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Numerical check of the Meyerhof bearing capacity equation for shallow foundations Van Baars, Stefan in Innovative Infrastructure Solutions (2018), 3(9), 1-13 Most geotechnical design codes and books use the equations of Meyerhof or Terzaghi to calculate shallow foundations. These equations are based on the failure mechanism published by Prandtl for shallow ... [more ▼] Most geotechnical design codes and books use the equations of Meyerhof or Terzaghi to calculate shallow foundations. These equations are based on the failure mechanism published by Prandtl for shallow strip foundations. The common idea is that failure of a footing occurs in all cases according to a Prandtl-wedge failure mechanism. To check the failure mechanism and the equations of the currently used bearing capacity factors and correction factors, a large number of ﬁnite-element calculations of strip and circular footings have been made. The ﬁnite-element calculations show that in cases of soils with high friction angles, soils without cohesion or a surcharge, footings with inclined loading or circular footings, not the Prandtl-wedge failure mechanism, but other failure mechanisms occur. In addition, the currently used equations for the bearing capacity factors and correction factors are too high. Therefore, new equations have been presented in this article. For some correction factors, for example, the inclination factors and the cohesion slope factor, an analytical solution is found. [less ▲] Detailed reference viewed: 26 (3 UL)De gevolgen van de restkracht bij een paalfundering Rica, Shilton ; Van Baars, Stefan in Geotechniek (2018), Maart Alhoewel de funderingspaal al lang in gebruik is, is de bepaling van de draagkracht nog steeds een complexe zaak. Uit een verzameling eerder gemaakte veldproeven, die in Nederland, België en Frankrijk ... [more ▼] Alhoewel de funderingspaal al lang in gebruik is, is de bepaling van de draagkracht nog steeds een complexe zaak. Uit een verzameling eerder gemaakte veldproeven, die in Nederland, België en Frankrijk waren uitgevoerd, werd de conclusie getrokken dat het berekende puntdraagvermogen, per 2017, met 30% gereduceerd moest worden. Bij deze proeven werd verondersteld dat de spanningen op de onbelaste paal na installatie kunnen worden verwaarloosd, en daarmee ook de resulterende restkracht in de paal. Uit andere veldproeven genoemd in de wetenschappelijke literatuur, blijkt echter dat deze restkracht niet te verwaarlozen is. Numerieke berekeningen uit deze publicatie ondersteunen deze conclusie. Deze verwaarlozing leidt nu tot een te klein puntdraagvermogen, en een te groot schachtdraagvermogen. Naast het probleem van de verwaarlozing van de restkracht speelt er nog een belangrijk probleem: de gehanteerde invloedszone bij de methode Koppejan is gebaseerd op een destijds aangenomen bezwijkmechanisme, die volgens numerieke berekeningen uit deze publicatie onjuist is. Dit leidt tot nog meer onnauwkeurigheden. [less ▲] Detailed reference viewed: 30 (3 UL)100 Jaar Prandtl-Wig: De draagkrachtfactoren Van Baars, Stefan in Geotechniek (2017), December In 1920 publiseerde Prandtl een artikel over het bezwijken van een materiaal onder een strookbelasting. De bezweken grondmoot is hierbij verdeeld in drie zones, die tezamen de zogenoemde Prandtl-wig ... [more ▼] In 1920 publiseerde Prandtl een artikel over het bezwijken van een materiaal onder een strookbelasting. De bezweken grondmoot is hierbij verdeeld in drie zones, die tezamen de zogenoemde Prandtl-wig vormen. Prandtl heeft nooit aan de funderingstechnische toepassing gedacht, maar geotechnici vertalen deze oplossing nu als een analytische oplossing voor de draagkracht van de grond onder een strookfundering. Deze oplossing is uitgebreid door Reissner met een bovenbelasting naast de funderingsstrook en door Keverling Buisman en Terzaghi voor het grondgewicht. Terzaghi schreef dit als eerste met drie gescheiden draagkrachtfactoren en Meyerhof als eerste met “inclinatie-” en “shape” factoren, voor de drie afzonderlijke draagkrachtcomponenten; cohesie, bovenbelasting en het grondgewicht. Deze schrijfwijze werd later door Brinch Hansen overgenomen. In dit artikel zijn een groot aantal numerieke berekeningen gemaakt om het bezwijkmechanisme van de Prandtl-wig en de bijbehorende draagkrachtfactoren te controleren. Het blijkt dat het Prandtl-wig bezwijkmechanisme juist is, maar niet in alle gevallen. Ook blijkt dat de analytische oplossingen voor de draagkrachtfactoren die tegenwoordig worden gebruikt, alleen juist zijn voor grond met een onrealistisch hoge dilatantiehoek. [less ▲] Detailed reference viewed: 65 (0 UL)Fatigue of rocks Pytlik, Robert Stanislaw ; Van Baars, Stefan in Johansson, Erik; Raasakka, Ville (Eds.) Proceedings of the 3rd Nordic Rock Mechanics Symposium, NRMS 2017 (2017, November) Cyclic loading on civil structures can lead to a reduction of strength of the used materials. For the materials concrete and especially steel, there are clear design codes about how to account for the ... [more ▼] Cyclic loading on civil structures can lead to a reduction of strength of the used materials. For the materials concrete and especially steel, there are clear design codes about how to account for the reduction of the material shear strength due to this cyclic loading, which is called fatigue. For the material rock, however, there are no design codes or standards for fatigue, in terms of shear strength reduction. For this reason, a large number of laboratory triaxial tests have been performed, in order to evaluate the fatigue of rocks by comparing the shear strength parameters obtained in cyclic triaxial tests with the static shear strength. Tests have been performed on artificial gypsum, a mixture of sand and cement (mortar) and soft sedimentary limestone. Correlations of the fatigue, for both the number of cycles and the cyclic stress ratio, have been obtained. All triaxial tests were conducted on dry samples (no pore pressure) in the natural state. The range of the confining pressure was between 0 MPa and 0.5 MPa. The frequency was kept low to allow for a precise application of the cyclic load and also accurate readings. The number of applied cycles was from a few cycles up to a few hundred thousand. The imperfections in the artificial gypsum have a significant impact on the results of the (especially cyclic) strength tests. Therefore another man made material was used – a mixture of sand and cement (mortar). As the first static test results were very promising, mortar was used in further tests. The cyclic tests, however, presented a similar, high scatter of results as for artificial gypsum. Due to the complex behaviour of the cohesive materials and high scatter of the results, many tests were required. Two different strategies were used to investigate the fatigue of the cohesive geomaterials: 1. the remaining shear strength curve: after a given number of cycles, a final single load test until failure, measures the remaining shear strength of the sample. 2. the typical S-N curve (Wöhler curves): one counts the number of constant loading cycles until failure. The fatigue of rocks can be seen as a reduction of the cohesion. In this way, the fatigue of a cohesive geomaterial can be described by (a reduction of) the remaining cohesion. An unexpected observation for both artificial gypsum and mortar was that unlike the number of cycles, the size of the cyclic stress ratio has little influence on the remaining shear strength, and therefore on fatigue. The remaining shear strength (or strength reduction) curve has been compared with the standard S-N curve, and is found to be rather similar for both artificial gypsum and mortar. The reason for this is this unexpected observation. Because of this, the S-N curve and the remaining shear strength curve should be theoretically identical. The results of the triaxial tests show however that, the S-N curve gives a bit steeper slope than the remaining shear strength curve, which would imply a shorter life and a faster reduction in comparison to the remaining shear strength curve, but this is only because, the prematurely failed samples are not included in the remaining shear strength curve, while a significant number of the S-N samples are prematurely failed samples from the remaining shear strength tests. [less ▲] Detailed reference viewed: 17 (1 UL)Historical overview of CPT-based design methods Rica, Shilton ; Van Baars, Stefan in Proceedings of the International Conference of Civil Engineering, ICCE 2017, Tirana 12-14 October 2017 (2017, October 12) The Cone Penetration Test (CPT) is used for many decades in order to evaluate the pile bearing capacity. Pieter Barentsen developed the CPT around 1930 in order to investigate the soil resistance. However ... [more ▼] The Cone Penetration Test (CPT) is used for many decades in order to evaluate the pile bearing capacity. Pieter Barentsen developed the CPT around 1930 in order to investigate the soil resistance. However, Boonstra was the first, in 1940, to used the cone resistance of the CPT as the unit pile bearing capacity. From this moment, the CPT became very important in the evaluation of the pile bearing capacity. An overview is given about the most common pile design methods, which are based on the in-situ Cone Penetration Test (CPT). This overview will start with the evolution of the CPT, followed by a brief presentation of the methods in use. An important part, in pile design methods, is the averaging procedure of the cone resistance over an influence zone around the pile tip. Since the pile tip is much wider than the tip of the CPT cone, the influence zone of the pile is also much larger, therefore the cone resistance has to be averaged over the influence zone around the pile tip. Unfortunately, the definition of this zone is different for each method in use. Finally, several methods for the pile tip bearing capacity near the interface of a soil layer will be discussed, including related methods from De Beer [1] and White & Bolton [2]. [less ▲] Detailed reference viewed: 40 (8 UL)The axisymetric failure mechanism of circular shallow foundations and pile foundations in non-cohesive soils Van Baars, Stefan in Computations and Materials in Civil Engineering (2017), 2(1), 1-15 In 1920 Prandtl published an analytical solution for the bearing capacity of a centric loaded strip footing on a weightless in-finite half-space, based on a so-called Prandtl-wedge failure mechanism ... [more ▼] In 1920 Prandtl published an analytical solution for the bearing capacity of a centric loaded strip footing on a weightless in-finite half-space, based on a so-called Prandtl-wedge failure mechanism. Reissner extended this solution for a surrounding surcharge and Keverling Buisman and Terzaghi for the soil weight. Meyerhof and other researchers presented correction factors for the shape of the shallow foundation, which would suggest that, the failure mechanism of circular shallow foundations, is related to the Prandtl-wedge failure mechanism. Meyerhof and Koppejan adapted this Prandtl-wedge failure mechanism also for pile foundations. The numerical calculations made in this article show that the Prandtl-wedge cannot be applied to circular shallow foundations and pile foundations in non-cohesive soils. The failure zone (plastic zone) below a loaded circular plate or pile tip, is far wider and deeper than the Prandtl-wedge. The calculations also show that there is, for these axisymmetric cases, failure both in and out of the standard x-y plane, but most of the failure is due to out-of-plane (tangential) failure. Therefore, this failure mechanism is different from the Prandtl-wedge failure mechanism. Also interesting are the circular and diagonal thin zones below the plate and around the pile tip, where there is no out-of-plane failure, although there is still in-plane failure. In these thin zones without out-of-plane failure, the tangential (out-of-plane) stresses are relatively high due to large shear strains, formed during previous shearing or sliding of the soil. [less ▲] Detailed reference viewed: 34 (4 UL)Pile load test at the west coast of Mexico Rica, Shilton ; Van Baars, Stefan ; in Proceedings of Pile 2017 (2017, September) Five pile tests have been performed, at the west coast of Mexico, in order to evaluate their pile bearing capacity. The Kentledge system (a test pile loaded in between two tension piles) has been used to ... [more ▼] Five pile tests have been performed, at the west coast of Mexico, in order to evaluate their pile bearing capacity. The Kentledge system (a test pile loaded in between two tension piles) has been used to execute the pile tests. The soil stratigraphy has been surveyed with standard penetration tests, cone penetration tests and borings, and consists of a ten to twelve meter soft clayey soil on top of a hard clay layer. Three identical pile tests have been performed on bored piles with a pile diameter of 0.6 m and a pile length of 30 m. In addition, two identical pile tests have been performed on driven piles with a squared cross section of 0.5 m × 0.5 m and a pile length of about 21 m. The aims of these tests were, first to evaluate the pile bearing capacity for both the bored and the driven pile types, in order to decide which pile type will be used finally, for the foundation of a factory; and second, to study the influence of the pile installation process on the pile bearing capacity of both pile types. During the testing of the bored piles, load measurements in different sections of the pile suggested that almost all bearing capacity came from the pile section in the upper soft clay layer. Since it is impossible to have such a relative large friction along the pile shaft in the soft soil, and because far more concrete was used for making the pile than expected, it had to be concluded that the liquid concrete has widened the pile diameter just above the hard soil layer, leading to a bulking effect in the pile. Therefore, the pile was leaning on this hard soil layer. For the driven test piles, the measurements showed a normal behaviour of both the pile shaft friction and the pile tip bearing capacity. [less ▲] Detailed reference viewed: 48 (8 UL)Numerical Check of the Meyerhof Bearing Capacity Equation for Shallow Foundations Van Baars, Stefan in Shehata, Hany; Rashed, Youssef (Eds.) Congress and Exhibition on Sustainable Civil Infrastructures (2017, July) In 1920 Prandtl published an analytical solution for the bearing capacity of a strip load on a weightless infinite half-space. This solution was extended with a surrounding surcharge by Reissner and with ... [more ▼] In 1920 Prandtl published an analytical solution for the bearing capacity of a strip load on a weightless infinite half-space. This solution was extended with a surrounding surcharge by Reissner and with the soil weight by Keverling Buisman. It was Terzaghi who wrote this with three separate bearing capacity factors for the cohesion, surcharge and soil-weight. Meyerhof extended this to the equation which is nowadays used; with shape and inclination factors. He also proposed equations for the inclination factors, based on his own laboratory experiments. Since then, several people proposed updated equations for the soil-weight bearing capacity factor, and also for the shape and inclination factors. The common idea is that failure of a footing occurs in all cases with a Prandtl-wedge failure mechanism. In order to check the failure mechanisms and the currently used equations for the bearing capacity factors and shape factors, a large number of finite element calculations of strip and circular footings have been made. These calculations proof that for some cases there are also a few other failure mechanisms possible. Also the currently used bearing capacity factors and shape factors are not correct. In fact, for footings on a soil with a higher friction angle, all three bearing capacity factors and all three shape factors can be much lower than the currently used values. This means that the currently used equations for the soil-weight bearing capacity factors and the shape factors are inaccurate and unsafe. Therefore, based on the finite element calculations, new equations have been presented in this paper. [less ▲] Detailed reference viewed: 55 (2 UL)Определение проницаемости органоминеральных грунтов с помощью диссипационных тестов, выполняемых пьезоконом Van Baars, Stefan ; in Russian Magazine Geoinfo (2017) В настоящее время испытания пьезоконом (CPTu, статическое зондирование с измерением порового давления) часто используются для предварительной оценки структурных и деформационных параметров грунтов. При ... [more ▼] В настоящее время испытания пьезоконом (CPTu, статическое зондирование с измерением порового давления) часто используются для предварительной оценки структурных и деформационных параметров грунтов. При использовании результатов тестирования пьезоконом стандартные исследования площадки, состоящие из испытаний статическим зондированием (CPT), бурения и лабораторных испытаний, могут быть оптимизированы. Коэффициент консолидации и гидравлическая проводимость (коэффициент фильтрации Кф) – параметры, необходимые для прогнозных оценок осадок во времени, могут быть получены с использованием диссипационных тестов, выполненных пьезоконом (т.е. тесты по рассеиванию порового давления, выполняемые после остановки зондирования). Тест на диссипацию основан на том, что скорость рассеивания избыточного порового давления (воды), возникающего во время вдавливания пьезокона через насыщенные водой глины и илы, зависит от коэффициента фильтрации грунтовой среды. Однако, интерпретация кривых диссипации часто проблематична, поскольку существующие методы анализа предполагают непрерывное снижение порового давления со временем, тогда как фактические кривые диссипации часто демонстрируют нестандартное поведение, интерпретация которого более сложна. В настоящей статье представлен метод интерпретации, который можно использовать для оценки коэффициента фильтрации независимо от формы кривой диссипации. Примеры результатов, полученных с использованием новой методики анализа, сравниваются с результатами, полученными с использованием лабораторных одометрических исследований. Перевод статьи на русский язык выполнен Петром Космиади. [less ▲] Detailed reference viewed: 35 (9 UL)The influence of superposition and eccentric loading on the bearing capacity of shallow foundations Van Baars, Stefan in Computations and Materials in Civil Engineering (2016), 1(3), 121-131 In 1920 Prandtl published an analytical solution for the bearing capacity of a centric loaded strip footing on a weightless in-finite half-space. Reissner (1924) extended this solution for a surrounding ... [more ▼] In 1920 Prandtl published an analytical solution for the bearing capacity of a centric loaded strip footing on a weightless in-finite half-space. Reissner (1924) extended this solution for a surrounding surcharge and Keverling Buisman (1940) for the soil weight. Terzaghi (1943) wrote this as a superposition of three separate bearing capacity components for the cohesion, surcharge and soil-weight. The first question is to what ex-tent the currently used components are correct. The second question is to what extent the superposition is correct, because the failure mechanisms for these three components are not the same. A number of finite element calculations show that there is indeed an error, which is luckily not too large and leads to predictions on the safe side. Meyerhof (1953) extended the equation of Terzaghi with correction factors for the shape of the footing and the inclination of the load. For eccentric loading however, there are no correction factors. The common practice is to reduce the contact area of the foundation such that its centroid coincides with that of the load, which means that, the area of the foundation outside the effective area, is completely neglected. Therefore the third question is, if this reduction of the foundation area is an accurate method to describe the reduction of the bearing capacity due to eccentric loading. A number of finite element calculations show that this is indeed the case. [less ▲] Detailed reference viewed: 97 (7 UL)Shear strength and stiffness degradation of geomaterials in cyclic loading Pytlik, Robert Stanislaw ; Van Baars, Stefan in Soils and Rocks (2016), 39(3), 273-283 Cyclic loading on civil structures can lead to a reduction of strength and stiffness in the loaded materials. The life span of many cyclically loaded structures such as wind turbines, high-speed train ... [more ▼] Cyclic loading on civil structures can lead to a reduction of strength and stiffness in the loaded materials. The life span of many cyclically loaded structures such as wind turbines, high-speed train tracks and bridges strongly depends on the foundation. The soils and rocks in the foundation can be subjected to cyclic loads from natural and human sources. In order to evaluate the fatigue behaviour of geomaterials, this paper presents static and cyclic triaxial test results for several geomaterials. It was concluded that cyclic loading on different geomaterials can cause different types of effects. The shear strength of cohesionless crumbled limestone increases during cyclic loading; while for cohesive materials, such as gypsum and mortar, the strength decreases. The strength decrease can be seen as a degradation of the cohesion. The most significant factor in the cohesion reduction was found to be the number of applied cycles. It was also noticed that the friction angle for sands does not reduce under cyclic loading. A fatigue limit was not found for cohesive geomaterials; neither a dependence of the strength reduction on the cyclic loading ratios. [less ▲] Detailed reference viewed: 112 (5 UL)A 3D shear material damping model for man-made vibrations of the ground Macijauskas, Darius ; Van Baars, Stefan in 13th Baltic Sea Region Geotechnical Conference (2016, September) 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 ▲] Detailed reference viewed: 48 (1 UL)100 Year Prandtl’s Wedge - Intermediate report Van Baars, Stefan Report (2016) The biggest problem for a shallow foundation, just as any other type of foundation, is a failure due to an overestimation of the bearing capacity. This means that the correct prediction of the bearing ... [more ▼] The biggest problem for a shallow foundation, just as any other type of foundation, is a failure due to an overestimation of the bearing capacity. This means that the correct prediction of the bearing capacity of the foundation is often the most important part of the design of a civil structure. That is why the publication of Prandtl in 1920 about the hardness of a plastic body, was a major step in solving the bearing capacity of shallow foundations, although it is well possible that he never realised this, because his solution was not made for civil engineering purposes, but for mechanical purposes. Over the last 100 year, a lot of extensions have been made, for example with inclination factors and shape factors, and many laboratory experiments have been done and also many numerical calculations have been made. Some even try to extrapolate the failure mechanism for shallow foundations to the failure mechanism around the tip of a pile. All this scientific work leads back to the first publication made by Ludwig Prandtl in 1920. This intermediuate report “100 Year Prandtl’s wedge” has been made for all those who are interested in these fundamentals of foundation engineering and their history. [less ▲] Detailed reference viewed: 219 (24 UL)Failure mechanisms and corresponding shape factors of shallow foundations Van Baars, Stefan in Atalar (Ed.) Proceedings of 4th International Conference on New Developments in Soil Mechanics and Geotechnical Engineering (2016, June) 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. In the 1940s, Keverling Buisman and Terzaghi extended the Prandtl-Reissner formula for the soil weight. Since then several people proposed equations for the soil-weight bearing capacity factor. In 1963 Meyerhof was the first to write the formula for the (vertical) bearing capacity of shallow foundations with both inclination factors and shape factors. The failure mechanisms belonging to the cohesion bearing capacity factor and the surcharge bearing capacity factor is for an infinite (2D) strip footing a Prandtl-wedge failure mechanism, but according to Finite Element Modelling (FEM) the failure mechanism belonging to the soil-weight bearing capacity factor is not. It looks more like a global failure mechanism. This means that the assumed superposition in the Terzaghi equation, and in the Meyerhof equation, is not automatically allowed. Additional FEM calculations show that in the case of a finite strip footing, and especially of round footings, the failure mechanism is again very different, and leads to much lower shape factors as factors based on a Prandtl-wedge failure mechanism. In fact the third direction, i.e. the tangential direction, which plays no important role in the failure mechanism for infinite strip footings, starts to play a major role in the failure mechanism and in the magnitude of the bearing capacity of the strip footing [less ▲] Detailed reference viewed: 88 (1 UL)Advanced Soil Mechanics Van Baars, Stefan 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 ▲] Detailed reference viewed: 183 (9 UL)The influence of the shaft friction and pile shape on the pile tip bearing capacity Van Baars, Stefan 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 ▲] Detailed reference viewed: 85 (4 UL)The Bearing Capacity of Footings on Cohesionless Soils Van Baars, Stefan 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 ▲] Detailed reference viewed: 71 (4 UL)Laboratory tests on Dutch limestone (Mergel) Pytlik, Robert Stanislaw ; Van Baars, Stefan 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 ▲] Detailed reference viewed: 102 (7 UL)Triaxiaalproeven op Limburgse mergel leveren verassende resultaten Pytlik, Robert Stanislaw ; Van Baars, Stefan 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 ▲] Detailed reference viewed: 92 (7 UL)PROPAGATION OF HARMONICAL VIBRATIONS IN PEAT Macijauskas, Darius ; Van Baars, Stefan 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 ▲] Detailed reference viewed: 88 (8 UL) |
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