Influence of different cutter spacings on rock fragmentation efficiency of Beishan granite by TBM

In tunnel construction by tunnel boring machine (TBM), the excavation efficiency is highly related to cutter spacing. Due to the inhomogeneous, discontinuous and anisotropic characteristics in rocks, there are limitations to study rock fragmentation process by using numerical simulation. The purpose of in-situ penetration tests is to optimize the operational parameters for a specific TBM, thus this kind of tests cannot consider the effect of different cutter spacings on rock breaking process. The full-scale cutting experiment can minimize the size effect by using a large rock sample and the cutter spacing is able to be adjusted during the experiment, therefore the full-scale cutting experiment has been widely used in recent years. In order to study the influence of different cutter spacings on excavation efficiency by TBM cutter, the mechanical rock fragmentation experimental platform of Beijing University of Technology is used to conduct a series of rock cutting tests with five sets of cutter spacings. In these tests, the diameter of the disc cutter with a constant cross section is 432 mm. The size of test granite rock sample is 1000 mm×1000 mm×600 mm, taken from Beishan, Gansu Province of China. During the experiments, the three dimensional forces acting on the rolling cutter are recorded, and the rock chips collected after each cutting pass are weighed respectively. The impact of cutter spacing on excavation efficiency is analyzed by use of the obtained average normal force, average rolling force and specific energy. The results show that when penetration is small, the cutter spacing has no obvious effect on the average normal force and rolling force. However, with the increasing penetration, the cutter spacing has significant impact on both the average normal force and the rolling force. For all of the cutter spacings arranged in the tests, the increase in penetration will produce more rock chips but not always improve the efficiency of rock fragmentation at the same time. For Beishan intact granite, when the ratio of spacing to penetration is around 30, the specific energy reaches the minimum value, which means that the highest rock fragmentation efficiency can be expected under this condition.