[H-RE12] New Developments in Engineering Geology

[HRE12-P05] Maximum temperature of muddy stone causes heaving in tunnel

*Yu-ki Sekiyama¹, Kazuma Mochizuki², Takafumi Imai², Hikaru Ueno², Yuzuru Yamamoto¹ (1.Kobe University, 2.Japan Railway Construction, Transport and Technology Agency)

Keywords:heaving, Vitrinite reflectance, Rock-Eval, smectite

Convergence (heaving) of road and railway tunnels that have a large impact on the economy and safety, frequently occur after construction. This phenomena are also problematic in the point of engineering point of view since additional constructions are necessary to carry out. Two-types of heaving are classified based on the possible mechanisms: one is swelling caused by smectite-swelling pressure, the other one is squeezing occurred by extrusion due to plasticization of ground and collapsing the tunnel spaces . However, the background mechanism which is clue to construct prediction has not been clearly understood. Especially, the swelling type of the heaving shows progressive strain/deformation of tunnel, which makes more difficult to detect immediately after construction and take countermeasures against it . Swelling-type heaving is caused by smectite in rocks. They are drying during the construction and being wet and swell up postdated. Therefore, quantitative measurement of the smectite content by XRD has been performed as one of the possible risk assessments. This study focused on the diagenetic property of clay minerals. That is, transition of smectite to illite, a non-swelling clay mineral occurred in the early phase of diagenesis and is completed ~150°C. Here, we measured paleo-maximum temperature using the Rock-Eval T_max (°C) and vitrinite reflectance R_O (%) and compared the result to the smectite contents. The samples were taken from three areas where Shinkansen-system has been constructed: (1) A Tunnel (Hokkaido); (2) Kaga Tunnel / Hiyama Tunnel (Hokuriku); and (3) Hiyama Tunnel (Kyushu). As the results, we obtained the following temperatures: (1) T_max = 412-439 °C, R_O = 0.43-0.59 %; (2) T_max = 421-441 °C, R_O = 0.43-0.59 %; (3) T_max = 444-450 °C, R_O = 0.69-0.87 %. Based on our data base, the T_max (°C) and R_O (%) were converted to the maximum experienced temperature T (°C): (1) T = 62-135 °C; (2) T = 93-139 °C, and (3) T = 140-161 °C. Based on the temperature distribution along the tunnels, several high-temperature intervals were identified. The followings are comparison of the T (°C) with the smectite content in such intervals, (1) T = 133-135 °C and the smectite content is 10 %; (2) T = 133-139 °C and smectite content 2 %; (3) T = 140-161 °C and smectite content 0 %. These results are consistent with the assumption of diagenesis: illite contents in the smectite-illite mixed layer reached 80 % at the temperature of 140 °C (Inoue,1986). In the intervals characterized by the lower temperature intervals, the comparisons are the followings: (1) T = 62-124 °C, smectite content is widely distributed 0-50 %; (2) T = 93-124 °C, smectite content is relatively high 13-22 %. This study confirmed that illitization occurred, which reduced significant amount of smectite at the interval experienced high temperature (>133°C). On the other hand, certain amount of smectite has been remaining at the intervals where represent low-maximum temperature (<124°C). Therefore, it is possible to evaluate the risk of swelling-type heaving based on the maximum temperature measurement. That is, low risk where experienced high temperature, while a risk area where experienced low temperature and is characterized by high smectite content.