Imperial College London

DrStavroulaKontoe

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Reader in Soil Dynamics
 
 
 
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Contact

 

+44 (0)20 7594 5996stavroula.kontoe Website

 
 
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Location

 

535Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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111 results found

Kontoe S, 2022, The seventeenth Mallet-Milne lecture, BULLETIN OF EARTHQUAKE ENGINEERING, Vol: 20, Pages: 2821-2823, ISSN: 1570-761X

Journal article

Kontoe S, Summersgill F, Potts D, Lee Yet al., 2022, On the effectiveness of slope stabilising piles for soils with distinct strain-softening behaviour, Geotechnique, Vol: 72, Pages: 309-321, ISSN: 1021-8637

The stabilisation of slopes with rows of discrete vertical piles is a commonly adopted method for both cuttings as well as embankment slopes. The majority of existing design procedures consider the pile only as an additional force or moment acting on the critical slip surface of the un-stabilised slope. Based on simplified models, existing design methodologies effectively ignore any interaction of the pile with the evolution of the failure mechanism, while they do not consider important aspects of soil behaviour for slope stability relating to strain softening response. This paper presents a numerical investigation that challenges the above-mentioned simplifications, demonstrating the importance of the soil-pile interaction. Two dimensional plane-strain hydro-mechanically coupled finite element analyses were performed to simulate the excavation of a slope, considering materials with both a strain softening and non-softening response. The impact of pile position and time of pile construction on the stability of a cutting were parametrically examined, comparing and contrasting the findings for the different material types. The results suggest that an oversimplification during design regarding the soil/pile interaction could entirely miss the critical failure mechanism.

Journal article

Vinck K, Liu T, Jardine RJ, Kontoe S, Ahmadi-Naghadeh R, Buckley RM, Byrne BW, Lawrence JA, McAdam RA, Schranz Fet al., 2022, Advanced in-situ and laboratory characterisation of the ALPACA chalk research site, Géotechnique, ISSN: 0016-8505

Low-to-medium density chalk at St Nicholas at Wade, UK, is characterised by intensive testing to inform the interpretation of axial and lateral tests on driven piles. The chalk de-structures when taken to large strains, especially under dynamic loading, leading to remarkably high pore pressures beneath penetrating CPT and driven pile tips, weak putty annuli around their shafts and degraded responses in full-displacement pressuremeter tests. Laboratory tests on carefully formed specimens explore the chalk's unstable structure and markedly time and rate-dependent mechanical behaviour. A clear hierarchy is found between profiles of peak strength with depth of Brazilian tension (BT), drained and undrained triaxial and direct simple shear (DSS) tests conducted from in-situ stress conditions. Highly instrumented triaxial tests reveal the chalk's unusual effective stress paths, markedly brittle failure behaviour from small strains and the effects of consolidating to higher than in-situ stresses. The chalk's mainly sub-vertical jointing and micro-fissuring leads to properties depending on specimen scale, with in-situ mass stiffnesses falling significantly below high-quality laboratory measurements and vertical Young's moduli exceeding horizontal stiffnesses. While compressive strength and stiffness appear relatively insensitive to effective stress levels, consolidation to higher pressures closes micro-fissures, increases stiffness and reduces anisotropy.

Journal article

Liu T, Ahmadi-Naghadeh R, Vinck K, Jardine RJ, Kontoe S, Buckley RM, Byrne BWet al., 2022, An experimental investigation into the behaviour of de-structured chalk under cyclic loading, Géotechnique, ISSN: 0016-8505

Low-to-medium density chalk can be de-structured to soft putty by high-pressure compression, dynamic impact or large-strain repetitive shearing. These process all occur during pile driving and affect subsequent static and cyclic load-carrying capacities. This paper reports undrained triaxial experiments on de-structured chalk, which shows distinctly time-dependent behaviour as well as highly non-linear stiffness, well-defined phase transformation (PT) and stable ultimate critical states under monotonic loading. Its response to high-level undrained cyclic loading invokes both contractive and dilative phases that lead to pore pressure build-up, leftward effective stress path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios that resemble those of silts. These outcomes are relatively insensitive to consolidation pressures and are distinctly different to those of the parent intact chalk. The maximum number of cycles that can be sustained under given combinations of mean and cyclic stresses are expressed in an interactive stress diagram which also identifies conditions under which cycling has no deleterious effect. Empirical correlations are proposed to predict the number of cycles to failure and mean effective stress drift trends under the most critical cyclic conditions. Specimens that survive long-term cycling present higher post-cyclic stiffnesses and shear strengths than equivalent ‘virgin’ specimens.

Journal article

Ahmadi-Naghadeh R, Liu T, Vinck K, Jardine RJ, Kontoe S, Byrne BW, McAdam RAet al., 2022, A laboratory characterisation of the response of intact chalk to cyclic loading, Géotechnique, ISSN: 0016-8505

This paper reports the cyclic behaviour of chalk, which has yet to be studied comprehensively. Multiple undrained high-resolution cyclic triaxial experiments on low-to-medium density intact chalk, along with index and monotonic reference tests, define the conditions under which either thousands of cycles could be applied without any deleterious effect, or failure can be provoked under specified numbers of cycles. Intact chalk's response is shown to differ from that of most saturated soils tested under comparable conditions. While chalk can be reduced to putty by severe two-way displacement-controlled cycling, its behaviour proved stable and nearly linear visco-elastic over much of the one-way, stress controlled, loading space examined, with stiffness improving over thousands of cycles, without loss of undrained shear strength. However, in cases where cyclic failure occurred, the specimens showed little sign of cyclic damage before cracking and movements on discontinuities lead to sharp pore pressure reductions, non-uniform displacements and the onset of brittle collapse. Chalk's behaviour resembles the fatigue response of metals, concretes and rocks, where micro-shearing or cracking initiates on imperfections that generate stress concentrations; the experiments identify the key features that must be captured in any representative cyclic loading model.

Journal article

Buckley R, Jardine R, Kontoe S, 2021, In situ testing in low-medium density structured chalk, 6th International Conference on Geotechnical and Geophysical Site Characterization

Conference paper

Buckley R, Kontoe S, Jardine R, Barbosa P, Schroeder Fet al., 2021, Pile driveability in low-to-medium density chalk, Canadian Geotechnical Journal, Vol: 58, Pages: 650-665, ISSN: 0008-3674

Pile driving in low- to medium-density chalk is subject to significant uncertainty. Predictions of “chalk resistance to driving” (CRD) often vary considerably from field driving behaviour, with both pile refusals and free falls under zero load being reported. However, recent field studies have led to better understanding of the processes that control the wide range of behaviour seen in the field. This paper describes the primary outcomes of the analysis of dynamic tests at an onshore and an offshore site and uses the results to propose a new method to predict CRD. The method is based on phenomena identified experimentally: the relationship between cone penetration resistance and CRD, the attenuation of local stresses as driving advances, and the operational effective stress interface shear failure characteristics. The proposed method is evaluated through back-analyses of driving records from independent pile installation cases that were not included in developing the method, but involved known ground conditions, hammer characteristics, and applied energies. The proposed method is shown to lead to more reliable predictions of CRD than the approaches currently applied by industry.

Journal article

Buckley R, Byrne BW, Doherty JP, Jardine R, Kontoe S, McAdam RA, Randolph MFet al., 2021, Measurements of distributed strain during impact pile driving, Piling 2020, Publisher: ICE Publishing

This paper reports the use of optical Fibre Bragg Grating (FBG) sensors to monitor the stress waves generated below ground during pile driving, combined with measurements using conventional pile driving analyzer (PDA) sensors mounted at the pile head. Fourteen tubular steel piles with a diameter of 508 mm and embedded length to diameter ratios of 6 to 20 were impact driven at an established chalk test site in Kent, UK. The pile shafts were instrumented with multiple FBG strain gauges and pile head PDA sensors, which monitored the piles’ responses under each hammer blow. A high frequency (5 kHz) fibre optic interrogator allowed a previously unseen resolution of the stress wave propagation along the pile. Estimates of the base soil resistances to driving and distributions of shaft shear resistances were found through signal matching that compared time series of pile head PDA measurements and FBG strains measured below ground surface. Numerical solutions of the one-dimensional wave equation were optimised by taking account of the data from multiple FBG gauges, leading to significant advantages that have potential for widespread application in cases where high resolution strain measurements are made.

Conference paper

Moeller JK, Kontoe S, Taborda D, Potts Det al., 2020, Maximum depth of liquefaction based on fully-coupled time domain site response analysis, 4th International Symposium on Frontiers in Offshore Geotechnics

Soil susceptibility to liquefaction is most commonly assessed in engineering practice using empirical correlations of in-situ tests with observed surface manifestations of liquefaction in case histories. This simplified design method further incorporates a correction factor for varying overburden pressure, derived from laboratory data, and provides expressions for earthquake induced shear stresses based on simplified one-dimensional equivalent linear site response analysis. The resulting factor of safety against liquefaction is only valid for the depths represented in the laboratory test data, case history data and the site response analyses, i.e. a maximum depth of 20 m. In order to evaluate the susceptibility of soils at larger depths, one-dimensional time-domain site response analyses are carried out, showing the extent of the liquefied zone for sand deposits of different depths. This study evaluates the performance of a bounding surface plasticity model in comparison with a nonlinear elastic cyclic model regarding the amplification and damping of certain frequency contents of shear waves propagating through deep soil deposits. These findings are of particular relevance for applications in offshore geotechnical engineering, where liquefaction in large depths can have severe effects on the load-carrying capacity of deep pile foundations.

Conference paper

Buckley R, Jardine R, Byrne B, Kontoe S, McAdam R, Ahmadi-Naghadeh R, Liu T, Schranz F, Vinck Ket al., 2020, Pile behaviour in low-medium density chalk: preliminary results from the ALPACA project, 4th International Symposium on Frontiers in Offshore Geotechnics

Conference paper

Tsaparli V, Kontoe S, Taborda D, Potts Det al., 2020, Resonance as the source of high vertical accelerations: field demonstration and impact on offshore wind turbines, 4th International Symposium on Frontiers in Offshore Geotechnics

Recent studies have demonstrated the significance of the vertical seismic acceleration component for offshore wind turbines, as their low natural period in this direction can result in significant excitation, potentially making this load case design-driving. Unexpectedly high vertical ground accelerations, well exceeding their horizontal counterparts, have also been recorded in a number of recent seismic events. This study explores the concept of resonance between the vertical seismic component and the natural frequency for compressional waves of fully saturated soil deposits, which can aggravate further the vertical accelerations at the top of structures of interest, using numerical analysis and monitoring data. The site response at a strong motion station that registered the second highest peak ground vertical acceleration during the 2011 Mw 6.2 Christchurch earthquake in New Zealand is modelled in finite element analyses. Two different depths are also considered: the first one is truncated at the interface of the softer surficial deposits with the stiff gravel horizon. This has been shown to be adequate for S-wave propagation modelling. Conversely, the second one models the full depth to bedrock. Despite the number of uncertainties involved, the results validate the concept of resonance in compression against field measurements and demonstrate the importance of the modelled depth in the case of vertical site response analysis.

Conference paper

Buckley R, Jardine R, Kontoe S, Barbosa P, Schroeder Fet al., 2020, Full-scale observations of dynamic and static axial responses of offshore piles driven in chalk and tills, Géotechnique, Vol: 70, Pages: 657-681, ISSN: 0016-8505

This paper describes and interprets tests on piles driven through glacial tills and chalk at a Baltic Sea windfarm, covering an advance trial campaign and later production piling. The trials involved six instrumented 1.37m diameter steel open-ended tubes driven in water depths up to 42m. Three piles were tested statically, with dynamic re-strike tests on paired piles, at 12-15 week ages. Instrumented dynamic driving and re-strike monitoring followed on up to 3.7m diameter production piles. During driving, the shaft resistances developed at fixed depths below sea-bed fell markedly during driving, with particularly sharp reductions occurring in the chalk. Shaft resistances increased markedly after driving and good agreement was seen between long-term capacities interpreted from parallel static and dynamic tests. Analyses employing the sites’ geotechnical profiles show long-term shaft resistances in the chalk that far exceed those indicated by current design recommendations, while newly proposed procedures offer good predictions. The shaft capacities mobilised in the low-plasticity tills also grew significantly over time, within the broad ranges reported for sandy soils. The value of offshore field testing in improving project outcomes and design rules is demonstrated; the approach described may be applied to other difficult seabed conditions.

Journal article

Buckley R, McAdam R, Byrne B, Doherty J, Jardine R, Kontoe S, Randolph Met al., 2020, Optimisation of impact pile driving using optical fibre Bragg grating measurements, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 146, Pages: 1-15, ISSN: 0733-9410

This paper reports the use of optical Fibre Bragg Grating (FBG) sensors to monitor the stress waves generated below ground during pile driving, combined with measurements using conventional pile driving analyzer (PDA) sensors mounted at the pile head. Fourteen tubular steel piles with a diameter of 508 mm and embedded length to diameter ratios of 6 to 20 were impact driven at an established chalk test site in Kent, UK. The pile shafts were instrumented with multiple FBG strain gauges and pile head PDA sensors, which monitored the piles’ responses under each hammer blow. A high frequency (5kHz) fibre optic interrogator allowed a previously unseen resolution of the stress wave propagation along the pile. Estimates of the base soil resistances to driving and distributions of shaft shear resistances were found through signal matching that compared time series of pile head PDA measurements and FBG strains measured below ground surface. Numerical solutions of the onedimensional wave equation were optimised by taking account of the data from multiple FBG gauges, leading to significant advantages that have potential for widespread application in cases where high resolution strain measurements are made.

Journal article

Tsaparli V, Kontoe S, Taborda D, Potts Det al., 2020, A case study of liquefaction: demonstrating the application of an advanced model and understanding the pitfalls of the simplified procedure, Geotechnique: international journal of soil mechanics, Vol: 70, Pages: 538-558, ISSN: 0016-8505

The complexity of advanced constitutive models often dictates that their capabilities are only demonstrated in the context of model testing under controlled conditions. In the case of earthquake engineering and liquefaction in particular, this restriction is magnified by the difficulties in measuring field behaviour under seismic loading. In this paper, the well documented case of the Canterbury Earthquake Sequence in New Zealand, for which extensive field and laboratory data are available, is utilised to demonstrate the accuracy of a bounding surface plasticity model in fully-coupled finite element analyses. A strong motion station with manifestation of liquefaction and the second highest peak vertical ground acceleration during the Mw 6.2 February 2011 event is modelled. An empirical assessment predicted no liquefaction for this station, making this an interesting case for rigorous numerical modelling. The calibration of the model aims at capturing both the laboratory tests and the field measurements in a consistent manner. The characterisation of the ground conditions is presented, while, to specify the bedrock motion, the records of two stations without liquefaction are deconvolved and scaled to account for wave attenuation with distance. The numerical predictions are compared to both the horizontal and vertical acceleration records and other field observations, showing a remarkable agreement, also demonstrating that the high vertical accelerations can be attributed to compressional resonance. The results provide further insights into the underperformance of the simplified procedure.

Journal article

Pelecanos L, Kontoe S, Zdravkovic L, 2020, The effects of dam-reservoir interaction on the nonlinear seismic response of earth dams, Journal of Earthquake Engineering, Vol: 24, Pages: 1034-1056, ISSN: 1363-2469

The objective of this study is to investigate the effects of dam–reservoir interaction (DRI) on the nonlinear seismic response of earth dams. Although DRI effects have for long been considered as insignificant for earth dams, that conclusion was mainly based on linear elastic investigations which focused only on the acceleration response of the crest without examining the seismic shear stresses and strains within the dam body. The present study explores further the impact of DRI focusing on the nonlinear behavior of earth dams. The effects of reservoir hydrodynamic pressures are investigated in terms of both seismic dam accelerations and nonlinear dynamic soil behavior (seismic shear stresses and strains). It is shown that although dam crest accelerations are indeed insensitive to DRI, the stress and strain development within the dam body can be significantly underestimated if DRI is ignored.

Journal article

Jardine R, Buckley R, Byrne B, Kontoe S, McAdam Ret al., 2019, Research to improve the design of driven pile foundations in chalk: the ALPACA project, Coastal Structures 2019, Publisher: Karlsruhe: Bundesanstalt für Wasserbau, Pages: 923-930

Large numbers of offshore wind turbines, near-shore bridges and port facilities are supported by driven piles. The design and installation of such piles is often problematic in Chalk, a low-density, porous, weak carbonate rock, which is present under large areas of NW Europe. There is little guidance available to designers on driveability, axial capacity, the lateral pile resistance which dominates offshore wind turbine monopile behaviour, or on how piles can sustain axial or lateral cyclic loading. This paper describes the ALPACA project which involves comprehensive field testing at a low-to-medium density chalk research test site. The project is developing new design guidance through comprehensive field testing and analysis combined with in-situ testing campaigns and advanced static-and-cyclic laboratory testing on high quality block and rotary core samples.

Conference paper

Jardine RJ, Buckley RM, Byrne BW, Kontoe S, McAdam RA, Andolfsson T, Liu TF, Schranz F, Vinck Ket al., 2019, Improving the design of piles driven in chalk through the ALPACA research project, Revue Française de Géotechnique, Vol: 158, ISSN: 0181-0529

Chalk is present under large areas of NW Europe as a low-density, porous, weak carbonate rock. Large numbers of offshore wind turbines, bridges and port facilities rely on piles driven in chalk. Current European practice assumes ultimate shaft resistances that appear low in comparison with the Chalk’s unconfined compression strength and CPT cone resistance ranges and can impact very significantly on project economics. Little guidance is available on pile driveability, set-up or lateral resistance in chalk, or on how piles driven in chalk can sustain axial or lateral cyclic loading. This paper describes the ALPACA (Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing) project funded by EPSRC and Industry that is developing new design guidance through comprehensive field testing at a well-characterised low-to-medium density test site, supported by analysis of other tests. Field experiments on 36 driven piles, sixteen of which employ high resolution fibre-optic strain gauges, is supported by advanced laboratory and in situ testing, as well as theoretical analysis. The field work commenced in October 2017 and was largely complete in May 2019.

Journal article

Lau K-K, Kontoe S, Anatolakis G, 2019, A critical comparison between stress and energy based methods for the evaluation of liquefaction potential, SECED 2019 Conference: Earthquake risk and engineering towards a resilient world

Selected liquefaction case histories in New Zealand during recent earthquakes were analysed using the conventional SPT and CPT stress based methods, and the energy based method recently proposed by Kokusho (2013). Several sites in the wider Christchurch region were examined considering strong motions from the 2010-2011 Canterbury Earthquake Sequence. The liquefaction potential was also examined at three sites in the Wellington and Marlborough regions for the 2013 Mw6.6 Lake Grassmere and 2016 Mw7.8 Kaikoura earthquakes. The methods were compared in terms of the critical liquefaction depth and layer thickness, data scatter and number of false-negative predictions. The Kokusho energy based method performed satisfactorily in assessing the liquefaction potential at the case history sites, giving comparable results to the stress based methods. Furthermore, the Kokusho method succeeded in identifying liquefaction potential at several sites in Christchurch where false-negative predictions were shown in the CPT stress based method.

Conference paper

Solans D, Kontoe S, Zdravkovic L, 2019, Monotonic and cyclic response of tailings sands, SECED 2019 Conference: Earthquake risk and engineering towards a resilient world, Publisher: https://www.seced.org.uk/index.php/proceedings

: The extensive mining production worldwide results in vast amounts of residues requiring the construction of new tailings dams. As site availability is limited due to environmental restrictions, tailings dams tend to be very large and, with heights of over 200 m in some cases, often raising stability concerns. Past experience has shown that failure of tailings dams during earthquakes can be catastrophic, with detrimental consequences for the neighbouring communities, environment and the economy. Prominent examples of such cases are the failure of the El Cobre N°1 dam in Chile, due to the 1965 earthquake, and more recently the Fundão tailings dam failure in Brazil in November 2015. This article investigates the monotonic and cyclic behaviour of tailings sands for different fines content and at a range of relative densities and confining pressures. Several aspects of the behaviour of tailings sands, such as compressibility, strength characteristics and cyclic response, are compared with those of natural sands. Based on the available laboratory test results and the interpretation performed, it is possible to distinguish certain features of this type of material, which are not typically observed in natural soil deposits, and to address common misleading comparisons between the behaviour of natural and tailings sands.

Conference paper

Buckley R, Jardine R, Kontoe S, Schroeder F, Barbosa Pet al., 2019, The design of axially loaded driven piles in chalk, XVII European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: Icelandic Geotechnical Society

The behaviour of driven piles in chalk is poorly understood; their installation resistance, set-up characteristics and response to cyclic and static loading all warrant further investigation. Current axial capacity design methods have poor reliability, particularly in low-medium density chalk. This paper gives an overviewof research which combined systematic investigations at an onshore chalk site in Kent, UK, with careful analysis of large scale offshore tests. The onshore studies involved reduced-scale open-ended driven piles and heavily instrumented closed-ended Imperial College Piles. The offshore analyses addressed static and dynamic pile tests conducted on full scale open-ended steel tubular piles driven in glacial till and low-to-medium density chalk. The understanding drawn from both streams of research form the basis for a new Chalk ICP-18 effective stress-based design approach, which centres on the key physical phenomena identified: (i) the close correlation between pile resistances and local variations in CPT cone resistance (ii) the marked effect of the relativedepth, h/R*, of the pile tip below any given chalk horizon (iii) the effective stress shaft interface shear failure characteristics and (iv) very significant capacity gains over time. The new method offersbetter predictions of field behaviourwith time than the current industry method.

Conference paper

Pelecanos L, Kontoe S, Zdravkovic L, 2019, Nonlinear seismic response of earth dams due to dam-reservoir interaction, The XVII European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: Icelandic Geotechnical Society

Field data showthat the seismic response of a dam with a full reservoir is different from a dam with an empty reservoir. This is due to“dam-reservoir interaction” (DRI), which is related to the asynchronous vibration of the dam and reservoir domains. It was for long considered that DRI effects are important for concrete dams and insignificant for earth dams. This was based on findings considering mainly dam crest accelerations, which for earth dams wereindeed found to be insensitive to DRI. However, other aspects of theresponse of earth dams, such as the deformation characteristics, should also be considered to fully characterise theseismicdam response. Therefore for a more complete study of the DRI effects on the seismic response of earth dams one should also consider the induced seismic shear stresses and strains, along with the magnitude of reservoir hydrodynamic pressures. This study considers a well-documented case study, the La Villita earth dam in Mexico, for which relevant field measurements are available allowing the development of a well-calibrated numerical model. A series of static and dynamic nonlinear finite element analyses are performed which consider the impact of the reservoir domain on the dam response. It is shown that although earth dam crest accelerations are indeed insensitive to DRI, the actual dynamic soil behaviour can be severely affected, developing large values of seismic shear stresses and strains within the dam body. This study highlights the importance of accurately considering DRI when assessing the seismic performance of earth dams.

Conference paper

Jardine R, Kontoe S, Liu T, Vinck K, Byrne B, Ross M, Schranz F, Andolfsson T, Buckley Ret al., 2019, The ALPACA research project to improve design of piles driven in chalk, XVII European Conference on Soil Mechanics and Geotechnical Engineering, Publisher: Icelandic Geotechnical Society

Chalk is present under large areas of NW Europe as a low-density, porous,weak carbonate rock. Large numbers of offshore wind turbines, bridgesand port facilities rely on piles driven in chalk. Current European practice assumesultimate shaft resistances that appear low in comparison with the Chalk’s unconfined compression strength and CPT cone resistance rangesand can impact very significantly on project economics. Little guidance is available on pile driveability, set-up or lateral resistance in chalk, or on how piles driven in chalk can sustain axial or lateral cyclic loading. This paper describes the ALPACA (Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing) projectfunded by EPSRC and Industry that is developingnew design guidance through comprehensive field testing at awell-characterised low-to-medium density test site, supported by analysis of other tests. Field experiments on 36driven piles, sixteen of which employ high resolution fibre-optic strain gauges, is supported by advanced laboratory and in-situ testing, as well as theoretical analysis. The field work commenced in October 2017 andwas largely complete inMay2019.

Conference paper

Solans D, Skiada E, Kontoe S, Potts DMet al., 2019, Canyon topography effects on ground motion: Assessment of different soil stiffness profiles, Obras y Proyectos, Vol: 25, Pages: 51-58, ISSN: 0718-2805

The effect of topography on ground motion has been well recognized during numerous earthquakes. Several studies present observational evidence from destructive earthquakes, where the damage is higher in the vicinity of hills and near slope crests. Furthermore, a number of numerical studies aimed to reproduce this phenomenon using different numerical methods, e.g. Finite Elements, Finite Differences and Boundary Elements have been carried out. Most of these investigations involve parametric studies, considering different variables. However, one of the assumptions of these studies is a homogeneous soil stiffness with depth, which is not in most cases realistic. This article investigates the effects of canyon topography on ground motion considering different soil stiffness profiles over a rigid bedrock. Three soil profiles with stiffness variation with depth are examined and compared to the case of a soil layer of uniform stiffness. An additional analysis of a two- layer medium lying above half-space is also considered. Time domain numerical analyses are carried out with the Imperial College Finite Element Program ICFEP, considering linear elastic soil behaviour over rigid bedrock. The input motions are wavelets of harmonic nature, modified by a Saragoni and Hart (1973) temporal filter. These wavelets with a characteristic. pulse period Tp in the range of 0.1 s to 2 s are analysed. This study confirms that the topographic amplification extrema are located between the natural periods of the corresponding one-dimensional free-field profile in agreement with recent previous studies. Furthermore, the amplitude of the topographic amplification peaks is shown to change for the different examined soil stiffness profiles.

Journal article

Jardine R, Buckley R, Byrne B, Kontoe S, Liu T, McAdam R, Schranz F, Vinck Ket al., 2019, Rationalising the design of piles driven in chalk through the ALPACA project, 2nd International Conference on Natural Hazards & Infrastructure

Conference paper

Pelecanos L, Kontoe S, Zdravkovic L, 2019, Seismic response of earth dams in narrow canyons, VII International Conference on Earthquake Geotechnical Engineering

It is nowadays well appreciated that dams built in narrow canyons exhibit a stiffer re-sponse than those in wide canyons, due to the confined geometry of the canyon banks. The numerical modelling of dams in wide canyons is usually considered as computational-ly less expensive than those in narrow canyons. This is because the former can be ideal-ised by a two-dimensional plane-strain model, while the latter requires a full three-dimensional analysis to appropriately consider the stiffening effect of the narrow canyon geometry. This paper presents a computationally-efficient way to consider the stiffening effect of a narrow canyon in a two-dimensional analysis by using an appropriately in-creased material stiffness.

Conference paper

Kontoe S, Han B, Pelecanos L, Zdravkovic Let al., 2019, Hydrodynamic effects and hydro-mechanical coupling in the seismic response of dams, VII International Conference on Earthquake Geotechnical Engineering, Publisher: Balkema

The seismic design of earthfill and rockfill dams routinely relies on methods of analysis which adopt simplifying assumptions regarding the dynamic response of the reservoir, while the dynamic interaction of the fluid and solid phases within the dam body is also typically ignored. In this paper, a simple numerical approach for the efficient simulation of hydrodynamic pressures in finite element analysis is presented and then used to assess the impact of hydrodynamic pressures on the seismic response of dams. The importance of both hydrodynamic pressures and of hydro-mechanical coupling is then discussed within the context of two well-documented case studies, of an earthfill and a rockfill dam, comparing the numerical predictions against field measurements.

Conference paper

Kontoe S, Han B, Pelecanos L, Zdravkovic Let al., 2019, Seismic response of earthfill and rockfill embankment dams, 3rd Meeting of EWG Dams and Earthquakes - an International Symposium, Publisher: LNEC

The seismic design of earthfill and rockfill dams routinely relies on methods of analysis, which adopt simplifying assumptions regardingthe damgeometry, soil behaviour and the dynamic interaction of the fluid and solid phases within the dam body. This paper explores such simplifying assumptions, which aretypically used for thenumerical modelling of earthfill and rockfill embankment dams,within the context of two case studies. First a clay core dam,theLa Villita dam in Mexico, is considered focusing mainly on the implications of 2D plane strain approximation in the case of dams built in relatively narrow canyons. In the second case study, of the rockfill Yele dam in China, the importance of hydro-mechanicalcoupling is explored by parametrically varying the permeability of the materials.

Conference paper

Norambuena R, Tsaparli V, Kontoe S, Taborda D, Potts Det al., 2019, The effect of irregular seismic loading on the validity of the simplified liquefaction procedures, Obras y Proyectos, Pages: 42-50, ISSN: 0718-2805

Soil liquefaction has been one of the major hazards for civil engineering projects relating to earthquakes. The simplified liquefaction procedure which is used to assess liquefaction susceptibility in practice is still based on semi-empirical methods. These rely on the assumption that irregular seismic motions can be represented fully by an equivalent number of cycles of uniform stress amplitude, which is based on the peak acceleration measured at ground surface. Most methodologies used to calculate the equivalent number of cycles are based on Miner's damage concept developed for the fatigue analysis of metals. Several researchers have questioned the validity of this concept, as soils have a highly non-linear response. The present work investigates numerically the concept of the equivalent uniform amplitude cycles. Effective stress-based non-linear finite element analyses are performed with a modified bounding surface plasticity model that allows to realistically simulate liquefaction, reproducing the cyclic strength of sands accurately. The seismic response of a 15 m deep uniform level-ground sand deposit is simulated with full hydro-mechanical coupling to establish the benchmark extent of liquefaction zone. In parallel, the analyses are repeated assuming drained conditions to compute the irregular time-histories, which are then converted to an equivalent number of uniform amplitude cycles. The constant amplitude series are then applied in single element simple shear test simulations, with initial conditions those corresponding to the 7 m depth in the deposit. The results in terms of the predicted triggering of liquefaction are contrasted to the predictions of the fully coupled benchmark analyses at the corresponding depth to assess the validity of the Seed et al. (1975) methodology, based on Miner's cumulative damage concept.

Journal article

Buckley R, Jardine R, Kontoe S, Lehane Bet al., 2018, Effective stress regime around a jacked steel pile during installation, ageing and load testing in chalk, Canadian Geotechnical Journal, Vol: 55, Pages: 1577-1591, ISSN: 0008-3674

This paper reports experiments with 102 mm diameter closed-ended instrumented Imperial College piles (ICPs) jacked into low- to medium-density chalk at a well-characterized UK test site. The “ICP” instruments allowed the effective stress regime surrounding the pile shaft to be tracked during pile installation, equalization periods of up to 2.5 months, and load testing under static tension and one-way axial cyclic loading. Installation resistances are shown to be dominated by the pile tip loads. Low installation shaft stresses and radial effective stresses were measured that correlated with local cone penetration test (CPT) tip resistances. Marked shaft total stress reductions and steep stress gradients are demonstrated in the vicinity of the pile tip. The local interface shaft effective stress paths developed during static and cyclic loading displayed trends that resemble those seen in comparable tests in sands. Shaft failure followed the Coulomb law and constrained interface dilation was apparent as the pile experienced drained loading to failure, although with a lesser degree of radial expansion than with sands. Radial effective stresses were also found to fall with time after installation, leading to reductions in shaft capacity as proven by subsequent static tension testing. The jacked, closed-ended, piles’ ageing trends contrast sharply with those found with open piles driven at the same site, indicating that ageing is affected by pile tip geometry and (or) installation method.

Journal article

Jardine R, Buckley R, Kontoe S, Barbosa P, Schroeder Fet al., 2018, Behaviour of piles driven in chalk, Chalk 2018, Publisher: ICE Publishing

Driving resistance is difficult to predict in chalk strata, with both pile free-fall self-weight ‘runs’ and refusals being reported. Axial capacity is also highly uncertain after driving. This paper reviews recent research that has explored these topics. Programmes of onshore tests and novel, high-value offshore, experiments involving static, dynamic and cyclic loading are described. The key findings form the basis of the Chalk ICP-18 approach for estimating the driving resistance and axial capacity of piles driven in low-to medium-density chalk. The new approach is presented and the highly significant impact of set-up after driving is emphasised. It is shown that Chalk ICP-18 overcomes the main limitations of the industry’s current design guidelines by addressing the underlying physical mechanisms. While further tests are required to enlarge the available test database, the new approach is able to provide better predictions for tests available from suitably characterised sites. A new Joint Industry Project is outlined that extends the research to cover further axial, lateral, static and cyclic loading cases.

Conference paper

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