optical stress measurement device represents a pioneering effort in the application of DIC technology to rock engineering, highlighting its potential to advance stress measurement techniques in the field.

Two New Methods for Defining Shut-In Pressure in Hydraulic Fracturing Tests

Hydraulic fracturing is one of the most common methods to determine in situ rock stress. The interpretation of the shut-in pressure to determine the minor principal stress is an important element of this method, and many different methods to interpret shut-in pressure have been studied and developed throughout the years. Each method has its advantages and disadvantages. With more than 50 years of research and development within the rock stress measurement field, especially in HF, SINTEF has established two practical ways of defining shut-in pressure. These methods are independent and termed zero flow and water hammer. The zero flow method has been used by SINTEF in more than 130 projects over the last 30 years. The methods clearly differ from the other methods as they are based on singular events in the development of pressure/flow versus time which enables us to read the shut-in pressure directly during testing. In this paper, a comparison is made between different methods for interpretation of shut-in pressure, including 12 existing methods and the 2 SINTEF methods. Comprehensive laboratory tests were performed, and a field test was selected from SINTEF’s database for demonstration and comparison of the methods. The SINTEF methods have been developed mainly for use in hard rock environment where the rock is a jointed aquifer and with low permeability. The application of the two methods has traditionally been hydroelectric power development, different types of tunnel, and cavern projects, and also in mineral mining. The methods have not been used in deep petroleum applications such as oil wells or offshore in porous rock types.

Rock Stress Measurement Methods in Rock Mechanics—A Brief Overview

The current brief review paper on rock stress measurement methods is very crucial factors in mining, civil infrastructure, geothermal energy, nuclear underground disposal, large underground oil storage caverns, etc as well as in geology and geophysical area. Measurement of in situ rock stress is a very challenging and difficult quantity and not possible to measure directly. Measure the deformation or displacements or hydraulic factors by perturbing the rock and converting the measured quantity into rock stress. There are two main categories for measuring methods: direct and indirect methods. The most common methods of direct in situ stress techniques are briefly described including advantages, disadvantages and limitations. Moreover, authors included the application of Artificial Intelligence (AI) for rock stress measurement methods.

Investigation, numerical model, and monitoring for Follo line - a metro project in Oslo

Just south of Oslo Central Station the new high-speed Follo Line railway tunnels pass beneath the existing Ekeberg road tunnels. This paper presents the construction methods, numerical model, and monitoring program used to assess the stability of the E6 road tunnels during the excavation of the Follo Line tunnels only a few metres below. The construction of the Follo Line was approved subject to three conditions: (1) There should be no negative effect on the stability of the Ekeberg tunnels, (2) The traffic flow in the Ekeberg tunnels must be maintained at all times and (3) Any risk of instability in the existing tunnels must be detected beforehand so that necessary precautionary action could be taken in good time. In order to deal with the challenges, SINTEF has carried out different investigations (including rock stress measurements), established a comprehensive numerical model, and a thorough monitoring program for analyses. The activities were combined in a so-called “rock mechanic tool”, and the tool has been used successfully in this project, operating smoothly to provide adequate information for excavation planning and decision making.

A Numerical Investigation for In-S itu Measurement of Rock Mass Mechanical Properties with a CCBO Probe and Evaluation of the Method’s Error in Estimating the In-S itu Stresses with the Overcoring Technique

Accurate determination of rock stress in great depths has become one of the most critical issues in civil and mining affairs along with the expansion of underground excavation spaces’ scale and deepening of under-excavation areas. The hydraulic fracturing method and coring-based laboratory methods including ASR, DSCA, and AE are main techniques in rock stress measurement in the earth’s depth. Thus far, previous studies have reported repeated samples from the values obtained from the above mentioned techniques. Nonetheless, the “Compact Conical Borehole Overcoring” (CCBO) technique is an example of the stress-relief method and this technique, is one of the most cost-effective and accurate methods for measuring the in-situ stresses simply by digging one borehole either in the tunnels’ wall or in the deep wells’ depth. The current study aims at analyzing the overcoring technique with the CCBO probe to identify errors regarding the number of sensors installed, as well as checking its ability to estimate mechanical parameters of mass rock in-situ. In this study, we have utilized the analytical method based on numerical simulations to determine the accuracy of the method’s dependence on the number of sensors installed on the probe. We performed numerical analysis using COMSOL finite element software, as well as MATLAB software. In addition, we attempted to propose possible relations for estimating mechanical parameters of mass rock using the probe, a method determined by the inventors.

Effect of Advancing Direction of Working Face on Mining Stress Distribution in Deep Coal Mine

Deep mining has become the normal state of coal mining; compared with the mine with shallow buried depth, the consequent high level of in situ stress and complex distribution have brought severe threats to the stability of the stope and the surrounding rock of the roadway. In this research, taking the 121304 working face of Kouzidong Mine as the engineering background, the characteristics of mining-induced stress distribution under complex in situ stress environment in deep mining are analyzed by using on-site measurement of the original rock stress and mining stress, establishing a theoretical model centered on the middle section of the working face, and establishing large-scale numerical calculation models for different advancing directions. It was found that under deep mining conditions, the maximum stress of the original rock is 25.12 MPa, and the direction is vertical. The advanced influence range of mining stress is about 150 m, and the abutment pressure presents a three-peak distribution characteristic in front of the working face. The research results provide important theoretical guiding value for guiding the mining of coal mines with similar geological conditions.

Rock Stress Measurements for Unlined Pressure Tunnels: A True Triaxial Laboratory Experiment to Investigate the Ability of a Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress

To avoid hydraulic failure of unlined pressure tunnels, knowledge of minimum principal stress is needed. Such knowledge is only obtainable from in situ measurements, which are often time-consuming and relatively costly, effectively limiting the number of measurements typically performed. In an effort to enable more stress measurements, the authors propose a simplified and cost-effective stress measuring method; the Rapid Step-Rate Test (RSRT), which is based on existing hydraulic testing methods. To investigate the ability of this test to measure fracture normal stresses in field-like conditions, a true triaxial laboratory test rig has been developed. Hydraulic jacking experiments performed on four granite specimens, each containing a fracture, have been performed. Interpretation of pressure-, flow- and acoustic emission (AE) data has been used to interpret fracture behaviour and to assess fracture normal stresses. Our experimental data suggest that the proposed test method, to a satisfactory degree of reliability, can measure the magnitude of fracture normal stress. In addition, a clear correlation has been found between fracture closure and sudden increase in AE rate, suggesting that AE monitoring during testing can serve as a useful addition to the test. The rapid step-rate test is also considered relevant for field-scale measurements, with only minor adaptions. Our findings suggest that the RSRT can represent a way to get closer to the ideal of performing more testing along the entire length of pressure tunnel, and not only at key locations, which requires interpolation of stress data with varying degree of validity.

Challenging Infrastructure Project Assisted by Monitoring and Numerical Modelling as the Follo Line Tunnels were Excavated Below Existing Tunnels

Just south of Oslo Central Station, the new high-speed Follo Line railway tunnels pass beneath the existing Ekeberg road tunnels. This paper presents the construction methods, numerical model, and monitoring program used to assess the stability of the E6 road tunnels during the excavation of the Follo Line tunnels only a few metres below. The construction of the Follo Line was approved subject to three conditions: (1) there should be no negative effect on the stability of the Ekeberg tunnels, (2) the traffic flow in the Ekeberg tunnels had to be maintained at all times and (3) any risk of instability in the existing tunnels must be detected beforehand, so that necessary precautionary actions could be taken in good time. To deal with the challenges, SINTEF developed a comprehensive analysis procedure, combining continuous rock stress measurements and displacement measurements with 2D and 3D numerical modelling. The rock stress change monitoring was used together with the numerical model to monitor the stability conditions in the Ekeberg tunnels as the Follo Line tunnels were excavated. This ensured that any risk of instability in the existing tunnels could be detected in advance to enable precautionary action to be taken. The successful completion of the new tunnels without any disturbance to the road tunnels shows that the procedure would be useful for dealing with similar applications in the future.

A Fiber Bragg Grating Borehole Deformation Sensor for Stress Measurement in Coal Mine Rock.

A borehole deformation sensor for long-term stress monitoring in coal mine rock based on optical fiber Bragg gratings (FBGs) is presented. The sensor converts borehole deformation into optical fiber strain by using four rings. For each ring, two FBGs are bonded with the ring to measure the borehole deformation, and a reference FBG free from mechanical load is introduced to remove the temperature effect. Two simple checks on the test data can be performed to improve the test accuracy. Laboratory and field tests were conducted to validate the accuracy and long-term performance of the sensor. The results show that the sensor is capable of measuring stress in rock with good accuracy, and it performs well over a long period of time in coal mines. The developed sensor provides an approach for the long-term monitoring of stress changes in coal mine rock.

Evaluation of Hydraulic Fracturing and Overcoring Methods to Determine and Compare the In Situ Stress Parameters in Porous Rock Mass

There are various methods to determine in situ stress parameters, but each method is having its own advantages and limitations. Among the methods available, the hydraulic fracturing method is the most commonly adopted procedure for in situ stress measurements because of its simplicity, reliability and economic viability. But, the limitations with this technique are that some assumptions implicit for this method such as that the rock mass is continuous and elastic. Moreover, the reliability and validity of this method becomes questionable in porous rocks that may encounter in underground tunnels. These limitations are experienced ever since the introduction of this method, especially in porous rocks. In general, the recognition of fracture initiation in the hydraulic fracturing test has not proved difficult. The relatively slow rates of pressurisation have ensured that when fracture initiation occurs, the sudden increase in volume has led to a marked drop in pressure in the porous section, which is easily recognised from the pressure record (Sjoberg et al. in Int J Rock Mech 40:999–1010, 2003). But the drop-in pressure in a porous rock is far more difficult to recognise. This is since pressure cannot be developed if the rate of leakage in the formation is equal to or higher than the flow rate applied for fracture initiation. This problem of non-generation of water pressure can be tackled by use of a high viscosity fluid. Stress measurement were conducted by hydraulic fracturing method by using high viscous liquid in porous rocks. The stresses evaluated by this technique were correlated with stress measured by overcoring method. The stress measured by overcoring method was used as bench-mark as this method does not suffer from the presence of porosity of the rock. This new technique will be helpful in conducting the stress measurements in porous rocks, which will be highly beneficial to both mining and hydropower related excavation.

Study on the in-situ stress Measurement and Distribution Law in the Li Lou Mining Area

Li Lou Iron Mine was the most productive underground mining iron mine in China. Up to now, no original rock stress measurement had been carried out. In order to understand the distribution characteristics and variation law of in-situ stress in mining area, a digital hollow inclusion strain gauge based on double temperature compensation was used to measure 7 measuring points at 3 levels, and three-dimensional in-situ stress data of 7 measuring points were obtained. According to the measured results, the average ratio of horizontal stress to vertical stress was 2.36, and the main stress field in Li Lou mining area was horizontal tectonic stress. The direction of maximum principal stress was NNE-SSE, in which the north was near NE and the south was near SE, which was basically parallel to the strike of fault structure in the mining area. The maximum horizontal principal stress, the minimum horizontal principal stress and the vertical principal stress changed linearly with the increase of depth.

Analysis of the stabilising effect of ribs of reinforced sprayed concrete (RRS) in the L\xf8ren road tunnel

The Løren tunnel is a road tunnel at Ring road 3 in Oslo, Norway. The tunnel has a length of 915 m in rock, has two tubes with three lanes and breakdown fields, and was first opened in 2013. For rock support in the case of weak rock masses, ribs of reinforced sprayed concrete (RRS) were used. The scope of this article is to present and analyse the results of a measurement programme carried out on three of these ribs. This is done by focusing on deformations in the rock and the support function of the ribs due to these deformations. The instrumented RRS had strain meters installed in the reinforcement and the concrete. From the surface above the RRS, multipoint borehole extensometers were placed to survey the soil and rock mass deformations caused by tunnel advancement. In addition, 2D and 3D rock stress measurements and rock property testing were conducted. The measurements and numerical modelling show that the deformations are too small to cause a considerable load on the installed support construction and that the 2D stress measurements seem to best fit the in-situ stress conditions. The rock mass quality in the area of this study is on the verge of where one usually starts using reinforced ribs. It is concluded that the RRS are not required because of deformations in the rock but, rather, because of the need to lock blocks, increase the friction in joints and prevent movement in larger filled joints. For this purpose, the RRS should probably be designed differently to get the most out of the materials used.

Analytical investigation for stress measurement with the rheological stress recovery method in deep soft rock

Due to the difficulty and weakness of current stress measurement methods in deep soft rock, a new rheological stress recovery method of the determination of the three-dimensional (3D) stress tensor is proposed. It is supposed that rock stresses will recovery gradually with time and can be measured by embedding transducers into the borehole. In order to explore the applicability and accuracy of this method, analytical solutions are developed for stress measurement with the rheological stress recovery method in a viscoelastic surrounding rock, the rheological properties of which are depicted as both the Burger’s model and a 3-parameter solid model. In such conditions, explicit analytical expressions for predicting time-dependent pressures on the transducer are derived. A parametric analysis is then adopted to investigate the influences of the grout solidification time and the mechanical properties of the grout layer. The results indicate that this method is suitable for stress measurement in deep soft rock, the characteristics of which are soft, fractured and subjected to high geo-stress.

A New Three-Dimensional Pressure Transducer for Measuring Soft Rock Stress

Commonly used methods of measuring rock stress, including hydraulic and borehole relief methods, are difficult to apply in deep soft rock masses. This study proposes a new method to measure stress in soft rock by taking advantage of the strong rheological behavior of soft rock masses. Rock stresses can be gradually measured over time by embedding pressure transducers into soft rock masses; this can be accomplished with a newly developed vibrating-wire pressure transducer that can monitor three orthogonal normal stresses. Based on the elastic theory, this study derives the relationship between applied pressures and frequency changes of the vibrating wire. Uniaxial and triaxial calibration tests were performed using a rock mechanical testing system to investigate the repeatability, linearity, and interference characteristics of the new transducer. The performance of the transducer was also verified in a model test using a true triaxial loading system, in which the transducer was tested under different stress states in a cement mortar body. The results indicate the transducer performs well and is applicable for soft rock stress measurement.

Experimental Investigation on the Mechanical Behavior of a New Three-Dimensional Pressure Transducer

Accurate measurements of in situ stress in deep soft rocks are needed to design the tunnel support system and to evaluate the stress redistribution process. Due to the difficulty and weakness of current methods in deep soft rock stress measurement, a new one, rheological stress recovery method to determine the three-dimensional stress tensor is proposed. It is supposed that rock stresses will recover gradually with time and can be measured by embedding transducers into the borehole. According to this method, an original three-dimensional pressure transducer to measure rock stresses is designed and manufactured. In order to investigate the accuracy of the measurement, calibration test and model test have been performed. Both uniaxial and tri-axial calibration tests are conducted to evaluate the transducer performance. For model test, the cement mortar was used to simulate the effect of the surrounding materials. This test considered different rock strengths as well as different types of loading. The results indicate that this transducer is good for stress measurement.

Research on Optimization of Bolt Support for Tilted Strata Roadway in Soft Rock

Bad and asymmetrical deformation occurred in soft tilted strata roadway for the existence of tectonic stress, we carried out the study on geostatic stress character and optimizing bolting support parameters in 2# coalbed of Zijin Colliery. Based on original rock stress measurement, broken rock zone measurement and survey of roadway deformation, characters of original rock stress and range of broken rock zone were given. According to existent problems, a variety of numerical computations were carried out by FLAC3D, failure mechanism of roof is shear-tensile failure. New support methods of bolt installed vertically in tilted roof and strengthening support of sidewalls and corners of roadway are figured out. The research was carried out for 600 meters. The monitoring results prove that the optimized support design is viable and the new bolting parameters are reasonable.

Influence of rock failure and damage on in situ stress measurements in brittle rock

The modified Doorstopper stress measurement technique consists in recording the recovered strains caused by stress relief at the bottom of a borehole drilled in a rock mass and converting the relieved strains into stresses by using the deformability parameters at the measuring point. This conversion is usually based on the assumption of a linear elastic behaviour. Hence, when plastic strains are induced at or near the borehole bottom prior to or during stress relief, the stress tensor calculated by assuming an elastic behaviour can be wrong. Plastic strains develop at the borehole bottom or in its vicinity, causing local damage. Core discing is probably the most visible manifestation of core failure, however, even if no visible signs of damage are present, plastic strains can be induced at or near the borehole bottom, affecting the recovered strains and the calculated stresses. Using Flac3D, the stress relief process for different 3D in situ stress states in an elasto-plastic cohesion softening friction hardening model is simulated in order to determine if the non-elastic behaviour can be detected from the strain recovery curves. Errors on the far-field stress state obtained by assuming an elastic behaviour are calculated. Finally, means of minimizing core damage in the presence of high in situ stresses are briefly explored and discussed.

Research on Comprehensive Prediction of Rock Burst in Sanshandao Gold Mine

The paper takes the deep exploitation of Sanshandao gold mine as the research subject , the methods of distribution of rock structure and lithology examination, in-situ rock stress measurement, physical and mechanical tests of rock, and fuzzy mathematic prediction are used to study rockburst tendency in deep development and exploitation of the mine, which fill the gaps for the past using only one qualitative indexes of rock to judge the possibility of rock bust, and make the evaluation results more close to the objective reality.

Principle of in-situ 3D rock stress measurement with borehole wall stress relief method and its preliminary applications to determination of in-situ rock stress orientation and magnitude in Jinping hydropower station

As a main constituent of geological body, the rock masses have distinct differences from other materials, one of which is that rock masses are initially stressed in their natural states. Hence, it is an extremely challenging and significant research project to know the present residual stress of the rock masses in the earth’s crust. Although some regularities of distribution of in-situ rock stresses can be deduced, the basic means to study the state of rock stress is in-situ stress measurement. After a brief review of several measuring methods of in-situ 3D rock stress, a new one, borehole wall stress relief method (BWSRM) to determine the in-situ 3D rock stress tensor in a single drilled borehole was proposed. Based on the principle of in-situ rock stress measurement with BWSRM, an original geostress measuring instrument was designed and manufactured. Preliminary experiments for determination of in-situ stress orientation and magnitude were carried out at an experimental tunnel in Jinping II hydropower station in China, where the buried depth of overburden was about 2430 m. The results showed that it was feasible to measure the in-situ 3D rock stresses with BWSRM presented in this paper. The BWSRM has a broad prospect for in-situ 3D rock stress measurements in practical rock engineering.

Technical auditing of rock mechanics modelling and rock engineering design

We present a procedure for technically auditing rock mechanics modelling and rock engineering design. As rock mechanics modelling becomes more sophisticated, with the ability to include more parameters and to be coupled with other disciplines such as hydrogeology, and as the requirements on rock engineering projects become ever more challenging, it is prudent, if not essential, to have some form of procedure for checking that the modelling and design are suitable within the limits of current knowledge. Accordingly, we present here an appropriate auditing procedure, and demonstrate its use with the examples of measurement of rock stress (as a soft audit) and modelling for the design of hydropower caverns in China (as a semi-hard audit), the auditing questions being tailored to the nature of the work being audited. The types of procedures demonstrated can be used from the initial project concept, through site investigation and modelling, to construction, monitoring and back analysis, allowing the whole process to be concurrently checked providing a transparent and traceable audit trail of results and associated decisions made.