Missouri University of Science and Technology
Scholars' Mine
International Conference on Case Histories in
Geotechnical Engineering
(2008) - Sixth International Conference on Case
Histories in Geotechnical Engineering
Aug 11th - Aug 16th
Lessons Learned from Slope and Trench Failures in
Japan
Yasuo Toyosawa
National Institute of Occupational Safety and Health, Tokyo, Japan
Sahaphol Timpong
National Institute of Occupational Safety and Health, Tokyo, Japan
Kazuya Itoh
National Institute of Occupational Safety and Health, Tokyo, Japan
Follow this and additional works at: http://scholarsmine.mst.edu/icchge
Part of the Geotechnical Engineering Commons
Recommended Citation
Toyosawa, Yasuo; Timpong, Sahaphol; and Itoh, Kazuya, "Lessons Learned from Slope and Trench Failures in Japan" (2008).
International Conference on Case Histories in Geotechnical Engineering. 33.
http://scholarsmine.mst.edu/icchge/6icchge/session02/33
This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International
Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright
Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact
[email protected].
LESSONS LEARNED FROM SLOPE AND TRENCH FAILURES IN JAPAN
Toyosawa Yasuo
National Institute of Occupational
Safety and Health
Tokyo, Japan 2040024
Timpong Sahaphol
National Institute of Occupational
Safety and Health
Tokyo, Japan 2040024
Itoh Kazuya
National Institute of Occupational
Safety and Health
Tokyo, Japan 2040024
ABSTRACT
This paper presents case histories of slope and trench failure accidents in Japan during the period of 1989 to 2001 based on the
database from construction industrial labor accident reports. The major risk factors of the failures are identified by considering the
location of slope and trench failures, types of construction, slope and trench geometries, scale of slope failure, time of occurrence and
characteristics of worker involved in the failures. It was found that it is necessary to improve the safety standards and provide more
safety education and training especially in the small construction projects to prevent the slope and trench excavations in future.
INTRODUCTION
Slope and trench excavations are one of the most hazardous
operations in geotechnical works. In many cases slope and
trench failures are likely to occur rapidly without any clear
signs of failure, most of construction workers have insufficient
time to escape from the excavation sites and hence, the labor
accidents generally take place. In Japan, the slope and trench
excavations have caused a number of fatalities and injuries of
construction workers throughout time. Figures 1 and 2 show
the example of labor accidents caused by slope and trench
excavations, respectively. The number of labor accidents and
fatalities caused by slope and trench excavations in Japan is
shown in Fig. 3. During the period of 1989 to 1996 about 24
and 37 workers were killed each year in trench and slope
excavations, respectively. However, the number has decreased
from 1996 onwards and it was about 11 and 25 workers per
year for slope and trench excavations, respectively. The
possible reasons for the decrease in the number of fatalities
were development of laws relating to safety measures on
construction sites and impacts of economy and employment
conditions. As can be seen in the figure, the number of labor
accidents caused by trench excavations is generally 2 times
higher than that of slope excavations. This is probably due to
the fact that the trench excavation work not only presents all
the special challenges of a confined space environment, but it
also puts the workers at risk of being struck by objects and of
being caught in a trench cave-in. The labor accidents are
costly in terms of direct and indirect cost to the construction
industry. The direct costs associated with each slope and
trench failures include compensation insurance, penalties and
fines for violating safety standards, rescue operation and body
recovery. The indirect costs include work schedule delays,
time and effort to training a new worker.
Paper No. 2.47
Fig. 1. Labor accident caused by slope failure.
Fig. 2. Labor accident caused by trench collapse.
1
70
Slope excavations
Trench excavations
Fatalities due to slope failures
Fatalities due to trench failures
Number of accidents (cases)
60
50
60
50
40
40
30
30
20
20
10
10
Number of fatalities (people)
70
0
0
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Year
Fig. 3. Number of labor accidents caused by slope and trench
excavations in Japan during 1989-2001.
destabilization of slope when the lower part of a natural slope
is removed or a part of the slope is steepened. There are two
general construction methods for stabilizing slope: retaining
wall and slope improvement methods. Based on the labor
accident reports, it was found that about 73% of the labor
accidents occurred during works involving slope stabilization
by the retaining wall method. In contrast, the number of
accidents that occurred during works using the slope
improvement method was smaller (about 15%). This is
probably due to the fact that the retaining wall construction
generally includes the excavation of steep and unstable slope,
and it also requires the workers to work in a narrow space
between the retaining wall and the excavated ground such as
assembling/dismantling of formworks, smoothing of base and
etc. Therefore, the workers have no sufficient time to escape
from such narrow space when slope failure occurred.
Hokkaido
In order to establish effective measures to prevent the labor
accidents caused by slope and trench excavation works in
future, this paper investigated the labor accidents and analyzed
them from various perspectives. The information of the labor
accidents occurred during the slope and trench excavations in
Japan during the period of 1989-2001 was obtained from the
construction industrial labor accident reports of the Japan
Construction Safety and Health Association (JCSHA). The
main information extracted from each report was location of
accident, types of construction, slope and trench geometries,
scale of failure, time of occurrence and characteristics of
worker involved in the accidents. It should be noted that some
reports have incomplete data.
Tohoku
Chubu
Chugoku
Kanto
Kinki
Shikoku
Okinawa
CHARACTERISTICS OF LABOR ACCIDENTS CAUSED
BY SLOPE AND TRENCH FAILURES
Fig. 4. Geographical regions of Japan.
110
Location of Slope and Trench Failures
Slope excavations
Trench excavations
100
90
Number of accidents (cases)
Japan is officially divided into 8 main geographical regions:
Hokkaido, Tohoku, Kanto, Chubu, Kinki, Chugoku, Shikoku
and Kyushu (including Okinawa) regions as presented in Fig.
4. Figure 5 shows the distribution of the location of labor
accidents in slope and trench excavations, the region with the
highest percentage of accident was Chubu, followed by Kanto,
Kinki and Kyushu regions. Since no data were available on the
total number of slope and trench excavations in each region,
so it was not possible to determine that slope and trench
excavations in these regions were statistically more or less
safe than other regions. However, it should be noted that most
of natural landslides and slope failures in Japan commonly
occur in these regions.
80
70
60
50
40
30
20
10
0
Hokkaido Tohoku Kanto
Chubu
Kinki Chugoku Shikoku Kyushu
Location
Fig. 5. Location of slope and trench failures.
Types of Construction
Figures 6 and 7 show the labor accidents classified by type of
slope and trench excavations, respectively. In case of slope
excavation most of slope failure occurred due to
Paper No. 2.47
As presented in the Fig. 7, the sewer, water supply and
drainage pipes installations are the most frequent type of
trench excavation when the trench collapse occurred. By
consider the project cost and construction period of slope and
2
trench excavations the possibility of labor accident was higher
in the construction work with a lower amount of project cost
and relatively short construction period. Figure 8 shows the
number of workers presented at the time of slope and trench
failures; approximately 85% and 91% of the accident occurred
in slope and trench excavation projects with less than 8
workers on the construction site. Because the number of
workers can be related to the size of the contractor, so the
possibility of accident is higher with the small contractor. This
may be explained by the fact that the small contractors may
not have operation chief and/or worker with appropriate
knowledge concerning construction safety and they may not
have sufficient budgets to provide adequate safety equipments.
Retaining wall method (73%)
Slope improvement method
(15%)
Others
(12%)
Slope and Trench geometries
Figure 9 shows the number of slope and trench failures
according to the slope angle. In case of slope excavation, most
of slope failure (50%) occurred when the slope angle was 60
to 75 degrees. Because the minimum construction cost for
trench excavation is generally achieved by adopting the
steepest possible trench wall slope angles, unsurprisingly,
about 77% of the accidents occurred in trenches with the slope
angle of 90 degrees as shown in the figure. According to the
study on natural slope failures in Japan by Monma et al.
(1999), the slope failures occurred most frequently with the
slope angle of about 40 to 49 degrees. Therefore, the
frequency of failure of steep slope during slope and trench
excavations is higher than that of the natural slope. Figure 10
shows the labor accidents caused by slope excavations
classified by slope height. It was found that most of slope
failures (55%) occurred with the slope height of 2 to 10m.
Figure 11 shows the number of accidents according to trench
depth, about 80% of trench collapses were reported in trenches
shallower than 3m and only 2% of accidents occurred in
trenches deeper than 5m. It should be noted that only 4% of
slope failures occurred in slope height less than 2m, however,
in case of trench excavation about 40% of trench collapses
was observed in trenches shallower than 2m.
100
Fig. 6. Types of slope excavation.
90
Slope excavations
Trench excavations
Building and foundation
(12%)
Gas line and cable
(4.6%)
Road and railroad
(2.6%)
Others (3.6%)
Sewer, water supply, and drainage pipes
(77.2%)
70
60
50
40
30
20
10
0
less than 60
Fig. 7. Types of trench excavation.
60-75
76-89
90
Slope angle (degrees)
70
Fig. 9. Number of slope and trench failures by slope angle.
60
Number of accidents (%)
Number of accidents (%)
80
Slope excavations
Trench excavations
Slope height
50
40
34%
30
< 2m
2-5m
5-10m
10-30m
30-50m
> 50m
29%
20
10
3.2%
4.3%
4.3%
0
1-4
5-8
9-12
13-19
Number of workers (people)
Fig. 8. Number of workers presented at the time of failure.
Paper No. 2.47
26%
Fig. 10. Number of slope failures by height of slope.
3
This indicated that even in shallow trenches the possibility of
accidents still exists due to the confined space environment of
trench excavation. Figure 12 shows the number of trench
failures based on the trench depth and length ratio, D/L. It was
found that 85% of accidents occurred in trenches with D/L
less than 0.6. Because longer trenches have a tendency to be
affected more by surcharge load from spoil pile and vibration
of construction machines.
Collapsed soil
3
7%
6%
4%
5%
19%
Trench depth
<1m
1-2 m
2.1-3 m
3.1-4 m
4.1-5 m
>5m
39%
< 50m
3
51-100m
3
101-150m
3
151-200m
3
201-400m
3
> 400m
59%
Fig. 13. Number of slope failures by collapsed soil volume.
Collapsed soil
13%
3
13%
5.1%
< 1m
3
1-2m
3
2.1-3m
3
3.1-5m
3
> 5m
17%
2.1%
0.68%
40%
15%
Fig. 11. Number of trench failures by depth of trench.
36%
D/L ratio
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1.0
1.1-1.5
19%
19%
Fig. 14. Number of trench failures by collapsed soil volume.
22
20
38%
3.7%
28%
Fig. 12. Number of trench failures by D/L ratio.
Number of accidents (%)
3.7%
Slope excavations
Trench excavations
18
8.6%
16
14
12
10
8
6
4
2
Scale of Slope Failure
0
Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec
Figures 13 and 14 show the estimated amount of collapsed soil
that observed in the slope and trench failures, respectively. As
expected, the amount of collapsed soil in the slope excavation
is generally larger than that of the trench excavation. It was
found that 59% and 55% of slope failures occurred when the
amount of collapsed soil was less than 50m3 and 2m3 for slope
and trench excavations, respectively. By comparing to the size
of excavations, the amount of collapsed soil is generally small,
therefore, the scale of slope failure at the time of accident
could be classified as small-scale slope failure. Because the
small-scale slope failure generally occurs rapidly, so the
workers tend to suffer from the labor accidents since they have
almost no time to escape from the working area especially in
the confined space environment in the retaining wall
construction and the trench excavation.
Paper No. 2.47
Month
Fig. 15. Number of slope and trench failures by month.
Time of Occurrence
The distribution of accidents based on month of the year is
shown in Fig. 15. It is also possible that the economic pressure
to complete the projects before the end of budget and before
the winter can lead the high number of accidents as shown in
March and November, respectively. Figure 16 shows the
number of slope and trench failures by the time of occurrence,
the accidents mainly occurred before lunch break (10-11a.m.),
after lunch break (2-3p.m.) and before the end of working time
(4-5p.m.). However the reason for this situation is unclear.
4
22
40
Slope excavations
Trench excavations
20
35
16
Number of fatalities (%)
Number of accidents (%)
18
14
12
10
8
6
4
Slope excavations
Trench excavations
30
25
20
15
10
5
2
0
8a.m. 9a.m. 10a.m.11a.m.12a.m. 1p.m. 2p.m. 3p.m. 4p.m. 5p.m.> 6p.m.
0
0-5
Time of occurrence
6-10
11-20
21-30
31-40 More than 40
Experience of worker (years)
Fig. 16. Number of slope and trench failures by time.
Fig. 18. Number of fatalities by experience of worker.
50
CONCLUSIONS
45
Slope excavations
Trench excavations
Number of fatalities (%)
40
35
30
25
20
15
10
5
0
15-19
20-29
30-44
45-54
55-64 More than 65
Age of worker (years)
Fig. 17. Number of fatalities by age of worker.
Characteristic of Workers involved in Labor Accidents
The distribution of worker age observed in the labor accidents
is shown in Figure 17. It was found that about 78% and 72%
of the accidents involving workers older than 45 years for
slope and trench excavations, respectively. This indicated that
the older workers tend to be involved in the labor accidents. In
addition, inexperienced workers tend to have more accidents
compared to experienced workers as shown in Fig. 18.
Because experienced workers can sometimes detect the
instability of slope for example from tension cracks and
changes in soil texture, however, inexperienced workers are
less likely to detect these signs. Although the experienced
workers may have received more training concerning
construction safety, it was found that about 30% of workers
working in construction site at least 21 years before they were
involved in the slope and trench failures. Because the
experienced workers could be related to the older workers and
in many cases the experienced workers tend to take
unnecessary risks.
Paper No. 2.47
The case histories of slope and trench failures in Japan during
the period of 1989-2001 were presented in this paper. The
failures are likely to occur in the small construction projects
conducted by the small contractors. Most of slope and trench
failures occurred in works involving the slope stabilization by
retaining wall method and the installation of pipelines. The
scale of slope failure at the time of accident is generally small
and the failure occurs rapidly so the workers have almost no
time to escape from the confined space working area. Most of
slope failures occurred when the gradient of slope was 60° to
75° with the slope height of 2-10m. In case of trench
excavations, most of trench collapses occurred in the trenches
shallower than 3m with slope angle of 90°. By considering the
time of occurrence, the slope and trench failures mainly
occurred before/after lunch break and before the end of
working time, the economic pressure can also lead the high
number of slope and trench failures. Based on the
characteristic of workers, the older and inexperienced workers
tend to be involved in more labor accidents. These findings
emphasized that it is necessary to improve the safety standards
and provide more safety education and training especially in
the small construction projects to prevent the failures in future.
REFERENCES
Japan Construction Safety and Health Association [19892004] “Construction industrial labor accident reports.
Construction Safety and Health Yearbooks”.
Monma, K., Chida, Y., and Ebihara, K. [1999]. “Actual
condition of landslide disaster”. Ministry of Construction
Public Works, Document of Public Works Research Institute
(PWRI), No. 3651, pp.180. (in Japanese)
5