Tertiary Creep Reproduction in Back-Pressure-Controlled Test to Understand the Mechanism and Final Failure Time of Rainfall–Induced Landslides Atitkagna DOK*, Hiroshi FUKUOKA, Takeshi KATSUMI* and Toru INUI* *Graduate School of Global Environmental Studies, Kyoto University Synopsis Fukuzono (1985) developed a method for predicting final failure time of a slope based on the findings from large scale flume test series that log of acceleration and velocity of surface displacement immediately before failure are proportional, d 2 x/dt 2 =A(dx/dt) α . He proposed a simple method for predicting failure time by plotting the inverse velocity of surface displacement (1/v). Yet, its mechanism is still unknown. To investigate the Tertiary creep mechanism in soils, a series of back-pressure controlled test on saturated sands were undertaken in ring shear apparatus. The tests were conducted under particular normal stress and shear stress with pore-water pressure changes to simulate the potential sliding surface condition in heavy rainfall. Sand and its mixture with clay material were used for specimen. Consequently, these tests could reproduce Tertiary creep to failure, in which similar log v-log α relationship and α value range was found. While, linear relationship of α and A values was found. Keywords: Tertiary Creep, ring shear apparatus, back-pressure-controlled test, rainfall– induced landslides. 1. Introduction Landslides are complex geo-disasters frequently triggered by earthquake and/or intense heavy rainfall or other related natural/ anthropogenic impacts. Such catastrophic disasters have not only claimed residents’ lives, but also resulted in property damages and other socio-economic consequences, which significantly interrupts the development of the communities and nations. Since the social resources for preventing those threatening potential landslides is limited in every country, the best solution recently met is safe evacuation immediately before the final catastrophic failure of the landslide. To realize an effective evacuation, reliable prediction methodology must be established. Accordingly, in landslide fields, failure-time prediction methods of landslide have been widely developed by many researchers including Saito and Uezawa (1960, 1966) as an initiation, Kawamura (1984), Fukuzono (1978, 1982, 1985), Voight (1988, 1989), Azimi (1988), Hayashi, et al. (1980). However, only two methods developed by Saito and Uezawa (1960, 1966) and Fukuzono (1985) were accepted world-wide. Based on Tertiary creep deformation theory through the findings getting from large scale flume tests for landslide studies, Fukuzono (1985) found logarithm of acceleration is proportional to the logarithm of velocity of surface displacement immediately before the failure, expressed as d 2 x/dt 2 =A(dx/dt) α , where x is surface displacement, t is time, and A and α are constant. Besides, he proposed a simple method for predicting the time of failure by the inverse velocity (1/v) mean. The curve of inverse velocity is 京都大学防災研究所年報 第 54 号 B 平成 23 年 6 月 Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 54 B, 2011 ― 263 ―