Using Thermal Monitoring and Fibre Optic Measurements to Verify Numerical Models, Soil Parameters and to Determine the Impact of the Implemented Investment on Neighbouring Structures

Abstract

Numerical modelling using Finite Element Method (FEM) is currently a standard procedure for engineering complex structures and determining structure–subsoil cooperation conditions. It is used for, among others, forecasting displacements, which are the calculation results most easily verified. Numerical modelling is also used to identify the impact on neighbouring structures and design a monitoring system and determine expected values, e.g., displacements. A numerical model enables one to optimally design the monitoring system for a facility under construction and the neighbouring structures through selecting a measurement technology, matching the scope of obtained results or choosing sensor and measurement point locations. The implemented monitoring may be based on various technologies, from thermal monitoring, laser scanning, fibre optic measurements, to classic surveying measurements. The walls must protect the soil from excessive displacement and protect the excavation against groundwater inflow. If the wall is not watertight, deepening the excavation may cause a sudden water inflow. Leak and erosion process thermal monitoring is a proven leak detection method. It is based on the tests utilizing heat and water transfer process relations, which are coupled processes. Another tool for verifying numerical models is the installation of DFOS (Distributed Fibre Optic Sensors) at the stage of executing structural elements (e.g., diaphragm walls, barrettes, foundation slab). It allows one to permanently monitor both temperature and displacements during element execution (concrete curing), and following facility construction and operation stages. The paper presents methods for designing selected monitoring elements of engineering objects, based on calculations using the Finite Element Method. The verification of numerical models, based on data obtained from DFOS, classic surveying measurements and thermal monitoring, is also presented.

1. Introduction

The construction of new structures and buildings within an urbanized area leads to increased interaction between existing buildings and infrastructure, and forces the designers to plan more and more complex structures, due to the limit in terms of actions between neighbouring structures. Appropriately early and precise identification of threats enables efficient prevention of construction failures, effectively limiting their adverse outcomes, including, primarily, the scale of required repairs and downtime during construction [1]. Automated structural and environmental monitoring of increasing complexity is required to determine the impact, information on exceeded permissible values, followed by taking protective measures.

This can be facilitated by modern measurement techniques that enable a monitoring network with a high measurement resolution to be obtained (at low costs relative to the investment costs), which provides information in a continuous and automated manner.

In an increasing number of cases, the proper and effective design of a monitoring network for a complex structure is based on numerical modelling, which is used to locate critical structural elements, estimate the expected displacement and strain values, etc., as well as to determine the location and range of measuring devices. Numerical modelling utilizes a developed arrangement diagram for soil and groundwater strata within the medium. A numerical model should involve the mapping (as real as possible) of soil strata deposited within the subsoil, and the materials shall be grouped into packages of similar deformation parameters.

It is particularly important in highly urbanized areas, where a very developed underground infrastructure collides with a diverse arrangement of geotechnical strata and several groundwater levels. Such cases require the development of spatial (3D) models. Next, after applying initial boundary conditions, the model is supplemented with structural elements of existing facilities and infrastructure. The development of a numerical model involves adopting several assumptions that should be verified using implemented monitoring (e.g., tightness of soil strata and elements used to limit seepage in the area of the investment site).

Based on the measurements obtained from the construction stage, the results of the FEM (Finite Element Method) modelling can be analysed and compared on an ongoing basis and the reasons for any discrepancies in the results can be sought. The discrepancies may be due to anomalies occurring in real-world conditions (impossible to detect from the structural design stage data) or technical aspects of the numerical modelling. Finding the cause (e.g., by comparing displacements) may allow for introducing necessary adjustments to the numerical model and result verification.

The authors have tried to describe all the above statements in detail below, referring to examples of using the monitoring design method based on numerical calculations.

2. Numerical Modelling

Numerical calculations are most usually conducted during the structure and their foundation engineering process. The developed models exhibit different detail in degree and scope, depending on the project preparation stage and in order to conduct the calculations. Another example of using numerical models involves expert activities in the event of issues arising within the site or in its vicinity (e.g., exceeding adopted monitoring limit values or damage to neighbouring structures).

Models developed to design a monitoring system or recreate failure causes shall contain important structural elements and its phased execution while mapping all static diagrams. Potential drainage shall also be taken into account, besides execution.

The following deep foundation numerical model staging scope shall be adopted as minimum:

• Subsoil initial stress generation;
• Embedded retaining walls execution;
• Dewatering the excavation;
• Excavation with successive wall propping (strutting or anchoring);
• Foundation slab execution;
• Building underground part of structure;
• Completion of excavation dewatering;
• Building above-ground part execution;
• Building operation period.

Figure 1 shows selected diagrams of the numerical model for the A14 station of the Warsaw Underground construction. The following division into stages was applied when developing the A14 underground station model:

• Subsoil initial stress generation;
• Execution of diaphragm walls down to −20.7 m b.g.l (below ground level)—Figure 1a;
• Excavation execution to level −4.55 m b.g.l.;
• Execution of the first row of anchors;
• Excavation execution to level −6.65 m b.g.l.;
• Execution of the second row of anchors;
• Excavation execution to level −11.85 m b.g.l.;
• Tubular struts at −10.85 m b.g.l.—Figure 1b;
• Excavation execution to level −14.60 m b.g.l.;
• Execution of foundation slab—Figure 1c;
• Execution of walls and columns to intermediate floor level;
• Execution of walls and columns up to underground station floor—Figure 1d;
• Excavation backfilling.

Determining the impact on neighbouring structures also requires studying the subsoil under these structures. This is why surveying cannot be limited to the footprint of the newly designed building, but must also cover the area required to analyse earth pressure on walls or anchor calculations and the subsoil near existing adjacent structures. The deep foundation impact range depends on the subsoil type and adopted excavation construction solutions.

Table 1. Various criteria of a deep foundation impact range.

Criterion	Source	Range	Additional Remarks
1	[3]	1.5 ÷ 2 Hw	non-cohesive soils: fine and medium sands, gravels
2	[4]	2 ÷ 2.5 Hw	London clays and glacial clays
		2 ÷ 3 Hw (max. 5 Hw)	stiff cohesive soils
3	[5]	2 ÷ 4 Hw	London clays and glacial clays
4	[6]	2.0 Hw	in sands
		2.5 Hw	in clays
		3.5 Hw	in silts
		100 m	no subsoil data
5	Russian standard [7]	30 m	at preliminary analysis
6	Russian standard [SP22.13330.2016] at preliminary analysis [8]	5 Hw	with the anchored excavation protection structure, but no more than 2 Lk, where Lk is the anchor length (free and fixed length)
		4 Hw	with the sheet pile wall that acts as a cantilever or propped (with steel struts), as well as in the case of an open-pit excavation
		3 Hw	with the diaphragm wall or pile wall technology, acting as a cantilever or propped (with steel struts)
		2 Hw	with the diaphragm wall or pile wall technology structure, and using the top-down excavation method

lists various criteria for determining a deep foundation impact range, usually expressed as its multiplied depth H_w [2]. In the event of a preliminary analysis of Warsaw soils, a distance equal to three excavation depths, counting from its footprint, shall be adopted as the minimum surveying range.

Adopted numerical model sizes shall cover the impact area and the structures of neighbouring buildings located within the impact range. Correctly adopted numerical model sizes shall be verified after the calculations. The sizes shall be selected so that the boundary conditions do not affect the obtained results. In the event of a model for deep foundations, the zero isoline for vertical displacements should fall within the model outline at each calculation stage. Figure 2 shows a correct spatial model example, as a view verifying the position of the zero isoline for vertical displacements.

The calculations shall also use appropriate constitutive models. In the case of the soil, the model should map non-linear soil behaviour and describe stiffness reduction together with increasing deformation—Figure 3. The Hardening Soil model can be used in simple cases, whereas, in more complex cases, its expansion with the small deformation range can be used, i.e., Hardening Soil Small (HSs) and Hardening Soil Brick (HSB), while it should be emphasized that the Hardening Soil Small model applicability is limited, and it should not be used when an analysis contains a load direction change—Figure 4. The Coulomb–Mohr model, i.e., elastic and ideally plastic, should not be used in calculations to determine displacements and the design impact range.

Apart from soil description, it is also worth using non-linear structural description models through taking into account phenomena associated with rheology or profiling for displacement determination.

The developed numerical models should be verified—preferably using a properly designed monitoring system, starting from the very beginning of construction. The verification of the model enables the determination of whether it is adequate for the implemented project, verifying the parameters of materials used in calculation or other assumptions adopted when developing it.

A calculation and monitoring data comparison example involves vertical displacements for a skyscraper foundation slab at one of the construction stages. Figure 5a shows a settlement map based on measuring benchmarks in the slab. Figure 5b shows settlements calculated in the ZSoil 2020 software v20.07. The general nature of the displacements and extreme values is the same, proving that model development and adopted calculation parameters are correct.

3. Monitoring Design Principles

The PN-EN 1990 [11] standard contains a recommendation to conduct “inspections at the engineering, execution and maintenance stages…” to complete a structure as closely corresponding to the requirements and assumptions adopted in the design. Although the application of standards is no longer mandatory, good practices invariably emphasize the significance of systematic, cyclical building condition assessment, stressing the importance of objectivity, reliability and completeness of the premises of such an assessment.

In terms of inspection, diagnostics scope depends on facility type. An urbanized environment, in particular, is characterized by increasingly dense underground infrastructure, more and more intensive interactions between neighbouring structures and growing consequences of potential, even short-term operational restrictions, associated with the need for observations, measurements or renovations.

The conditions above indicate the growing importance of information required for a reliable and appropriately complex impact assessment. In addition, the ever lower operational risk acceptance threshold forces engineers to look for new, innovative solutions in terms of monitoring, which could satisfy the requirements of today and tomorrow that are predicted for the designed service life of a structure.

Underground foundations and co-existing infrastructure are subject to various impacts throughout successive construction and operation stages. The most important ones are associated with the broadly understood interaction with the surrounding soil. Such impacts are determined by factors that can significantly change their properties over time. These are, e.g., external loads, but also groundwater impact changes and often underestimated changes in the values of soil material parameters. This is why correctly assessing the condition of adjacent soil and factors that can potentially impact its changes is extremely important to determine the impact properly. In the case of an untight wall and a change in the conditions due to caused by seepage soil deformations, the stress in structural elements can significantly increase relative to actual initial values, exceeding design values periodically or permanently.

An appropriately early identification of threats enables the efficient prevention of failures, which allows stakeholders to effectively limit their adverse outcomes, including, primarily, the scale of required repairs and the duration and degree of operational and functional restrictions. When defining monitoring conditions, please note that it is not only limited to a set of observations, although observations are the key component. Monitoring (from the Latin “monitor”, “admonitor”—adviser, warning/reminding person) means an activity focused on the observation, notification and detection of threats. The selection of proper monitoring system solutions requires a pre-determination of the type of potential threats, their spatial distribution, technically identifiable associated processes and phenomena, type and scope of physical parameters enabling the detection and tracking of threat development, as well as their possible development rate. Moreover, it is crucial to precisely set out the principles of information/notifications on a threat occurrence or possibility/risk of occurrence. From a functional perspective, a monitoring system should consist of two fundamental components, namely, observational (measurement) and warning (supporting observation result interpretation and suggesting necessary actions). Designers and investors of buildings are increasingly faced with the dilemma of choosing the technology to solve the base monitoring system.

The following can be used to assess the impact of executing a deep foundation structure on subsoil displacements:

• Criteria based on statistically elaborated measurement results, collected in various regions of the world, for various geological and soil conditions, as well as for different execution technologies;
• Local criteria based on statically elaborated measurement results for various soils, technologies and adjacent infrastructure type;
• Relationships based on elaborating numerical modelling results [12];
• Numerical models developed for the analysed investment project [2].

In many cases, the spatial or temporal dynamics of controlled process variability are significant enough to virtually eliminate the possibility of using a traditional measurement network, due to time-consuming measurements and result interpretation potential. Under such conditions, it is particularly justified to apply a solution that enables automatic measurement, result archiving, and preliminary interpretation, within a practically acceptable time frame. Modern, automated monitoring systems guarantee obtaining the required spatial and temporal density, even in the case of extremely vast structures and the observation of processes exhibiting high variability in the time domain. Regardless of the decision on the selection of measurement technologies, please note that ensuring a sufficiently high result, in terms of credibility level, requires a monitoring system to be fitted with certain reasonable redundancy, which would enable mutual measurement result verification, measurement error evaluation, gross error elimination and system immunization to the outcomes of failure and malfunctions of basic components, at least in terms of its most important functions.

Designing a monitoring system and selecting appropriate sensor locations should be based on analysing forecast results obtained from numerical modelling. When designing sensors (also linear sensors), the sensor operating range and limit threshold can be determined using the results from model grid nodes, which simulate sensor location. Monitoring these values enables the analysing and designing of thresholds, when a model is subjected to a load variant. Exceeding such thresholds is limited through the permissible displacement of adjacent structure or underground infrastructure facilities. Such values are set out in national standards and guidelines.

Device type and range selection can be based on numerical calculation results. Various devices enable verifying assumptions at different structure construction phases. Surveying and inclinometric monitoring should be conducted from the beginning of implementation.

Calculations required to determine expected values within the implemented monitoring should involve characteristic parameters and appropriately collected loads. Combinations considering structure dead load and potential operational loads, arising from the construction, are the best for construction stage monitoring. Apart from safety, monitoring is aimed at confirming the calculation assumptions of the analyses.

Determining warning and alarm values is an important aspect of a monitoring design—it should be based on the condition of the existing monitored structure and its subsoil. The value obtained in numerical calculations, related to the design, cannot be directly adopted, because if the model is developed correctly, these are expected values and will occur in the course of the implementation. Boundary values specified by standards and, potentially, the facility’s owner or administrator guidelines, should be compared to the results of numerical calculations. From this, it can be determined whether the implementation of the new project threatens the existing building or structure. Additional analyses may also be performed to show which displacements will have an adverse effect on adjacent infrastructure. Only these values can be accepted as warning or alarm values. The warning and alarm thresholds should be set to limit values, not expected values. Sensor range and locations should be selected using the numerical simulation result analysis. Figure 6 shows a diagram of a numerical model with correctly selected dimensions, used to determine the impact on the neighbouring building and underground infrastructure, in the form of a large-diameter collector.

Numerical simulations make it possible to determine the displacements for any model node. Figure 7a shows a numerical model for determining expected displacement values, in the vicinity of the building under construction. Figure 7b shows the line along which the displacement values will be identified, to determine expected values on sensors placed at this location. Figure 8 shows determined vertical displacements along the assumed location, shown in Figure 7b.

4. Selected Monitoring Technologies

4.1. Thermal Monitoring

Thermal monitoring utilizes heat and water transfer process relations, which are coupled processes. According to original assumptions, this method was developed to monitor leakages and erosion processes in damming structures, such as dams and embankments [13]. It is currently recommended for this purpose by the International Commission on Large Dams (ICOLD) [14] and has also been successfully used in damming structures in Poland [15]. Selected tools within this method have been adapted, with very good outcomes, for detecting leaks in deep excavation retaining walls [16,17]. The thermal monitoring method is currently the most accurate and recommended method for detecting leakages in deep excavation retaining walls [18,19].

Thermal monitoring is based on analysing the variability in temperature flow in the ground. Privileged seepage paths, such as wall leaks and erosion processes in soil (washing out of soil particles with water flow), lead to changes in the water flow rate and direction, thus, causing characteristic disturbances to the soil’s thermal field, which enable detection and analysis of both leaks and, indirectly, erosion processes [13,20,21,22,23].

The thermal monitoring method can be divided into two types. Both methods, a measurement procedure description and the instrumentation, were presented in the publications by Radzicki [13,15]. The first one is a passive method, where groundwater temperature changes are measured and analysed. The first method is the passive method—uses a heat source in the medium, e.g., heating “warm diaphragm wall” or cut-off wall, water of different temperature, e.g., in the tank and ground, in the collector (pipeline) and ground.

The second method is an active one—it introduces heat to the medium, because there is already a “cold wall” or the wall is made of, e.g., sheet piling, or the difference in soil and structure temperature is too low. Active thermal testing involves introducing probes integrated with heaters into the ground and studying the distribution of generated heat—determining the difference in cooling time of individual probes. Analysing the heater heat dissipation characteristics enables leakage detection.

Embedded retaining walls can constitute elements of underground building structures, but must certainly ensure excavation wall stability and protect the excavation against groundwater inflow [2,24]. The application of a tight wall aims to limit the drainage range to its outline and reduce the structure’s impact on neighbouring buildings and infrastructure [25]. Introducing vertical cut-off walls into the ground often leads to groundwater damming and a change in their flow direction to outside of the wall, hence, a change in the water and soil conditions within the area of the implemented construction project.

Water inflowing to the excavation can be limited by extending the seepage path (using diaphragm walls or sheet piling), constructing walls or piling up to the ceiling of poorly permeable strata, or executing a horizontal cut-off wall within the diaphragm wall outline, below the foundation slab floor elevation. If the wall is not tight, excavation sinking with simultaneous drainage within the footprint set out by walls may lead to a sudden inflow of groundwater inside it. It is a process that can cause soil washout by groundwater inflowing from outside the wall and, hence, lead to the destruction of its structure due to erosion and suffosion [26,27]. This, in turn, may result in the formation of a hydraulic piping, and ultimately, in wall failure and soil deformations outside of the excavation, provoking a failure, or even a construction disaster, related to the building under construction or adjacent structures [2].

When analysing leakage, two groups can be distinguished [2]. The first group consists of subsoil surveying errors. The second group consists of wall execution errors—leaks caused by diaphragm wall execution errors. A wall opening may be caused by ground-fall inside the diaphragm, improper section abutment formation due to selecting incorrect separators, routing the contractor pipe above the bentonite slurry, and pollutants penetrating into the diaphragm.

A leak may occur at different excavation outline locations and may cause destructive phenomena in the soil medium surrounding the excavation. An example of a potential leak zone includes walls connected with the horizontal impermeable soil stratum in the ground or a tight cut-off wall instead if it is missing (e.g., in the “jet grouting” technology).

Figure 9 shows leak or piping cases in the area of a deep excavation, discussed by Clough and O’Rurke [28], Popielski [2] and Kłosiński [29]. Symbol 1 marks the seepage direction of water flowing through the wall or bottom of the excavation, while symbol 2 marks the zone of potential erosion, leading to a relaxation and change in parameters, or even formation of a cavity in the ground. Diagram (A) shows a leak through walls, (B and C) horizontal cut-off wall looseness, (D) subsoil ground stratum looseness, (E) leak along the outline of the structure—privileged seepage path, (F) leak caused by water damming due to wall, and (G) illustrates a development scenario for an erosion process at a certain distance from the excavation, due to a leak event. Case (G) is a schematic illustration of an internal erosion process that can occur even at a significant distance from the excavation, and is associated with the intensification of groundwater flow and erosion phenomena towards the excavation. This process can also influence the foundation conditions of buildings and infrastructure (e.g., pipelines, collectors and tunnels).

The occurrence of the aforementioned leaks and erosion processes may lead to excavation inundation or flooding, wall deformation, destruction of the bottom of the excavation or wall, change in the water and soil conditions of excavation surroundings and the foundation conditions of adjacent buildings, ground settlement outside of excavation and ground collapse [2]. Possible consequences of such events include extended construction time, repair and/or need for retaining wall reinforcement, redesigning the constructed building due to changes in the subsoil around the excavation. On the other hand, ground deformations outside the excavation may damage nearby buildings (Figure 10) or underground infrastructure and lead to compensation proceedings.

The accurate detection of any leaks in the wall of the planned excavation, prior to commencing excavation sinking, is very important in executing an underground building section, in an efficient and timely manner and avoiding the aforementioned issue.

A leak in the embedded retaining wall, natural tight soil layer or horizontal cut-off wall executed below the bottom of the excavation, can be detected through testing in the course of draining the zone inside the outline of the planned excavation, and prior to its sinking. Tightness can be assessed through measuring the temperature (thermal monitoring method) using probes, enabling temperature measurements [13,16].

The passive method is the most common and most cost-efficient thermal monitoring method for deep excavations. Cement-based materials are used for constructing deep excavation walls (e.g., diaphragm walls, pile walls). Hydration heat is released during concrete aging. This leads to increased temperature (“warm wall”) in the wall and the subsoil in the vicinity of the wall elements. Due to the low thermal conductivity of the soil, the subsoil temperature remains elevated for a prolonged time. Water flowing through untight wall spots causes an intensified, advective inflow of external heat (cooler groundwater from outside the wall), which leads to a local, rapid temperature drop in the heated soil near the leak. Depending on the case, hydration heat may be used for leak detection for up to several weeks after completing the wall.

Excavation execution with leak verification, using any thermal monitoring method, involves the following stages:

• Execution of embedded retaining walls (diaphragm walls);
• Installation of vertical temperature measurement profiles within the outline of the planned excavation, drainage wells and piezometers;
• Pumping water from inside the wall, which generates differential pressure between the external and internal water level and its flow at untight wall locations;
• Checking water discharge from the well and water levels in piezometers outside the wall and within the excavation outline;
• Using the thermal monitoring method to detect soil temperature field disturbances caused by leaks;
• Precise sealing of detected leaks from the surface e.g., using grouting;
• Excavation execution.

The tests are conducted using sensors inserted into the soil. The sensors are used for developing vertical soil temperature profiles. A diagram illustrating the application of a thermal monitoring method for detecting structural leaks within the planned deep excavation is shown in Figure 11.

Figure 12 and Figure 13 show selected temperature distributions, resulting from conducted numerical simulations. In the case of the model without leaks, the temperature increase in the soil along the diaphragm wall length is the same in each direction and results from the thermal conditions and the process of heat conduction only—Figure 14a. When it comes to the results of the variant with a leak, the range in temperature spread is greater and is located in the leak area—Figure 14b. The range of a diaphragm wall thermal impact on soil temperature distribution is approx. 2.0–4.0 m. In the diaphragm wall leak case, analysed in Figure 12, it can be seen that the water flowing through the opening is heated up. This is the case when a hydration process occurs in the wall and the seepage process is relatively slow. In such cases, locating a leakage is possible by searching for the maximum temperature.

In the event of an intensive leak, or when it is not intensive but the diaphragm wall is cooled down, cold water flows into heated soil in the excavation. In such a case, leaks may be detected based on decreasing temperatures, relative to the heated soil, through the heat dissipated from the diaphragm wall prior to commencing drainage.

Figure 13, Figure 15 and Figure 16 show the results of thermal monitoring modelling in the active method version, when locating leaks through a diaphragm wall. Preparing the structures for measurements is similar to the passive method’s case and is shown in Figure 11. The active method involves using different devices—the thermal tests are based on introducing probes integrated with heaters into the soil and studying the distribution of generated heat.

Figure 13 illustrates cross-sections through a fragment of the numerical model, used for the analysis. Dashed lines mark the cross-section locations. Figure 13a outlines a diaphragm wall with a gap, which illustrates a leak, measuring probes (red points) and a soil drainage well within the diaphragm wall outline (blue point). Figure 13b shows a vertical section. The diagram shows a diaphragm wall outline with a gap that illustrates a discontinuity, measurement profile (red line) and wells (blue line).

Calculations of heat transfer and flow due to drainage require a coupled analysis of transient seepage and thermal conditions. Besides adopting the parameters related to the water flow, i.e., seepage coefficient and selected model describing soil saturation changes for the calculations, one should also introduce heat conductivity coefficients and volumetric heat capacity. These parameters should be determined through laboratory testing. Obtained results include pressure, seepage rate vector and temperature distribution within the model, for given time instants.

Examples of analysis results are shown in Figure 15 (as a cross-section) and Figure 16 (as a projection). Temperature distributions within the models for various time instants, after the start of the water heating and pumping, are presented. The white line in the cross-sections marks the groundwater table position. Figure 15a shows that the heat below the groundwater table dissipates much faster than above, due to full soil saturation and water flow. The application of linear sensors within the thermal monitoring method may also determine the groundwater table position.

Based on Figure 15 and Figure 16, it can be observed that the temperature recorded by the sensors drops the fastest in the vicinity of the leak. Measuring the sensor cooling time enables the determination of the water flow rate in the soil. Comparing Figure 16 and Figure 17, it can be additionally seen that the heat dissipation direction coincides with the groundwater flow direction, which is quite obvious. Therefore, using a larger number of sensors makes it possible to determine, besides seepage rate, the groundwater flow direction. Laboratory testing devoted to this issue is presented in [31].

4.2. Distributed Fibre Optic Sensors

4.2.1. Technology Description

One of the modern measuring technologies used in an increasing number of cases in the construction industry involves Distributed Fibre Optic Sensors (DFOS). They were initially used in aviation and aeronautics due to the high degree of responsibility and implementation of novel solutions. They were used for, among others, measuring the deformations of aircraft wings or spacecraft fuselage plating elements [32,33,34]. Currently, fibre optic sensors are increasingly used in other fields of engineering, including infrastructural facilities, such as bridges, tunnels and deep foundations. They exhibit very high-resolution measurements at low purchase and installation costs, in terms of the optical fibre itself. They can replace classic vibrating wire, electrical resistance and electromechanical sensors, owing to the possibility of geometrically continuous measurements along the entire sensor length, with a minimum resolution of 1 cm and a maximum measurement distance of up to 100 km (depending on the needs). Classic sensors are point-type sensors; therefore, their sizes, unit cost and installation method prevent achieving such high ranges and resolutions, as in the case of DFOSs [35].

DFOS ensure good spatial resolution, owing to high sensitivity. They enable real-time indications of not only, e.g., that a pipeline is subject to disturbing mechanical stress at an unknown location, but also, of the precise location of the fault—in some cases with an accuracy of centimetres, which is sufficient in the perspective of large-diameter linear structures. The service team can be immediately sent to fix the issue, before it leads to serious damage.

Fibre optic sensors can measure deformations or temperature, depending on the applied measuring device. This primarily depends on the frequency of the analysed signal, and discrete peaks within the electromagnetic spectrum are determined as Rayleigh, Raman and Brillouin peaks [36]. DFOS fibre optic sensors use natural imperfections in the optical fibres, which lead to backscattering of the light at a specific wavelength.

4.2.2. Technology Description II

Some important structures, where fibre optic sensors play a significant role, are foundations of road and rail structures and high skyscrapers. Their monitoring system can provide resourceful information to understand the interactions of structures within the soil environment.

DFOSs were used, e.g., in London, where the fibre optic sensors were fitted to eight reinforced concrete test piles, at three various sites in the city. Fibre optic cables were fixed to reinforcement baskets, using mounting bands at intervals of approx. 1 m. The instrumentation of rebars consisted of a single DFOS circuit, with temperature and strain cables that could be simultaneously monitored, from a single optical reflectometer (BOTDR) channel. In addition to DFOS cables, all eight piles were fitted with conventional instruments that consisted of VWSG (strain gauge) pair and recoverable extensometers, at several locations along the reinforcement. The reinforcement baskets were also fitted with temperature sensors, at 30 cm intervals, to measure the concrete hydration temperature. These three instrumentation systems were used as reference points to access DFOS data accuracy. The authors of [34] present detailed test results, confirming that DFOS sensors not only ensure continuous measurements, but also enable data to be obtained that would traditionally require the application of three various measurement systems.

Important elements of a city’s critical infrastructure can be located within the impact zone of deep foundations. A fibre optic system was used to monitor the condition of the existing collector, in the course of implementing the Grand Paris Express project. The reinforced concrete collector was located in the impact zone of newly implemented construction projects—underground tunnels and stations. For this purpose, it was decided to subject it to, among others, monitoring, using fibre optic sensors. The cited example was presented in [37]. Similar solutions were applied in Warsaw, when developing a monitoring system project for the period of repairing and modernizing, followed by exploitation of a Burakowski sewage collector.

4.2.3. DFOS in the Burakowski Sewage Collector

The repair and modernization of existing underground urban infrastructure and the construction of new network (utilities) sections lead to an increased degree in their interactions. Direct access to the structure that is required during the technical condition assessment is becoming increasingly difficult, or even impossible in many cases. However, it is necessary to constantly maintain a certain technical level of the infrastructure that guarantees its efficiency and safety.

One of the examples of a typical underground infrastructure modernization is sewage collector and pipeline modernization, using technologies based on existing pipes or tunnels. Such a renovation process is often associated with placing lining or panels inside the existing structure (increasingly more often made of plastics), adapted to the cross-section of the structure. This completely cuts off access to the existing structures of a collector or pipe, making it impossible to assess its condition. Therefore, the process requires a monitoring system (Structural Health Monitoring SHM) that enables the acquisition of measurement data and a collector’s technical condition assessment based on such information. It also provides advantages that allow for the monitoring of the new structure, placed outside the existing one.

The application of DFOS (fibre optic sensors) that continuously measure displacement and temperature changes along network sections with a high resolution, enables the creation of a permanent and deformation-sensitive monitoring system, at large distances along the axis of the structure, and at low costs of the very sensors, relative to the investment project (renovation) costs.

Assuming a constant and systematic modernization of the network, and other underground infrastructure facilities in cities, it is possible to construct a “neural system” of modern cities in the future. It would be based on the real-time transmission of data from linear units, while simultaneously signalling unexpected or progressing failure.

Given the high resolution of the fibre optic technology, the system would also be able to record changes in the direct surrounding of the structure. The authors believe this to be the right direction for the development of underground infrastructure SHM, assuming that the applied monitoring technologies would be standardised and added to a smart city management system.

Identification of the benefits and advantages of developing a network monitoring system in cities lead to attempts of using sensors on plastic pipes, where the installation methods and displacement result verification were checked [38]. This was followed by implementing a pilot monitoring system section, based on the DFOS technology in Warsaw, with cooperation between the Municipal Water and Sewage Company in Warsaw (MPWiK) and the Warsaw and Kraków Universities of Technology. The analysis covered a 4.8-km-long, concrete-casing sewage collector, in a bad technical condition, which was subjected to renovation by reinforcing the existing structure using GRP panels [39].

Due to the significant role of the collector within the Warsaw wastewater transfer system and the renovation method cutting off access to the collector’s concrete structure, the following issues have been defined, which require solutions:

• Assessment of the modernized collector structure’s technical condition;
• Tracking the development of cracks identified in the course of the technical condition inspection, covering the existing concrete casing of the collector;
• Maintaining the original collector structure during conducted renovation work, especially maintaining identified scratches and cracks, and the tracking of new ones occurring at key work stages, i.e., GRP panel placement inside the collector, diaphragm gap grouting process, grout setting process;
• Monitoring the cooperation between GRP panels and the concrete casing after completed renovation and during operation of the renovated section, e.g., under extreme operating conditions (complete filling).

Monitoring covered the section of the renovated Burakowski sewage collector fragment section, which experienced the highest structural damage. The pilot section, fitted with sensors, was approx. 146-m-long and spread between two inspection chambers.

Two sensor lines were installed in the L and R collector horizontal axis (at 3 and 9 o’clock), and one line along the collector axis in the top T (12 o’clock) (Figure 18), in pre-prepared longitudinal grooves in the concrete structure of the existing collector.

The installation of sensors in grooves and their embedding in a chemical anchor enables them to be protected against mechanical damage when installing GRP panels, and ensures correct cooperation between the sensors and the existing concrete casing of the collector. Figure 18 and Figure 19 show a sensor installation diagram, and Figure 20 shows photos taken during the installation of the sensor in the collector.

Deformation measurements were conducted during renovation work, in four measurement sessions: successively—prior to placing GRP panels inside the collector (baseline measurement) —after placing the GRP panels (prior to grouting), then after the grout filling between panels and the existing structure, and during operation, after filling the collector with sewage. Measurement phasing allowed one to observe the operation of the structure, subjected to a gradual load change.

Figure 21 shows examples of measurement results, obtained using DFOS, located at the top (T) of the collector part. The cracks have cramped due to additional load introduced inside the collector (P4—during operation + filling with sewage). Although these changes are very minor (0.01 to 0.02 mm), they are clearly identifiable. The results above prove the very high sensitivity of fibre optic sensors, which enables the detection of deformations and damage, at a very early development stage. They would probably be unnoticeable, even during a site visit inside the collector (Figure 22).

The numerical model for a collector section (taking into account its renovation stages) was based on geotechnical surveying, groundwater histogram and the material parameters of the collector structure. The obtained results confirmed approximated values of displacements, caused by placing panels inside the collector, at the section between the inspection chambers and backfilling of the space between the panels and the existing structure with grout (Figure 23 and Figure 24).

5. Conclusions

Monitoring conducted in an ongoing investment project can be based on numerous technologies, from fibre optic measurement, thermal monitoring and laser scanning, through various strain gauges, inclinometers and pressure sensors, to classic surveying measurements. Surveying measurements, together with currently achievable accuracies of elevation determination, still remain the most reliable source of information on displacements experienced by the subsoil and the structures located near the newly constructed building. A numerical model can be used to forecast displacements during individual structure execution stages and determine the correct location for measurement points. On the other hand, it should be verified based on surveying monitoring results, conducted since the beginning of the construction phase. An example of the validity of using numerical calculations in monitoring design are the projects presented in this paper. Thermomonitoring enables the detection of leaks in diaphragm walls or sheet piling. It can detect leaks much faster and more accurately than simple water level measurements in piezometers. Numerical calculations can complement field measurements to better determine the scale of the leak and its possible impact on the condition of the soil in the excavation area. The results obtained in the Burakowski collector, using DFOS, confirmed the approximate values of displacements caused by placing the plates inside the collector, and confirmed the predicted character of displacements from the numerical model. Pre-emptive monitoring (prior to project implementation commencement), often with the use of unusual measurement point locations, arising from the individual nature of structural actions, is required in the event of determining the impact of neighbouring investment projects.