Field measurements on piled raft foundations in Japan

Field measurements on piled raft foundations in Japan Yamashita, K.

R & D Institute, Takenaka Corporation, Japan

Keywords: piled raft, settlement, load sharing, measurement, case history, seismic loading ABSTRACT: This paper offers field measurements on the settlement and the load sharing of piled rafts in Japan. In the first part of this paper, the measurement results obtained from the selected structures on piled rafts, completed in 1984 to 2009, are overviewed and general aspects of the foundation settlements and the load sharing between raft and piles are discussed. In the second part of this paper, the two buildings on piled rafts subjected to the seismic loading during the 2011 Tohoku Pacific Earthquake are presented and the effects of the strong seismic motion on the behavior of the piled rafts are discussed. 1

INTRODUCTION

In designing raft foundations, engineers frequently encounter situations in which the bearing capacity of the raft is quite adequate, but the settlements are estimated to be excessive. In such cases, it is proposed that the combined use of a raft, along with a limited number of piles, could be an economical countermeasure (Burland et al., 1977; Randolph, 1994). In recent years, there has been increasing recognition that the use of piles to reduce raft settlements can lead to considerable economic savings without compromising the safety and the performance of the foundations (Poulos, 2001). For designing piled rafts, the interaction of the raft-soil-pile system must be carefully considered in order to predict the settlement and the load sharing between the raft and the piles. Detailed investigations of several high-rise buildings in Germany, mainly in Frankfurt, have been carried out (Katzenbach et al., 2000). However, not so many case histories exist on the monitoring the load sharing between the raft and the piles as well as the settlements. Thus, the accumulation of field evidence is required to develop more reliable design methods for piled rafts (Mandolini et al., 2005). Piled raft foundations have been used in Japan for many buildings, including tall buildings in excess of 150 m, since a piled raft was first used in the four-story building in 1987. Field measurements on the settlements and the load sharing between the raft

and the piles have been carefully carried out (Yamashita et al., 2011a; Yamashita et al., 2011b). In the first part of this paper, the field measurement results obtained from the selected structures on piled rafts in Japan, completed in 1984 to 2009, are overviewed. Among these, piled rafts with ground improvement, which were employed to cope with liquefaction beneath the raft, are included. Based on the measurement results, general aspects of the foundation settlement and the load sharing between the raft and the piles are discussed. At present, it is required to develop more reliable seismic design methods for piled rafts in Japan and other countries where major earthquakes occur frequently. However, only a few case histories on monitoring seismic soil-pile-structure interaction behavior exist (Mendoza et al., 2000). In the second part of this paper, the two case histories of the piled rafts subjected to the seismic loading during the 2011 Tohoku Pacific Earthquake are presented. The effects of the strong seismic motion on the behavior of the piled rafts are discussed. 2

CASE HISTORIES IN 1980S AND 1990S

2.1 Introduction Early in the 1980s in Japan, there existed no case histories on monitoring the settlement and the load sharing behavior of piled rafts. Therefore, to investigate the effectiveness of settlement reducing 79

Figure 1. Schematic view of the concrete silo with soil profile.

piles and the load sharing between the raft and the piles, field measurements on the foundation settlement and the axial loads of the piles, the contact pressures beneath the raft were carried out. In the 1980s and 1990s, piled rafts were applied to several structures, mainly small-scale structures (Kakurai, 2002). These foundations may be called ‘piled rafts of the first generation in Japan’. 2.2 Piled rafts applied to small-scale structures A piled raft was first applied to the small silo on a trial basis before applying to an actual building (Kakurai et al., 1987). Figure 1 shows a schematic view of the concrete silo and foundation with a soil profile. The subsoil consists of a soft alluvial stratum from the ground surface to a depth of 44 m. The silty soil layer between depths of 9 to 15 m is normally consolidated. The average contact pressure over the raft was 74 kPa. Consequently, a raft foundation with five friction piles was used aiming at reducing the excess consolidation settlement in the normally consolidated silty soil. Figure 2 shows the increase in the total load, the total pile load and the foundation settlements with time. The sum of the measured total pile-head load was 43% of the total structure load; the rest of the total structure load 57% was supported by the raft. The average settlement reached 31 mm at the end of observation. Based on the favorable results, a piled raft was first used in the construction of a four-story office building in Saitama City (former Urawa City) in 1987 (Yamashita et al., 1991). Figure 3 shows a schematic view of the building and foundation with a soil profile. The soil profile to a depth of 5 m is made of diluvial clay called Kanto Loam. The average contact pressure of the building was 61 kPa. A raft with friction piles was employed primarily to 80

Figure 2. Increase in total load, total pile load and foundation settlements with time.

reduce the differential settlements. The friction pile was located just below the column where the superstructure load was presumed to be concentrated. Figure 4 shows a layout of the piles. The piles were constructed by inserting a steel-H member into a pre-augered borehole filled with mix-in-place soil cement. The measured foundation settlements were 3.0 to 10.5 mm and the maximum angular rotation of the raft was 1/1760 radian at the end of construction. The relationship between the sum of the pile-head load and the total load in the tributary area is shown in Fig. 5. The ratio of the sum of the measured pile-head loads to the building load estimated in the tributary area of the three piles was 0.51 at the end of construction and slightly increased to 0.56 three to twelve months after the end of construction. The results of the field measurements on the piled rafts supporting the three structures, i.e. the concrete silo, the four-story building and the five-story

N-value 0

Unconfined compressive strength (kPa)

Consolidation yield stress (kPa)

50 0

500

100

0

500

0

Figure 3. Schematic view of the four-story building with soil profile.

Figure 4. Foundation plan with locations of monitoring devices.

building (Yamashita et al., 1994), were summarized as follows: ・The measured maximum settlements were 11 to 35 mm at the end of observation. ・The ratios of the load carried by the piles to the total load were estimated to be 0.43 to 0.56 at the end of observation. The ratios of the load carried by the piles to the total load were approximately equal to those to the effective load because the buoyancy forces were negligible. Based on the field measurements, it was found that piles were effective in reducing overall and differential settlements of the foundation and a raft could share about half of the structure load. 3

CASE HISTORIES IN 2000S

3.1 Introduction In the early 2000s, a basic framework of design method, including seismic design method, for piled rafts has been established in Japan (AIJ, 2001). To confirm the validity of the design method, field measurements on the settlements and the load sharing between the raft and the piles have been carried out for the selected structures. Table 1 shows the case histories of the piled rafts completed in the 2000s (Yamashita et al., 2011a; Yamashita et al., 2011b). 3.2 Design method The design method for the piled rafts listed in Table 1 was based on the following common design criteria. The design criteria under working load conditions are as follows:

Figure 5. Increase in total load, total pile load and foundation settlements with time.

・It has to be proved that the factor of safety against the ultimate bearing capacity of a piled raft foundation is larger than 3.0. The ultimate bearing capacity of the piled raft foundation can be replaced with the ultimate bearing capacity of the raft foundation alone (ignoring the effect of the piles). ・It has to be proved that the maximum settlement and the maximum differential settlement are less than the allowable values. The maximum differential settlement can be replaced with the maximum angular rotation. Typical values for the limited allowable angular rotation are 1/1000 to 1/500 radian in Japan. The settlements of the foundations, as well as the load sharing between raft and piles, were computed by the simplified method of analysis developed by Yamashita et al. (1998). The deformation parameters of the soil were determined based on the shear 81

Table 1. Case histories of piled rafts in 2000s. 㻿㼠㼞㼡㼏㼠㼡㼞㼑

㻿㼕㼠㼑

㻯㼛㼚㼟㼠㼞㼡㼏㼠㼕㼛㼚㼜㼑㼞㼕㼛㼐

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㻼㼕㼘㼑㼟㻺㼡㼙㼎㼑㼞

㻸㼑㼚㼓㼠㼔㻔㼙㻕

㻰㼕㼍㼙㼑㼠㼑㼞㻔㼙㻕

㻼㼕㼘㼑㻌㼠㼥㼜㼑

㻝㻝㻙㼟㼠㼛㼞㼥㻌㼛㼒㼒㼕㼏㼑㻌㼎㼡㼕㼘㼐㼕㼚㼓

㻭㼕㼏㼔㼕

㻞㻜㻜㻠㻙㻜㻡

㻢㻜㻚㻤

㻝㻤㻝

㻟㻚㻜，㻟㻚㻢

㻝㻣㻚㻜

㻠㻜

㻞㻣㻚㻡，㻞㻢㻚㻥

㻝㻚㻝㻛㻝㻚㻠＊㻙㻝㻚㻡㻛㻝㻚㻤＊

㻯㼍㼟㼠㻙㼕㼚㻙㼜㼘㼍㼏㼑㼏㼛㼚㼏㼞㼑㼠㼑㻌㼜㼕㼘㼑

㻝㻟㻙㼟㼠㼛㼞㼥㻌㼔㼛㼟㼜㼕㼠㼍㼘

㻻㼟㼍㼗㼍

㻞㻜㻜㻠㻙㻜㻡

㻡㻝㻚㻟

㻝㻢㻥

㻢㻚㻠

㻞㻚㻡

㻝㻣

㻝㻥㻚㻜

㻜㻚㻤㻙㻝㻚㻜

㻮㼛㼞㼑㼐㻌㻌㼜㼞㼑㼏㼍㼟㼠㼏㼛㼚㼏㼞㼑㼠㼑㻌㼜㼕㼘㼑

㻴㼍㼐㼞㼛㼚㻌㼑㼤㼜㼑㼞㼕㼙㼑㼚㼠㼍㼘㻌㼔㼍㼘㼘

㻵㼎㼍㼞㼍㼗㼕

㻞㻜㻜㻡㻙㻜㻣

㻝㻥㻚㻜

㻞㻡㻥－㻠㻠㻞

㻤㻚㻜－㻝㻟㻚㻠

㻠㻚㻜

㻟㻣㻝

㻞㻞㻚㻜－㻞㻡㻚㻣

㻜㻚㻢㻙㻜㻚㻤


㻠㻣㻙㼟㼠㼛㼞㼥㻌ｒ㼑㼟㼕㼐㼑㼚㼠㼕㼍㼘㻌㼠㼛㼣㼑㼞

㻭㼕㼏㼔㼕

㻞㻜㻜㻢㻙㻜㻥

㻝㻢㻝㻚㻥

㻢㻜㻜

㻠㻚㻟

㻞㻚㻡

㻟㻢

㻡㻜㻚㻞

㻝㻚㻡㻙㻝㻚㻥㻛㻞㻚㻥＊


㻣㻙㼟㼠㼛㼞㼥㻌㼛㼒㼒㼕㼏㼑㻌㼎㼡㼕㼘㼐㼕㼚㼓

㼀㼛㼗㼥㼛

㻞㻜㻜㻟㻙㻜㻠

㻞㻥㻚㻠

㻝㻜㻜

㻝㻚㻢，㻞㻚㻞

㻝㻚㻡

㻣㻜

㻞㻥㻚㻤，㻟㻜㻚㻠

㻜㻚㻢㻙㻜㻚㻥


㻝㻥㻙㼟㼠㼛㼞㼥㻌㼞㼑㼟㼕㼐㼑㼚㼠㼕㼍㼘㻌㼎㼡㼕㼘㼐㼕㼚㼓

㻷㼍㼓㼛㼟㼔㼕㼙㼍

㻞㻜㻜㻡㻙㻜㻢

㻣㻡㻚㻤

㻞㻡㻣

㻟㻚㻞

㻟㻚㻜

㻞㻤

㻢㻞㻚㻤

㻝㻚㻞㻛㻝㻚㻤＊，㻝㻚㻟㻛㻞㻚㻞＊


㻝㻞㻙㼟㼠㼛㼞㼥㻌㼞㼑㼟㼕㼐㼑㼚㼠㼕㼍㼘㻌㼎㼡㼕㼘㼐㼕㼚㼓

㼀㼛㼗㼥㼛

㻞㻜㻜㻣㻙㻜㻤

㻟㻤㻚㻣

㻝㻥㻥

㻠㻚㻤

㻝㻚㻤

㻝㻢

㻠㻡㻚㻜

㻜㻚㻥㻙㻝㻚㻞


＊

modulus at very small strains. The secant shear modulus was set to 0.30 times the shear modulus at very small strains in the analysis. The design criteria under seismic loading conditions are as follows: ・It has to be proved that the factor of safety against the ultimate bearing capacity of the piled raft is larger than 1.5 under vertical loading together with lateral loading. ・It generally has to be proved that the factor of safety against the ultimate bearing capacity of the piles is larger than 1.5 against the maximum axial load assumed in the design load sharing. The influence of the lateral loading on the piled raft has to be considered, i.e., the maximum bending moment and the shear force on the cross-sections of the pile evaluated by the analytical method developed by Hamada et al. (2009) should be less than the design structural strength of the piles. 3.3 Thirteen-story hospital on soft clay The hospital building is located in Osaka (Yamashita et al., 2011a). Figure 6 shows a schematic view of the building and the foundation with a soil profile. The building consists of a thirteen-story high-rise section and a four-story low-rise section. The high-rise part is a steel-framed structure, while the low-rise section and the basement are a reinforced concrete construction. The soil profile down to a depth of 8 m is made of loose sand and silty sand. From the depth of 8 to 21 m, there lie soft sandy silt and silty clay layers. The average contact pressure over the raft was

82

㻼㼕㼘㼑㼐㻌㼞㼍㼒㼠㼣㼕㼠㼔㻌㼓㼞㼛㼡㼚㼐㻌㼕㼙㼜㼞㼛㼢㼑㼙㼑㼚㼠㻰㼕㼍㼙㼑㼠㼑㼞㻌㼛㼒㻌㼑㼚㼘㼍㼞㼓㼑㼐㻌㼎㼍㼟㼑

169 kPa in the high-rise section and 114 kPa in the low-rise section. In the low-rise section, a raft foundation was proposed because the consolidation yield stresses of the soft sandy silt and the silty clay below the raft were slightly larger than the average contact pressure. In the high-rise section, to reduce consolidation and differential settlement, a piled raft foundation consisting of seventeen 19-m long PHC (pretensioned spun high-strength concrete) piles in the inside and 198 steel-H piles built in the soil-cement diaphragm walls in the perimeter was proposed. The piles were constructed by inserting the PHC piles into a pre-augered borehole filled with mixed-in-place soil cement. Figure 7 shows the foundation plan with a layout of the piles and the locations of the monitoring devices. The two piles at locations of 3C and 4C were installed with a couple of LVDT-type strain gauges at the pile head. Four earth pressure cells and one piezometer were installed beneath the raft. The vertical ground displacement below the raft was measured near the center of the high-rise section by differential settlement gauges. The settlements of the foundation were measured at the monitoring points of the selected columns by an optical level. Figure 8 shows the vertical ground displacement measured at the depth of 7.5 m, which is approximately equal to the foundation settlement, with the settlement of the raft measured at column 3C by an optical level. The ground displacement reached 20.6 mm at the end of observation, 52 months after the end of construction. The maximum angular rotation was 1/1440 radian at the edge of the high-rise section.

㻿㼠㼑㼑㼘㻙㻴㻌㼜㼕㼘㼑㼟㻌㼕㼚㻌㼟㼛㼕㼘㻙㼏㼑㼙㼑㼚㼠㻌㼐㼕㼍㼜㼔㼞㼍㼓㼙㻌㼣㼍㼘㼘㼟㻌

Figure 7. Layout of piles with locations of monitoring devices.

Figure 6. Schematic view of the building and foundation with soil profile.

Figure 9. Measured axial loads of piles 3C and 4C.

Load 　(kN)

Figure 8. Measured settlement of ground and raft.

㻟㻜㻜㻜㻜㻞㻜㻜㻜㻜㻝㻜㻜㻜㻜㻜

㼀㼛㼠㼍㼘㻌㼘㼛㼍㼐㻾㼍㼒㼠㻌㼘㼛㼍㼐㻮㼡㼛㼥㼍㼚㼏㼥系列1 系列5 系列6

㻜㻞㻜㻠㻜㻢㻜㻤㻜㻝㻜㻝㻞㻝㻠㻝㻢㻝㻤㻞㻜㻞㻞 Time （day) 㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜㻜

㻼㼕㼘㼑㻌㼘㼛㼍㼐㻌㻝㻜㻜㻡㻜㻜㻜

㻱㻝

㻱㻞

㻞㻜㻜

㻠㻜㻜

㻢㻜㻜

㻱㻟

㻱㼒㼒㼑㼏㼠㼕㼢㼑㻌㼘㼛㼍㼐㻌

㻤㻜㻜㻝㻜㻜㻜㻝㻞㻜㻜㻝㻠㻜㻜㻝㻢㻜㻜㻝㻤㻜㻜㻞㻜㻜㻜

㻱㻠

㼃

系列5

Figure 10. Measured contact pressures and pore-water pressure.

Figure 9 shows the measured axial loads of the two piles. Figure 10 shows the development of the measured contact pressures of the raft and the pore-water pressure beneath the raft. Figure 11 shows the time-dependent load sharing among the piles, the soil and the buoyancy in the tributary area of two columns, 3C and 4C. The sum of the measured pile-head loads and the raft load in the tributary area varied from 28 to 29 MN 25 to 52 months after the end of construction. The sum of the design vertical load of the two columns, which corresponds to the sum of the dead load and the live

Figure 11. Load sharing between raft and piles in tributary area.

load, is 23.8 MN. The sum of the measured pile-head loads and the raft load in the tributary area is generally consistent with the sum of the design load of the two columns. The ratios of the load carried by the piles to the effective load were estimated to be 0.60 to 0.62 and those to the total load were estimated to be 0.45 to 0.46, respectively, 25 to 52 months after the end of construction. 3.4 Forty-seven-story residential tower on medium sand 83

㻡㻰㻌

㻡㻰㻌

㻝㻢㻝㻚㻥㼙㻌㻿㻼㼀㻌

㼀㼞㼕㼎㼡㼠㼍㼞㼥㻌㼍㼞㼑㼍㻌

㻿㻙㼣㼍㼢㼑㻌

㻺㻙㼂㼍㼘㼡㼑㻌㼂㼟㻔㼙㻛㼟㻕㻌㻢㻜㻜㻜㻌㻌㻌㻡㻜㻌㻜㻌㻌㻌㻌㻡㻜㻜㻡㻜㻜㻌

㻝㻜

㻰㼑㼜㼠㼔㻌㻔㼙㻕

㻞㻜㻟㻜

㻿㼕㼘㼠㻿㼍㼚㼐㻌㼍㼚㼐㻌㼓㼞㼍㼢㼑㼘

㻡㻜

㻸㼍㼙㼕㼚㼍㼠㼑㼐㻌㼞㼡㼎㼎㼑㼞㻌㼎㼑㼍㼞㼕㼚㼓㼟㻌㻡㻚㻟㼙㻌

㻡㻚㻤㼙㻌


㻠㻚㻟㼙㻌

㻿㼍㼚㼐

㻝㻤㼙㻌㻿㼕㼘㼠㻿㼍㼚㼐㻿㼍㼚㼐㻌㼍㼚㼐㻌㼓㼞㼍㼢㼑㼘

㻠㻜

㻣㻰㻌

㼀㼞㼕㼎㼡㼠㼍㼞㼥㻌㼍㼞㼑㼍㻌

㻜

㻳㻸±㻌㻜

㻣㻰㻌

㻿㼍㼚㼐

㻟㻞㼙㻌㻠㻝㼙㻌

㻿㼕㼘㼠㻿㼍㼚㼐㻌㼍㼚㼐㻌㼓㼞㼍㼢㼑㼘

㻡㻞㻚㻠㼙㻌㻡㻠㻚㻡㼙㻌

㻢㻜

㻣㻜㼙㻌㻿㼑㼠㼠㼘㼑㼙㼑㼚㼠㻌㼓㼍㼡㼓㼑㼟㻌


The forty-seven-story residential tower is located in Nagoya (Yamashita et al., 2011a). Figure 12 shows a schematic view of the building and the foundation with a soil profile. The building is a reinforced concrete structure with a base isolation system. The soil profile below a depth of 4 m from the ground surface consists of diluvial medium to dense sand-and-gravel and sand to a depth of 17 m, underlain by medium silt to a depth of 27 m. Between depths of 43 and 53m, there lies medium silt. The average contact pressure over the raft was 600 kPa. The reinforced concrete raft is founded on the medium-to-dense sand and gravel. To reduce the excessive settlement due to the medium silt layers, a piled raft foundation consisting of thirty-six 50-m long cast-in-place concrete piles was proposed. Figure 13 shows the foundation plan with a layout of the piles the locations of the monitoring devices. Two piles, 5D and 7D, were provided with a couple of LVDT-type strain gauges at the pile head and at the pile toe. Eight earth pressure cells and one piezometer were installed beneath the raft surrounding the two piles. The vertical ground displacements below the raft relative to the reference point at a depth of 70 m using differential settlement 84

Figure 14. Measured vertical ground displacements.

gauges. Figure 14 shows the measured vertical ground displacements where the initial values of the displacements were taken right before the casting the foundation slab. The ground displacement measured at a depth of 5.3 m is approximately equal to the foundation settlement. The measured ground displacement reached 23.4 mm at the end of the construction. Thereafter, the displacement increased very slightly to 24.2 mm at the end of observation, 17 months after the end of construction. Based on the measured settlements by an optical level 17 months after the end of construction, the measured settlements were 12 to 29 mm and the maximum angular rotation of the raft was 1/1000 radian. Figure 15 shows the development of the measured axial loads on pile 7D. The load transferred to the pile toes was relatively small and the instrumented piles show the behavior of friction piles. Figure 16 shows the development of the measured contact pressures of the raft and the pore-water pressure beneath the raft after the end of construction. Figure 17 shows the time-dependent load sharing among the piles, the soil and the groundwater buoyancy in the tributary area of column 7D and the ratio of the load carried by the pile to the effective

■

Figure 16. Measured contact pressures and pore-water pressure.

Figure 15. Measured axial loads of the piles 7D.

㻠㻜㻟㻜㻞㻜㻝㻜㻜

㼀㼛㼠㼍㼘㻌㼘㼛㼍㼐㻾㼍㼒㼠㻌㼘㼛㼍㼐㻮㼡㼛㼥㼍㼚㼏㼥系列3 㻰㼑㼟㼕㼓㼚㻌㼘㼛㼍㼐㻌㻞㻢㻚㻟㻹㻺

㻜㻞㻜㻠㻜㻢㻜㻤㻜㻝㻜㻝㻞㻜㻜㻜㻜㻜㻜㻜㻜

㻼㼕㼘㼑㻌㼘㼛㼍㼐㻌㻱㼒㼒㼑㼏㼠㼕㼢㼑㻌㼘㼛㼍㼐㻌

（a）Load sharing between raft and pile

（b）Ratio of load carried by pile

Figure 17. Load sharing between raft and piles in tributary area of column 7D.

load versus time together with that to the total load versus time. The sum of the measured pile-head load and raft load on the tributary area was 29.6 MN on columns 7D at the end of observation. The design vertical load of columns 7D, which correspond to the sum of the dead load and the live load, was 35.6 MN. Therefore, the sum of the measured pile-head load and the raft load on the tributary area was consistent with the design load. The ratios of the load carried by the piles to the effective load on the tributary area of columns 5D and 7D were estimated to be 0.93 and 0.87, respectively, both at the end of

construction and at the end of observation. 3.5 Piled raft with ground improvement The seven-story office building is located in Tokyo (Yamashita et al., 2011b). Figure 18 shows a schematic view of the building and the foundation with a soil profile. The building is a steel-framed structure. The soil profile down to a depth of 11 m from the ground surface is made of soft silt and loose sand. Between depths of 11 m to 42 m, there lies a thick soft to medium silt stratum, underlain by a diluvial very dense sandy layer.


85

An assessment of a potential of liquefaction during earthquakes was carried out using the simplified method (Tokimatsu and Yoshimi, 1983). It indicated that the loose sand had a potential of liquefaction during earthquakes with the peak horizontal ground acceleration of 2.0 m/s2. Therefore, to cope with the liquefiable sand and ensure bearing capacity of a raft, grid-form deep cement mixing walls (TOFT method) shown in Fig. 19 were employed below the raft. As to the TOFT method, typical compressive strength of the soil cement is 2 N/mm2 and the high-modulus soil-cement walls confine loose sand so as not to cause excessive shear deformation to the loose sand during earthquakes. The effectiveness of the TOFT method was confirmed during the 1995 Hyogoken-Nambu earthquake (Tokimatsu et al., 1996). The average contact pressure over the raft was 100 kPa. In order to reduce overall and differential settlement due to consolidation of the soft cohesive soil, a piled raft consisting of seventy 30-m long PHC piles with grid-form deep cement mixing walls was employed. Figure 20 shows a layout of the piles and the grid-form soil-cement wall with the locations of the monitoring devices. Two piles were installed with a couple of LVDT-type strain gauges. Two earth pressure cells and a piezometer were installed beneath the raft. The vertical ground

displacements below the raft were measured by differential settlement gauges. The settlements of the foundation were measured by an optical level. Figure 21 shows the measured vertical ground displacements below the raft at three depths relative to a reference point at a depth of 46 m. The ground displacement at a depth of 3.0 m reached 21.9 mm at the end of observation, 72 months after the end of construction. The settlements of the foundation measured by the optical level were 17 to 31 mm and the maximum angular rotation of the raft was 1/1200 radian four years after the end of construction. Figure 22 shows the time-dependent load sharing among the piles, the soil, the soil-cement walls and the buoyancy in the tributary area, where the total

Figure 19. Grid-form deep cement mixing walls (TOFT method).

Grid-form soil-cement walls

㻟㻞㻚㻠㼙

㻝㻜㻚㻤㼙

㻯

㻝㻜㻚㻤㼙

㻮

㻭

◎◎

○

● ●

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Office building ○

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Dining hall ◎ ◎

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Pile diameter

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㻝㻜㻚㻌㼙

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Tributary area

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㻰

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W

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E1

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E2

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◎ ◎

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d=0.9m

○ d=0.8m

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d=0.7m

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㻺

①

㻝㻜㻚㻤㼙

②

㻝㻜㻚㻤㼙

③

㻝㻜㻚㻤㼙

④

㻝㻜㻚㻤㼙

⑤

㻝㻜㻚㻤㼙㻝㻝㻤㻚㻤㼙

⑥

㻝㻜㻚㻤㼙

⑦

㻝㻜㻚㻤㼙

⑧

㻝㻜㻚㻤㼙

⑨

㻝㻜㻚㻤㼙

⑩

㻝㻜㻚㻤㼙

⑪

d=0.6m

Monitoring devices ●

○ ○ ○

●

□ △

㻝㻜㻚㻤㼙

Instrumented pile

Earth pressure cell Piezometer

☆ Settlement gauges

⑫

Figure 20. Layout of piles and grid-form deep cement mixing walls with locations of monitoring devices.

Figure 21. Measured vertical ground displacement.

86

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Figure 22. Time-dependent load sharing between raft and piles in tributary area.

■

㻜㻌

㼀㼕㼙㼑㻌（㼐㼍㼥㼟）㻞㻜㻜㻌㻠㻜㻜㻌㻢㻜㻜㻌㻤㻜㻜㻌㻝㻜㻜㻜㻌㻝㻞㻜㻜㻌㻝㻠㻜㻜㻌㻝㻢㻜㻜㻌㻝㻤㻜㻜㻌㻞㻜㻜㻜㻌㻞㻞㻜㻜㻌㻞㻠㻜㻜㻌㻞㻢㻜㻜㻌

㻜㻌

㻿㼑㼠㼠㼘㼑㼙㼑㼚㼠㻌㻔㼙㼙㻕

load is assumed to be equal to the sum of the design column load. The ratio of the load carried by the piles to the effective load was estimated to be 0.54 at the end of construction and increased to 0.72 at the end of observation. This was supposed to be caused by a decrease in raft resistance due to the consolidation settlement of the soft silt layers below the raft. Meanwhile, the ratio of the load carried by the piles to the total load was estimated to be 0.50 at the end of construction and 0.66 at the end of observation.

㻝㻜㻌

㻞㻜㻌

㻟㻜㻌

㻠㻜㻌

Figure 23. Measured foundation settlements. 㻝㻚㻟㻜㻌

4

DISCUSSIONS ON MEASUREMENTS

4.1 Settlement The vertical ground displacements just below the rafts were successively measured by the settlement gauges from the beginning of construction to 17 to 72 months after the end of construction as for six of the structures in Table 1. The measured settlements at the end of construction, seoc, were 12 to 23 mm, while those at the end of observation, seobs, reached 17 to 24 mm. Figure 23 shows the relationship between the foundation settlement and the elapsed time for the six structures. The measured settlements were relatively small, less than 30 mm, and within a limited range. Figure 24 shows the ratio of the measured settlement from the beginning of construction to that at the end of construction, st/seoc, versus the elapsed time from the end of construction for the five structures. The value of st/seoc ranged from 1.03 to 1.20. The increase in the settlements after the end of construction is supposed to be due to live load and primary creep of soil in which no consolidation settlement would occur except for the seven-story building.

㻿㼠㻛㻿㼑㼛㼏㻌

㻝㻚㻞㻜㻌

㻝㻚㻝㻜㻌

㻝㻚㻜㻜㻌

㻌㻜㻌

㻞㻜㻜㻌㻠㻜㻜㻌㻢㻜㻜㻌㻤㻜㻜㻌㻝㻜㻜㻜㻌㻝㻞㻜㻜㻌㻝㻠㻜㻜㻌㻝㻢㻜㻜㻌㻝㻤㻜㻜㻌㻞㻜㻜㻜㻌㻞㻞㻜㻜㻌㻞㻠㻜㻜㻌㼀㼕㼙㼑㻌㼍㼒㼠㼑㼞㻌㻱㻚㻻㻚㻯㻌㻔㼐㼍㼥㼟㻕㻌

Figure 24. Ratio of measured settlement after the end of construction to that at the end of construction.

㻱㼚㼐㻙㼎㼑㼍㼞㼕㼚㼓㻌㼜㼕㼘㼑㼟㻌㻌 ○ ● 㻌㻝㻝㻙㼟㼠㼛㼞㼥㻌㼎㼡㼕㼘㼐㼕㼚㼓㻘㻌㻝㻟㻙㼟㼠㼛㼞㼥㻌㼎㼡㼕㼘㼐㼕㼚㼓㻘㻌㻠㻣㻙㼟㼠㼛㼞㼥㻌㼎㼡㼕㼘㼐㼕㼚㼓㻌 □ ■ 㻌㻝㻥㻙㼟㼠㼛㼞㼥㻌㼎㼡㼕㼘㼐㼕㼚㼓㻌㻔㼣㼕㼠㼔㻌㼓㼞㼛㼡㼚㼐㻌㼕㼙㼜㼞㼛㼢㼑㼙㼑㼚㼠㻕㻌 □㻌 ■㻌㼅㼍㼙㼍㼟㼔㼕㼠㼍㻌㻒㻌㻴㼍㼙㼍㼐㼍㻘㻌㻞㻜㻝㻝㻌㻔㼣㼕㼠㼔㻌㼓㼞㼛㼡㼚㼐㻌㼕㼙㼜㼞㼛㼢㼑㼙㼑㼚㼠㻕㻌㻲㼞㼕㼏㼠㼕㼛㼚㻌㼜㼕㼘㼑㼟㻌 ○㻌㻴㼍㼐㼞㼛㼚㻌㼑㼤㼜㼑㼞㼕㼙㼑㼚㼠㼍㼘㻌㼔㼍㼘㼘㻌 ◇㻌㻷㼍㼗㼡㼞㼍㼕㻌㼑㼠㻌㼍㼘㻚㻘㻌㻝㻥㻤㻣㻧㻌㼅㼍㼙㼍㼟㼔㼕㼠㼍㻌㻒㻌㻷㼍㼗㼡㼞㼍㼕㻘㻌㻝㻥㻥㻝㻧㻌㼅㼍㼙㼍㼟㼔㼕㼠㼍㻌㼑㼠㻌㼍㼘㻚㻘㻌㻝㻥㻥㻠㻌＋ □㻌㼅㼍㼙㼍㼟㼔㼕㼠㼍㻌㻒㻌㼅㼍㼙㼍㼐㼍㻘㻌㻞㻜㻜㻥㻌㻔㼣㼕㼠㼔㻌㼓㼞㼛㼡㼚㼐㻌㼕㼙㼜㼞㼛㼢㼑㼙㼑㼚㼠㻕㻌

4.2 Load sharing Mandolini et al. (2005) pointed out that the pile spacing ratio, which is the ratio of the pile spacing to the pile diameter, plays a major role in load sharing between the raft and the piles based on a review of the available experimental evidence by the monitoring of full-scale structures. Figure 25 shows the ratios of the load carried by the piles to the effective load in the tributary area αp’ at the end of observation versus the pile spacing ratio s/d as for the ten case histories, where s is average center-to-center spacing between the instrumented pile and the adjacent piles and d is the shaft diameter of the pile. Although the value of αp’ varied from 0.43 to 0.93, it was found that the value of αp’ generally decreased

Figure 25. Ratio of load carried by piles to effective load versus pile spacing ratio.

as the pile spacing ratio was increased. The value of αp’ seems to have significantly decreased as the pile spacing ratio was increased from about four to six. At the pile spacing ratio of larger than about six, the value of αp’ seems to decrease gradually. Furthermore it appears that the value of αp’ depended on the type of piled rafts, i.e. the value of αp’ for the raft with end-bearing piles is approximately 0.6 to 87

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■

㻝㻥㼙㻌

㻢㻜㼙㻌

㻿㻼㼀㻌㻿㻙㼣㼍㼢㼑㻌㻺㻙㼂㼍㼘㼡㼑㻌㼂㼟㻔㼙㻛㼟㻕㻌㻜㻌㻌㻌㻡㻜㻌㻜㻌㻌㻌㻡㻜㻜㻌

㻳㻸±㻜㻌㻿㼍㼚㼐㻌㻿㼍㼚㼐㻌㼍㼚㼐㻌㼓㼞㼍㼢㼑㼘㻌

▽㻤㻚㻜㼙㻌 ▽㻝㻝㻚㻠㼙㻌 ▽㻝㻟㻚㻠㼙㻌

㻝㻜㻌㻿㼍㼚㼐㻌

㻰㼑㼜㼠㼔㻌㻔㼙㻕㻌

0.7 and that for the raft with friction piles is 0.4 to 0.6. However, it should be noted that the value of αp’ for the seven-story building is somewhat larger than the others, possibly, because a decrease in raft resistance was caused by the consolidation settlement of the soft silt below the raft. Based on the measurement results, it is suggested that piled rafts work more effectively with a pile spacing ratio of larger than about six on the assumption that no consolidation settlement occurs in the soil below the raft, where at least 30% of the effective load could be carried by the raft.

㻿㼍㼚㼐㼥㻌㼟㼕㼘㼠㻌㻞㻜㻌㻿㼕㼘㼠㼥㻌㼟㼍㼚㼐㻌㻿㼍㼚㼐㻌

㻝㻞㻚㻡㼙㻌

㻞㻝㻚㻡㼙㻌

㻿㼍㼚㼐㼥㻌㼟㼕㼘㼠㻌㻟㻜㻌

㻿㼍㼚㼐㻌㻿㼍㼚㼐㼥㻌㼟㼕㼘㼠㻌

㻟㻜㻚㻡㼙㻌

㻠㻜㻌

㻟㻞㻚㻜㼙㻌㻟㻣㻚㻜㼙㻌

㻿㼕㼘㼠㻌㻠㻝㼙㻌

㻹㼡㼐㼟㼠㼛㼚㼑㻌

㻿㼑㼠㼠㼘㼑㼙㼑㼚㼠㻌㼓㼍㼡㼓㼑㼟㻌㻤㻜㼙㻌

SEISMIC BEHAVIOUR OF PILED RAFTS

5.1 The 2011 Tohoku Pacific Earthquake The 2011 Tohoku Pacific Earthquake, with an estimated magnitude of Mw=9.0 Moment Magnitude scale, struck the East Japan at 14:46 on Mar. 11, 2011. According to the JMA, an earthquake epicenter was located about 130 km east-southeast off the Oshika Peninsura at a depth of 23.7 km. The earthquake struck the sites of the two buildings, e.g. the hadron experimental hall and the twelve-story building listed in Table 1. The following shows the field measurement results on the piled rafts before and after the earthquake as well as discussions on the effects of the seismic motion on the foundation behaviour. 5.2 Hadron experimental hall in Ibaraki The hadron experimental hall is located at JPARC (Japan Proton Accelerator Research Complex) in Ibaraki Prefecture (Yamashita et al., 2011a). Figure 26 shows a schematic view of the building and the foundation with a soil profile. The building is a steel reinforced concrete structure. The subsoil below a depth of 6 m consists of diluvial dense sand-and-gravel and medium to dense sand to a depth of 16m. Between the depths of 23 and about 40 m, lie cohesive layers with unconfined compressive strengths of 180 to 480 kPa, underlain by a weathered sandy mudstone. Figure 27 shows the foundation plan with a layout of the piles and the locations of the monitoring devices. The average contact pressures over the raft were 259 kPa in the experimental line, 350 kPa in the beam line and 442 kPa in the beam dump. The foundation levels are between depths of 8.0 to 13.4 m. A reinforced concrete mat was founded on the dense sand-and-gravel and medium to dense sand. To reduce the settlement due to the compression of the cohesive layers below the depth of 23 m to an acceptable level, a piled raft foundation consisting of 88


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N

㻱㼤㼜㼑㼞㼕㼙㼑㼚㼠㼍㼘㻌㼘㼕㼚㼑

㻹㼛㼚㼕㼠㼛㼞㼕㼚㼓㻌㼐㼑㼢㼕㼏㼑㼟

㻼㻟

㻢㻜㼙

5

㻵㼚㼟㼠㼞㼡㼙㼑㼚㼠㼑㼐㻌㼜㼕㼘㼑㻱㼍㼞㼠㼔㻌㼜㼞㼑㼟㼟㼡㼞㼑㻌㼏㼑㼘㼘㻼㼕㼑㼦㼛㼙㼑㼠㼑㼞 ★ 㻿㼑㼠㼠㼘㼑㼙㼑㼚㼠㻌㼓㼍㼡㼓㼑

㻼㻞

★㻼㻝

㻮㼑㼍㼙㻌㼐㼡㼙㼜㻮㼑㼍㼙㻌㼘㼕㼚㼑

㻡㻢㼙

㻞㻜㼙


Ｅ

㻱㼤㼜㼑㼞㼕㼙㼑㼚㼠㼍㼘㻌㻴㼍㼘㼘㻌

Photo 1. Ground subsidence along the building.

371 PHC piles with diameters varying from 0.60 to 0.80 m. Forty-four months after the end of construction, the 2011 Tohoku Pacific Earthquake struck the site of the building (Yamashita et al., 2012a). The distance from the epicenter to the building was about 270 km. At the site 0.9 km south from the building,

Ｋ

㻮㼑㼍㼙㻌㼘㼕㼚㼑㻌㻝㻞㻚㻡㼙㻌㻞㻝㻚㻡㼙㻌

㻿㼍㼚㼐㻌㻿㼍㼚㼐㻌㼍㼚㼐㻌㼓㼞㼍㼢㼑㼘㻌㻿㼍㼚㼐㻌㻿㼍㼚㼐㼥㻌㼟㼕㼘㼠㻌㻿㼕㼘㼠㼥㻌㻿㼍㼚㼐㻌㻿㼍㼚㼐㻌

㻟㻜㻚㻡㼙㻌

㻿㼍㼚㼐㼥㻌㼟㼕㼘㼠㻌

㻠㻝㻚㻜㼙㻌

㻿㼕㼘㼠㻌㼃㼑㼍㼠㼔㼑㼞㼑㼐㻌㼙㼡㼐㼟㼠㼛㼚㼑㻌

㻜㻌

Figure 28. Measured vertical ground displacements.

the peak ground acc elerations at a depth of 6 m below the ground surface were recorded to be 3.24 m/s2 for the horizontal direction and 2.77 m/s2 for the vertical direction (Hashimura et al., 2011). Photo 1 shows the ground subsidence along the northeast side of the experimental hall, which reached a maximum of 1.2 m after the earthquake. Figure 28 shows the measured vertical ground displacements relative to the reference point. The ground displacement at the depth of 12.5 m was approximately equal to “foundation settlement” in case that it was initialized just before the casting of the foundation mats. The foundation settlement reached 20.7 mm just before the earthquake. Figure 29 shows the profiles of the measured vertical ground displacements before and after the earthquake. The foundation settlement increased 4.1 mm to 24.8 mm 28 days after the earthquake when the monitoring system was restored. It can be seen that the increments in the ground displacements

（a）Pile P1 in beam line

（b）Pile P2 in beam dump Figure 30. Measured axial loads of piles.

Figure 29. Profiles of vertical ground displacements.

occurred mostly by the compression of the silty soil between depths o f 23 and 41 m. The compression of the silty soil seemed to be caused by the vertical cyclic loading due to the inertial force acting on the building. Figure 30 shows the measured axial loads of the piles. Figure 31 shows the measured contact pressures and pore-water pressure beneath the raft. After the earthquake, the axial loads of pile P1 decreased only slightly and the contact pressures near pile P1 increased slightly. On the other hand, the axial load of pile P2 at pile head increased 30% and the contact pressures near pile P2 increased 39%, possibly, because the frictional resistance at the interface of the outside wall of the structure and the back-filled sand was considerably reduced by the subsidence of the back-filled sand due to the strong seismic motion. The pore-water pressures were not affected by the seismic motion.

（a）Near pile P1（beam line）

（b）Near pile P2（beam dump） Figure 31. Measured contact pressures and pore-water pressure.

89





Figure 32. Load sharing between raft and piles in tributary area of pile P1.

Figure 33. Load sharing between raft and piles in tributary area of pile P2.

Figure 32 shows the time-dependent load sharing among the piles, the soil and the buoyancy on the tributary area of pile P1. Figure 33 shows those on the tributary area of pile P2. The ratio of the load carried by the piles to the effective load on the tributary area of pile P1 was estimated to be 0.85 and that of pile P2 was 0.67 just before the earthquake. The former decreased only slightly to 0.82 and the latter decreased slightly to 0.57 28 days after the earthquake. In one year after the earthquake,

the ratios of the pile load to the effective load were quite stable. 5.3 Twelve-story base-isolated building in Tokyo The twelve-story residential building is located in Tokyo (Yamashita et al., 2011b). The building is a reinforced concrete structure with a base isolation system of laminated rubber bearings. Figure 34 shows a schematic view of the building and the


90

㻵㼚㼟㼠㼞㼡㼙㼑㼚㼠㼑㼐㻌㼜㼕㼘㼑

㻝㻚㻜㼙

㻱㼍㼞㼠㼔㻌㼜㼞㼑㼟㼟㼡㼞㼑㻌㼏㼑㼘㼘

㻜㻚㻤㼙

㻼㼕㼑㼦㼛㼙㼑㼠㼑㼞

㻞㻚㻡㼙㻞㻚㻡㼙

㻹㼛㼚㼕㼠㼛㼞㼕㼚㼓㻌㼐㼑㼢㼕㼏㼑㼟

㻝㻚㻞㼙

㻿㼑㼠㼠㼘㼑㼙㼑㼚㼠㻌㼓㼍㼡㼓㼑㼟

㻭㼏㼏㼑㼘㼑㼞㼛㼙㼑㼠㼑㼞

◆

◆ 㻭㻞（㻳㻸㻙㻝㻡㼙） ◆ 㻭㻝（㻳㻸㻙㻝㻚㻡㼙）

◆ 㻭㻟（㻳㻸㻙㻡㻜㼙）

㻤㻚㻜㼙

㻼㼕㼘㼑㻌㼐㼕㼍㼙㼑㼠㼑㼞

㻜㻚㻤㼙

㻤㻚㻤㼙

㻭㼏㼏㼑㼘㼑㼞㼍㼠㼕㼛㼚㻌㻔㼙㻛㼟㻞㻕㻌

㻲

㻝㻜㻚㻟㼙

㻟㻟㻚㻞㻡㼙

㻰

㻱㻝

㻱㻟

㻡㻮

㻮㻝㻜㻚㻡㻡㼙

㻰㻝

㻱㻞

㻰㻞㻱㻠

㻱㻡

㼃

㻣㻮㻱㻢

㼀㼞㼕㼎㼡㼠㼍㼞㼥㻌㼍㼞㼑㼍㻭

①

③ 㻥㻚㻜㻞㻡㼙

⑤ 㻤㻚㻠㼙

⑦ 㻥㻚㻜㻞㻡㼙

㻟㻜㻚㻜㻡㼙

Figure 35. Layout of piles and grid-form deep cement mixing walls with locations of monitoring devices.

㼀㼕㼙㼑㻌㻔㼟㻕㻌

Figure 36. Time histories of EW accelerations of ground and structure.

㻹㼍㼞㻚㻌㻝㻝㻘㻌㻞㻜㻝㻝㻌

foundation with a typical soil profile. The soil profile down to a depth of 7 m is made of fill, soft silt and loose silty sand. Between depths of 7 m to 44 m, there lie very-soft to medium silty clay strata underlain by a diluvial very dense sand-and-gravel layer. The average contact pressure over the raft was 199 kPa. To improve the bearing capacity of the subsoil beneath the raft, as well as to cope with the liquefiable silty sand, the grid-form deep cement mixing walls were embedded in the overconsolidated silty clay below a depth of 16 m. Consequently, a piled raft consisting of sixteen 45-m long PHC piles (SC piles in top portion) were employed. The locations of the monitoring devices are shown in Figs. 34 and 35. As for the seismic observation, the NS, EW and UD accelerations of the free-field ground were recorded by the vertical array consisted of borehole-type triaxial servo accelerometers and those of the building on the raft, the first and the twelfth floors were recorded by triaxial servo accelerometers. The axial loads of the piles and the contact pressures between the raft and the soil as well as the pore-water pressure beneath the raft were also recorded during the earthquake in common starting time with the accelerometers. Thirty months after the end of construction, the 2011 Tohoku Pacific Earthquake struck the building site. The distance from the epicenter to the building site was about 380 km. The seismic responses of the soil-structure system were successfully recorded (Yamashita et al., 2012b). Figure 36 shows the time

Figure 37. Vertical ground displacements at a depth of 5.8 m.

histories of the EW acceleration of the ground and the structure. The peak accelerations of the first and the twelfth floors were 0.527 and 0.619 m/s2, respectively, whereas the peak horizontal ground acceleration of 1.748 m/s2 was observed near the ground surface. The peak acceleration of the first floor was reduced to 30% from that near the ground surface by the base-isolation system and the kinematic soil-foundation interaction. Figure 37 shows the measured vertical ground displacements just below the raft, which were approximately equal to the foundation settlements. The foundation settlement, initialized just after the casting of the slab, reached 17.3 mm on Mar. 10, just before the earthquake. The settlement increased very slightly to 17.6 mm on Mar. 15, 4 days after the earthquake. Thereafter, the settlements were stable. Figure 38 shows the development of the measured axial loads of piles 5B and 7B. The axial loads at the 91



㻢㻚㻜㼙㻌㻝㻢㻚㻜㼙㻌

(a) Pile 5B 㻹㼍㼞㻚㻌㻝㻝㻘㻌㻞㻜㻝㻝㻌

Figure 40. Contact pressures between raft and deep mixing walls and those between raft and soil.

(b) Pile 7B



㻭㼤㼕㼍㼘㻌㼘㼛㼍㼐㻌㻔㻹㻺㻕㻌

㻼㼞㼑㼟㼟㼡㼞㼑㻌（㼗㻼㼍）㻌

Figure 38. Measured axial loads of piles 5B and 7B.

㻯㼛㼚㼠㼍㼏㼠㻌㼜㼞㼑㼟㼟㼡㼞㼑㻌（㼗㻼㼍）㻌

㻯㼛㼚㼠㼍㼏㼠㻌㼜㼞㼑㼟㼟㼡㼞㼑㻌（㼗㻼㼍）㻌

㻢㻚㻜㼙㻌㻝㻢㻚㻜㼙㻌㻠㻢㻚㻡㼙㻌



㻭㼤㼕㼍㼘㻌㼘㼛㼍㼐㻌㻔㻹㻺㻕㻌

(a) Pile 5B

Figure 41. Fluctuations of contact pressures and pore-water pressure.

㻹㼍㼞㻚㻌㻝㻝㻘㻌㻞㻜㻝㻝㻌㼀㼕㼙㼑㻌㻔㼟㻕㻌

(b) Pile 7B Figure 39. Fluctuations of axial loads of at pile head.

pile head were 14.7 MN and 8.3 MN on Mar. 10 and became 14.8 MN and 8.2 MN on Mar. 15 on piles 5B and 7B, respectively, so that there was little change in the pile-head load before and after the earthquake. Figure 39 shows the fluctuations of the axial loads at the pile head during the earthquake. The ratios of the maximum amplitude of the pile-head load to that measured just before the earthquake were 5.8 and 12.4 % on piles 5B and 7B, respectively. Figure 40 shows the development of the measured 92

㻰㼑㼟㼕㼓㼚㻌㼏㼛㼘㼡㼙㼚㻌㼘㼛㼍㼐㼟㻌㻟㻢㻚㻜㻹㻺

㻼㼕㼘㼑㼟㻌

㻰㼑㼑㼜㻌㼙㼕㼤㼕㼚㼓㻌㻌㼣㼍㼘㼘㼟㻌㻌㻾㼍㼒㼠㻌㻿㼛㼕㼘㻌㻮㼡㼛㼥㼍㼚㼏㼥㻌

Figure 42. Time-dependent load sharing among piles, deep mixing walls and soil in tributary area.


㻱㼒㼒㼑㼏㼠㼕㼢㼑㻌㼘㼛㼍㼐㻌

㼀㼛㼠㼍㼘㻌㼘㼛㼍㼐㻌

Figure 43. Ratios of load carried by piles to effective load and total load in tributary area.

㻸㼛㼍㼐㻌㻔㻹㻺㻕㻌

contact pressures between the raft and the deep mixing walls and those between the raft and the soil together with the pore-water pressure beneath the raft. The contact pressures between the raft and the deep mixing walls were 296 to 316 kPa and those between the raft and the soil were 39 to 63 kPa on Mar. 10. The contact pressures, except for the value from D1, increased very slightly on Mar. 15. Figure 41 shows the fluctuations of the contact pressure between the raft and the deep mixing walls and that between the raft and the soil together with the pore-water pressure. The amplitude of the contact pressure between the raft and the deep mixing walls was significantly larger than that between the raft and the soil as in the case of the static measurements. The excess pore-water pressure was considerably smaller than the contact pressures between the raft and the soil. Figure 42 shows the time-dependent load sharing among the piles, the deep mixing walls, the soil and the buoyancy in the tributary area of columns 5B and 7B. Figure 43 shows the ratio of the load carried by the piles to the effective load and that to the total load in the tributary area versus time. The ratio of the load carried by the piles to the effective load was 0.67 on Mar. 10 and little change in the ratio could be observed just after and 13 months after the earthquake. Figure 44 shows the time histories of the load sharing among the piles, the deep mixing walls, the soil and the buoyancy during the earthquake. Each load was initialized to the measured value on Mar. 10. The fluctuation of the total load was relatively small and carried mainly by the piles. Figure 45 shows the interaction curve of axial load and bending moment of the SC pile corresponding to the allowable and the ultimate bending moment for an axial force in design, together with the relationship between the axial load and the bending moment at pile head observed during the earthquake. The bending moments were obtained by combining the components in NS and

㻼㼕㼘㼑㼟㻌

㻰㼑㼑㼜㻌㼙㼕㼤㼕㼚㼓㻌㼣㼍㼘㼘㼟㻌

㻿㼛㼕㼘㻌㻮㼡㼛㼥㼍㼚㼏㼥㻌


Figure 44. Load sharing among piles, deep mixing walls and soil during earthquake.

EW directions. It is found that the change in the axial force is small and the observed bending moments for both piles are significantly smaller than the allowable bending moment of the pile.

㼁㼘㼠㼕㼙㼍㼠㼑㻌

㼁㼘㼠㼕㼙㼍㼠㼑㻌

㻭㼘㼘㼛㼣㼍㼎㼘㼑㻌

㻹㼑㼍㼟㼡㼞㼑㼐㻌

(a) Pile 5B (1.2m in diameter)

㻭㼘㼘㼛㼣㼍㼎㼘㼑㻌

㻹㼑㼍㼟㼡㼞㼑㼐㻌

㼁㼘㼠㼕㼙㼍㼠㼑㻦㻌㼡㼚㼕㼠㻌㼟㼠㼞㼑㼟㼟㻌㼍㼠㻌㼠㼔㼑㻌㼑㼐㼓㼑㻌㼛㼒㻌㼏㼛㼚㼏㼞㼑㼠㼑㻌㼞㼑㼍㼏㼔㼑㼟㻌㼠㼔㼑㻌㼏㼛㼙㼜㼞㼑㼟㼟㼕㼢㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔㻌㻔㻝㻜㻡㻌㻺㻛㼙㼙㻞㻕㻌㻌㻭㼘㼘㼛㼣㼍㼎㼘㼑㻦㻌㼡㼚㼕㼠㻌㼟㼠㼞㼑㼟㼟㻌㼍㼠㻌㼠㼔㼑㻌㼑㼐㼓㼑㻌㼛㼒㻌㼟㼠㼑㼑㼘㻌㼜㼕㼜㼑㻌㼞㼑㼍㼏㼔㼑㼟㻌㼠㼔㼑㻌㼥㼕㼑㼘㼐㻌㼟㼠㼞㼑㼟㼟㻌㼕㼚㻌㼠㼑㼚㼟㼕㼛㼚㻌㼍㼚㼐㻛㼛㼞㻌㼠㼔㼍㼠㻌㼛㼒㻌㼏㼛㼚㼏㼞㼑㼠㼑㻌㼞㼑㼍㼏㼔㼑㼟㻌㻞㻛㻟㻚㻡㻌㼠㼕㼙㼑㼟㻌㼠㼔㼑㻌㼏㼛㼙㼜㼞㼑㼟㼟㼕㼢㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔㻌

(b) Pile 7B (1.0m in diameter)

Figure 45. Interaction curves of axial load and bending moment of SC pile.

93

6

CONCLUSIONS

The settlement and the load sharing behaviour of piled rafts were investigated by monitoring full-scale structures for a long term. Through the investigation, the following conclusions can be drawn: ・The measured settlements of the large-scale piled rafts were 14 to 24 mm 17 to 72 months after the end of construction on the seven structures completed in 2005 to 2009. The measured settlements were relatively small, less than 30 mm, and within a limited range. ・The ratios of the load carried by the piles to the effective load on the tributary area, αp’, were estimated to be 0.43 to 0.93 at the end of observation. It was found that the value of αp’ generally decreased as the pile spacing ratio was increased. At the pile spacing ratio of larger than about six, the value of αp’ seems to decrease gradually. The ratios of the load carried by the piles were found to be quite stable for a long period after the end of construction. ・During the monitoring period, the 2011 Tohoku Pacific Earthquake struck the sites of the buildings. As for the two buildings in Ibaraki and Tokyo, no significant change in the foundation settlements and the load sharing between the raft and the piles was observed after the earthquake. Further research is required to develop more reliable seismic design methods for piled rafts particularly in highly seismic areas such as Japan. 7

ACKNOWLEDGEMENTS

Dr. J. Hamada of Takenaka R&D Institute and Mr. T. Yamada of former Takenaka R&D Institute were involved in various aspects of design and field measurements. Messrs. N. Nakayama, H. Ito, H. Abe, Y. Soga and H. Yamamoto of building design departments of Takenaka Corporation were involved in the foundation design for the projects. The author would like to thank them all for their contribution to this work. The author also would like to thank Dr. M. Kakurai from Pile Forum Co., Ltd for his valuable assistance and useful discussions. REFERENCES Architectural Institute of Japan (2001). “Recommendations for Design of Building Foundations”, 7.2 Piled raft foundation, pp. 339-348 (in Japanese). Burland, J.B., Broms, B.B. and de Mello, V.F.B. (1977). “Behaviour of foundations and structures”, Proc. 9th ICSMFE, Vol.2, pp. 495-546.

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Hamada, J., Tsuchiya, T. and Yamashita, K. (2009). “Theoretical equations to evaluate the stress of piles on piled raft foundation during earthquake”, J. Structural Const. Eng. (AIJ), Vol. 74, No. 644, pp. 1759-1767 (in Japanese). Hashimura, H., Uryu, M., Yamazaki, T., Nakanishi, R., Kirita, F. and Kojima, K. (2011). “A study on the response characteristics of base-isolated structures for earthquake motion (Part 1 & Part 2) ”, Proc. Annual Meeting of AIJ, pp. 621-624 (in Japanese). Kakurai, M., Yamashita, K. and Tomono, M. (1987). “Settlement behavior of piled raft foundation on soft ground”, Proc. 8th ARCSMFE, pp. 373-376. Kakurai, M. (2002). “Study on load-transfer mechanism of vertically loaded piles”, Thesis, Tokyo Institute of Technology (in Japanese). Katzenbach, R., Arslan, U. and Moormann, C. (2000). “Piled raft foundation projects in Germany”, Design applications of raft foundations, Hemsley J.A. Editor, Thomas Telford, pp. 323-392. Mandolini, A., Russo, G. and Viggiani, C. (2005). “Pile foundations: Experimental investigations, analysis and design”, Proc. 16th ICSMGE, Vol.1, pp. 177-213. Mendoza, M. J., Romo, M.P., Orozco, M. and Dominguez, L. (2000). “Static and seismic behavior of a friction pile-box foundation in Mexico City clay”, Soils & Foundations, Vol.40, No.4, pp. 143-154. Poulos, H.G. (2001). “Piled raft foundations: design and applications”, Geotechnique 51, No.2, pp. 95-113. Randolph, M. F. (1994): Design methods for pile groups and piled rafts, Proc. 13th ICSMFE, pp. 61-82. Tokimatsu, K. and Yoshimi, Y. (1983). “Empirical correlation of soil liquefaction based on SPT N-value and fines content”, Soils & Foundations, Vol.23, No.4, pp. 56-74. Tokimatsu, K., Mizuno, H. and Kakurai, M. (1996). “Building damage associated with geotechnical problems”, Special Issue of Soils & Foundations, pp. 219-234. Yamashita, K. and Kakurai, M. (1991). “Settlement behavior of the raft foundation with friction piles”, Proc. 4th Int. Conf. on Piling and Deep Foundations, pp. 461-466. Yamashita, K., Kakurai, M. and Yamada, T. (1994). “Investigation of a piled raft foundation on stiff clay”, Proc. 13th ICSMFE, Vol.2, pp. 543-546. Yamashita, K., Yamada, T. and Kakurai, M. (1998). “Simplified method for analyzing piled raft foundations”, Proc. the 3rd International Geotechnical Seminar on Deep Foundations on Bored and Auger Piles BAPⅢ, pp. 457-464. Yamashita, K., Yamada, T. and Hamada, J. (2011a). “Investigation of settlement and load sharing on piled rafts by monitoring full-scale structures”, Soils & Foundations, Vol.51, No.3, pp. 513-532. Yamashita, K., Hamada, J. and Yamada, T. (2011b). “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, pp. 1-10. Yamashita, K., Hashiba, T. and Ito, H. (2012a). “Settlement and load sharing behavior of a piled raft subjected to strong seismic motion”, Proc. 11th Australia - New Zealand Conference on Geomechanics. Yamashita, K., Hamada, J., Onimaru, S. and Higashino, M. (2012b). “Seismic behavior of piled raft with ground improvement supporting a base-isolated building on soft ground in Tokyo”, Soils & Foundations (Acceptance).