Analytical study on soil behavior and its countermeasures using a pipe jacking method in deep strata

In Japan, when urban infrastructures need to be constructed, the difficulty of utilizing the ground or shallow strata will necessarily lead to the more frequent use of the deep strata. The common construction methods are open-cut, pipe jacking, and shield methods. In recent years a new pipe jacking method has been established that can be adapted to 20 m or more below the ground. Using this method, the drivage machine and the jacking pipe continue to move underground until the completion of the driving. Therefore an over-cutting area (so-called tail-void) must be formed to lower the friction between the ground and the pipe. The over-cutting area is filled with lubricants. However, because the stress release of the ground continues to advance when the over-cutting area is formed, there are some challenges required to cope with the stability of the surrounding ground. In order to utilize the pipe jacking method in the deeper strata layers, the theory, analysis and installation of over-cutting area have to be systemized, and such systematic data must be stored. Therefore, the conditions of the over-cutting area in the deep pipe jacking method are discussed using numerical analyses. Moreover, in the deep strata, surrounding soil is compacted by large earth-covering pressure and its self-support properties are improved. Therefore, strength and Young's modulus are larger and the soil may exhibit mediumhard rock properties. Generally, if medium-hard rock properties are discussed, cracks (weak points) need to be considered as important deformation factors. Therefore, effects of the condition of soil, such as cracks and/or fracture zones on the deformations after jacking, are discussed by a numerical analysis and their problems and tasks are mentioned. As a result, the over-cutting area deformation is large and it is difficult to construct there by the pipe jacking method in the fractured ground. Moreover, as for the measurement for fractured ground, pressure in the over-cutting area and improvement of the surrounding soil was analyzed and the suppressive effect of deformations in the over-cutting area was found.