Acta Geotechnica

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.

Embedded retaining walls equipped with ground heat exchangers is one kind of energy geostructures, harvesting the shallow ground energy for heating and cooling buildings. However, past investigations mainly focus on the thermal assessment of energy walls, but pay limited attention to mechanical wall behavior and associated ground response. At present, there is no standard preliminary code for assessing the long-term mechanical performance of energy walls. In this study, a comprehensive literature review is conducted to summarize the existing studies on the long-term mechanical behavior of both conventional retaining walls and thermo-active ones, respectively. The review identifies three most critical mechanical factors for energy walls (i.e., horizontal wall movement, ground settlement, and basement heave), while their permissible values can be referred to standard preliminary design criteria of conventional retaining walls in the short-term and limited long-term field measurements. For elaborating the application of design criteria, a parametric study is implemented to evaluate the long-term mechanical performance of energy walls in stiff clay under various thermal conditions, including the influence of thermal solicitations, station temperature and asymmetrical operation mode. The vivid assessing results show that the thermal-induced mechanical movements due to geothermal operation may cause unfavorable serviceability issues (e.g., cracks), but are unlikely to bring critical damages to the structural performance. The identified critical mechanical factors and evaluation of thermal influences in this study provide some guidance for the mechanical assessment of long-term energy wall behavior, particularly in the light of limited field measurements and demonstration cases.

Soil plugging in open-ended piles leads to an increase in compressive bearing capacity but also influences pile driving resistance. Many different factors affect the tendency for soil plugging, for example, pile diameter, penetration depth and installation method. In this paper, the influence of the installation method on soil plugging is investigated. In situ measurements during the installation of two instrumented tubular piles are carried out to investigate the internal and external stresses acting on the pile during the installation process. Furthermore, the cone penetration resistance inside one pile is measured during the installation, and the accelerations and strains at the pile head are monitored to predict the bearing capacity. The installation method is varied between vibratory and impact pile driving. The results show that a significant increase in horizontal stresses inside the pile occurs during impact driving which leads to the conclusion that a soil plug is formed. During vibratory pile driving, no stress increase was observed.

In this paper, a numerical method is proposed to simulate multi-scale fracture propagation driven by fluid injection in transversely isotropic porous media. Intrinsic anisotropy is accounted for at the continuum scale, by using a damage model in which two equivalent strains are defined to distinguish mechanical behavior in the direction parallel and perpendicular to the layer. Nonlocal equivalent strains are calculated by integration and are directly introduced in the damage evolution law. When the weighted damage exceeds a certain threshold, the transition from continuum damage to cohesive fracture is performed by dynamically inserting cohesive segments. Diffusion equations are used to model fluid flow inside the porous matrix and within the macro-fracture, in which conductivity is obtained by Darcy’s law and the cubic law, respectively. In the fractured elements, the displacement and pore pressure fields are discretized by using the XFEM technique. Interpolation on fracture elements is enriched with jump functions for displacements and with level set-based distance functions for fluid pressure, which ensures that displacements are discontinuous across the fracture, but that the pressure field remains continuous. After spatial and temporal discretization, the model is implemented in a Matlab code. Simulations are carried out in plane strain. The results validate the formulation and implementation of the proposed model and further demonstrate that it can account for material and stress anisotropy.

Suffusion is defined as fine soil particles' detachment by hydraulic forces and their subsequent transport through pre-existing pores between the coarse particles. Contact erosion would occur at the interface between two soil layers if the coarse–fine soil combinations were submitted to a flow parallel to the interface. In some foundations with complex soil structures, suffusion and contact erosion can interact simultaneously, especially in double-layered or multilayered soil foundations with a cut-off wall. To describe the progression of suffusion and contact erosion, investigate the interaction between them, and explore the influence of retention ratio (Dc35/d85SA) between soil layers, nine double-layered internal erosion tests were performed in this study. The results reveal that the effect of the retention ratio on suffusion and contact erosion in double-layered alluvial foundation with a cut-off wall exhibits a consistent trend. Specifically, at both the onset of erosion and blowout, when Dc35/d85SA < 1, the critical hydraulic gradients increased at almost a constant value of Dc35/d85SA, in contrast, as Dc35/d85SA exceeds 1, a corresponding decrease in the critical hydraulic gradients was observed. Besides, when Dc35/d85SA < 1, the initial mode of erosion was contact erosion at the downstream interface; when Dc35/d85SA > 1, the initial mode of failure was either suffusion at the tip of the cut-off wall or simultaneous suffusion and contact erosion.

Terrestrial subsurface exploration equipment has developed over time to take advantage of the planet’s gravitational acceleration and energy availability. Today, heavy surface equipment provides the reaction forces needed to overcome the ground penetration resistance. Energy is so readily available that the depth-limiting parameter is often the surface anchor weight. To access the lunar subsurface, mankind will need to overcome its low gravity environment with a limited power supply. This paper reports on the performance of an earthworm-inspired subsurface penetration probe developed to mimic the annelid’s anterior end. The device can be described as a miniature cone penetration probe equipped with a soft membrane that is actuated by an external hydraulic system. Subsurface penetration tests are conducted using lunar mare simulant at high void ratio. The results show up to 80% reduction in penetration resistance at a maximum peak power demand of 0.2 watts. Moreover, the penetration resistance, penetration energy, and power demand can all be adjusted by tuning the probe inflation volume and penetration depth interval. Thus, the earthworm-inspired penetration process can be adapted ‘on the flight’ to site-specific conditions.

When a foundation pit is hydraulically connected with surroundings, the dewatering inside the excavation would both induce water-level decline and enclosure wall deflection, which together cause ground settlement outside the excavation. However, the current studies have not fully revealed the settlement behaviour under the combined actions of the above two factors; meanwhile, the individual effect of the two factors on the ground settlement is still indistinguishable. In this study, in situ pumping test and numerical simulations were both conducted to ascertain the above issues. Specifically, a fluid–solid coupling numerical model was developed to simulate a practical foundation pit dewatering test; measured ground settlement and groundwater drawdown were adopted to validate the numerical model; then, a series of numerical simulations were performed to revel the characteristics of ground settlement due to the combined actions of groundwater drawdown and wall movement; on this basis, the individual impacts of the two factors on the ground settlement were separated. Results show that the settlement ratio caused only by enclosure wall movement (ηb) varies in the range of 2.5–43%, depending on the pumping location, pumping time and hydraulic connectivity in strata; ηb is overall greater during the pumping of phreatic aquifer compared to the pumping of confined aquifer, while with the pumping time elapsed, ηb would both decrease apparently regardless of the pumping location.