Stability of tunnels adjacent to dynamic compaction operation using three-dimensional modeling

Dynamic Compaction is one of the best methods of deep soil improvement. Dynamic compaction operations generate large vibrations; Therefore it should be designed and implemented in a way that does not damage adjacent underground spaces. In this research, for a tunnel with constant diameter, four different locating depths and three different impact energies are modeled numerically using finite element code ABAQUS. In this way six impact distances from tunnel axis are considered. In order to determine safe distances of dynamic compaction from tunnel axis, peak particle velocity (PPV) values in tunnel which is defined by reliable standards are considered. Proposed method consists of repeated dropping of a heavy weight tamper in a predetermined pattern on the weak ground that is going to be compacted. Ground vibrations caused by dynamic compaction can damage adjacent underground spaces such as tunnels and buried structures. At First, by numerical modeling of dynamic compaction, critical zone of the tunnel was determined then variations of maximum PPV location for different depths of tunnel and different impact energies were indicated. Finally safe impact distances from tunnel axis for different depths of tunnel with different impact energies in critical zone were determined by allowable PPV that defined by reliable German, British and Swiss standards.
Methodology and Approaches
Dynamic compaction has been modeled by many researches with different numerical methods such as finite element. In this research, ABAQUS finite element code in three dimensional space was applied in three steps. In the first step, gravity analysis was performed to exert initial stresses. In the second step, excavation of the tunnel and installation of lining were performed. And in third step, Dynamic implicit analysis for drop of tamper on soil in ten number of blows with time duration 60s for each impact were considered. This research shows that the critical zone of tunnel is located at the first quarter of tunnel in side of dynamic compaction. Increase in depth of the tunnel, safe impact distances from tunnel axis were reduced. By considering constant impact energy and impact distance by, maximum PPV location at the end of critical zone will move up by increase in depth of the tunnel. Also, assuming constant depth of the tunnel and impact distance, maximum PPV point location was fixed with decrease in impact energy.