A grade crossing of Changda Railway would rebuild to a grade separation. In this project, foundation pit of frame-shaped culvert has a length of 10m. According to geotechnical investigation and surveying report, this area is quaternary alluvium. The type of slope soil belongs to cohesive soil and sandy soil. Depth of groundwater level is 3.5m. At the south of the foundation pit, there is a large telegraph pole iron frame. A row of digging piles has been put at the edge of foundation pit. The diameter of digging piles is 1m, distance between them is 2m.
The type of this foundation pit support is temporary. In later stage of building project, a retaining wall can be built beside slope. On the premise of safety and usability, the cost of foundation pit support engineering should be decreased as much as possible. So self-drilling anchor bolt
is applied. At the position that under existing digging piles, ten self-drilling anchor bolts should be drilled. Each of the self-drilling anchor bolt should has a length of 18m, tensioning force is 165kN. Under these self-drilling anchor bolt, another row of them shoud be drilled, this time, each self-drilling anchor bolt should has a length of 10m with tensioning force of 60kN. The 2 rows of anchor bolts both should be downward with angle of 10.
Support construction should work in with earth excavation. When excavator excavating, it should left layer with depth of 0.2m for roof bolter. Allowable deviation of self-drilling anchor bolt hole distance is 100mm. Before drilling, mesopore and bit should be checked that if they are unimpeded. When bolt is drilled in the designed depth, slurry that is made of pure cement slurry with water-cement ratio of 0.4 should be grouted from mesopore. Plate and anchor head should be installed in time to fix bolt temporarily. When the strength of cement paste reaches 5.0MPa, tensioning force should be the designed value and tension the anchor head.