Foam used during EPB tunnelling in saturated sand, parameters determining foam consumption

The amount of foam injected during drilling with an EPB-shield in saturated sand is quite often based on experience and/or empiric relations. A method is presented in to calculate the amount of foam needed to create a muck with limited or no grain stress. The results show that, as expected, the volume of the foam to be injected is much larger in dry soil compared to saturated soil. In saturated soil the amount of foam to be injected depends on various parameters. The permeability of the soil in front of the EPB-shield appears very important. This paper describes the dependencies and shows that recommended foam injection ratio’s from literature may be too small when used in permeable sandy soil. The FIR of the foam in a mixture may be much smaller than of the original foam

Slurry infiltration tests for slurry shield tunnelling in saturated sand

Tunnelling in saturated sand will cause excess pore pressures in the sand. This was the case during the construction of all tunnels in the Netherlands. This excess pore pressure influences the stability of the tunnel face. Therefore the magnitude of the excess pore pressure is of importance. Furthermore, it requires more information on the penetration process of the bentonite slurry as it will occur at the front of the tunnel face. This paper deals with preliminary infiltration tests to investigate some aspects of the infiltration. Results will be compared with theory

Is 1 + 1 = 2 ? Results of 3D model experiments on piled embankments

Most design models for basal reinforced piled embankments design the geosynthetic reinforcement (GR) as a single biaxial layer. In practice, however, the required strength and stiffness is frequently divided into two ore more GR layers with or without a layer of granular material in between. This paper compares three geogrid-systems : (1) one GR layer consisting of one biaxial geogrid and a (2) one GR layer consisting of two uniaxial geogrids installed directly upon each other and (3) two geogrids with a fill layer in between. Five model tests have been carried out to study the differences between these three systems. This test series has been presented earlier by Van Eekelen et al. 2012a. The present paper presents part of the results. It was shown that the behaviour of the first two systems is the same: the GR stiffness of the biaxial geogrid is apparently the same as the summed stiffness in each direction of the two uniaxial grids. It was also shown that the application of a fill layer between the two GR layers results in a slightly more linear dependency of the net load on the fill aan in the end of the tests, in slightly more arching. However, the differences are very small. The GR stiffness, for example, has much more influence than whether or not a fill layer has been applied between the two GR layers. It is therefore concluded that the stiffness and strength of two or more GR layers can be summed, thus 1 + 1 = 2 indeed

The influence of the leakage length on the initation of backward erosion piping

The earliest relation describing the risk of backward erosion piping, from Bligh in 1915, calculates an overall gradient over the dike. This gradient should be lower than a certain value depending on the material of the dike foundation for the dike to be safe. Later developments rationalize this number, showing the influence of grain size, permeability, density and added the influence of the thickness of the aquifer. However, the principle remains the same. The actual overall gradient is compared to a critical gradient. This contribution will show, using analytical groundwater flow calculations that for the beginning of backward erosion piping, not only the overall gradient but also the leakage length of the semi-confined aquifer, usually present at the landward side of the dike, is of importance. This leakage length determines the piezometric head on the landward side of the dike. This is not very new, since this is also used in the 'blanket theory' developed and used in the USA, but a slightly different approach results in some new conclusions. It will also ba shown that the influence of the leakage length on the pipe progression and in ultimo on the breach of the dike is less. This means that with a long leakage length the difference in overall head between pipe initiation and breakthrough is larger than for the situation with a shorter leakage length

Similitude in soil-structure and soil-soil interaction

Scaling laws are often presented as a criterion that has to be fulfilled. For example: a pile diameter should be larger than 20 times the diameter of the grains to have similitude between the model tip force and the tip force of the corresponding prototype. This paper investigates another approach: What is the mechanism that requires a certain ratio between pile diameter and grain size and can this influence be quantified? If so, it is sometimes possible to calculate the systematic error made and correct the results of model tests. This appeared possible for the influence of the grain size on the pile tip resistance and for the influence of viscous forces in clay. It was not possible when undrained behavior in the model corresponds with drained behavior of the prototype or in situations where flow is important and the solution of Darcy’s equation results in singularities

TBM pressure models: calculation tools

Mechanized tunnel construction in soft ground has evolved significantly over the last 20 years, especially on the matter of settlement control. This was achieved by guiding the TBM operation to control the main factors that induce soil displacements, namely the face pressure and the closure of the soil-lining void. Nowadays, TBMs can be operated within strict serviceability requirements. However, several mechanisms of the excavation cycle are still not taken into account when estimating the induced soil deformations. Therefore, it is important to properly model the processes around a TBM, but in order for such models to be assimilated in the state of practice, they should be combined in a design framework where the operational characteristics can be assessed together with the induced soil displacements and lining forces for different project conditions. This paper presents the first step of this general project by focusing on the tail void grouting pressures. A model for the grout flow is associated with a finite element model to calculate the induced soil displacements in a dynamic equilibrium between the boundary pressures and the soil-lining gap. These two elements are combined in a calculation tool with a user friendly input-output layout

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

Session report: dams, embankments and slope stability

This session report discusses 13 papers that were presented at the 8th International conference on physical modelling in geotechnics 2014 in the session 'Dams, embankments and slope protection'. It concentrates on whether scaling is possible and what the consequences of scaling are in the various papers. Some tests do not really appear to be scaled model tests, because it is difficult to define prototype that behaves as the model. However, such models can still be useful to study mechanisms that are relevant in prototype

Pile tunnel interaction: literature review and data analysis

The underground space of densely populated cities contains deep foundations and tunnels. Considering the possibility that a new tunnel can be built close to existing piles it is necessary to assess the possible effects of that interaction. Most case studies have shown limited damage on pile supported structures; however, these constructions deal with great uncertainty as the mechanism of pile tunnel interaction is not completely understood. Physical tests at full and reduced scale are a valuable tool to improve that understanding and validate prediction methods. A descriptive review of studies on that matter is presented followed by a quantitative comparison of the results of tunnelling induced axial forces and settlements on the piles. Gathering and analysing these data provided a deeper understanding of the influencing geometrical and structural parameters as well as indicating where further research is needed