W. Broere

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.

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%.

Structural analysis for shallow tunnels in soft soils

Generally, studies on structural design for bored tunnels focus on moderate to deep tunnels (cover-to-diameter ratio C/D ≥ 2). Such tunnel design methods cannot be used for shallow-situated bored tunnels because the influence of buoyancy is discounted, and actual loads on the tunnel lining are not taken into account properly. This paper proposes a new model that has more accurate loads on the tunnel lining combined with finite-element analysis for shallow tunnels. Internal forces and deformations of various shallow bored tunnels are investigated. The relationship between the optimal thickness-to-diameter ratio d/D of the tunnel cross section and the cover-to-diameter ratio C/D is also studied.

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.

A two-stage numerical analysis approach for the assessment of the settlement response of the pre-damaged historic Hoca Pasha Mosque

ABSTRACT The current article presents a case study of the settlement response of the historic Hoca Pasha Mosque that involves uncertainties arising from the complex excavation activities, soil properties, building materials, and geometry and the presence of pre-existing cracks in the mosque’s walls. The objective is to demonstrate the added value of a two-stage numerical analysis approach for the assessment of the settlement response of the building. The first stage comprises analyses of the structural behavior using the monitored settlements for each wall. The second stage examines the behavior of the complete building as a whole. The effects of soil-structure interaction and the pre-existing cracks are considered through discrete interface elements. It is shown that executed simulations can reasonably reproduce the overall settlement response, resulting stresses and the pre-existing crack activities. The parametric analyses in the second stage also produce generalizable results, of use beyond the specific case. Namely, as the soil/structure stiffness ratio increases the settlement-induced vulnerability increases. Including soil-structure interaction in the analyses reduces tensile strains due to differential settlements. Including pre-existing cracks reduces tensile strains in the vicinity of the cracks but results in an increase of stresses in neighboring sections.

Detection of Anomalies in Diaphragm Walls

If a calamity with a retaining wall occurs, the impact on surrounding buildings and infrastructure is at least an order of magnitude more severe than without the calamity. In 2005 and 2006 major leaks in the retaining walls of underground stations in Amsterdam and Rotterdam occurred. After these cases had been thoroughly studied it was concluded that the diaphragm walls had anomalies in the joints between the D-wall panels. These anomalies are hard to locate using regular leakage detection systems and may cause major damage to the surroundings if an erosive leak progresses. Such a risk is hard to handle for a project and may stand in the way of the application of D-walls close to adjacent buildings. Between 2009 and 2014, within the GeoImpuls program, research was done to develop measurements to detect these anomalies. Three techniques have been found to be effective in evaluating the quality of the concrete in the joint area between D-wall panels. During D-wall production the slurry refreshing operation and concrete casting can be verified using Distributed Temperature Sensing (DTS). After curing of the concrete the Cross-hole Sonic Logging (CSL) method, applied to the joint area, can be used to assess the concrete quality in the joint. Anomalies can be localized both vertically and horizontally. The size and contents of the anomaly can be estimated based upon laboratory reference tests and site experience of several projects. If verification of anomalies detected with CSL or DTS measurements is required, Electrical Resistivity Tomography (ERT) can be considered to evaluate the hydrological parameters of the anomaly, although the reliability of these results is much lower than with the DTS and CSL measurements. Based upon the information of the tests, it can be decided if mitigating repair works prior to excavation of the building pit are necessary or that reparation of the anomaly can safely take place while excavating the building pit. The paper will give a short description of all three methods and the executed validation measurements. The paper concludes with practical recommendations for implementation of the measurements.

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.

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.