Use Of Grouting To Actively Control Tunnel Deformation Induced By Adjacent Excavation

For safeguarding metro structures adjacent to excavation,much attention has been paid to passive control measures,such as strengthening support system of excavation and optimization of excavation scheme.The passive control measures have limited effect.In addtion,the passive measures are not timely and active to control tunnel deformation and unable to recover tunnel deformation during the construction process of excavation.Notably,the the passive measures significantly increase the cost and construction period of excavation.Grouting is a method of actively controlling tunnel deformation.Some attentions have been paid to case studies on application of grouting to practical engineering,and system control theory and methodology have not yet formed.Based on in-situ tests,engineering cases and numerical simulations,the theory and methodology of active grouting to simultaneously restore the horizontal and vertical deformation of the tunnel is systematically studied.The main contents are as follows:(1)In-situ tests of soil deformation caused by single-hole grouting,hole-by-hole grouting in a single row of holes and row-by-row grouting in multiple rows of holes were carried out,revealing the mechanism and development of soil deformation caused by grouting.The excess pore water pressure caused by grouting decreases with increasing distance and it can be expressed by a power function.Due to the dissipation of pore water pressure,the soil deformation recovers to a certain extent after grouting,i.e.,the grouting efficiency is less than 1.When multiple rows of holes are grouted row by row,the soil in the first grouting area has a reactive support effect and a shielding effect on the soil deformation on the same side and the opposite side of the subsequent grouting,respectively.Based on the theory of circular hole expansion and the idea of upper and lower limits,a theoretical solution to the horizontal displacement of the soil caused by grouting with multiple rows of holes is proposed when considering the shielding effect and the reactive support effect.This solves the problem that the deformation of the soil on both sides caused by grouting with multiple rows of holes calculated by the existing theoretical solution is consistent.(2)In-situ tests,applications and numerical simulations for grouting to actively control tunnel deformation were conducted.A scheme of using active grouting to replace all or some of passive control measures is put forward to achieve timely and effective control of tunnel deformation.In addition,the horizontal displacement distribution of the tunnel along the longitudinal direction caused by single-hole grouting can be expressed by Gaussian curves.The tunnel ring close to the grouting hole corresponds to a high grouting efficiency.As the grouting distance increases,soil deformation and grouting efficiency decay rapidly.A single-hole grouting with large volume and continuous grouting along local holes tend to cause excessive local deflection of the tunnel.In order to restore the tunnel deformation uniformly and efficiently,a grouting principle of “close,more grouting holes,small grout volume,interval grouting(within the same row),from far to near(within multiple rows of holes)”is proposed.(3)Comprehensively considering the horizontal and vertical rigid body displacement recovery rate,the ellipticity recovery rate of the tunnel,and the growth rate of the force and deformation of the retaining system when grouting is used to actively control the tunnel deformation caused by excavation,a comprehensive evaluation system of grouting effect is proposed.Based on the engineering case of grouting to restore tunnel deformation,finite element simulation of grouting to control tunnel deformation was carried out.The results show that in order to increase the recovery rate of tunnel ellipticity,the horizontal centerline of the vertical grouting body should be located above that of the tunnel when the tunnel is in the uplift zone and transition zone.Meanwhile,the horizontal centerline of the grouting body should be flush with that of the tunnel when the tunnel is in the settlement zone.In addition,when the tunnel is in the settlement zone,a scheme of inclined grouting at the side and below the tunnel(or multi-point grouting below the tunnel)combined with vertical grouting is proposed to simultaneously control the horizontal deformation and settlement deformation of the tunnel.When inclined grouting is used,it is necessary to inject the grout into a rubber bag.When the tunnel is in the uplift zone,the combination of vertical grouting and implementing unloading holes on the side(away from excavation)and below the tunnel can simultaneously control the horizontal deformation and uplift deformation of the tunnel.In order to reduce the adverse effect of grouting on the retaining system of the excavation,the combination of cut-off walls and grouting is proposed.The cut-off wall has a shielding effect and a reactive support effect on the grouting,so the cut-off wall can not only improve the beneficial effect of grouting on tunnel deformation,but also reduce the adverse effects of grouting on the retaining system of excavation.The closer the wall is to the grouting position,the better the control effect will be.