1 Design and construction of new seismic retrofit method for pile foundation using steel shell reinforced concrete and ground improvement -Application for the pile foundation of Tokyo Monorail- Satoshi Matsuki 1 , Masanori Shibata 2 , Katsutoshi Fujisaki 3 , Takahiro Arai 4 , Takashi Obara 5 and Masayuki Ishido 6 1 Chief engineer, Kajima Corporation, Civil Engineering Design Division 2 Construction Manager, Kajima Corporation, Tokyo Civil Eng. Branch 3 Deputy Manager, Kajima Corporation, Civil Engineering Administration Division 4 Senior Researcher, Kajima Research Institute, Civil Structure Group 5 Researcher, Kajima Research Institute, Soil and Ground Group 6 President, Monorail Engineering ABSTRACT: In recent years, the necessity for the seismic retrofit to the foundation of existing structures is increasing. In carrying out seismic retrofit to the pile foundation of Tokyo Monorail, there were difficulties such as narrow construction site due to construction in the canal and the limited head room under the railway in service. In order to solve these problems, the seismic retrofit method for pile foundation using steel shell reinforced concrete and ground improvement was developed. In this construction method, seismic performance is enhanced by reinforcing the pile heads of the foundation with steel shell and concrete, and by reinforcing the middle and bottom of piles with ground improvement, which increases horizontal resistance. This construction method was applied to the pile foundation of Tokyo Monorail and the construction was completed without any problems. 1 INTRODUCTION With the occurrence of great earthquakes such as the 1995 Hyogoken-Nambu earthquake and the 2011 off the Pacific Coast of Tohoku earthquake, there has been an increasing need for seismic retrofit of existing structures and their foundations. In the seismic retrofit of pile foundations of railway bridge piers in urban areas, however, only a limited number of methods are available because of the restrictions on the site or on the overhead clearance. A seismic retrofit method was developed for application to the bridge pier foundations in a canal under the Tokyo Monorail to
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Design and construction of new seismic retrofit method for pile foundation using steel shell reinforced concrete and ground improvement
-Application for the pile foundation of Tokyo Monorail-
1Chief engineer, Kajima Corporation, Civil Engineering Design Division 2Construction Manager, Kajima Corporation, Tokyo Civil Eng. Branch 3Deputy Manager, Kajima Corporation, Civil Engineering Administration Division 4Senior Researcher, Kajima Research Institute, Civil Structure Group 5Researcher, Kajima Research Institute, Soil and Ground Group 6President, Monorail Engineering
ABSTRACT: In recent years, the necessity for the seismic retrofit to the foundation of existing structures is increasing. In carrying out seismic retrofit to the pile foundation of Tokyo Monorail, there were difficulties such as narrow construction site due to construction in the canal and the limited head room under the railway in service. In order to solve these problems, the seismic retrofit method for pile foundation using steel shell reinforced concrete and ground improvement was developed. In this construction method, seismic performance is enhanced by reinforcing the pile heads of the foundation with steel shell and concrete, and by reinforcing the middle and bottom of piles with ground improvement, which increases horizontal resistance. This construction method was applied to the pile foundation of Tokyo Monorail and the construction was completed without any problems.
1 INTRODUCTION
With the occurrence of great earthquakes such as the 1995 Hyogoken-Nambu earthquake and the 2011 off the Pacific Coast of Tohoku earthquake, there has been an increasing need for seismic retrofit of existing structures and their foundations. In the seismic retrofit of pile foundations of railway bridge piers in urban areas, however, only a limited number of methods are available because of the restrictions on the site or on the overhead clearance. A seismic retrofit method was developed for application to the bridge pier foundations in a canal under the Tokyo Monorail to
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solve the above problems. This paper describes the construction and design of the method that improves seismic performance by increasing the horizontal resistance of piles by reinforcing the pile heads using steel shells and concrete and by improving the ground in which piles are embedded (hereinafter referred to as the “ground improvement method by the reinforcement with steel shells and concrete).
Photo 1. Foundation of viaduct of Tokyo Monorail in the canal
2 OUTLINE OF THE METHOD
The seismic retrofit method is applicable to group pile foundations. A structural outline of the method and the ground conditions are shown in Figure 1. Organic soft and silty soils with an N value of 0 through 1 are distributed from the sea bottom to the bearing layer. The method comprehensively reinforces all of the pile heads by combining a steel shell and concrete at the head, and ensures the safety of piles against shear and flexural failures. The soils around the pile are strengthened by improving the soils using jet grouting to increase the horizontal resistance of piles and the vertical resistance of soils at the bottom surface of reinforcing concrete and to reduce the stress occurring in the pile body during an earthquake. Thus, safety is ensured against the shear and flexural failures of piles.
The ground is improved while a platform is on the existing footing. Jet grout columns are therefore arranged in the shape of a doughnut. Overlapping jet grout columns makes an integrated caisson-type foundation after retrofit. The steel shell also provides earth retention and temporary coffering functions. Then, the material costs can be reduced.
FIG. 1. Structural outline and soil conditions
A.P.-3.280
5,514
3,750
3,250
8,750
886
850
A.P.-7.030
A.P.-10.280
ExistingPile
Section(B-B)
Steel Shell
Concrete
Jet grout columnf 3500
A.P.+2.234
103007600
f 700A.P.-21.766A.P.-20.880
A.P.+2.1m(H.W.L)A.P.±0.0m(L.W.L)
SoilImprovement
Concrete
Steel Shell
Gravel
Organic Soft Soil
Silty Organic Soft Soil
Sand
Silt
Seabottom
Section(A-A)
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3 DEVELOPMENT OF THE METHOD
3.1 Examination of the design method The method is applied to a new structure. The design method, therefore, had to be
built based on existing design criteria. Examined in this study was a design method that was in compliance with the “Design Standards for Railway Structures”(RTRI) for application to the bridge pier foundations of the Tokyo Monorail. After the ground was improved, the entire foundation was expected to be like a caisson foundation. The positions would be explicit where the mode of vibration would be relatively simple and non-linear. Design was therefore developed based on the nonlinear spectrum method. A flow of design steps is shown in Figure 2. In the nonlinear spectrum method, response ductility factor is calculated based on the designated seismic coefficient spectrum in the yield state using the equivalent natural period and seismic coefficient in the yield state that were obtained in static nonlinear analysis. The responses of the bridge pier body, pile and soil cylinder were calculated to evaluate seismic safety. A prerequisite for forming a structure was “no reduction of compressive load-carrying capacity in the case of cracking in jet grout columns under alternate loading and unloading during an earthquake”. Verifications were made in the tests described in Section 3.2.
FIG. 2. Design flow chart
3.2 Performance evaluation of jet grout columns in testing (1) Examination of test method
In loading tests using a scale model that simulated the structure, reproducing the surrounding ground was difficult in terms of scale. Then, a two-dimensional FEM analysis was performed to examine whether the stress condition equivalent to that in the actual structure could be reproduced or not in a beam-shaped specimen at the positions subjected to severe compressive stress in the actual structure (evaluation positions). (For the analysis method, refer to Section 4.1). As a result, it was verified that the stress condition at the positions could generally be reproduced by adjusting
Definition of present situation
Determination of structural dimensions of existing structures
Determination of soil conditions and design seismic forces
Assumption of reinforcement measures and cross section
Preparation of nonlinear static analytical model for whole structural system
Determination of load-displacement relationship for whole structural system
Response calculation in reinforced structure
Seismic safety evaluation of reinforced structure
Check results for elements
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START
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the point of loading on the beam-shaped specimen (shear span ratio).
FIG. 3. Analytical results on loading position
(2) Specimen
An outline of the specimen and the points of loading are shown in Figure 4. The specimen was an approximately 1/6 scale model of the actual structure. The half cross section was modeled. The specimen was 670 mm in width, 950 mm in column height and 5,890 mm in length. Steel shells with a thickness of 2.0 mm and a length of 1,550 mm were attached at both ends of the specimen. Reinforcing concrete was applied in 930mm-long areas and jet grout columns that were generated using fluidized soils in 620mm-long areas in the steel shell. Six prestressing bars (diameter: 36 mm, length: 5,790 mm) simulating prestressed concrete piles in the actual structure were arranged in the specimen. (The axial stiffness and flexural stiffness were set at the level equivalent to that in the prestressed concrete piles.) The unconfined compressive strength of the jet grout columns placed in the specimen during the loading test was 2.5 N/mm2 and the compressive strength of reinforcing concrete was 50.1 N/mm2.
FIG. 4. Outline of specimen and points of loading
(3) Loading method and parameters measured
The loading method is shown in Figure 5. Loading was applied based on a failure load Pd = 183.6 kN, which was obtained by calculation in a preliminary two-dimensional FEM analysis. The load was incremented in nine steps from a failure load of (i) Pd/3 = 61.2 kN to (ii) Pd/2 = 91.8 kN, (iii) Pd/1.5 = 122.4 kN, (iv) Pd = 183.6 kN, (v) Pd x 1.5 = 275.4 kN, (vi) Pd x 2.0 = 367.2 kN, (vii) Pd x 2.2 =
Steel shell
Evaluation positions
Actual structure
Beam-shaped specimen
She
arin
g sp
an
Grouted SoilConcrete
Steel Shell
Unit
SoilMix
High-Strength Steel Bar
half section
Cross section PC steel bar
Cross section
Plan
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403.9, (viii) Pd x 2.4 = 440.6 kN and (ix) Pd x 2.6 = 477.4 kN. In the preliminary FEM analysis, the design strength of the soil cylinder was set at 1.3 N/mm2. Loading and unloading were applied three times at each step. Measured were the load applied, displacement, strain of the soil cylinder and the state of cracking of jet grout column.
FIG. 5. Loading method
(4) Test results
As the main data, the load-displacement relationship, the ultimate condition of cracking (at the final step) and the relationship between the load and the strain of soil cylinder are shown in Figures 6 through 9. 1) Failure condition
Figures 6 and 7 show the deformations (cracking and crushing) of the soil cylinder induced by loading. Loading caused negative flexural cracking in the first round of unloading at step (i). Next, positive flexural cracking occurred in the second round of loading at step (i). Subsequently, positive and negative flexural shear cracking occurred in this order at step (ii). At step (iii), positive bond splitting occurred along the prestressing steel and then the negative one occurred. Finally, positive compressive failure occurred in the second round of loading at step (ix) and the ultimate state was reached. In the ultimate state, no brittle failure occurred and cohesive behavior with ductility was confirmed. 2) Failure safety factor
The design failure load Pd when the soil cylinder had a design strength of 1.3 N/mm2 was 183.6 kN as described earlier. The unconfined strength of the soil cylinder during the test was 2.5 N/mm2. The design failure load Psd was set at 183.6 x 2.5/1.3 = 353.1 kN (Figure 6). (Nonlinearity of materials was considered in the analysis but proportion (an elastic body) was applied in this study for evaluation conservatively.) Compared with the design failure load Psd, the maximum load in the test was 480.9 kN. Then, a failure safety factor of 480.9/353.1 = 1.36 was confirmed. 3) Strain of soil cylinder
Figures 8 and 9 show the relationship between the load and the strain of the soil cylinder at the top and bottom edges and on sides.
The data collected at the measurement points where the maximum values were obtained at the top and bottom edges and on sides were selected (Figure 7). The mean failure strain of the soil cylinder (in compression) obtained in an unconfined compression test was -3,950 x 10-6. The strain was first exceeded at -387.9 kN during
-400 -300 -200 -100
0 100 200 300 400
0 10 20 30 40 50 60 70
1) 61.2 (fracture load/3)
2) 91.8 (fracture load/2)
3) 122.4 (fracture load/1.5
4) 183.6 (fracture load)
5) 275.4 (fracture load*1.5)
until fracture
Loading steps
Load
(kN
)
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the first round of unloading at step (vii) at the bottom edge on the side. Subsequent cyclic loading that was applied beyond the failure strain area did not immediately cause the load bearing capacity to decrease. Sufficient load bearing capacity in compression was verified.
FIG. 6. Load-displacement relationship
FIG. 7. Cracking in jet grout column at ultimate state
FIG. 8. Load-strain curve of jet grout column (top and bottom)
FIG. 9. Load-strain curve of jet grout column (side)
Crack induced by positive loading Crack induced by negative loading
Shear and flexural crack flexural crack
Strain gauge
Compression failure Bond splitting crack along the prestressing steel
Side
Bottom
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-6000 -5000 -4000 -3000 -2000 -1000 0 1000
荷重[kN]
ひずみ [×10-6] （引張）（圧縮）
平均破壊ひずみ -3,950Average Failure Strain -3,950
(Compression) STRAIN [×10-6]
LOA
D [k
N]
(Tension)
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4 SEISMIC RETROFIT DESIGN FOR THE BRIDGE PIER FOUNDATION PILES AT THE TOKYO MONORAIL
4.1 Analytical model A static nonlinear analysis was adopted considering the material nonlinearity in a
two-dimensional FEM model. The elements constituting the structure and a cross section are shown in Figure 10. The bridge pier, piles, footing and the front and rear sides of the steel shell were modeled using beam elements. The reinforcing concrete and jet grout columns were modeled by solid elements. The steel shells on sides were modeled by shell elements. The ground was modeled using nonlinear springs. The analytical model is shown in Figure 11. No-tension springs were adopted as soil springs in the direction perpendicular to the surface in jet grout columns, on the bottom surface of the pile and the front and rear surfaces of the soil cylinder for consideration conservatively.
In the analysis, the tensile stress occurring in the soil cylinder was ignored. The bond between the pile and reinforcing concrete and that between the pile and soil cylinder were ignored conservatively.
FIG. 10. Analytical model
FIG. 11. Soil spring model
4.2 Check method
The safety of members was checked based on the stress resultant occurring in
鋼殻
地盤改良体
5,800
1,423
2,954
1,423
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1,599 4,402 1,599
地盤改良体
8,052
2,549
2,954
2,549
9,174
2,386 4,402 2,386
鋼殻
補強
5,800
7,600
橋脚（ビーム要素）
フーチング
鋼殻・前背面
PC杭
補強コンクリート
改良体・上部
改良体・下部
鋼殻・側面
地盤
（ソリッド要素）
（ソリッド要素）
（ビーム要素）
（ビーム要素）
（ソリッド要素）
（シェル部材）
（ビーム要素）
（ソリッド要素）
（換算断面）
（換算断面）
ｺﾝｸﾘｰﾄ
Pier(Beam Element)Pile-cap
Steel Shell/Front,Back
PC Pile
Concrete
Upper Soil Improvement
Lower Soil Improvement
Steel Shell/Side
Ground
(Solid Element)
(Beam Element)
(Beam Element)
(Solid Element)
(Shell Element)
(Beam Element)
(Solid Element)
(Solid Element)
SteelShell
Jet Grout Column(Equivalent Section)
Jet Grout Column(Equivalent Section)
kSVB
kSVB
kSVB
kSVB
kH
kH
kV
PCPile
SoilImprovement
▽A.P.-7.030
▽A.P.-10.280
▽A.P.-19.030
YC1(Silty OrganicSoftSoil)
N=0
YC2(Silt)
N=1
▽A.P.-19.880
▽A.P.-20.766
Yg (Sand)N=25
Tog (Gravel)N=50
kSHD
kSVD
kSHD
kSVD
kSHD
kSVD
kSHD
kSVD
kSHD
kSVD
kSHD
kSVD
kH
Front
kS
kSVB
kSVB k H
kVkS
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each element of structures. Parameters that were checked in members are listed in Table 1.
Table 1. Summary of seismic safety assessment Element Specification Determinant Check Results
Joint Plate SM490 t=12mm Temporary - 216.3cm2 Required Area
171.9cm2
Bolt 12-M20 (F10T) Temporary - 0.79 1.0
4.3 Seismic retrofit design
The seismic retrofit effect of the method was examined in the case where it was applied to the bridge pier foundation of the Tokyo Monorail. Horizontal seismic coefficient-horizontal displacement curves before and after retrofit are shown in Figure 12. In the structural response analysis before retrofit, yielding occurred at a horizontal seismic coefficient of approximately 0.4. In response to a level-2 earthquake, the horizontal seismic coefficient was 0.54 and the horizontal displacement was 622 mm. It was found in the structural response analysis after retrofit that the seismic coefficient in the yield state was approximately 0.8, nearly double that before retrofit. As for the preceding failure mode, piles yielded before retrofit but bridge piers yielded after retrofit (Figure 13). Thus, the restoration capacity increased.
FIG. 12. Load-displacement curve before and after seismic retrofit
0
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HorizontalSeismicCoeficientKh
D isp(m m )
kh-δC urve（LongitudinalD irection）
Yield PointDisp：402mm
kh=0.79
Yield PointDisp：296mm
kh=0.41
Shear FailureDisp：456mm
kh=0.51
ResponseDisp：622mm
kh=0.54
Afterseism ic
retrofitBeforeseism icretrofit
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FIG. 13. Change in failure mode after seismic retrofit
5 IMPLEMENTATION OF THE METHOD
5.1 Implementation conditions The seismic retrofit work was carried out for bridge piers of the Tokyo Monorail
that were located in the Keihin Canal. The water depth was approximately 4 m and the depth from the sea bottom to the bearing later (gravel layer) was approximately 17 m at the site. The total length of the pile that carried the footing was approximately 22 m including the depth of pile embedment. A head clearance of 3 m was available as the construction work was done below railway tracks in service. 5.2 Implementation method
Figure 14 shows a flow of implementation steps.
FIG. 14. Implementation steps
(1) Installation of steel shell
A steel shell was composed of pieces with a maximum height of 700 mm. Sixteen layers were assembled in the field and immersed using their self weights. The steel shell was immersed to the bottom end of the mud layer to reinforce pile
A.P.-3.280
5,514
3,750
3,250
8,750
886
850
A.P.-7.030
A.P.-10.280
A.P.+2.234
103007600
f 700A.P.-21.766A.P.-20.880
A.P.+2.1m(H.W.L)A.P.±0.0m(L.W.L)
SoilImprovement
Concrete
Steel Shell
Gravel
Organic Soft Soil
Silty Organic Soft Soil
Sand
Silt
Seabottom
Yeild zone before seismic retrofit Yeild zone after
seismic retrofit Load
Construction machine Existing pier
Pier foundation Steel shell
Reinforcing concrete
Slab concrete
Jet grout column (upper part)
Jet grout column (lower part)
Clearance 3m
Seabottom Organic soft soil
Slity organic soft soil
Silt
Sand gravel (N>50)
JETCRETE method
Rod drawing up by rotating
4m
17m
1m
22m
A.P.+2.1m(H.W.L.)
A.P.±0.0m(L.W.L.) Installation of steel shell
Placing reinforcing concrete
Placing bottom slab concrete
Construction of Jet grout column (upper part)
Construction of Jet grout column (lower part)
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heads and to prevent the ground improvement material jetted in ground improvement work from flowing out. In the case where immersion was difficult because of great resistance to penetration, water jetting at the tip of the steel shell was employed as an auxiliary method. In the steel shell, monitoring pipes were inserted for ground improvement and guide pipes were installed for collecting sludge. The diameter of the piping was set at 200 mm. It was verified in a full-scale test conducted separately that no sludge discharged caused any clogging. (2) Concrete placement in bottom slab
Bottom slab concrete of a thickness of 150 mm was placed under water at the sea bottom (top end of the soft soil layer) in the steel shell. The objective was to prevent the blowout of sludge and ensure that the sludge was led to the guide pipe. (3) Construction of jet grout columns
For improving the ground, the JETCRETE method, a kind of jet grouting method was adopted. In the method, small monitoring pipes are used that can be attached to 45mm-diameter boring rods. The construction machinery was smaller than in conventional jet grouting methods. Construction was carried out smoothly even in a narrow space as at the construction site in this study (Photograph 2).
In order to increase the efficiency of collecting the sludge that was produced during the application of JETCRETE, the upper section of the soil cylinder (in the mud in the steel shell) was preliminarily generated. Eight jet grout columns were constructed in the sequence shown in Figure 15 avoiding continual construction of adjacent jet grout columns to minimize the effect of construction on prestressed concrete piles. It was of concern that applying a jet grouting method to cohesive soils was likely to cause highly cohesive sludge to clog and blow out to the surrounding ground. Installing the guide pipes enabled smooth collection of sludge.
Photo 2. Construction machine
FIG. 15. Layout plan of jet grout column
Exsisting footing
Jetcrete column (diameter D=3.5m)
Steel shell
Foundation pile
Survey boring
*Numbers shown in the figure corresponds to construction sequence.
1
5
3 8
7 4
6
2
11
(4) Placement of reinforcing concrete Seawater was removed from inside the steel shell above the slab concrete and
concrete was placed. Light-weight (specific weight: 1.75), high-consistency concrete was adopted to prevent the increase of load on existing piles and avoid manual work in the steel shell. 5.3 Quality assessment of jet grout columns
Surveys were conducted to verify the effect of ground improvement on or later than the 28th day after the construction of jet grout columns. Cores were bored at positions 0.3 D (where D is a design diameter of soil cylinder of 3.5 m) away from the center of the soil cylinder in the radial direction. Examples of cores sampled are given in Photograph 3. The rate of core sampling was more than 95% on the average, higher than 90% (AIJ), a target for quality check in the deep mixing method.
Figure 16 shows the results of unconfined compression test conducted for cores. The mean unconfined compressive strength qu was 5.3 N/mm2 (minimum value: 1.9 N/mm2) while the design strength was 1.3 N/mm2. The Technical Guidelines for the deep mixing method (AIJ) concerning the core strength of soil cylinder and actual strength Qu stipulate that Qu = qu - 1.3σ (where σ is the standard deviation on the assumption of a normal distribution). The actual strength Qu estimated based on the test results was 2.4 N/mm2. The coefficient of variation of core strength was 42% and in the range in which the above equation was applicable. It was then determined that the actual strength was higher than the design strength. The percentage of the core strengths lower than the design strength (defective fraction) was 5% or less. In view of the fact that strength varies greatly because of the characteristics of the method, it is important to make a quantitative evaluation of the effects of strength fluctuations on the strength characteristics of the entire jet grout system.
Figure 17 shows the relationship between qu and the modulus of deformation E50. E50 is in the range between approximately 200 and 400 times qu. The relationship is similar to the general relationship for cement improved soils. E50 is 1602 N/mm2 on the average (σ = 664), well above the design value of 700 N/mm2.
It was thus verified that high-quality jet grout columns that met the performance requirements could be constructed using the JETCRETE method.
Photo 3. One of the core samples (colored by phenolphthalein reaction)
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FIG. 16. Result of unconfined compression test
FIG. 17. Relationship between qu and the modulus of deformation E50
6 CLOSING REMARK
The method was applied to the seismic retrofit of bridge piers of an existing railway. Construction work was carried out for four bridge piers in various work types over an eight-month construction period. The displacement of tracks was constantly monitored during the construction work. No displacement occurred due to construction and no adverse effects were incurred on the operation of the railway services. It was verified that adopting the JETCRETE method requiring no large machinery provided the ease of construction under the railway in service and enabled the construction of high-quality jet grout columns. The authors will further rationalize the method based on the results obtained in this project and will apply the method more widely to bridge piers in rivers.
REFERENCES
Railway Technical Research Institute (RTRI). (1999). “Design Standards for Railway Structures and Commentary -Seismic Design-” Architectural Institute of Japan (AIJ). (2006). “Recommendations for Design of Ground Improvement for Building Foundations”
