The construction of longspan rectangular mined tunnel adjacent to rainwater box culvert in waterrich silt and silty clay stratum is always difficult. Hence, the combined technology of horizontal jet grouting pile and fullsection curtain grouting is proposed to consolidate the tunnel and surrounding soil to increase the formation stiffness. The tunnel excavation is divided into 6 parts by using benching method. The small duct grouting is used for advanced support; and the dismantling of temporary supports and construction of secondary lining are carried out in sections. During the construction, the deformations of the tunnel and box culvert are monitored. The results show that soil consolidation and conditioning by using horizontal jet grouting pile and fullsection curtain grouting can make the deformation of box culvert under control, and the deformation of primary support structure is small (6.8 mm) during construction, so as to show the feasibility of the advanced reinforcement and excavation and supporting measures; and also can effectively reduce the risk of mined tunnel construction under similar geological conditions. The results can provide reference for similar projects in the future.