عنوان مقاله [English]
Application of micro-piles to stabilize unstable slopes has been widely considered in recent years. A micro-pile is a small-diameter (typically less than 300 mm), drilled, and grouted as non-displacement pile that is typically reinforced. The main objective of this study is to evaluate the influence of the micro-pile group on increasing safety factor as well as finding the optimum position of micro-piles across the embankments slopes. In this study, numerical analyses were performed to investigate the efficiency of micro piles on the behavior of embankments using ABAQUS Finite Element code. The analyses have been carried for plane strain conditions and under earthquake loads. Based on acquired results, it was implied that using micro pile groups on the lower part of embankment slope is the optimal approach to reinforce embankments. This can increase the embankment safety factor by 33% in static mode and by 34% in dynamic mode. The impacts of the parameters of micro pile inclination angle, diameter, length, micro-pile spacing, and the type of soil on safety factor in dynamic mode have been investigated. Results show that increasing the length and diameter of the micro piles increase the safety factor. By increasing the micro pile angle in respect to vertical axis up to 30°, safety factor increases but any additional increase of the angle decreases the factor. Furthermore, by increasing the spacing of micro piles up to 4 times the diameter, causes improving the safety factor, but more space has a negative impact. By strengthening the type of the adhesive soil (simultaneous increase of embankment adhesion from 8 KPa to 16 KPa and elasticity module from 30 MPa to 60 MPa), embankment safety factor increases by 40%. Furthermore, by strengthening the sandy soil (simultaneous increase of internal friction angle from 32 to 40 and soil elasticity module from 20 MPa to 45 MPa), the safety factor of embankment increases by 41 percent.
In facing with problematic soils such as loose soil with low bearing capacity, high permeability, and soil with liquefaction potential, two approaches are common: the use of bearing elements, and modification mechanical-physical properties of the soil. Each of the above approaches has its own characteristics. Some innovative technics have combined the two approaches containing the advantages of both. Among these techniques, one can refer to the application of micro piles accompanied by grouting. In addition to functioning as a bearing element, this technique also improves mechanical properties of the soil due to grouting .
Micro pile is a replacement pile being accompanied by boring and grouting. Micro piles are often reinforced using steel components. They are commonly used to increase the foundations' strength against static and seismic loading, reinforcing the slopes, and stabilizing the walls of excavated areas . At first, using micro piles was solely considered to improve the weak foundation of buildings. By developing this technique, the application was extended to other geotechnical areas such as stabilizing the slopes and reducing the potential of liquefaction. Presently, the instruction recommended by US.FHWA is widely accepted reference by designers and contractors in this field .
Recent seismic tests have shown that micro pile systems can be used in seismic areas. However, engineering data on the seismic performance of micro pile systems is limited and more research is needed to create and assess seismic designing techniques and engineering guidelines. Before applying micro piles as a measure to reinforce the embankments, experimental and numerical studies are required to confirm the efficiency of this method. Numerical studies conducted by engineers in recent years show the efficiency of micro piles in this application. In this regard, one can refer to the two-dimensional study of seismic behavior of micro pile groups in sandy soil conducted by Maghsoodi et al.  and three-dimensional elastoplastic study of the seismic performance of inclined micro piles by Sharour et al. . Furthermore, employing micro pile networks in rapid excavation of a railway project by Bruce and Janes , investigating the stability of the slopes reinforced with micro piles by Sun et al.  could be mentioned in this field. Dynamic and static analysis of micro piles to reinforce high embankments of railway on loose foundation by Esmaili et al. , and comprehensive parametric study to achieve better perception of seismic behavior of micro piles in slopes by Elahi et al.  have been carried out to get better understanding of this subject.