Today large numbers of cultural heritages, mainly masonry structures, are in danger of collapse throughout the world. For conservation and restoration of these structures, it is greatly important to evaluate their stability including ground conditions. However, the conventional stability analyses using numerical method have neglected or rarely considered ground conditions and the interaction between masonry building and ground. In this study, therefore, newly developed NMM-DDA was applied to the stability analysis for one of the Prasat Suor Prat towers of Angkor Thom in Cambodia. NMM-DDA is expected to consider and simulate the stability of both masonry buildings and ground at the same time, which is difficult for NMM and/or DDA alone. In NMM-DDA simulations, masonry structures and ground are modelled using DDA blocks and NMM elements, respectively. The results obtained from the numerical simulations were compared with the on-site investigation by JSA, and the applicability of newly developed NMM-DDA to stability analyses of masonry structures was also discussed in this paper. INTRODUCTION
Today large numbers of cultural heritages, mainly masonry structures, are in danger of collapse throughout the world. The ruins of Angkor, located around Siem Reap in Cambodia, are one of such heritages. Angkor is the remains of the Khmer Empire, which flourished approximately from the 9th century to the 13th century. It was invaded and ruined by the Ayutthaya Kingdom in the 15th century, and it had been forgotten until rediscovered by a French expedition in the 19th century. After rediscovery, conservation and restoration work has been conducted by many countries including Japan .
For the conservation and restoration of masonry buildings, it is greatly important to evaluate their stability including ground conditions. In the case of Angkor, the monuments tend to be influenced by the repeated ground conditions of wet and dry, since Cambodia belongs to the tropical monsoon climate zone, where rainy and dry seasons are rotated every year.
Hence, to estimate the stability of masonry structures, considering the ground conditions is greatly significant.
Masonry structures contain many discontinuous planes, therefore, considering the influence of them is required to investigate the mechanical behaviour of each stone. The stress transmission mechanism of masonry structures plays important roles for their stability and can be investigated by only discontinuum-based numerical approaches such as DDA (Discontinuous Deformation Analysis) [2, 3] and NMM (Numerical Manifold Method) [4, 5], because these methods can treat separation and contact of the blocks easily and properly. DDA and NMM have been widely used to analyse both dynamic and static problems. For example, DDA has been applied to rockfall simulation  and earthquake response analysis , and NMM for tunnel excavation analysis  and stability analysis of cut slopes . Also for stability analysis of heritages, DDA has been applied to Masada , and both DDA and NMM have been separately applied to a pyramid . However, it is difficult for DDA or NMM alone to analyse the stability of both masonry buildings and ground at the same time.
Field conditions in extreme rock drilling include high temperature and high pressure, yet most research neglects the effect of temperature or pressure without proving their real impacts. This work quantifies the effect of temperature and side confining pressure on rock drilling parameters. This research is significant because until now little data has been generated under controlled conditions that isolate temperature and pressure in rock drilling. Carthage Marble and Crab Orchard Sandstone were subjected to 232 º C and drilled at various RPM and Penetration per Revolution settings, with substantial reductions in drilling forces as compared to ambient recorded. Samples exposed to side confining pressure also showed a reduction in drilling forces as compared to ambient, with Torque reduced less than Weight on Bit.
Field conditions in rock drilling include elevated temperature and pressure, yet little drilling research that replicates these conditions has been conducted. Recent work at West Virginia University in conjunction with the Department of Energy’s Extreme Drilling Laboratory and URS Corp. has conducted rock drilling experiments at both elevated temperature and elevated pressure. The high temperature experimental work is of interest to modelers, engineers and researchers, and is of great importance due to the lack of available data in the literature. The elevated pressure test is a preliminary simulation of the effect of principal stress difference as found in under-balanced drilling conditions. The experimental simulation is accomplished by exerting side confinement to the rock sample.
2. STILETTO DRILLING APPARATUS
A drilling apparatus, nicknamed “Stiletto”, capable of measuring Weight on Bit, Torque, Rate of Penetration and RPM while drilling rock samples was custom built for this work, and is shown in Error! Reference source not found.. The sample holder was designed so that the sample is centered above the load cell that measures Weight on Bit. A thrust bearing allows the torque arm to rotate freely, reliably transferring torque to the torque measuring load cell, which engages the torque arm at its free end tip. Both load cells were calibrated using the original manufacturer’s data and the Shunt Calibration method. In all tests reported in this work, a standard 12.7 mm (1/2 inch) carbide tipped masonry drill bit as shown in Error! Reference source not found. was used to drill all samples. The drill press used was a radial drill press capable of a wide range of RPM and Penetration per Revolution settings. These were calibrated independently, and the settings reported are those determined by calibration. The RPM and Penetration per Revolution were imposed on the samples, while Weight on Bit and Torque were measured. Signals from the sensors were sent to the Signal Processing Board, which contained all of the electronics, load cell readers, and Data Acquisition Board. Signals were relayed to a laptop, where a custom LabVIEW program displayed them graphically and saved the data automatically for later reduction at a rate of 50 milliseconds per cycle. This arrangement is shown in Figure 2.
Kao, C.S. (Department of Civil Engineering, University of Minnesota) | Labuz, J.F. (Department of Civil Engineering, University of Minnesota) | Ince, N.F. (Department of Electrical and Computer Engineering, University of Minnesota) | Kaveh, M. (Department of Electrical and Computer Engineering, University of Minnesota) | Biolzi, L. (Dipartimento di Ingegneria Strutturale, Politecnico di Milano)
A surface instability apparatus was used to produce spalling in a laboratory setting, and damage in the rock was monitored by acoustic emission (AE) and digital image correlation (DIC) techniques. Lateral displacement served as the feedback signal to control the post-peak response with a closed-loop, servo-hydraulic load frame. A clustering analysis using the concept of a hierarchical dendrogram applied to low signal to noise ratio events provided "super AE" locations that matched the crack trajectories. DIC was used to investigate incremental displacement fields during surface spalling. Real-time images were successfully captured under high stress levels through modification of the device. Displacement fields were computed in the early stage of loading and around peak stress. Young’s modulus was reasonably estimated with the axial strain measured by DIC. Fracture from spalling phenomena was revealed by contours of incremental horizontal displacement around peak stress. A concentration of deformation leading to fracture was identified, as was a region of relaxation behind the damage zone.
In highly stressed rock, axial splitting may develop in an area adjacent to a free surface. Fracture usually appears in a form of numerous microcracks parallel to the opening (Fig.1), and a spalling phenomenon is the consequence of linkage and clustering of microcracks that splits the rock into parallel slabs. In underground excavations, rock bursts have been attributed to this type of failure . Laboratory uniaxial compression tests also frequently show axial splitting in rock specimens [c.f. 2, 3]. However, the cylindrical shape of the specimen in a typical uniaxial compression test limits observation and measurements of surface spalling. In this study, axial splitting tests were performed in a device called the surface instability apparatus or SIA (U.S. patent 5,024,103), which overcomes limitations in uniaxial compression when investigating axial splitting and spalling phenomena. The SIA allows the control of axial splitting near the only free surface within a twodimensional deformation state with accurate measurements of force and displacement, as well as the possibility to observe microcracking . In this study, the SIA was modified to accommodate both acoustic emission (AE) and digital image correlation (DIC) techniques to monitor damage development and incremental displacement fields during initiation and propagation of the failure process from axial splitting.
2. EXPERIMENTAL PROCEDURE
The concept of the SIA is to simulate a plane-strain condition with zero minor principal stress, i.e. a free surface. The SIA consists of six major parts: two rigid side walls, one back wall, one front wall with an open window, and top and bottom plates (Fig. 2a). The two side walls are bolted into the front and back walls to create a rigid frame. The machined right-orthogonal prismatic rock specimen is wedged between two rigid side walls. The secondary principal stress is developed passively in the Z-direction due to the Poisson’s effect, as the specimen is loaded axially (vertically) in the Y-direction (Fig. 2b). The front wall does not contact the specimen and guarantees a free face, while the rear wall ensures that the lateral deformation and spalling take place in the exposed front face.
Das, K.C. (Department of Mathematics, Indian Institute of Technology) | Deb, D. (Department of Mining Engineering, Indian Institute of Technology) | Kavala, S. (Department of Mining Engineering, Indian Institute of Technology)
Rock bolts have been widely used as a primary support system to stabilize rock masses around tunnels, underground mine galleries, slopes and others structures. To model the interaction between fully grouted bolts and the rock mass a numerical procedure is developed called „enriched finite element method (EFEM)‟. Conceptually if a solid finite element is intersected by a grouted rock bolt, it becomes an „enriched‟ element. Nodes of enriched elements have additional degrees of freedom which are used to determine displacements and stresses in the bolt. Stiffness of enriched elements is formulated based on properties of the rock mass, bolt rod and grout, orientation of the bolt and borehole diameter. This paper quantitatively evaluates bolt performance in different shapes of underground openings viz circular, rectangular and D-shaped, using the proposed enriched finite element method (EFEM) combined with elasto-plastic behaviour of rock mass and grout material. In addition, a comparative study of bolt performance is also presented considering both coupled and decoupled behaviour of rock bolts.
Rock bolts have been widely used as a primary support system to stabilize slopes, hydro dams and underground structures such as tunnels and mine workings and others structure made in rock masses. The term “rock bolt” is defined in geomechanics as a form of mechanical support that is inserted into the rock mass with the primary objective of increasing its stiffness and/or strength with respect to tensile or shear loads. In general, rock bolts reinforce rock masses through restraining the deformation within rock masses and reduces the yield region around the excavation boundary. During the last four decades, different types of rock bolts have been practiced, out of which fully grouted active/passive bolts were the most common types. For a fully grouted passive rock bolt installed in deformable rock masses, a neutral point exists on the bolt, where shear stress at the interface between the bolt and grout material vanishes. Based on neutral point concepts, shear stresses and axial loads developed along a bolt rod are analytically formulated by many researchers. Bolt grout interactions around a circular tunnel in Hoek-Brown medium have been formulated analytically considering a bolt density factor. Considering different approaches to bolt performance Stille presented a closed form elasto-plastic analytical solutions of grouted bolts. Based on the shear lag model (SLM), Cai et al. derived an analytical solution of rock bolts for describing the interaction behaviours of rock bolt, grout material and rock mass. Brady and Lorig. numerically analyzed the interactions of bolt grout in Mohr Coulomb media using the finite difference method (FDM) technique. In addition, numerous studies have been published on the analytical solution of stresses and displacements around a circular tunnel considering elasto-plastic rock mass with Mohr-Coulomb yield criterion. Elwi and Hrudey and d‟Avila et al., proposed embedded finite element method for reinforcing curved layers and concrete structures respectively. Finite Element Method (FEM) and/or FDM based procedures are also developed for the analysis of the said problem and have been presented in many references for solving geotechnical problems.
Over the past decade mining companies have adopted aggressive mining strategies when designing their new block cave mines, ultimately driven by NPV. Block heights have typically doubled or even quadrupled compared to those of the past and similarly drawpoint spacings have increased by up to 30%. The poor track record experienced from several of these new mines brings into question whether the ‘state of the art’ in cave flow and recovery prediction tools can be confidently applied, both empirical and numerical methods. Empirical models are by definition only applicable when applied within the constraints of the data that supports them. Thus there is an urgent need to both re-examine and expand the empirical models to incorporate the experiences from these ‘outlier’ mines or to develop new models. Numerical models, on the other hand, face different challenges, the greatest being the inability to model the required level of detail on a mine-wide scale, i.e. computational limitations.
Unlike most mining methods, block cave mines demand a large capital investment for the development and construction of a significant portion (if not all) of the mine prior to the commencement of any production. Thus mining engineers are tasked to design and optimize the resource recovery to ensure the maximum return on investment whilst still tempered by practical mining and geotechnical constraints. In the past, one of these mining constraints has been the height of the block to be caved. The main reasoning behind the imposed block height constraint is to ensure that the orebody is able to cave easily, that dilution entry is minimized and that the drawpoint brow integrity is maintained until all of the economic ore above is extracted. Benchmarking studies conducted by Florez and Karzulavic for the International Caving Study II , clearly show the increasing trend to develop caves with higher block heights. The latest proposed block heights being up to twice the maximum presented on the chart in Figure 1.
The main mechanisms of cave flow and mixing within the caved ore column are arguably either poorly understood or over simplified in the predictive modelling tools used in estimating the resource recovery. To date the basis for most cave flow and resource recovery prediction tools are based on empirical rules derived from observations of laboratory scale physical model experiments (using sand or rock aggregates), limited mine scale field trials (using marker recovery experiments), mine observations and numerical methods (numerical and stochastic models).
The main reason for relying on empirical rather than mine-calibrated numerical methods has simply been due to the vast scale of the problem, as mine scale modelling of interparticular flow behaviour demands extensive computation power.
However recent experiences from modern, higher block height caves as well as recent advances in cave flow research in conjunction with the development of improved modelling tools and sophisticated flow monitoring systems have challenged the conventional wisdom regarding cave mass flow. Consequently the suitability of using existing empirical rules, for recovery predictions where the block heights are ‘poles apart’ from those from which they have been derived, is justifiably questionable.
There are multiple models predicting relationships among porosity, bulk density, sonic velocity, and effective stress in shales that are currently in use for geomechanical model construction, including, most importantly, pore pressure and fracture gradient predictions. If fact, some of the pore pressure routines used as building blocks in various algorithms are based on eclectic models lacking common underlying physical principles. In this paper I make an attempt to construct a unified model by using the core principle of exponential decay of mud porosity with effective stress and show that this principle can support not only the typical undercompaction-related pore pressure generation mechanism, but also the systems undergoing varying amounts of unloading. Moreover, shale bulk density variation with depth, which relates directly to the vertical stress magnitude, can also be derived from the same principle. The key element of the model is the sonic velocity vs effective stress model, which incorporates log-based information on clay content in shale and allows continuous shale pore pressure profile computation in wells.
Formation fluid pressures in excess of hydrostatic has been routinely observed in deep hydrocarbon exploration wells worldwide. The overpressure development in siliciclastics-dominated sedimentary basins is most typically attributed to shale compaction disequilibrium, i.e., undercompaction mechanism due to an inhibited drainage of shales upon loading [1, 2, 3]. Other sources of overpressure, such as clay diagenesis, organic matter maturation, and thermal expansion are invoked locally [e.g., 3, 4] and with varying success of numerical prediction due to nonuniqeness of sonic velocity versus effective stress relationship during unloading. Despite the importance of understanding the origin of overpressures, there are relatively few publications on relationships among shale porosity, sonic velocity, and effective stress. Moreover, there are multiple schemes and algorithms for pore pressure prediction relying on some empirical models of porosity, velocity, and effective stress that lack a common principle and, hence, unstable or erroneous predictions and need for local calibration [3, 5]. My intent here is to show that 1) such a common principle can be found, 2) various schemes of pore pressure prediction can be consistenly unified, and 3) there is enough empirical evidence supporting the models and concepts proposed.
2. SHALE COMPACTION MODEL
Mechanical compaction of clay-rich mud is well known to be a function of vertical effective stress [6, 7, and 8]. Alpin et al  derived the following relationship between porosity and vertical effective stress. Therefore my final normal compaction trend (NCT) model is a combination of Eqs. (1) and (3) spliced around 5 MPa (725 psi). The dramatic difference between the exponential model (3) and the power law model (4) is even more obvious at greater Sve values, i.e., greater depths (Figure 2). We can see that reducing compaction modulus from the typical NCT values around 27.5 MPa (4000 psi) results in farther deviation. One possible explanation for such a discrepancy is that Traugott model was developed from density logs through siliciclastic sequences including sands, fraction of which is not accounted for in the model.
Pierre shale was tested under an undrained condition within the University of Minnesota Plane-Strain Apparatus, which was modified to allow application and measurement of pore pressure. A rectangular, prismatic specimen was carefully machined and assembled with porous stones between the upper and lower steel platens; the specimen, porous stones, and platens were held together with a custom jig and a thin layer of polyurethane was applied to prevent confining fluid from penetrating the shale. Once air was removed from the system, equal back pressure was applied to the top and bottom of the specimen. The back pressure was incrementally (1 MPa) increased to 8 MPa, and held constant for ten days, with confining pressure 0.35 MPa higher than back pressure. The maximum B-value achieved was 0.63. The undrained, plane strain compression test was performed with an initial confining stress of 4 MPa, and an average lateral displacement rate of 10-4 mm/s; peak stress was reached in about one hour. Measurements of axial and lateral displacements, axial load, confining and pore pressures provided the necessary information to calculate the undrained and drained elastic parameters of the rock within the framework of poroelasticity.
The presence of pore fluids can affect the deformation process and facilitate or delay material failure . Dilation of rock in undrained deformation induces a reduction of pore pressure and growth of the limit stress value . On the other hand, a contractive response induces an increase of pore pressure and reduction of the failure stress . For undrained deformation processes with the condition of isochoric deformation, the limit surfaces are reached for lower shear stress values than those for drained deformation. Further, depending on the material density and initial pressure, there can be several limit states with adjacent domains of stability and instability . The overall project concerns the testing of watersaturated soft rock such as shale under plane-strain compression, where drained properties are difficult to determine because of low permeability. Dilatant hardening and contractant softening will be investigated from undrained experiments, where pore pressure will be measured throughout the deformation process. This paper presents the results of an undrained, plane strain experiment on Pierre shale. Poroelastic parameters of the rock were calculated from measurements of axial and lateral displacements, axial load, confining and pore pressures. Values of porosity, Skempton (B-value) coefficient, and bulk modulus of the fluid used for saturation were estimated prior to compression testing. Poroelastic theory introduced by Biot [4,5] and developed by Rice and Cleary  and Detournay and Cheng  was used to determine the undrained and drained elastic parameters.
2.1. Plane-Strain Apparatus An apparatus for determining the constitutive response of soft rock, named the University of Minnesota Plane- Strain Apparatus, was designed and built based on a passive stiff-frame concept . The biaxial device is unique because it allows the failure plane to develop and propagate in an unrestricted manner. In addition, the apparatus was modified to allow pore pressure to be applied and monitored during an experiment.
Cemented Paste Backfill is gaining popularity as a method for filling underground mine stopes. However, the behavior of the paste is not well understood. This paper proposes a modeling method which represents the paste as a series of sequential layers. Layer size is controlled by the rise rate of the backfill in the stope. The time-dependent behavior of the fill is implemented as each layer can be aged and assigned the age appropriate material properties. These properties were obtained by a laboratory testing program summarized in the paper. The paper presents a brief validation exercise as well as a comparison between the model and in-situ instrumentation results.
Cemented paste backfill (CPB) is a popular backfilling method for underground mining operations due to its delivery speed and versatility, engineered strength, and for decreasing the area needed for and the amount of risk associated with surface disposal. However not much is known about how the CPB behaves in an underground opening, particularly during the early curing ages of the CPB. This lack of understanding poses several problems for mining engineers including how to design backfill barricades and backfilling procedures. For instance, most mines use a two staged pour which is divided into a plug pour and a final pour. There is a delay in-between the two pours, usually 3 to 7 days, to allow the paste in the plug to harden and gain strength. This delay and subsequent paste strengthening is used to protect the backfill barricade when the rest of the stope is filled. There is a large potential for a mine to save money if they can reduce their stope cycle times or move to a continuous pour procedure. Increased understanding of CPB behaviour would also decrease the uncertainty associated with the barricade fences themselves, allowing for better and cheaper fence designs. However, these cost savings cannot be realized unless a there is a reasonable and reliable method for determining what stresses the backfill barricade will experience. These stresses are dictated by a range of parameters including: binder content, binder type, tailings type, additional aggregates, filling rate, stope geometry, etc.
Numerical and analytical models have provided a starting point for investigating in-situ pressures within the stope and at the backfill barricade. However, there is a lack of in-situ stress measurements to test the validity of these models. The University of Toronto has established a large field project which involves the instrumentation of several test stopes at three mine sites: Barrick Gold Corporation’s Williams Mine, Inmet Mining Corporation’s Cayeli Bakir Mine, and Xstrata Copper Canada’s Kidd Mine. This paper presents a numerical modelling exercise of the 2010 test stope at the Williams Mine (Thompson et al, 2010).
The modelling was conducted using Itasca’s Flac3D modelling software and attempts to incorporate the stope filling rate , the time-dependent strength behaviour of the paste, and the actual 3D stope geometry. The model tries to match the actual filling strategy used to fill the stope and compares these results with the readings from the in-situ instrumentation.
Two sets of data from a well in a field in Kazakhstan have been analyzed in this paper: seismic travel velocity times and drilling operational data. From both the different sources data rock strengths versus depth was calculated and compared for the formations drilled through. The representative Apparent Rock Strength Log (ARLS) was created by combining results of rock strengths from both data sources. The representative ARSL was then used to optimize drilling of the next well in this field using a commercially available drilling optimization simulator. By applying different bit designs and types and drilling operational parameters versus depth in combination with the ARSL rate of penetration (ROP) is predicted. Simulating different operational parameters and bits result in the best optimum combination and maximum ROP in the different sections of the well and the least expensive drilling for the entire new well to be drilled in the same area. This paper shows the approach and the simulation results and recommendations for the next well to be drilled in this area in Kazakhstan. The rock strength obtained from two independent sources showed similarities indicating that seismic data could possibly be useful in preplanning and optimization of exploratory drilling.
It is well known throughout the oil and gas industry that drilling costs have always been a major part of field development expenses. Optimization of drilling can bring significant savings to projects that are under development. Alternately drilling costs that are budgeted at lower values due to optimization can make untapped projects look profitable, particularly the ones that are marginal due to various factors e. g. unstable crude prices, recession, instability of any kind. Drilling optimization is becoming a key factor in pre-planning phases of drilling campaigns. Seismic data is a crucial input in exploration and characterization of petroleum reservoirs. The amount of easy-to-find reservoirs is decreasing rapidly while the need of oil and gas is increasing. The industry is facing difficulties in finding smaller, more subtle reservoirs with higher risks and costs. From seismic data, not simply information about structural geometry and bulk rock volumes of potential reservoirs is needed, but it is also desirable to characterize their detailed properties such as rock strength of formations to drill for preplanning phase, to minimize risks and optimize drilling. Rock physics analysis combined with seismic attributes has become a key strategy in quantitative seismic interpretation. The success of a typical drilling simulation is dependent on the quality of its Apparent Rock Strength Log (ARSL), which quantifies the unconfined compressive strength of the formation as a function of depth. Once the ARSL is finalized it serves as a basis for drilling optimization to obtain the highest ROP and the lowest $/meter drilled for the whole well interval. In the case presented drilling data for the only well drilled in Kazakhstan was available. In order to check the validity of the ARSL generated from drilling data seismic data in that area proved to be useful.
Ferrero, A.M. (Department of Civil Engineering, University of Parma) | Migliazza, M. (Department of Civil Engineering, University of Parma) | Alejano, L. (Department of Natural Resources and Environmental Engineering) | Rodriguez, Dono A. (University of Vigo)
The development of new methods for the geostructural survey that do not need direct contact with the rock mass have been developed by several authors and applied to rock slopes. The new idea of this work was to apply a non-contact method based on photogrammetric techniques at the tunnel excavation for a complete rock mass 3D geometrical reconstruction. The core of the method is the possibility to obtain all the information by taking just few photographs, reconstructing automatically a digital surface model (DSM) of the excavation surface and then applying a code (Rockscan) for the determination of the discontinuity orientation and location on the excavation surface. The rock mass knowledge allowed to perform both equivalent continuum and discrete modelling and to compare the results with in situ convergences. The method has been developed through the application to two real cases: a mining gallery in dolomite underground quarry in Tassullo (TR) and a railway tunnel face in the Parma province (North Italy). The paper describes the method, the developed code and their application to the tunnels, showing the influence of the rock mass structure to the anisotropic convergence.
The quality of the design of tunnels is closely linked to the degree of knowledge of the rock mass, which is based above all on the investigation of samples that are gradually enriched during the excavation phases which allow access to the uncovered rock faces. The traditional structural survey can be a complex and dangerous operation when dealing with a low quality rock mass due to possible unstable ground conditions. Moreover in certain delicate cases the excavated surface must be rapidly consolidated and this makes the survey even more difficult to accomplish. Finally the tunnel section can be some meters high making it impossible to reach it all without a special lifting system. Using a photogrammetric technique, we can obtain a complete front description that can lead to a more accurate discrete numerical model of the rock mass. This paper presents the development of a rock mass characterization method based on photogrammetric surveys carried out in two underground sites: a mining gallery in a dolomite quarry in Tassullo (TR) and a railway tunnel in the Parma Province (North Italy). The numerical models developed in the second case include a 3D Finite Element Method (FEM) and a 2D Distinct Element Method (DEM) model.
2. GEOSTRUCTURAL SURVEY
A photogrammetric survey of rock walls of an underground excavation is conducted using a similar block scheme to that of a traditional photogrammetric flight conducted for cartographic purposes in that a series of photographic strips are taken at an almost constant distance from the excavation wall. The main drawback of this phase concerns the reduced dimensions of the section. Compared to a survey on the surface, in which there is great freedom in the choice of the distance from a wall, which also depends on the final precision required, an underground survey is constrained by the dimensions of the tunnel.