1. Introduction
  Voids are normally formed behind the tunnel lining on the crown portion of aged tunnels due to geological reasons. It is strongly recommended that those voids should be filled with a grouting material to improve the stability of the tunnel.
  Conventionally, air mortar or air milk is widely used as a material to grout voids for its low cost. Since the fluidity of such material is very high, however, the material can easily flow into small cracks in the ground or aged linings. Therefore voids can not be sufficiently filled. Also, the grouted material may separate from the base material when water is present inside the void. If a large amount of material is pumped from a yard mixing plant, the pumping pressure may crush the trapped air in the material and require measures to maintain its quality.
  Taking into account such problems with the conventional material, the authors developed a new grouting method called "Aqua Grout Method" which uses a new grouting material.

Table 1 Example of proportion

S.A.A stands for Set Accelerating Agent

Components for 1 m3
Cement	Bentonite	polymer +S.A.A	Water
279 kg	356 kg	9.5 kg	760 kg

Fig.1 Void behind tunnel lining

2. Design of New Material
  The components used for the new material are cement, bentonite, super absorbent-polymer, set accelerating agent and water. For reference, an example of the proportion of the material is shown in Table 1. The proportion may be designed to satisfy the conditions in each case.
  The super absorbent polymer absorbs water, and particles may expand more than 50 times in volume to strongly resist separation in water even under high pressure. Bentonite plays an important role in generating the aspect (Thixotropical) in which the high fluidity is guaranteed under pressure or vibration while viscosity becomes high when the pressure is released. Due to such aspect, the material easily flows through the pumping pipe and steadily stands inside the void once it is released.
  Since all components are prepared as powder, the procedure to manufacture the grouting material is very simpleDJust mix all components with water in a suitable mixer. The area required for the mixing plant can be reduced by more than 30% from that for the conventional grouting material.

3. Experimental Verification of Performance
  Photo 1a shows the material standing on a tableCduring a table flow testC when the material was just released from the cylindrical mold. Photo 1b shows that the material is becoming fluid and expanding on the flow table as the table was dropped cyclically.
  In order to confirm the material's resistance to segregation of water and its performance after being filled into a limited void, the material was pumped into water inside a case similar to the void for the purpose of checking whether the case could be filled without causing material segregation in water. It was determined that water remained pure even when the material was being pumped and the case was being filled. (see photo 2).
  Photo 3 shows a test of filling performance under gravity. The colgate pipe half-cut in the axis direction simulates an actual void with 500mm in height, 800mm in width and 2000mm in length. The material was grouted from the centerD It was observed that the colgate pipe was completely filled with the material without an excessive amount of leakage.

Photo 2 Pumping test in water

Photo 3 Filling ability test

Photo 1a Table flow test (before)

Photo 1b Table flow test (after)

Fig.2 Construction facilities

4. Actual Applications
  The Aqua Grout Method was introduced into backfill works on the crown of actual railroad tunnels in Kyoto prefecture. The facilities of construction consisted of mixers, pump, pressure gages and construction stage as shown in Fig. 2. All the components of the material were poured into the mixers at a plant and pumped into the grout position.
  Although the tunnels were more than 90 years-old and made of bricks with deteriorated joints, the grouted material never leaked out through the joints while it was grouted. Also, the water originally dropping through the joints was still clear even while material was being grouted and it was easily observed that the segregation did not occur in water.
  In terms of quality management, the collected specimens through the construction were tested in a laboratory and all the results met the requirements including the compressive strength of 2N/mm² in 28 days.

5. Conclusions
  The Aqua Grout Method with the new material has been determined to be very effective as a back-fill method for the voids behind the lining of aged tunnels. Furthermore, the authors will continue the research of this method and contribute to rehabilitation of aged tunnels.