Simultaneous backfill grouting, pressure development in construction phase and in the long-term

Abstract

Over the years it has become clear that simultaneous backfill grouting is of major importance to the design of the tunnel lining and TBM process control. Considerable research has been conducted in the last 20 years to get a grip on these two aspects. Good grouting is the key to successful settlement control, reducing the differential displacement between segments and rings, the moments in the lining (in both ring- and longitudinal direction), and for optimization of the tunnel boring process. Simultaneous backfill grouting was carried out in shield tunnelling for the first time in 1982, in the construction of No. 4 line of the Osaka Subway in Japan. The conventional method using mortar grouting had been expected to cause a ground settlement of 50-100 mm when tunnelling through very sensitive soft clay. The use of simultaneous backfill grouting kept the settlement in a 10-30 mm range (Hirata, 1989). Since then, this method has been introduced in many regions of the world, such as Asia, Europe and America, reducing the settlement associated with shield tunnelling. The next step was the optimization of the material properties of the grout. Extensive research resulted in the two-component grout TAC (in Europe: ETAC) which gave: (1) A more efficient tunnel boring process because there is no clogging of the grout in the injection system. (2) Faster and uniform support of the tunnel lining. (3) Settlements in the range of 0-15 mm, through the better control of the grout injection. In the paper the two-component injection system is explained and a typical example for the composition and the behaviour of the grout is given. A figure with the conceptual grout pressure change versus time is presented for different soil types and different grouting pressures. Grouting pressure due to simultaneous backfill grouting starts acting on the circumference of the lining immediately after the passage of the shield tail. The grouting pressure distribution becomes uniform shortly after the grouting because the grout is in a plastic state. With hardening, the grout holds the earth pressure and the water pressure in the ground and conveys them to the tunnel lining. After hardening of the grout, the lining pressure changes depending on the compression of the grout, the deformation of the lining, stress relaxation, and so on, and then reaches a steady value. The magnitude of the pressure change is depends on the ground conditions, e.g. hard or soft soil, and also on the magnitude of injection pressure. In the case of soft soil, the lining pressure approaches the initial stress with time, regardless of the injection pressure. In the case of hard soil, the lining pressure approaches the active earth pressure. A case history, with the development of the grouting pressure from the construction phase until 6 months later is presented and discussed. An overview is given, with field measurements, of about 35 cases with grout pressures at two days after tail passage. These cases have been grouped as: soft clay soil, stiff clay soil and sand/sandy gravel. Finally, from measured grout pressure distributions for three cases the resulting bending moments in the lining have been calculated. A back analysis was performed based on the conventional Japanese design method. The conclusion of this back analysis was that the lining seems to be over designed using the conventional Japanese design method. It was concluded that, it is necessary to improve and develop a more rational design method for the future. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.