Patent Description:
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 loading 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 characterized 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 reinforcement 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.

The documents <CIT>, <CIT> and "<NPL>," show methods for monitoring of stress changes in excavations.

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 excavation. 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 computer 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 comprises determining strain data of the excavation by at least six strain measurement devices, such as borehole extensometers, wherein the strain measurement devices are arranged surrounding the excavation to determine the strain data along different directions with respect to each other, receiving the determined strain data on a processing unit arranged in connection with the strain measurement devices, and back-calculating, or inverse calculating, by the processing unit, a stress change based on the received strain data.

Back-calculation herein refers to a process in which input parameters are adjusted until the calculated result coincides with the actual result found, that is, with the determined strain data and/or a stress change related determined therefrom.

For example, in case of determining stress in a plane, strain data may be determined in at least three different directions, however, any two of the directions not in opposite directions (having angle of <NUM> degrees therebetween). According to the invention, for determining stress in three dimensions, strain data is determined in at least six different directions, however, any two of the directions not in opposite directions (having angle of <NUM> 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 displacement(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 determined, such as calculated, based on the strain data.

Preferably, the strain measurement devices may be arranged such that the directions, 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 <NUM> degrees with respect to each other.

In various embodiments, the back-calculating may include the use of superposition 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 determination of the strain data, determining unit loads. The unit loads may be determined 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 determine 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 factors.

In various embodiments, the directions of two of the devices are different by an angle in the range of <NUM> to <NUM> degrees, optionally, by an angle of <NUM> or <NUM> degrees. Preferably the directions are arranged in <NUM> 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 devices 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, inclinometers, 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 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 preferably 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 <NUM> or <NUM> 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 system.

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 a system comprising at least six strain measurement devices and 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 accumulates.

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 explicitly stated.

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.

Some embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.

<FIG> illustrates schematically an excavation <NUM> according to an embodiment of the present invention. The dashed lines perpendicular with respect to the longitudinal direction <NUM> of the excavation <NUM> illustrate the progress of the excavation activities, such as mining, in the excavation <NUM>. At <NUM> is shown a stress state or level of the excavation <NUM> in accordance with the progress. The initial stress state or level <NUM> is shown as a horizontal dashed line.

Regarding the initial, or virgin or in situ, stress state or level <NUM>, 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 influencing the stress in rock mass as for example heat, liquid or gas pressure or presence of weakening zone within the rock.

<FIG> illustrates schematically a system according to an embodiment not according to the present invention. The system comprises at least three strain measurement devices 15A-15C, or a set <NUM> 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 <NUM>, preferably into the surroundings <NUM> of the excavation <NUM>. The strain measurement devices 15A-15C are arranged to determine the strain data along different directions with respect to each other such as shown in <FIG>. In the embodiment of <FIG>, the first strain measurement device 15A has an angle of <NUM> degrees relative to the horizontal, the second strain measurement device 15B has an angle of <NUM> degrees relative to the horizontal, and the third strain measurement device 15C has an angle of <NUM> degrees relative to the horizontal. Furthermore, the system comprises a processing unit <NUM>, 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 <NUM> is preferably at least in communication connection with the strain measurement devices 15A-15C.

In various embodiments, the strain measurement devices <NUM>, 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 multipoint borehole extensometers to measure strain in multiple locations. The extensometers may be, for example, <NUM> meters long and installed to angles of <NUM>, <NUM>, and <NUM> degrees measured from horizontal. The extensometers may include six anchor points placed in every <NUM> 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 borehole extensometers. The borehole extensometers may be at least <NUM> or <NUM> meters 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 <NUM> to <NUM> degrees, optionally, by an angle of <NUM> or <NUM> degrees.

Such as in <FIG>, the strain measurement devices 15A-15C of a set <NUM> may be arranged such that the directions thereof intersect inside the excavation <NUM>. 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 <NUM> as shown in <FIG> by dashed lines.

According to the invention, the system comprises at least six strain measurement devices. Increasing the number of strain measurement devices reduces the effect of noise of the measurements.

The strain measurement devices <NUM>; 15A-15C may, preferably, be arranged at least <NUM> to <NUM> centimeters away from the wall of the excavation <NUM> 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 <NUM>; 15A-15C may, preferably, be arranged close to the excavation <NUM>, such as not farther than two to five times the size, such as width, of the excavation <NUM>, in order to be able to measure the changes in the stress state.

In various embodiments, the strain measurement devices <NUM>; 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 <NUM>; 15A-15C.

<FIG> illustrate schematically a system according to an embodiment of the present invention. The system is similar to one shown in <FIG>, however, it comprises at least six strain measurement devices arranged into two sets <NUM> and <NUM> of such devices 15A-15C. Furthermore, the at least six strain measurement devices forming the two sets <NUM>, <NUM> of at least three measurement devices may be arranged in opposite directions with respect to a longitudinal direction <NUM> of the excavation <NUM>. This is illustrated in <FIG> in which the sets <NUM>, <NUM> are in an angle <NUM>, <NUM> with respect to a perpendicular direction <NUM> relative to the longitudinal direction <NUM>. The angle differs from <NUM> and <NUM> degrees and is preferably in the range of <NUM>-<NUM> degrees, more preferably in the range of <NUM>-<NUM> degrees, and most preferably <NUM> degrees. This allows determining the stress changes in three dimensions.

<FIG> further illustrates how to sets <NUM>, <NUM> may be arranged into the surroundings <NUM> of the excavation <NUM>.

In various embodiments, the system may be configured, such as by the processing unit <NUM>, to provide an alert if a stress threshold is exceeded. The threshold may be linked to the stress state or level is illustrated at <NUM> in <FIG>.

<FIG> shows a flow diagram of a method in accordance with an embodiment of the present invention.

Step <NUM> refers to a start-up phase of the method. Suitable equipment and components and measurement devices are obtained, and systems assembled and configured for operation.

Step <NUM> refers to determining strain data of the excavation <NUM> by at least three strain measurement devices <NUM>; 15A-15C, wherein the strain measurement devices <NUM>; 15A-15C are arranged surrounding the excavation <NUM> to determine the strain data along different directions with respect to each other.

Step <NUM> refers to receiving the determined strain data on a processing unit <NUM> arranged in connection with the strain measurement devices <NUM>; 15A-15C.

Step <NUM> refers to back-calculating, by the processing unit <NUM>, a stress change based on the received strain data.

In various embodiments, the back-calculating may include the use of superposition 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 regression are the stress tensor components or loading factors.

In various embodiments, variables to be solved are at least two principal stresses (e.g. σ<NUM> and σ<NUM>) and a first direction (e.g. α), 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. σ<NUM>, σ<NUM> and σ<NUM>) and a first and a second direction (e.g. α and β), and, preferably, in a third direction γ, 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 <NUM> based on a plurality of stress states or levels <NUM> determined during the excavation work as shown in <FIG>. The determination of the initial stress state or level <NUM> may be based on back-calculating from the one of the of plurality of stress states or levels <NUM> by taking into account the stress changes determined during the excavation work.

Method execution is stopped at step <NUM>. The method may be performed or executed continuously, intermittently, repeatedly, or on demand. For example, the processing unit <NUM> may be arranged to poll the measurement devices <NUM>; 15A-15C with a sampling of once per hour or at least once per four hours.

<FIG> 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 <NUM> of the strain data, determining <NUM> 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 <NUM>, in the computing device <NUM>, such as inside the excavation <NUM> 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 intervals, 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 consecutive 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 <NUM>. There are several known such methods, for example, based on continuum and discontinuum 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 Discrete Element Method (DEM) with codes as UDEC and 3DEC and Discrete Fracture Network (DFN).

In various embodiments, the stress changes are determined in three dimensions, for example, depending on the number and configuration, such as orientations, of the strain measurement devices <NUM>, <NUM>; 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, homogeneous, 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 <NUM> 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 Δεtot = Δεσ,z + Δεσ,x + Δεσ,xz, where Δεtot is the total strain difference and Δεσ,i is the strain component difference from corresponding loading. For each of these components the strain may be expressed as strain from unit load multiplied by a load factor. In this case the whole formula changes to Δεtot = Lz Δεσ,z,<NUM> + Lx Δεσ,x,<NUM> + Lxz Δεσ,xz,<NUM>, where Li is the loading factor for a loading component, and Δεσ,i,<NUM> the strain difference component from corresponding unit loading.

Regarding the back-calculation, with use of measured data from the excavation <NUM> surroundings <NUM> and numerical modelling of unit load, it is possible to calculate the loading factors and, thus, the change of the stress tensor around the excavation <NUM>. The above equation may be written as εT = εl,<NUM> L, where εT is a vector of measured bolt strain data (length N measured data, for instance), εl,<NUM> a matrix of strain components from corresponding unit loading (size <NUM> times N, for instance), and L a vector of loading factors for loading components (length <NUM>, 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 Lz, Lx and Lxz can be obtained. Then, a solution may be found with use of multiple linear regression. A dependent variable of the multiple linear regression may be the determined strain and the explanatory variables 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, Generalized least square, Percentage least square, Iteratively 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 <NUM>, according to various embodiments, may comprise an input for external units which may be connected to a communication interface of the unit <NUM>. External unit may comprise wireless connection or a connection by a wired manner. The communication interface provides interface for communication with external units such as the strain measurement device <NUM>, <NUM>; 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 <NUM> 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.

Claim 1:
A method for real-time monitoring of stress changes in an excavation (<NUM>), such as in a mine or a tunnel, the method comprising:
- determining (<NUM>) strain data of the excavation by at least six strain measurement devices (<NUM>, <NUM>; 15A-15C), wherein the strain measurement devices (<NUM>, <NUM>; 15A-15C) are arranged surrounding the excavation (<NUM>) 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 (<NUM>, <NUM>; 15A-15C) form two sets (<NUM>, <NUM>) of three measurement devices (15A-15C) arranged in opposite directions with respect to a longitudinal direction (<NUM>) of the excavation (<NUM>), wherein one (<NUM>) of the two sets (<NUM>, <NUM>) is arranged to deviate from a perpendicular plane relative to the longitudinal direction (<NUM>) of the excavation (<NUM>) and into one side of the plane, and the other (<NUM>) of the two sets (<NUM>, <NUM>) is arranged to deviate from the perpendicular plane relative to the longitudinal direction (<NUM>) of the excavation (<NUM>) and into the other side of the plane,
- receiving (<NUM>) the determined strain data on a processing unit (<NUM>) arranged in connection with the strain measurement devices (<NUM>, <NUM>; 15A-15C), and
- back-calculating (<NUM>), by the processing unit (<NUM>), a three-dimensional stress change as a tensor based on the received strain data, and on strain components determined based on pre-determined unit loads with respect to said directions and on corresponding loading factors.