Coupled nonlinear thermo-hydro-mechanical finite element simulations were carried out to investigate the behavior of energy micropiles subjected to thermal loading cycles. Two kinds of problems were analyzed: The case of an isolated micropile, for which comparison with previous research on medium-size isolated energy pile is provided, and the case of large groups of micropiles, with the aim of investigating the interaction effects. In both problems, micropiles were considered installed in a thick layer of very soft, saturated clay, characterized by isotropic or anisotropic hydraulic conductivity. Two advanced existing hypoplastic models, one incorporating the thermal softening feature, were used to describe the clay behavior in both problems. The settlements of the micropile head were found to increase during thermal cycles under constant mechanical load, showing a sort of ratcheting. For micropile groups, the settlement increase rate was faster as the spacing between micropiles was reduced. The excess pore water pressures developed at the micropile-soil interface played a significant role on the deformation and displacement fields of the soil-micropile systems, especially in the case of micropile groups, affecting the shear strength developed at the micropile-soil interface. The consolidation process was faster when the hydraulic conductivity was anisotropic, meaning that the development of excess pore water pressure was reduced in this case. As the spacing between the micropiles increased, i.e., as thermal interaction decreased, the heat flux exchanged by a micropile of the group during one cycle approached the heat flux exchanged by an isolated micropile in the same period.

In order to study the force characteristics and reinforcement mechanisms of the bank protection capacity of micropile groups under rain seepage, two different scale models were employed using model tests and the finite element method. Focusing on the stress within the micro-piles, the lateral soil pressure against the piles, the displacement at the pile tops, and the overall stability of the embankment reinforced by the micro-piles, engineers can assess the performance and durability of the structure during rainwater scouring. The study shows that rainfall leads to increased soil saturation, which in turn reduces the soil's shear strength. When subjected to loading after rain, micropiles within the same row exhibit similar strains. The thrust from potential landslides at the top of the slope causes the rear row of piles to experience greater flexural deformation. The difference between the soil pressure values of the same row after rainwater infiltration is small, and the overall soil pressure value increases in a stepwise manner with the increase of loading volume. The micropile support helps reduce soil displacement. The displacement of the middle and front row of piles is significantly lower compared with the back row by 32.3 % and 35.7 %, respectively. The pile group can limit the soil displacement within a certain range, which is beneficial for improving the stability of bank slopes under rainfall scouring. The micropile group enhances the overall slip resistance of the bank slope and can inhibit the development of the slip and crack surface of the bank slope to a certain extent. In engineering design, it is crucial to determine an appropriate pile spacing. A too small spacing can prevent the piles from achieving their optimal bending strength, whereas too large a spacing may lead to the risk of the bank slope as a whole experiencing overturning damage.

Seismic retrofitting of existing bridges has been in practice for years to meet the stringent seismic requirements set forward by revised design codes. For retrofitting, however, bridge piers are often prioritized while less attention is given to the bridge foundations, which are equally prone to damage under seismic loadings. The current work presents a series of experimental studies in assessing the performance of 2 x 2 pile groups reinforced with micropiles in terms of head-level stiffness and damping under low-to-high levels of static and dynamic loadings, encompassing the influence of loading-induced soil nonlinearity. Practical micropiles inclinations of 0 degrees, 5 degrees, and 10 degrees with respect to the vertical are studied. Experimental results reveal that the head-level stiffnesses of pile groups reinforced with micropiles, contrary to the general expectations, become smaller than the pile group without micropiles at higher levels of applied loading. To elucidate the governing mechanism for such experimentally obtained results, three-dimensional nonlinear finite-element analyses were carried out. Results from the numerical analyses support the experimental results, suggesting that the presence of micropiles may not always increase the head-level stiffness of soil-foundation systems, particularly at higher levels of applied loading where the soil nonlinearity generated at the vicinity of piles and micropiles governs the overall head-level stiffnesses.

Micropile groups (MPGs) are typical landslide resistant structures. To investigate the effects of these two factors on the micropile-soil interaction mechanism, seven sets of transparent soil model experiments were conducted on miniature cluster piles. The soil was scanned and photographed, and the particle image velocimetry (PIV) technique was used to obtain the deformation characteristics of the pile and soil during lateral loading. The spatial distribution information of the soil behind the pile was obtained by a 3D reconstruction program. The results showed that a sufficient roughness of the pile surface was a necessary condition for the formation of a soil arch. If the surface of the pile was smooth, stable arch foundation formation was difficult. When the roughness of the pile surface increases, the soil arch range behind the pile and the load-sharing ratio of the pile and soil will increase. After the roughness reaches a certain level, the above indicators hardly change. Pile spacing within the range of 5-7 d (pile diameters) was suitable. The support effect was poor when the pile spacing was too large. No stable soil arch can be formed, and the soil slips out from between the piles.