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

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

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

3D character of backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water-retaining structures which are founded on a sandy aquifer. Nowadays, piping research and safety assessments are often based on experimental or numerical modelling using arbitrary model widths or even two-dimensional (2D) assumptions. This technical note shows the influence of this limitation through a series of small-scale experiments with varying model widths. The flow pattern proves to be highly three-dimensional (3D), influencing both the pipe geometry and critical gradients leading to piping failure. A 2D model is unable to capture the important aspects of the erosion mechanism and a correction factor needs to be applied if the minimum width for correctly simulating a 3D situation is not accomplished

Piping in loose sands: the importance of geometrical fixity of grains

Piping is one of the possible failure mechanism for dams and levees with a sandy foundation. Water flowing through the foundation causes the onset of grain transport, due to which shallow pipes are formed at the interface of the sandy layer and an impermeable blanket layer. In the past, the mechanism has been investigated predominantly in densely packed sands, in which the process was observed to start at the downstream side (backward erosion). Recently performed experiments in loose sand (van Beek et al. 2009) showed a different failure mechanism (forward erosion). In this article additional experiments of piping in loose sands are described for investigating the relevance of the forward process for practice. In these experiments the type of process was found to be dependent on the presence of shear resistance between sand box cover and top sand grains, that causes grains to be fixed geometrically. Without this shear resistance the process was found to be forward, whereas with this shear resistance the process was found to be backward oriented. The change in degree of fixity and relative density as a result of loading is investigated with electrical density measurements. The experiments show that the forward process is not relevant for levees in practice, in which the cohesive blanket layer causes the sand grains to be fixed properly

Vertically inserted geotextile used for strengthening levees against internal erosion

The effectiveness of a vertical inserted geotextile against internal erosion, in particular backward erosion, has been investigated. Some small- and medium-scale tests were performed as well as a field-scale test and compared with test series without any erosion mitigating measure. All tests demonstrated that the geotextile shield was very effective to prevent backward erosion. Numerical analysis showed that it was more effective than an impermeable sheet pile of the same length. The paper describes the technique in more detail as well as the results of numerical calculations. Furthermore, the test facilities are describe

Governance above the state: explaining variation in international authority

Hooghe, E.A.E.B. [Promotor]Marks, G.W. [Promotor