Adam Bezuijen

Analytical Model for Fracture Grouting in Sand

A conceptual, analytical model has been developed to describe the fracture grouting process in sand. The objective of the model is to improve understanding about this process in sand and to model propagation of the fractures. The results can be used to assess the parameters that control the fracture process. It is assumed that the complicated shape of a fracture in sand can be simplified to a geometrical shape (such as a tube or a plane) as a first approximation. Filtration of the grout appears to have a significant influence on the fracture shape when grout is injected into permeable subsoil such as sand. By assuming a pressure at which a fracture starts and a minimum pressure for propagation, it appeared possible to calculate the width-to-length ratio of the fracture independent of other soil properties. Quantification of the flow inside a fracture and the filtration processes resulted in a model that has been used to study differences in fracturing behavior in model tests and field tests on fracture grouting in sand. It was concluded that the width-to-length ratio of the fractures in a permeable soil decreases if the injection pressure of the grout or the permeability of the grout cake is decreased.

Levee Failure Due to Piping: A Full-Scale Experiment

Piping is considered as an important failure mechanism for water retaining structures in the Netherlands. A recently performed study on the safety of Dutch levees raised some doubts with respect to the validity of the current calculation model. A large research program has therefore started to investigate the process of piping in more detail. After laboratory experiments and desk studies, the model was validated in a full-scale experiment (seepage length 15 m). This paper describes the piping process as observed in this experiment. Different phases were found: seepage, retrograde erosion, widening of the channel and failure. Once sand craters were formed, stabilization of sand transport was not observed, although quantities of transported sand were very low. Ongoing erosion resulted in a piping channel from the downstream to the upstream side in a few days. Widening of the channel due to continued erosion finally resulted in significant deformation and failure of the levee.

Data Analysis of Pile Tunnel Interaction

AbstractTunnel construction in interaction with deep foundations is often a matter of concern in urban areas. However, a great deal of uncertainty is still evident in the differences between regulations for minimum tunnel clearance and in the design of preventive measures for the pile–tunnel interaction effects. To address this problem, this paper presents, analyzes, and compares over 50 reports of case studies, physical models, and full-scale and centrifuge tests on a wide range of layouts and soils. The effects of new tunnels on existing piles are described in terms of mechanisms, induced settlements, and structural forces. The data analysis reveals patterns that could not be found within the limited number of tests of each separate study. Quantitative measurements of pile settlements and axial stresses have been related to the pile position, working load, and tunnel volume loss. The results indicate that most piles do not reach failure and that for piles at a lateral distance of more than two times the...

Field Measurements to Investigate Submerged Slope Failures

Many flood defences in The Netherlands have been disapproved for flow slides of the Holocene subsoil. Traditionally these flow slides are assumed to be induced by static liquefaction. Only in recent times it has been recognized that flow slides may also concern breach flows, which do not necessarily require loosely packed sand. For both static liquefaction and breach flow the inaccuracy of the currently applied methods to determine in situ density lead to high computed probabilities of failure, which is one of the main problems in the safety assessment of flow slides. In order to reduce this uncertainty, based on a literature study a number of methods were selected and applied on four test locations: two sites where flow slides occurred and two sites where no flow slides occurred, but for which high probabilities on flow slides were calculated based on current Dutch assessment rules for liquefaction and breach flow. For these sites CPT’s and electrical resistivity cone tests available from earlier investigations, were extended with seismic CPT’s and interpreted for relative density and state parameter. The results of this study lead to the conclusion that some of the historical flow slides in The Netherlands may have been the result of static liquefaction in loosely packed sand. For many other slopes, however, it is more reasonable to assume that the failures must have been breach flows in medium or densely packed layers.

Analytical Groundwater Flow Calculations for Understanding the Flow and Erosion in a Coarse Sand Barrier

The coarse sand barrier (CSB) is a promising method to avoid ongoing backward erosion piping resulting in increased safety of a dike for this failure mechanism. Experiments are performed at different scales at Deltares, the Netherlands. These experiments show a significant increase in the critical head for structures with a CSB compared to structures without a CSB. The increase of critical head cannot only be ascribed to the lower erodibility of the coarser particles in the barrier, but also to the reduction of hydraulic load on these coarse particles in the barrier, resulting from the permeability contrast of barrier material and surrounding sand. To investigate the influence of a CSB on the flow pattern numerical and analytical calculations have been performed. This paper focusses on the analytical calculations. It will be shown that these can explain the increase in strength and the measured scaling effects.

Scale Effects in Coarse Sand Barrier Experiments

The coarse sand barrier is considered as a promising measure to prevent backward erosion piping from causing failure of embankments. A pipe is allowed to progress backwards until it encounters the coarse sand barrier, which prevents it from progressing unless the head difference over the embankment is significantly increased. A three stage experimental programme supported by groundwater flow modelling is carried out to investigate the feasibility of this method. The hypothesis is that the strength of the barrier is characterised by a local gradient at the interface between the barrier and the pipe. Major questions are: can the horizontal gradient as measured in laboratory tests be used to characterise the strength of the barrier material, over which distance should a horizontal gradient be determined, and is this distance the same for models at different scales? This paper presents the background theory and demonstrates the effects using scale dependent criteria. Preliminary results of small- and medium-scale experiments are used to compare the two approaches.

The use of partially hydrolysed polyvinyl alcohol for the production of high drug-loaded sustained release pellets via extrusion-spheronisation and coating: In vitro and in vivo evaluation

Partially hydrolysed polyvinyl alcohol (PVA) was evaluated as a pelletisation aid for the production of pellets with a high acetaminophen and metformin hydrochloride concentration (>70%, w/w). Mixtures with varying drug concentration and PVA/microcrystalline cellulose (MCC) ratios were processed via extrusion-spheronisation, either after addition of PVA as a dry powder or as an aqueous solution. Finally, high drug- loaded metformin pellets were coated with a methacrylic acid copolymer (Eudragit™ NM 30D) and evaluated for their sustained release potency in vitro and in vivo. The plasticity index of the wet mass increased by the addition of PVA to the formulation, which resulted in enhanced extrusion-spheronisation properties, even at a high drug load. Although the MCC concentration was successfully lowered by adding PVA, the inclusion of MCC in the formulation was essential to overcome problems related to the tackiness effect of PVA during extrusion. Overall, wet addition of PVA was superior to dry addition, as pellets with a higher mechanical strength and narrower particle size distribution were obtained. Pellets containing 87% (w/w) metformin hydrochloride were successfully layered with 20% (w/w) coating material, yielding sustained release pellets with a final drug load of 70% (w/w). In addition, the sustained release characteristics of the PVA-based pellets with a high drug content were confirmed in vivo as no difference with the Glucophage™ SR reference formulation was observed.

Wet and Dry Effects on the Hydraulic Conductivity of a Polymer Treated GCL Prototype

Geosynthetic clay liners (GCLs) are widely used to isolate pollutants because of their low hydraulic conductivity to water. However, the performance of clay barriers may be impaired by prolonged exposure to electrolytic liquids which may lead to the compression of the diffuse double layer. The consequences are the increase of permeability and the loss of self-healing capacity. Moreover, the efficiency of the liners can further deteriorate by repeated wet and dry cycles, which may lead to desiccation of the bentonite and associated cracking. Modified bentonites have been introduced to improve the resistance of clay barriers to aggressive solutions. This study deals with a polymer-amended clay, HYPER clay. HYPER clay is treated with an anionic polymer and dehydrated and it shows enhanced performance in presence of electrolyte solutions. The effect of wet and dry cycles on the hydraulic conductivity to seawater of needle-punched GCLs prototypes of treated and untreated bentonite was investigated. The prototype samples containing HYPER clay 8% showed lower permeability compared to those containing untreated bentonite. However, the temperature suggested from the standard used in this study is extremely high and it does not represent the temperature in the field.

TBM pressure models: observations, theory and practice

Mechanized tunnelling in soft ground has evolved significantly over the last 20 years. However, the interaction between the tunnel boring machine (TBM) and the ground is often understood through idealized concepts, focused mostly on the machine actions in detriment of the reactions from the ground. These concepts cannot be used to explain several mechanisms that have been observed during the construction of mechanized tunnels. Therefore, this paper presents the path from field observations to the theoretical developments to model the TBM-ground interaction more realistically. Some ideas on how these developments can be applied into practice are presented. Finally, a discussion is proposed about how an effective collaboration between academia and industry can alleviate the current concentration of knowledge in the state of practice.

3D FEM simulation of groundwater flow during backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, a 3D numerical approach of the groundwater flow leading to the erosion mechanism of backward erosion piping is presented and discussed. Comparison of the 2D and 3D numerical results explicitly demonstrates the inherent 3D nature of the piping phenomenon. In addition, the influence of the dike length is investigated and discussed for both piping initiation and piping progression. The results clearly indicate the superiority of the presented 3D numerical model compared to the established 2D approach. Moreover, the 3D numerical results enable a better understanding of the complex physical mechanism involved in backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures.