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Permeation grouting - injections at low-pressure values of either microfine cements or chemical products - is frequently adopted to increase the mechanical/hydraulic properties of soils, and standard design approaches are currently either empirical or based on simplified analytical solutions. In this paper, some fundamental hypotheses of these analytical solutions are discussed by carrying out a campaign of finite-element numerical analyses, in which the injection phase in a water-saturated soil is analysed, a Newtonian rheology for the grout is implemented and the hypothesis of immiscibility for the two liquids is assumed. The effect of injection source geometry is discussed, as well as the role of gravity and capillarity. The authors analyse the conditions, in terms of injection flow rate, grout viscosity, soil intrinsic permeability and retention curve, under which the analytical solutions provide reliable results. The numerical results have been compared with the simplified analytical solution herein derived for a one-dimensional spherical geometry, in terms of the 'characteristic curves' for the system: the relationship between injection pressure/grout front advancement and injection time.

Low-pressure injection of nanosilica aqueous suspensions is often adopted to either waterproof or increase the liquefaction resistance of granular soils. The basic principle behind this ground improvement technique consists in filling the soil pores with a low-viscosity fluid that changes its consistency with time, first into a gel, then into a solid. From an application point of view, the simulation of the time-dependent permeation process is crucial to relate the in situ distance covered by the grout to the operational parameters. A comprehensive investigation was performed, combining laboratory experiments with theoretical approaches, to characterize the phenomenon and then derive predictive relations useful for designing treatment executions. The time-dependent rheological properties of different nanosilica aqueous suspensions were first quantified by means of rheometric tests, then described with Bingham's law. Grout permeation in granular media was then simulated by suitably modifying Darcy's law to incorporate the temporal evolution of Binghamian grout rheology. After validating the modified Darcy's law employment for nanosilica grout flows with respect to laboratory experimental data, simplified analytical equations, capable of predicting the temporal evolution of the distance covered in situ by the grout and the flow rate-injection pressure relation, are provided.

Permeation grouting, i.e. injections at low pressure, of microfine cements is frequently adopted in tunnelling and underground structures to either increase the mechanical properties or reduce the hydraulic conductivity of soils. From an applicative point of view, the time dependent permeation process, crucial to assess the spatial contour of the final content of the injected microfine cement, is highly affected not only by operational parameters, geometry of injection sources and particulate phase nature of the grout under exam, but also by its time-dependent rheological properties. This latter aspect is not deeply investigated in literature, especially in the ranges of shear rates, times and water-cement ratios commonly adopted during permeation grouting treatments. To this aim, in this paper, a comprehensive investigation has been performed, combining laboratory experiments with theoretical approaches. The time-dependent rheological properties of microfine cements characterized by different water-cement ratios have been first quantified by means of rheometric tests and described with a Bingham’s law. The microfine cement permeation in granular media has then been experimentally investigated and so the employment of a Darcy’s law modified to incorporate the temporal evolution of Bingha-mian grout rheologies has been validated for microfine cement flows.

Nanosilica aqueous suspensions are mechanically flexible and environmentally non-toxic materials. These are injected into soils at low pressures as fast remedials to stop piping induced by excavations, for the sealing of contaminants or to reduce the seismic-induced liquefaction potential. To adopt the technique, it is crucial to firstly predict the temporal evolution of the permeation process highly affected not only by operational parameters, but also by the time-dependent rheological properties of the employed grout. In this paper, a simplified method to design permeation grouting treatments with eco-friendly nanosilica grouts is proposed. Its employment is discussed focusing on how to rationally optimize the injection process in terms of nozzle spacing, injection time, grout compositions and pump pressure. This can be a useful tool in practical applications also to interpret the experimental results obtained from trial in situ tests, always recommended before grouting. The final goal is to guarantee a more efficient design approach for these treatments, currently absent in the scientific literature, taking not only times and costs but also environmental impact into account.

A block impact model based on the elasto-viscoplastic macro element approach is developed for regular base prismatic blocks. This model upgrades a previously conceived model for spherical boulders introducing (i) a rotational degree of freedom; (ii) a moment-rotation relationship; (iii) a toppling mechanism. The model can improve rockfall simulations by considering block geometry and the exchange between translational and rotational energies. Model parameters were calibrated by using laboratory tests on vertical impacts. Parametric analyses were carried out to investigate for both vertical and inclined impacts, the effects of block shape and orientation. The influence of these factors on the impact force, the maximum penetration depth as well as the exchange between translational and rotational energies is discussed. A comparison with the available results for small and large scale laboratory tests shows model capabilities and put in evidence the “nonlinear” relationship between maximum acceleration (or equivalently the maximum contact force) and impact translational velocity. For vertical impacts the trend of the maximum penetration depth is a function of the block shape. Prismatic blocks can experience larger values of maximum penetration than spherical blocks characterized by coincident masses and kinetic energies. In case of bouncing of a prismatic block the increment of normal maximum displacement with respect to spherical blocks ranges from about 66% for triangular base prisms to 132% for hexagonal base blocks. In case of no bouncing, the increments range from about 82% for triangular blocks to −32% for hexagonal blocks. Maximum normal forces also depend on block shape and orientation. In case of a vertex impact with no bouncing, triangular blocks show a decrement in the maximum force of about 43% with respect to the spherical block. The increment of initial block angular velocity generates a reduction in both maximum penetration depths and impact forces.

In spite of the increasing diffusion of tunnel boring machines, conventional tunnelling is still preferred for economic reasons in case of short tunnels, unconventional cross sections or irregular tunnel trajectories. In conventional tunnelling, the mechanical response of the tunnel front is a main concern and, when tunnels are excavated in cohesive soils, this is dominated by the time factor, related to geometry, to the mean excavation rate and to the hydro-mechanical properties of the materials involved. This is particularly evident during excavation standstill: front displacements progressively increase with time and, in many cases, the system response under long-term conditions becomes unstable. In conventional tunnelling, a common technique employed to improve the system response (under both short- and long-term conditions) is the installation of fibreglass tubes within the advance core. In this paper, the mechanical response of both unreinforced and reinforced deep tunnel fronts in cohesive soils is experimentally analysed. In particular, the results of a series of 1 g small-scale tests, taking into account both the influence of the excavation rate (the unloading time) on the system response and the evolution with time of the tunnel face displacements, induced by a rapid reduction in the horizontal stress applied on the tunnel face, are reported.

Chalks are highly porous rocks formed of mainly carbonate grains bonded together by calcite bridges. From a microstructural point of view they are very similar to calcarenites. The above characteristics make them prone to water induced weathering, frequently featuring large caverns and inland natural underground cavities. This study is aimed to determine the main physical processes at the base of the short - and long-term weakening experienced by these rocks when interacting with water. We present the results of microscale experimental investigations performed on calcarenites from different sites in Southern Italy. SEM, thin sections, X-ray CT observations and related analyses are used for both the interpretation - definition of the structure changes and the identification - quantification of the degradation mechanisms. Two distinct types of bonding could be identified within the rock: depositional bonds and diagenetic bonds. The diverse mechanisms linked to these two types of bonding explain both the observed fast decrease in rock strength when water fills the pores (shortterm effect of water), identified with a short-term debonding (STD), and a long-term weakening of the material, when the latter is persistently kept in water saturated conditions (long-term effect of water), identified with a long-term debonding (LTD). To highlight the microhydro-chemo mechanical processes of formation and annihilation of the depositional bonds and their role in the evolution of the mechanical strength of the material, mechanical tests on samples prepared by drying partially saturated calcarenite powder, or a mix of glass ballotini and calcarenite powder were conducted. Finally the use of transparent chemical consolidants as a more sustainable alternative to grouting is explored experimentally.

Grouting and jet grouting are geotechnical consolidation techniques commonly employed to improve the mechanical behaviour of soils. Although these techniques are common, the micromechanical processes taking place at the local level are not yet totally understood and modelled. In this work, such a problem has been approached from a micromechanical perspective via the discrete element method by considering the local interaction among soil grains and pseudo-fluid particles. Homogeneous representative elementary volumes of a virtual analogue of silica sand have been first generated and tiny rigid frictionless particles have been subsequently injected through them, to simulate the grouting in granular materials. Various injection pressures, initial soil pressures and initial soil densities have been considered. The different diffusion patterns, the flow rate, the consequent increase in local stresses and the consequent reduction in local porosity have been discussed. To overcome the DEM computational restrictions and to speed up the injection simulations, a novel procedure based on the replication of pre-equilibrated cells has been adapted for both the initialization and injection phase. Finally, a qualitative laboratory-scale pressure grouting test has been reproduced to validate the results.