Deep Mixing Method(DMM),an in-situ admixture stabilization technique using cement and/or lime as a binder,has been applied in many construction projects for various improvement purposes(Kitazume,Terashi,2013).DMM was put into practice in Japan in the middle of 1970s to improve soft marine deposits,and was spread into China,South East Asia,and recently to the other part of the world.Two decades of practice has made equipment improved,binders changed,and applications diversified.Lime is replaced with cement in Japan.The special machines used to stabilize soft soil are basically composed of several mixing shafts and blades,and a binder-supplying system.In one operation,a column of treated soil is constructed in a ground.Through a series of construction steps,any arbitrary shape of improved mass(such as block,wall and grid types)can be formed in the ground.For liquefaction mitigation,the block and grid types of DMM

have been applied.

Figure 1 shows an application at Kushiro port for liquefaction mitigation of backfill,where sandy ground was stabilized by a block type improvement with 100% improvement area ratio.The ratio of the improved soil area to the whole site area.The design field strength qu,was 100 kPa,which was considerably low level compared with that for clay soil improvement.The revetment was subjected to the 1994 Toho-Oki earthquake,which caused huge damages with liquefaction and cracks at unimproved area.However,due to the ground improvement,negligible damage took place at the revetment,which has confirmed the high applicability of DMM for earthquake disaster mitigation(Yamazaki,2000).

DMM was applied to foundation of a building at the Kobe Port,where sandy ground was improved by a grid type improvement to prevent excessive pore water pressure generation during earthquakes.A 14-story building

Figure 1.Application of DMM at Kushiro port(Yamazaki,2000)

located on Meriken Wharf in Kobe was experienced the 1995 Hyogoken-Nambu earthquake.The soil profile at the site consisted of 10～12 m of soft reclaimed sand and gravel layers over the seabed.The seabed soil consisted of alternating layers of clay,sand and gravel.The building was supported by cast-in-place reinforced concrete piles with a diameter of 2.5 m extending to dense diluvial sand and gravel at a depth of 33 m.Its section and plan diagrams are shown in Figure 2.A grid type improvement was applied to prevent liquefaction in the upper loose fill.The improvement area ratio was approximately 20%.The unconfined compressive strength of the improved soil after about six weeks curing was 4～6 MPa(Tokimatsu et al,1996; Suzuki et al,1996; Namikawa et al,2007).

Figure 2.Application of DMM at Kobe port

Figure3a shows the damage of quay wall near the building after the earthquake.The concrete caisson type quay walls on the west,south and east displaced horizontally by 1 m,2 m,and 0.6 m respectively and settled by 0.5 m,0.6 m and 0.3 m.Sand boils and ground fissures were observed at the ground surface.In the building,however,there was no crack at the surface of the improved ground as shown in Figure 3b.The head of the cast-in-place piles supporting the building was found to be intact.Moreover,negligible differential settlement was observed on the first floor of the building.These have indicated that the cement stabilization improvement could mitigate the damage to pile foundation and superstructure.

Figure 3.Damage due to the earthquake

Yodo River flows from Lake Biwa to Osaka Bay through Osaka City.Due to the Hyogoken-Nambu Earthquake in January 1995,the river dike was heavily damaged for the length of 1.8 km because of slope failure due to ground liquefaction(Kamon,1996).A representative cross section of the damaged dike is shown in Figure 4a.The top portion of the river dike sank down about 3 m.The damaged dike had to be restored very quickly,because there was a risk of flooding during the rainy season which usually commenced in June.The ground condition at the site is shown in Figure 4b.The ground consisted of a sandy layer and a clayey layer.As the SPT N-value of the sandy layer was smaller than 10,the liquefaction might take place again in earthquake attack in the future.Because there were many residential houses in the neighborhood along the river dike,it was necessary to avoid noise and vibratory problems during the construction.This was one of the reasons why the deep mixing method was applied.The cross section of the improved ground is shown in Figure 4c,where grid type improvement

Figure 4.Application of grid type deep mixing at the Yodo River dike

was applied to prevent the liquefaction of the ground and to improve the stability of river embankment.The grid of the stabilized soil columns was about 5 m by 5.4 m.The design strength of the stabilized column,q_u was 500 kPa.Assuming the strength ratio of the field stabilized soil and laboratory stabilized soil,q_uf/q_ul was assumed to be 0.25 in the design,and 90 or 100 kg/m³ of blast furnace slag cement type B were mixed to achieve the design strength for the sandy layer and clay layer respectively.

Soon after the aftermath of the 2011 Tohoku earthquake and tsunami,the Cement Deep Mixing(CDM)Association,the Dry Jet Mixing(DJM)Association and Chemical Grouting Co.Ltd.conducted field surveys in the Tohoku and Kanto areas to investigate any deformation and damage in the deep mixing improved ground and the performance of the improved grounds.Though few slight deformations were found in some grounds,as a whole no severe deformation and damage was found in the improved ground and superstructures even they were subjected to quite large seismic force(Kitazume,2012; Tokunaga et al,2015).Here,two applications in the field surveys are briefly introduced.

1.5.1 Road embankment

The road embankment at Soga,Chiba Prefecture,was improved by the grid type deep mixing method for liquefaction prevention.The original ground beneath the embankment contains large amount of fine sand to the depth of -7 m,which was anticipated to be highly liquefied due to earthquake motion.The improved ground has about 5.6 m in width,6.0 m in height,and the improvement area ratio is 50%,and whose unconfined compressive strength,q_u is 200 kN/m²(Figure 5a).No damage was found in the embankment and the improved ground is shown in Figure 5b,even subjected to the seismic force of 5.0 upper in Japanese Magnitude-Shindo(seismic intensity scale).As contrast,Figure 5c shows heavy damage on road without improvement located in the neighborhood due to liquefaction.

Figure 5.CDM improved ground at Soga,Chiba Prefecture

1.5.2 River embankment

The foundation for the river embankment in Chiba Prefecture was improved by the grid type DM method as shown in Figure 6a,where the width and height of the improved ground were 21.0 m and 21.0 m respectively The improvement area ratio and the design

Figure 6.DJM improved ground for the river embankment in Chiba Prefecture

strength were 50.6% and the design unconfined compressive strength,q_uck of 600 kN/m² respectively.No damage was found in the embankment and the improved ground,as shown in Figure 6b.