Abstract. 1 Introduction

Transcription

1 Numerical simulation for up-lifted manhole during liquefaction S. Takada*, J. Ueno* "Department of Civil Engineer ing, Kobe University, 1 Rokko-dai, Nada, Kobe 652, Japan * Civil Engineering Department, Konoike Construction Co. Ltd., 3-6-1, Kitakyuhoji-machi, Chuo-ku, Osaka 541, Japan Abstract Present study shows the special consideration for the manhole which was uplifted when Kushiro-Oki Earthquake Jan. 15th, A dynamic FEM effective-stress analysis is used to simulate the behavior of a manhole and to clear the mechanism of damage for buried structures subjected to liquefaction phenomena. 1 Introduction A big earthquake (M=7.8) attacked Kushiro \ City, the northern part of Japan, at 20:12 on January 15, At that time the weather was so cold, as the temperature went down below -10 C. Therefore the ground from surface to 0.5m-1.0m in depth was frozen. Seismic intensity is considered to be about V-JMA intensity( gals). Two main causes for seismic damage of various structures are considered as liquefaction in coastal zones and slope instability in terrace geological soil condition at suburban areas[1]. We, visiting jm@ggg there for damage investigation 16 hours after Photo 1: Up-lifted manhole

2 116 Soil Dynamics and Earthquake Engineering the occurrence of the Earthquake, found the surprising phenomena of sewage manholes, which broke the asphalt pavement and rose up over 1m from surface as shown in Photo 1. The behavior of up-lifted manholes will give an important information for the study on earthquake countemeasures of buried structures under liquefaction environments. 2 Sketch and field investigation results for up-lifted manhole Figure 1 shows locations and elevations of up-lifted 22 manholes in Kushiro- Cho sewerage pipelines. These manholes have been up-lifted perpendicularly without jetting sands, however liquefaction phenomena such as sand volcanoes were observed at the road side nearby[2]. Reason of the up-lifting can be considered as buoyancy force by liquefaction in deeper sand layer. These manholes are grouped into A, B and C by their location as shown in Figure 1. Distribution of the elevation of up-lifted manholes in each group is shown in Figure 2. The maximum elevation above the ground surface turned out to be 1.5 m in group A and a manhole located center in each group give the maximum elevation. Figure 3 is a sketch of a manhole-pipeline system known by an excavating field test done on 10 months later from the Earthquake. It turns out that the sewerage pipelines were also up-lifted as well as the manholes without shear cutting at connecting points between pipelines and manholes. Figure 4 shows results of remote control camera-in-pipe test along the sewerage Hume concrete pipeline between No.4 and No.5 manhole shown in Figure 1. This figure indicates the position of 22 pipe segments(0 300mm), location of pipe cracks and location of invasion water. An original position of the manholes and the sewerage pipe are shown in Figure 5 along with their position after the earthquake. Camera-test indicated that the pipeline was pulled out and separated at 7 joints. As each joint has 90mm allowance capacity for pull ing-out, total joint displacement can be estimated as more than 630 mm. However, Figure 5 teaches that total residual elongation should be less than 630mm. We can then conclude that the joints of the Hume concrete pipelines had been subjected to pulling-out and pushing-in simultaneously during rising up process. Next we would like to discuss about the depth of sand layer below the ground surface which would have related with the liquefaction phenomena. Based on the data of soil profile, the assumed depth of a fro/en surface layer and dimension of the manhole, an analytical model is able to be constructed as shown in Figure 6, A condition for the up-lifting of the manhole can be written as follows: W, +V/2 + R sf (1)

3 Soil Dynamics and Earthquake Engineering 117 Here, Wi: Weight of a manhole, W^: Weight of water inside a manhole, R: Resisting force for up-liftduc to friction of surrounding soil and F: Buoyancy by liquefaction. Assuming that the friction force is 0.1N (ton/m~, N: blow counts of penetration test)[3) and a unit weight of liquefied sand is 1.9 ton/in^, we can estimate the height of liquefied sand layer from the bottom of manhole is 2.17m. We can say that more than 90% of sandy layer below the ground water would be liquefied to lift up the manhole. 3 Dynamic FEM simulation for the behavior of a manhole under liquefaction environment Here we have tried to simulate the behavior of the up-lifted No.4 manhole of sewerage system shown in Figure 1. (1) Analytical method We have been developing a computer program to analyze non-linear dynamic behavior of structures buried in possible liquefied ground. This program consists of two parts. On the first part, pore water pressure is calculated. Dynamic FEM computational technique (2-dimensional) based on equation (2), is used to calculate the effective soil stress. M]{Aii} + c]{au) + K]{Au) = - M Aa (2) For non-linear stress-strain relation, a hysteric loop of the Hardin-Drnevich type with modified Masing law 4] is used and Wilson B method is applied for a numerical integration,. Regarding dilatancy and consequent excess pore water pressure, following equations[5] are employed. Vj m*otl"^~r - /O\ N + A ^ ' m, = ab(ojb-' ^ d" = nt7 (5) Here, vj: a volumetric strain, m^: a coefficient of volumetric consolidation, m*, A, n, a and b: the material coefficients, r]: shear stress ratio (shearstress/effective stress), N: the number of cyclic shear stress, dva: a volumetric strain due to one cycle of drainage cyclic shear deformation and du: a pore water pressure due to one cycle of un-drainage cyclic shear deformation. On the second part, dissipated pore water pressure and consequent settlement of ground are calculated based on the following equations.

4 118 Soil Dynamics and Earthquake Engineering ^ +?^ + f+ F* = 0 (0) dx dy dx > + S- + + F>=0 (7) dx dy dy '»T> \ *\ / *\,, '»,,\ Here, P: the pore water pressure, k*, kyi permeability, u, v: displacement and p: a density of water. In order to solve in FEM, equation (9) is made from above equations with discreti/ation by Galerkin method. (8) [c,f /{Pt + *}\ *... \~(. \ ( \ _ I i-\ I \y> I w I Here, JRJ: a matrix of permeability, Cij: a matrix of related pore water pressure with nodal force, JKJ: a stiffness matrix, {Pt}: a vector of pore water pressure and (Auj: a vector of nodal displacement. (2) Analytical model Figure? is a model ground and manhole used for the simulation based on the soil profile and the estimated results of the depth of the liquefied sandy layer in previous chapter. Physical properties of the soil such as relative density, friction angle and permeability coefficient are listed in Table 1 for three horizontal ground layers and for one back-fill soil layer. A sandy layer and the back filled sand with depth of 150cm below the surface were assumed to be non-liquefied because of the effect of the freeze. This frozen state on surface layer would give an important influence for liquefaction in deeper liquefable sand layers, because it prevents excess pore water pressure to dissipate through surface layer. It also affects the behavior of the manhole, because it binded the manhole firmly until the manholes began to move. The frozen layer played a role of guide for the manhole to move perpendicularly. FEM meshes are shown in Figure 8, which indicating possible liquefied layers by shading area. Acceleration seismogram was not recorded at the site during the Earthquake. As an input ground motion at the bottom of the model, accelerogram recorded when Lorn a Prieta Earthquake, adjusted to the maximum 100 gal, was employed.

5 (3) Simulated results Soil Dynamics and Earthquake Engineering 119 Figure 9 shows time histories of the excess pore water pressure ratio(exccss pore water pressure divided by an effective soil stress) at center and edge points on the bottom of the manhole. The ratio is turned out to reach up to one at 8 seconds after the seismic excitation has began and to start to dissipate immediately after the end of the excitation. The dissipation of the excess pore water pressure starts earlier at the edge point of the manhole than the center point. Acting forces on the manhole during the process of liquefaction are considered as shown in figure 10. Notations in Figure 10 are as follows ; Us: Static water pressure, Ud: Dynamic excess water pressure which can be calculated by the FEM simulation, Wm: weight of manhole and Qi and Qi.max : Resisting force for up-lifting by the frozen soil layer and its maximum value. Here, resisting force for up-lifting by the liquefied layer may be negligible due to small values. Factor of safety for the up-lifting of the manhole can be obtained by next formula. r _ Wm + Qi.roaxA Here, A' and A are the surface area of the manhole contacted with the fro/en soil layer and the liquefied sand layer respectively. Figure 11 is a time transient of the factor of safety for the up-lifting of the manhole showing that the manhole starts to float above ground surface when the factor becomes less than one. An elevation of the up-lifting of the manhole can be obtained by next formulae. W^t) =JT dtj (f(t) g / Wm)dt (11) f(0 - (Ud + U,) A - (Wm + Qi._ ' A j (12) Equation (11) means a double integral of an acceleration time history of the manhole which is obtained by the force f(t) acting on the manhole divided by its mass. Figure 12 is a calculated elevation Wr(t) of the manhole during the process of liquefaction. The manhole has been floating gradually and reaches to the maximum elevation of 180 cm at 15 seconds after the occurrence of earthquake in this simulation. An observed elevation of the up-lifted manhole was 160cm, which gives fairly good agreement with the simulation. Next, in order to know the effect of fro/en surface layer and the peat layer, the model ground shown in figure 13 is used. Figure 14 shows the excess pore water pressure ratios on point A, B, C, a, b and c. Input was same as that of figure 9, But adjusted to the maximum 250 gal. It is clear that the saturated layer doesn't become liquefied and the manhole rises up less than the observed case in the

6 120 Soil Dynamics and Earthquake Engineering field when the frozen layer should not exist, that is the ground water were not sealed tightly by the fro/en ground. 4. CONCLUSIONS Present study states up-lifted damage of a manhole-buried pipeline system during 1993 Kushiro-Oki Earthquake and gives an analytical simulation on the behavior of a manhole which was subjected to lift up to 160 cm by liquefaction phenomena. Fbllowings are the summary of this paper. (1) In consideration of ground condition in the vicinity of up-lifted manhole during the 1993 Kushiro-Oki Earthquake, it becomes clear that the deeper sandy soil layer gives an important roll for the floating of the manhole. (2) Pipe-in camera test and an excavating field test have cleared that buoyancy force by liquefaction was acting not only on the manhole but also on the pipeline itself. (3) Numerical simulation for the manhole can give fairly good agreement with an observed value as for floating elevation. (4) Simulated results show that the fro/en layer gives an important effect for the up-jiit of the manhole. A developed computer program to analy/e non-linear dynamic behavior of structures buried in possible liquefied ground, will provide a useful tool when an earthquake countermeasure for buried structures under liquefaction environments is taken into consideration. REFERENCES 1. Takada, S. and J. Ueno: Quick Report on Lifeline Damage During 1993 Kushiro-Oki Earthquake, Jan Kiso-Ji ban Consultant Co. LTD. : Report on Damage Investigation During 1993 Kushiro-Oki Earthquake, Jan Japan Road Association : Seismic Standard for Substructure in Road Bridge, Feb Y. Sato, Pradan, Tei, B.S and F. Tatsuoka: Cyclic Stress-Strain Relation in Tri-Axial Un-drai nage Test, The 22nd Conference on Soil Mechanics and Foundation, pp , June Nishi, K., M. Kanatani, 1. Matsui and J. Tom a: Evaluation of Ground Stability during Earthquake(No. l)-development of Evaluation Methodology for Stability of Sand and/or Gravel Ground based on Dynamic Analysis- Report of Central Research Institute of Electrical Power, Sept

7 Soil Dynamics and Earthquake Engineering 121 Group A no damage Group B Nichii JJ,. O.'lO 0.13 Q7ir\ /'V JS o*i! y v Sand volcano Koba-2chome Group C Numbers indicate elevation of manhole Figure 1: Location and elevation of up-lifted 22 manholes Group A 0.0m 1.0m 0.5m 0.0m 1.5m 1.0m 0.5m 0.0m Group B Group C Figure 2: Distribution of up-lifted elevation of manholes J Gap?. 6ca Sewarage pipeline.. ''... - Figure 3: Sketch of manhole - pipeline system Diredtion of surve' Invasion water Crack Dip ( More than 10cm ) Figure 4: Test result of field camera test

8 122 Soil Dynamics and Earthquake Engineering Manhole No.4 Manhole No.5 After earthquake g % / g Frozen layer ^ #(50-150cm)^ YM'/////'////////////////t. 175cm 195cm Liquefied layer Xcm F 30cm Concrete Figure 6: Model of ground and manhole o^ '\ Before earthquake Figure 5: Deformation of pipeline I (D <3> t7 0)Fill <z>peat layer ^Sand layer gback fill Manhole Table 1: Physical property of soil layers No. (D Relative density Friction angle Figure 7: Model of ground and manhole manhole V 5.7m liqih;fab le 1 "/ ay I E er 3 MM > 9.5m Figure 8: Analytical model of ground and manhole

9 i Transactions on the Built Environment vol 14, 1995 WIT Press, ISSN c 3 ^ _ Factor of safety 31 (fo' C B «ress pore water ] pressurer; itio 3 o 3 0DO - v_ i a_ ^ X, rq s? a. EXCESS PORE WATER PRESSURE RATIO ")4 Elevation of up-lift (m) o Ef. ^S 3 fro (D ^& i ) TIME(SEC) o 1 1 m o CL (D CD CD * "/ I- 3 CO o t 1 11 I -g OQ c3 a- = 31 "" iw' 2 (A m O OCD> i. (D CD crq to CO