JOINT CONFERENCE PROCEEDINGS 9th International Conference on Urban Earthquake Engineering/ 4th Asia Conference on Earthquake Engineering March 6-8, 2012, Tokyo Institute of Technology, Tokyo, Japan
PILED RAFT FOUNDATION WITH GRID-FORM GROUND IMPROVEMENT SUBJECTED TO THE 2011 EARTHQUAKE Akihiko Uchida1), Takeshi Yamada2), Nobuyuki Odajima3), and Kiyoshi Yamashita4) 1) Chief Researcher, R & D Institute of Takenaka Corporation, Dr. Eng. 2) Chief Researcher, R & D Institute of Takenaka Corporation 3) Chief, Tokyo Head office, Takenaka Corporation 4) R & D Institute of Takenaka Corporation, Dr. Eng.
[email protected],
[email protected],
[email protected],
[email protected]
Abstract: The grid-form ground improvement by the deep cement mixing method is one of the countermeasure methods against soil liquefaction. The improvements brought by this method were confirmed during the 1995 Hyogoken-Nambu Earthquake. The number of building foundations adopting this method has increased in the last years. During the 2011 Tohoku Pacific Earthquake, which occurred on March 11, serious liquefaction damages were observed in the reclaimed lands of the Tokyo Bay area. Since then, liquefaction countermeasures have been recognized to be of importance for reclaimed lands. In this paper, the building foundation improved by the grid-form ground improvement method is introduced. The field investigation results of the ground around the building after the 2011 earthquake are presented. The effect of the liquefaction prevention of the grid-form ground improvement ground is discussed based on the dynamic nonlinear analysis for foundation soil layers with an actual acceleration record. Comparing the field investigation result with the simulation analysis, it is concluded that the grid-form ground improvement prevented the liquefaction of the original soil surrounded by the grid-form walls during the 2011 earthquake.
1． INTRODUCTION The grid-form improvement, shown in Figure 1, for improving loose grounds using the deep cement mixing method is one of the countermeasure methods against soil liquefaction. This method prevents liquefaction of unimproved sand deposits encased within walls of grid by reducing shear deformation of these deposits during earthquakes. Photo 1 shows a soil cement mixing machine. Figure 2 shows a typical procedure for constructing cement mixing walls. The grid-form improved parts are the result of mixing the original soil with cement slurry in overlapping columns, each one with a diameter of 1 m. The normal compressive strength of the soil cement is typically from 1 to 2 N/mm2. The performance of such improved grounds during the 1995 Hyogo-ken Nambu Earthquake was already reported by Suzuki et al. (1996) and Tokimatsu et al. (1996). Although severe liquefaction damage was observed on its surrounding ground, the building’s foundation improved by this method experienced no liquefaction damage. The number of buildings in which this method was applied increased in the last years. When Tohoku Pacific Earthquake occurred on March 11, 2011, serious liquefaction damages were observed around the reclaimed lands of the Tokyo Bay area. Since then, liquefaction countermeasures have been recognized to be of importance for reclaimed lands. In this paper, a building foundation improved by the grid-form ground improvement method is introduced. The
Figure 1 Grid-form improved ground
Photo 1 Mixing machine (four axle type)
Penetration
Penetration
Tip treatment
Tip treatment
Withdrawing
Withdrawing
・Discharge of soildified material ・Stirring and mixing
・Discharge of soildified material ・Stirring and mi xing
（a）two axle type machine （b）four axle type machine Figure 2 Construction procedure ①
Mat foundation
⑳
GL-（m） Yu
10 20
Ground improvement
Yl
30 40 Na
50 60 70
Ta
PHC pile
80
Figure 3 Outline of foundation and soil layers N-value
N-value
0 20 40 60
0 20 40 60 0
0
Holocene Yu Sand
Holocene Yl Clay
2． OUTLINE OF BUILDING FOUNDATION AND SOIL PROFILE Figure 3 shows a section of the superstructure and foundation of the building. The superstructure of the building is a four-story parking facility, which was constructed in Urayasu-city, Chiba in 2006. In plan, the superstructure is rectangular with 213 m in length and 71 m in width. The mean dead load of the building on the ground surface is about 45 kN/m2. Landfill and a sand layer are distributed over the ground from the surface to a depth GL-14~16 m as shown in Figures 3 and 4, and a soft clay layer is distributed over the lower part. The bottom depth of the clay layer changes greatly in the site, where the bearing stratum with SPT-N
Fill
F 10
Holocene Yu Sand
10
20
20
30
Na Pleistocene Sand Ta
F
Holocene Yl Clay
30
Depth（m）
Fill
Depth（m）
ground field investigation results around the building after the 2011 earthquake are also presented. The effect of the grid-form ground improvement on preventing liquefaction is discussed based on the dynamic nonlinear analysis of foundation soil layers and recorded data.
40
(a) Boring No.1
40
50
Pleistocene Na Clay 60
Pleistocene Ta Sand
70
80
(a) Boring No.2 Figure 4 Soil profiles and SPT N-values
Fines content. Fc (%)
SPT N-value
細粒分、Fc(%) 0 100
N値 10
0
Safety factor of liquefaction, FL値 FL 0
20
0
0
5
5
5
10
深度(m) Depth (m)
0
Depth (m) 深度(m)
Depth (m) 深度(m)
value over 50 appears at GL-39 m and at GL-72 m at the line 1 and line 20 of the building, respectively. SPT-N values of the landfill and sand layer from the surface to around GL-14 m are around 10. The water table is at GL-1.8 m. Figure 5 indicates the liquefaction potential of the site according to AIJ recommendations for foundations (2001). When the surface acceleration on the site is estimated at 200 cm/s2, the safety factor against liquefaction (FL) becomes less than 1.0 in the depth from GL-1.8 m to GL-12 m. This result suggests that the liquefaction is likely to occur in this site during a large earthquake. Therefore, to prevent liquefaction of the landfill and sand layer on the site, the grid-form ground improvement method was selected to insure a good performance of the piled-raft foundation adopted for the building (Yamashita et al., 2011). To decrease settlement of the superstructure, pre-stressed high-strength concrete (PHC) piles with diameters from 500 mm to 1000 mm and lengths from 33 m to 60 m were used. The length of piles and soil improvement were selected based on data of 13 soil borings on the site. Figure 6 shows the typical 15.6 m×16.5 m grid-form ground improvement adopted for the building. The specifications of the grid-form ground improvement were decided based on AIJ recommendations for ground improvement (2006) and the simplified method proposed by Taya et al. (2008). The unconfined compressive strength of the improved soil assumed for design was 1.8 N/mm2. Ground improvement was performed by using the two-axle type mixing machine. After completion of the ground improvement, improved soil samples were recovered and tested. The unconfined compressive strength of these samples after cured for four weeks was in the range 3.3 9.8N/mm2 (with an average value of 5.8 N/mm2).
10
1
2
10
15
15
15
20
20
20
Figure 5 Liquefaction triggering analysis according to AIJ recommendations (boring No.6. αmax=200 cm/s2)
Photo 2 Sand boils observed in Urayasu city
3． POST-EARTHQUAKE FIELD SURVEY Extensive soil liquefaction was observed in the reclaimed land of Urayasu-city after the 2011 Tohoku Pacific Earthquake (M=9.0), where sand boils were
Photo 3 Building just after earthquake (2011.3.13)
A
No.6 Instrumented pile Settlement gauge Earth pressure cell Piezometer
No.1
A B No.2
A’
Figure 6 Grid-form plan and location of monitoring devices
End of construction ▼
Earthquake ▼
0
Settlement （mm）
confirmed around the site as shown in Photo 2. Photo 3 shows the building condition just after the earthquake (photo taken on March 13, 2011). While small settlement was confirmed in the ground side 3 to 4 m away from the building, no ground damage such as sand boils were observed during the field investigation around the building.
-10 -20 -30 -40 -50 06/03 06/09 07/03 07/09 08/03 08/09 09/03 09/09 10/03 10/09 11/03 11/09 Year/Month
Figure 7 Measured settlement of ground（point A） 4． FOUNDATION EXAMINATION BASED MEASUREMENTS
PERFORMANCE ON LONG-TERM 15 10 5
Contact pressure
Water pressure
0 -5 06/03 06/09 07/03 07/09 08/03 08/09 09/03 09/09 10/03 10/09 11/03 11/09 Year/Month
(a) point A 20
15
Contact pressure
Water pressure
10 5 0 -5 06/03 06/09 07/03 07/09 08/03 08/09 09/03 09/09 10/03 10/09 11/03 11/09 Year/Month
(b) point B Figure 8 Measured contact and water pressures
157
Acceleration (cm/s2)
Pressure （ｋPa）
In order to confirm the performance of the piled raft foundation of the building, long-term measurements of the settlement has been carried out since March 2006 just before constructing the foundation slab. Figure 6 shows the location of instruments for measuring the performance of the foundation. The measured parameters are the settlement of the foundation ground below the raft, axial load of piles, earth pressure and water pressure beneath the raft foundation. The recorded settlement of the foundation ground since the beginning of construction is shown in Figure 7. Before the earthquake, the foundation was in an almost stable state where the settlement of the foundation ground was 16 mm. Two months after the earthquake, the subsidence reached 23 mm. The earthquake induced an increase of settlement of 7 mm. The earth pressure and the water pressure under the raft are shown in Figure 8 where no remarkable change was seen after the earthquake. This result suggests that the raft foundation of the building has been in contacted with the soil surrounded by the grid-form ground improvement. In other words, the superstructure of the building performed in harmony with the foundation ground. Based on the field survey and the settlement records before and after the earthquake, it is thought that liquefaction did not occur on the soil surrounded by the grid-form ground improvement during the earthquake event.
Pressure （ｋPa）
20
0
-157
Figure 9 Time history of recorded acceleration at Urayasu (K-Net) 5． CONFIRMATION OF LIQUEFACTION PREVENTION FOR GRID-FORM GROUND IMPROVEMENT In order to investigate the effect of the liquefaction prevention for the grid-form ground improvement, a simulation analysis was conducted using the time history of the recorded surface acceleration in Urayasu city during the 2011 Tohoku Pacific Earthquake. Figure 9 shows the EW component of this record (K-Net Urayasu station, maximum acceleration of 157 cm/s2). Two-dimensional FEM analysis (Super-FLUSH) was conducted for a model of the grid-form improved soil parts (walls) and the original soil. Figure 10 shows the FEM model along the short side of the foundation ground（section A－A’）. The improved parts, laying in the longitudinal direction of the building, formed the confining wall elements. The shear modulus of the walls laying in the transverse
direction of the building was considered relative to the grid interval of 16.5 m. These walls in both directions were connected at the corner of the grids. In this way, the confining effect of the grid-form ground improvement was represented in the analysis. The soil properties of the original soil layers were based on the PS logging result as shown in Table 1. The soil nonlinearities were determined by laboratory test results. The improved ground properties were linear. The initial shear modulus of the improved soil was considered based on another study. The evaluation of the liquefaction was determined by FL value. It is calculated by the equation (1). The shear stress of soil layers was evaluated from the response analysis and the liquefaction strength was estimated through the N-value based on the AIJ recommendations. The factor of
14.6m
14.6m
14.6m
14.6m
7.6m
un-improved ground ① ↓
Improved wall
40m
15.6m 0.5 0.5
15.6m 0.5 0.5
④original soil surrounded
15.6m 0.5 0.5
15.6m 0.5 0.5
↓0.5
8.6m 0.5 0.5 0.5
40m
G.L. ±0.0m ①
-5.9m
②
-10.7m ③ -14.0m ④ -16.75m
Energy transmission
エ ネ ル ギ ー 伝 達 境 界
エ ネ ⑤ ル ギ -28.9m ー 伝 ⑥ -31.8m 達 境 ⑦ 界 -40.75m ⑧ -45.25m
Energy transmission
Improved 面外壁 wall
⑨ -51.8m ⑩ -54.7m ⑪ -60.85m ⑫ -67.1m ⑬工学的地盤
入力地震動
Figure 10 Finite element model (section A-A’) Table 1 Material properties used for analysis thickness m
Layer
Soil type
1 2 3 4 5 6 7 8 9 10 11 12 13
Fill Sand Sand Clay Clay Clay Clay Sand Sand Clay Sand Clay Sand
5.90 4.80 3.30 2.75 12.15 2.90 8.95 4.50 6.55 2.90 6.15 6.25
14
Improved soil
14.00
water table GL-1.8m boundary Shear wave velocity initial shear modulus poisson's Density Nonlinearity G0 depth Vs ratio Note G/G0 ～γ , h～γ kN/m 3 2 GL-m m/s MN/m 5.90 18.0 140 35 0.489 8-1 improved 10.70 18.5 140 36 0.489 layer 8-2 14.00 18.5 140 36 0.489 16.75 17.0 140 33 0.493 8-3 28.90 15.5 140 30 0.493 31.80 17.0 210 75 0.493 8-4 40.75 16.0 170 46 0.493 8-5 42.25 18.5 230 98 0.493 51.80 19.0 250 119 0.493 8-9 54.70 18.0 230 95 0.493 60.85 19.0 270 139 0.493 67.10 18.0 270 131 0.493 20.0 420 353 0.473 linear (h=2%) 14.00
18.0
-
the shear stress, γn, which relates the equivalent shear stress, τeff , and the maximum shear stress, τmax , was determined from the response analysis, and was calculated by the equations (2) and (3). FL=R/(τeff/σ’ν) (1) τeff=γn・τmax (2) γn = 0.1(M-1) (3) where FL is the safety factor of liquefaction; R is the liquefaction strength; σ ’ ν is the effective overburden pressure; M is the magnitude of earthquake (=9.0) . Figure 11 compares the analysis results along the soil depth of the grid-form ground improvement, original soil surrounded with the grid and un-improved ground area. The improved and un-improved soils performed almost in the same way in terms of acceleration and horizontal displacement. On the other hand, the shear stress of the original soil surrounded by the grid-form walls was much smaller than that of the grid-form ground improvement as shown in Figure 11(c). This was explained by the rigidity of the improved ground. Figure 12 indicates the liquefaction safety factor of the original soil surrounded by the grid-form improved walls
700
0.260
linear (h=2%)
and un-improved soil. FL values of the un-improved soil were less than 1.0 at some depths, suggesting that liquefaction would occur during the earthquake. However, the FL values of the original soil surrounded by the grid-form improved walls were higher than 1.0 along the depth, suggesting that liquefaction would not happen in such conditions. These analysis results almost agreed with the situation investigated after the earthquake, and it may be confirmed that the grid-form ground improvement succeeded to prevent liquefaction during the 2011 Tohoku Pacific Earthquake.
6． SUMMARY The outline of an actual building foundation was introduced where the grid-form ground improvement was adopted as a liquefaction countermeasure. Post-event field investigation results around the site after the 2011 Tohoku Pacific Earthquake were also reported. The effect of the liquefaction prevention of the grid-form ground improvement was confirmed based on the dynamic
Acceleration (cm/s2 ) 0
100
Displacement (cm)
Acceleration (cm/s2 ) 200 0
100
200 0
5
10
Shear stress (kN/m2 )
Displacement (cm)
15 0
5
10
15 0
25 50 75 100 0
Shear stress (kN/m2 ) 200
400
600
0
-10
Depth (m)
-20
-30
-40
-50
-60
0
Displacement (cm) 5
-70
(a) Acceleration
Original soil un-improved by groundOriginal soil surrounded surrounded by lattice lattice improvement
10
(b) 凡例用 Displacement
Original soil un-improved by gridground grid-formsurrounded improvement lattice lattice Figure 11 Analysisimprovement results
15
(c) Shear stress
un-improved ground
Original soil
surrounded by un-improved ground lattice lattice improvement
the original soil surrounded by the grid-form improved walls during the earthquake. The analysis showed that the liquefaction would happen in the un-improved soil during the earthquake. These results agreed approximately with the field investigation. (3) It was confirmed that t grid-form ground improvement succeeded to prevent the liquefaction during the earthquake.
Safety factor of Safety factor of liquefaction, FL liquefaction, FL 1 2 3 0 1 2 3 0
0 2 4 6
Improved layer
Acknowledgements The authors would like to gratefully acknowledge Oriental Land Co., Ltd. for their permission to publish the investigation results.
8 10 12 14
16 ①un-improved ground
④ Original soil surrounded
Figure 12 Distributions of safety factors of liquefaction
nonlinear analysis for foundation soil layers with recorded acceleration. The major findings are as follows. (1) The original soil surrounded by the grid-form ground improvement did not experience liquefaction during the earthquake based on the field investigation carried out around the building after the earthquake and on the long-term measurement of the foundation performance. (2) The nonlinear analysis by using the site acceleration record suggested that the liquefaction did not occur in
References Architectural Institute of Japan (2001): Recommendations for Design of Building Foundations (in Japanese). Architectural Institute of Japan (2006): Recommendations for Design of Ground Improvement for Building Foundations (in Japanese). National Research Institute for Earth Science and Disaster Prevention: K-NET, http://www.k-net.bosai.go.jp/ Suzuki, Y., Saito, S., Onimaru, S., Kimura, T., Uchida, A. and Okumura, R. (1996): Grid –shaped stabilized ground improved by deep cement mixing method against liquefaction for a building foundation, Tsuchi-to-kiso, JGS, Vol.43, No.3, pp.46-48 (in Japanese). Taya, Y., Uchida, A., Yoshizawa, M., Onimaru, S., Yamashita, K. and Tsukuni, S. (2008) : Simple method for determining lattice intervals in grid-form ground improvement, Japanese Geotechnical Journal , Vol.3, No.3, pp.203-212 (in Japanese) Tokimatsu, K., Mizuno, H. and Kakurai, M. (1996): Building damage associated with geotechnical problems, Special Issue of Soils and Foundations, pp.219-234. Yamashita, K., Hamada, J. and Yamada, T. (2011): Field measurements on piled rafts with grid-form deep mixing walls on soft ground, Geotechnical Engineering Journal of the SEAGS & AGSSEA, Vol.42, No.2, 1-10
