Earthquake-resistant design method of purification facilities to tolerate ground behavior by liquefaction

Transcription

1 Earthquake-resistant design method of purification facilities to tolerate ground behavior by liquefaction Toyokazu Nakano 1 Murata Kouichi 2 ASTRACT Many water purification facilities are located on a weak foundation near the river. However, such the foundation will be liquefied easily, when an earthquake occurs. Therefore, when conducting seismic strengthening works of water purification facilities, it is necessary to take countermeasures against liquefaction, and it is an important issue to reduce the expenses. In this paper, we show a new design method to overcome the issue. We applied this method to the sedimentation basins and the rapid filters (24,m 3 /day. As a result, We were able to hold down the cost of construction to half (about 5 billion yen of a new construction. We show the points below. (1 Tolerance of the displacement of facilities by ground sinkage We tolerate the displacement of facilities by the liquefaction, within the limits where water purification functions are not affected (e.g. relative vertical displacement is less than 5cm. This didn t have to conduct the expensive foundation improvement work. (2 Seismic isolation effect by liquefaction Liquefaction ground reduces response acceleration during an earthquake. This reduced the force acting on facilities. (3 Utilizing of analysis result of dynamic effective stress analysis "Dynamic effective stress analysis" was introduced to accurately calculate the amount of the ground deformation and the decrement width of the earthquake response acceleration by soil liquefaction. 1 Toyokazu Nakano, Staff Officer for, designing Dept., Engineering Div., Osaka Municipal Waterworks ureau,2-1-1, Nanko-kita, Suminoe-ku, Osaka, Japan, t-nakano@suido.city.osaka.jp 2 Kouici Murata, Manager for, designing Dept., Engineering Div., Osaka Municipal Waterworks ureau,2-1-1, Nanko-kita, Suminoe-ku, Osaka, Japan, k-murata@suido.city.osaka.jp 1

2 1. Introduction Osaka Municipal Waterworks ureau has introduced the concept of asset management to reduce and standardize the cost incurred by projects to improve water supply facilities. Aging facilities have been systematically renovated and earthquake-resistant design has been applied while effectively utilizing existing structures. Many water purification facilities and other important facilities in Japanese urban areas that withdraw water from rivers (surface water are located on soft ground, most of which occupies liquefiable layers and alluvial cohesive soil layers. In applying earthquake-resistant design to these facilities, it is essential to take measures for coping with soft ground. This paper presents a process in which this design method was reviewed to ensure economical and effective application of earthquake-resistant design to water purification facilities located on soft ground liquefiable ground in particular. 2. Earthquake-resistant design concept Unlike the conventional techniques to prevent liquefaction by employing the soil improvement method, etc., this concept is intended to reasonably apply earthquake-resistant design to water purification facilities located on liquefiable ground by tolerating liquefaction (i.e., displacement of structures caused by ground deformation to a certain extent. 3. Feature of this design method 3.1 Reduction of the expensive foundation improvement work Some conventional methods to prevent ground liquefaction have been to improve the nature of liquefiable soil based on the soil improvement method, etc. While these methods were very costly, this concept helps eliminate ground improvement measures; advantages derived from this concept are highly significant. 3.2 Seismic isolation effect by liquefaction It is also noteworthy that liquefiable layers have the effect to attenuate earthquake response of the ground Liquefiable layer Mounding Fig.1:Standard location of water purification facilities surface (hereinafter referred to as base isolation effect. The base isolation effect can be effectively utilized if liquefiable layers are retained; the scale of seismic retrofit can also be reduced. In many cases, water purification facilities are partially buried in the mounding on the surface of liquefiable ground, as shown in Fig. 1. ecause such facilities are partially exposed above ground, the influence of inertial force attributed to seismic motion on the structures is relatively large. Thus, effective utilization of the base isolation effect is highly promising. 3.3 Disadvantages of this concept Water supply facilities designed to tolerate liquefaction on ground subject to permanent deformation would also be subject to deformation if located right above the deformed ground. Most large-scale facilities have many expansion joints (hereinafter referred to as joints. Thus, liquefaction could cause joints to open up and watertightness to be lost in many cases. 3.4 Design concept how to cope with a dilemma In enabling the foundation ground of water purification facilities to tolerate liquefaction, the biggest challenge is to overcome the disadvantages described in 3.3. It is therefore essential to accurately calculate residual displacement of facilities and joints after liquefaction, and confirm 2

3 that residual displacement is within the limits where water purification treatment functions are not affected. If residual displacement is within the tolerable limits, or if residual displacement can be limited to the tolerable limits, this design concept can deliver the cost advantages. 3.5 Utilizing of analysis result of dynamic effective stress analysis We employ the method for analyzing residual deformation caused by liquefaction based on dynamic effective stress analysis involving cyclic triaxial tests, in order to accurately calculate residual displacement of water purification facilities and existing joints after liquefaction. Several liquefaction analysis programs capable of handling dynamic effective stress analysis (e.g., LIQCA, FLIP have been increasingly used in practical applications. In this paper, LIQCA (Ver. 2D95 is applied as an example. Notably, LIQCA is capable of factoring in ground subsidence, etc. attributed to dissipation of excess pore water pressure after an earthquake. 4. Application to earthquake-resistant construction We applied this method to the sedimentation basins and the rapid filters (24,m 3 /day. 4.1 Target facilities The outlines of these facilities are shown in Table-1. oth of the facilities were constructed over 5 years ago, and they should be renewed in the near future. In the general designs in those days, facilities were constructed to resisting against earthquakes of.2 lateral seismic coefficient. Therefore, probably these facilities do not have earthquake performances against Level 2 earthquakes as in the facilities recently constructed. Moreover, liquefaction was not taken into consideration in the designs in those days. Target Coagulation settling basin Sand filtration basin Table1: Outlines of Target Facilities Configuration/dimensions (external dimensions Length: 126m,width: 97m, depth: 6m (Length: 125m,width: 22m depth: 6m 4basins Length: 114m, width: 37m depth: 5-8m (Length:14m,width:1m, depth:5m 2basins Foundation type Spread foundation Pile foundation Construction year Designed subgrade The designed subgrade is shown in Fig.-2. As for the designed subgrade, the engineering bedrock is at m from the ground level, and the part above the bedrock consists of sand layers and cohesive soil layers. The average N values for each of surface layers above the engineering bedrock is less than 1, and therefore very soft. Furthermore, all sand layers are considered to be liquefied at a Level 2 earthquake Design earthquake motions There are two patterns in our design seismic waveforms during a Level 2 earthquake : the inland-zone active fault earthquake (Uemachi fault zone earthquake estimated at magnitude 7.5 (Fig. 3 and the subduction-zone earthquake Nankai Trough Quake estimated at magnitude 8.6 (Fig. 4. We adopt the former (Fig. 3 data, because it showed relatively large values both in response 3

4 Layer A Layer Layer C Layer D Response 加速度 acceleration (gal (gal Response acceleration (gal 応答加速度 (gal acceleration and response displacement of ground around structures in the event of an earthquake m m 1.1m 3.85m Ac1 Ac1 2.m 2.m 上部 As1( As1( (top 上 4.55m 4.55m 上部 As1( As1( (middle 中 2.45m 2.45m 上部 As1( As1( (bottom 下 2.5m 上部 Ac2( Ac2( 上 (top 上部 Ac2( 2.6m Ac2( (bottom 下 2.95m As2( 下部 As1 上 (top 1.8m 1.5m As2( 下部 (bottom Ac2 下 Foundation 基盤 (Dg (Dg Dg Fig.5:Columner section of soil at a facility subject to analysis facility 基盤 (Dg subject to analysis Fig.4: The design seismic waveforms during a Level2 earthquake (subduction -zone 5. Earthquake-resistant performance specifications 5.1 Marginal performances of structural members against damage The limits on earthquake performances of structural members were determined by taking into consideration of affects on functions and repairability.the limits of structural members of structures are shown on Table2. Structural members Mounding 盛土 粘土 Clay Sand 砂 シ Silty ルト sand 質砂 Sand 砂 シルト Silty 質 clay 粘土 Sand 砂 粘土 Clay Sand and gravel 砂礫 N-value N 値 Soil layer with the 液状化層 possibility of liquefaction A 層 層 C 層 Soil layer with the 液状化層 possibility D 層 of liquefaction Fig.2: Columnar section of soil at a 最大値 =-995.4gal Maximum value 995.4gal Table2: Marginal performances against against damage Suspended solid Rapid filter contact clarifier Max. load bearing capacity or less (bending, shear Yield resistance or less (bending, shear 5.2 Marginal performances against residual displacement during earthquakes The earthquake performances of this design are verified as to the structural members and also the residual displacement of structures after an earthquake, because this design admits the displacement of structures to a certain degree. The limits of the residual displacement of structures are shown on Table Performance against overall displacement (Displacement in the vertical direction The limits on residual displacement (vertical were determined by taking into consideration of overflow of water for the suspended solid contact clarifier, and limits of unevenness on filter layers 時間 (sec Surface of seismic bed rock (input wave 基盤入力波形 2 4 Time(sec Fig.3: The design seismic waveforms during a Level2 earthquake (inland-zone Maximum 最大加速度 =269.4gal value 269 gal Time 時間 (s (sec Surface AT1-EW of seismic bed rock (input wave 4

5 Depth (m Depth (m Depth (m for the rapid filter. (Table3 * Inclination of the filtration basin causes filtration trouble, etc. due to uneven thickness of sand/gravel layers. According to the City of Osaka s specifications, relative vertical displacement must be limited within 5 cm, as shown in Fig. 5. (Displacement in the horizontal direction Ground liquefaction may result in a lateral Sand layer 砂利層 Filtration ろ過 Fig.5: Tolerable relative vertical displacement of a rapid sand filter flow. If the ground displacement exceeds the displacement limits of connections between structures and connecting pipes (refer to below, it will also be necessary to take measures against the lateral flow Marginal performances against displacement of joints The tolerable joint opening/unevenness is specified based on the expansion ability derived from installation of water stops/joints (couplings.(table Marginal performances for connections with connecting pipes The limit performance is specified based on the expansion ability of flexible pipes installed at connections with structures.(table3 Table3: Marginal performances against residual displacement 未処理水 Untreated water Suspended solid contact clarifier Rapid filter Displacement in the vertical direction 5 cm or less 5 cm or less Displacement in the horizontal direction Admitting to a certain degree displacement of joints Opening: 2 cm, unevenness: 5cm connections with connecting pipes Residual displacement between structures and ground = 2 cm 6. Results of structural analyses on existing structures 6.1 Results of analyses on ground responses Fig. 6 shows the depth distribution diagrams of response analysis results for maximum response acceleration, maximum response displacement, and maximum shear strain. The maximum value on the engineering bedrock was gal, but the value reduced by half to gal on the ground level. Furthermore, the distribution of maximum response acceleration shows obvious decreases in acceleration in the sand layer, which is easily liquefiable. These results have proved the base isolation effect of liquefied sand layers. 砂層 Gravel layer 集水室 Catchment chamber Treated 浄水 water Within 5cm 以内 5cm (tolerable ( 許容値 level Maximum response acceleration (gal Maximum response displacement (cm Shear strain (% 最大応答加速度 (gal 最大応答変位 (cm せん断ひずみ (% Layer A 層 A A 層 A 層 Layer A Layer A 深度 1 ( m Layer Layer 層 層 層 Layer 1 1 C 層 C 層 C 層 Layer C Layer C Layer C 2 D 層 D 層 D 層 Layer D 2 Layer D Layer D 2 Fig.6: Ground response analysis results in the event of a level 2 earthquake 5

6 5.35m Coagulation setting basin 6.2 Scope of damage to structures (results of earthquake response analysis Receiving well A A Fig. 7 shows the analysis results of A-A, -,and C-C cross sections of the coagulation settling basin and rapid sand filter Scope of damage to the coagulation settling basin Figs. 8 and 9 show the verification results of flexure and Intermediate ozone contact basin shear fracture. C The coagulation settling basin is buried to the half of Layer A. Thus, as shown in Fig. 6, the impact of ground Rapid sand filter strain/response displacement is relatively small. Due to the C base isolation effect attributed to liquefaction of Layer or Fig.7: Cross section analysis (example deeper layers, maximum response acceleration is only 4-5 gal. Thus, the impact of the inertial force is also small. In addition, flexure fracture would not occur because the coagulation settling basin has a number of joints; it was found that the scope of damage to structural members would be highly localized. 32. Portions subject to flexure fracture fracture or shear fracture Sludge removal pit Joint Joint Joint Joint Joint Joint Joint Joint Joint Joint Joint Fig.8: Damage to structural members of the coagulation settling basin (A-A cross section Layer A Layer Scope of damage to the rapid sand filter Fig.1 shows the verification results of flexure and shear fracture. It was found that the entire pile foundation of the rapid sand filter would be subject to shear fracture. It was also found that damage of a certain scale would occur because (i the structures are restrained by bearing piles (therefore transmission of seismic motion is facilitated on the surface of seismic bedrock, and the base isolation effect attributed to liquefaction cannot be fully utilized and do not have joints, and in particular, (ii the bottom slab of the pipe gallery reaches Layer whose ground strain and response displacement are large, etc. 7. Residual deformation (results of earthquake response analysis Described below are results of deformation 6 Portions subject to fracture or shear fracture 46.8 Service tunnel Joint Joint Joint Joint Fig.9: Damage to structural members of the flocculation basin (- cross section 37.m 13.5m Pipe 管廊 gallery 13.5m φ43 Φ 43 ペデスタル杭 pedestal pile Fig.1: Damage to structural members of the rapid sand filter (C-C cross section Layer A 層 A Layer 層 Layer C 層 C Layer D 層 D Layer A Layer 曲げ破壊 Portions subject or せん断破壊箇所 to fracture or shear fracture

7 analysis on A-A, -, and C-C cross sections of the coagulation settling basin and the rapid sand filter in Fig. 7 after an earthquake (t =4s. 7.1 Residual deformation of the coagulation settling basin Residual deformation of the rapid sand filter It was found that, in the settling basin (Fig. 11,localized subsidence (which would not affect installation of sludge collectors would be 27 cm (angle of inclination: 1.77 at the sludge removal pit, while in the flocculation basin (Fig. 12, the ground would be floated by up to 11 cm in the service tunnel (overall float: up to 7 cm. It was also found that joints would not be subject to large openings, and that displacement would be within tolerable levels (e.g., displacement variance of up to 5 cm at tube connections..2cm.3cm.3cm.5cm Open the joint 1.2cm 4cm 11cm Tube (Tube connectio 7cm 27cm Fig.11: Residual deformation of the coagulation settling basin (A-A cross section 7.2 Residual deformation of the rapid sand filter We performed analysis on the assumption that there are no existing piles that cause shear fracture (spread foundation, because the main objective of this analysis is to calculate the maximum value of residual deformation. Fig. 13 shows the results. It was found that the surrounding liquefiable soil would move to the point right below structures whose effective overburden pressure is small, causing the lightweight pipe gallery to be elevated in particular, resulting in inclination of the filtration basins (located on both sides of the pipe gallery by 9 cm (= 13-4cm Reinforcement method to limit ground deformation/flow while tolerating liquefaction To limit the inclination down to a tolerable level, a decision was made to set up continuous 7cm 5cm (Tube connectio.7cm.7cm.5cm.6cm 15cm 11cm 7cm Fig.12: Residual deformation of the rapid sand filter (C-C cross section 4cm 13cm 13cm 4cm Fig.13: Residual deformation of the rapid sand filter (C-C cross section walls by using steel sheet piles and enclose both sides of the facility, as shown in Fig.14. As a result, the inclination of the filtration basin was successfully controlled to 3 cm, as shown in Fig.15. 7

8 Preliminary space Position change L=21. Regulati ng type cm 3cm 3cm cm Steel sheet ( Ⅳtype Steel sheet (Ⅳtype Fig.14: Installation of continuous walls made from steel sheet piles Fig.15: Residual deformation after installation of continuous walls made from steel sheet piles 8. Anti-earthquake reinforcement methods 8.1 Reviewing seismic retrofit methods Ground deformation adaptation type reinforcement method We selected the ground deformation adaptation method. Figs show examples of general measures taken to improve structures. Figs. 2 and 21 show examples of measures taken for water purification equipment. (A-1 Multiple collars (A-2 Flexible pipe ( Extensi on Internal surface (C-1 Earthquake-resistant water shut plate (C-2 Flexible coupling Fig.16: Measures taken for pipes connections Fig.17: Measures against overflow Fig.18: Measures taken for joints (internal surface of basins External surface (D Architectural expansion joint cover Joint cover (E Regulating type vertical baffle plates Fig.19: Measures taken for joints (external walls Lightweight reinforcement method The basic concept of this design method is to retain soft soil layers (e.g., liquefiable layers. For this reason, the question whether the weight increase due to seismic retrofit affects long-term ground stability has to be taken into account separately. It is therefore desirable to apply a reinforcement method that is lightweight and does 8 Fig.2: Measures taken for water purification systems (F Enhancing adaptation to unevenness in filtration systems Effluent 排水トラフ trough 入 Inlet 出 Outlet Vertical baffled channel flocculation basin ろ過 Filtration Effluent trough Cross section of the rapid sand filter Fig.21: Measures taken for water purification systems

9 6.2 L= not significantly affect the empty volume of basins, within the limits that ground is not floated due to liquefaction, to keep the consolidation tolerance level/ground tolerance bearing capacity within the normal levels. Figs. 22 and 23 show examples of measures taken. (GPost-installed shear reinforcing bars This method offers an advantage of avoiding reduced capacity due to increasing RC thickness for components whose shear proof stress is considered insufficient. RC (HReducing weight of the under drain system/eliminating the gravel layer Cross section of the rapid sand filter Fig.22: Measures to reinforce components Fig.23: Measures taken for water purification systems 8.2 Construction method employed to take measures We reviewed effective methods to apply earthquake-resistant design by combining the reinforcement method presented in 8.1. above, while taking into account the scope of damage to structural members and residual deformation. ased on safety verification of various reinforcement patterns, we came up with seismic retrofit solutions for the flocculation basin (Fig. 24, overall coagulation settling basin (Fig.25, and rapid sand filter (Fig. 26. Sand Gravel Old Perforated plate Made of ceramics Sand New Made of resin (A-2 Flexible pipes Method of increasing RC Method of increasing RC thickness thickness (D Expansion cover (G Shear reinforcing ( Extension (C Flexible (E Regulating type baffle bars couplings, etc. plates (A-1 Multiple collars Fig.24: seismic retrofit of the coagulation setting basin (A-A cross section (ERegulating type baffle plates (E Method of increasing Method of increasing RC thickness RC thickness (GShear reinforcing bars (C Flexible couplings, etc. Fig.25: seismic retrofit of the flocculation basin (- cross section Steel sheet (Ⅳtype Method of increasing Method of increasing RC thickness RC thickness (GShear reinforcing bars Concrete for (H Reduction in weight of the under preventing sand boil drain system Underground water (F High-mobility effluent trough communicating tube Steel sheet (Ⅳtype Fig.26: seismic retrofit of the rapid sand filter (C-C cross section 9

10 9. Reduction in the construction cost The construction cost incurred by applying this earthquake-resistant design turned out to be less than half (about 5 billion yen of the construction cost incurred by scrap and build projects for preventing liquefaction based on the conventional soil improvement method, etc. 1. Conclusion In this paper, we presented a technique to reasonably apply earthquake-resistant design to water purification facilities located on soft ground liquefiable ground in particular. The focus of this technique is not to prevent liquefaction by means of the soil improvement method, etc. but to tolerate liquefaction to a certain extent. This technique also has the potential to enable effective utilization of the base isolation effect attributed to liquefaction. We also presented an example of practical application of this earthquake-resistant design in Osaka City. Main results derived from this review can be summarized as follows: (1 Dynamic effective stress analysis was introduced to accurately calculate the residual deformation of water purification facilities caused by liquefaction and attenuation range of the earthquake response (base isolation effect. Meanwhile, cyclic triaxial test results were reflected in the ground model. We were able to introduce a liquefaction analysis technique that has been hardly applied in the field of earthquake-resistant design for water supply facilities. (2 Regarding the target earthquake-resistant performance, we specified the marginal performances of structural members against damage in compliance with the new earthquake-resistant guidelines, and presented a method to specify marginal performance against residual displacement after an earthquake. (3To demonstrate the practicability, we applied this earthquake-resistant design technique in a project to renovate water purification plant(24,m 3 /day. We were able to present a specific example of detailed design. The technique proposed in this paper offers a solution to economically and effectively take measures against liquefaction, which have been highly costly in conventional techniques, despite tight fiscal budgets of local governments. The City of Osaka will continue to employ this technique in renovating/applying earthquake-resistant design to water supply facilities with various ground conditions/facility types and enhance practical application of this technique. Reference [1] Gentaro Yoshizawa, Kazuya Yamano and Haruyuki Iwata; 21. Earthquake-Resistant Design Method of Purification Facilities in the Liquefaction Ground. I-416, the 65th Japan Society of Civil Engineers Annual Academic Lecture Overviews. 1