The 2011 earthquake off the Pacific coast of Tohoku was a magnitude 9.0 Mw (Category 9.0 on the Richter scale) undersea megathrust earthquake. The March 11 event was the most powerful known earthquake ever to have hit Japan, and one of the five most powerful earthquakes in the world since modern record-keeping began in 1900. The Tohoku event resulted in a major tsunami that brought destruction along the Pacific coastline of Japan and resulted in the loss of thousands of lives and devastated entire towns. The degree and extent of damage caused by the earthquake and resulting tsunami were enormous, with most of the damage being caused by the tsunami. The aftermath of the 2011 Tohoku earthquake and tsunami included both a humanitarian crisis and massive economic impacts. The tsunami created over 300,000 refugees in the Tohoku region. After the earthquake, three organiza- Soil Mixing Performance in the 2011 Tohoku Earthquake The survey included a total of 978 DM projects, 855 wet-type DM projects and 123 dry-type DM projects. The soil improvement purposes of the sites are quite varied. Some were for improving slope stability, others sought to reduce ground settlement or to mitigate liquefaction. The shapes of the improved sites also varied, depending on the ground conditions and the improvement purposes. Some had block shapes, others had wall or (23 ft) wide and 8.9 m (29 ft) high, and where the design strength, q , and the uck improvement area ratio, as, are 1.0 MPa (145 psi) and 97% for the upper part, 0.6 MPa (87 psi) and 58% for the lower part, as shown in Figure 1. The ground improvement was carried out in 2005 using an on-land DM machine installed on the small verge. No damage was found in the embankment and the improved ground, even those subjected to the large ground motion of the seismic force of 5.0 upper in Japanese Magnitude-Shindo (seismic intensity scale) as shown in Figure 2. River Embankment at the Tone River Figure 1. Naka River Seawall cross section tions — Cement Deep Mixing Association, Dry Jet Mixing Association and Chemical Grouting Co. Ltd — conducted field surveys in the Tohoku and Kanto regions to investigate the performance of 800 sites with previously improved ground using Deep Mixing (DM) Method. The results revealed no serious deformation and damage in the improved grounds and superstructures even though they were subjected to quite large seismic forces. The Deep Mixing (DM) Method, in-situ soil admixture stabilization method using lime or cement as binder, was developed in the 1970s in Japan. There are two types: the Cement Deep Mixing (CDM) Method in which slurry form binder is used, and the Dry Jet Mixing (DJM) Method in which dry powdered binder is used. Due to various advantages such as rapid improvement, negligible adverse influence of noise and vibration during execution, both methods have been frequently applied to land and marine constructions for various improvement purposes in Japan. grid shapes. Among them, a total of 834 DM improved sites were surveyed in-situ after the earthquake: 29 (Aomori Prefecture), 16 (Iwate Prefecture), 21 (Akita Prefecture), 21 (Yamagata Prefecture), 47 (Miyagi Prefecture), 6 (Fukushima Prefecture), 83 (Ibaragi Prefecture), 96 (Chiba Prefecture), 70 (Saitama Prefecture), 310 (Tokyo Prefecture) and 135 (Kanagawa prefec- ture). Following are two examples of the results of the survey. Seawall at the Naka River Part of the seawall at the Naka River, Tokyo Prefecture, was improved by the DM Method. The sheet-pile wall installed at the front of the seawall was improved by DM to increase the horizontal resistance and the embankment stability. Jet grouting was also applied between the pipe and the DM ground to increase the lateral resistance. The DM improved ground was about 7.0 m AUTHOR: Masaki Kitazume Tokyo Institute of Technology DEEP FOUNDATIONS • JULY/AUG 2012 • 77 A part of the river embankment at the Tone River, Ibaragi Prefecture, was improved by the block type of soil mixing. The original ground beneath the embankment consisted of stratified layers to depth of -30 m (-98 ft), and some of them were sand layers with fine particles. The ground was anticipated to be liquefied during (a) (b) Figure 2. Comparison of DM (a) improved ground and (b) unimproved ground at the Naka River
