This page presents some highlights of my research activities
- Research related to my PhD thesis
In my doctoral thesis I developed an integrated landslide risk assessment system for National Roads and Motorways aiming at the assessment of the failure risk of highway geotechnical assets (embankments and soil and rock cuttings) and the relevant consequences on traffic flow and travellers’ safety. Although important parts of this thesis have not yet been submitted for publication in journals or conferences, this thesis already gave me 5 journal and 3 conference papers (journal papers 21-25, conference papers 4,6-7).
According to this system, the traffic dislocation risk derives from the combination of three factors, a) the stability of earth-structures, where special attention is given to the action of water, as being the most common triggering factor for failure, b) the extent of failure and c) the consequences on traffic flow and travellers’ safety. As regards to the first factor, a separate sub-system for the assessment of the failure hazard of earth slopes, rock slopes and highway embankments (typical, on sloping ground and on soft ground) has been proposed covering the most likely failure types. The role of the most common triggering factors for failure, that is, climatic conditions and earthquake is emphasised. The second factor, that is, the extent of failure, is based on proposed analytical and semi-empirical models. Where necessary, existing models published in international journals and conference proceedings supplement the system. Finally, the failure extent mentioned above is compared with the width of roadway elements for the assessment of the consequences on traffic flow, whilst the potential velocity impact of vehicle onto the failed mass comprises indicator for the safety of travellers. It is worth mentioning that, the critical review on the existing rock mass classification and risk assessment systems in my PhD thesis (see also journal paper 24 and journal paper 21 respectively) revealed serious weaknesses.
- The Generalized Coefficients of Earth Pressure: A Unified Approach
The journal paper 8 offers an extension of Cauchy’s first law of motion to deformable bodies with internal resistance with application to earth pressures. In this respect, a unified continuum mechanics approach for deriving earth pressure coefficients for all soil states (“at rest”, active, passive and intermediate on both active and passive “side”), applicable to cohesive-frictional soils and both horizontal and vertical pseudo-static conditions is proposed. The validity of the proposed coefficients is strongly supported by the fact that, under static conditions they are transformed into the well-known Rankine’s expressions for cohesive-frictional soils for the active and passive state. Numerical applications as well as comparisons with widely used solutions (e.g., Rankine’s and Mononobe–Okabe’s), design code practices (EN 1998-5; AASHTO), and results from centrifuge tests further support the validity of the proposed coefficients. In the framework of this work, analytical expressions for the calculation of the depth of neutral zone in the state “at rest”, the depth of tension crack in the active state (as it is affected by the seismic excitation), the required wall movement for the mobilization of the active or passive state and the mobilized shear strength of soil are also given. The latter is important component even for the active and passive states, as during a seismic excitation, the active or passive earth pressure appears to be greater and smaller respectively because less shear strength is mobilized. The required wall movement depends on the roughness of the wall, the depth considered with respect to the height of the wall, the difference between the coefficient of earth pressure at rest and the coefficient of active (or passive) earth pressure and the two elastic constants of soil.
- Analytical solutions in probabilistic geotechnical engineering based on random fields
The journal paper 13 presents an analytical solution for calculating the probability of failure of rock slopes against planar sliding. This solution in based on the theory of random fields accounting for the influence of spatial variability on slope reliability. As shown, the spatial correlation of shear strength can have an important influence on slope performance expressed by the probability of failure. This is a significant observation, since ignoring the influence of spatial correlation in design may lead to non-conservative estimations of slope reliability. To the best knowledge of the authors, this work constitutes the first analytical approach of random fields in geotechnical engineering.
In the journal paper 11, the estimation of the joint roughness coefficient (JRC) has been embedded in the framework of random fields. Although this method is one of higher complexity in regard of the presumed background knowledge, it encodes naturally subtler information about the rock surface roughness. It is noted that, the proposed random field approach considers automatically the scale of the problem (no correction factor is needed), whilst the JRC estimates appear to be more stable as compared to the existing solutions.
- Two- and three- dimensional soil slope stability analysis in closed-form
Closed-form solutions satisfying both equilibrium of moments and forces have been proposed for the stability analysis of earth slopes in two- and three- dimensions (see journal papers 15 and 16 respectively). The failure surface is considered circular and spherical (or spheroidal) in the two- and three- dimensions respectively. For the case of homogeneous earth slopes, these solutions lead to the exact answer to the problem, as no assumptions are necessary regarding the internal state of stress (see Appendix in the journal papers 15 and 16). The usefulness of the 3D version of the problem is far beyond the fact that it gives the exact safety factor value for a c-phi, homogeneous slope against purely rotational slide along a spheroid surface. This solution is a versatile tool for studying various important research topics objectively such as the effect of the third dimension and tension crack on the stability of soil slopes. In this respect, it was found that, the width of failure corresponding to the minimum safety factor value is not always infinite, but it is affected by the triggering factor for failure e.g. water acting as pore pressures and/or as hydrostatic force in the tension crack (conclusion appearing in in the accepted paper acc1). More specifically, it was found that, when a slope is near its limit equilibrium and under the influence of a triggering factor, the minimum safety factor value corresponds to a near spherical failure mechanism, even if the triggering factor (e.g. pore-water pressures) acts uniformly along the third dimension of slope. Moreover, it was found that, the effect of tension crack is much greater when the stability of slopes is studied in three dimensions; indeed, safety factor values comparable to the 2D case are obtained.
My involvement with this topic of research also led me to the production of two mathematical papers, the “equal-area projection: spheroid to sphere to plane” and the “area of spherical lune formed by two random planes” (see journal papers 14 and 17 respectively).
- The effect of targeted field investigation on the reliability of geotechnical engineering structures - Numerical random field approach
This research topic refers to the numerical investigation of the effect of targeted field investigation on the reliability of different geotechnical engineering structures, i.e. pile foundations, earth-retaining structures (studying both the active and passive states) and shallow foundations (studying both the elastic settlement and bearing capacity analysis problems). The targeted field investigation refers to sampling from a specific point or a set of points of the ground semi-space (i.e., adopting a sampling strategy) so that the statistical uncertainty in the design to be minimised. The latter, generally, is attributed in the international literature to limited material soil testing. The sampling strategy leading to the minimum statistical error is called optimal. This research activity was based on the Random Finite Element Method (RFEM), properly considering soil sampling in the analysis. The RFEM method combines finite element method with the random field theory. Contrary to the common belief that statistical uncertainty decreases with increasing number of samples, this research clearly shows that the statistical error in the design of geotechnical engineering structures can only be minimized by targeted field investigation. Additionally, it shows that the benefit from a targeted field investigation is by far greater as compared to the benefit gained from the use of characteristic values in a limit state design framework; the characteristic value concept is used by Eurocode 7 (EN 1997). Relatively, 4 journal papers have already been published (journal papers 3-5 and 9), while our research on this topic continuous.
- Designing passive rockfall measures based on computer simulation and field data
Currently, there are two design guides for catchment areas based on field work, the “Evaluation of Rockfall and its Control” by Ritchie (1963) referring to deep ditches and the “Rockfall Catchment Area Design Guide” by Pierson et al. (2001) referring to flat areas inclined up to 1V:4H. Where necessary, rockfall simulation programs are used by practitioners for studying possible rockfall events and the effectiveness of measures. These Monte-Carlo based software, however, do not take into account the random irregularities on slope faces and the fact that rocks bounce on them before hitting the ground producing impact distances greater than zero. In the journal paper 20 and conference paper c5 an effective design guide for deep ditches and concrete walls or fences combining a) the impact distances of 11,250 rocks recorded by Pierson et al. (2001) for various slope heights and gradients with b) computer simulations for studying rockfall trajectories after the first impact of rocks has been proposed.
- Designing geotechnical engineering structures based on non-conventional factoring strategies
The design of geotechnical engineering structures using non-conventional factoring strategies was subject matter of research in the journal papers 12 and 19 and the book chapter ch1. In these works, it was shown that, the consideration of factoring strategies related to e.g. pore water pressures or seismic coefficient, in addition to the traditional safety factor with respect to the shear strength of soil, gives a more comprehensive insight into the problem of safety of geotechnical engineering structures. Indeed, application of the problem in the limit state framework of EN 1997 showed that the proposed methodology may result in a safety level of geotechnical engineering structures significantly lower than the respective one obtained using the limit-state method in its traditional form.
- The equivalent modulus of elasticity of layered soil mediums for designing shallow foundations
A critical review of the available methods for calculating the equivalent elastic constants (Eeq, νeq) for the case of transversely loaded horizontally stratified soil mediums has been offered in the journal paper 10. The main finding of the paper in question is that, the vast majority of the existing methods return Eeq values that greatly differ from the value effectively representing the original stratified medium, while most of them ignore the Poisson’s ratio. Thus, the use of the current methods may easily lead to either non-economic or unsafe designs. This paper received extremely positive feedback by the reviewers i.e. Reviewer 1: “The work represents a commented and critical compilation that would be useful to practitioners, academics and researchers. On top of that, the manuscript is well written and organized, and the conclusions presented are sound, relevant, and may act as catalyser for future research on the topic.”, Reviewer 2: “The review seems very comprehensive and includes works already published in the 40s of the twentieth century and extending to the present. It is a work of great merit and of undoubted practical interest for the readers of the Journal.”, Reviewer 3: “The manuscript is appropriate to this journal and I believe that it has practical important applications”.
- Elastic settlement analysis of flexible and rigid foundations
Not surprisingly, the well-known Schmertmann’s method has been abandoned in the forthcoming revised version of Eurocode 7 (EN 1997); a critical review of the method in question is offered in the journal paper j7. The new EN 1997 norm recommends the stress–strain method deriving from pure elasticity. In this respect the journal papers j1 and j2 offer a convenient way for calculating the settlement of shallow foundations with the stress-strain method. The first one based on the well-known strain influence factor concept, while the second one introduces the strain influence area concept.
The effectiveness of the “characteristic point” point has also been studied through comparison of the uniform elastic settlement of 210 cases with rigid footings (values obtained from 3D finite element analysis) with the settlement of the respective flexible footings at various points on the plan-view of footings (analytical modelling was carried out based on the theory of elasticity). In the ur2 paper it was found that the location of characteristic point may greatly vary from what we know so far. The same 3D FE models also allowed me to examine the validity of the approximate relationships for the settlement analysis of rigid rectangular footings (ur3), finding that for ν<0.45 the correct relationships are ρRigid≈0.9ρAverage and ρRigid ≈0.75ρCenter; also it was found that for ν=0.5 a different relationship stands.
Regarding also the elastic settlement analysis of shallow foundations, taking into account the effect of footing shape on the modulus of elasticity (see c2), it was found that the problem is practically insensitive to the aspect ratio of footings. Thus, the problem is reduced to finding the settlement of the respective square or the equivalent circular footing (see j2).
- A comparative assessment of the methods‑of‑moments for estimating the correlation length of one‑dimensional random fields
This paper (journal paper j6) has been published in Elsevier’s Archives of Computational Methods in Engineering journal with 7.242 Impact Factor. It examines in depth the effectiveness of eight methods-of-moments for estimating the spatial correlation length.
In my doctoral thesis I developed an integrated landslide risk assessment system for National Roads and Motorways aiming at the assessment of the failure risk of highway geotechnical assets (embankments and soil and rock cuttings) and the relevant consequences on traffic flow and travellers’ safety. Although important parts of this thesis have not yet been submitted for publication in journals or conferences, this thesis already gave me 5 journal and 3 conference papers (journal papers 21-25, conference papers 4,6-7).
According to this system, the traffic dislocation risk derives from the combination of three factors, a) the stability of earth-structures, where special attention is given to the action of water, as being the most common triggering factor for failure, b) the extent of failure and c) the consequences on traffic flow and travellers’ safety. As regards to the first factor, a separate sub-system for the assessment of the failure hazard of earth slopes, rock slopes and highway embankments (typical, on sloping ground and on soft ground) has been proposed covering the most likely failure types. The role of the most common triggering factors for failure, that is, climatic conditions and earthquake is emphasised. The second factor, that is, the extent of failure, is based on proposed analytical and semi-empirical models. Where necessary, existing models published in international journals and conference proceedings supplement the system. Finally, the failure extent mentioned above is compared with the width of roadway elements for the assessment of the consequences on traffic flow, whilst the potential velocity impact of vehicle onto the failed mass comprises indicator for the safety of travellers. It is worth mentioning that, the critical review on the existing rock mass classification and risk assessment systems in my PhD thesis (see also journal paper 24 and journal paper 21 respectively) revealed serious weaknesses.
- The Generalized Coefficients of Earth Pressure: A Unified Approach
The journal paper 8 offers an extension of Cauchy’s first law of motion to deformable bodies with internal resistance with application to earth pressures. In this respect, a unified continuum mechanics approach for deriving earth pressure coefficients for all soil states (“at rest”, active, passive and intermediate on both active and passive “side”), applicable to cohesive-frictional soils and both horizontal and vertical pseudo-static conditions is proposed. The validity of the proposed coefficients is strongly supported by the fact that, under static conditions they are transformed into the well-known Rankine’s expressions for cohesive-frictional soils for the active and passive state. Numerical applications as well as comparisons with widely used solutions (e.g., Rankine’s and Mononobe–Okabe’s), design code practices (EN 1998-5; AASHTO), and results from centrifuge tests further support the validity of the proposed coefficients. In the framework of this work, analytical expressions for the calculation of the depth of neutral zone in the state “at rest”, the depth of tension crack in the active state (as it is affected by the seismic excitation), the required wall movement for the mobilization of the active or passive state and the mobilized shear strength of soil are also given. The latter is important component even for the active and passive states, as during a seismic excitation, the active or passive earth pressure appears to be greater and smaller respectively because less shear strength is mobilized. The required wall movement depends on the roughness of the wall, the depth considered with respect to the height of the wall, the difference between the coefficient of earth pressure at rest and the coefficient of active (or passive) earth pressure and the two elastic constants of soil.
- Analytical solutions in probabilistic geotechnical engineering based on random fields
The journal paper 13 presents an analytical solution for calculating the probability of failure of rock slopes against planar sliding. This solution in based on the theory of random fields accounting for the influence of spatial variability on slope reliability. As shown, the spatial correlation of shear strength can have an important influence on slope performance expressed by the probability of failure. This is a significant observation, since ignoring the influence of spatial correlation in design may lead to non-conservative estimations of slope reliability. To the best knowledge of the authors, this work constitutes the first analytical approach of random fields in geotechnical engineering.
In the journal paper 11, the estimation of the joint roughness coefficient (JRC) has been embedded in the framework of random fields. Although this method is one of higher complexity in regard of the presumed background knowledge, it encodes naturally subtler information about the rock surface roughness. It is noted that, the proposed random field approach considers automatically the scale of the problem (no correction factor is needed), whilst the JRC estimates appear to be more stable as compared to the existing solutions.
- Two- and three- dimensional soil slope stability analysis in closed-form
Closed-form solutions satisfying both equilibrium of moments and forces have been proposed for the stability analysis of earth slopes in two- and three- dimensions (see journal papers 15 and 16 respectively). The failure surface is considered circular and spherical (or spheroidal) in the two- and three- dimensions respectively. For the case of homogeneous earth slopes, these solutions lead to the exact answer to the problem, as no assumptions are necessary regarding the internal state of stress (see Appendix in the journal papers 15 and 16). The usefulness of the 3D version of the problem is far beyond the fact that it gives the exact safety factor value for a c-phi, homogeneous slope against purely rotational slide along a spheroid surface. This solution is a versatile tool for studying various important research topics objectively such as the effect of the third dimension and tension crack on the stability of soil slopes. In this respect, it was found that, the width of failure corresponding to the minimum safety factor value is not always infinite, but it is affected by the triggering factor for failure e.g. water acting as pore pressures and/or as hydrostatic force in the tension crack (conclusion appearing in in the accepted paper acc1). More specifically, it was found that, when a slope is near its limit equilibrium and under the influence of a triggering factor, the minimum safety factor value corresponds to a near spherical failure mechanism, even if the triggering factor (e.g. pore-water pressures) acts uniformly along the third dimension of slope. Moreover, it was found that, the effect of tension crack is much greater when the stability of slopes is studied in three dimensions; indeed, safety factor values comparable to the 2D case are obtained.
My involvement with this topic of research also led me to the production of two mathematical papers, the “equal-area projection: spheroid to sphere to plane” and the “area of spherical lune formed by two random planes” (see journal papers 14 and 17 respectively).
- The effect of targeted field investigation on the reliability of geotechnical engineering structures - Numerical random field approach
This research topic refers to the numerical investigation of the effect of targeted field investigation on the reliability of different geotechnical engineering structures, i.e. pile foundations, earth-retaining structures (studying both the active and passive states) and shallow foundations (studying both the elastic settlement and bearing capacity analysis problems). The targeted field investigation refers to sampling from a specific point or a set of points of the ground semi-space (i.e., adopting a sampling strategy) so that the statistical uncertainty in the design to be minimised. The latter, generally, is attributed in the international literature to limited material soil testing. The sampling strategy leading to the minimum statistical error is called optimal. This research activity was based on the Random Finite Element Method (RFEM), properly considering soil sampling in the analysis. The RFEM method combines finite element method with the random field theory. Contrary to the common belief that statistical uncertainty decreases with increasing number of samples, this research clearly shows that the statistical error in the design of geotechnical engineering structures can only be minimized by targeted field investigation. Additionally, it shows that the benefit from a targeted field investigation is by far greater as compared to the benefit gained from the use of characteristic values in a limit state design framework; the characteristic value concept is used by Eurocode 7 (EN 1997). Relatively, 4 journal papers have already been published (journal papers 3-5 and 9), while our research on this topic continuous.
- Designing passive rockfall measures based on computer simulation and field data
Currently, there are two design guides for catchment areas based on field work, the “Evaluation of Rockfall and its Control” by Ritchie (1963) referring to deep ditches and the “Rockfall Catchment Area Design Guide” by Pierson et al. (2001) referring to flat areas inclined up to 1V:4H. Where necessary, rockfall simulation programs are used by practitioners for studying possible rockfall events and the effectiveness of measures. These Monte-Carlo based software, however, do not take into account the random irregularities on slope faces and the fact that rocks bounce on them before hitting the ground producing impact distances greater than zero. In the journal paper 20 and conference paper c5 an effective design guide for deep ditches and concrete walls or fences combining a) the impact distances of 11,250 rocks recorded by Pierson et al. (2001) for various slope heights and gradients with b) computer simulations for studying rockfall trajectories after the first impact of rocks has been proposed.
- Designing geotechnical engineering structures based on non-conventional factoring strategies
The design of geotechnical engineering structures using non-conventional factoring strategies was subject matter of research in the journal papers 12 and 19 and the book chapter ch1. In these works, it was shown that, the consideration of factoring strategies related to e.g. pore water pressures or seismic coefficient, in addition to the traditional safety factor with respect to the shear strength of soil, gives a more comprehensive insight into the problem of safety of geotechnical engineering structures. Indeed, application of the problem in the limit state framework of EN 1997 showed that the proposed methodology may result in a safety level of geotechnical engineering structures significantly lower than the respective one obtained using the limit-state method in its traditional form.
- The equivalent modulus of elasticity of layered soil mediums for designing shallow foundations
A critical review of the available methods for calculating the equivalent elastic constants (Eeq, νeq) for the case of transversely loaded horizontally stratified soil mediums has been offered in the journal paper 10. The main finding of the paper in question is that, the vast majority of the existing methods return Eeq values that greatly differ from the value effectively representing the original stratified medium, while most of them ignore the Poisson’s ratio. Thus, the use of the current methods may easily lead to either non-economic or unsafe designs. This paper received extremely positive feedback by the reviewers i.e. Reviewer 1: “The work represents a commented and critical compilation that would be useful to practitioners, academics and researchers. On top of that, the manuscript is well written and organized, and the conclusions presented are sound, relevant, and may act as catalyser for future research on the topic.”, Reviewer 2: “The review seems very comprehensive and includes works already published in the 40s of the twentieth century and extending to the present. It is a work of great merit and of undoubted practical interest for the readers of the Journal.”, Reviewer 3: “The manuscript is appropriate to this journal and I believe that it has practical important applications”.
- Elastic settlement analysis of flexible and rigid foundations
Not surprisingly, the well-known Schmertmann’s method has been abandoned in the forthcoming revised version of Eurocode 7 (EN 1997); a critical review of the method in question is offered in the journal paper j7. The new EN 1997 norm recommends the stress–strain method deriving from pure elasticity. In this respect the journal papers j1 and j2 offer a convenient way for calculating the settlement of shallow foundations with the stress-strain method. The first one based on the well-known strain influence factor concept, while the second one introduces the strain influence area concept.
The effectiveness of the “characteristic point” point has also been studied through comparison of the uniform elastic settlement of 210 cases with rigid footings (values obtained from 3D finite element analysis) with the settlement of the respective flexible footings at various points on the plan-view of footings (analytical modelling was carried out based on the theory of elasticity). In the ur2 paper it was found that the location of characteristic point may greatly vary from what we know so far. The same 3D FE models also allowed me to examine the validity of the approximate relationships for the settlement analysis of rigid rectangular footings (ur3), finding that for ν<0.45 the correct relationships are ρRigid≈0.9ρAverage and ρRigid ≈0.75ρCenter; also it was found that for ν=0.5 a different relationship stands.
Regarding also the elastic settlement analysis of shallow foundations, taking into account the effect of footing shape on the modulus of elasticity (see c2), it was found that the problem is practically insensitive to the aspect ratio of footings. Thus, the problem is reduced to finding the settlement of the respective square or the equivalent circular footing (see j2).
- A comparative assessment of the methods‑of‑moments for estimating the correlation length of one‑dimensional random fields
This paper (journal paper j6) has been published in Elsevier’s Archives of Computational Methods in Engineering journal with 7.242 Impact Factor. It examines in depth the effectiveness of eight methods-of-moments for estimating the spatial correlation length.