Suggestions, Methods and Examples of Monitoring of Rock Structures and Excavation of Rock Mass

Rock mechanics projects, excavations and rock mass monitoring are day-by-day concerns of professionals and scientists of rock engineer. Technological advances observed in the 20 and 21 centuries provided high precision equipment capable of establishing deformation and estimating the rock mass stress remotely and in real time. In addition, in order to confirm and study the data obtained with theses equipment, numerical programs of modeling became more accessible to schools, research centers and private companies. Monitoring an excavation requires, besides understanding fully the rock structure, precise definitions and goals: why, how, where. This article discusses concepts of monitoring, modeling and calibration, as well as presents examples of applications where these questions were successfully answered.


Introduction
The main objectives of excavations monitoring are to gather the deformation data and to estimate the state of stress "in situ" and "over time" that will enable to understand the geomechanic rock mass behavior. Therefore, the monitoring process should allow the investigators:  To evaluate the excavations safety;  To confirm the premises assumed in the project;  To analyze and understand the rock mass mechanical behavior and its failure phenomenon; How to cite this paper: da Gama  To gather "in situ" and more reliable data for the project;  To provide data for numerical model calibration;  To confirm premises used in the excavation process;  To control the quality of data obtained;  To provide data for modifying and improving the project;  To evaluate the effect of corrective measures and variations in the excavation methodology. The possibility of a rock mass failure around excavations cannot be eliminated or accessed with high accuracy. However, monitoring procedures provide ways to reduce these risks to acceptable levels. "In situ" data are the most reliable information since they are not subjected to project simplifications or laboratory analysis. The scale effect and the unpredictable geological complexities are not neglected but are, naturally, taking into account [1]. However, instrumentation and monitoring any excavation require not only a wide knowledge about the rock mass itself, but also to decide why, when, how, where, and, what should be monitored. The continuous advances in the computer science such as the processing speed and storage capacity of microcomputers allow more and more the application of theoretically well-known mathematical methodologies, in solving geomechanic problems. Today, programs using the method of finite elements [2], finite differences [3], boundary elements [4] and discrete elements [5] [6] are relatively easy to implement in most microcomputers available in the market. In addition, new improvements in the numerical methodologies were made available since the application of the theory of continuous equivalent [6] bringing up the possibility of simulating larger displacements and even the collapse of systems. In the field of rock mass instrumentation, several pieces of equipment were developed to evaluate virgin stress state like the [7] Hydraulic Fracture Equipment and the LANDIS system, which uses laser beams to measure displacements [8]. One can say that each geomechanic field has developed, somehow, independently, and not many researchers have dedicated their efforts to integrate numerical modeling and rock mass instrumentation. [9] was one of the first to present a methodology that combined the instrumentation to numerical simulation (SPDR) using data obtained in real time. [10] discusses the wide use of numerical modeling methodologies. However, he points out that the prediction of the mechanical behavior depends heavily on the reliability of data used. [8] shows a methodology where the rock mass instrumentation and the boundary element numerical modeling are applied in the induced stress and displacements monitoring in an underground gold mine. It is very complex to accurately quantify geological structures, as well as geomechanic properties such as stress state, water level and pressure, permeability, plastic and elastic parameters, fluidity and resistance of a rock mass. To overcome these difficulties, the auscultation together with a numerical analysis is highly effective during and after the excavation procedures, in order to monitor the stability of the structures, and to re-evaluate the geological and geomechanical parameters used in the analysis. It would reduce the differences between the expected and the actual behaviors al- lowing the investigator to design structures holding safety factors more reliable, avoiding, in many cases, the application of multipliers to the safety coefficients. Stress and strain measurements have been done, for many years, with the help of gauges that are read visually by an operator. This kind of instrumentation is, invariably, not directly connected to any system of analyses. In addition, the data are not gathered continuously over time. Moreover, their installation is time consuming and it can only be installed in places with relative stability. Therefore, they cannot be used in places of greater interests of study. This article depicts the monitoring procedures using a Monitoring System of underground excavation (Sismo), [11] and the calibrated modeling of an underground Shrinkage Stoping Zinc mine. The instrumentation made it possible to study the effect of blasting in small galleries and to predict the rupture of small pillars. A numerical model was built with the software UDEC (Universal Distinct Element Code -Itasca Consulting Groupe) version 3.0. The model was used to study the effect of distressing and the related deformations around an underground opening excavated with drilling and blasting.

Monitoring Rupture Processes
The main objective of a monitoring system [12] is to acquire data, by measuring deformations (strain) and estimating stress, that can be used in order to accurately understand the rock mass geomechanic behavior. [13] showed the advantages of monitoring based upon the convergence and confining method. [14] showed that normal stresses vary rapidly close to excavating surfaces while tangential stresses vary gradually. He also showed that the shear stress may Variation directions close to the excavation walls. Back-analysis, in its simplest way can be understood as the reversed process of the ordinary analysis, which means, to introduce known values of forces, stresses, displacements and strain into a model to obtain the material's mechanical parameters. That emphasizes the importance of a good system of monitoring when back-calculation is used. When the material's parameters are obtained, the model may be used to design, geometry and size, of future excavations and to propose better methodologies of excavations [15]. This kind of investigation allows using a statistical distribution of the data back analyzed. Controlled modeling objectives, basically, to ascertain about excavation models created from the description of a physical mean, from the rheology study, from the auscultation and from the numerical model. The fact of establishing a geomechanic model, that is a simplification of a real model, does not mean that we have to use the same conventional parameters of description, the same assays and the same points of instrumentation. A geomechanic model implies:  Improving lab and in situ testes with the objective of obtaining data that are more reliable.  Improving the instrumentation, being able to monitor closely and follow variations in the measurable quantities.  Improving the geotechnical-structural mapping in a way to better represent the structures that condition the failure.  Gathering all the data collect in a quantitative technique of calculus, allowing the analysis of the stresses and strains. (Validated model).  Use of a valid model to simulate geometry and methodologies of excavations.
The scheme of a calibrated model can be seen in Figure 1.

Monitoring Conditions and Types
The success of a monitoring program depends upon some keys factors discussed by [1]. With regard to the monitoring technique, few aspects should be emphasize: the objectives of the monitoring program; the equipment employed and how they are installed; the place chosen for installation; the frequency and time span of data recording; and the type and quality of the data obtained. Monitoring can be said to be qualitative, such as visual observation, or quantitative, when instrumentations are used to measure or estimate information directly (deformations, stress) or indirectly by indicators (acoustic emission, seismography, etc.).

Sismo -Underground Excavation Monitoring System
The underground excavation monitoring system (SISMO) [16] is a direct measuring mythology that aims to gather quantitative data of strain and estimated stress variation. It can be subdivided into three integrated parts: (Linear Variable Differential Transducer) sensor with real time transmitting capabilities. The LVDT can be coupled with telescopic rods, invar wire in single pairs or multiple points. Stress variation sensors similar to flat jacks of different shapes and sizes, to be installed inside pillars and excavation walls and PVT (Stress Variable Transducer) to be placed inside drill holes and natural discontinuities. Both systems are capable of real time data transmission  Data acquisition Analogical to digital transducer board system utilizing from 6 to 12 channels. Each channel is used for one set of measurement, strain or stress variation.

Data Processing Software
A software called Estavel [16], was written in C++ to compile and to process the data received in any single channel or in simultaneously in all the 12 channels. Depending upon the arrangement of the measuring instruments it would be possible to work together with coupled strain/stress measurements of 12 different points. The data collect is then subjected to an initial statistical analysis by applying the Gradient Method [16].

Monitoring and Modeling of a Calibrated Excavated Gallery
Herein, this paper discusses a gallery dug into a massive dolomite. The main objectives of this study were:  Monitoring Variations in stress and deformation in real time during the excavation with explosives;  Feed the numerical model established with the software UDEC with the monitored data;  Comparing the monitored data with data from numerical model calibrated.

Geological Context of the Monitoring and the Place of Installation of the SISMO
As shown in Figure 2, after the opening of a "stopping", a great collapse due to instability the walls took place. These walls are parallel to the areas of sub-vertical shear. The removal of ore from the instability of the walls of the stopping was done through a gallery-dug perpendicular to the shear zone vertical and sub-parallel to the shear zone of sub-horizontal. The gallery parallel to that was instrumented with the System of Monitoring SISMO. The depth of these galleries is 122 m and the concentrations of zinc ore from that mine, stay in areas of shear. These shear zones have the general direction of NE-SW stress of several kilometers thick and about 50 m. It is reddish dolomite, shear and breccias, which separates the overlapping gray dolomite (cover -hanging wall) of dolomite roses (Lumpfish -footwall). A perpendicular gallery in the shear zones of the sub-vertical ore is horizontal. Geomaterials

Installation of the Instrumentation of the Sismo
The

Results of Monitoring
Monitoring during the SISMO application took place for 337.5 hours. The variation of stress (Δσ) and deformation (Δγ) (convergence) was recorded at constant intervals of 10 seconds. Figures 3-6 show the most relevant results. Initially treated by the method of moving averages to a constant sampling of 10 data recorded every second. This treatment helped prevent the spread of data.    Test 001 - Figure 3 shows the results of variation in deformation with time (Δγ) and Figure 4 shows the results of the variation of stress (Δσ). In

Results
The results in 006 test results were compared to the UDEC and the theoretical model of rheology behavior of the excavation. The curves obtained in the tests clearly show a visco-elastic behavior of the excavation, i.e., the deformation increases with time, "recovering" these deformations over time. This rheology model has the following theoretical formulation for the deformation behavior of cm function of time: where: ε strain, applied stress σ, E modulus of elasticity, initial time t 1 Figure 11 and Figure 12 show a comparison between, the two models loading and testing done in UDEC 006 all sensors. Figure 11 is on the convergence and  Probably the site is the test of shear zones within past, this portion of the mass is favored by the relaxation of stress. It is interesting to note that beyond the zone of sub-vertical shear, the instability of the walls of excavations is striking; however, in the direction perpendicular to them, i.e. in the direction of excavation tools and models here, this instability is not in the size of the excavation.

Conclusions
The The results confirm, at this stage of experience with the SISMO, the expectations with respect to their effectiveness and versatility in the acquisition and interpretation of data.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.