Volume 4, Issue 3 September 2019

Special Contribution

Piled Raft Foundation Combined with Deep Mixing Wall Grid
Kiyoshi Yamashita
Executive ManagerTakenaka Research and Development Institute, Chiba, JapanAbstract. An advanced piled raft system, which can be used in very soft clayey soils and/or liquefiable soils, called “piledraft foundation combined with deep mixing wall grid” has been developped. The piled raft system was employed for a12-story building and field monitoring of the soil-structure system was performed. During the monitoring period, the2011 Tohoku Earthquake (M9.0) struck the site (about 380 km from the epicenter). In this paper, the static and dynamic
monitoring results of the advanced piled raft system, and a numerical simulation using a 3-dimentional finite element(FE) model to examine the performance under strong earthquakes were presented.
1. Introduction
In recent years, piled raft foundation, which means a piled foundation combining piles and raft response in a design, hasbeen used in many countries. In Japan, since a basic design framework for piled rafts was established in the early 2000s,
piled rafts have been employed for lots of buildings including high-rise buildings over 200 m in height. At the same time,
field evidence, especially the settlement and the load sharing between piles and raft, was accumulated by monitoring
full-scale structures, and the effectiveness of piled rafts in
reducing average and differential settlements has been
confirmed (Yamashita et al., 2011a).
2. Piled raft combined with ground improvement
The most effective application of piled rafts occurs when theraft can provide adequate load capacity but the settlement
and/or differential settlements of the raft alone exceed theallowable values, hence favorable situations may be soil profiles
consisting of relatively stiff clays and relatively dense sands(Poulos, 2017). On the other hand, unfavorable situations are
profiles with very soft clayey soils and/or requifiable soils near
the surface of the raft, which may be subjected to consolidation
settlement and/or seismic liquefaction. To cope with this, anadvanced piled raft system, i.e., a piled raft foundation
combined with gound imprpvement, has been developed(Yamashita et al., 2011b).
Improved soilcolumns

φ1,000mm

Liquefiable soils
Soft clayey soils
DMW grid(a) Piled raftfoundation

Fig. 1. Advanced piled raft system

Penetration

800mm(b) Piled raft combinedwith DMW grid

Tip treatment

Withdrawing

・Discharge ofsoildified material
・Stirring and mixing

Couple of soil columns lapped andcontinuously arranged

Liquefiable layer
Improved soilcolumns(a) Schematic of DMW grid(b) Construction procedure (2-axle type)Fig. 2. Deep mixing wall (DMW) grid3Volume 4, Issue 3 September 2019
Fig. 1 illustrates the advanced piled raft system called “piled raft combined with deep mixing wall (DMW) grid”. The DMWgrid (illusutrated in Fig. 2), which is expected to restrict the shear deformation of the soil within the grid, is used as acountermeasure of soil liquefaction under the raft as well as to increase the bearing capacity of the raft foundation.
Moreover, the DMWs are expected to resist the inertial force from the structure during strong earthquakes.
3. Static and Dynamic Monitoring of 12-story Building on Soft GroundFig. 3 illustrates a schematic view of a 12-story residensial building and its foundation with a typical soil profile. Thebuilding located in Tokyo is a reinforced-concrete structure with a seismic base-isolation system. The total load in thestructural design was 198.8 MN (average pressure over the raft was 200 kPa). The subsoil consists of very soft to medium
clayey soil layers to a depth of 43 m, underlain by dense sandy layers. The groundwater table appears about 1.8 m below
the ground surface, and it was ascertained that the silty sand near the ground surface had a potential for liquefaction
with a peak ground acceleration (PGA) of 0.2g.
Hence, to prevent liquefaction of the silty sand as well as to provide adequate load capacity of the raft, a piled raftcombined with DMW grid (which was extended to a depth of 16 m with the bottom being embedded in the stiffer siltyclay) was employed. Sixteen PHC piles, whose toes reached the very dense sand-and-gravel layer, were used to reduce
the settlement to acceptable levels. Fig. 4 illustrates the foundation plan with the layout of the piles and the DMW grid(area replacement ratio is 0.25). The effectiveness of the piled raft system was confirmed by comparing performance andcost with conventional large-diameter cast-in-place concrete pile foundation.
Field monitoring on the settlement and the load sharing between piles and raft was performed both statically anddynamically (Yamashita et al., 2012). The locations of the monitoring devices are shown in Fig. 4. Two piles, 5B and 7B,
were provided with LVDT strain gauges. Fig. 5 shows the measured vertical ground displacement just below the raft after
the casting of the raft. The vertical ground displacement might be approximately equal to settlement of the raft, and itrefers as raft settlement here. Fig. 6 shows the time-dependent vertical load sharing among the piles, the DMWs, the soil
and the buoyancy in the tributary area of columns 5B and 7B indicated in Fig. 4. During the monitoring period, on March
11, 2011, the 2011 Tohoku Earthquake (M=9.0, about 380 km from the epicenter) struck East Japan, and the seismic
responses of the 12-story building were successfully recorded. The PGA recorded at the site was 0.18 g as shown in Fig.
7. Just before the earthquake, the raft settlement was 17.3 mm, and the ratio of the load carried by the piles to the netload (the gross load minus the buoyancy) was estimated to be 0.67 where the ratio of the net load carried by the DMWs
to the net load was 0.26, fairly greater than that carried by the soil. After the earthquake on March 15, the raft settlement
increased by only 0.3 mm to 17.6 mm, and thereafter, the raft settlements ranged from 17 to 18 mm. Furthermore, almost
no change in load sharing among the piles, the DMWs and the soil was observed after the earthquake. Thus, both theraft settlement and the load sharing were quite stable after the earthquake.
1.2m

Instrumented pile

1.0m

Earth pressure cell

0.8m

Piezometer

12FL

Settlement gauges

FillSilty sand

4.8m

BFL 5.8m

F

Borehole-type
triaxial accelerometer
6.0m◆ A3（GL-50m）

8.8m

1FL

◆ A1（GL-1.5m）

0.8m

Triaxial
accelerometer

38.7m

GL±0

S-wave velocity
Vs (m/s)
0500 1000◆ A2（GL-15m）

Accelerometer◆SPT
N-Value050

2.5m 2.5m

Monitoring devices

8.0m

Pile diameter

1.5m

D4050Clayey silt

16.0m 16.0m

15.0m

27.1m

E1B

5B
D1E2E3E5D2E4E67B

Silty clay

Clayey silt
Sandy silt
SandSand andgravel

60 Sand Sandand gravelSand

10.55mDepth (m)Depth (m)3016.0m
DMW grid

10.3m

Silty clay

2033.25m

1048.0m 46.5m
50.0m

Tributary area

Y50.0mA①60.0mSettlement gauges③9.025m

A8.4m

X⑤⑦9.025m

30.05m70Fig. 3. Schematic of 12-story building andfoundation with soil profile

Fig. 4. Foundation plan with layout of piles andDMW grid4Volume 4, Issue 3 September 2019

Tohoku Earthquake
Mar. 11, 2011

E.O.C.
Sep. 16, 2008

Feb. 28, 2017-20

50-1040030Load (MN)

Vertical ground disp. (mm)E.O.C.
Sep. 16, 2008

1020Raft settlement

Tohoku Earthquake
Mar. 11, 2011

Feb. 28, 2017

Design column loads 36.0MN

Piles

20DMWs10RaftSoil
Buoyancy

300500

1000

1500
2000
Time (days)

2500

3000

03500

0Fig. 5. Measured vertical ground displacements justbelow raft

500

1000

1500
2000
Time (days)

2500

3000

3500

Fig. 6. Load sharing among piles, DMWs and soil in thetributary area

Acceleration (m/s2)

Fig. 7 shows the acceleration time histories recorded near
the ground surface, on the raft and the 1st and 12th floors
in the EW direction. The duration of the seismic motion
was longer than 600 s. The peak acceleration on the 1stfloor was significantly reduced compared to that on theraft, which indicates that the base-isolation system acted
quite well. In addition, the peak acceleration on the raft
was clearly reduced compared to that near the groundsurface due to the input losses caused by the dynamic soilstructure interaction.
Fig. 8 shows the relationship between the axial force andthe bending moment measured at the pile head of Pile 5B,
which were obtained by combining the components in NSand EW directions. In order to check the performance ofthe piles, the design interaction curves of the SC pile(which was used for top portion) are also shown in Fig. 8.
It was found that the increments in axial force wereremarkably small due to the action of the base-isolation
system, and that the measured bending moments werefairly small compared to the damage limit state values (theunit stress at the edge of the concrete is almost in theelastic condition).

Base isolation

Input losses

Fig. 7. Acceleration time histories recorded on theground and structure (EW)

4. Performance Under Strong Earthquakes throughNumerical Simulation

15
Bending moment (MNm)

To examine the seismic performance of the piled raftsystem under strong earthquakes, a numerical simulation
was conducted using a nonlinear 3-dimentional FE model
illustrated in Fig. 9 (Yamashita et al., 2018). As strong
earthquake motions for the present analysis, input motionof Level 2 earthquake (with mean return period ofapproximately 500 years) was employed. Fig. 10 shows thecode-defined acceleration response spectrum of the Level
2 motion on the engineering bedrock. Fig. 11 shows theacceleration time history of the input motion using Kobephase data on the bedrock (2E). The input motion wasapplied horizontally in the NS direction at a depth of 75 m.

Ultimate
limit
10Damagelimit
5

Measured
0-15

01530
45
Axial load (MN)

6075Fig. 8. Relation of axial force with bending
moment (Pile 5B)
5Volume 4, Issue 3 September 2019

10.0

Acceleration (m/s2)

5% damping
8.0

Level 2 (target)

6.0
2011 Tohoku
EW

4.0
2.0

NS0.0
0.01

0.1

1

10

Period (s)Fig. 10. Acceleration response spectra of Level 2
motions and 2011 Tohoku Earthquake on bedrock

Fig. 9. 3-dimentional FE model

Acceleration (m/s2)

To verify the deformation parameters of the DMWs
5.0
max = (2.90, 36.61s)
and the soil and to validate the simulation method, a
2.5
numerical simulation was carried out using themoderate earthquake motion recorded during the0.0
2011 Tohoku Earthquake. The response spectra of the-2.5
input accelerations on the bedrock (which wasmin = (-3.56, 33.55s)-5.0
derived from the records at a depth of 50 m) is shown
0204060
80100in Fig. 10. The peak horizontal displacement profiles ofTime (sec)the ground and superstructure at the center and thepeak bending moment profiles of Pile 5B are shown inFig. 11. Input acceleration time history (Kobe phase)
Fig. 12, compared with the observed values. The
displacements are relative ones to the reference point
at a depth of 50 m. It is seen that the simulated values of ground and structure response and those of pile bending
moment agree well with the observed ones, and that the dynamic 3-dimentional FE simulation method is capable ofreproducing the observation records and that the deformation parameters of the unimproved and the stabilized soils atrelatively small strains were appropriate.
Then, the seismic response analyses under Level 2 earthquakes were conducted, where numerical cases without theDMWs were also conducted. Fig. 13 shows the peak bending moment profiles of Pile 5B. The bending moment near thepile head in the case with DMWs was significantly small in comparison with that in the case without DMWs. To clarify thedifference in pile bending moment in the cases with and without DMWs, the ground deformation and the shear force
acting at the pile head during the shaking were examined.

Acceleration (m/s2)
-1.0
0.0
1.0

2.0

15

DMWs

45

Depth (m)

30

DMWs

30

2030

45
40

60

60
Simulation

Observed (Structure)
Observed (Ground)(a) Acceleration

0.6

101515Depth (m)Depth (m)DMWs

Bending moment (MNm)-0.4 -0.2 0.0 0.2 0.4

300075-0.60104530

15Displacement (cm)-505-10Height (m)Height (m)-2.04575

Pile 5B
Simulation

Observed (Structure)
Observed (Ground)(b) Displacement

50
Simulation

Observed(c) Bending moment

Fig. 12. Profiles of peak response of ground and structure and pile bending moment (EW)6

Volume 4, Issue 3 September 2019
Fig. 14 shows the profiles of the peak horizontal displacement of the DMWs and ground under the mid-side of the raft.
It is seen that the displacements near the raft bottom of the DMWs were markedly smaller than those of the ground in
the case without DMWs. Fig. 15 illustrates the equilibrium of lateral external forces and resistant forces at the raft bottom
when the bending moment of Pile 5B was at its maximum. Regardless of the presence of the DMWs, the inertial forcesof the superstructure were very small compared to the earth pressure on the raft side. This occurred because of theaction of the base-isolation system. In the case with DMWs, the external forces were carried mostly by the DMWs andthe load carried by the piles was much smaller than that in the case without DMWs. In addition, it is noted that the tensile
failure of the DMW grid seemed to be limited to the bottom portion as illustrated in Fig. 16.
Thus, both the ground displacement near the raft bottom and the shear force acting at the pile head were decreased by
the presence of the DMWs. As a result, the bending moments near the pile head were significantly smaller than those in
the case without DMWs.

-10

Bending moment (MNm)-5051015-30001015

Depth (m)Depth (m)-152030Z

Pile 5B
DMWs
no DMWs

40-20

Displacement (cm)-1001020303045Z60

DMWs
no DMWs

M＋
Y(N)

Y(N)

7550

Fig. 14. Peak displacement profiles of DMWs andground under the mid-side of raft

Fig. 13. Peak bending moment profiles (Pile 5B)

Superstructure

Superstructure

3.0

4.7

Shaking
direction

Z
M＋

No. of failure
Gauss points
8
7,6
5,4
3,2,10Y(N)(Unit MN)

Raft

Raft

-17.5
Piles Soil
1.8 9.5

-49.9
DMWs
53.1

-15.4

-25.3

Piles Soil
14.8 21.1

Y(N)(a) DMWs

X(E)(b) no DMWs

Fig. 15. Equilibrium of lateral forces at raft bottom
when bending moment (Pile 5B) was at its maximum

Fig. 16. Extent of tensile failure in DMW grid7

Volume 4, Issue 3 September 2019
5. Concluding Remarks
Based on the static and dynamic monitoring of the 12-story building, it was found that the piled raft combined with DMW
grid showed a good performance under the moderate seismic motion as well as ordinary conditions. In addition, throughthe numerical simulation using a 3-dimentional FE model under strong earthquakes, the ground displacement near theraft bottom and the shear force acting at the pile head were found to be significantly decreased by the presence of thegrid. This indicates that the DMW grid was quite effective in reducing the sectional force of the piles in the piled raftsystem.
The DMW grid could be designed more rationally by following the principles of a performance-based design. The reason
is that the DMW grid can be regarded as supplementary structural elements in the foundation system, and minor damage
to the grid can be tolerated under strong earthquakes, provided that the required foundation performance has beensatisfied.
REFERENCES
Poulos, H. G. 2017. Tall Building Foundation Design, CRS Press.
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 (3), 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 (2), 1-10.
Yamashita, K., Hamada, J., Onimaru, S. and Higashino, M., 2012. Seismic behavior of piled raft with ground improvement
supporting a base-isolated building on soft ground in Tokyo, Soils & Foundations, Vol. 52 (5), 1000-1015.
Yamashita, K., Shigeno, Y., Hamada, J. and Chang D. W., 2018. Seismic response analysis of piled raft with grid-form deep
mixing walls under strong earthquakes with performance design concerns, Soils & Foundations, Vol. 58 (1), 65-84.

◆ A brief CV of Dr. Kiyoshi Yamashita
Dr. Yamashita obtained his Bachelor and Master of Engineering from Tokyo Institute ofTechnology. He joined Takenaka Corporation in 1977 as a research engineer. He was a deputy
general manager in Takenaka R&D Institute from 2009 to 2012. His reserch interests are designand performance of piled raft foundations under both ordinary and seismic conditions. He
engaged in foundation design of around 50 buildings in which piled rafts were employed. He
also planned and executed long-term field monitoring of the selected piled rafts. He has
published more than 150 technical papers including more than 20 journal articles and 40
conference papers. He received the JGS (Japanese Geotechnical Society) Awards in 1996, 2013
and 2018.

8