Keywords

6.1 Introduction

Taking into account that hazard (or cause) is ‘a potential threat to humans and their welfare’, risk (or consequence) is ‘the probability of hazard occurrence’, and disaster is ‘the realisation of hazard’, then disaster risk management may be defined as the process of systematic application of management policies, procedures, and practices to the tasks of identifying, analysing, assessing, treating, and monitoring risk (Fig. 6.1a). It involves:

Fig. 6.1
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Natural disaster risk management: a components of risk assessment; b emergency management cycle

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  • a formal, quantitative evaluation of potential injury or loss over a specified period of time;

  • the prospect of future malperformance of a safety or security systems.

Many so-called “natural” hazards have both natural and human components. While hazard can exist even in an uninhabited region, the risk can occur only in an area where people and their possessions exist.

Emergency is realisation of a hazardous event, which requires the organisation and response of the society other than in normal condition. Emergency management (Fig. 6.1b) comprises measures such as organised analysis, planning, decision making, and assignment of available resources with an aim to:

  • prevent,

  • mitigate (lessen the effects of),

  • prepare for,

  • respond to, and

  • recover from the hazard effects (Milutinović 2006).

For building a culture of natural disaster risk management, the greatest energy should be devoted in building the three pillars of sustainability:

  • leadership development (people asset);

  • capacity development (physical asset);

  • awareness development (public awareness, training, and education programs).

Preventive measures and activities are the best mean against natural disasters and of protection of human lives and social goods.Footnote 1,Footnote 2 Prevention arrangements are of long-term nature comprising constant governmental and professional activities, in order to establish consistent scientific bases and their practical application in prevention and mitigation of natural disaster risk.

In this respect, the chapter is dealing with the risk management considering hazardous geodynamic natural phenomena (landslides and earthquakes) and discussing several important aspects of problems related to geotechnical engineering:

  • the up-to-date approaches to identification, analysis, and assessment of risks imposed by natural disasters involving geodynamic events;

  • the treatment of these risks by application of the contemporary design, geotechnical, and constructional measures for slopes and associated supporting structures, foundations, and tunnel structures;

  • the monitoring of the risks along with the development of the appropriate monitoring and early-warning systems.

6.2 Potentially Hazardous Geodynamic Natural Phenomena

Natural hazard events have both immediate and longer term effects upon people, physical structures, and economic activities. Geodynamic natural phenomena, such as landslides and earthquakes, are among the most hazardous natural threats to human lives and property. The recent studies have revealed that these hazardous natural phenomena resulted in the greatest number of human deaths, as well as in the highest economic losses, when compared to other natural disasters (Fig. 6.2).

Fig. 6.2
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Source Munich Re 1996 (Milutinović 2006)

Natural disasters worldwide 1960–1995: a death toll (3,000,000); b economic losses (439 billion US$)

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6.2.1 Landslides—Characteristics and Special Problem Areas for Emergency Management

  • Speed of onset is mostly rapid; warning period may vary; little or no warning may be available if the cause is earthquake; however, some warning may be assumed in the case of landslides arising from continuous heavy rain; minor initial landslips may give warning that heavy landslides are to follow; natural movement of land surface can be monitored, thus providing long warning of future possibility of landslides;

  • Damage to structures and system can be severe (building may be buried or villages swept away); rivers may be blocked, causing flooding; crops may be affected; sometimes areas of crop-producing land may be lost altogether (e.g. in the major slippage of surface soils from a mountainside); when landslides are combined with very heavy rain and flooding, the movement of debris (e.g. remains of buildings) may cause high levels of damage and destruction;

  • Search and rescue actions are associated with difficulties in access and movement in affected areas; risk of follow-up landslides may hamper response operations;

  • Rehabilitation and recovery may be complex and costly; in severe cases it may not be possible and/or cost-effective to rehabilitate the area for organised human settlement.

6.2.2 Earthquakes—Characteristics and Special Problem Areas for Emergency Management

  • Earthquake onset is sudden, usually no warning; following a major earthquake, secondary shocks may give warning of a further earthquake activity;

  • Major effects arise mainly from violent ground movement (vibration), fracture, or slippage; especially they include widespread loss of or damage (usually very severe) to structures, lifeline systems, essential services, and life support systems, as well as considerable casualty due to lack of warning;

  • Severe and extensive damage, creating the need for urgent counter measures, in particular search and rescue, and medical assistance;

  • Difficulty of access and movement;

  • Response problems may be severe, extensive, and difficult (e.g. rescue from a high occupancy building collapses, or in a circumstances where additionally a chemical or radiation hazard exists, etc.);

  • Victim identification may often be very difficult;

  • Recovery requirements may be very extensive and costly; recover from the seismic hazard effects may possibly take 5–10 years or even more.

6.2.3 General Counter Measures

While the characteristics, nature, and extent of effects vary according to the particular natural hazard and the particular physical characteristics of the area affected, the most of counter measures are somewhat ‘generic’, and are based mainly on set of measures aimed at impending the occurrence of a disaster event and/or preventing such an occurrence having harmful effects (prevention), and/or set of measures and activities aimed at reducing the impact of a disaster on a nation or community (mitigation). Whilst it may be possible to prevent some disaster effects, other effects will unavoidably persist, but they can be reduced provided appropriate measure is taken.

  • Typical non-structural measures:

    • Development and improvement of legal framework (emergency/disaster legislation, land-use regulations, building regulations) and institution building;

    • Development of research and training centres, as well as continuous education and improvement of knowledge of scientists, engineers, and planners;

    • Development and installation of warning systems;

    • Public awareness and training.

  • Typical structural measures:

    • Construction of structures to resist the forces generated by environmental hazards;

    • Strengthening of existing structures to make them more resistant against the environmental hazard forces.

The structural measures shall be developed based on:

  • Adequate site planning;

  • Assessment of forces created by the potential environmental hazards;

  • The planning and analysis of structural measures to resist such forces;

  • The design and proper detailing of structural components;

  • Construction with suitable materials;

  • Good workmanship under adequate supervision.

In the subsequent sections, the contemporary achievements in prevention of landslide occurrence, in investigation of the punching of columns through footings which is the newly-considered phenomenon both under static and dynamic conditions, and in reduction and mitigation of seismic risk for tunnel structures as crucial elements in transportation network are presented, highlighting the state-of-the-art approaches to assessment of forces induced by the potential hazard, geotechnical and construction strategies of prevention and mitigation of adverse natural hazard effects, as well as development of monitoring and early-warning systems.

6.3 Contemporary Approaches to Risk Management for Landslides

Landslides belong to the greatest hazards for the population, material property, and environment. As the population expands, both in terms of habitation and usage of areas, the risks of the emergence of landslides and considerable damage also increase (Fig. 6.3).

Fig. 6.3
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Landslides on the slopes and inclines (Protić and Bonić 2018)

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In order to minimise the risk of human casualties and material damage, it is necessary to have institutions, which would react timely and efficiently. This comprises detecting of the signs of sliding, issuing warnings to the population and competent institutions, evacuation, and eventually, remediation of the damage.

6.3.1 Preventive Measures for Landslides

For the purpose of prevention of potential landslides, it is necessary to implement preventive measures in the most efficient way. The preventive measures include:

  • Mapping of the landslides into the existing maps. In the critical situations, it is necessary to predict what areas would be at the highest risk. There must exist a National Cadastre of landslides, which would be used as a basis for production of prediction maps.

  • An Early Warning System. It is a method of geotechnical monitoring used for the assessment of stability of inclines and slopes. It includes various techniques of instrumental observation and monitoring in the real time. The system must be connected to the Emergency situation sector of the local self-governments and at the national level.

  • Education and capacity building. Training of the local emergency centres and wider public for reporting and recording of landslides in the territories of local self-governments is a significant step in the organised social action focused on mitigation of natural disaster effects. One of the best ways to inform the population on the importance of a systematic approach to the problems related to the landslides is to present the principle of good and bad practice and provide specific instructions and advice during the training.

  • Improvement in the legislative power. Observation of legal regulations in the area of space and urban planning, research and remediation of structures and land are the foundations for reducing the hazard of landslides for the population, material and other property, both at the state level and the level of cities and municipalities. For that reason, it is important to engage professionals and companies registered for landslide inspection and remediation.

With adequate preventive measures, and with functioning of a global system and data bases, the onset of soil movement would be observed more easily and the system would react on time and warn the competent services.

Starting from all this, it is necessary to form a contemporary data base about the processes and phenomena, whose existence and effects can indirectly or directly endanger the stability and function of the civil engineering structures over time, for the purposes of designing and construction of new and maintenance of the existing structures, especially for the housing buildings and the structures of transportation infrastructure. The basis should contain: an inventory of (registry) of phenomena, their history of development, maps of hazards and risks, data on investigation and success of remediation, quality and quantity assessment of the hazards for an area and structures in it, data on the monitoring during construction and operation, as the prevention for timely discovery of instability phenomena and timely undertaking of adequate remedial measures.

The characteristic example is the Tamara cyclone, which hit Serbia and surrounding countries in 2014. In the period from 13th to 18th May 2014 Tamara cyclone hit southeast and central Europe, causing floods and landslides. Serbia and Bosnia and Herzegovina sustained the greatest damage due to the fact that the rainfall exceeding the historical records. Namely, in some localities, the amount of rainfall in three days of May (14th–16th of May) exceeded the monthly average rainfall for more than four times. Until 20th of May, no less than 62 persons lost their lives. The rainfall set the torrential floods and rockslides into motion, and numerous rivers from the Sava and Morava basins flooded (Fig. 6.4a). Over 2000 landslides occurred (Fig. 6.4b). According to the official data, over 1.6 million people were affected in Serbia and Bosnia and Herzegovina.

Fig. 6.4
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Tamara cyclone: a flooded areas in Serbia; b areas in which landslides occurred (www.tanjug.rs)

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Considering that the reaction units in emergency situations in Serbia are the crisis municipal centres, a number of municipalities proclaimed an emergency during the Tamara cyclone. However, the lack of an appropriate landslide cadastre and prediction maps of landslide hazard for most of the Serbian territory, or at the level of large territorial units (municipalities/cities), made the operation of emergency centres difficult, when an urgent reaction in the affected areas was necessary.

For that reason, a campaign of mapping and recording of landslides was conducted by the most eminent competent institutions dealing with this issue. Mapping of the landslides was organised at the level of municipalities and coordinated so that all the municipalities have even criteria and forms for registering of landslides and damage assessment. For every recorded landslide, the following data needed to be entered: exact location, type of phenomenon, dimensions of phenomenon, sketch of the predicted land cross-section, motion mechanism, activation date and previous activity status, as well as the relative and quantitative assessment of hazard and damage level. Yet, given that the data on the landslides were collected by various institutions, the reports differed to a great extent, and considering that the field campaign was focused on the most critical locations, many small landslides remained unregistered.

Regarding that landslides are a great hazard and that the competences of the governmental bodies and public institutions in the Republic of Serbia are divided, there was a need to collect and integrate the data into a single system according to the standards and requirements of the European Union, and to make them publicly accessible to mechanical search services. In that sense, it was necessary to produce uniform reports on such events and use them for the most contemporary hazard analysis and to develop an early warning system. This is a systematic approach to the issue of control and remedy, which provides a more quality spatial planning.

For that reason, completion of the landslide cadastre in certain municipalities was organised in the framework of the project “Harmonisation of data on landslides and training of local self-governments for their monitoring” under the working title “BEWARE” (BEyond landslide aWAREness), which began in May 2015 (Abolmasov et al. 2015). It is a sub-project of the UNDP initiative for improvement of vitality and readiness for a response to emergency situation in the Republic of Serbia, financed by the Government of Japan, and coordinated through the UNDP Office in the Republic of Serbia for assistance and renovation of the flooded areas and the Ministry of Mining and Energy of the Republic of Serbia. The goals of the BEWARE project are:

  • Contribution to the methodology of acquiring, processing, and production/completion of a data basis of landslides through harmonisation and standardisation of data; recording of landslides in target municipalities: production of the map of hazards and risks.

  • Strengthening of governmental bodies, primarily of the Ministry of Mining and Energy and the Geological institute, for regular landslide monitoring in agreement with the good practice in EU states.

  • Production of BEWARE (GIS—geographic information system) web protocol, which represents a platform for inspection and reporting of landslides, and accompanying material including the hazard maps.

  • Building personnel and material–technical capacities of involved municipalities, which can regularly monitor and register landslides in their territories, which is an active participation in completion of the national database of landslides.

A very important role in terms of sustainability of the project is given to the representatives of local self-governments, who are trained for registration of current and future emergence of instability in the territory of their municipalities and cities. For this purpose, appropriate equipment and material was provided and necessary training was conducted. The equipment of conducting of the Project consisted of one tablet device with pre-installed BEWARE android application, one navigation device, and a computer with appropriate software. The task of the user—representative of a local self-government is to record the emergence of a landslide by making a field visit, by locating and memorising the landslide using tablet, and by filling in the accompanying form of the cadastre list and taking photographs of the location. For the purpose of obtaining as real data on a landslide as possible, it is recommended to visit and record the main scarp and toe of the landslide, as well as different characteristic elements, deformations and ridges, wells, and damage on the buildings and infrastructure. The project was realised in 27 target municipalities, which were the most affected by the landslides in 2014 (Fig. 6.5a). Distribution of registered landslides by municipalities, in which the BEWARE project was realised, is displayed in Fig. 6.5b.

Fig. 6.5
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BEWARE project: a municipalities in Serbia where the project was realised; b number of registered landslides by municipalities in which the project was realised (Abolmasov et al. 2015)

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The BEWARE project, allows, among other things, formation of a global database, which can be continuously completed and updated by the trained personnel from the local self-governments. This provides timely information about the changes, which could indicate the onset of danger, so as to react in a proper way and on time.

Further field investigations and analyses are taken over by the appropriate state institutions, such as the Geological Institute and other, which make the prediction maps of hazards and risks. Hazard comprises probability of emergence of a dangerous event (landslide in this case) with specific characteristics, in a specific time, and place. The risk is the measure of expected losses due to the hazard, which took place in a specific area during a specific time interval. The expected losses refer to injuries and human casualties, and material damage.

An example of the hazard map is given for the municipality of Krupanj, which sustained the most damage from landslides from all the mentioned municipalities. The map was made using the AHP method (Analytic Hierarchy Process) and it is displayed in Fig. 6.6.

Fig. 6.6
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Hazard map for the municipality of Krupanj (Krušić et al. 2015)

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The AHP method, as an expert or empirical method, belongs to the so-called multi-criteria analysis (Marjanović et al. 2013), and it uses simple matching of important factors (geological, geomorphological, climatic, hydrological–hydrogeological, and environmental factors) based on a quantitative assessment of their impact on the sliding process (using a predefined scale for quantification). The mentioned factors affect the final model through their weighted coefficients (points), whereby each value indicates the impact of the individual parameter and all the factors are normalised (scaled to the same scale, in this case 0–100%, i.e. 0–1). Since their impact is simultaneously determined, they demand mutual evaluation, that is, quantification of each individual member in respect to any other. Assessment is carried out by experts who have experience with a given type of landslide in a given area of research, independently from one another, in order to harmonise their criteria. In this way, the weighted average coefficients for each of the factors are obtained. By their simple summation in the GIS environment, a finite quantitative model of landslide susceptibility for a given terrain in a given area is obtained. It can be classified into a qualitative model with specified classes, for example, a class of high, medium, and low susceptibility. Finally, it is necessary to evaluate the model by comparing the final model with the cadastre of landslides (by comparing the spatial distribution of the landslide from the cadastre and, for example, the class of high susceptibility to sliding). The described AHP method procedure facilitates reduction and control of subjectivity in assessment of input parameters. Considering the hazard map for the municipality of Krupanj (Fig. 6.6), the definite land model (susceptibility to sliding) has been defined upon the following input parameters: altitude, exposition to sun, energy of relief, slope inclination, distance from the border of units with different hydro-geological function, distance from water courses, vegetative cover, and geology.

Slopes can also be monitored by a number of modern methods: Aeria photogrammetry, radar shooting, and today the most commonly used method is where the field equipment is installed on the landslide, from which it is possible to read the data on the movement of certain points. Using the obtained data on movements of points on the landslide and registered scarps it is possible to reconstruct the potential sliding surface, which is of prime importance in the landslide remediation.

The limitations of traditional deterministic approach in slope stability analyses are that it does not consider the uncertainty of input parameters and does not provide information on the probability of slope failure. The newer approach is based on a probabilistic concept, where each input parameter is defined by the range of possible values and a probability distribution function. The results of a number of performed comparative stability analyses clearly indicate the advantages of the probabilistic approaches, which is therefore increasingly applied in practice (Davidović et al. 2012b, 2015).

Considering embankment and cutting slopes along transportation networks, construction with suitable materials and good workmanship under adequate supervision are of paramount importance among preventive measures against sliding occurrence. Moreover, by virtue of a number of contemporary slope-stability software, the slope stability analyses both under static and dynamic conditions are indispensable nowadays in the design stage (Davidović et al. 2012a, 2014).

6.3.2 Remedial Measures for Landslides

The remedial measures are procedures bound to minimise the damage and restore the situation into its original condition. Permanent rehabilitation measures are a series of activities that are carried out after detailed geotechnical research of the terrain and the elaboration of the rehabilitation project with the aim of permanent stabilisation.

As water is the most common cause of landslides, the most effective permanent remedial measures involve the drainage of water from the surface and outside the body of the landslide. And here are very important measures that involve the development of drainage trenches and channels or the installation of horizontal drainage pipes, whereas in the first stage (emergency operation) only drainage channels can be made, and then, by setting drainage pipes in them, they become permanent measures of rehabilitation.

In addition, permanent rehabilitation measures include various support structures: retaining walls made of stone, concrete, reinforced concrete, and of reinforced soils, gabions, anchors, diaphragms, retaining structures on micropiles, etc. (Prolović et al. 2011; Davidović et al. 2017).

Diaphragm represents a thin flexible wall in the ground, built by special technology (Bonić et al. 2015b). Diaphragm construction is most often done by protecting the sides of the trench by not classically bracing it, but by using slurry. The clay suspension (slurry) is a mixture of clay and water. Working with the slurry requires a special system for its mixing, purification, insertion into the trenches, and capture during the construction of the trench, which makes the procedure for securing the sides of the trench very expensive. However, there are frequent cases when the geotechnical conditions at the given location are not so complex, that is, when the foundation pits are of smaller depth, in a cohesive soil, without presence or with a poor supply of groundwater. In such locations, construction of diaphragms can be considered with a simpler technology.

Bonić et al. (2015a) proposed a technical solution for the construction of diaphragms according to a simplified procedure that allows the quick and easy execution of works based on the system of construction of successive lamellas. The solution envisages the construction of the diaphragm lamellas up to 4.00–6.00 m deep, 2.00–2.50 m wide, and with thickness of 0.40–0.60 m and exceptionally up to 1.20 m. These limits have been adopted for the following reasons. The depth of the excavation is limited by the average values of the short-term excavation depth without the special securing of its sides, but it is also limited by the maximum depth of excavation using the universal excavator. Eventually, larger depths can also be excavated in materials with higher values of cohesion. The recommended width of the lamella is within the minimum range for diaphragms for two reasons. First, in this way, a minimum front of vertically excavated material without a support is achieved, which maximises the arc effect of the surrounding soil upon excavation. Owing to this effect, considerably longer short-term excavation depths without the special securing are also often achieved. In addition, the aforementioned average values of the dimensions of the lamella correspond to the volume of the mixer by which the concrete is brought to the construction site, thus allowing the fastest and the most economical construction of one lamella and the diaphragm as a whole. Of course, these dimensions can be changed and adapted to geotechnical and construction conditions at a given location.

In order to form the ending part of the lamella and the joint with the next lamella, it is envisaged to produce a special finishing sheet pile of ferrocement, although it can also be made of other materials (steel, various composites, etc.), which represents practically the only anticipated formwork. The sheet piles are 1 m long and of U-shaped cross-section, with two or more stirrups, for better connection and joint work of adjacent lamellas, as well as for better resistance to water seepage. The bonding of the lamellas of the diaphragm is reinforced by the formation of a special vertical bond beam, which couples the two adjacent lamellas. An additional monolithisation of the diaphragm can be achieved by creating a common header at the top of the diaphragm.

The technology of construction of diaphragms comprises the stages (Fig. 6.7):

Fig. 6.7
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Newly-proposed technology for the diaphragm construction in cohesive soils: a narrow trench excavation; b joint of adjacent lamellas (view from above); c finishing sheet pile (Bonić et al. 2015a)

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  • Excavation of the trench by the excavator up to the designed depth;

  • Placing the pre-assembled reinforcement cage in the trench by the excavator;

  • Placing the pre-prepared finishing sheet pile by inserting it into the bent sheet-metal guide-frames on the reinforcement cage by the excavator;

  • Filling the lamella with fresh concrete mass into which the additive for accelerated binding is added;

  • When the fresh concrete mass is sufficiently hardened, the next lamella is dug up.

It should be noted that all the above-mentioned operations on the diaphragm construction are performed from the surface of the field, outside the trench, which is a significant fact in terms of the workers’ safety considering working conditions in narrow excavations.

6.4 Contemporary Approaches to Risk Management for Foundation Structures

Control of the punching of columns through footings is mandatory part of the design of reinforced concrete footings exposed to notable concentrated forces through the columns. Behaviour of column footings and foundation slabs under load depends in general case on the soil characteristics, type, characteristics of the material of the footing, and intensity of the load. Typically, high concentrated loads in the columns may lead to the abrupt failure of those footings, i.e. punching the column through the footing, which is the specific phenomenon for the static conditions and, in particular, when the structure is exposed to an earthquake excitation (Fig. 6.8). Although foundations have essential influence on the behaviour of the structure and to the soil, standards do not pay enough attention to their analysis, and in some standards the specific details of the foundation analysis are not even mentioned. Complexity of behaviour of the soil and complexity of the subsoil–structure interaction lead to the fact that in majority of the standards is adopted the empirical method of design of the punching shear resistance of reinforced footings and reinforced foundation slabs.

Fig. 6.8
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Damage to the slab due to the punching of the column through the foundation (Wallace et al., ND)

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Recently, the punching shear resistance and punching behaviour of the reinforced concrete footings and foundation slabs have been investigated by experimental testing program on real soil, in in situ conditions, conducted from 2009 to 2014 (Bonić et al. 2012), by comparison of experimental and calculation results (Bonić and Folić 2013), as well as by comparison of testing results with numerical simulations in ANSYS (Vacev et al. 2015) taking into account the materially nonlinear behaviour of soil, concrete, and reinforcement (Bonić et al. 2010a, b). The influence of the main parameters, such as compressive strength of concrete, flexural reinforcement ratio, shear slenderness of the footing, soil reactive pressure distribution, stiffness of the soil–footing system, and mechanism of footing punching were in the focus of these studies (Fig. 6.9).

Fig. 6.9
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Saw-cut of the failed footings (Bonić et al. 2017)

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The key result of the conducted parametric studies was determination of the factors, whose influence is dominant in assessment of column footing punching. In addition, a modification of the punching capacity expression in Eurocode 2 is proposed (Bonić et al. 2017), which can contribute with its application to daily engineering practice.

6.5 Contemporary Approaches to Seismic Risk Management for Tunnel Structures

Historically, underground facilities have experienced a lower rate of damage in comparison with surface structures, and initially, tunnel structures were designed with no regard to seismic effects. Namely, being confined by the surrounding medium (soil/rock), these structures have long been assumed to have good seismic performance. Therefore, in a quite long time, tunnel damages did not take enough attention as it was the case with surface structures. Nevertheless, some underground structures have experienced significant damage in recent large earthquakes, including the 1995 Kobe earthquake in Japan, the 1999 Chi–Chi earthquake in Taiwan, as well as the 1999 Kocaeli and the Duzce earthquakes in Turkey (Fig. 6.10). As the tunnel number and its earthquake-induced damage and failure increased, the widely accepted idea that tunnels and underground structures are invulnerable to earthquakes has appeared to be illusive, and this problem has attracted the attention of experts and scientists around the world, reviving the interest in the associated design and analysis methods.

Fig. 6.10
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Seismically induced tunnel damage: a Kobe earthquake, Japan, 1995 (Lanzano et al. 2008); b Mid North Iwate earthquake, Japan, 1998 (Johansson and Konagai 2007); c Chi-Chi earthquake, Taiwan, 1999 (Hashash et al. 2001)

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Summarising the results based on the damage reports of 50 destructive earthquakes around the world in the period from 1989 to 2003, Bommer et al. (2006) have shown that, while the primary contributor to the damage of buildings is ground shaking, when it comes to transportation systems and utilities ground failure (liquefaction, slope instability, and fault displacement), as the secondary effect of seismic shaking, becomes more important and needs more serious attention (Fig. 6.11).

Fig. 6.11
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Primary causes of damage in 50 world earthquakes in the period from 1989 to 2003: a buildings; b transportation systems; c utilities (Bommer et al. 2006)

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6.5.1 State-of-the-Art Approaches to Assessment of Tunnel-Lining Forces Induced by the Seismic Hazard

In the first attempts of assessing the seismic response of tunnel structures the so-called free-field deformation approach was used, in which ground deformations due to seismic waves (in the absence of a structure or excavation) are estimated and the tunnel structure is designed to accommodate these strains (it is assumed that the structures experience the same strain rate as the ground in the free-field). Although this approach can enable a first-order estimation of the anticipated deformation of the structure, however, it does not account for the interaction of the tunnel structure with the surrounding ground. The presence of a tunnel structure considerably modifies the free-field ground motion leading to a different seismic response of the tunnel lining. The interaction effects between a structure and surrounding ground layers sometime can change strong ground motion, which is input to the structure, thus causing larger external forces to the structure. This phenomenon is related to the combined effects of kinematic interaction and dynamic (inertial) interaction. The kinematic interaction is influenced by the inability of a structure to match the free-field deformation, since the stiffness of the structure impedes a development of the free-field motions. The dynamic interaction is caused by the existence of structural mass that makes the effect of inertial force on the response of the surrounding ground. In the case of tunnel structures, the inertial interaction is less important than the kinematic one, because the mass of the structure is negligible against that of the surrounding environment. Therefore, analysis of the tunnel–ground interaction, concerning both the tunnel and the ground stiffness, is required in order to find the accurate tunnel response, which represents the basis of the contemporary soil–structure interaction approach. Besides the ratio of the ground and the lining stiffness, another aspect which sensibly affects the response of the tunnel is the shear stress transmission at the ground–lining interface. Numerous approaches are commonly based on the assumption that the soil is perfectly bonded to a structure (no-slip condition). However, the contact between the soil and the structure is usually imperfect, since slippage as well as separation often occur in the interface region (full-slip condition). In many situations in practice, the condition of a partial slip exists. Nevertheless, solutions are usually derived for the two extreme contact conditions: full-slip and no-slip. It is the usual practice to consider both of the extreme cases and apply the more critical one.

The comparative analytical and numerical studies on the soil–tunnel structure interaction effects considering representative of two main soil classes—stiff soil of good conditions and soft saturated soil of poor conditions (Zlatanović et al. 2014, 2015) indicated that the importance of the relative rigidity between the tunnel and the ground is the predominate factor that influences the soil–structure interaction (SSI) effects, which should not be omitted in analysis of the dynamic response of tunnel structures, having in mind that the interaction effects between structure and surrounding medium can result in larger external loading to the structure. These effects are the most prominent for the case of flexible linings in soft soils. Larger deformation is caused in the softer ground subjected to larger strong ground motion due to the smaller stiffness (Fig. 6.12).

Fig. 6.12
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Earthquake-induced ovaling deformation of the circular tunnel cross-section (displacement enlarged 25 times): a dense sand; b soft clay (Zlatanović et al. 2015)

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Numerous approaches are commonly based on the assumption that the soil behaves in a linear elastic manner. However, the soil region immediately adjacent to the tunnel structure can experience extensive strain level, thus causing the coupled soil–tunnel system to behave in a nonlinear manner. Accordingly, there was a need to modify the linear approach in order to provide a reasonable estimation of the ground response under seismic impact, in which case the soil is cyclically loaded. Experimental results have suggested that some energy is dissipated, even at a very low strain level, thus indicating that the damping ratio of a soil is never zero. It is also suggested that both the soil’s shear modulus and the damping ratio are dependent on the shear-strain level. The results of the analyses related to the comparison of the seismically induced soil–tunnel structure interaction effects considering linear and nonlinear soil behaviour (Zlatanović et al. 2016) drawn the conclusion that linear analysis underestimates the soil shear strain, and consequently also underestimates the soil displacements induced by seismic shear-wave propagation, producing a significant underestimation of the tunnel lining’s cross-sectional forces. Nonlinear analysis, on the other hand, due to a prediction of a significantly larger soil shearing (soil displacements), results in a higher relative displacement between the top and the bottom of the circular tunnel cross-section compared to the linear analysis. This leads to significantly greater ovalisation of the tunnel structure (Fig. 6.13), and therefore, to higher values of the internal lining forces. In addition, it seems that the linear analysis can result in a more conservative estimation of the internal forces in the tunnel lining, as the peak acceleration and the level of soil nonlinearity increase, which is a consequence of the constant soil shear modulus and the constant damping ratio assumption typical for the linear soil behaviour.

Fig. 6.13
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Earthquake-induced ovaling deformation of the circular tunnel cross-section: a linear SSI analysis; b nonlinear SSI analysis (Zlatanović et al. 2016)

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Contemporary development of computer software and the finite element method (FEM) concept has provided a powerful tool toward investigating the seismic response of tunnels. Considering that the dynamic FE analyses are quite complex and they require large computer capacities, a simplified approach is needed, i.e. certain assumptions and idealisations considering numerical models must be made (Zlatanović et al. 2013). Although the models may be simple, they should be able to capture the most significant aspects of the seismic response of tunnels. In order to estimate the ability of numerical models to simulate the soil–structure interaction effects, numerical analyses have been performed with the aid of the software ANSYS (Zlatanović et al. 2017), comparing the discrete beam–spring and the continuous finite element models (Fig. 6.14). Unlike the beam–spring model that cannot analyse static and dynamic effects simultaneously except by simple superposition, whose results do not yield a correct solution in a comprehensive way, the continuous FE model allows to account both for static and dynamic loads in a single analysis and gives results that are in a good agreement with the real physical state.

Fig. 6.14
figure 14

Numerical simulation of the soil–tunnel structure interaction effects: a discrete beam–spring model; b continuous FE model (Zlatanović et al. 2017)

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6.5.2 Contemporary Laboratory Equipment for Experimental Testing

Development of research centres and implementation of shaking tables in experimental research of tunnel structures in terms of seismic impact are also very important segments of preventive measures in seismic risk management. However, geotechnical models cannot be placed directly on a vibrating platform. Over the past two decades, several research groups around the world have been working on the development of laminar shear boxes, which will enable shear strains of the model due to earthquake action, which are of particular importance in nonlinear analyses, to be identical to deformations of prototype in real conditions. Laminar boxes can be of a rectangular cross-section (Fig. 6.15a) and of a cylindrical shape (Fig. 6.15b).

Fig. 6.15
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Laminar boxes: a rectangular laminar box [Soil Dynamics and Dynamic Testing Laboratories, Institute of Earthquake Engineering and Engineering Seismology (IZIIS), University “Ss. Cyril and Methodius” of Skopje, Republic of Macedonia (www.iziis.edu.mk)]; b circular laminar box [used in Japan on the largest shaking table in the world, which can simulate 3D soil movements (Suzuki et al. 2008)]

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Laminar shear box (Chunxia et al. 2008) consists of a number of rigid frames. Each of the frames rests on individual bearings, which are connected to the outer frame, so that the weight of the box through these bearings is transferred from the platform to the outer frame. This arrangement allows full utilisation of the shaking table’s shear capacity. For the case of one-dimensional shaking, the bearings, which are connected to the outer frame, prevent unwanted movements in the transverse and vertical direction, allowing only free displacements in the longitudinal direction. Also, the bearings allow very smooth relative movements of the frames relative to each other, which impose the laminar box to deform in a shear beam manner, thus following the deformation shape of the soil model in the box. This leads to the mitigation of the effect of the restricted lateral spread created by artificial boundaries of laminar box that do not exist in the prototype condition. The base of the box, made of steel plate, must be firmly connected to the platform before the test itself, either by screwing or welding, and should be sprinkled with a coarse sand layer in order to prevent the occurrence of slipping at the contact of the soil model and the base of the box. The interior of the laminar box should be coated with a thin layer of flexible latex, which prevents the penetration of the soil into the gaps between the frames, as well as has a waterproofing role in the case of testing the saturated soil model. Laminar boxes can be adapted to a wide range of tests in the field of geotechnical earthquake engineering (amplification, liquefaction and cyclic mobility phenomenon, excess pore water pressure generation, dissipation rates, soil–structure interaction, etc.). The model can be equipped with various sensors: strain gauges, accelerometers, velocity meters, earth pressure transducers, pore-water pressure transducers, transducers for measuring displacements, settlement meters, and load cells.

6.5.3 Geotechnical and Construction Strategies to Mitigate Seismic Effects on Tunnel Structures

The damage to subway tunnels during the Hyogoken-Nanbu (Kobe) Earthquake in Japan in 1995 has stimulated a sharp rise in research activities considering possible measures for mitigation of seismic effects on tunnel structures.

  1. (1)

    Soil improvement techniques—“Jet grouting”

In spite of the fact that there is no possibility of designing a tunnel that would completely resist large deformations of soil induced by earthquake, however, techniques of stabilising the surrounding soil such as densification, reinforcement, or grouting can to a great extent be effective in mitigating the consequences of seismic effects.

Jet grouting represents an efficient soil stabilisation technique that is applied in the most diverse geotechnical conditions, from soft clays and silts to sands and gravels. The essence of the technology is the use of a jet of the water cement mixture, which under pressure fragments the soil and mixes with it in situ, thus forming a column that represents a mixture of earth and binding material. The resulting columns in the array, the so-called “soilcrete”, are characterised by high compressive strength and low water permeability. The diameter of the columns depends on the speed of rotation and rise of the drilling tool. It is the safest method that over the last two decades is increasingly widespread around the world as a very fast procedure, which can be applied in the vicinity of existing structures without interrupting their service and results in significant cost savings (Hayward Baker Inc. 2004).

In tunnel construction, the jet grouting technique is used to stabilise the ground around the tunnel excavation, in particular at the tunnel crown, which is of particular importance in the application of the tunnelling method when the entire face of a tunnel profile is excavated. Soil reinforcement can be achieved by vertical columns (Fig. 6.16a), horizontal columns that form a protective arch under whose protection the tunnel profile can be excavated (Fig. 6.16b), or inclined columns (Fig. 6.16c). This soil improvement technique can be used both for the construction of new and for the seismic retrofit of existing tunnel structures.

Fig. 6.16
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Soil improvement around tunnel by the jet-grouting technique: a vertical columns (Aбpaмчyк et al. 2004); b horizontal columns (ROTEX OY 2006); c inclined columns (Dash et al. 2003)

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  1. (2)

    Tunnel flexible joints and ductile tunnel lining design

In the case of tunnel structures, sudden changes in stiffness are quite often considering both the construction itself and the surrounding medium. These changes usually occur:

  • at the contact of tunnels with accompanying objects, such as transit stations;

  • at locations of connection or crossing of two or more tunnels;

  • in the case of heterogeneous geological structure of the surrounding ground;

  • in places of local restrictions on tunnel structures from movements.

At these locations, differences in stiffness may yield a difference in movements of certain parts of the structure or different layers of the surrounding medium, which can cause the concentration of stress at these points (additional moments and shear stresses). The most suitable solution in these situations is the application of flexible joints, usually in the form of flexible steel plates or polymeric materials. These joints are particularly useful for connecting the tunnel portals with the remaining part of the structure (Hashash et al. 2001).

In order to enable tunnel structure to adapt to the anticipated movements of the surrounding ground, it must have a certain ductility, which can be achieved by jointed (segmented) tunnel linings, among which there could be distinguished straight-jointed liners (Fig. 6.17a) and staggered-jointed liners (Fig. 6.17b). By segmented linings, the reduction of the moment in tunnel structures up to 50% may be achieved (Mizuno and Koizumi 2006). The segments are connected in the ring by the longitudinal (segment) joints, whereas the rings are assembled in the tunnel lining by the transverse (circumferential, ring) joints. Steel dowels are often used as joints in combination with rubber seals in order to ensure the waterproof ability of the structure. Differences in the stiffness between the lining segments and the connecting joints greatly affect the internal forces in the segments of the tunnel lining. Due to the existence of a large number of joints, dynamic properties of the tunnel structure, and therefore the interaction of the structure and the soil, can be very complex.

Fig. 6.17
figure 17

Types of jointed tunnel linings: a straight-jointed linings; b staggered-jointed linings (Popović 1987)

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  1. (3)

    Seismic isolation of tunnels

One of the measures for mitigating the seismic effects on tunnels is the coating of the tunnel structure with isolating material, in order to minimise the shearing forces at the interface of the tunnel lining and the surrounding ground. The seismic isolation material has to be long-lasting and stable in terms of settlements. In the case when the tunnel structure passes through two layers of soil with different stiffness, at the contact of two layers, apart from flexible seismic joints, the tunnel should also be provided with isolation from the surrounding ground, which ensures the reduction of forces in the tunnel structure, and consequently, mitigation of earthquake-induced deformation of the tunnel (Lanzano et al. 2008).

Kim and Konagai (2001) investigated the effect of expansion and contraction of materials based on silicone, rubber, and bitumen for seismic isolation of tunnels. The ability of contraction and expansion of isolating material greatly contributes to a better seismic isolation effect considering tunnel structures. The conclusion of these studies is that materials characterised by low values of the Poisson’s coefficient and shear modulus show the best seismic isolation effect. In the case of bituminous materials, however, the appearance of softening with increasing strains is noted.

The results of the analysis performed by Keshvarian et al. (2004) showed that the rubber-based isolation of great thickness and length results in the most efficient reduction of the internal forces and stresses in the tunnel lining, as well as of the amount of displacements in the tunnel structure and the surrounding ground. Based on these results, it is concluded that composite tunnel linings of concrete and rubber are very efficient in absorbing the energy released during seismic activity.

Hasheminejad and Miri (2008) carried out analyses of different variants of the multilayered cylindrical tunnel lining. In the first case, the polymeric isolation layer was placed between the tunnel structure and the surrounding ground. In the second case, the isolation was placed at the inner surface of the tunnel structure. In the third case, the polymer damping layer was constrained by a substantially stiffer layer of concrete. Polymers of different attenuation characteristics were considered. According to the results of the analysis, the viscoelastic isolating material of a low shear modulus value for the third case of structural arrangement is proved to be the most efficient from the aspect of seismic isolation effect, in particular for the case of S-wave propagation perpendicular to the longitudinal axis of the tunnel. In the case of wave propagation parallel to the axis of the tunnel, especially in the range of medium and high frequencies, the stress concentration caused by dynamic influences drastically increases, which points to the need for further research in the field of seismic isolation of tunnels.

6.5.4 Intelligent Monitoring of Seismic Damage of Tunnels

In the study of Bairaktaris et al. (2000), a fibre-optic-based deformation monitoring system is presented. The main advantages of fibre-optic sensors over conventional seismic deformation sensors are that they reduce connectivity problems significantly, offer galvanic isolation essential to modern computer-based systems, and their sensitivity is better than that of conventional strain gauges. This greatly improved operational sensor system can be integrated into a reinforced concrete tunnel. The sensor can be either embedded into the structure or be retrofitted onto the surface of the structure to be monitored. The uniqueness of the technology is the use of carbon composite materials for fibre-optic sensors, which have numerous advantages such as: excellent resistance to aggressive chemical environments (e.g. the alkaline environment of concrete), excellent mechanical behaviour, flatness so they can easily be placed on the surface of civil engineering structures, and very low total volume that enables sensors to be embedded in concrete without perturbing the structure.

The Intelligent Seismic Monitoring System consists of the Deformation Monitoring System (it uses fibre-optic sensors for providing real-time measurements of the deformation of the tunnel lining and the rail track) and the Decision Support System that is the core of the system (it comprises a seismic stability module, a serviceability module, and an expert system). The Seismic Stability Module records deformation at critical lining locations, and based on it, assesses the remaining dissipative capacity at these locations and signals a warning the tunnel to be shut down if this capacity is not sufficient for the tunnel to sustain expected aftershocks. The module receives measurements of the deformations in critical sections of the tunnel in real time and reconstructs the corresponding hysteresis loops. The relevant energy terms are computed, so that the current state of damage can be assessed. The Serviceability Module receives real-time measurements of rail deformations and estimates the safe speed of train. The Expert System coordinates the modules, provides easy access to the system via a user interface, and takes care of the maintenance of the database.

6.6 Concluding Remarks

Geodynamic natural phenomena, such as landslides and earthquakes, are among the most hazardous natural threats to human lives and property. One of the basic principles in the modern approach to natural disaster risk management is the tendency towards the strengthening of preventive measures, which results in numerous advantages, both in terms of safety and in economic terms. Natural disaster prevention arrangements are of long-term character considering permanent governmental and professional activity for the needs of establishment of consistent scientific bases and their practical application in prevention and mitigation of disaster risk.

When it comes to geotechnical engineering, over the past decade or two, a great progress has been made in the field of preventive and mitigation measures in managing the risks of landslides and earthquakes, primarily due to the advancement of computer technology that, consequently, has enabled the development of contemporary methodologies and software for relevant and reliable risk analysis and assessment, modern laboratory equipment for conducting very serious and complex research, modern design strategies and construction technologies, as well as contemporary monitoring and early-warning systems. Although a major step forward in this field has been achieved over the past period, however, there are still many necessities and opportunities for further research in this area.