The influence of a thin weak soil layer on the pile bearing capacity.

in International Conference on Deep Foundations and Ground Improvement, Urbanization and Infrastructure Development: Future Challenges (2018, June 05)

Most pile design methods for estimating the pile bearing capacity, are based on data from the cone penetration tests (CPT), such as the Dutch (Koppejan) method, the Belgium (De Beer) method or the French ... [more ▼]

Most pile design methods for estimating the pile bearing capacity, are based on data from the cone penetration tests (CPT), such as the Dutch (Koppejan) method, the Belgium (De Beer) method or the French (Bustamante & Gianeselli) method. In all these methods, the unit cone resistance of the CPT has to be averaged within an influence zone around the pile tip, in order to be able to calculate the pile tip bearing capacity. The influence zone is defined as the zone in which the parameters of the soil have an influence on the pile tip bearing capacity. For the French method this zone is between 1.5 times the pile diameter, D, above and below the pile tip; while for the Dutch method this zone is between 8 D above to a maximum of 4 D below the pile tip. Numerical simulations on piles installed in homogeneous sandy soil, show that these definitions of the influence zone are not accurate. The simulations show an influence zone from 1 or 2 D above, to 5 or 6 D below the pile tip, depending on the soil parameters. In this research, also the influence of a weak thin soil layer on the pile bearing capacity has been studied. All simulations are made for two different thicknesses of the thin soil layer. Also the soil strength parameters and soil stiffness parameters have been varied, in order to show the influence of this thin soil layer. This influence zone has been confirmed by laboratory tests. [less ▲]

The bearing capacity of shallow foundations on slopes

in Proceedings of NUMGE 2018 9th European Conference on Numerical Methods in Geotechnical Engineering (2018, June)

In 1920 Prandtl published an analytical solution for the bearing capacity of a soil under a strip load. Over the years, extensions have been made for a surrounding surcharge, the soil weight, the shape of ... [more ▼]

In 1920 Prandtl published an analytical solution for the bearing capacity of a soil under a strip load. Over the years, extensions have been made for a surrounding surcharge, the soil weight, the shape of the footing, the inclination of the load, and also for a slope. In order to check the current extensions of a loaded strip footing next to a slope, many finite element calculations have been made, showing that these extensions are often inaccurate. Therefore new factors have been proposed, which are for both the soil-weight and the surcharge slope bearing capacity, based on the numerical calculations, and for the cohesion slope bearing capacity, also on an analytical solution. [less ▲]

100 Years of Prandtl's Wedge

Book published by IOS Press (2018)

The biggest problem for a shallow foundation, just as for 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 for 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 by 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 years, a lot of extensions have been made, for example with inclination factors and shape factors. Also many laboratory experiments have been done and 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 by Ludwig Prandtl in 1920. This book, “100 Years of Prandtl’s Wedge”, is intended for all those who are interested in these fundamentals of foundation engineering and their history. The Appendices include a copy of Prandtl’s Über die Härte plastischer Körper and of Reissner’s publication of 1924, Zum Erddruckproblem. [less ▲]

De gevolgen van de restkracht bij een paalfundering

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 ▲]

Fatigue of rocks

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 ▲]

Pile load test at the west coast of Mexico

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 ▲]

Numerical Check of the Meyerhof Bearing Capacity Equation for Shallow Foundations

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 ▲]

Shear strength and stiffness degradation of geomaterials in cyclic loading

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 ▲]

A 3D shear material damping model for man-made vibrations of the ground

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 ▲]

Failure mechanisms and corresponding shape factors of shallow foundations

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 ▲]