Method, system and computer program product for real-time monitoring of stress changes in an excavation

Abstract

A method for real-time monitoring of stress changes in an excavation (100), suchas in a mine or a tunnel comprises determining (410) strain data of the excava-tion by at least three strain measurement devices (15, 25; 15A-15C), whereinthe strain measurement devices (15, 25; 15A-15C) are arranged surroundingthe excavation (100) to determine the strain data along different directions withrespect to each other, receiving (420) the determined strain data on a pro-cessing unit (50) arranged in connection with the strain measurement devices(15, 25; 15A-15C), and back-calculating (430), by the processing unit (50), astress change based on the received strain data.

Description

METHOD, SYSTEM AND COMPUTER PROGRAM PRODUCT FOR REAL- TIME MONITORING OF STRESS CHANGES IN AN EXCAVATION FIELD OF THE INVENTION The present invention relates in general monitoring of excavations. In particular, however not exclusively, the present invention concerns monitoring stress changes in excavations. BACKGROUND A rock body is subjected to stress in almost any point. The in situ stress depends mostly on kind of loading which the rock mass is subjected to and has been subjected to in prior. The most important loads are tectonic processes and load- ing by weight of overlying strata. There are many other loads which may be influencing the stress in rock mass as for example heat, liquid or gas pressure or presence of weakening zone within the rock. In general, stress may be char- acterized by a stress tensor. It may be defined in a point of space as a matrix of nine perpendicular stress components. For safety and economical reasons, it is important to have knowledge about the stress around excavations, such as mines, shafts, and tunnels. It is, however, not enough to determine the initial stress since the level of the stress changes as the excavation work progresses. As is apparent, it is too late to recognize that the stress level got to high when the excavation starts to collapse. In accordance with known solutions, the collapsing can be prevented or controlled by providing reinforcements into the excavation. On the other hand, installing too much rein- forcement just for safety reasons increases the construction cost which is, of course, not desirable either. Thus, there is still need to develop solutions for determining the stress in excavations. SUMMARY An objective of the present invention is to provide a method, a system, and a computer program product for real-time monitoring of stress changes in an ex- cavation. Another objective of the present invention is to at least alleviate some of the drawbacks in the known solutions. The objectives of the invention are reached by a method, a system, and a com- puter program product as defined by the respective independent claims. According to a first aspect, a method for real-time monitoring of stress changes in an excavation, such as in a mine or a tunnel, is provided. The method com- prises determining strain data of the excavation by at least three strain meas- urement devices, such as borehole extensometers, wherein the strain measure- ment devices are arranged surrounding the excavation to determine the strain data along different directions with respect to each other, receiving the deter- mined strain data on a processing unit arranged in connection with the strain measurement devices, and back-calculating, or inverse calculating, by the pro- cessing unit, a stress change based on the received strain data. Back-calculation herein refers to a process in which input parameters are ad- justed until the calculated result coincides with the actual result found, that is, with the determined strain data and / or a stress change related determined there- from. For example, in case of determining stress in a plane, strain data may be deter- mined in at least three different directions, however, any two of the directions not in opposite directions (having angle of 180 degrees therebetween). In case of determining stress in three dimensions, strain data may be determined in at least six different directions, however, any two of the directions not in opposite directions (having angle of 180 degrees therebetween). Furthermore, the method may comprise determining a displacement and / or an angular displacement in a direction, or a plurality of displacements in various directions, and calculating a strain change based on the determined displace- ment(s). Optionally, the calculated strain(s) may then be utilized to produce the strain data. The stress state or the change of stress state may then be deter- mined, such as calculated, based on the strain data. Preferably, the strain measurement devices may be arranged such that the di- rections, or imaginary lines extending along the directions, intersect inside the excavation. However, the directions are such that they do not align, such as two directions having 180 degrees with respect to each other. In various embodiments, the back-calculating may include the use of superpo- sition to calculate the strain change based on strain components included the determined strain data. Furthermore, the stress change may be determined based on pre-determined unit loads with respect to said different directions and, optionally, corresponding pre-determined loading factors. In some embodiments, the method may comprise, preferably prior to the deter- mination of the strain data, determining unit loads. The unit loads may be deter- mined with respect to each of the directions related to the strain measurement devices. Preferably, the unit loads are stored in the computing device, such as inside the excavation or outside thereof. In addition, the stored unit loads may be utilized in the back-calculation to deter- mine the stress change. In various embodiments, the back-calculating may, alternatively or in addition, comprise utilizing multiple linear regression, wherein a dependent variable of the multiple linear regression is the determined strain and explanatory variables of the multiple linear regression are the stress tensor components or loading fac- tors. In various embodiments, the directions of two of the devices are different by an angle in the range of 10 to 45 degrees, optionally, by an angle of 15 or 20 de- grees. Preferably the directions are arranged in 45 degrees with respect to each other. In preferable embodiments, the determining may comprise determining strain data of the excavation by at least six strain measurement devices. The at least six strain measurement devices may form two sets of three measurement de- vices arranged in opposite directions with respect to a longitudinal direction of the excavation. In various embodiments, the method may comprise determining the initial stress level based on a plurality of stress levels determined during the excavation work. In various embodiments, the strain measurement devices may be strain gauges, extensometers, such as borehole extensometers, linear variable differential transformers (LVDTs), rebar rock bolts instrumented with strain gauges, incli- nometers, or an array of convergence measurement points. In various preferable embodiments, the strain measurement devices may be borehole extensometers. According to a second aspect, a system for monitoring of stress changes in an excavation is provided. The system comprises at least three, or at least six, strain measurement devices arranged into the excavation, wherein the strain measurement devices are arranged to determine the strain data along different directions with respect to each other. The strain measurement devices may pref- erably be borehole extensometers. and a processing unit arranged to receive the determined strain data and to back-calculate a stress change based on the received strain data. Furthermore, the borehole extensometers may be at least 10 or 20 meters in length. In various embodiments, the system may be configured to provide an alert if a stress threshold is exceeded. The alert may be issued as a visible and / or audible alert, and / or by a signal transmitted from the processing unit to an external sys- tem. According to a third aspect, a computer program product for real-time monitoring of stress changes in an excavation is provided. The computer program product comprises program instructions which when executed by a processing unit cause the processing unit to perform the method in accordance with the first aspect. The present invention provides a method, a computer program product and a system for remote monitoring of an electronic treatment device for treating a patient, such as a negative pressure treatment device. The present invention provides advantages over known solutions in that online and real time access to in situ stress state can be used to optimize the mining sequence, to reduce ore dilution and to limit ore losses. It can also support more precise reinforcement design and give the ability to detect and to react to unexpected changes while maintaining a higher level of safety and avoiding collapses. Stress state change monitoring is able to give feedback about the success of mining sequencing and sufficiency of ground control methods. With the real-time monitoring of the stress state, it is possible to increase the safety of underground mines especially if the stress changes cause significant risks. By monitoring the actual change in real time, the designing and sequencing of the mining can be turned into an iterative process. The determined stress state gets more accurate as the strain data ac- cumulates. Various other advantages will become clear to a skilled person based on the following detailed description. The terms “first”, “second”, etc., are herein used to distinguish one element from other element, and not to specially prioritize or order them, if not otherwise ex- plicitly stated. The exemplary embodiments of the present invention presented herein are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used herein as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as characteristic of the present inven- tion are set forth in particular in the appended claims. The present invention itself, however, both as to its construction and its method of operation, together with additional objectives and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF FIGURES Some embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. Figure 1 illustrates schematically an excavation according to an embodiment of the present invention. Figure 2 illustrates schematically a system according to an embodiment of the present invention. Figures 3A and 3B illustrate schematically a system according to an embodi- ment of the present invention. Figure 4 shows a flow diagram of a method in accordance with an embodiment of the present invention. Figure 5 shows a flow diagram of a method in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF SOME EMBODIMENTS Figure 1 illustrates schematically an excavation 100 according to an embodi- ment of the present invention. The dashed lines perpendicular with respect to the longitudinal direction 5 of the excavation 100 illustrate the progress of the excavation activities, such as mining, in the excavation 100. At 30 is shown a stress state or level of the excavation 100 in accordance with the progress. The initial stress state or level 31 is shown as a horizontal dashed line. Regarding the initial, or virgin or in situ, stress state or level 31, the in situ stress depends mostly on kind of loading which the rock mass is subjected to and has been subjected to. The most important loads are tectonic processes and loading by weight of overlying strata. There are many other loads which may be influ- encing the stress in rock mass as for example heat, liquid or gas pressure or presence of weakening zone within the rock. Figure 2 illustrates schematically a system according to an embodiment of the present invention. The system comprises at least three strain measurement de- vices 15A-15C, or a set 15 of at least three of such devices 15A-15C, for the two-dimensional case or at least six for the three-dimensional case, arranged into the excavation 100, preferably into the surroundings 20 of the excavation 100. The strain measurement devices 15A-15C may be arranged to determine the strain data along different directions with respect to each other such as shown in Fig. 2. In the embodiment of Fig. 2, the first strain measurement device 15A has an angle of 60 degrees relative to the horizontal, the second strain measurement device 15B has an angle of 45 degrees relative to the horizontal, and the third strain measurement device 15C has an angle of 15 degrees rela- tive to the horizontal. Furthermore, the system may comprise a processing unit 50, such as a computer or computing device, arranged to receive the determined strain data and to back-calculate a stress change based on the received strain data. Thus, the processing unit 50 is preferably at least in communication con- nection with the strain measurement devices 15A-15C. In various embodiments, the strain measurement devices 15, 15A-15C may be strain gauges, extensometers, such as borehole extensometers, linear variable differential transformers (LVDTs), rebar rock bolts instrumented with strain gauges, inclinometers, or an array of convergence measurement points. In some embodiments, the strain measurement devices 15A-15C may be multi- point borehole extensometers to measure strain in multiple locations. The extensometers may be, for example, 20 meters long and installed to angles of 20, 50, and 70 degrees measured from horizontal. The extensometers may in- clude six anchor points placed in every 3.33 meters. Such devices may give five displacement or strain values, one between each two anchor points. In preferable embodiments, the strain measurement devices 15A-15C are bore- hole extensometers. The borehole extensometers may be at least 10 or 20 me- ters in length. Furthermore, the borehole extensometers may be such as they are anchored in a plurality of positions of the extensometers, and, optionally, arranged to measure strain at the plurality of positions thereof. In some embodiments, the directions of two of the devices may be different by an angle in the range of 10 to 45 degrees, optionally, by an angle of 15 or 20 degrees. In various embodiments, such as in Fig. 2, the strain measurement devices 15A- 15C of a set 15 may be arranged such that the directions thereof intersect inside the excavation 100. This thus preferably refers to imaginary lines extending from the devices 15A-15C along their directions and thereby the imaginary lines, in fact, may be arranged to intersect in the excavation 100 as shown in Fig. 2 by dashed lines. In various embodiments, the system may comprise at least six strain measure- ment devices. Increasing the number of strain measurement devices reduces the effect of noise of the measurements. The strain measurement devices 15; 15A-15C may, preferably, be arranged at least 10 to 100 centimeters away from the wall of the excavation 100 in order to avoid the damage of excavation work, such as damage due to blasts, on the measurements, such as related to excavation damaged zone close to the wall. Furthermore, the strain measurement devices 15; 15A-15C may, preferably, be arranged close to the excavation 100, such as not farther than two to five times the size, such as width, of the excavation 100, in order to be able to measure the changes in the stress state. In various embodiments, the strain measurement devices 15; 15A-15C may be arranged not to intersect or pass through rock joints or weak zones in the surrounding rock mass so as to avoid movements therein affecting the devices 15: 15A-15C. Figures 3A and 3B illustrate schematically a system according to an embodi- ment of the present invention. The system is similar to one shown in Fig. 2, however, it comprises at least six strain measurement devices arranged into two sets 15 and 25 of such devices 15A-15C. Furthermore, the at least six strain measurement devices forming the two sets 15, 25 of at least three measurement devices may be arranged in opposite directions with respect to a longitudinal direction 5 of the excavation 100. This is illustrated in Fig. 3A in which the sets 15, 25 are in an angle 16, 26 with respect to a perpendicular direction 6 relative to the longitudinal direction 5. The angle differs from 0 and 180 degrees and is preferably in the range of 10-75 degrees, more preferably in the range of 15-60 degrees, and most preferably 45 degrees. This allows determining the stress changes in three dimensions. Figure 3B further illustrates how to sets 15, 25 may be arranged into the sur- roundings 20 of the excavation 100. In various embodiments, the system may be configured, such as by the pro- cessing unit 50, to provide an alert if a stress threshold is exceeded. The thresh- old may be linked to the stress state or level is illustrated at 30 in Fig. 1. Figure 4 shows a flow diagram of a method in accordance with an embodiment of the present invention. Step 400 refers to a start-up phase of the method. Suitable equipment and com- ponents and measurement devices are obtained, and systems assembled and configured for operation. Step 410 refers to determining strain data of the excavation 100 by at least three strain measurement devices 15; 15A-15C, wherein the strain measurement de- vices 15; 15A-15C are arranged surrounding the excavation 100 to determine the strain data along different directions with respect to each other. Step 420 refers to receiving the determined strain data on a processing unit 50 arranged in connection with the strain measurement devices 15; 15A-15C. Step 430 refers to back-calculating, by the processing unit 50, a stress change based on the received strain data. In various embodiments, the back-calculating may include the use of superpo- sition to calculate the strain change based on strain components included the determined strain data. Optionally, the strain components may be determined based on unit loads and corresponding loading factors. Alternatively or in addition, the back-calculating may comprise utilizing multiple linear regression, wherein a dependent variable of the multiple linear regression is the determined strain and explanatory variables of the multiple linear regres- sion are the stress tensor components or loading factors. In various embodiments, variables to be solved are at least two principal stresses (e.g. 01 and 02) and a first direction (e.g. a), such as related to an angle of or between the two principal stresses. Alternatively or in addition, variables to be solved may be three principal stresses (e.g. 01, 02and gs) and a first and a second direction (e.g. a and B), and, prefer- ably, in a third direction y, such as related to angles of or between the principal stresses. The angles may refer to pitch, yaw and roll, respectively. In an embodiment, there may be only two angles determined, and one of the stresses or stress components may be assumed to be in a vertical direction, such as parallel or opposite with respect to the direction of the gravity. The two other stresses or stress components may be assumed to lie in a horizontal plane. In various embodiments, the method may further comprise determining the initial stress state or level 31 based on a plurality of stress states or levels 32 deter- mined during the excavation work as shown in Fig. 1. The determination of the initial stress state or level 31 may be based on back-calculating from the one of the of plurality of stress states or levels 32 by taking into account the stress changes determined during the excavation work. Method execution is stopped at step 499. The method may be performed or executed continuously, intermittently, repeatedly, or on demand. For example, the processing unit 50 may be arranged to poll the measurement devices 15; 15A-15C with a sampling of once per hour or at least once per four hours. Figure 5 shows a flow diagram of a method in accordance with an embodiment of the present invention. The method may comprise, preferably prior to the determination 410 of the strain data, determining 402 unit loads or stresses. The unit loads or stresses may be determined with respect to each of the directions related to the strain measurement devices. Preferably, the unit loads or stresses are stored, at 404, in the computing device 50, such as inside the excavation 100 or outside thereof. In addition, the stored unit loads or stresses may then be utilized in the back- calculation to determine the stress change. While, the determination of the stress change may be performed substantially continuously or in certain inter- vals, such as once in couple of hours or so, the determination of the unit loads or stresses may be performed only once or periodically, such as during consec- utive phases of the excavation work. In various embodiments, numerical modelling methods may be used in the back- calculations for determining the strains around the excavation 100. There are several known such methods, for example, based on continuum and discontin- uum methods, and combinations thereof. Examples of continuum methods are methods like Finite Element Method (FEM), Finite Difference Method (FDM) and Boundary Element Method (BEM). Examples of discontinuum methods are Dis- crete Element Method (DEM) with codes as UDEC and 3DEC and Discrete Frac- ture Network (DFN). In various embodiments, the stress changes may be determined in two or three dimensions, for example, depending on the number and configuration, such as orientations, of the strain measurement devices 15, 25; 15A-15C. In various embodiments, the stress state change may be back-calculated using linear regression of strain change observations, the elastic constitutive relation and the superposition principle. Thus, an assumption of continuous, homogene- ous, isotropic and linearly elastic rock (CHILE) conditions may be done. In this case the loading stress tensor acting on a rock body of the excavation 100 may be divided to its components and sum up the results of strains or displacements. Regarding the superposition of loading, the total strain of selected sections within the medium may be calculated by simple summing up the components. In plane stress, the total strain (difference) may be calculated based on Aewor = Ago + Atox + Ao xz, Where Age is the total strain difference and Aesis the strain component difference from corresponding loading. For each of these compo- nents the strain may be expressed as strain from unit load multiplied by a load factor. In this case the whole formula changes to Aewt = Lz Aoz1 + Lx A€ox1 + Lxz Ago xz,1, Where Liis the loading factor for a loading component, and Aes; 1 the strain difference component from corresponding unit loading. Regarding the back-calculation, with use of measured data from the excavation 100 surroundings 20 and numerical modelling of unit load, it is possible to cal- culate the loading factors and, thus, the change of the stress tensor around the excavation 100. The above equation may be written as e7 = €,1 L, where er is a vector of measured bolt strain data (length N measured data, for instance), €,1a matrix of strain components from corresponding unit loading (size 3 times N, for instance), and L a vector of loading factors for loading components (length 3, for instance). Furthermore, by taking combinations of three lines of the whole set of equations, it is possible to find loading factor vector for each of these combinations, and as a result points in space of L., Lx and Lx. can be obtained. Then, a solution may be found with use of multiple linear regression. A dependent variable of the mul- tiple linear regression may be the determined strain and the explanatory varia- bles of the multiple linear regression may be the stress tensor components or loading factors. Several linear regression estimation methods have been developed and are known to a skilled person. One of the most used method is the Least square estimation and related methods which include Ordinary least square, General- ized least square, Percentage least square, lteratively reweighed last squares, Instrumental variables, Optimal instruments and Total least squares. Next family of methods is the Maximum likelihood estimation methods which includes also Least absolute deviation, Ridge regression and Adaptive estimation method. With respect to the numerical modelling utilized in the back-calculation, methods such as related to the known Kirsch equations, and solution thereto, and / or Mohr's circle may be utilized. The processing unit 50, according to various embodiments, may comprise an input for external units which may be connected to a communication interface of the unit 50. External unit may comprise wireless connection or a connection by a wired manner. The communication interface provides interface for communi- cation with external units such as the strain measurement device 15, 25; 15A- 15C and / or external systems for outputting the alert, if any. There may also be connecting to the external system, such as a laptop or a handheld device. There may also be a connection to a database of the system or an external database. The processing unit 50 may comprise one or more processors, one or more memories being volatile or non-volatile for storing portions of computer program code and any data values and possibly one or more user interface units. The mentioned elements may be communicatively coupled to each other with e.g. an internal bus. The processor is at least configured to implement at least some method steps as described. The implementation of the method may be achieved by arranging the processor to execute at least some portion of computer program code stored in the memory causing the processor, and thus the processing unit 50 and / or the system, to implement one or more method steps as described. The proces- sor is thus arranged to access the memory and retrieve and store any infor- mation therefrom and thereto. For sake of clarity, the processor herein refers to any unit suitable for processing information and control the operation of the pro- cessing unit, among other tasks. The operations may also be implemented with a microcontroller solution with embedded software. Similarly, the memory is not limited to a certain type of memory only, but any memory type suitable for storing the described pieces of information may be applied in the context of the present invention.

Claims

CLAIMS 1. A method for real-time monitoring of stress changes in an excavation (100), such as in a mine or a tunnel, the method comprising: determining (410) strain data of the excavation by at least six strain meas- urement devices (15, 25; 15A-15C), wherein the strain measurement devices (15, 25; 15A-15C) are arranged surrounding the excavation (100) to determine the strain data along different directions with respect to each other, and wherein: none of said different directions is parallel relative to any other of said directions, and / or the at least six strain measurement devices (15, 25; 15A-15C) form two sets (15, 25) of three measurement devices (15A-15C) arranged in opposite di- rections with respect to a longitudinal direction (5) of the excavation (100), receiving (420) the determined strain data on a processing unit (50) ar- ranged in connection with the strain measurement devices (15, 25; 15A-15C), and back-calculating (430), by the processing unit (50), a three-dimensional stress change as a tensor based on the received strain data, and on strain com- ponents determined based on pre-determined unit loads with respect to said directions and on corresponding loading factors.

2. The method of claim 1, wherein the strain measurement devices (15, 25; 15A-15C) are arranged such that the directions intersect inside the excavation (100).

3. The method of claim 1 or 2, wherein the back-calculating (430) includes the use of superposition to calculate the strain change based on strain compo- nents included the determined strain data.

4. The method of any one of claims 1-3, wherein the back-calculating (430) comprises utilizing multiple linear regression, wherein a dependent variable of the multiple linear regression is the determined strain and explanatory variables of the multiple linear regression are the stress tensor components or loading factors.

5. The method of any one of claims 1-4, wherein the directions of two of the devices (15A-15C) are different by an angle in the range of 10 to 45 degrees, optionally, by an angle of 15 or 20 degrees.

6. The method of any one of claims 1-5, comprising determining an initial stress state or level (31) based on a plurality of stress levels (32) determined during the excavation work. / . The method of any one of claims 1-6, wherein the strain measurement de- vices (15, 25; 15A-15C) are borehole extensometers.

8. Asystem for real-time monitoring of stress changes in an excavation (100), the system comprising at least six strain measurement devices (15, 25; 15A-15C) arranged into the excavation (100), wherein the strain measurement devices (15, 25; 15A- 15C) are arranged to determine the strain data along different directions with respect to each other, wherein none of said different directions is parallel relative to any other of said directions, and / or the at least six strain measurement de- vices (15, 25; 15A-15C) form two sets (15, 25) of three measurement devices (15A-15C) arranged in opposite directions with respect to a longitudinal direction (5) of the excavation (100), a processing unit (50) arranged to receive the determined strain data, and to back-calculate a three-dimensional stress change based on the received strain data, on strain components determined based on pre-determined unit loads with respect to said directions, and on corresponding loading factors.

9. The system of claim 8, wherein the strain measurement devices (15, 25; 15A-15C) are borehole extensometers, and are at least 10 or 20 meters in length.

10. The system of claim 8 or 9, configured to provide an alert if a stress thresh- old is exceeded.

11. A computer program product for real-time monitoring of stress changes in an excavation, the product comprising program instructions which when exe- cuted by a processing unit (50) cause a system comprising the processing unit (50) and at least three strain measurement devices (15, 25; 15A-15C) to perform the method according to any one of the preceding claims 1-7.

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