Geopolymer Materials for Low-Pressure Injections in Coarse Grained Soil: Multiscale Approach to the Study of the Mechanical Behaviour and Environmental Impact

The term soil improvement is commonly referred to the modification of soil structure in order to obtain a material with better physical and mechanical properties such as strength, stiffness or permeability. With this purpose, one of the most commonly used applications, particularly in coarse-grained soils, is the low pressure injection of cementitious mixtures. In recent years, there has been a growing demand for solutions with limited environmental impact and limited CO2 emissions and, in this regard, the cement present in the injected grout is evidently the weak point of traditional solutions. In this work, the experimental study of geopolymer materials as a substitute of cement mixture for low-pressure injection for coarse-grained soils improvement is presented. The study started with a focus on the geopolymer fresh mixture properties (density, viscosity, horizontal ellipsis ) and the evolution over the time of the mechanical properties (compression and tensile strength and stiffness) comparing three different mix designs at three different monitoring temperatures. The same evaluations were repeated on sand samples injected with the different types of mixtures previously analyzed. For a selected mix design, a permeation test was carried out under controlled conditions to test the pumpability and effectiveness of geopolymer injection. Finally, to deepen the chemical interaction between the injected mixture and interstitial water, an injection test was carried out using a scaled model of a real injection system. The experimental study carried out was aimed both at the analysis of the characteristics of the geopolymer material and at its physical interaction with coarse-grained soil, passing through the measurement of the mechanical characteristics of the geopolymer material and of the solid sand skeleton mixed with geopolymers. Finally, the possible chemical interaction of the mixtures with groundwater was also evaluated in order to highlight any environmental issues. The results shown provide a preliminary but sufficiently broad picture of the behavior of geopolymer mixtures for low-pressure injection for coarse-grained soil improvement purposes both from physical-mechanical and chemical points of view

Front-face pressure drop during the standstill phase for EPB mechanized tunnelling in coarse-grained soils

Mechanized tunnelling with EPB-TBMs bases its success on the ability to guarantee a uniform and constant front-face pressure during the excavation, using the excavated soil properly treated with chemical additives. This front-face pressure, measured with pressure sensors inside the working chamber, is kept constant during the excavation phase by balancing the amount of incoming soil, the foam injected, and the amount of soil extracted through the screw conveyor in the working chamber. During the standstill phase, required to install the tunnel lining elements, particularly in the coarse-grained soils, the pressure inside the chamber tends to decrease until the next resumption of the excavation. This effect, mainly linked to the permeability of the soil outside the TBM, tends to increase settlements induced on the surface and, therefore, should be minimized. In this paper, a very simple calculation model proposed by Bezuijen and Gerheim Souza Dias in 2017 for tunnelling in saturated sandy soils, is applied to an Italian tunnelling project. A comparison of model predictions and data recorded real time on the site during the excavation is illustrated and discussed. The predictive ability of the calculation model proposed for saturated sand seems to be useful also for TBM applications in gravelly soil under the phreatic surface, highlighting the weight of the main factors affecting the specific boundary value problem; moreover, it can be a useful tool to address the problem and to individuate technical countermeasures finalized to minimize the phenomenon and its negative effects

Grain morphology and strength-dilatancy of sands

The presented experimental study, from micro to macro scale, assesses the effects of grain shape on the strength-dilatancy behaviour and physical properties of seven natural sands of different origins and mineralogical composition. The micro-scale has been characterized by processing 2D image of grains, using the fractal analysis of particle contour to define their shape. The macroscopic material properties are evaluated performing direct shear tests at three different normal stresses, and measuring void ratios limit and angles of repose. An innovative method to interpret the experimental data obtained has pinpointed interesting relationships linking the morphological features of grains to the mechanical and physical properties of sands. Data confirm that the increase in grain shape irregularity produces an increase in all the variables investigated: void ratios limit, friction angles and dilatancy parameters

Prediction of tunnelling-induced effects on a historic building in Rome

Tunnelling in urban environments often poses serious concerns regarding potential detrimental effects induced on the surrounding structures and infrastructures, as in the case of the soon-to-be-built new Flaminio railway station. The station's project involves the conventional excavation of three adjacent shallow tunnels, whose roofs are located very close to the foundation of a historic building. To minimize the risk of damaging the building, the excavation is preceded by ground improvement works and the installation of a very stiff pre-support system made of concrete-filled steel pipes just above the tunnels’ roofs. This paper presents a Class A prediction of tunnelling-induced effects, obtained developing a three-dimensional numerical model that accounts for the complexities of the problem's geometry and construction phases. The soil mechanical behaviour is described by an advanced constitutive model (the Hardening Soil model with small strain stiffness). The results provide useful insights about the soil-tunnel-building system's behaviour. The expected settlements are very little and the potential damages to the building practically negligible. In addition to drastically reducing the risk of local instabilities, the heavy ground improvement works and the installation of the pre-support – whose effects are quantified through comparative analysis – further reduce settlements and distortions induced on the building. Finally, the results are compared with the predictions obtained adopting a simple linear elastic perfectly plastic constitutive model (Mohr-Coulomb model) both using the parameters employed at the design stage, calibrated assuming precautionary medium–high strain levels, and attempting to capture a more realistic level of induced strains. A comprehensive monitoring system will record a significant amount of data concerning the soil-structure interaction, enabling to better understand the behaviour of the system and to test the accuracy of the numerical predictions

Study on the environmental impact of chemicals used in mechanized tunneling techniques

This paper focuses on the environmental impact of soil conditioning agents used in the mechanized excavation of tunnels. The effects of the chemical composition, dosage and injection mode of the different additives, as well as the effects of soil characteristics and the influence of the aerobic and anoxic environment on chemical biodegradation, through tailored laboratory tests were individually investigated. Results are reported and discussed, thus providing useful information regarding the interaction be-tween the soil and the chemical additives used.. A comparison between the experimental results and those obtained through a scale model of a site for the disposal of the treated soil is also reported. In addition, useful information regarding the biodegradation of the tested additive are discussed

The management of the soil conditioning process for the excavation of the Rome Metro C line

Tunnel excavations in urban areas are often performed with TBM and Earth Pressure Bal-ance technology which requires the continuous injection of chemicals in the soil condi-tioning process. Several advantages can be achieved by performing laboratory activities prior to starting the excavation in order to predict risks such as clogging in fine grained soils or severe cutter-head wear in coarse soils and to define an optimal range of values for injection parameters. Since the in-lab reproduction of all the conditions occurring dur-ing the excavation is hardly possible, measurement performed directly on site and TBMs monitoring data analysis are relevant tools to improve the knowledge on soil conditioning process and chemicals management. For the project of the Rome Metro C line, laboratory tests and on-site measurement were performed and useful information on excavation were drawn from the analysis of the recorded data. The results provided insights on ad-vantages, limits and differences of results between laboratory-scale and on-site soil condi-tioning

Classification of foam and foaming products for EPB mechanized tunnelling based on half-life time

During tunnel excavation with Tunnel Boring Machines (TBMs) and earth pressure balance (EPB) technology, conditioning agents are commonly used to modify the physical and mechanical properties of soils so that they are appropriate for the application of the front-face support pressure during excavation. The consistency of the conditioned soil must be stable during the time necessary to advance and install the lining. Hence, a stability analysis of the injected foams is an important aspect. To assess the stability of a foam a classification is proposed based on the results of a standard foam generation procedure and on half-life tests using products from the main suppliers. A further classification of the foaming agents used according to their ability to produce a stable foam in terms of Foam Expansion Ratio is also proposed

Chemical interaction between fine-grained soil and foaming agents in tunnelling with TBM-EPB

The use of Tunnel Boring Machines (TBM) and particularly the Earth Pressure Balance (EPB) technology is one of the most commonly used way to perform mechanized tunnel excavation. The possibility to perform the excavation with high performance, avoiding risk for workers and reducing the induced subsidence, particularly important in urban area, are the main reason of the growing of this technology. The success of the excavation is manly related to the quality of the soil conditioning process obtained by injecting chemicals under the form of foam to the front of excavation and mixing it to the excavated soil. For fine grained soils the interaction between clayey particles and chemicals lead to the variation of the soil features and, particularly, to the reduction of the natural adhesion between soil and cutterhead metallic surface. This paper reports the results of an experimental research activity carried out to measure the temporary and permanent effects of chemical interaction between the chemicals and the clay particles both in terms of physical and mechanical properties. To this aim different clayey soil samples of different mineralogical composition, from European tunnel excavation projects, are treated with foams generated employing different commercial products. Laboratory apparatuses and test procedures specifically developed are employed to quantify accurately the adhesion between soils and metallic surfaces. The systematic comparison between the results obtained on natural and conditioned soil sample provide useful insights on the effects of chemicals on adhesion and on permanent effects on physical and mechanical properties

Handbook on Tunnels and Underground Works. Volume 1: Concept - Basic Principles of Design

The book provides a new, global, updated, thorough, clear and practical risk-based approach a global, updated, thorough, clear and practical new risk-based approach to tunnelling design and construction methods, and discusses detailed examples of solutions applied to relevant case histories. It is organized in three sequential and integrated volumes: • Volume 1 ‘Concept – Basic Principles of Design’ • Volume 2 ‘Construction – Methods, Equipment, Tools and Materials’ • Volume 3 ‘Case Histories and Best Practices’ The book covers all aspects of tunnelling, giving useful and practical information about design (Vol. 1), construction (Vol. 2) and best practices (Vol. 3). It provides the following features and benefits: • updated vision on tunnelling design, tools, materials and construction • balanced mix of theory, technology and applied experience • different and harmonized points of view from academics, professionals and contractors • easy consultation in the form of a handbook • risk-oriented approach to tunnelling problems. The tunnelling industry is amazingly widespread and increasingly important all over the world, particularly in developing countries. The possible audience of the book are engineers, geologists, designers, constructors, providers, contractors, public and private customers, and, in general, technicians involved in the tunnelling and underground works industry. It is also a suitable source of information for industry professionals, senior undergraduate and graduate students, researchers and acade