Jelke Dijkstra

New Method of Full-Field Stress Analysis and Measurement Using Photoelasticity

Photoelastic measurements provide a means to obtain a meaningful representation of the stress state in a granular material over the full area of a plane-strain sample without the need to place stress transducers inside the sample. This method uses the property of non-crystalline materials to become optically anisotropic when put under stress. To measure the resultant relative retardation of a light beam transmitted through a model built from glass grains and a liquid with a matching refractive index in the pores, a full-field polariscope has been built. This setup is able to characterize the stress state in the full-field of the sample with only seven intensity measurements. A plane-strain pile penetration test is used as an example.

Numerical model of elasto-plastic electro-osmosis consolidation of clays

Electro-osmosis consolidation is an innovative and effective ground improvement method for soft clays. But electro-osmosis is also a very complicated process, as the mechanical behaviour, and the hydraulic and electrical properties of the soil are changing rapidly during the treatment process; this makes electro-osmosis hard to describe and simulate. Traditional electro-osmosis consolidation theory cannot provide a satisfactory solution because it does not directly consider the mechanical behaviour, nor the nonlinearity of the mechanical, hydraulic and electrical properties of the soil. A numerical solution for the electro-osmosis consolidation of clay in multi-dimensional domains is presented, with the coupling of the soil mechanical behaviour, pore water transport and electrical fields being considered. Three fully coupled equations are solved using classical finite elements. Furthermore, the Modified Cam Clay model is incorporated to describe the elasto-plastic behaviour of clay. The proposed model is first verified against a classical one-dimensional analytical solution for electro-osmosis consolidation to demonstrate its accuracy and efficiency. Then, an assessment of the impact of nonlinear soil behaviour is made by comparing numerical results with the results of an electro-osmosis laboratory test. The results show that the numerical solution can well predict the nonlinear behaviour of clay during electro-osmosis consolidation.

Eulerian simulation of the installation process of a displacement pile

During the jacked installation of a displacement pile large deformations occur around the pile. The change in soil conditions near the pile after pile installation has finished has to be incorporated in the subsequent bearing capacity analysis. This paper presents the results of such an analysis. Two simulation methods to model the installation phase in an Eulerian framework are presented and compared with experimental data. In the first approach the pile is preembedded, while the soil is pushed through the fixed mesh. The second approach moves the pile geometry such that pile penetration is emulated, while keeping the mesh fixed. The results show that the second approach approximates the experiment within 20%, while the first approach overpredicts the measured result by 55%.

Towards consistent numerical analyses of embankments on soft soils

A consistent automated parameter derivation method for an advanced constitutive model, Creep-SCLAY1S, which considers anisotropy, structure and creep in soft soil is presented. The algorithms are i...

Under Pressure: A Laboratory Investigation into the Effects of Mechanical Loading on Charred Organic Matter in Archaeological Sites

The present publication investigates what happens to archaeological sites when they are built over. Focus is put on the degradation of charred organic materials by static loading. It is assumed that materials lose archaeological value if their fragments become too small to be recovered, or too distorted to be classified at species level. Several charred ecofacts of a few millimetres in size (wood fragments, hazelnut shells, and seeds) have been selected and subjected to individual particle strength tests. Assemblages of these particles have also been compressed one-dimensionally and scanned at several stages of testing using laboratory based X-ray microtomography. Microscopic damage by splitting or crushing is found to be limited at the macroscopic yield stress. It initiated at stresses less than 80 kPa for the weakest assemblages, and in all cases at stresses below 320 kPa. (80 kPa represents the load of a 6 m high sand embankment on soft soil that has half-settled underneath the groundwater table, while 320 kPa corresponds to stresses applied beneath the pile foundation level of high-rise buildings.) Sand seeded with charred particles has also been tested to illustrate the beneficial effect of embedment of charred particles in sand during static one dimensional loading.

Experimental Investigation Into Plugging of Open Ended Piles

This paper presents the results of a model pile test in a transparent photoelastic medium. This medium is composed of crushed glass particles in a refractive index matching liqu id. The test setup allows for quantitative photoelastic measurements. For the measurement of the photoelastic data the phase stepping technique is used. Stresses in the granular medium located next to and in the plug were recorded at varying installation depths of the pile during monotonic jacking of the transparent pile in combination with the pile head load. The horizontal stress rati o in the plug for this particular test is found to be equal to the ve rtical stress. A distinct stress arch below the pile base is obse rved during penetration. The high local stress increase in and below the plug move with the pile as the plug travels downward into the soil and do not change much in absolute magnitude.

Investigation of in-situ soil density change by resistivity measurements

Additional in-situ measurements during piezocone penetration tests can provide important information at a low cost due to recent advances in measurement technology. Resistivity measurements, commonly used in geophysical measurements, can be adapted to a standard piezocone penetration test (CPTU) to supply data about the in-situ properties of the soil. Change in soil density in extensive sand layers can therefore be determined. A series of laboratory multi-frequency AC resistivity tests with a novel electrode configuration have been performed with a model probe to investigate possibilities of in-situ measurement of volumetric properties of sand in a controlled saturated sand sample. The results show that soil density change of saturated sand can be measured efficiently and with relative accuracy with resistivity measurements at multiple frequencies. A measurement frequency spectrum of 100 Hz to 100 kHz is recommended for in-situ tests.

Experimental Investigation Into Pile Diameter Effects of Laterally Loaded Mono-Piles

This paper presents the results of model tests on laterally loaded mono-pile foundations in sand. The tests have been performed in a geotechnical centrifuge. The objective of the research is to quantify large diameter effects of these mono-piles on the lateral capacity and the stiffness response for cyclic lateral loading. These large diameters are out of the validity range of the commonly used design methods. For this reason prototype pile diameters up to 4.4 m with a length over diameter ratio of 5 have been investigated. The results show an increase in pile diameter from Ds = 2.2 m to Dl = 4.4 m leads to a significant increase in static lateral capacity and stiffness from cyclic load tests. All tests have been performed with constant L/D = 5, Id = 60% and a load eccentricity up to e = 4.8 m. However, the current test series needs to be extended to higher initial densities and the load control should be more strictly regulated before a clear diameter dependence, for pile diameters > 2.2 m, is proven.Copyright

Quantification of the failure mechanism of a granular material next to a displacement pile

This paper presents the results of a model pile penetration test in a photoelastic material. In this test the soil is represented by an assembly of photoelastically sensitive glass particles. This allows for the determination of stresses in the assembly by the photoelastic method. Displacements around the pile are measured in the same setup using digital image correlation. Tests were performed in a medium dense particle assembly. The development of stresses and strains, in particular the development of horizontal stress around the pile tip and pile shaft, is quantified and presented in this paper.