J. Mt. Sci. (2017) 14(9): 1751-1762 e-mail: jms@imde.ac.cn http://jms.imde.ac.cn https://doi.org/10.1007/s11629-016-3976-5

1751

Abstract: Gas disasters always restrict the tunnel

constructions in mountain area, which becomes a

major geological barrier against the development of

underground engineering. China is rich in coal

resources and has a large amount of gas with a wide

range of distribution. However, China experienced

not only adverse effects on coal mining but also gas

disasters in underground engineering construction,

such as tunnels and chambers. With the increased

number of tunnels passing through coal-bearing

strata, the incidence of gas accidents is also rising.

Therefore, the significance of preventing and

mitigating gas disasters should be emphasized, and an

effective risk assessment method for gas disasters

should be established. On the basis of research on

over 100 gas tunnels in China, a relatively ideal gas

disaster risk assessment method and system for

tunnels are established through the following

measures. Firstly, geo-environmental conditions and

gas situations were analyzed during construction.

Secondly, qualitative analysis was combined with

quantitative analysis. Finally, the influencing factors

of gas disasters, including geological conditions, gas,

and human factors, were investigated. The gas tunnel

risk assessment system includes three levels: (1) the

grading assessment of a gas tunnel during the

planning stage, (2) the risk assessment of gas tunnel

construction during the design and construction

stages, (3) the gas tunnel outburst risk assessment

during the coal uncovering stage. This system was

applied to the dynamic assessment of gas disaster

during the construction of the Zipingpu tunnel of

Dujiangyan–Wenchuan Highway (in Sichuan,

Southwest China). The assessment results were

consistent with the actual excavation, which verified

the rationality and feasibility of the system. The

developed system was believed to be back-up and

applied for risk assessment of gas disaster in the

underground engineering construction.

Keywords: Tunnel engineering; Gas disasters; Risk

assessment; Index system; Zipingpu tunnel

Introduction

China is a mountainous country;

approximately 75% of the land is mountainous or

heavily mounded. A considerable number of tunnel

Received: 5 April 2016 Revised: 8 February 2017 Accepted: 6 July 2017

Developing a risk assessment system for gas tunnel disasters in China

KANG Xiao-bing1* http://orcid.org/0000-0003-2537-639X; e-mail: kangxiaobing09@cdut.cn

LUO Sheng2 http://orcid.org/0000-0003-1768-8756; e-mail: 1847044883@qq.com

LI Qing-shan1 http://orcid.org/0000-0002-9550-3238; e-mail: 527244406@qq.com

XU Mo1 http://orcid.org/0000-0002-7470-7274; e-mail: xm@cdut.edu.cn

LI Qiang1 http://orcid.org/ 0000-0001-7613-0942; e-mail: 576575724@@qq.com

* Corresponding author

1 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China

2 College of Engineering, Sichuan Normal University, Chengdu 610071, China

Citation: Kang XB, Luo S, Li QS, et al. (2017) Developing a risk assessment system for gas tunnel disasters in China. Journal of Mountain Science 14(9). https://doi.org/10.1007/s11629-016-3976-5

© Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag GmbH Germany 2017

J. Mt. Sci. (2017) 14(9): 1751-1762

1752

projects are inevitably necessary when traffic lines

cross mountains. The development of economy

requires the development of traffic. Meanwhile, the

development and construction of high-grade

highways and high-speed railways have increased

the necessity for improved tunnel engineering. The

increasing number of tunnels may encounter coal

formation during their construction and the

occurrence of gas tunnels. As long as gas exists

inside a tunnel (regardless whether the gas has

appeared sooner or later, the length of time,

location, and quantity), the tunnel is considered as

a gas tunnel (TB10120-2002). In China, the

Yesanguan Tunnel of the Yi-Wan Railway, the

Hongshiyan Tunnel of the He-Wu Railway, the

Huayingshan Tunnel of the Guang-Ying Highway,

and the Zhipingpu Tunnel of the Du-Wen Highway

are recently built gas tunnels that presented the

risks of gas suffocation, combustion, explosion, and

outburst during their construction. Information on

gas tunnels that are over 3 km long and built

during the past five years are listed in Table 1, and

the distribution positions are showed in Figure 1.

Due to the complexity of geological conditions and

constraint on in situ investigations, tunnel

constructions always encounter various poor

geology and geo-hazards, such as high stress, rock

burst, large deformation, karst geology, gushing

mud, and gas disasters (Doyle 2001; Tang et al.

2003).

Gas is a commonly acknowledged hazard in

the construction of tunnels, shafts, and

underground chambers. However, current civil

engineering and engineering geology literature

does not provide enough information on the nature

of underground gases, or guidance toward avoiding

potential problems (Doyle 1976). On the basis of

the current understanding on gas disasters, if

engineers can master and manage relevant

information systematically and in a timely manner,

as well as implement effective predictions before

the occurrence of a possible danger, then gas

tunnel disasters using existing control measures

will fully and confidently eliminated (Kang et al.

2013).

Danger refers to the possibility of suffering

from damages. The nature expression of danger is

dangerous. In danger assessment (also called safety

assessment and frequently called risk assessment

in other countries), systematic and scientific

theories and methods are used to analyze system

security qualitatively or quantitatively, as well as

identify the best control and countermeasures for

danger sources (Chapman and Cooper 1987). The

Table 1 Summary of gas tunnels that are over 3 km long constructed during the past 5 years in China

No. Tunnel name Tunnel length (m)

Gas condition Progress (completed in )

Notes

T1 Yundingshan 1# Left 3577, right 3574

Total gas emission is high. October 2012 Yuan 2014

T2 Longquanshan 7328 The highest gas concentration is 86540 ppm, whereas the average is 5000–6000 ppm.

April 2014 Gan 2014

T3 Tianpingzhai Left 3680, right 3702

Gas content is less than 10%. October 2011 Yang 2014

T4 Meilingguan 8271

Gas pressure is greater than 0.20 MPa, gas concentration peaks at 1.3%, and the absolute emission rate reaches up to 3.03 m3/min.

March 2013 Su et al. 2011

T5 Laoshishan 8120 Gas leaks during excavation. April 2016 Chang et al. 2013 T6 Wushaoling 20050 CH4 content is 0.02–0.37 mL/g. March 2013 Cao and Wang 2013

T7 Sanlian 12214 Gas pressure is 2.533–3.229 MPa, gas content is 7.54–12.87 m3/t, and gas emission rate is 4.606–15.816 m3/min.

April 2012 Hu 2012

T8 Xujiawan 4652 The highest gas concentration in borehole is 15820 ppm, and the emission rate reaches up to 0.6 m3/min.

September 2013 Shi 2012

T9 Huangjialiang 11632 The highest gas concentration in borehole is 28450 ppm.

February 2016 Hu 2013

T10 Majiapo 3950 The absolute emission rate reaches up to 2.06 m3/min.

December 2012 Han et al. 2011

T11 Liujiazhuang 7583 The absolute emission rate reaches up to 3.06 m3/min.

November 2014 Liu 2014

J. Mt. Sci. (2017) 14(9): 1751-1762

1753

risk is a basic and natural element of life and

defined as “the chance of something happening

that will have an impact on objectives” meaning

risk can be either positive or negative (Rausand

2011). In some literature, risk can also be defined

as an expression of the impact and the possibility

of a mishap in terms of potential mishap severity

and probability of occurrence (Clifton 2005;

Pamukcu 2015). The system can be ensured safety

through danger assessment.

Danger assessment is one of the topics of

interest in disaster science both locally and

internationally. It is also an important aspect of

disaster forecasting, prevention, and reduction, as

well as the basis of regional geological disaster risk

research (Aleotti and Chowdhury 1999). The

reasonableness and accuracy of the assessment

results are directly related to disaster prevention

and reduction benefit rates, and are the bases of

the government in developing appropriate policies.

Accidents and other associated problems occur

during the construction and exploration of the

underground structures and are very often related

to uncertainties related to the geological conditions

and construction conditions. To help eliminate or

at least reduce these accidents, it is necessary to

systematically assess and manage the risks

associated with construction and operation of

underground structures (He et al. 2015). By the use

of risk assessment potential problems can be

clearly identified such that appropriate risk

mitigation measures can be implemented in a

timely manner (Brown 2012; Sousa and Einstein

2012).

Researchers have been studying disasters that

have occurred around the world for a long time;

however, they see hazard assessment as a new

interdisciplinary area in disaster research. The

increasing number of disaster losses has led to the

rapid development of related disciplinary theories

and technologies. Specialized regional geological

hazard assessment and analysis have been

conducted with the “international decade for

disaster prevention and reduction” campaign and

have gradually received interest worldwide during

the 1990s (William and Petak 1993). Since then, an

increasing number of relevant research results

have emerged.

In many studies on geological disasters,

research on landslides and debris flows is the most

comprehensive and comprises most systems

primarily because these two geological disasters are

the most widely distributed, serious, and

influential (Kang 2009). By contrast, gas tunnel

hazard assessment research remains minimal and

mostly addresses unpaired systems.

Figure 1 The distribution position of 11 gas tunnels.

J. Mt. Sci. (2017) 14(9): 1751-1762

1754

1 Risk Assessments of Gas Tunnels

1.1 Gas tunnel construction situation

Gas disasters frequently occur during the

construction of highway and railway tunnels. In

order to avoid such disasters in coal mine areas,

tunnel designers always tried to choose a tunnel

route without cutting across coal formation in the

past. From 1949 to 1999, China had built 18 gas

tunnels, which only accounted for 0.18% of the

total number of tunnels in the country. With the

development of transportation in China, the

probabilities of passing through and building

tunnels in coal seams and gas areas increase.

Moreover, the number of tunnels passing through

multiple coal seams and areas with high amounts

of gas is also increasing. According to partial

statistics, over 110 gas tunnels have been

constructed in China since 2000. Among these, 60

have a length of over 3 km, which is longer than the

total number of gas tunnels built before 2000

(Kang 2009).

As the number of tunnel constructions in coal

measure strata continuously increase, the

incidence of gas accidents also rises. The most

serious gas disaster accident in China occurred on

December 22, 2005 at around 2 p.m.. During the

construction of the right section of the Zipingpu

Tunnel (the original Dongjiashan Tunnel) in the

Dujiangyan–Wenchuan Highway, Sichuan, an

extremely serious gas explosion occurred. A total of

44 people died, and 11 others were injured (Kang

2009). A direct economic value of 20.35 million

yuan was lost. During the explosion, one worker

was thrown 168m away from the tunnel by the

pressure of the shock waves caused by the

explosion in the tunnel and the formation of the

“barrel effect.” Numerous vehicles were also

affected by the high-pressure shock waves. The 70-

t formwork trolley outside the tunnel entrance was

seriously deformed and pushed 47 m away from

the entrance by the high-pressure shock waves.

1.2 Research status of gas tunnel risk assessment

With the increase in tunneling depths and the

occurrence of worsening conditions, deep, large-

scale and rapid mining may lead to more

complicated dynamic features for tunnels, making

them vulnerable to dynamic disasters such as

coal/gas outbursts and gas exploded with

subsequent heavy damage and casualties (Zhen

et al. 2014). Although a number of gas tunnels were

constructed in China in the last century, the

researches related to reasonable definitions,

monitoring methods, construction safety

technologies, and emergency prevention measures

about gas tunnel risk assessment remain limited.

Therefore, the risk assessment of gas tunnel

disasters is only at the preliminary stage, and

research results are mostly obtained from coal and

gas outburst hazard assessment and coal mine

system analyses (Liu and Xing 1999; Yu 1992).

Existing gas tunnel data are mostly about gas

combustion and explosion in tunnel engineering

instead of about coal and gas outburst disasters

that happened in coal mine systems. Therefore, a

gap remains in the risk assessment of gas tunnels.

With regard to gas tunnel risk assessment, the

“gas tunnel railway technical specifications”

(TB10120-2002) (Professional Standard of the

People’s Republic of China 2003) provides several

provisions. For example, the definition and grading

of gas tunnels are the same as those for gas tunnel

risk assessment. The related contents of the

specifications are as follows. A gas tunnel is defined

as a tunnel that crosses or is near coal-bearing

strata and other gas-containing formations. It is

divided into three categories, namely, low-gas

tunnel, high-gas tunnel, and outburst tunnel. The

categories of a tunnel are determined based on the

highest gas volume concentration of the tunnel

working area. A gas tunnel working area is divided

into non-gas, low-gas, high-gas, and outburst work

areas. Low and high-gas work areas are determined

by the absolute amount of gas emission. When the

amount of gas emission is less than 0.5m3/min, the

entire work area is classified as a low-gas area;

when it is greater than or equal to 0.5 m3/min, the

entire work area is classified as a high-gas area.

Meanwhile, an outburst must satisfy at least four

indices, namely, (1) a gas pressure of P≥0.74 MPa,

(2) an initial velocity of coal gas diffusion of △P≥10,

(3) a coal ruggedness coefficient of f≤0.5, and (4) a

destructive type of coal class III. According to the

“gas tunnel railway technical specifications”, a

hierarchical partitioning gas tunnel flowchart is

shown in Figure 2. It must be said that the coal

J. Mt. Sci. (2017) 14(9): 1751-1762

1755

ruggedness coefficient is a physical and mechanical

properties (intensity f) index of coal, which is one

of the important indexes of outburst danger

assessment in coal mine area. And under the action

of tectonic stress, coal seams break and crumple. In

order to predict and prevent the outburst of coal

and gas, destructive type of coal class is divided

into five types by the coal mining industry in China.

The preceding specifications only consider

absolute gas emission on gas grading and zoning,

which is a single-index assessment system.

However, outburst assessment is different. An

assessment system does not reflect the synthetic

effect of gas tunnel disaster factors and cannot fully

evaluate the gas situation inside a tunnel (i.e.,

incomplete assessment criteria). Hence, the

grading and zoning assessment system of a tunnel

should be further improved. The present study is

based on the “gas tunnel railway technical

specifications” (TB10120-2002) and a large

number of existing gas tunnel engineering projects.

This paper conducted a preliminary study on the

risk assessment of gas tunnels and proposed a

hazard assessment system for gas tunnels.

1.3 Content of gas tunnel risk assessment

Considering the different natural conditions

and human activities in various regions, the

intensity of geological disasters also varies. General

regional geological hazards require vulnerability

and final risk assessments. A vulnerability

assessment contains a landslide that is subjected to

a control scale and other factors that vary in scope.

However, during the same landslide, some places

may be seriously damaged, whereas other places

Figure 2 Classification and partition technique flowchart of a gas tunnel.

Gas tunnel

classification

 

1. Low-gas tunnel

 2. High-gas tunnel

 

3. Outburst tunnel

 

Gas tunnel working area division

One index (gas emission) Four indices

 

1. Non-gas

working area

2.Low-gas

working area

3.High-gas

working area

 

4. Outburst

working area

Determined by the highest

gas concentration of the

tunnel working area

Gas outbur st forecast before

exposing the coalstr atum

1. No prominent danger 2. Outburst danger

Gas tunnel r ailway technical specifications

J. Mt. Sci. (2017) 14(9): 1751-1762

1756

are insignificantly damaged. This phenomenon is

related to the capacity for disaster prevention and

bearing of a place. In fact, risk analysis could be

described as a structured process that identifies

both the likelihood and consequences of hazards

arising from any given facility or activity (Summers

2000). Risk analysis can be conducted according to

the formula (risk = hazard × vulnerabi1ity), which

is provided by the Department of Humanitarian

Affairs in 1992 (UNDHA 1992).

Tunnel engineering belongs to a linear

underground project. In a small underground

space, once a gas disaster (mainly gas explosion)

occurs, the workers in the tunnel generally die on

the spot, and machinery and equipment are

severely damaged. After a gas disaster, the damage

rating of the affected object (people and equipment)

is the maximum, and the loss rate is substantially

100%. Vulnerability assessment is insignificant;

hence, this study only involves gas tunnel hazard

assessment.

The risk assessment of gas disasters performed

in this study involves the following three levels. In

planning the route selection stage, engineers can

only conduct an overall risk assessment of a tunnel

from the macro level for gas tunnel classification

assessment because of the limited information

available. During both the survey and design stages

and the construction stage, engineering conditions

should be extensively understood. Based on

detailed survey and construction data, risk

appraisal is performed for the different parts of a

tunnel according to the requirements; this process

describes gas tunnel engineering risk assessment.

When construction is close to the coal seam in the

highlighted danger zone of a tunnel, engineers deal

with tunnel face outburst risk assessment. Based

on the assessment results, engineers implement

timely corresponding engineering measures to

uncover coal construction and guarantee the safety

of construction; this process describes the tunnel

face outburst risk assessment.

During each stage of tunnel construction, the

obtained data and engineering requirements are

used to perform the corresponding risk assessment,

evaluate the possibility of occurrence of a gas

hazard (e.g., combustion, explosion, asphyxia, etc.),

and offer a scientific basis for the next stage to

ensure construction safety and progress as well as to

provide a good operating environment for the tunnel.

2 Risk Assessment System for Gas Tunnels

Tunnel construction projects, tend to be large,

complex, and expensive infrastructure

undertakings that encompass various types of risks

throughout the project lifecycle that arise from the

uncertain nature of the underground (Pamukcu

2015). Any tunnel construction project will have to

undergo the stages of planning and selecting lines,

reconnaissance, and design and construction

before it is completed. The information, problem,

and results are different for each stage. Based on

the concept of gas tunnel risk assessment, a gas

tunnel risk assessment system aims to work at

different stages, including the following three levels:

(1) the grading assessment of a gas tunnel during

the planning stage; (2) the risk assessment of gas

tunnel construction during the design and

construction stages; and (3) the gas tunnel

outburst risk assessment during the coal

uncovering stage (Figure 3). This risk assessment

system is not only in line with the project but also

presents scientific research ideas from the macro-

level to the micro-level.

During the planning stage, the overall risk

assessment of a tunnel can only be performed from

the macro-level due to lack of available data; this

process describes the gas tunnel classification

assessment. During the survey design and

construction stages, the requirement for

engineering conditions is higher. According to

detailed investigation and construction data, the

risks at the different parts of a tunnel should be

evaluated; this process describes the risk

assessment for gas tunnel construction. When the

construction area is close to the coal seam in a

prominently dangerous area of a tunnel, this paper

performs the tunnel face outburst risk evaluation.

Based on the evaluation result, engineers

implement the timely corresponding engineering

measures to uncover coal construction and

guarantee the safety of construction; this process

describes the tunnel face outburst risk assessment.

2.1 Gas tunnel classification assessment

When the gas content is high, the

concentration is also high; thus, gas accidents are

likely to occur, and the risk is great. Evaluating gas

J. Mt. Sci. (2017) 14(9): 1751-1762

1757

tunnel classification during the early part of the

planning and selection stages provides a qualitative

classification of the proposed tunnel; it is also the

first step in a risk assessment system. A tunnel is

regarded as engineering in its entirety. The grade

assessment of a tunnel is performed according to

the most favorable gas occurrence conditions (i.e.,

the highest gas content in the tunnel) to evaluate

the tunnel gas level.

Gas tunnel classification assessment is based

on the gas content from the gas disaster of the

studied tunnel. This assessment corresponds to the

classification of gas tunnels in the technical

specifications of a railway gas tunnel (Figure 2).

Based on the background of the geological

environment and the controlling effect of gas

interaction, people investigated the possibility of

the occurrence of a gas disaster in a relatively

stable environment without considering the effects

of human engineering activities in the future. The

susceptibility of a gas tunnel disaster is a

probability value, and the result of the assessment

does not have a clear time scale and is not

concerned with the gas accumulation stage of the

tunnel. The degree of gas tunnel disaster

probability is high, which roughly corresponds to a

special geological environment background that

determines if a gas disaster can easily occur.

Numerous factors affect gas tunnel disasters.

In addition to the factors that affect coal seam gas,

other factors related to the tunnel exist. Factors

that affect the generation and preservation of gas in

coal seams mainly include the coal-bearing strata

and the surrounding rock of a coal seam (i.e.,

stratigraphic lithology), geological structure,

metamorphic degree of coal, intrusion of magma

into coal seams, hydrogeological conditions, and

seam depth. The influencing factors related to the

tunnel include tunnel depth, tunnel length, tunnel

thickness through the coal seam, and distance

between the tunnel construction area and the coal

seam.

Based on the aspects of gas generation,

preservation conditions, and distribution pattern,

Figure 3 Risk assessment of a tunnel gas disaster.

J. Mt. Sci. (2017) 14(9): 1751-1762

1758

the geological environment of the gas tunnel status

and the regional gas distribution in China is

determined to be closely related according to the

statistical analysis of gas tunnels in China. Over

97.5% of gas tunnels are located in 20 large

regional gas areas in China (Kang 2009). High-gas

tunnels are located in high-gas areas, and low-gas

tunnels are located in low-gas areas. Therefore,

having a good command of regional gas zoning

characteristics is important to determine gas

tunnel classification; moreover, it can be used as an

important index of macro judgment during the

planning and selection stages.

In the aforementioned situation, incorporating

all the elements that reflect the conditions of gas

generation into a potential hazard analysis is

impossible and unnecessary. On the basis of a

comprehensive analysis of gas geological

conditions in the research area, as well as the

principle of combining dominant factors,

comprehensive principles, operational principles,

and qualitative and quantitative analyses, this

paper considered the influences of various factors

and the possible combinations of the various

factors on the occurrence of gas tunnel disasters

and identified gas area, tunnel depth, geological

structure, coal seam thickness, and groundwater

level as gas tunnel classification indices (Table 2).

Then, this paper established the assessment indices

and criteria of gas tunnel classification, predicted

the most serious gas disasters that could occur in

the tunnel excavation process, proposed the

corresponding measures, and provided the basis

for organization and construction.

2.2 Risk assessment of gas tunnel construction

Given the special characteristics of gas tunnels,

only a small portion of a coal seam exists, that is,

only a part of a tunnel poses a gas hazard. If the

entire tunnel is classified to be of construction

grade based on the gas tunnel classification system,

then complying with the “coal mine safety

regulations” (State Administration of Work Safety

and State Administration of Coal Mine Safety 2016)

and relevant provisions will considerably increase

the investment costs and extend the time limit.

Moreover, given the complex geological conditions

of a tunnel, along with the continuous development

of construction, deviations also occur between the

gas tunnel classification and the actual situation.

Conducting a risk assessment of gas tunnel

construction is necessary to ensure the safety and

progress of a construction project.

Gas tunnel construction risk assessment

during the design and construction stages is based

on the assessment of gas tunnel classification. The

grading assessment results have four grades:

micro-gas tunnel, low-gas tunnel, high-gas tunnel,

and gas outburst dangerous tunnel. For micro-gas

tunnels, risk and outburst risk assessments are not

performed. The design and construction stages of

the gas tunnel construction risk assessment are

divided into three parts, namely, gas tunnel

construction area division, superposition of

construction factors, and generation of

construction risk assessment results. This paper

followed the principles of assessment index

selection and integrated the factors of construction

area division and the human factors into the

construction process. Then it proposed that the risk

factors of gas disasters in tunnel construction are

divided into four categories, namely, basic

geological factor, gas factor, engineering factor, and

human factor. Ten assessment indices (Figure 4)

were used to predict gas hazard risk in tunnel

excavation.

According to past experience in mature

geological hazard assessment and the reference for

the type of gas tunnel disasters (i.e., the “gas tunnel

railway technical specifications”) in work area

classification, this paper divided the gas tunnel

construction risk into four grades during a

comprehensive analysis of the various factors

based on the assessment. The grades are secured

(I), safe (II), slightly dangerous (III), and

dangerous (IV). This paper followed the principle

that a greater the more dangerous of the factors.

Secured (I): The majority of the factors are in

favor of safety. The gas tunnels are in a safe

condition. No accident occurs. Gas disaster and

Table 2 Classification assessment indices of a gas tunnel

Assessment index

Gas area Nature of gas zone

Geological structure Closed coal seam degree

Coal seam thickness Thickness (m) Tunnel depth Buried depth (m) Groundwater level Spring flow (L/s)

J. Mt. Sci. (2017) 14(9): 1751-1762

1759

accident do not occur in the present situation and

in the construction process.

Safe (II): The majority of the factors are in

favor of safety. The gas tunnels are in a safe

condition. No gas accident occurs. Several small

status quo and dangerous conditions may occur

during the construction process, such as choking

and partial combustion; however, no gas accident

threatens the health of workers.

Slightly dangerous (III): More than half of the

factors are not conducive to safety. Some factors

have reached a dangerous status. Several slightly

hazardous situations have occurred in the tunnel.

Gas disasters may occur under status quo

conditions and during the construction process.

Dangerous (IV): The majority of the factors are

not conducive to safety. Most factors tend to have

dangerous values. The occurrence of a gas disaster

is possible. Several accidents occur under status

quo conditions and during the construction process,

such as explosions or coal and gas outburst, which

result in casualties.

Gas tunnel zoning assessment corresponds to

the gas tunnel work area division of the “gas tunnel

railway technical specifications” (Figure 2). Hence,

the disaster probability assessment of each tunnel

section under natural geological conditions

provides the basis and background value of

construction hazard assessment. Construction risk

assessment factors are more complex than those

for zoning assessment.

During the tunnel survey and design and

construction stages, people performed numerous

explorations and test works in the tunnel site and

obtained detailed information on the geological

conditions of the tunnel. Risk assessment is mainly

for different tunnel segments. Therefore, a linear

tunnel project should be evaluated in sections. A

detailed and accurate classification assessment

index for gas tunnel construction risk assessment

can provide clear guidance for the next

construction project. During the construction of

tunnels, information becomes available regarding

the geological conditions crossed by the tunnel,

behavior of the excavation as well as information

on the construction. This information should be

used to update the predictions during the design

phase. In the context of the developing

Figure 4 Risk assessment index of gas tunnel construction.

Gas factors

Geological

factors

Absolute gas emission

Gas tunnel zoning

evaluation

Correction factors

during construction

Risk assessment

o f tunne l

cons t ruc t ion

Human

factors

Gas pressure

Geological structure

Coal body structure

type

Groundwater

condition

Control measures

Gas management

Experience in gas

tunnel construction

Technology skills in

tunnel construction

Distance from the

working area to the

coal seam

Engineering

factor

J. Mt. Sci. (2017) 14(9): 1751-1762

1760

methodology, great emphasis will be placed on

updating the geological conditions for the part of

tunnel that has not been excavated based on the

geological conditions encountered during

excavation. This will then be used to update the

construction strategy for the remaining

unexcavated part of the tunnel (He et al. 2015).

2.3 Tunnel face outburst risk assessment in gas tunnels

Gas tunnel construction risk assessment

results have four levels, namely, a safe area, a

relatively safe area, a slightly dangerous area, and a

danger zone. The tunnel disaster of gas at working

face creates a risky working environment for

workers, and causes amass of casualties in

construction around the world (Zhang et al. 2015).

When tunnel construction occurs in a slightly

dangerous area and the hazardous area proceeds

toward the gas stratum, the tunnel face (i.e., the

“highlighted assessment”) should be evaluated. An

outburst assessment provides gas tunnel disaster

possibility assessment for a tunnel face.

This paper successively conducted forecast

assessment for the following indices: maximum

depth, geological structure, dynamic phenomena

when drilling forward, highest gas pressure,

maximum initial velocity of gas emission from

boreholes, and the coal structure type (Table 3).

Considering the comprehensive hypothesis of coal

and gas outburst (coal and gas outburst is the

result of the combined effect of three factors,

namely, ground pressure, high-pressure gas, and

coal structure) as a guide, the selection and

identification of six indices of outburst risk

assessment is according to the actual situation of

gas tunnel and coal mine research, combined with

the gas tunnel railway technical specifications

(TB10120-2002) and the risk assessment

achievements. When the six indicators are all grade

I, the tunnel face presents no prominent danger.

When three or more indicators are level III, the

working face is an outburst zone, and the

construction workers should take outburst

prevention measures. The remaining cases are

classified as outstanding threat areas.

Outburst assessment is based on the latest

data of the tunneling process. A special assessment

of outburst hazards, which that can lead to

disasters that will directly harm the workers,

should be conducted. Construction risk assessment

would be modified, and outburst disasters in the

tunnel face would be predicted to provide

protection for tunnel driving.

3 Engineering Application

The Zipingpu Tunnel is located in the

Dujiangyan–Wenchuan Highway Section of State

Road 317 in Sichuan. The length of the left tunnel is

4090 m, and that of the right tunnel is 4060 m.

The gas emission from the tunnel through the coal

seam and the coal measure strata in the initial

survey design is less than 0.5 m3/min. Based on the

“technical specifications for a railway gas tunnel”

(TB10120-2002) (i.e., the relevant provisions of the

relevant highway tunnel code without gas

classification), the tunnel is a low-gas tunnel.

However, at 2 p.m. on December 22, 2005, a huge

gas explosion occurred during the construction of

the right tunnel hole.

The Zipingpu Tunnel is located in the high-gas

zone of the Dabashan Mountain of the Longmen

Mountain in the Sichuan Basin. It has a buried

depth of over 550 m. Three main geological

structures are crossed by the tunnel, namely, the

Gongjia Syncline, Shajin Syncline, and Gongjia

Anticline. The lithology of the tunnel mainly

Table 3 Assessment system for outburst danger indices

Assessment index Level

Ⅰ Ⅱ Ⅲ

Depth (m) ＜300 300–500 ≥500

Geological structure No General Complex outburst construction Dynamic phenomena when drilling forward

No dynamic phenomenon Collapse hole, drill clip Orifices, splitting sound, thunder

Gas pressure (MPa) ＜0.35 0.35–0.74 ≥0.74

Initial velocity of gas emission from borholes (L/m∙min)

＜4 4–6 ≥6

Coal structure type Original structure of coal Fragmentation of coal Crushed coal, coal mylonite

J. Mt. Sci. (2017) 14(9): 1751-1762

1761

comprises gray and dark gray mudstone, gray black

carbonaceous mudstone, a thin coal layer of soft

rock (the thickness of the coal seam can reach 0.4

m, and the seam is rich in gas), and less yellow

sandstone and siltstone to hard rock. The aquifer of

the tunnel site is mainly Triassic Xujiahe sandstone

fissure water aquifer, followed by quaternary loose

heap alluvium pore aquifer. The water content of

the sandstone fractured aquifer is moderate, and

the groundwater is a multi-layer fissure water

controlled by lithology. Water content is generally

scarce and a small amount of linear water flows.

According to the principle and method of gas

tunnel classification assessment, the Zipingpu

Tunnel exhibits the risk of gas outburst. Engineers

determined that the working face of the absolute

gas emission rate in the Zipingpu Tunnel reaches

3.19 m3/min. Hence, the Zipingpu Tunnel is a high-

gas tunnel. When the left tunnel is excavated to

K14+878, three cracks on the lower left of the

tunnel face emerged; the cracks have a length of 2-

5m, and a maximum width of approximately 2 cm,

and they are along the circumferential direction of

the tunnel in which a terminal has a small hole of

approximately 10 cm2 The fracture and the small

hole produced a hissing sound, and spurted gas

sharply outward. A portable gas detection

instrument and optical gas detection instrument

was used to detect the hole and crack, respectively;

the gas concentration reached more than 10%.

Then, the small hole also discharged black liquid

mixed with tiny coal and carbonaceous mudstone

block; the spray distance was 3.0 m. Site analysis

concluded the possibility of a great danger of

outburst.

Based on the gas tunnel classification

assessment, the gas tunnel is identified through the

“highest level of gas content in the tunnel.” The

outburst risk assessment of left tunnel face at

K14+878 was performed using the comprehensive

assessment method; the assessment results are

shown in Table 4. The four indicators for level Ⅲ

(i.e., geological structure, dynamic phenomenon of

pre-drilling, gas pressure, and coal structure type)

were detected, whereas the index of the depth for

the level I (i.e., initial velocity of gas emission from

boreholes), was not detected. Comprehensive

assessment of the left tunnel face at K14+878 with

outburst danger was performed. The Zipingpu

Tunnel is accordingly a gas outburst tunnel. The

assessment results were verified by the

construction process, and the results are consistent.

This method can be applied in practical

engineering.

4 Conclusion

Gas tunnel classification assessment,

construction risk assessment, and the assessment

of the outburst factors of a tunnel face exhibit

similarities and differences. They do not

fundamentally differ in terms of methods, but

different factors at different stages are considered.

These three phases constitute the gas disaster risk

assessment system for a tunnel project. In this

study, develop a fast and accurate assessment

method for gas disasters in tunnel line selection,

design, and construction phases. Engineers then

implement appropriate engineering prevention and

control measures. This study is not only consistent

with the construction procedure of a project but

also presents scientific ideas from the macro level

to the micro level.

Acknowledgments

The authors would like to acknowledge the

support by the National Natural Science

Foundation of China (Grant No. 41302244). And

also would like to express appreciation to the

reviewers and editors for their valuable comments

and suggestions that helped to improve the quality

of the paper.

Table 4 Outburst risk assessment of left tunnel face at K14+878

Index Index value Index Level

Depth 270 m Ⅰ

Geological structure Complex outburst construction

Ⅲ

Dynamic phenomena when drilling forward

Orifices, Hold the drill bit

Ⅲ

Gas pressure (MPa) 0.83 MPa Ⅲ Initial velocity of gas emission from borholes (L/m.min)

Test failed -

Coal structure type Crushed coal, coal mylonite

Ⅲ

J. Mt. Sci. (2017) 14(9): 1751-1762

1762

References

Aleotti P, Chowdhury R (1999) Landslide hazard assessment: summary review and new perspectives. Bulletin of Engineering Geology and the Environment 58(1): 21-44

Brown ET (2012) Risk assessment and management in underground rock engineering – an overview. Journal of Rock Mechanics and Geotechnical Engineering 4(3): 193-204. https://doi.org/10.3724/SP.J.1235.2012.00193

Chang XW, Wang GN, Liu YP (2013) Research on Gas Characteristics and Partition Prediction of Gas of Laoshishan Tunnel. Journal of Railway Engineering Society (4): 79-84. (In Chinese)

Chapman CB, Cooper DF (1987) Risk Analysis for Large Projects: Models, Methods and Cases. John Wiley & Sons Publishers.

Clifton AE (2005) Hazard analysis techniques for system safety.John Wiley & Sons Inc., USA. pp 14, 27.

Cao LM, Wang H (2013) Prevention in construction of Wushaoling tunnel gas disaster and Countermeasures. Inner Mongolia Science Technology & Economy (10): 92-93. (In Chinese)

Doyle BR (1976) Hazardous gases underground, applications to tunnel engineering. Copyright 2001 by Marcel Dekker.

Doyle BR (2001) Hazardous gasses underground. Marcel Dekker, New York. p 375.

Gan GY (2014) The geological characteristics of the storage status of the oil and gas along the Chengdu-chongqing passenger railway and comprehensive evaluation on the tunnel gas hazards. Southwest Jiaotong University geological engineering master degree thesis, Chengdu. (In Chinese)

Han YF, Xie SH, Yang L (2011) Detection of coal seam gas content in majiapo tunnel and gas project rating. Journal of Shijiazhuang Tiedao University (Natural Science) 24(4): 74-77. (In Chinese)

He M, Sousa RLE, Müller A，et al. (2015) Analysis of excessive deformations in tunnels for safety evaluation. Tunneling and Underground Space Technology 45: 190-202. https://doi.org/ 10.1016/j.tust.2014.09.006

Hu CT (2012) Discussion of coal seam uncovering and outburst prevention technology for high gas working area of Sanlian tunnel. Engineering construction (2): 56-59. (In Chinese)

Hu GS (2013) Gas monitoring and prevention measures of Huangjialiang Tunnel. Technological Development of Enterprise 32(28): 10-13. (In Chinese)

Kang XB (2009) Study on Gas Disaster Risk Assessment System of Tunnel Engineering. Chengdu University of Technology geological engineering Ph. D. Dissertation, Chengdu. (In Chinese)

Kang XB, Xu M, Luo S, et al. (2013) Study on formation mechanism of gas tunnel in non-coal strata. Natural Hazards 66(2): 291-301. https://doi.org/10.1007/s11069-012-0484-y

Liu YB (2014) Construction technology of high gas tunnel in Guizhou section of Shanghai-Kunming railway line for passenger traffic. Sci-Tech Innovation & Productivity 241: 96-99. (In Chinese)

Liu YC, Xing GL (1999) A Discussion On Classification of Mine Gas. Mine Safety (8): 34-35. https://doi.org/10.13347/j.

cnki.mkaq.1999.08.016 (In Chinese) Pamukcu C (2015) Analysis and management of risks

experienced in tunnel construction. Acta Montanistica Slovaca 20(4): 271-281.

Professional Standard of the People's Republic of China (2003) Technical Code for Railway Tunnel with Gas(TB10120-2002). Beijing: China Railway Publishing House. (In Chinese)

Rausand M (2011) Risk assessment; theory, methods and application. John Wiley & Sons Inc., USA. pp 4-6.

Shi L (2012) Gas formation mechanism and Construction Countermeasures in the Xujiawan Tunnel. Railway Standard Design(s): 123-125. (In Chinese)

Sousa RL, Einstein HH (2012) Risk analysis during tunnel construction using Bayesian Networks: Porto Metro case study. Tunneling and Underground Space Technology 27(1): 86-100. https://doi.org/10.1016/j.tust.2011.07.003

State Administration of Work Safety and State Administration of Coal Mine Safety (2016) Coal Mine Safety Regulations. Beijing: China Coal Industry Publishing House. (In Chinese)

Su PD, Li ZB, Fan XL (2011) Prediction and study of shallow reservoired natural gas in Meilingguan tunnel of Lanzhou-Chongqing railway. Subgrade Engineering (1): 28-30. (In Chinese)

Summers J (2000) Analysis and management of mining risks.Proceedings of the MassMin, Australia. pp 63-64.

Tang YQ, Ye WM, Yu H (2003) Marsh gas in shallow soils and safety measures for tunnel construction. Engineering Geology 67(3): 373-378. https://doi.org/10.1016/S0013-7952(02) 00207-7

UNDHA (United Nations, Department of Humanitarian Affairs) (1992) Internationally Agreed Glossary of Basic Terms Related to Disaster Management. Geneva: United Nations Department of Humanitarian Affairs.

Wen ZJ, RinneM, Han ZZ, et al. (2014) Structure model of roadway with large deformation and its basic research into engineering theories. TehničkiVjesnik 21(5): 1065-1071.

William J, Petak AA (1993) Natural disaster risk assessment and mitigation policies. Xiang LY, Cheng XT, et al. (Translates), Beijing: Seismological Press.

Yang DZ (2014) Analysis on the effect of highway tunnel gas migration and gas-soild coupling. Southwest Jiaotong University architectural and civil engineering master degree thesis, Chengdu. (In Chinese)

Yu QX (1992) Mine Gas Control. Xuzhou: China University of Mining and Technology Press. (In Chinese)

Yuan H (2014) Research on gas occurrence and migration in tunnel of Cheng-de-nan express way. Southwest Jiaotong University geological engineering master degree thesis, Chengdu. (In Chinese)

Zhang H, Pera LS, Zhao Y, et al. (2015) Researches and applications on geostatistical simulation and laboratory modeling of mine ventilation network and gas drainage zone. Process Safety and Environmental Protection 94: 55-64. https://doi.org/10.1016/j.psep.2014.10.003