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Keynote Lecture: Dike failures; Risks, Causes and Costs Van Baars, Stefan Speeches/Talks (2010) Detailed reference viewed: 51 (0 UL)Near field wave transformation in clay and peat Van Baars, Stefan in Fifth international conference on recent advances in geotechnical earthquake and soil dynamics, San Diego (2010, May) A series of simple impact tests is performed, both on clay and on peat, in order to compare the current wave theory with the obtained data. The waves at the surface are measured with three 3D geophones ... [more ▼] A series of simple impact tests is performed, both on clay and on peat, in order to compare the current wave theory with the obtained data. The waves at the surface are measured with three 3D geophones. With this the wave propagation and the energy dissipation in three directions has been studied both for clay and peat. Since the impact produces a group of waves, only the first arrival time can easily be detected. By transferring the wave information into a 3D energy flow, also the average arrival time and the end-of-wave arrival time can be determined. The results of all tests are very similar, the short wave group splits up in three phases: the first phase consists of compression waves, in the second phase the Rayleigh waves dominate and in the last phase, remarkably, a kind of diagonal Love waves dominate. The second phase starts with a sudden flip of the particle motion due to the arrival of the second wave. In clay 78% of the energy is found in the second phase, while in peat 72% of the energy is found in the third phase. [less ▲] Detailed reference viewed: 38 (0 UL)The Causes and Mechanisms of Historical Dike Failures in the Netherlands Van Baars, Stefan ; in E-water (2009) In a historical perspective, flood protection in the past was not given high priority - plague and periods of famine and war took precedence. Poverty and a lack of knowledge made it difficult to create ... [more ▼] In a historical perspective, flood protection in the past was not given high priority - plague and periods of famine and war took precedence. Poverty and a lack of knowledge made it difficult to create safer dikes. Dike engineering did improve in Napoleonic era due to the French Central Government, but many dikes still failed. A historical overview of the causes and mechanisms of dike failures in the Netherlands has been drawn up, and resulted in a list of 337 recorded events leading to an assumed total of 1735 dike failures in the Netherlands between 1134 and 2006. Storm surges were, generally speaking, the primary cause of dike failure, followed by high water and ice drift. Two-thirds of all dikes failed as a result of the inner slope protection or the crest of the dike being eroded. The main causes for this were run-over and wave overflow, which could have been prevented, had the dikes been higher. The second cause of dike failure was ice drift. All other mechanisms were of minor importance, but were in the past either difficult to determine or even know about. [less ▲] Detailed reference viewed: 74 (0 UL)Trillingen tijdens het boren van de Hubertustunnel Van Baars, Stefan ; in Geotechniek (2009) Tijdens het boren van de Hubertustunnel in Den Haag zijn geen schadelijke trillingen opgetreden, maar wel onverwachte hinderlijke trillingen. Bij het boorproces in bijvoorbeeld rotsformaties zijn ... [more ▼] Tijdens het boren van de Hubertustunnel in Den Haag zijn geen schadelijke trillingen opgetreden, maar wel onverwachte hinderlijke trillingen. Bij het boorproces in bijvoorbeeld rotsformaties zijn dergelijke trillingen bekend, maar men had dit niet verwacht in het hier aanwezige strandzand. Uit analyse van de gemeten trillingen blijkt dat de trillingen voornamelijk zijn ontstaan door de stroeve voortgang van de TBM in het zand ten gevolge van het stick-slip effect. Het vooruit schokken van de TBM veroorzaakt hoofdzakelijk trillingen in boorrichting: schuifgolven langs de wand en drukgolven aan het boorfront. [less ▲] Detailed reference viewed: 51 (0 UL)Numerical modelling of visco-plastic soft soils Van Baars, Stefan in 11th Baltic Sea Geotechnical Conference on Geotechnics in Maritime Engineering (2008, September) In many cases geotechnical calculations are made with linear or non-linear elastoplastic soil models. The most common case in which these types of soil models are not sophisticated enough is during the ... [more ▼] In many cases geotechnical calculations are made with linear or non-linear elastoplastic soil models. The most common case in which these types of soil models are not sophisticated enough is during the modelling of creep of soft soils. Most constitutive soil models modelling creep are based on volumic strain reduction over time. In this paper the creep is modelled through shear stress reduction (the viscous behaviour). The key point of viscous modelling is that this type of stress reduction (Mohr-circle squeezing) also takes place below the Coulomb failure line. In other words, viscosity is a sort of slow and rate-depending plasticity without reaching the Coulomb criterion; therefore also the word visco-plasticity can be used. This paper shows that visco-plastic modelling of soft soils will produce more realistic results and shows how this visco-plastic modelling works. [less ▲] Detailed reference viewed: 50 (0 UL)Adaptation of Finite Element Models for Probabilistic Design Van Baars, Stefan in 11th Baltic Sea Geotechnical Conference on Geotechnics in Maritime Engineering (2008, September) Both a circular slip surface stability calculation (Bishop or Fellenius) and a Finite Element Method calculation (c-phi reduction) produce a safety factor but not a probability of failure or reliability ... [more ▼] Both a circular slip surface stability calculation (Bishop or Fellenius) and a Finite Element Method calculation (c-phi reduction) produce a safety factor but not a probability of failure or reliability index. A Monte-Carlo method leads, because of the many slip surface calculations, to the most accurate calculation of the probability of safety, but this method is far too time consuming for daily FEM calculations. The idea is therefore to replace the Monte-Carlo approach by a First Order Reliability Method (FORM), based on a limited number of FEM calculations. A possible problem with this FORM is, that variable soil parameters create a correlated and therefore variable failure mechanism. However, FORM gives an indication of this non-linear behaviour and in most cases an accurate probability of failure of the structure can be calculated. This article gives a complete example of the calculation of the probability of failure of a dyke ring along a river, based on a limited number of Phi-C reduction calculations with Plaxis. The input data for this calculation is based on a limited number of given soil test data. [less ▲] Detailed reference viewed: 60 (0 UL)Vertical Caisson Breakwaters Cyclically Loaded ; ; Van Baars, Stefan in 11th Baltic Sea Geotechnical Conference on Geotechnics in Maritime Engineering (2008, September) Aim of this paper is analysing the breakwater foundation behaviour observed during centrifuge tests performed at the laboratory of GeoDelft (The Netherlands). Dynamic centrifuge tests were performed on a ... [more ▼] Aim of this paper is analysing the breakwater foundation behaviour observed during centrifuge tests performed at the laboratory of GeoDelft (The Netherlands). Dynamic centrifuge tests were performed on a model caisson breakwater, founded on medium-dense sand. The structure was subjected to cyclic horizontal loading simulating regular and irregular wave loading, until failure occurred. The tests were aimed at finding a critical failure mechanism, investigating the influence of the loading scheme and analysing the development of instantaneous pore pressures and residual pore pressures in the sandy subsoil. Test results showed high instantaneous values of excess pore pressures underneath the structure, not leading to a complete “liquefaction flow” failure. Dilation phenomena of sand induced local “recovering” of strength during each cycle and failure occurred according to a progressive mechanism with large cyclic strains. [less ▲] Detailed reference viewed: 66 (0 UL)Dutch peat dyke failure during the dry summer of 2003 Van Baars, Stefan in Proc. International symposium Drougt and Construction SEC 2008 (2008, September) ABSTRACT - In August 2003, after a once in a twenty years dry summer, one of the dykes along a canal in The Netherlands failed at night. Because of this dyke breach, the water in the canal started to run ... [more ▼] ABSTRACT - In August 2003, after a once in a twenty years dry summer, one of the dykes along a canal in The Netherlands failed at night. Because of this dyke breach, the water in the canal started to run into a housing quarter of the village Wilnis, which is about 30 km Southeast of Amsterdam. Like many other small dykes in The Netherlands, the complete dyke in Wilnis consists of peat. Since peat has a relatively low unit weight, a peat dyke has a higher risk of being pushed aside by water pressure than sand or clay dykes. Peat also has a much higher water content than clay and sand, which makes it more vulnerable to drought. When the stability of this dyke is checked, it becomes clear that failure of the dyke was a realistic threat. The dyke was far too light and the drought reduced this weight even a little bit more, which caused the failure. RÉSUMÉ – En août 2003, après une période de sècheresse bidécennale, une des digues le long d’un canal aux Pays-Bas a rompu durant une nuit. A cause de la rupture de cette digue, un quartier du village de Wilnis situé á 30 km au sud-est d’Amsterdam a été inondé. Comme beaucoup des petites digues aux Pays-Bas, cette digue est entièrement constituée de tourbe. Puisque la tourbe est très légère, une digue constituée de ce matériau a un risque plus important de glisser par rapport á une digue en sable ou en argile. De plus, la teneur en eau de la tourbe est beaucoup plus grande que pour un sable ou une argile ce qui la rend plus vulnérable au séchage. Si la stabilité de cette digue est contrôlée, il est clair que le risque de rupture de cette digue était très sérieux. Elle était beaucoup trop légère et la sècheresse a diminué un peu plus le poids ce qui a causé la rupture. [less ▲] Detailed reference viewed: 124 (0 UL)The Stability of Flood Defences on Permeable Soils: The London Avenue Canal Failures in New Orleans ; Van Baars, Stefan ; in Sixth International Conference on Case Histories in Geotechnical Engineering (2008, August) The two failures of the London Avenue Canal floodwalls contributed largely to the flooding of central New Orleans due to hurricane Katrina. In this paper, both failures are analyzed and compared to each ... [more ▼] The two failures of the London Avenue Canal floodwalls contributed largely to the flooding of central New Orleans due to hurricane Katrina. In this paper, both failures are analyzed and compared to each other since the flood defenses are both located on permeable soils. Photo’s observation and calculations are used for the analysis. Both failures are caused by the permeable sand layer below the floodwall that allowed high pore water pressures to develop below the floodwall. However, the south breach seems to be caused by the piping failure mechanism and the north breach by loss of stability. At the South breach, the impermeable top layer was thicker than at the North breach, increasing the stability. The North beach was less vulnerable for piping and the lack of stability caused a large breach. The London Avenue Canal failures are a clear yet tragic example of the failure of flood defenses on permeable soils. The failures show that multiple failure mechanism may occur and since there are many flood defenses on permeable soils world wide, the lessons from Katrina can be used to prevent future catastrophes. [less ▲] Detailed reference viewed: 102 (0 UL)A probabilistic tool for geotechnical assessment of jack-up penetration ; ; et al in Proceedings of the 6th International conference on offshore site investigation and geotechnics (2007, September) Prediction of jack-up leg penetration requires assesment of uncertainties in calculation models and their input parameter values, including quality and extent of site investigation information. Commonly ... [more ▼] Prediction of jack-up leg penetration requires assesment of uncertainties in calculation models and their input parameter values, including quality and extent of site investigation information. Commonly, uncertainy assesment is achieved by sensitivity analysis. This paper presents a probabilistic approach for a more accurate prediction of jack-up leg penetration behaviour, as an add-on for conventional Society of Naval Architects and Marine Engineers (SNAME) calculation models.... [less ▲] Detailed reference viewed: 24 (1 UL)Determination of organic soils permeability using the piezocone dissipation test Van Baars, Stefan ; in Environmental & Engineering Geoscience (2007) The hydraulic conductivity, which is among other parameters, needed to predict time-dependant settlement, can be estimated using a piezocone dissipation test. This test is commonly used together with cone ... [more ▼] The hydraulic conductivity, which is among other parameters, needed to predict time-dependant settlement, can be estimated using a piezocone dissipation test. This test is commonly used together with cone penetration tests (CPTs), soil borings and laboratory tests in foundation site investigations. The piezocone dissipation test is based on the fact that the rate of decay of the large excess pore water pressures generated during penetration of the piezocone through saturated clays and silts depends on the hydraulic conductivity of the material. However, interpretation of the dissipation curve is often problematic as existing methods of analysis assume a continuous decrease of pore pressure with time, whereas actual dissipation curves often exhibit non-standard behavior, the interpretation of which is more complex. This paper presents a new method of interpretation that can be used to estimate the hydraulic conductivity regardless of the shape of the dissipation curve. Examples of results using the new analysis method are compared with results obtained using laboratory oedometer testing. [less ▲] Detailed reference viewed: 85 (1 UL)Problemen waterkeringen New Orleans tijdens Katrina ; Van Baars, Stefan in Land+Water (2007) In de zomer van 2005 werd New Orleans zwaar getroffen door orkaan Katrina. Dat het waterkeringsysteem heeft gefaald is iedereen bekend, maar wat daar de redenen voor waren is veel minder bekend. De ... [more ▼] In de zomer van 2005 werd New Orleans zwaar getroffen door orkaan Katrina. Dat het waterkeringsysteem heeft gefaald is iedereen bekend, maar wat daar de redenen voor waren is veel minder bekend. De opgetreden waterstanden hadden immers volgens Nederlandse maatstaven gekeerd moeten kunnen worden. Nu blijkt echter dat er een groot aantal problemen waren met het waterkeringsysteem. De belangrijkste worden in dit artikel beschreven. [less ▲] Detailed reference viewed: 53 (1 UL)New Orleans tijdens orkaan Katrina Van Baars, Stefan in Land+Water (2007) New Orleans kent een grote overeenstemming met delen van Nederland. Het is een delta, het ligt gedeeltelijk lager dan zeeniveau en het kent ook een dubbele bedreiging: zowel van de zee als van de rivieren ... [more ▼] New Orleans kent een grote overeenstemming met delen van Nederland. Het is een delta, het ligt gedeeltelijk lager dan zeeniveau en het kent ook een dubbele bedreiging: zowel van de zee als van de rivieren. In de zomer van 2005 werd New Orleans zwaar getroffen door orkaan Katrina. In het verleden zijn er al vaker overstromingen geweest, maar nooit zo ernstig als deze keer. De beelden hiervan zijn over de hele wereld gegaan. In dit artikel wordt beschreven wat er mis ging. Omdat de situatie van New Orleans in meerdere opzichten op Nederland lijkt kunnen uit deze ramp lessen worden getrokken die wellicht ook voor Nederland toepasbaar zijn. [less ▲] Detailed reference viewed: 64 (0 UL)Viscous Soft-Soil Modelling Van Baars, Stefan in Proc. GeoDenver conference 2007 (2007, February) Viscosity is in fact a relaxation of the shear stresses over time, which results for constant stress states in a corresponding elastic strain relaxation. In other words, viscosity is a sort of slow ... [more ▼] Viscosity is in fact a relaxation of the shear stresses over time, which results for constant stress states in a corresponding elastic strain relaxation. In other words, viscosity is a sort of slow plasticity before the Coulomb criterion is reached. Creep settlement can be regarded as one of the viscous soil phenomena, but also the slow movement (a few mm or cm every year) of many natural slopes in mountainous area’s. Therefore a viscous constitutive model is derived from the standard principles that the reduction rate of the relative shear stress (the viscosity) is a function of this relative shear stress and that the reduction rate approaches zero while the stress-state approaches the K0-state. As an example a User-Defined (Soft) Soil Model is developed in order to model the continuous moving slopes. Although the model works fine for viscous behavior and creep problems, it seems as if the slope movements do not depend on viscosity but on dilatancy. [less ▲] Detailed reference viewed: 57 (0 UL)Soil Mechanics ; Van Baars, Stefan Book published by VSSD (2007) Detailed reference viewed: 180 (5 UL)Implementation and validation of a simple probabilistic tool for finite element codes Van Baars, Stefan in Numerical Methods in Geotechnical Engineering: Sixth European Conference on Numerical Methods in Geotechnical Engineering (Graz, Austria, 6-8 September 2006) (2006, August) Both a circular slip surface stability calculation (Bishop or Fellenius) and a FEM calculation (c-phi reduction) produce a safety factor but not a probability of failure or reliability index. The mayor ... [more ▼] Both a circular slip surface stability calculation (Bishop or Fellenius) and a FEM calculation (c-phi reduction) produce a safety factor but not a probability of failure or reliability index. The mayor problem with the probabilistical calculations is that variable soil parameters create a correlated and therefore also variable failure mechanism. The question is how much the value of the calculated probability of failure is influenced by this correlation. The Monte-Carlo method leads, because of the many slip surface calculations, to the most accurate calculation of the probability of safety. The disadvantage of this method is that it is far too time consuming for daily FEM calculations. The idea is therefore to replace the Monte-Carlo approach by another approach with only a limited amount of calculations. In this paper several methods are discussed and validated. An indication is given about the error, the number of necessary stability calculations and the optimal estimation points for an additional stability calculation. In this way a minimum number of FEM calculations is needed to obtain a certain desired accuracy. [less ▲] Detailed reference viewed: 70 (2 UL)The horizontal failure mechanism of the Wilnis peat dike Van Baars, Stefan in Geotechnique (2005), 55(4), 319-323 Detailed reference viewed: 119 (2 UL)Geotechnical applications and conditions of the observational method Van Baars, Stefan ; in Heron (2005), 50(3), Information observed during construction can be used to optimise the remaining parts of the construction or structure. In this article general conditions are indicated for the implementation of the ... [more ▼] Information observed during construction can be used to optimise the remaining parts of the construction or structure. In this article general conditions are indicated for the implementation of the Observational Method in geotechnical engineering. The most important condition is that the uncertain field condition (the observational data) must be clear to measure. For example in case of a brittle soil behaviour, there is no clear indicator to show that the load is approaching the maximum resistance. If a project fulfils all general conditions, a financial risk analysis is necessary to find out whether the Observational Method is profitable. The success of implementation varies strongly per geotechnical discipline. In this article a simplified approach for a risk analysis is given and a number of geotechnical cases are discussed for which the Observational Method can lead to a financial profit. Most of these cases are already informally implemented in daily practice. [less ▲] Detailed reference viewed: 55 (1 UL)Design of sheet pile installation by vibration Van Baars, Stefan in Geotechnical and Geological Engineering (2004), 22(3), 391-400 In the Netherlands 70% of the steel sheet piles are installed with vibrators. This is because vibrators have a high production rate and therefore a low installation cost. This method only works for soft ... [more ▼] In the Netherlands 70% of the steel sheet piles are installed with vibrators. This is because vibrators have a high production rate and therefore a low installation cost. This method only works for soft peat and clay layers and saturated sand layers which are not densely compacted. During pile installation several problems might occur. In this paper the attention is paid to avoiding jamming of the pile before it has reached its planned depth. In most of these cases this is caused by a design error which has let to a vibrator which is too light. Over the years different design tools, such as computer models, design graphs and design equations, have been developed in order to predict the minimum required vibrator force. In this paper a new design equation is presented, which consists of three parts: the mass of the sheet pile, the friction along the shaft and the soil resistance at the tip of the pile based on the cone penetration resistance. This equation and several other design tools for sheet pile installation, have been compared with 18 field tests. It was found that the general design graphs of the Dutch Union of Foundation Contractors (NVAF), the design equation of the Dutch Civil Engineering Centre for Construction Research and Design Rules (CUR) and the German design equation of the EAU1996 gave very poor results. The computer model Vibdrive from Holeyman (1996), the design graphs for specific locations from the NVAF and the new presented design equation from the author gave reasonable predictions. Since the computer program is, because of its complexity, not regarded as a design tool for daily design work and the local design graphs only exist for a small number of locations in The Netherlands, the new design equation is according to the field tests the best method for the calculation of the vibrator needed for sheet pile installation. [less ▲] Detailed reference viewed: 79 (0 UL)Soft Soil Creep Modelling of large settlements Van Baars, Stefan in 2nd International Conference on Advances in Soft Soil Engineering and Technology (2003) The most commonly used model in The Netherlands to predict settlements is the Koppejan Model. This model gives reasonable predictions, but sometimes there are large discrepancies with reality. This is ... [more ▼] The most commonly used model in The Netherlands to predict settlements is the Koppejan Model. This model gives reasonable predictions, but sometimes there are large discrepancies with reality. This is caused (among others) by: 1) The inaccurate calculation of the degree of consolidation, 2) The omission of the consolidation process for creep, which causes superposition errors for (temporarily) surcharge calculations, and 3) The omission of the waterpressures caused by creep. The problems disappear when the settlement process is solved with an incremental calculation. The results of the incremental calculations are compared with several 1-D compression tests and show good results. The Incremental Koppejan Model can easily be implemented in finite element models. In this paper also an example for a 3-D Elasto-Plastic Soft-Soil-Creep Model is presented. [less ▲] Detailed reference viewed: 80 (0 UL) |
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