INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Dynamic centrifuge model test for performance-based design of grid-form deep mixing walls supporting a tall building Junji Hamada & Tsuyoshi Honda Takenaka Research & Development Institute, Takenaka Corporation, Inzai, Chiba, Japan ABSTRACT Dynamic centrifuge model tests in the 50 g field were conducted to investigate a failure behavior of DMWs in liquefiable sand during large earthquakes. A miniature model of DMWs was made of soil-cement with an unconfined compressive strength of about 4,000 kPa in order to investigate the behavior/toughness of the DMWs after yield and failure on seismic performance. The DMWs model that support the superstructure’s weight of 206 kPa, the natural period of 0.6 second were set in a laminar shear box, and repeatedly tested by increasing the acceleration level of the input motion recorded at TAFT earthquakes. The relationship between shear stress and shear strain of the DMWs, namely the nonlinearity of the DMWs was investigated. No significant settlement of the structure was observed even if the normal and shear stresses in the DMWs were assumed to have locally reached the tensile or shear criteria of soil-cement. 1 INTRODUCTION potential for liquefaction mitigation. However, the local failures of the DMWs are not acceptable in the existing Grid-form DMWs (Deep cement Mixing Walls) which is method of allowable stress design. If a local failure can be one of the ground improvement technique are recently appropriately taken into account, the DMWs can be employed as a new method to increase bearing capacity designed more rationally, using a performance-based of foundations in soft grounds as well as a seismic design method. So far, a lot of shaking table tests countermeasure against seismic liquefaction. In the on cement mixing walls have been carried out by using method, the high-modulus soil-cement walls confine the centrifuges (e.g., Babasaki et al., 1992). Most of them loose sand enclosed by the DMWs so as not to cause have been used acrylic resin models, the shear modulus excessive shear deformation during an earthquake. The of which is equivalent to that of real soil-cement in order method was developed in the late 1980s, and has to investigate an effect of preventing liquefaction during demonstrated that the grid-form DMWs effectively earthquakes. Several researchers recently conducted prevented liquefaction and liquefaction-induced damage centrifuge model tests on grid-form DMWs using real soil- in the 1995 Hyogoken-Nambu earthquake. Namely, cement models (Tamura et al., 2015; Khosravi et al., during the earthquake, quay walls were heavily damaged 2016; Honda et al., 2015). Khosravi et al. (2016) used the due to liquefaction-induced lateral flow, however, a 14- soil-cement with an unconfined compressive strength of story building with a foundation of cast-in-place concrete 450 to 770 kPa and investigated the effect of the area piles and the DMWs that was surrounded by the quay replacement ratio and improved depth on the global walls survived and no damage was observed in the pile responses of the soft clay ground. Unfortunately, the foundation (Tokimatsu et al., 1996). In Japan, the grid- above tests were conducted without setting form DMWs have been widely used in a lot of buildings superstructure models on the DMWs. Accordingly, they since 1995. The DMWs have been also applied to support were not able to reach a conclusion about how the inertial buildings directly, and are capable of restricting the force from superstructure affected the performance of the settlement of buildings during or after earthquakes to an DMWs. acceptable level. Recently piled raft foundations This paper describes centrifuge model tests in 50 g combined with the grid-form DMWs have been applied to field in order to investigate failure behavior of DMWs the real buildings. The field measurements on long-term subjected to dynamic loading from superstructure of a tall and seismic behaviors of the buildings supported by piled building and ground oscillation during large earthquake. In rafts with the DMWs were also performed to confirm the this study, the miniature model of DMWs was made of validity of their foundation designs (Yamashita et al., soil-cement in order to investigate the behavior/ 2012). Furthermore, Tanikawa et al. (2015) proposed a toughness of the DMWs after yield and failure during mechanical joint adding dents and bumps at a top surface large earthquakes. The relationship between shear stress of the DMWs in order to ensure transmitting large lateral and shear strain of the DMWs, namely the nonlinearity of forces such as unsymmetrical earth pressure or seismic the DMWs by gradually increasing the input motion step inertia force of superstructure into the DMWs. by step, was also investigated. Before these tests, Some researchers conducted numerical analyses comparative tests between acryl grid-form DMWs model using elasto-plastic models to investigate seismic and soil cement model with low-rise building (rigid weight) behaviors of grid-form DMWs (Namikawa et al., 2007; were conducted and described in reference (Hamada and Shigeno et al., 2017), and concluded that the local failures Honda, 2017). of the DMWs were not the causes of the reduction in the Laminar shear box 1000 AH62~65 AH8 50 170 150 DH1 DH2 300 260 AV4 AV3 AV1 AH64 AV2 AH4 AH1 AH2 AH3 AH65 AH7 370 20mm 260mm AH52~55 AH0 260mm (a) Top View AH45 Accelerometer（V, H） AH35 200mm Piezometer AH34 AH55 AH44 AH63 Displacement transducer（V, H） AH33 AH54 AH62 AH43 DV2 DV1 AH32 AH53 AH42 AV4 AH7,8 AV3 AH52 Laminar shear box DH2 AH6 WL=GL-20mm DMWs (Soil cement) AH16 AH26 AH45 50 AH35 AH5 W5 AH15 AH25 AH44 W25 W15 AH24 50 AH34 AH4 W4 AH14 200 AH43 W14 W24 AH23 50 AH33 AH3 W3 AH13 Dr76% AH42 W13 W23 AH22 AH32 AH2 W2 AH12 50 W12 W22 80 Dr95% AV1 DH1 AV2 AH1 W1 W11 W21 AH0 －direction +direction (Unit:mm) Shaking direction (b) Side View (c) DMWs Model and attached gauges Figure 1. Schematic view of the DMWs and high-rise building model with ground. 2 CENTRIFUGE MODEL TESTS steel structure model to increase a friction between the structure model and the DMWs. Centrifuge model tests were performed at an acceleration Silica sand No.6 produced in Iide, Yamagata of 50 g using the 7 m radius centrifuge at Takenaka R. & Prefecture, Japan was used as the soil for all test cases. D. Institute. The scaling ratio is 1/50. The superstructure The physical properties of the Iide sand are following; 50 was modeled to be a tall building, the natural period of 0.6 percent diameter, D50 of 0.28 mm, uniformity coefficient, sec in prototype scale. The shaking table tests were Uc of 1.9, specific gravity of soil particles of 2.657, conducted with liquefiable sand deposit. minimum density of 1.452 g/cm3, maximum density of 1.744 g/cm3. Relative density of the saturated model 2.1 Setting up Model Ground, DMWs and Structures ground is 76 %. Model improved soil-cement walls were also made from Iide sand, kaolin clay and blast-furnace Figure 1 shows schematic view of the tests. The DMWs slag cement type B, W/C=60%. Unconfined compressive models made of soil-cement was set in a laminar shear strength of the soil cement is from 1500 to 2000 kPa in box, and a lot of accelerometers, piezometers and curing time of 7 days and about 4000 kPa in curing time displacement transducers were set on the DMWs models, of 28 days. grounds and structures. The inner dimensions of the The DMWs model was produced according to laminar shear box are 1000 mm long, 300 mm width, 350 following steps. Step1: the soil-cement that has not set mm height. A 2 mm-thick membrane was attached inside was poured in the mold made of panel boards. Step 2: the box to make it watertight. Size of the DMWs model is after a week, the boards were removed. Step3: the DMWs 20 mm thick of soil-cement wall (1.0 m thick in prototype model was cured a month under wet condition. scale) and spaced 240 mm center-to-center apart (12 m in prototype). Sand papers were attached beneath the Table 1. Peak accelerations and residual settlements Residual Peak Acceleration (cm/s2) settlement (mm) Step Ground DMWs Structure number Base Structure inner DMWs Free field Transverse Longitudinal Substructure Superstructure AH1 AH5 AH25 AH35, AH45 AH55, AH65 AH6 ave.(AH7,AH8) DV1 DV2 1 51 67 73 70, 67 63, 61 68 160 1.9 1.9 2 163 162 185 168, 162 137, 131 153 313 16.7 (18.6) 18.1 (20.0) 3 197 210 386 215, 212 165, 156 186 391 10.1 (28.7) 10.2 (30.3) 4 347 334 418 317, 322 195, 213 215 501 35.5 (64.2) 34.1 (64.4) 5 477 511 759 412, 448 239, 235 238 519 37.5 (101.7) 36.2 (100.6) ) 1 2 AH1(Base) AH5(inner DMWs) AH25(Free field) 0.5 0 -0.5 -1 Acceleration (m/s Acceleration 2 4 6 8 10 12 14 16 18 20 Time (s) ) 1 2 AH5(inner DMWs) AH6(Substructure) AH55(Long.) AH45(Trans.) 0.5 0 -0.5 -1 Acceleration (m/s Acceleration 2 4 6 8 10 12 14 16 18 20 Time (s) ) 2 2 1.5 AH6(Substructure) AH7(Superstructure) 1 0.5 0 -0.5 -1 -1.5 -2 Acceleration (m/s Acceleration 2 4 6 8 10 12 14 16 18 20 Time (s) Figure 2 Time histories of accelerations (Step 1, input acc.