4-1 Technology Development of Anti-seismic Structures for

Sewer Networks Yukihiro ISHIKAWA Bureau of Sewerage, Tokyo Metropolitan Government

ABSTRUCT

Japan has experienced several major earthquakes in the past two decades: the Southern Hyogo

Earthquake (1995), the Niigata-Chuetsu Earthquake (2004) and the Great East Japan

Earthquake (2011). These earthquakes damaged many kinds of infrastructure, including sewage

facilities, and resulted in a major long-term impact on civic life and urban activities.

Almost 90 years has passed since the Great Kanto Earthquake of 1923, and there is a 70

percent chance that a shallow-focus earthquake could hit directly under Tokyo within the next

30 years.

The Tokyo Bureau of Sewerage is working to develop seismic technologies to maintain

sewerage infrastructure as well as ensure safe evacuation and toilet usability in the event of a

major earthquake.

Here, we would like to introduce our seismic retrofitting measures for the joints connecting

sewage pipes to manholes (which have shown significant damage during earthquakes) and a

trenchless seismic retrofitting technology to prevent the manhole uplifting that results from

liquefaction. We would also like to briefly introduce a new technology currently under

development that prevents sediment influx into manholes during earthquakes.

KEYWORDS: Trenchless seismic retrofitting, liquefaction countermeasures, manhole

uplift suppression, new technology

INTRODUCTION

Sewage works suffered significant damage in the Hyogo (1995), Niigata Chuetsu (2004) and

Great East Japan (2011) earthquakes. Ruptured sewer lines (Photo 1) and uplifted manholes from

liquefaction (Photo 2) made toilet facilities at evacuation sites unusable and blocked road traffic,

thereby seriously hindering post-disaster civic life and emergency restoration efforts. Final

treatment of sewage was stopped, with workers implementing stopgap measures such as the

construction of temporary settling tanks on facility grounds and in neighboring canals and, for a

limited period, carrying out only primary treatment of sewage waste.

The metropolitan Tokyo area is estimated to have a 70 percent chance of experiencing a

magnitude-7 class earthquake within the next 30 years. If a magnitude 7.3 earthquake occurred in

the northern part of Tokyo Bay, an average of 27.1 percent of sewer pipes are expected to suffer

damage. If the earthquake halted sewage functions and a major rainfall occurred, the system

would not be able to carry off rainwater, potentially wreaking enormous damage.

Earthquake measures are therefore an urgent issue and require further strengthening of efforts

to mitigate damage and accelerate recovery and restoration.

Heavy traffic in urban areas such as Tokyo makes the regulation of road lanes during

excavation difficult. Moreover, open-cut excavation for the implementation of seismic

technologies in sewer lines presents difficulties since water, gas and electric lines are also buried

under roadways.

Given these conditions, the Tokyo Bureau of Sewerage is working to develop trenchless

seismic retrofitting technologies for its sewer lines.

Plan to Seismically Strengthen the Tokyo Sewer System

As part of the Tokyo 2020 Plan announced in 2011, the metropolitan government is actively

promoting earthquake measures in its aim to create a disaster-resistant city and enhance the

metropolis’ credibility.

When selecting sewer lines on which to carry out seismic retrofitting work, the Bureau of

Sewerage gives priority to “points” of attention such as sewer lines and manholes that carry

wastewater from disaster center hospitals, schools that serve as evacuation centers, and parks that

serve as evacuation facilities.

Specifically, the Bureau has been surveying the joints connecting manholes and sewer pipes of

700 millimeters or less at 2,500 evacuation facilities to determine whether retrofitting is possible.

At the end of March 2012, 78 percent had been surveyed, with a planned completion by the end

of March 2014.

Photo 1: Ruptured connection between

manhole and sewer pipes

Photo 2: Uplifted manhole (2004

Niigata Chuetsu Earthquake)

Moreover, the Bureau is also promoting another seismic measure “point”: suppressing manhole

uplifting to ensure that emergency vehicles can pass over roads during a disaster. These

measures are also being applied to manholes on sidewalks to ensure passage to evacuation

centers and the like.

Specifically, these measures are being applied in areas at high risk for liquefaction to roads

designated as essential emergency transport routes and evacuation routes in order to mount

recovery activities during in an earthquake disaster. Over a three-year period beginning in 2008,

the Bureau implemented measures to prevent uplifting of manholes along 500 km of roads

designated at emergency transport routes. From April 2011, we have been working to implement

these same measures for manholes along the 1,500 km of roads that access these emergency

routes.

Given the danger of an earthquake directly hitting the metropolis, the Bureau will need to

extend its seismic measures in stages, expanding out from such “points” to “lines” and

“surfaces.”

Seismic strengthening of “lines,” or the actual sewer pipes, is currently being carried out as part

of reconstruction work and is by necessity a long-term effort. As such, there is need to proceed

with such “line” work—seismic retrofitting and reconstruction—in parallel.

Fig. 1. Overall image of seismic retrofitting and reconstruction in stayed-in districts

ます・取付管の耐震化老朽管きょの更生・敷設替え

地区内残留地区

マンホールの浮上抑制マンホールと管きょの

接続部の可とう化

緊急車両の通行機能を確保

* Stayed-in Districts Areas in which there is no danger of fire spreading in a disaster and for which large-scale evacuations are not required.

Designated “Stayed-in” Districts Rehabilitate or re-lay superannuated pipes

Seismically strengthen catch basins and collecting sewers

Add flexibility to manhole-sewer pipe joints

Evacuation centers, etc.

Emergency transport roads

Suppress manhole uplift

Ensure that emergency vehicles can pass

Water reclamation center

In addition to work on lines from evacuation centers, etc. it also necessary to expand work to

maintain other essential “points” such as national and city organizations responsible for disaster

prevention; police, fire and other public facilities; facilities essential to post-disaster recovery of

electricity, gas and other lifelines; major railway stations, who must deal with stranded

commuters; and convenience stores and similar places that provide services during a disaster.

“Areas” refer to seismic retrofitting of sewer systems in the designated “stayed-in” districts (Fig.

1) (areas where it is safe to remain in a disaster) of Tokyo’s disaster prevention plan, largely

those in central Tokyo and around major stations. Since basically the entire sewer system needs

to be seismically retrofitted, by increasing the priority of reconstruction work, the Bureau must

promote speedy and efficient seismic retrofitting and superannuated pipe measures.

Developing Technology for Making the Tokyo Sewer System Earthquake-Resistant

Seismic retrofitting of sewer pipes consists of three basic types of work: strengthening the pipes

themselves, strengthening the joints connecting manholes and pipes, and measures to prevent the

uplift of manholes. Tokyo Bureau of Sewerage has been carrying out joint research with a

number of private-sector enterprises to develop seismic technologies with respect to latter two

types of work, and we would like to introduce them here.

1. Development of a Technology to Strengthen the Joints Connecting Manholes and Sewer

Pipes

In the 1995 Southern Hyogo Earthquake, sewer pipes suffered damage from ruptures, ejections,

joint slippage and cracks. In particular, the damage was often seen at the joint between the

manhole and the sewer pipe and in the first pipe leading from the manhole.

The joints connecting manholes to sewer pipes were rigid and not able to absorb the difference

in the movement between the two parts in response to ground motion.

After surveying the damage in the Hyogo earthquake, the Japan Sewage Works Association

published its guidelines and commentary for seismically retrofitting sewage facilities based on

two levels (Level 1, Level 2) of ground motion and indicated new thinking about

seismic-resistant design.

Using these guidelines, the Tokyo Bureau of Sewerage began collaborative research with

private enterprises from 1998 to develop a seismic technology for the joints connecting manholes

and sewer pipes.

(1) Request to Develop a Seismic Retrofitting Method for Existing Manholes

1) Background to the Request

Given the damage to sewage facilities in the 1995 Southern Hyogo Earthquake, the Tokyo

Bureau of Sewerage made a request to private-sector enterprises in 1998 to develop seismic

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The scope of application is as follows:

1) Pipe types: clay, reinforced concrete, rigid PVC pipe, fiber-reinforced plastic mortar pipe

2) Existing pipe size: inside diameter of 250‒700mm

3) Manhole diameter: inside diameter of at least 900mm

4) Thickness of manhole wall: 300mm or less

5) Manhole cover: 600mm or more

6) Manhole depth: 5m or less

(2) Use of a Method to Seismically Retrofit Joints in Rehabilitated Pipes at Manholes

1) Decision on the Method to Be Used

There is a large concentration of superannuated pipes in central Tokyo that have well exceeded

their statutory 50 years of useful life and, due to advancements in urbanization and more

sophisticated lifestyles, many of these pipes have insufficient flow capacity for handling

wastewater volumes and increased rainwater runoff. To address this problem, measures to deal

with superannuation are required.

In the 2004 Niigata Chuetsu Earthquake, sewers were unusable due to large-scale damage to

sewage lines and there was a shortage of toilet facilities at evacuation centers, reiterating the

importance of sewers as a lifeline.

With the aim of seismically retrofitting the joints between manholes and rehabilitated sewer

lines, in 2009 we assessed a method developed in the private sector and decided to implement it

in our facilities.

2) Overview of Method

This method applies to the joints between manholes and sewer lines for rehabilitation, adding

seismic resistance for pipe ejection, projection and refraction by giving a more flexible structure

[Top View] [Side View]

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(2) Target Values

The uplifted manholes resulting from large-scale earthquakes in Japan in recent years have

interfered with road traffic such as the passing of emergency vehicles. The Japan Sewage Works

Association’s 2006 Earthquake Measures Manual provides guideline criteria for emergency

measures with respect to passage interference (Table 1).

On the other hand, it is said that the maximum allowable height difference for the passage of

traffic is 15 cm. At present, it is not possible to calculate the amount of subsidence that would

occur from liquefaction, but if subsidence is estimated at 5 cm, then manhole uplift would need

to be kept to below 10 cm to stay with the 15 cm limit. We therefore aimed to achieve manhole

uplift of less than 10 cm.

In addition, we also set a development goal of working from inside the manhole to remove the

need for excavation, to be able to restore traffic as early as possible, and to create a structure that

would not interfere with operations and maintenance once implemented.

Table 1: Classification of degree of damage (excerpt)

Facility Survey topic Level of damage

Roads

Height

difference

between road

and manhole

Light Medium Heavy

Feel bumps when

passing over in a

vehicle (Difference

of 3cm or less)

Interferes with

operation of vehicle

(3cm-10cm)

Vehicles unable

to pass (more

than 10cm)

(3) Overview of Method

In this method, a dissipation valve is installed from the inside wall of the manhole. When pore

water pressure increases in the area around the manhole, dissipation valves release the pressure

by ejecting a pressure-sensitive plate, allowing the pore water to be released into the manhole

and preventing manhole uplift. (Fig. 6, 7)

This technology is not affected by the size, depth or shape of existing manholes and allows for

the selection of the best position and optimal number of valves. An installed valve consists of the

valve socket, a mesh screen to prevent sediment inflow and a pressure-sensitive plate. When the

pressure-sensitive plate is exposed to pore pressure water above a certain level, the plate is

ejected, which allows water to flow into the manhole to dissipate pressure. Installation work uses

a trenchless method and a specialized drilling device that stops just before penetrating through

the manhole wall. The dissipation valve is press-fitted into the manhole and the space between

the dissipation valve and the drilled space is filled.

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CONCLUSIONS

The Great East Japan Earthquake of March 11, 2011 produced shaking in Tokyo of Upper 5

(Japanese Seismic Intensity Scale) and liquefaction. Post-quake surveys focused on districts

where liquefaction occurred were able to confirm that the sewer systems that had undergone

seismic retrofitting had no major damage and were able to maintain flow.

To contribute widely to work for making earthquake-resistant sewerage works, we will

continue to expand the scope of implementation for seismic technologies and aim to establish

simple construction methods adapted to site conditions, as well as promote the development of

new technologies in needed areas.

REFERENCE

1. 2010 Management Plan, Metropolitan Tokyo Bureau of Sewerage (published February2010)

2. 2010 Technology Development and Promotion Plan, Metropolitan Tokyo Bureau of Sewerage

(published January 2011)

3. 46th Sewage Research Conference (2009), “Development of a Seismic Resistance Method for

the Joints of a Rehabilitated Manhole (Taishin Ippatsu-kun),” Tomohiro Hanahara, Yuji

Imasaki, pp. 158-160 (Japanese)

4. 48th Sewage Research Conference (2011), “Seismic Resistance Work for Sewer Lines

(Trenchless Method to Control Uplift of Manholes),” Hidetaka Takazawa, pp. 109-111

(Japanese)

5. 48th Sewage Research Conference (2011), “Method to Strengthen Seismic Resistance in

Existing Large-diameter Pipes,” Yukihiro Ishikawa, Kenzo Sato, Shohei Iwasaki, Tsutomu

Tanaka, Kohei Nakatani, pp. 112-114 (Japanese)