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Centrifuge Modeling and Mitigation of ManholeUplift due to Liquefaction
Gi-Chun Kang1; Tetsuo Tobita, M.ASCE2; Susumu Iai3; and Louis Ge, M.ASCE4
Abstract: Because low-compacted trench backfill around a manhole is normally liquefiable, the manhole could suffer uplift damage associ-ated with soil liquefaction during a strong earthquake. In this study, 22 dynamic centrifuge models were tested to investigate the response ofa buried manhole subjected to a dynamic load. The models were tested under 20g, and a substitute pore fluid was used to avoid the scaling lawconflict between the dynamic and diffusion processes. It was found that excess pore water pressure is one of the contributing factors to themagnitude of the manhole uplift. Using this result, new mitigation methods against the uplift in liquefied ground were developed. Theireffectiveness was also examined through the tests. A model manhole mitigated with the proposed methods was tested alongside regular modelmanhole. From the test results, the magnitude of manhole uplifts with the mitigation methods decreased as buildup of the excess pore waterpressure was restrained in high-compacted backfill or excess pore water was dissipated into the manhole during strong shaking.DOI: 10.1061/(ASCE)GT.1943-5606.0000769. © 2013 American Society of Civil Engineers.
CE Database subject headings: Centrifuge models; Soil liquefaction; Earthquakes; Manholes; Uplifting.
Author keywords: Dynamic centrifuge modeling; Liquefaction; Earthquakes; Manhole; Mitigation.
Introduction
A manhole is a top opening to an underground utility vault housingan access point for making connections or performing maintenanceon a public utility or other services including sewers, telephone,electricity, storm drains, and gas. During past earthquakes, damageto many sewer manholes was reported, together with ground set-tlement [Okamoto 1984; Japanese Society of Soil Mechanics andFoundation Engineering (JSSMFE) 1994; Koseki et al. 1997b;Japanese Geotechnical Society (JGS) 2003, 2004; Yasuda et al.2004, 2009;Yasuda andKiku 2006;Wakamatsu 2007;Konishi et al.2008; Japan Association for Earthquake Engineering (JAEE) 2010].In addition, a manhole that is buried at a shallow depth and sub-merged under the groundwater table may become unstable anduplift/float to the ground surface because of soil liquefaction. Lowcompacted backfill soil in a trench around amanhole usually has lowrelative density and hence is prone to liquefaction during strongshaking.
Manhole uplift and backfill settlement are typical and strikingdamage patterns due to strong earthquakes as shown in Fig. 1. Thiswas observed as early as the 1964 Niigata earthquake and Alaskaearthquake (Hall and O’Rourke 1991). Uplift failure of undergroundoil tanks and wastewater purification tanks was observed after the1990 Philippines, Luzon earthquake (Hamada 1991). Studies afterearthquakes indicated that the primary cause of the uplift is the liq-uefaction of the low compacted backfill. From a detailed soil in-vestigation in Nagaoka City, Ojiya City, and Kawaguchi Town afterthe 2004 Niigataken Chuetsu, Japan, earthquake, the relative density(Dr) of the backfill soil on the damaged areas where manholes werefound uplifted ranged from 38–41% (Yasuda et al. 2009). Increasedattention has been paid to manhole uplifts since the recent majorearthquakes, such as the 1993 Kushiro-oki, Japan, earthquake(Yasuda and Kiku 2006; Konishi et al. 2008); the 1993 Hokkaido-nansei-oki, Japan, earthquake (JGS 1997); and the 1994 Hokaido-toho-oki, Japan, earthquake (Yasuda and Kiku 2006). In particular,more than 1,400 sewer manholes were found uplifted by more than 1m above the ground surface after the 2004 Niigata-ken Chuetsu,Japan, earthquake (Yasuda andKiku2006;Konishi et al. 2008). In themost recent earthquake, the 2010Maule, Chile, earthquake, the upliftof a sewage facility (manholes and sewage tank) was reported, and themaximumuplift for the sewage tank in San Pedro del Valle was about1.2 m (JAEE 2010). Hence, mitigation on sewer systems has beenidentified as an important issue; the Ministry of Land Infrastructureand Transport of Japan formed a technical committee to investigatethe damage mechanisms and select appropriate restoration work(Technical Committee on the Sewer Earthquake Countermeasures2005).
Several methods against uplift of newly constructed manholeshave been proposed and implemented. To prevent liquefaction bycompacting backfill material (Yasuda 2003) or improving backfillmaterial with cement-mixed soil (Ishinabe et al. 1999; Yasuda et al.2001). Other methods for newly installed manholes are using soilbags with backfill (Yoshida et al. 2006) or attaching hoops on theperimeter of a manhole to prevent movement caused by the liquefiedsoils around it (Kiku et al. 2007). These proved to be effective after
1Postdoctoral Research Fellow, Dept. of Civil, Architectural, and En-vironmental Engineering,Missouri Univ. of Science andTechnology, Rolla,MO 65409; formerly, Researcher, Disaster Prevention Research Institute,Kyoto Univ., Kyoto 611-0011, Japan (corresponding author). E-mail:[email protected]
2Assistant Professor, Disaster Prevention Research Institute, KyotoUniv., Kyoto 611-0011, Japan. E-mail: [email protected]
3Professor, Disaster Prevention Research Institute, Kyoto Univ., Kyoto611-0011, Japan. E-mail: [email protected]
4Associate Professor, Dept. of Civil Engineering, National TaiwanUniv., Taipei 10617, Taiwan; formerly, Associate Propessor, Dept. of Civil,Architectural, and Environmental Engineering, Missouri Univ. of Scienceand Technology, Rolla, MO 65409. E-mail: [email protected]
Note. This manuscript was submitted on March 4, 2011; approved onMay 13, 2012; published online on May 16, 2012. Discussion period openuntil August 1, 2013; separate discussions must be submitted for individualpapers. This paper is part of the Journal of Geotechnical and Geoenvir-onmental Engineering, Vol. 139, No. 3, March 1, 2013. ©ASCE, ISSN1090-0241/2013/3-458–469/$25.00.
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the 2007 Niigataken Chuetsu-oki, Japan, earthquake (Morii andNishino 2008; JGS 2009). These methods have been primarily ap-plied to newly constructedmanholes. To apply the proposedmethodsto existing manholes, the soil in the trench should be replacedcompletely. However, considering the costs, it may not be a realisticapproach. Therefore, developing mitigation methods for existingmanholes still remains challenging.
Centrifuge Modeling
Geotechnical centrifuge modeling has been used to simulate dy-namic and seismic events (Schofield 1981; Arurlanandan and Scott1993; Ko 1994). In a dynamic centrifugemodel test, according to thescaling rules for n g centrifugal field (Ko 1988), the gravity, fre-quency, and acceleration are to be increased by n times while thelength and time are reduced by n. The stress, strain, velocity, andfluid pressure in the prototype soil mass are preserved. In modelscale, this is able to reproduce the same effective confining stresslevel as imposed in the prototype.
In this study, the geotechnical centrifuge at the Disaster Pre-vention Research Institute, Kyoto University, Japan, was used. Thecentrifuge has an in-flight platform radius of 2.5 m and a capacityof 24 g-ton. It is equipped with a one-dimensional shake table (al-lowable displacement: 65 mm) that can operate under centrifugalaccelerations of up to 50g. To investigate the uplift behavior of amanhole in liquefied ground, the shaking was applied at a 20gcentrifugal field.
Details of the Model Manhole
The standardNo. 1 precast manhole is typically used in Japan [JapanSewage Works Association (JSWA) 2001] as shown in Fig. 2. Itconsists of four parts including a ring and cover on top, manholecone, shaft, and base slabwith inlet and outlet. Properties of theNo. 1manhole are listed in Table 1, whose apparent unit weight is assumedto be 8.25 kN/m3.
Four model manholes were used in the tests. They are made ofaluminum, and each cylinder has the dimensions shown in Fig. 3.
The manhole cone and pipes connected to the manhole were notconsidered for simplicity. Model 1 [Fig. 3(a)] has an outer diameterof 55mm, a length of 150mm, and a wall thickness of 5 mm inmodelscale. Model 2 [Fig. 3(b)] has a length of 100 mm in model scale.The apparent unit weight of Model 1 was 9.57 kN/m3 includingthe sensors mounted inside [Figs. 3(h and i)] and that of Model 2 is9.98 kN/m3 as summarized in Table 2.
A mitigation method was applied to centrifuge Models 3 and4. Model 3 [Figs. 3(c and e)] has a filtering net with a diameter of10 mm, whereas Model 4 [Figs. 3(d and f)] has a filtering net of 15mm in model scale. The length of the pipe connected with the fil-tering net in the manhole is 100 mm in model scale [Figs. 3(c, d, andg)]. To examine the effectiveness of the method, the tests wereconducted with a deeper groundwater table of 1 m so that the porewater did not flow into the manhole before shaking. A mesh(75 mm) made of stainless steel was attached at the filtering netto prevent sandy soil from seeping into the manhole, as shown inFigs. 3(e and f).
Fig. 1. Uplifted manholes and settled backfill after the 2007 NotoPeninsula, Japan, earthquake
Fig. 2. Cross section of the standard No. 1 manhole
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Preparation of the Viscous Fluids
It is well known that a scaling conflict exists between the dynamicand consolidation events in centrifuge modeling (Ko 1988). Porefluid having a higher viscosity can be used to satisfy the scaling law of
the diffusion process. Under a 20g centrifugal field, the metolose(Shin-Etsu Chemical Co. 1997) is mixed with lukewarm tap waterto increase the viscosity by 20 times that of water (20 MPa). Themetolose used in this study is Type SM-100 in the form of whitepowder. It is tasteless, odorless, and physiologically harmless;however, its viscocity is sensitive to temperature change (Shin-EtsuChemical Co. 1997). With the metolose, high viscosity can beobtained without changing other significant fluid parameters such asdensity or surface tension for testing up to 100MPa (Dewoolkar et al.1999). In this study, a 2% solution of the metolose, which hasa viscosity of 100 MPa at 20�C, was produced several days beforemodel preparation. A cup and bob type viscometer [Viscotester,VT03-F (RION Co. 1997)] was used to measure the viscosity. Thesolution of 100MPa was diluted with water to obtain a viscocity of 20MPa at room temperature. Before model construction, the solution wasdeaired in a vacuum chamber for approximately 24 h until no airbubbles appeared at the water surface. Monitoring and adjusting thetemperature in the solution are important to maintain the specified
Table 1. Standard No. 1 Manhole (RC) Properties
Parameter Value
Length, h (m) 3.00Diameter, d (m) 1.05Wall thickness, t (m) 0.08Mass of cap, Wcap (kN) 0.92Mass of base slab, Wbase (kN) 3.43Total weight, Wt (kN) 20.6Volume, V (m3) 2.49Apparent unit weight, gm (kN/m3) 8.25
Note: Unit weight of RC (gc) 5 23.50 kN/m3.
Fig. 3. (a–d) Model manhole and mitigation methods used in the tests; filtering nets installed in (e) Model 3 and (f) Model 4; (g) pipes to guidepressurized pore water into the manhole during shaking; (h) pore water pressure transducer installed at the bottom of model manhole; (i) accelerometerand pore water pressure transducer installed on the inside
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viscosity of the porefluid. Therefore, a digital thermometer probewasembedded in the model to monitor the temperature of the model soiland adjust the air conditioner in the centrifuge chamber [Fig. 4(f)].
Model Construction
Silica sands, whose properties are known from previous work (Tobitaet al. 2011), were used in this study. It is classified as poorly gradedsand (SP) with Gs5 2:66, emax 5 1:19, and emin 5 0:71. The modelsand was prepared in a rigid container, which has nominal innerdimensions of 0:453 0:153 0:30m3 with a glass wall on one side. Itis noted in Fig. 4 that two manholes were installed side by side. Themitigation method was only applied to the manhole on the right. Themodel soil was prepared per the following procedures.1. A layer of soil with relative density ðDrÞ5 85% was first
prepared by compacting moist silica sands up to 200 mm fromthe bottom of the container [Fig. 4(a)]. The term native groundis used to represent this soil in the subsequent discussion.
2. To install the model manholes, a trench with volume 0:11530:1153 0:160m3 was excavated using an aluminum mold ofthe same dimension with a perimeter of the trench to preventthe reclaimed surface from collapsing during excavation [Figs.4(b and c)].
3. After excavation, a 10-mm-thick gravel layer was depositedon the bottom of the trench, and themanholewas placed on topof the gravel layer [Figs. 4(d and e)]. Crushed stone witha particle diameter range of 5–10 mm was placed under themodel manhole as a gravel layer.
4. After saturating the soil in the container up to the specifiedgroundwater table with viscous pore fluid, the same silica sandwas water-pluviated and backfilled in the trench to a relativedensity (Dr) of approximately 36% [Figs. 4(e and f); Table 3].This soil is labeled as backfill in this paper.
From constant head permeability tests, the permeability of thesilica sandwith the distilledwaterwas 3:643 1023 cm/s forDr5 85%and 8:803 1023cm/s for Dr5 36%. Because of this difference(2.4 times) between native ground and backfill, seepage from thebackfill to the native ground can be minimized during shaking.
In each test, the model was prepared carefully so that it had thesame initial conditions. Special care was taken while saturatingthe model ground. After constructing the model, viscous water wasadded to ensure that both native ground and backfill were fullysaturated so that the water poured into the backfill was not absorbedin the native ground.
Instrumentation and Measurement
A total of eight accelerometers, eight pore water pressure trans-ducers, and two laser displacement transducers were installed at thelocations shown in Fig. 5. Accelerometer A0 recorded the inputmotion, whereas A1 and A5 were placed at the top of the manholes,and A2 and A6 were installed at the inner bottom of the manholes.
Table 2. Model Manhole (Aluminum) Properties (Prototype Scale)
Parameter
Value
Length, h 5 3 m Length, h 5 2 m
Diameter, d (m) 1.10 1.10Wall thickness, t (m) 0.10 0.10Mass of sensors, Ws (kN) 0.67 0.67Mass of base slab,Wbase (kN) 1.70 1.70Total weight, Wt (kN) 27.30 18.98Volume, V (m3) 2.85 1.90Apparent unit weight,gm (kN/m3)
9.57 9.98
Note: Unit weight of aluminum (ga) 5 26.50 kN/m3.
Fig. 4. Procedure for the construction of the model ground
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Tab
le3.
Sum
maryof
Centrifug
eMod
elTests(Prototype
Scale)
Test
number
Mod
eltype
Mod
elleng
th
GWL(m
)
Inpu
tacceleratio
n(m
/s2)
Num
ber
ofload
cycles
Relativedensity
Cross-sectio
nal
area
oftrench
(m3)
App
arentu
nit
weigh
tof
mod
el(kN/m
3)
Notes
Left
(num
ber)
Right
(num
ber)
Left
(m)
Right
(m)
Backfi
ll(%
)
Native
grou
nd(%
)
CS1
1—
33
06.78
3036
852:332:33
3:0
9.57
GWL5
0m
CS2
13
33
17.25
3036
852:332:33
3:0
9.57
GWL5
1m
CS3
14
33
17.15
3036
852:332:33
3:0
9.57
GWL5
1m
CS4
14
33
1.7
7.19
3036
852:332:33
3:0
9.57
GWL5
1.7m
CS5
1—
33
36.60
3036
852:332:33
3:0
9.57
GWL5
3m
CS6
14
33
12.05
3036
852:332:33
3:0
9.57
Inpu
tacceleration5
2.05
m/s2
CS7
14
33
14.64
3036
852:332:33
3:0
9.57
Inpu
tacceleration5
4.64
m/s2
CS8
14
33
16.97
3036
364:533:03
3:0
9.57
Nativegrou
nd:D
r536%
CS9
14
33
16.47
3085
852:332:33
3:0
9.57
Backfi
ll:Dr585%
CS1
01
43
31
6.87
1536
852:332:33
3:0
9.57
Num
berof
loadingcycles
515
CS1
11
43
31
6.91
6036
852:332:33
3:0
9.57
Num
berof
loadingcycles
560
CS12
14
33
17.05
3036
652:332:33
3:0
9.57
Nativegrou
nd:D
r565%
CS13
14
33
16.97
3036
—2:332:33
3:0
9.57
Trench:
acrylic
box
CS14
14
33
16.89
3036
—2:332:33
3:0
9.57
Nativegrou
nd:sandmixed
with
clayey
material
CS15
22
22
16.89
3036
852:332:33
3:0
9.57
Bottom
ofthemanho
le:latticeshaped
foun
datio
nCS16
22
22
16.51
3036
852:332:33
3:0
9.57
Bottom
ofthemanho
le:aluminum
plate
CS17
2—
22
16.63
3036
852:332:33
3:0
9.57
Bottom
ofthemanho
le:liquefiablesoil
CS1
81
43
31
6.95
3036
852:332:33
3:0
11.27
gm5
11.27kN
/m3
CS1
91
43
31
7.06
3036
852:332:33
3:0
13.08
gm5
13.08kN
/m4
CS2
01
43
31
6.93
3036
852:332:33
3:0
15.47
gm5
15.47kN
/m5
CS2
11
43
31
6.79
3072
852:332:33
3:0
9.57
Com
pacted
backfill
CS2
21
43
31
6.62
3036
852:332:33
3:0
9.57
Mitigatio
nmeasure
(Fig.10)
Note:GWL5
grou
ndwater
depth;
bold
indicatestestsaremutually
comparable;
theothers
werecomparedin
previous
works
(Tob
itaet
al.20
11).
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A3 andA7were located at the backfill surface, whereas A4measuredthe native ground surface acceleration. Manhole uplift was measuredbyD1 andD2with a capacity of625mmat a distance of 80mm fromthe target. The laser displacement transducer was out of range whenuplift displacement exceeds a maximum of 50 mm (model scale),such as CS1, CS11, and CS12 shown in Table 4. Ground surfacesettlements were directly measured by a ruler before and after eachexperiment. Uplift displacement was also measured by a ruler todetermine the final uplift displacement. The difference betweenmeasured uplift displacements by the sensor and the rulerwas found tobe minimal as listed in Table 4. Pore water pressure transducers wereplaced perpendicular to the direction of shaking to minimize theinfluence of hydrodynamic pressure as a result of shaking. In short,they were attached to the glass wall with double-sided tape (Fig. 4).Porewater pressureTransducersP1 andP4were located in thebackfillat a depth of 2 m below the ground surface. P2 and P5were located inthe backfill at the same depth at the base of the manholes. P3 and P6were attached at the inner bottom of the manhole so that pore waterpressure during uplift could be measured. P7 and P8 were located inthe native ground at a depth of 0.4 m below the manholes.
Test Procedures
After confirming that all equipment and sensors functioned properly,centrifugal acceleration was increased gradually to 20g. The modelwas spun at 20g for 5 min to reach the desired consolidation state.
Centrifugal acceleration was kept at 20g for dynamic loading. For thesake of simplicity, the input motion was applied by a sinusoidal wavewith amplitudes of 2:0527:25m/s2 and a frequency of 1.25 Hz inprototype scale. The seismic behavior of the manhole under realisticearthquake motion may be different with that under the sinusoidalwave. However, it is reasonable to verify the effectiveness of theproposed mitigations against uplift. Table 3 summarizes 22 test casesconducted in this study.
Test Results
Manhole Uplift
The uplift mechanism was investigated through the series of ex-periments. Fig. 6 shows the recordings of normal manhole structures(Model 1) for CS3, with the groundwater table at 1 m below theground surface; CS21, where the backfill soil was compacted byshaking of themanhole; andCS22, where the resistance force againstuplift was applied. Maximum peak accelerations measured in eachcase are 7.15, 6.79, and 6.62 m/s2, respectively. As shown in Fig. 6,the manholes started to lift up at 7.5 s when the excess pore waterpressure measured in the middle of backfill (P1) reached the initialvertical effective stress indicated bys0
v. At 7.5 s,markedby thedottedvertical line in Fig. 6, excess pore water pressure at the bottomof the manhole (P3) exceeded the initial vertical effective stress, s0
vm
Fig. 5. Centrifuge model test setup (CS1)
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Table 4. Summary of Test Results (Prototype Scale)
Testnumber
Uplift displacement
Model type Laser displacement transducer Ruler Settlement of backfill (ruler)
Left (number) Right (number) Left (m) Right (m) Left (m) Right (m) Left (m) Right (m)
CS1 1 — Out of range — 1.10 — 0.183 —
CS2 1 3 0.96 0.92 0.85 0.87 0.190 0.203CS3 1 4 0.95 0.77 0.86 0.71 0.200 0.19CS4 1 4 0.49 0.38 0.45 0.34 0.180 0.14CS5 1 — 0.00 — 0.00 — 0.050 —
CS6 1 4 0.00 0.00 0.00 0.00 0.073 0.06CS7 1 4 0.20 0.18 0.17 0.17 0.175 0.16CS8 1 4 1.07 0.93 0.81 0.93 0.217 0.255CS9 1 4 0.00 0.00 0.02 0.00 0.018 0.018CS10 1 4 0.23 0.14 0.20 0.15 0.168 0.165CS11 1 4 Out of range Out of range 1.60 1.60 0.220 0.2CS12 1 4 Out of range 0.94 1.03 0.94 0.170 0.16CS13 1 4 0.23 0.18 0.22 0.18 0.230 0.227CS14 1 4 0.82 0.62 0.74 0.59 0.205 0.275CS15 2 2 0.50 0.48 0.46 0.45 0.200 0.188CS16 2 2 0.52 0.29 0.47 0.29 0.150 0.133CS17 2 — 0.60 — 0.51 0.18 0.148 —
CS18 1 4 0.57 0.37 0.54 0.34 0.178 0.195CS19 1 4 0.42 0.30 0.37 0.27 0.168 0.123CS20 1 4 0.13 0.05 0.11 0.05 0.150 0.12CS21 1 4 0.13 0.08 0.12 0.07 0.023 0.083CS22 1 4 0.70 0.25 0.68 0.31 0.210 0.195
Note: Bold indicates tests are mutually comparable; the others were compared in previous works (Tobita et al. 2011).
Fig. 6. Test results for CS3, CS21, and CS22 for the normal model manhole (Model 1)
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(at the bottom of the manhole). This indicates that at least the bottomhalf of the backfill was fully liquefied for the manhole to be uplifted.However, in CS21 [Fig. 6(b)], the excess pore water pressure at thebottomof themanhole (P3) didnot exceed the initial vertical effectivestress (s0
vm), and it might lead to a small magnitude of manhole upliftin CS21. The corresponding residual uplift displacements were 0.13and 0.70 m compared with the uplift displacement of CS3 (0.95 m).
This decrease in uplift displacement is believed to be the result ofthe backfill compaction in CS21 and the resistance force in CS22.Therefore, the uplift started when excess pore pressure around themanhole approached 100% initial vertical effective stress. When theinput motion approached its maximum value, the equilibrium offorces acting on the manhole, such as the uplifting force, the weightof the manhole, and friction, was lost because of soil liquefactionduring strong shaking. Also, liquefied backfill was moving towardthe bottom of the manhole because the apparent unit weight of amanhole is smaller than that of the backfill, causing a gradient ofinitial effective overburden pressure at the depth of the manhole base(Tobita et al. 2011).
Pore Pressure Buildup during Shaking
The excess pore water pressure (EPWP) in the backfill is consideredtobeoneof themost influential factors (Koseki et al. 1997a, b; Yasudaand Kiku 2006) in manhole uplift. It was investigated by comparingthe uplift displacement and excess pore water pressure ratio(EPWPR) time histories. Fig. 7 shows the EPWPR time histories bythe depth of the groundwater table (CS15 0m, CS25 1m,CS45 1:7m, and CS55 3m). The EPWPR is plotted using a 2-smoving average filter to remove spikes during shaking. The buildupof excess pore water pressure in saturated ground (CS1) was thefastest, and the corresponding uplift displacement (1.1 m) was the
Fig. 7. Time histories of the excess pore water pressure ratio by 2-smoving average filter in backfill for CS1, CS2, CS4, and CS5
Fig. 8. Relationship between EPWPR at 7 s after the beginning of tests (Fig. 7: vertical dotted line) and residual uplift ratio for primary factors:(a) groundwater depth; (b) amplitude of input acceleration; (c) number of load cycles; (d) relative density (Dr) of backfill; (e) cross-sectional areaof a trench; (f) apparent unit weight of a manhole
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largest (Table 4). This indicates a strong correlation between themagnitude of the uplift and the speed of excess pore water pressurebuildup. Thus, to investigate parameters affecting the increase inEPWPR, the EPWPR at 7 s after starting the measurement indicatedby the thick vertical dotted line shown in Fig. 7 is extracted for eachtest case shown inFig. 8. TheEPWPR inCS5was not built upbecausethe ground is dry sand.
Fig. 8 shows the relationship between the EPWPR and upliftdisplacment for each factor considered in the centrifuge model tests.The uplift displacement (Df ) is normalized by the groundwater depth(h). The EPWPR was higher when the depth of the groundwatertable was shallower [Fig. 8(a)]. It also shows that the correspondingamplitude of input acceleration is larger [Fig. 8(b)] and the relativedensity (Dr) of thebackfill is higher [Fig. 8(d)].TheEPWPR is slightlyincreased with the increase in the cross-sectional area of a trench asshown in Fig. 8(e). Conversely, the apparent unit weight of a manholedoes not seem to be relevant to an increase of EPWPR. As for thenumber of load cycles, there is no influence on the increase of EPWPRas shown in Fig. 8(c). However, the uplift displacement may continueto increase until the shaking stops as the EPWPR remained high.
Uplift Mitigation
Excess Pore Water Dissipation into the Manhole
In the previous section, the increase of excess pore water pressure inliquefied ground during earthquakes was shown to be one of theimportant factors affecting the uplift displacement of a manhole. Itwas also found that reduction of excess pore water pressure is the keyto reduce the uplift displacement of a manhole. Hence, a mitigationmethod was proposed to dissipate the pressurized pore water pressurethrough themanhole (Konishi et al. 2008). Themethod is effective for
both existing and new manholes. Fig. 9 shows a schematic view ofthe mechanism for the proposed mitigation. The mitigation deviceconsists of a filtering net installed at the connection between a sewagepipe andmanhole. Also, a pipe to guide porewater into themanhole isconnected to the filtering net in the manhole shown in Fig. 9. Beforeshaking, the water level in the pipe connected to the filtering net is thesame as the groundwater table around the manhole. During shaking,excess pore water pressure around the manhole gradually increases.The pressurized pore water is guided into the manhole through thefiltering net and pipe as a result of the pressure difference betweenthe outside and inside of the manhole. To verify the effectiveness ofthe mitigation method, a series of the centrifuge model tests wasconducted using Model 3 and Model 4 manholes.
The mitigation method was found to have a fatal flaw in that itwas hard to dissipate the pressurized pore water into the manholewhen it started to uplift because the head difference,Dh (initial value50.2 m), was increased with the manhole uplift (Fig. 9). Therefore,another device was proposed to eliminate the defect, and it was usedin model CS22. In Fig. 10, a beam structure was installed with themanhole and connected between a manhole and the shallow nativeground. Because the beam structure restrains the upward movementof the manhole by connecting relatively stiff native ground and in-creases the resistance force against the uplift, the device helps acceleratethe dissipation of the excess pore water pressure into the manholeduring the manhole uplift.
Mitigation by Backfill Compaction
Backfill compaction is another effective mitigation method againstthe uplift of buried structures and soil liquefaction during earthquakes(Yasuda 2003; Tobita et al. 2011). However, the backfill in the trenchfor existing manholes was difficult to be further compacted becausesewer pipes connected to the manhole could be damaged during
Fig. 9. Schematic view for proposed mitigation device: (a) before shaking; (b) during shaking
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compaction. Therefore, a new method for both existing and newlyconstructed manholes was proposed to compact backfill without dam-aging sewer pipes.
Fig. 11 shows the model manholes installed with a vibrator forbackfill compaction on the top of the rigid container. The vibratorwas connected to the manholes by beam structures, and the backfillwas compacted by shaking the top of the manholes. After con-structing the model and before applying the centrifugal acceler-ation, the backfill was compacted by shaking the manhole as shownin Fig. 11. During shaking, the manhole may uplift because ofliquefaction of backfill. However, the device to connect between themanhole and vibrator is fixed at the top of the manhole and the
manhole could not uplift. Fig. 12 shows effects of the compaction byshaking the manhole using a vibrator. The relative density (Dr) ofbackfill was recalculated by settled volume within the trench. Therelative density (Dr) increased from 36–72% in about 1 min asa result of shaking the manhole. The effectiveness of compactedmodel ground is verified through a centrifuge model test.
Effectiveness of Proposed Mitigation against Uplift
Fig. 13 shows the uplift displacement of CS2, CS3, CS21, andCS22.CS2 and CS3 were conducted to examine the effect of the size offiltering net. They also represent standard tests without mitigation
Fig. 10. Centrifuge model test setup to increase the resistance force (CS22)
Fig. 11. Model manholes for compaction of backfill
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implementation. CS21 and CS22 both represent the cases with com-pacted backfill and the uplift mitigation device. The maximum inputaccelerations for selected tests were in the range of 6:6227:25m=s2
(Table 3). For convenience, the effect of input motion amplitudewas neglected. In CS2 and CS3 of Fig. 13, uplift displacements forModel 1 (no mitigation device) were almost at the same level as0.95 m (reference uplift displacment). The uplift displacement forModel 3 of CS2 (0.92 m) is larger than that of Model 4 of CS3(0.77 m). It indicates that, the larger the filtering net, the more ef-fective the mitigation. However, the greater the uplift displacement,the less that can be reduced by the mitigation methods. It is hard todissipate the pressurized pore water into themanholewhenmanholeswere uplifted rapidly such as in CS2 and CS3. In CS21, the upliftdisplacements were 0.13 m for Model 1 and 0.08 m for Model 4. Thisindicates thatmitigation effected by backfill compaction is significantly
greater than that by excess pore water pressure dissipation in CS21. InCS22, the uplift displacements were 0.25m forModel 4 and 0.70m forModel 1 (no mitigation device). This fact indicates that the device forthe increase of the resistance force against uplift used in Fig. 10 iseffective in reducing the uplift displacement and accelerating thedissipationof the excess porewater pressure into themanhole (Model 4)during manhole uplift.
Fig. 14 shows the dissipated pore water in the manhole afterstrong shaking. Pore water at a depth of 68 cm in the prototype scalewas guided into Model 4 for CS22 [Fig. 14(a)]. Conversely, porewater at a depth of 2 cm in the prototype scalewas guided intoModel4 of CS3, as shown in Fig. 14(c), where the filtering net was foundineffective as a result of the large magnitude of manhole uplift. InFig. 14(b), no water was measured in Model 1 (no mitigation).
Conclusions
In this study, excess pore water pressure in liquefied ground duringearthquakes was found to be one of the most influential factors to themanhole uplift after a series of centrifuge model tests. From the testresults, practical applications and two mitigation methods wereproposed for both existing and new manholes. The proposed miti-gation method can reduce excess pore water pressure in liquefiedground. Also, the backfill compaction can control the amount ofexcess pore water pressure during shaking. The effectiveness of themitigation methods was investigated and validated.
The seismic performance of a buried sewer manhole was simu-lated using centrifuge model tests. From the test results, the manholestarted to lift upwhen the EPWP in the backfill and that of the bottomof the manhole exceeded the initial vertical effective stress. In otherwords, the manhole started to lift up when soil liquefaction tookplace. The EPWPRwas increased in the shallow groundwater table,large magnitude of input acceleration, and high relative density (Dr)of backfill. In these cases, the largemagnitude of uplift displacementoccurredwith a rapid buildup of the EPWPR.Based on these factors,the EPWPR was identified as one of the important factors affectingthe uplift behavior.
It was found that the EPWPR is the key to reduce the upliftdisplacement of a manhole. Consequently, two mitigation methods,designed for both existing and new manholes against uplift behav-ior, were proposed. The first mitigation method can dissipate pres-surized pore water during strong shaking. The other one is backfillcompaction by shaking the manhole.
The most prominent mitigation method was through backfillcompaction. The uplift displacement (reference uplift displacement)in loose backfill (Dr� 36%) was about 0.95 m, whereas the upliftdisplacement in compacted backfill (Dr� 72%) by shaking themanhole was about 0.13 m.
The mitigation method by dissipating the excess pore water wasalso effective. However, the reduction in uplift displacement was
Fig. 12. Compaction of backfill by shaking the manhole (model scale)
Fig. 13. Results of centrifuge model tests for mitigation methods
Fig. 14. Pore water dissipated in the manhole: (a) Model 4 for CS21; (b) Model 1 for CS21; (c) Model 4 for CS2
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relatively small because the pipe connected to the filtering net up-lifted together with the manhole. Thus, another device to increasethe resistance force against the uplift that was connected between themanhole and native ground using a beam structure was proposed tohelp accelerate the dissipation of pore water into the manhole duringthe manhole uplift. Using the device, the uplift displacement wassignificant.
Acknowledgments
This research was sponsored by the Institutional Program for YoungResearcher Overseas Visit from the Global COE in the DisasterPrevention Research Institute of Kyoto University, Japan. Theauthors acknowledge the assistance of Dr. Yasuhiko Konishi andhis staff at Nihon Suido Consultants, Co. Ltd., for cooperation inproviding significant ideas and information.
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