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STUDY ON A LIQUEFACTION COUNTERMEASURE FOR FLUME STRUCTURE BY SHEET-PILE WITH DRAIN KAZUTAKA OTSUSHI1 , TOMOO KATO2 , TAKASHI HARA3,a , ATSUSHI YASHIMA3 , YU OTAKE4,b , KAZUHIKO SAKANASHI4 and AYUMI HONDA4 1 Construction Technology Department, Sumitomo Metal Industries Ltd., Sunayama 16-1, Kamisu, Ibaraki 314-0255, Japan. E-mail:
[email protected] 2 Kiso River Canal Integrated Management Office, Japan Water Agency, Makai-ji Higashi, 26-1 Sofue-cho, Inazawa, Aichi 495-0036, Japan. E-mail: Tomoo
[email protected] 3 Civil Engineering, Gifu University, Yanagido 1-1, Gifu, Gifu 501-1193, Japan. E-mail: a t
[email protected] 4 Structure Engineering Division, CTI Engineering Co. Ltd., Nihonbashi-hamacho 3-21-1, Chuo-ku, Tokyo 103-8430, Japan. E-mail: b
[email protected]
Abstract: Canals are important as lifeline facilities that supply water to urban areas. However, the countermeasure in popular use, such as improvement of entire liquefiable ground of the canal by chemical feeding, is hugely expensive. Against this background, the authors investigated a reasonable liquefaction countermeasure for an existing flume canal using sheet-pile with drain. In this study, several shaking table tests in the 1 g gravitational fields were conducted, and the experimental reproducibility of the analysis was checked to confirm the effectiveness of the proposed countermeasure. The results indicated that the proposed method was effective in reducing the deformation (i.e., uplift/sinking displacement and inclination) of the canal, and good reproducibility for dynamic effective stress FEM analysis using LIQCA code with respect to the results of the experiment was confirmed. Keywords: Liquefaction countermeasure; flume structure; sheet-pile with drain.
1. INTRODUCTION
Early execution of seismic countermeasures for infrastructure elements has been called for since the prediction of several large earthquakes in the near future throughout Japan. The flume structures treated in this study play an important role as a lifeline by supplying drinking water to urban areas. In this study, the application of steel sheet-pile with drain as a reasonable liquefaction countermeasure that reduces residual deformation (i.e., uplift/sinking displacement and inclination) in the flume structures (as shown in Fig. 1) have been investigated by authors.
Figure 1. Target structure.
Ground Improvement Technologies and Case Histories Edited by C. F. Leung, J. Chu and R. F. Shen. Published by Research Publishing Services. c 2009 by Geotechnical Society of Singapore (GeoSS). Copyright ISBN: 978-981-08-3124-0 doi:10.3850/GI123 437
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Figure 2. Layout of sheet-pile with drain.
1.1. Proposal of a reasonable liquefaction countermeasure
The option of enclosing the canal structure in steel sheet-pile walls containing drain holes (as shown in Fig. 2) was studied by the authors (Kita et al., 1992). Steel sheet-piles with drain has steel channels with a number of outlets to allow water to drain from sand deposits, making them capable of reducing excess pore water pressure generated by earthquake. This method offers high applicability in urban areas due to the characteristics of its construction performance (i.e., space-saving, rapid, low-vibration and low-noise construction), prevents shear deformation of the liquefied soil from the outside of the sheet-pile enclosure and reduces the excess pore water pressure inside it.
2. EXPERIMENTAL STUDY 2.1. Test procedure
The layout and conditions of the instrumentation for the model tests are shown in Fig. 3 and Table 1, respectively. The soil container used for the tests was a rigid box measuring 2,800 mm in length, 900 mm in height and 695 mm in width. The dimensions of the model were determined on the basis of the similitude law with respect to the dynamic response of saturated soil (Iai, 1988) to represent an approximate one-twentieth geometrical scale model. Clean silica sand with a mean diameter of 0.13 mm was used to create saturated sand deposits. The viscosity of the pore fluid was about 9.5 times that of water in consideration of similarity to the permeability of saturated soil. The model ground consisted of a loosely saturated sand layer with an embankment made of gravel on it. The canal models were two rigid boxes. The sheet-pile models were
Figure 3. Layout of instrumentations for shaking table tests.
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Study on a Liquefaction Countermeasure for Flume Structure by Sheet-Pile with Drain
Table 1. Type of countermeasure
Conditions for shaking table tests.
Loose sand deposit Dr (%)*
Unit weight*
43.0 39.4 38.7
18.8 18.8 18.8
without countermeasure with normal sheet-pile with sheet-pile with drain
439
Type of canal model Supply canal Drainage canal
dimensions (L)*
(H)*
(W)*
450 400
300 300
695 695
Specific gravity 0.90 0.50
Acceleration (gal)
Note: *Dr: relative density, *Unit weight: (unit: kN/m3 ), *(L): length, (H): height, (W): width (unit: mm) 400 200 0 –200 – 400
0
5
10
15
Time(sec)
Figure 4.
Input motion (Maximum acceleration: 318 gal).
made of steel plates, and were set up with hinged supports at the bottom of the soil container. A pre-fabricated thin drain mat was attached to the inner surface of the sheet-pile models so as not to influence the flexural rigidity. The motion input in the shaking table tests is shown in Fig. 4. This study assumes the worst-case scenario of both the Tokai and Tonankai earthquake occurring simultaneously. The value used for the seismic wave in the base ground was estimated by the Central Disaster Prevention Council. The deduced seismic wave at the bottom of the liquefiable sand deposit was then calculated by applying SHAKE (a one-dimensional equivalent linear analysis code) to the wave in the base ground. In these shaking table tests, the time scale of the seismic wave was adjusted based on the similitude law, and the amplitude of acceleration was adjusted to be twice the size of the original wave to clarify the resulting phenomena. 2.2. Results and discussions of experiment
Figure 5 and Picture 1 illustrate the residual deformation of the canal models after shaking, respectively. In the case without countermeasure, huge deformation was observed in both the water-supply and drainage channels, indicating that the seismic performance requirements of the canal after an earthquake were not satisfied. The residual deformation of the canal without countermeasure was mainly induced by settlement of the embankment and the extension of shear deformation in the liquefied soil to the area under the canal, where the liquefied soil suffered lateral compression from the surrounding soil. This deformation was caused by the difference in the effective overburden pressure between the area below the canal model and the area outside it. Moreover, a cavity was created below the canal model. This was also a factor in the uplift of the canal due to the buoyancy and movement of pore water toward the soil below the canal model — a phenomenon that is not accompanied by the soil deformation (Koseki et al., 1997).
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Vertical Displacement (mm)
100
without countermeasure
80 with normal sheet-pile
60 40
with sheet-pile with drain
20
δ5
δ6
0
δ8 δ7
–20 – 40
δ5
δ6
δ7
δ8
Figure 5. Residual deformation of the canal models.
Water-supply canal
Drainage canal
Water-supply canal
Drainage canal
Photo 1. Deformed configuration of the model after shaking.
without-countermeasure
with normal sheet-pile
Uplif displacement (δ7+δ8)/2 (mm)
with sheet-pile with drain 50 40 30 20 10 0 0.1
1
10 Time (sec)
100
1000
Figure 6. Transition of uplift displacement of drainage canal.
Meanwhile, the deformation of the canal models was greatly reduced in the case with countermeasures. In particular, the effect was remarkable in the case of sheet-pile with drain. The sheet-pile enclosure serves to damp the shear deformation of the liquefied soil, containing it to within a narrow range inside the enclosure. Moreover, in the case of sheetpile with drain, the excess pore water pressure in the vicinity of the sheet-pile dissipates during shaking. As a result, the strength of the soil is retained between the canal model and the sheet-pile, and shear deformation of liquefied soil inside the sheet-pile enclosure is not easily generated. The deformation levels of the canal models were therefore greatly reduced compared with the other two cases.
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Study on a Liquefaction Countermeasure for Flume Structure by Sheet-Pile with Drain
5sec
10sec
15sec
with normal sheet-pile
5sec
10sec
15sec
2 Excess pore water pressure (kN/m )
with sheet-pile with drain
441
4
3
2
1
0
0
100
200
300
400
500
600
Distance from Sheet-pile (mm)
Figure 7. Distribution of the excess pore water pressure.
Figure 6 shows the transition in the uplift displacement of the drainage canal. This displacement increased with the passage of time during shaking, although the increment of uplift displacement gradually decreased. After the motion input ended, the uplift displacement decreased only in the case of sheet-pile with drain. Conversely, in the other two cases, the uplift displacement kept growing due to the buoyancy force. In this respect, the distribution of the excess pore water pressure inside the sheet-pile enclosure at the same depth is illustrated in Fig. 7. In the case of sheet-pile with drain, a hydraulic gradient was created in the excess pore water pressure. This is because the pore water inside the sheet-pile enclosure dissipated not only vertically but also horizontally toward the drain in front of the sheet-pile. Hence, the excess pore water pressure was dissipated promptly, so uplift due to the upward movement of pore water was reduced after shaking. 3. ANALYTICAL STUDY (LIQCA NUMERICAL CODE)
The governing equations for coupling problems between the soil skeleton and pore water pressure were obtained based on two-phase mixture theory (Biot, 1962) and u − p (displacement of the solid-phase pore water pressure) formulation was adopted in the two-dimensional analysis. The finite element method (FEM) was employed for the spatial discretization of the equation of motion, while the finite difference method (FDM) was employed for the spatial discretization of the pore water pressure in the continuity equation. In this simulation, a cyclic elasto-plastic model (Oka et al., 1999) was used to represent the sand and gravel. The boundary conditions were as follows: (i) The bottom of the ground was fixed. (ii) The side boundary of the dynamic analysis was assumed to be fixed in the horizontal direction. (iii) The drainage boundary was set on the sand layer while the other surfaces were impermeable, and the inner surface of the sheet-pile only in the case of sheet-pile with drain was set as the drainage boundary. 4. REPRODUCIBILITY OF EXPERIMENTAL RESULTS
The parameters for the constitutive model adopted and the liquefaction resistance curve obtained from the element simulation are shown in Table 2 and Fig. 8 respectively. In this
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Table 2. Parameters for constitutive model. ρ(kN/m3 ) e λ κ G0 /σm0 M ∗f ∗ Mm B0 B1 Cf γrP∗ γrE∗ D0∗
Density Initial void ratio Compression index Swelling Index Initial Shear Modulus ratio Failure stress ratio Phase transformation stress ratio Hardening parameter Hardening parameter Hardening parameter Reference strain parameter Reference strain parameter Dilatancy parameter
18.8 0.84 0.050 0.020 775 1.68 1.15 3000 80 0 0.0035 0.0080 4.0
n Cd
4.0 2000
Experiment
0.5
Cyclic shear stress ratio
Dilatancy parameter Parameter of anisotropy
Property-1 0.4
Property-2
0.3 0.2 0.1 0
1
10
100
Number of loading cycles
Figure 8. Liquefaction resistance curve.
Figure 9. analysis.
without countermeasure
40
Experiment
20
Property-2
0 -20 -40 -60
Property-1 0
3
6 9 Time (sec)
12
15
Vertical displacement (mm)
Vertical displacement (mm)
simulation, the physical properties and liquefaction parameters were determined considering not only the liquefaction resistance curve but also the stress-strain relationships and effective stress path indicated by the results of undrained cyclic torsional shear tests performed before the shaking table tests. In particular, it is necessary to consider that these shaking table tests were conducted under a level of confining stress lower than that in the undrained cyclic torsional shear tests. Liquefaction resistance increased with reductions in the initial effective mean principal stress (Koseki et al., 2005). A time history of vertical displacement (δ5) and a comparison of the experiment and the analysis are shown in Fig. 9 with sheet-pile with drain
40
Experiment
20
Property-2
0 -20 -40 -60
Property-1 0
3
6 9 Time (sec)
12
15
Time history of vertical displacement (δ5), comparison between experiment and
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Study on a Liquefaction Countermeasure for Flume Structure by Sheet-Pile with Drain
Table 3.
443
Results of experiment and analysis. Water-supply canal Exp.
Ana.
Sinking * without countermeasure −6.5 with normal sheet-pile −7.9 with sheet-pile with drain −3.1
−11.2 −10.0 −3.6
Exp.
Ana.
Inclination* 5.3 1.5 0.0
5.5 2.4 0.4
Drainage canal Exp. Ana. Uplift* 30.7 15.0 8.2
16.7 16.1 10.5
Exp.
Ana.
Inclination* 7.1 0.5 0.1
5.0 1.0 0.3
Note: *Sinking displacement; δ5 (unit: mm), Uplift displacement; (δ7+δ8)/2 (unit: mm), * Inclination (unit: degrees)
and Table 3, respectively. Good reproducibility for dynamic FEM analysis with respect to the results of the experiment (except for the influence of the cavity created in the case without countermeasure) was confirmed by adopting Property 2 considering not only the liquefaction resistance curve but also the various factors outlined above. 5. CONCLUSIONS
Several shaking tests and confirmation of the experimental reproducibility of the analysis were conducted in this study. The following results were derived: (i) The liquefaction countermeasure using sheet-pile with drain is effective in reducing the deformation (i.e., uplift/sinking and inclination) of flume structure. A key factor is the significant restriction of shear deformation in the ground near the sheet-pile due to the retention of soil strength stemming from the drainage effect. (ii) Good reproducibility for dynamic FEM analysis (i.e., dynamic effective stress FEM analysis using LIQCA code) with respect to the results of the experiment is confirmed. (iii) It is important to appropriately determine the parameters of the adopted constitutive model by considering not only the liquefaction resistance curve but also the various other factors involved. In future studies, the authors plan to confirm the effectiveness of the proposed method with respect to actual-scale structures based on this analysis. Reference 1. H. Kita, T. Iida, M. Nishitani and S. Noda, Experimental Study on Countermeasure for Liquefaction by Steel Sheet Piles with Drain, in Proc. 10th WCEE, Vol. 2 (1992), pp. 1701–1706. 2. S. Iai, Similitude for Shaking Table Tests on Soil-Structure-Fluid Model in 1g Gravitational Field, in Report of the Port and Harbor Research Institute, Vol. 27, No. 1 (1988). 3. J. Koseki, O. Matsuo and Y. Koga, Uplift Behavior of Underground Structure Caused by Liquefaction of Surrounding Soil during Earthquake, Soil and Foundations 37(1) (1997), 97–108. 4. M. A. Biot, Mechanics of deformation and acoustic propagation in porous media, Journal of Applied Physics 33(4) (1962), 1482–1498. 5. F. Oka, A. Yashima, A. Tateishi, Y. Taguchi and S. Yamashita, A cyclic elasto-plastic constitutive model for sand considering a plastic-strain dependence of the shear modulus, Geotechnique 49(5) (1999), 661–680. 6. J. Koseki, T. Yoshida and T. Sato, Liquefaction Properties of Toyoura Sand in Cyclic Tortional Shear Tests under Low Confining Stress, Soil and Foundations 45(5) (2005), 103–113.
