Launch of a Seismic Risk Assessment Service for Buried Linear Structures― Visualization of the Risks and Values of Invisible Structures Underground ―September 1, 2021
Tokyo Electric Power Services Co., Ltd.
Tokyo Electric Power Services Co., Ltd., (in Koto-ku, Tokyo: President Yasuhiro Kubo) has developed a seismic
risk assessment for buried linear structures and launch this service today. Through the assessment we have developed
this time, a seismic risk analysis of buried linear structures can be carried out to break down and visualize not only theentire structures but also the areas in the structures having a high risk of breakage. Therefore, we believe that theowners of the structures can obtain information on the risk level and amount of loss, which makes it possible todevelop an optimum renewal plan within the framework of a limited budget and time. This assessment method wasdeveloped under the commission of Tokyo Electric Power Company Holdings, Incorporated.

1. Background
The PML (*) is used as an indicator of the damage to a building caused by an earthquake. However, needless to say,
not only buildings but also underground structures are at seismic risk, and such underground structures are below
ground. Therefore, it has been very difficult to perform risk assessment, in particular, of linear undergroundinstallations, which are subject to this assessment. For example, headrace channels used at hydraulic power plants andwater intake pipes used at thermal power plants are installed linearly as long as some hundreds of meters to several
kilometers. If even a part of such structure breaks due to an earthquake, it not only causes water leakage but also
affects power generation. In addition, it requires a long time and huge costs to renew such long-range structures at onetime. Thus, the assessment of seismic risk for such linear underground structures has been a major issue for theirowners.
* PML: Probable Maximum Loss
How much a building will be damaged in the event of the largest credible earthquake is expressed in the ratio (%) to the replacement cost of thebuilding (the cost required if the entire building is reconstructed). The estimated size of an earthquake is based on the largest ground motion that isgenerally predicted to have a more than 10 percent chance of occurring in 50 years (a great earthquake that occurs once in approximately 475 years).

2. Outline of the Seismic Risk Assessment
First, this assessment is also carried out based on the PML that is used for the assessment of the building. As thematerial, structure, and ground characteristics of the surrounding area vary depending on the section even in one
structure, the entire structure is broken down into elements at an interval of 1–3 meters, and the assessment is carriedout for each element to assess the PML of the entire structure from the sum. The procedure is as follows:(1) A fragility curve is set for each element(2) The rate and amount of loss are calculated for each element(3) The amounts of loss of all elements are totalized to calculate the amount of loss of the entire facility(4) The amount of loss of the entire facility is divided by the total amount of loss to calculate the rate of lossof the entire facility(5) The steps (1)–(4) are carried out in both total and effective stress analyses (*2) to calculate each rate ofloss and PML value*2 Total stress analysis and effective stress analysis
Total stress analysis is a method of analysis in which the earth area and the pore water are handled together, concerning the stressgenerated in the ground. Effective stress analysis is an analysis method in which the earth and the pore water are handled separately,
concerning the stress generated in the ground. Effective stress analysis enables to represent the liquefaction phenomenon of theground.

The details of each procedure are as follows:1

Fragility curve
MCS result

Damage probability

Damage probability(1) A fragility curve is set for each element
A seismic response analysis of a structure is carried out to figure out the damage probability of each element
divided at an interval of 1–3 m. In addition, concerning the method of vibration, trial calculation is carried out 100
times in the Monte Carlo simulation (*3, hereinafter referred to as “MCS”) at each seismic movement
acceleration level to represent the result using a fragility curve.

Fragility curve
MCS result
Peak bedrock acceleration (cm/s2)

Peak bedrock acceleration (cm/s2)

Figure 1: Fragility curve (Element example 1)

Figure 2: Fragility curve (Element example 2)*3 Monte Carlo simulation (MCS)
A method for determining how a model reacts to a variable number entered randomly. In this assessment, MCS is carried out todetermine what the damage probability will be when a physical property of rock mass is set randomly at each peak groundacceleration level. The symbols as shown in the figures can be obtained by carrying out trial calculation many times, and applying thelognormal distribution function to them creates fragility curves (the solid lines in the figures).

(2) The rate and amount of loss are calculated for each element
To figure out the rate and amount of loss, a probabilistic seismic hazard analysis needs to be carried out. Theanalysis is carried out to determine how much the peak bedrock acceleration will be in the structure when an
earthquake occurred and to show it graphically.
Annual frequency of exceedanceSpot A
Spot B

Seismic movement during 475 years of return period

Peak bedrock acceleration (cm/s2)

Figure 3: Probabilistic seismic hazard curve (Example)

As the final objective of this assessment is to determine the PML value, the peak acceleration during 475 yearsof the return period is read graphically. Figure 3 shows the seismic hazard curves of two specific spots, and thepeak bedrock acceleration during 475 years is 686 cm/S2 at the spot A and 688 cm/S2 at the spot B.
Next, the rate of loss (damage probability) is calculated. If the calculation is carried out for the above spot A(686 cm/S2), taking the above figures 1 and 2 for example, the damage probability of the element in Figure 1
becomes 0.98 and that in Figure 2 becomes 0. Assuming that each member costs one million yen, the amount ofloss of the element in Figure 1 becomes one million yen x 0.98 = 980,000 yen, and that in Figure 2 becomes 0 yen.
In this way, the amount of loss of each element can be figured out by checking the peak bedrock acceleration of
the spot against the fragility curve of each element and multiplying the member cost of each element by thedamage probability.
(3) The elements obtained in step (2) are totalized to calculate the amount of loss of the entire facility(4) The amount of loss of the entire facility obtained in step (3) is divided by the amount of cost required if the entire
facility is renewed at a time to calculate the rate of loss of the entire facility
This is the rate of loss of the entire structure at the peak bedrock acceleration and the PML value.

2(5) The steps (1)–(4) are carried out in both total and effective stress analyses to calculate each rate of loss (= PMLvalue)
The above steps (1)–(4) are carried out in both total and effective stress analyses to calculate the PML value ineach case. Although the assessed risk generally tends to be larger in the effective stress analysis than in the total
stress analysis, in some cases, the PML value calculated was confirmed to be higher in the total stress analysis
depending on the element. Therefore, just to be sure, we decided to use two types of analyses for this assessment.
(6) The PML values are assessed
In general, the PML values calculated are compared with the table below for building damages.

PML value
0–10 %
10–20 %
20–30 %
30–60 %
Higher than 60 %Degree of risk
Very lowLow
MediumHigh
Very highExtent of building damage
Minor
Local damage
Medium damage
Major damage
Collapse

Table 1: The relationship between PML value and seismic damage (in the case of buildings)

Also in this assessment, smaller PML value means smaller risk of seismic damage to the structure. As the PML
values of the buildings, which were designed according to the Building Standards Act (Revised Seismic DesignMethod) in 1981 and later are generally approximately 10–20 %, we consider that 20 % is one of the standards also inthis assessment. Furthermore, it is also possible to determine to renew only a certain part, focusing on the rate of lossof the respective elements.
For all of these reasons, we consider that this assessment enables the owners of underground structures to establish,
design, and implement an optimum renewal plan in light of the budget and timing. In addition, it can also be utilized
for maintenance, such as by evaluating the effect of aseismic reinforcement based on the difference in the PML value,
which can be determined through comparison between before and after the aseismic reinforcement.
Although we originally started to develop this method for assessing buried linear structures for electric power, weconfirmed that it could be applied to underground linear structures for purposes other than electric power, such asutility conduits and cable tunnels. Therefore, we expect that this assessment method can be utilized by manycustomers, such as gas companies and telephone carriers who own and manage similar buried objects.
This assessment method won the Takahashi Prize from the Japan Electric Power Civil Engineering Association inFY 2020. This prize is given for the development, research, invention, etc. of technology related to hydraulic power
for electricity generation and other civil engineering technology related to power plants, which are found to haveespecially contributed to the advancement and improvement of such technology.
《Inquiries》
For inquiries about this matter and sales, please contact at the following number and E-mail:〇Tokyo Electric Power Services Co., Ltd.
PR Department: Hasegawa and Tanaka
Phone: +81-3-6372-5692
E-mail: msr-tanaka@tepsco.co.jp

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