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See detailAdvanced Soil Mechanics
Van Baars, Stefan UL

Book published by Epubli (2016)

Advanced Soil Mechanics This book Advanced Soil Mechanics is part of the education of Civil Engineering at the faculty of Science, Technology and Communication of the University of Luxembourg. This book ... [more ▼]

Advanced Soil Mechanics This book Advanced Soil Mechanics is part of the education of Civil Engineering at the faculty of Science, Technology and Communication of the University of Luxembourg. This book can be seen as a continuation of introductory courses of Soil Mechanics. This book contains the major principles and design methods used in Geotechnical Engineering, such as for soil improvement, geotextiles, tunnelling, shallow and pile foundations, sheet piles, anchors, struts, dewatering and safe design. [less ▲]

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See detailThe influence of the shaft friction and pile shape on the pile tip bearing capacity
Van Baars, Stefan UL

in The 17th Nordic Geotechnical Meeting Reykjavik Iceland 25th - 28th of May 2016 (2016, May)

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space, based on a sliding soil part, with three sliding zones, which is ... [more ▼]

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space, based on a sliding soil part, with three sliding zones, which is nowadays called the Prandtl wedge. This solution was extended by Reissner in 1924 with a surrounding surcharge. Keverling Buisman (1940) , and many researchers after him, extended the Prandtl-Reissner formula for the soil weight, but this part can be neglected for the deep pile foundations. It was Terzaghi (1943) who wrote the formula with bearing capacity factors and Meyerhof (1953) who started to write this formula with both inclination factors and shape factors. Because of the this development for shallow foundations , many researchers thought that failure of a pile tip in a deep sand layer will also show a Prandtl-wedge type of failure and that the stresses on the pile tip are constant and depend only on the shape factor, the friction angle of the soil and the vertical effective stress near the pile tip ( ), so not on the shape and size of the pile tip. This means that a Cone Penetration Test gives the average stress of a real pile and can in principle be used without a reduction for calculating the bearing capacity of a pile, just as Boonstra (1940) showed with his field test and just as the method of Van Mierlo & Koppejan (1952) assume and also many more recent predicting models do. The problem is that many researchers (Jardine et al, 2005, Lehane et al, 2005, Clausen et al, 2005) and recent field tests show that bearing capacity design based on unreduced Cone Penetration Test data are more than 30% too high (Van Tol et al. , 1994, 2010, 2012). Therefore all this has been modelled and studied, as far as possible, with Finite Element Modelling (Plaxis 2D axial-symmetric) . Many remarkable results were found. The shape and size of the pile tip did not matter indeed. But the currently used surcharge shape factor is incorrect. There is also no Prandtl-wedge type of failure at the pile tip, but a zone of plasticity, but still the surcharge bearing capacity factor of Reissner is correct. Also the stresses below the pile tip are not constant, but higher near the centre of the pile. Additional calculations show that the pile shaft friction does not influence the stresses at the pile tip, but the normal stresses of the pile tip do influence the shear stresses along the shaft. [less ▲]

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See detailThe Bearing Capacity of Footings on Cohesionless Soils
Van Baars, Stefan UL

in The Electronic Journal of Geotechnical Engineering (2015), 20

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space. This solution was extended by Reissner in 1924 with a surrounding ... [more ▼]

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space. This solution was extended by Reissner in 1924 with a surrounding surcharge. Keverling Buisman (1940) extended the Prandtl-Reissner formula for the soil weight. It was Terzaghi (1943) who wrote this in the form which is nowadays used. Since then several people proposed equations for the soil-weight bearing capacity factor. In this paper, we show that all those equations assume a Prandtl failure mechanism, while Finite Element Modelling (FEM) of strip footings on cohesion less materials proofs a global failure mechanism. Also these equations result in a higher bearing capacity than found with FEM. This means that the currently used equations for the soil-weight bearing capacity factor, and also the corresponding shape factor, are inaccurate and unsafe. Therefore new equations for the soil-weight bearing capacity factor, the soil-weight shape factor and the surcharge shape factor have been presented in this paper. [less ▲]

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See detailLaboratory tests on Dutch limestone (Mergel)
Pytlik, Robert Stanislaw UL; Van Baars, Stefan UL

in Schubert, W.; Kluckner, A. (Eds.) Future Development of Rock Mechanics - Proceedings of the ISRM Regional Symposium EUROCK 2015 & 64th Geomechanics Colloquium (2015, October 09)

In this note, results of triaxial laboratory tests on very weak sedimentary limestone from the construction of the “Geusselt A2” tunnel in Maastricht in the Netherlands are presented. The main purpose of ... [more ▼]

In this note, results of triaxial laboratory tests on very weak sedimentary limestone from the construction of the “Geusselt A2” tunnel in Maastricht in the Netherlands are presented. The main purpose of the triaxial tests was to evaluate the strength of this rock. Particularly interesting was that the strength parameters obtained in the laboratory, were much lower than what was expected after preliminary visual inspections. The two most popular models in soil and rock mechanics, the Mohr-Coulomb and Hoek-Brown failure criteria, were used to estimate the strength parameters and both did not give satisfying results. Still the Mohr-Coulomb model is the best model to use. [less ▲]

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See detailTriaxiaalproeven op Limburgse mergel leveren verassende resultaten
Pytlik, Robert Stanislaw UL; Van Baars, Stefan UL

in Geotechniek (2015), 3(19), 10-13

De civiele werken van de A2-tunnel in Maastricht zijn onlangs voltooid. Tijdens de bouw werd de Limburgse mergel in de bouwput als een stijve, stevige grondlaag beoordeeld, terwijl het na het verwijderen ... [more ▼]

De civiele werken van de A2-tunnel in Maastricht zijn onlangs voltooid. Tijdens de bouw werd de Limburgse mergel in de bouwput als een stijve, stevige grondlaag beoordeeld, terwijl het na het verwijderen als een cohesieloos zand werd aangezien. Om het sterktegedrag van deze mergel beter te begrijpen is door de Universiteit van Luxemburg aanvullend onderzoek gedaan. Uit triaxiaalproeven blijkt verrassenderwijze dat de sterkteparameters van verkruimelde mergel weinig afwijken van intacte mergel. De toplaag van de mergel heeft een zeer kleine cohesie en een grote hoek van inwendige wrijving. Alhoewel hierdoor de mergel onder druk zeer sterk is, is de mergel vrijwel niet in staat om trek op te nemen. [less ▲]

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See detailPROPAGATION OF HARMONICAL VIBRATIONS IN PEAT
Macijauskas, Darius UL; Van Baars, Stefan UL

in International Journal of GEOMATE (2014), 7(2), 1101-1106

In order to check the reliability of man-made vibration prediction methods, vibration tests were performed on one of polders in the North-West of the Netherlands. The polder was chosen because it has a ... [more ▼]

In order to check the reliability of man-made vibration prediction methods, vibration tests were performed on one of polders in the North-West of the Netherlands. The polder was chosen because it has a rather homogenous, thick and soft peat top layer. Here sufficient harmonical vibrations could be generated by a rather small shaker. The shaker was designed and manufactured in order to produce harmonical vibrations at the soil surface. It consists of two counter rotating electric vibrators (with rotating eccentric masses) in order to produce a vertically oscillating force. For the recordings of the vibrations, six 2D or 3D geophones were placed on the soil surface and one 2D geophone was placed on top of the shaker. The measured vibration amplitudes of the vertically oscillating shaker were compared with 1. Two different analytical methods used for the design of vibrating machine foundations, 2. The Confined Elasticity approach and 3. The Finite Element Method, for which Plaxis 2D software was used. Also the measured vibration amplitudes at the soil surface were compared with Barkan-Bornitz’s solution and Finite Element Modeling. [less ▲]

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See detailThe inclination and shape factors for the bearing capacity of footings
Van Baars, Stefan UL

in Soils and Foundations (2014)

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space. Prandtl subdivided the sliding soil component into three zones: two ... [more ▼]

In 1920 Prandtl published an analytical solution for the bearing capacity of a maximum strip load on a weightless infinite half-space. Prandtl subdivided the sliding soil component into three zones: two triangle zones on the edges and a wedge shaped zone in between the triangle zones that has a logarithmic spiral form. The solution was extended by Reissner in 1924 with a surrounding surcharge. Nowadays a more extended version of Prandtl’s formula exists for the bearing capacity. This extended formulation has an additional bearing capacity coefficient for the soil weight and has additional correction factors for inclined loads and for non-infinite strips loads. This extended version is in some countries known as “The equation of Meyerhof”, and in other countries as “The equation of Brinch Hansen”, because both men have separately published solutions for these additional correction factors. In this paper, we numerically solve for the stresses in the wedge zone and derive the corresponding bearing capacity coefficients and inclination and shape factors. The inclination factors are also analytically solved for. [less ▲]

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See detailFatigue of geomaterials
Pytlik, Robert Stanislaw UL; Van Baars, Stefan UL

in Oka, Fusao; Murakami, Akira; Uzuoka, Ryosuke (Eds.) et al Computer Methods and Recent Advances in Geomechanics (2014, September 23)

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See detailLandslides in urban areas of Luxembourg, caused by weak Rheatian Clay
Van Baars, Stefan UL; Bautista, Miguel; Becker, Rolf

in Lollino; Manconi; Guzzetti (Eds.) et al Engineering Geology for Society and Territory Volume 5 (2014, September)

Luxembourg is geologically divided into two parts: Oesling in the North and Gut-land in the Middle and South. Oesling is part of the Ardennes plateau. Gutland was formed in the Triassic and Jurassic ages ... [more ▼]

Luxembourg is geologically divided into two parts: Oesling in the North and Gut-land in the Middle and South. Oesling is part of the Ardennes plateau. Gutland was formed in the Triassic and Jurassic ages and is much younger than Oesling. It consists mainly of sedimentary rocks. Luxembourg has a variety of interesting, weak or problematic soils, such as the swelling gypsum layers, the layered schists of Wiltz and especially the weak Keu-per-Rhaetian-clay. The Rhaetian clay layer is mostly rather thin and is found at a relatively constant altitude and the band where it comes to the surface is identified by the varying erosion erratically found throughout Gutland. Approximately two third of all landslides are found along this line. Hence it was decided to investigate the Rhaetian clay in the geotechnical laboratory of the University of Luxembourg. Samples were taken from a pit at Rue de Mühlenbach on the north side of the city of Luxembourg and from a sliding slope of a building pit in Schutrange. The friction angle was found to be 8° at Mühlenbach and 3° at Schuttrange, which are both record low friction angles, which explains the high number of landslides in Luxembourg. [less ▲]

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See detailVibrations due to hydroshield tunnelling
Van Baars, Stefan UL

in Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014 (2014, June)

The Hubertus tunnel in Den Hague, the Netherlands, is a 1.5 km long bored tunnel, mostly below groundwater level, constructed with a hydro-shield Tunnel Boring Machine in 2006 and 2007. The vibrations due ... [more ▼]

The Hubertus tunnel in Den Hague, the Netherlands, is a 1.5 km long bored tunnel, mostly below groundwater level, constructed with a hydro-shield Tunnel Boring Machine in 2006 and 2007. The vibrations due to the tunnelling have been recorded both inside the tunnel and at ground surface. No harmful vibrations were observed while the Hubertus tunnel was being bored, although unexpected vibration nuisance did occur. This is an effect that is widely known when tunnelling in rock, but was not expected when tunnelling in beach sand. Analyses of the recorded vibrations showed that the vibrations were mainly caused by the trembling motion of the TBM in the sand as a result of the stick-slip effect. This creates shear waves along the TBM shaft and pressure waves at the bore front. Both waves travel in an axial direction. [less ▲]

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See detailSeismic Techniques for Surveying the Underground of Shallow Foundations
Van Baars, Stefan UL

in Journal of Civil Engineering and Architecture (2014), 8(5), 604-612

For civil structures founded on shallow foundations, the ground underneath the foundation often holds the greatest risks of the total structure. This can be due to of a very soft soil layer, an ... [more ▼]

For civil structures founded on shallow foundations, the ground underneath the foundation often holds the greatest risks of the total structure. This can be due to of a very soft soil layer, an inhomogeneous subsurface or a hidden dangerous object. It would be most favorable when a cheap and quick kind of seismic “tap-and-listen” technique can be used to detect those risks. The problem is however that an applied pulse or blast always creates a combination of compression-, shear- and surface waves. These types of waves have different wave velocities and will return therefore at different time intervals. For a shallow subsurface technique, all these waves will overlap, which makes the interpretation very hard. Both the single pulse technique and the single-frequency, multiple wave technique (constant vibration) have been studied, but both techniques have their limitations. It can be concluded from finite element calculations that it will be difficult or even impossible to design good seismic techniques for surveying the underground of shallow foundations for hidden shallow objects like water pipelines, undetonated bombs, dead bodies, coffins, potholes, etc.. The main reason is that the relative amount of reflected energy is simply too low in comparison to the energy of the still present original wave. [less ▲]

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See detailThe landslide of Kirf; a chain of governance failures
Van Baars, Stefan UL; Sosson, Mike; Stefan, Jung et al

in Zhang et al (Ed.) Geotechnical Safety and Risk IV (2014, January)

In January 2011 a landslide occurred in the German village of Kirf, which is between the town Saarburg and the border of Luxembourg. This slope failure caused a destruction of a federal main road over 50 ... [more ▼]

In January 2011 a landslide occurred in the German village of Kirf, which is between the town Saarburg and the border of Luxembourg. This slope failure caused a destruction of a federal main road over 50 meters. Stability calculations were made by a geotechnical consulting company, commissioned from the Federal Road Administration, in order to design the repair works and also to find out what really had happened. These calculations showed remarkably high safety factors. Nevertheless the consulting company blamed the families of the four houses down the slope for the failure, because they had made gabion walls in the toe of the slope. A research team of the University of Luxembourg discovered that this hilly area originally was called “On the slide” and has never been stabile according to their own calculations. So, the real question is why so many apparently uncorrelated mistakes were made in a single case. [less ▲]

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See detailThe soil of Luxembourg and the weak Rhaetian clay
Van Baars, Stefan UL

in Cahier Scientifique - Revue Technique Luxembourgeoise (2014), 2013(2), 34-37

Luxembourg is geologically divided into two parts: Oesling in the North and Gutland in the Middle and South. Oesling is part of the Ardennes plateau. Gutland was formed in the Triassic and Jurassic ages ... [more ▼]

Luxembourg is geologically divided into two parts: Oesling in the North and Gutland in the Middle and South. Oesling is part of the Ardennes plateau. Gutland was formed in the Triassic and Jurassic ages and is much younger than Oesling. It consists mainly of sedimentary rocks. Luxembourg has a variety of interesting, weak or problematic soils, such as the swelling gypsum layers, the layered schists of Wiltz and especially the weak Keuper-Rhaetian-clay. The Rhaetian clay layer is mostly rather thin and is found at a relatively constant altitude and the band where it comes to the surface is identified by the varying erosion erratically found throughout Gutland. Approximately two third of all landslides are found along this line. Hence it was decided to investigate the Rhaetian clay in the geotechnical laboratory of the University of Luxembourg. Samples were taken from a pit at Rue de Mühlenbach on the north side of the city of Luxembourg and from a sliding slope of a building pit in Schutrange. The friction angle was found to be phi = 8° at Mühlenbach and phi = 13° at Schuttrange, which are both record low friction angles, which explains the high number of landslides in Luxembourg. [less ▲]

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See detailDecomposition of measured ground vibrations into basic soil waves
Macijauskas, Darius UL; Van Baars, Stefan UL

in Pietruszczak, Stan; Pande, Gyan (Eds.) Proceedings of the 3rd International Symposium on Computational Geomechanics (ComGeo III), Krakow, Poland, 21-23 August, 2013 (2013, August 21)

Man-made vibrations from different types of sources are usually measured on the surface of the ground or building. The measured signal is always the superposition of all travelling basic waves. For a ... [more ▼]

Man-made vibrations from different types of sources are usually measured on the surface of the ground or building. The measured signal is always the superposition of all travelling basic waves. For a homogeneous half space there are three basic waves – the Compressional (P-wave), Shear (S-wave) and Rayleigh wave (R-wave). Depending on the measuring equipment, only the accelerations or velocities in time of the superposed wave can be measured, but not the distribution of the individual basic waves. Additional problems are that each of the basic waves has its own velocity, besides the body and surface waves have different attenuation laws. By using the rules of superposition of harmonic waves and also the propagation laws of the P-, S- and R-waves, it should be theoretically possible to split the measured superposed signal into the basic waves, because mathematically a system of equations can be assembled which describes the displacements at multiple measuring points in time. In this paper this problem has been solved for a homogenous, elastic and isotropic soil, which is disturbed by a harmonically oscillating disc on the surface. A numerical simulation was performed using a finite element method. The displacements in time were recorded in 10 points on the surface and a system of superposed equations was assembled and solved. The findings prove that each of the three basic waves has its own phase shift with the source, something which was not known before. [less ▲]

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See detailDe Luxemburgse bodem en de zwakke Rhät klei
Van Baars, Stefan UL; Becker, Rolf; Bautista, Miguel

in Geotechniek (2013)

Luxemburg kent zeer interessante, zwakke en problematische bodemsoorten zoals de zwellende gips, de gelaagde schist van Wiltz en vooral de zwakke Keuper-Rhät-klei. Die laatste komt als een dun laagje in ... [more ▼]

Luxemburg kent zeer interessante, zwakke en problematische bodemsoorten zoals de zwellende gips, de gelaagde schist van Wiltz en vooral de zwakke Keuper-Rhät-klei. Die laatste komt als een dun laagje in een groot deel van het land aan de oppervlakte. Ongeveer 2/3de deel van alle grondafschuivingen spelen zich hier af. Vandaar dat besloten is om de Rhät-klei te onderzoeken. Uit triaxiaalproeven die in het geotechnisch laboratorium van de Universiteit van Luxemburg gedaan zijn, blijkt dat deze klei een zeer kleine hoek van inwendige wrijving heeft en daardoor tot de zwakste kleien ter wereld gerekend mag worden. [less ▲]

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See detailSeismic techniques for surveying the underground of shallow foundations
Van Baars, Stefan UL

in Conference Testing and Design Methods for Deep Foundations 2012 kanazawa (2012, September)

For civil structures founded on shallow foundations, the ground underneath the foundation often holds the greatest risks of the total structure. In this paper the surveying of the subsurface below shallow ... [more ▼]

For civil structures founded on shallow foundations, the ground underneath the foundation often holds the greatest risks of the total structure. In this paper the surveying of the subsurface below shallow foundations has been studied for three mayor risks; the risk of soft soils, the risk of an inhomogeneous subsurface and the risk of dangerous hidden objects. The research question here is whether a kind of seismic technique can be used to detect those risks. As an example one could look at non-seismic wave detection techniques. There are “only-listen” detection techniques like passive sonar, infrared camera detection, and multi-microphone noise localization, but these have not been used as an example for seismic techniques in this paper. And there are “tap-and-listen” detection techniques, like active sonar, radar, echoscopy and the deep geophysical surveying techniques of the oil and gas exploration. This last technique is of course already a seismic technique in which the applied pulse or blast creates a combination of compression, shear and surface waves. These waves have all different wave velocities and will return at different time intervals. For a shallow subsurface technique though, all these waves will overlap unfortunately. Even for a single wave technique this problem is difficult to avoid, so it is best to create a very short and single pulse. For the creation of the wave both the shortness and the singleness is very difficult. Instead of a single pulse, also single-frequency, multiple waves (constant vibration) can be used as intentional wave initiation. Then there will be even more an overlap of waves, but maybe the effects of interference and resonance can lead to positive results in the detection. For the quantification of the risk of (dangerous) hidden objects, it can be concluded from Finite Element Calculations that the recording of the hidden objects by the “tap-and-listen” technique will very likely not work for smaller objects in an inhomogeneous subsurface. Also when we try to preserve the reflected energy as much as possible by tapping and listening at the same position on the ground surface right above the (hidden) object, it does not work. Even if we assume no frictional or viscous energy dissipation in the calculation, the relative amount of reflected energy is simply too low in comparison to the energy of the still present original wave. This means that it will be difficult or even impossible to design good seismic techniques for surveying the underground of shallow foundations for hidden shallow objects like water pipelines, undetonated bombs, dead bodies, coffins, potholes, etc. [less ▲]

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See detailInvestigations into the Safety Level of Marine Steel Sheet Pile Structures with Consideration of Corrosion
Odenbreit, Christoph UL; Osório, Paulo; Van Baars, Stefan UL et al

in Proceedings of the 20th Australasian Coastal and Ocean Engineering Conference 2011 and the 13th Australasian Port and Harbour Conference 2011 (2011, October)

In this paper, several reliability analyses of a sheet pile wall in a marine environment were performed, in which the influence of a wide range of corrosion models was assessed. The influence of the ... [more ▼]

In this paper, several reliability analyses of a sheet pile wall in a marine environment were performed, in which the influence of a wide range of corrosion models was assessed. The influence of the corrosion was compared in structures with different conditions (two different sheet pile profiles were used, as well as two values of the sheet pile length and also two different values of the surcharge). To perform the reliability analyses, the First Order Reliability Method (FORM) was used and reliability analyses were performed using a combination of different programs. The effect that the variation of the coefficient of variation v of the probability distributions has on the measured level of safety is not linear. For certain ranges of v, the influence on the safety level is not very significant, whereas in other ranges it causes an abrupt reduction of the safety, meaning that access to data of thickness loss due to corrosion of good quality (that represent reality as accurately as possible) is very important to avoid erroneous conclusions about the safety. [less ▲]

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See detailCauses of major geotechnical disasters
Van Baars, Stefan UL

in 3rd International Symposium on Geotechnical Safety and Risk, München, (2011, June)

Many disasters are related to geotechnical failure. Both for risk management and for future research it is important to know what new knowledge is needed to prevent these disasters. Therefore elev-en ... [more ▼]

Many disasters are related to geotechnical failure. Both for risk management and for future research it is important to know what new knowledge is needed to prevent these disasters. Therefore elev-en geotechnical disasters or failing projects of the last ten years in modern countries are studied and com-pared in this paper. All examples show that the disasters or failing projects had nothing to do with a large spread of the strength or load in a foreseen failing mechanism. There was also not an unknown failure mechanism or a lack of existing scientifical knowledge. All cases show a lack of available knowledge (or incompetence) of the designing part of the construction management. The mistakes which were made were often of a level not higher than a BSc or MSc teaching level. In none of these cases these mistakes were tackled by an internal project auditing and in none of these cases these mistakes were tackled by an exter-nal project design control, for example for a building permit. The biggest risk parameter in geotechnical design is therefore not the spread of load or strength parame-ters, but by far the existence and quality of the internal project auditing and the external project design control. [less ▲]

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See detailModelling of frictional soil damping in finite element analysis
Van Baars, Stefan UL

in The Second International Symposium on Computational Geomechanic (2011, April)

In soil dynamics, the soil is often described as a viscous material. In a viscous material however, the dissipated energy is assumed to be proportional to the wave frequency, which is absolutely not ... [more ▼]

In soil dynamics, the soil is often described as a viscous material. In a viscous material however, the dissipated energy is assumed to be proportional to the wave frequency, which is absolutely not applicable to soils. It is therefore better to use a concept of damping based on dry particle friction. A non-viscous model based on this concept results in a damping ratio that becomes constant for small deformations for both sand and clay, and is also independent of frequency or shear strain amplitude. This behaviour corresponds with laboratory measurements and requires only one damping parameter which can be obtained from laboratory tests. This model is implemented in Plaxis as a User-Defined Soil Model to analyse the problem of a strip footing subjected to a dynamic load. The initial results are rather remarkable. . For example, a Power Spectral Density plot of the velocities shows that not all frequencies seem to be damped equally and at some distance the input frequency is not even present. [less ▲]

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See detailKeynote Lecture: Dike failures; Risks, Causes and Costs
Van Baars, Stefan UL

Presentation (2010, June 01)

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