Effective grouting materials for tunneling through unconsolidated ground

Abstract

Generally, urban tunnels are exposed to severe excavation conditions e.g. in unconsolidated sandy layers, with a thin earth cover, right under existing structures and near underground structures. However, further cost reduction and handling of diverse cross-sections have been recently required, and accordingly NATM has been introduced under such conditions. The fact that NATM has been adopted under such conditions owes greatly to the development of auxiliary methods. In particular, forepoling method (this method involves the driving of pipes and the injecting of grouting materials into the ground ahead of face prior to excavation) has been frequently used to stabilize the ground around cutting face and to control the settlement of ground surface. However, through the applying the forepoling method to actual ground, it was found that grouting materials are not always injected radially and uniformly. In many cases of actual injection, the grouting material was only injected within the inside of the steel pipes or runs away, and, as a result, the ground was not improved sufficiently. It is considered that this occurred for two reasons; one is because the adopted material was not appropriate the ground to be improved, and the other is because ground conditions changed as a tunnel proceeded. Consequently, it was necessary to prepare a few types of grouting material to excavate ground safely, but it takes much time to exchange grouting materials. Moreover, all grouting materials available have at least one major disadvantage, for example lack of penetration, the shortage of strength or adverse environmental effects. In order to solve these problems, many mixing tests and in situ injection tests have been performed, resulting in the development of a new grouting method. In this method, two types of grouting materials, one is Solution-type grout suitable for sandy ground and the other is Suspension-type grout appropriate for clay or fissured ground, are available and they can be exchanged in a short time according to the change of ground condition. Furthermore, both materials have the high penetrability and the early development of strength. In this paper, the method is outlined. Then in situ and laboratory injection tests were conducted to verify the penetrability of the two types of grouting materials used in the new method, the conditions of completed columns of solidified soil, and to compare the materials with conventional grouting materials. Finally, it is shown that the adoption of the new grouting method to an actual construction site was successful. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.