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Deformations analysis and monitoring in underground constructions: automatic systems to measure convergence and preconvergence effects
Abstract
Due to the considerable number of uncertainties and to the anisotropic nature of rock masses, tunnel design assumptions should be always validated during the construction phase. This procedure could be performed by means of a regular monitoring of the rock mass deformation around the tunnel face, with particular attention to convergence and preconvergence phenomena. This paper presents the experimental data obtained through an automatic monitoring system composed of two devices, called PreConv Array and Cir Array, respectively developed for the direct monitoring of pre-convergence ahead of the excavation front, and the convergence of the tunnel profile in a specific section. Each device is connected to an acquisition control unit that queries the sensors and automatically sends the data to an elaboration center at predefined time intervals. The automated elaboration approach provides a complete and immediate description of the rock mass behavior.
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The design and construction of underground facilities in major cities in Vietnam, such as Hanoi and Ho Chi Minh city, have currently been expanded. Digging deep excavation pits for underground spaces or subway stations can lead to settlements of the surrounding buildings. Under the soft soils or water-saturated clay soils, the construction of underground spaces causes additional settlements to the surrounding soil mass. Therefore, it is necessary consider the technological settlement, which is the effect of the construction process of deep excavations, to forecast the total settlement of surrounding buildings. This study proposes an equation to predict the building subsidence due to the influence of deep excavation accounting the technological settlement.
Various empirical equations have been proposed to predict the ground settlement profile caused by the excavation of conventional circular tunnels. However, ground movement for the underground box structure has not been fully studied. In this study, ground settlement induced by underground box installations is investigated using two-dimensional finite element analyses. A new formulation to assess the settlement profile applicable to underground box structure is proposed based on parametric analyses of the changes in ground condition, geometric condition of structure, and construction conditions. This paper also presents a method to predict the maximum surface settlement around an underground box structure with artificial neural networks (ANNs), taking into account nine input variables that have direct physical significance. A MATLAB-based multi-layer back propagation neural network model is developed, trained, and tested with parameters obtained from numerical analyses. The maximum settlement from the ANN model, in conjunction with a new formulation to construct the settlement profile, turns out to be promising, by predicting a settlement profile compatible with field measurement data.
Dealing with engineering structures that are not easily replaceable requires frequent assessment of the damage state of the construction in order to estimate its durability and reliability. The procedures that allow damage to be detected and identified are broadly defined as Structural Health Monitoring (SHM). In this work, a SHM network has been deployed in a road tunnel that intersects a massive landslide, whose movements are causing the formation of cracks along the tunnel lining. The monitoring system measures in real time the displacements across major cracks and the rotation of the tunnel segments; data are gathered and made easily accessible through a web-based platform. The mechanisms by which the tunnel deforms under the landslide-induced stress have been defined through the analysis of three years of monitoring data. The factors triggering an increase in deformation rates and causing damage to the structure have also been investigated. This evidence will support the design of mitigation works to extend the life-span of the tunnel.
In the last 25 years in Italy, over 1000km of tunnels have been constructed in urban and extra-urban areas. Despite being constructed in very complex and difficult grounds; generally these were completed (from the design stage to the construction and actual opening of activity) with industrial methods, and within the technical times and costs foreseen by the design laid out by the tenders. This extraordinary result has been made possible since the public administrations have adopted modern design and construction approaches in their contract specifications, based on strictly scientific criteria and which leave nothing to the typical vagueness of those design and construction systems from the twentieth century, which remedied cavity instability solely by means of partitioning the excavation face. Today we know that full-face advancement is all the more necessary the more difficult the excavation conditions are. Memory serves to illustrate the criteria that helped define modern design and construction concepts and an example of their application for constructing a great twin tunnel (258 m2 + 258 m2 in section) in the urban area of Rome.
Optimization of the support used when constructing tunnels in soft surrounding rock has long been a hotspot in engineering. To control the deformation of soft rock and ensure construction safety, this paper proposes a support scheme involving weakening the anchor bolts while enhancing the rigidity and strength of the primary supports. This was realized by combining the large deformations that often occurred during the construction of the Youfangping tunnel of the Gucheng-Zhuxi expressway. Moreover, the scheme was analyzed and compared with the original scheme and one involving weakening the anchor bolts. In addition, the displacement deformations, force conditions on the anchor bolts, development of plastic zones, stress on the shotcrete, and force conditions on the secondary lining structure were analyzed via numerical simulation for the different support schemes. Concomitantly, three groups of experimental sections were selected on-site to monitor and measure the deformations and force conditions of the surrounding rock for these three support schemes. The numerical simulations and field-monitoring results show that weakening the anchor bolts has a small effect on the overall support provided by the support system. However, it can simply the process and reduce engineering costs. Moreover, increasing the rigidity and strength of the initial supports can effectively control the large deformations in the surrounding rock. Therefore, a support scheme in which anchor bolts are weakened while the rigidity and strength of the initial supports are enhanced, as presented in this paper, proves to be feasible method of support optimization. It also provides a useful reference for optimizing other similar tunnels along the expressway.
This paper introduces InSAR (Interferometric Synthetic Aperture Radar) as an advanced tool for measuring and monitoring surface ground movements over time, with interest in all phases of a tunnel project, both in urban and non-urban areas. Following a preliminary overview of the technology used to compile radar images of the earth's surface, the multi-image techniques (Persistent Scatterers Interferometry, PSI) and the InSAR algorithm (SqueeSAR) are briefly outlined. Two examples of InSAR data applied to tunnelling projects are presented. In the first case, the integration of InSAR surface measurements into monitoring by conventional methods is discussed as a tool for providing useful information to study the relationship between tunnelling and surface settlements. In the second case, the temporal evolution of ground displacements provided by SqueeSAR is applied in order to understand the link between tunnel excavation and surface movements, along a slope under passed by two large tunnels.
Tunnel construction commonly causes deformations of the surrounding ground, which can endanger buildings and other structures located in the vicinity of the tunnel. The prediction of these deformations and damages to buildings is difficult, due to limited knowledge of geotechnical conditions and due to uncertainty in predicting the response of the structures to the settlements. This motivates the development of a probabilistic model for the prediction of tunneling-induced damage to buildings. We propose such a model, based on the classical Gaussian profiles for the approximation of the subsidence trough and the equivalent beam method for modeling the response of the building walls. In practice, settlements are commonly monitored through deformation measurements. To account for this, we present a Bayesian method for updating the predicted settlements when measurements are available. Finally, we show how maximum allowable settlements, which are used as threshold values for monitoring of the construction process, can be determined based on reliability-based criteria in combination with measurements. The proposed methodology is applied to a group of masonry buildings affected by the construction of the L9 metro line tunnel in Barcelona.
Nowadays, with the increasing population of large cities, the need to expand public transportation, especially metro systems, is greatly increasing in urban areas. Therefore, excavation of new tunnels near the existing ones or other excavations located nearby has become inevitable. Excavation of such tunnels in urban areas should be done by considering the effects of these tunnels on buildings and other urban structures. Significant factors affecting interaction between tunnels, as well as the characteristics of surface settlement, are the existence of mixed ground (soil-rock) at tunnel faces or fault zones in the direction of tunnel excavation which have not been clearly investigated by researchers. These parameters have a great effect on the amount of maximum surface settlement and shape of surface settlement curve. Although several studies aim to analyse the interaction between newly excavated and existing tunnels and its effects on surface settlements, this subject certainly needs further investigation. This study mainly focuses on the effects of the interaction between twin tunnels mainly opened in fault zones and mixed ground on the basis of surface settlement measurements by using Earth Pressure Balance Machine (EPBM). Both numerical and empirical methods are used in this study. Observed data are used to test the validity of the results obtained from three-dimensional numerical modelling. The results from numerical methods were in good agreement with the real data. The results of this study reveal that the amount of maximum surface settlement and shape of surface settlement curve are strongly related to spacing between tunnels, fault zone thickness and type of tunnel face material. The interaction factor is almost zero when spacing is larger than 4D (D is tunnel diameter). Independent of fault zone thickness, the effects of the fault on longitudinal surface settlement continue 25 m from both sides of the fault centre.
An extensive programme of geotechnical instrumentation and monitoring was carried out during construction of the Ingula hydro power caverns to validate design assumptions and monitor long term creep effects. This paper supports the use of geotechnical monitoring during construction and discusses the results compared to the predicted convergence and where the monitoring allowed for rapid assessment of problems encountered during construction. Differences between the predicted convergence and the monitoring results necessitated a review of the numerical models used for design, to ascertain the sensitivity of changes in the construction sequencing and geotechnical parameters encountered during construction on the models and derive a new set of predicted convergence.
Building deformation and the potential damage of buildings caused by tunnel construction in urban areas have become a major concern in the planning and construction process of any tunnelling project. The currently adopted design procedure is highly conservative, and can lead to costly projects. In 1997 Potts and Addenbrooke introduced design charts to consider the influence of the building's own stiffness, thus leading to more realistic predictions of tunnel-induced deformation. Their relative stiffness approach was based on a parametric study using plane-strain finite element (FE) analyses in which the building was modelled by weightless, elastic beams. This paper presents the results of both two- and three-dimensional parametric FE studies that extended their building model to include a wider variety of building features such as building weight, the nature of the soil-structure interface, and the building dimension in the direction of the tunnel axis. By incorporating these additional building features into their approach it is shown how the relative stiffness expressions can be modified to be dimensionless. New design charts that take account of the wide variety of building characteristics are presented for these new relative stiffness expressions.
Ground deformation monitoring in tunnelling is a common means for selecting and controlling the excavation and support methods among those predicted in design, ensuring safety during tunnel excavation (including personnel safety inside the tunnel and safety of structures located at ground surface) and construction quality. This paper describes the types of ground deformation measurement often used in tunnelling, the difficulties in obtaining these measurements, the methods most commonly used for their evaluation, and the use of deformation measurements (a) in the selection of tunnel excavation and support systems and (b) in establishing early warning systems against ground collapse or damage to structures at ground surface.
Integrity is a common measure of the health of a tunnel. However, the relationship between integrity and developing anomalies is still unclear. Thus, this study acquires comprehensive recordings of lining cracks of a tunnel to enhance our understanding of tunnel degradation and lining crack evolution. Detailed anomaly variation of a deforming mountain tunnel that is influenced by slope movement were obtained from 2008 to 2015 utilizing high resolution images of lining surface. During the recording, the length of cracks within monitoring section increased at a rate of 54.46–86.85 m per 180 days. New cracks appeared within the recording period emerged mostly at the same locations as the existing cracks found in the beginning of the recording, suggesting that crack development is highly relate to the location in the tunnel. The evolution of cracks also varies with the preexisting crack patterns. Lining crack patterns that stretched over different construction units, which implies the effect of regionally or overwhelmingly external factors to tunnel degradation, can be characterized as transverse cracks, closed cracks and inclined cracks. Cracks with similar patterns evolve similarly in their lengths, densities and spalling area, except for transverse cracks at deep overburden. For advances in life cycle base tunnel maintenance, a tunnel in operation should be divided into zones based on the patterns of existing lining cracks or environmental boundaries at the beginning of monitoring. Subsequent interpretation can be performed to verify and modify tunnel zoning, revealing representative properties of the evolution of the cracks with a view to identifying integrity criteria and methods of repair.
This paper describes the analysis of the effectiveness and reliability of a new type of inclinometric chain, which is still under development by the authors, and is intended to be applied in the underground slope monitoring field. In the first part, the paper describes the new instrumentation which should allow for a deeper and detailed understanding of the type, location and origin of slope movements that should, in turn, help in understanding the triggering causes and the evolution mechanisms of landslides, and provide an innovative and substantial contribution to their stability analysis and control. The second portion of the paper is dedicated to a comparison between the classic instruments and the new MUMS device, demonstrating the advantages of measurement automation and economy in the use of the proposed device, which could also be equipped with other electronic instruments that would allow the measurement of other interesting physical quantities (such as pore pressure, temperature, stresses, etc.) together with displacement components.
In this chapter the most important geological problems that can occur in underground excavations are described. In fact the excavations progressively modify the state of stress in the ground and, as a consequence, the ground is forced to reach a new equilibrium state through deformations. The entity of these deformations and, in general, the behaviour of the masses being excavated depend on: the lithological nature, which determines the mechanical characteristics of the matrix; the structural features (stratification, schistosity, fracturing etc.) which determine the mechanical properties of the mass itself; the state of stress existing before the excavation and the hydrogeological characteristics of the medium.
Other aspects described in this chapter, that can be very relevant for a good underground construction performance are: the risk of natural gas finding, the presence of aggressive water, weathering and swelling minerals, the increase in temperature with depth (geothermal gradient), the seismicity, the radioactivity and the presence of hazardous minerals.
Tunnel monitoring and measurement is a very important part of the tunnel construction safety andit is one of the important contents of tunnels on Railway Passenger Dedicated Lines segment acceptance, Its purpose is through the tunnel clearance convergence and settlement observation,to analysis data, optimize tunnel construction and design, and determine a reasonable safety step and support time, protect tunnel construction and operational safety, Although the domestic and foreign on tunnel monitoring measurement work are very seriously, but at present the requirements of monitoring level and information construction are far away. This article based on 3D laser scanning technology, proposed tunnel 3D laser scanning measurement and data visualization technology, through the engineering application and practice, the results show that the technique can be real-time, accurate and comprehensive to get the deformation data of tunnel space,and can analysis and counter-analysis the deformation of tunnel construction, it has greatapplication value of engineering for early warning forecast the risk and guidingtunnel construction.
The 9 km long Chenani-Nashri Tunnel, currently under construction, is the longest road tunnel in India and is part of the planned four-lane widening of the NH-1A between Udhampur and Banihal in the state of Jammu and Kashmir. Bypassing the existing NH-1A from km 89 to km 130, the tunnel crosses a sub-Himalayan formation with a maximum overburden of 1,050 m. With an escape tunnel running parallel to the main tunnel, excavation is performed by Leighton-Welspun Contractors using the drill and blast method. Geodata Engineering (GDE) is providing consultancy services for detailed design and construction supervision including 3D-geotechnical monitoring. Back-analyses of already-excavated sections are performed to better understand the behaviour of the heterogeneous rock mass in which the tunnels are excavated. The numerical models are fed with the actual geological and geomechanical conditions encountered during excavation and the monitoring results. The 3D-monitoring system, specially implemented by GDE for this project, has played a key role in understanding the real rock mass behaviour, allowing the highlighting of potential risks, selecting the correct tunnel support class, checking of the effectiveness of countermeasures, identification of the tunnel stretches in which the final lining needs to be reinforced and providing cost-effective solutions to speed-up the construction process.
Tunneling induced settlement is often a controlling factor in the design of a subway or underground facility due to the impact on adjacent or overhead infrastructure. A method is proposed that uses an acceptable ground surface settlement profile to generate a tunnel system configuration that reproduces the acceptable/tolerable settlement profile. A brief discussion of existing methods of tunnel settlement analysis and two (2) case studies illustrating the proposed method are presented. The chosen case studies are urban subgrade rail tunnels (subways) constructed in Istanbul, Turkey and Bangkok, Thailand. Each case study is used to illustrate the use of the bifurcated static back calculation – iterative finite element model prediction/solution method.The initial settlement profiles used for both case histories were measured settlement profiles obtained from the literature ( and ). These settlement profiles were assumed to be the tolerable settlement profiles for the subway systems. For each case history, the iterative static back calculation – finite element model ingested the individual literature obtained design settlement profile to identify an optimum tunnel configuration (depth and size) that will reproduce the design settlement profile. Comparisons were made between the as-built tunnel configuration and the tunnel configuration predicted using the proposed method.
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