- Email: [email protected]
Slope Failure of Embankment in Sanyo Expressway Due to Passage of Typhoon no. 14 in 2005
SOILS AND FOUNDATIONS Japanese Geotechnical Society
Vol. 49, No. 5, 797–806, Oct. 2009
SLOPE FAILURE OF EMBANKMENT IN SANYO EXPRESSWAY DUE TO PASSAGE OF TYPHOON NO. 14 IN 2005 HIDEKAZU MURATAi), KAZUYA TAKEKUNIii) and YUKIO NAKATAiii) ABSTRACT Typhoon No. 14 in 2005 caused signiˆcant rainfall in Yamaguchi Prefecture of Japan, and, due to this, an embankment of Sanyo Expressway at Hataki in Iwakuni city failed. Detailed investigation was conducted after this event in order to identify the mechanism of this failure, which consisted of the ˆeld studies as well as laboratory tests and numerical analyses. It was consequently shown that the combined eŠects of heavy rainfall and the local geological and topographic conditions as well as the reduction of drainage capability due to the breakage of perforated pipe in the underdrainage system were the causes of the failure. Key words: case history, embankment, expressway, rainfall, seepage, slope stability (IGC: C7/E6) PM on 6th of September due to the rainfall, and no distortion was noticed during the inspection at around 11 PM on 6th of September. The concerned part of the Sanyo Expressway was opened to tra‹c on 25th June, 1992. During the 13-year period thereafter until the failure, it experienced heavy rainfall several times as listed in Table 2. However, neither slip failure nor any instability had occurred. Obviously, the never-experienced intensity of precipitation due to the present typhoon triggered the failure. This point is further evidenced by Fig. 3 in which the time history of accumulated rainfall is illustrated. It is noteworthy that the present rainfall lasted for more than 60 hours and the total precipitation exceeded 500 mm. Photo 2 illustrates the aerial view of the damaged area to depict three existing faults. The most important faults herein are called the Otake fault and the Iwakuni fault. Being sandwiched between these faults, the damage area has a small Hataki active fault which crosses the expressway embankment in the N-S direction. The preliminary geological study prior to the expressway construction reported spring water at the crossing of the fault on the expressway route. Furthermore, the site of the present embankment failure is identical with the location of debris ‰ow in the past.
TYPHOON NO. 14 IN 2005 Typhoon No. 14 of 2005 started on 29th of August in the Paciˆc Ocean and proceeded towards North West while getting greater in its size as shown in Fig. 1. When it landed at the Kyushu Island of Japan on September 6th, its maximum wind velocity was 35 meters/s and its range of strong wind (À15 m/s) reached 750 km on its eastern side and 700 km on the west. The eŠects of this typhoon were characterized by heavy rainfall; recording 1321 mm in the extreme case at Mikado in Miyazaki Prefecture of Kyushu. Yamaguchi Prefecture experienced the typhoon No. 14 from the morning of September 5th till the morning of 7th. Since this typhoon was large in size and low in velocity, its eŠects lasted for a long time. Table 1 indicates the rainfall records at diŠerent places in the prefecture. The locations of the sites in this table are depicted in Fig. 2 together with the place of embankment failure which is the topic in this paper. DETAILS OF RAINFALL-INDUCED FAILURE OF SANYO EXPRESSWAY EMBANKMENT The embankment failure as shown in Photo 1 occurred at around 0:50 AM on 7th September, 2005 during the typhoon. Half of the expressway, which was the eastbound lanes, collapsed over a length of 50 m. The depth of failure was 23 m and the total volume of the failed soil mass was 13,800 m3, which was deposited on the adjacent local road with a thickness of 4 m. Unfortunately 2 houses were buried under the soil resulting in the death of 3 residents. The expressway had been closed since 2:55 i) ii) iii)
DESIGN OF EXPRESSWAY EMBANKMENT
Body of Embankment Three typical cross sections of the failed embankment are illustrated in Fig. 4, and their locations are found in the plan view in Fig. 5. It is evident that the failed part of
Vice president, Yamaguchi University, Japan. Group Leader, West Nippon Expressway Company Limited, Engineering Department, Engineering Division, Japan. Professor, Yamaguchi University, Japan (nakata＠yamaguchi-u.ac.jp). The manuscript for this paper was received for review on December 3, 2007; approved on July 27, 2009. Written discussions on this paper should be submitted before May 1, 2010 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 797
MURATA ET AL.
798
Fig. 1.
Table 1.
Route of 2005 Typhoon No. 14 in the Paciˆc Ocean
AMEDAS rainfall records in Yamaguchi Prefecture during typhoon No. 14 (September 6th, Japan Meteorological Agency) Site
Terayama
Rakanzan
Hirose
Shinobu
Wada
Iwakuni
Kuga
Kudamatsu
Cumulative rainfall Daily precipitation Maximum hourly precipitation
524 449 68
540 472 59
397 352 55
313 244 31
357 337 47
347 295 47
444 382 45
322 293 31
Unit: mm
Fig. 2.
Location of AMEDAS precipitation monitoring stations in Yamaguchi Prefecture and place of expressway embankment failure
SLOPE FAILURE OF EMBANKMENT
Photo 1.
Failure of expressway embankment
Table 2. Heavy rainfall records at stations near embankment failure (June 1992–Sept. 2005) Site Terayama Kuga* No. 1 2 3 4 5 6 7 8 9 10
Daily precipitation mm (0AM-12PM) 449 382 219 204 182 182 171 163 157 156 150
2005/9/6# 2005/9/6# 1993/7/17 1993/7/27 1992/8/4 2004/8/30 2005/7/3 1993/7/2 1993/7/28 2001/6/19 1997/6/28
Site
Hourly precipitation mm
Terayama Kuga* No. 1 2 3 4 5 6 7 8 9 10 10 10
68 60 59 58 49 47 45 44 44 40 40 40 40
799
2005/9/6# 2002/9/16 1993/7/27 2004/7/8 2005/7/3 1992/8/8 2005/9/6 # 2004/8/30 1992/8/8 1996/8/14 2003/7/7 2003/7/23 2005/7/2
* Japan Meteorological Agency at Kuga, # Present typhoon event
the expressway was situated on a high embankment, while the adjacent parts on lower ˆll. The upslope part of the damaged section was cut in order to construct the embankment. The gradient of the embankment slope was H:V＝1.8:1 and its typical height was 23 m. The lower part of the earth ˆll had concrete blocks, while the long slope of the central part was covered by vegetation with a frame structure with anchorage at the bottom.
Drainage System Drainage from the embankment was considered to be very important at the time of construction. This is particularly true because of the evidence of past debris ‰ow.
Accordingly, the embankment and the surrounding area were divided into three zones as described in Fig. 6 which were named A-1, A-2, and A-3, respectively. Among them, A-1 was responsible for surface precipitation in the north (downstream side) slope of the embankment. This slope was divided into three smaller slopes and drainage trenches were installed in the transverse direction at their bottoms. The cross section of the trenches was 300 mm by 300 mm. The A-2 zone is located on an adjacent slope and its surface water ‰owed into another trench (300 mm by 300 mm) which was installed along the boundary with A-1. The A-3 catchment zone was responsible for the precipitation inside the expressway as well as the cut slope. The water ‰owed under the expressway through a concrete Colgate pipe which measured 1,500 mm in diameter and 75.3 m in length. Finally, all the water drained out of these three zones ‰owed into a bigger trench (600 mm by 600 mm) and then into a local drainage network. The capacity of the drainage channels was examined by using the rainfall record at Terayama in Table 2 which is the most signiˆcant based on experience. When the hourly precipitation is 68 mm, the analysis shows that the 300-mm trench for A-1 receives 0.145 m3/s which is 54z of the capacity of 0.269 m3/s. Similarly, the trench of the same size for A-2 receives 0.125 m3/s which is 46z of the capacity. Moreover, the Colgate pipe tunnel for A-3 receives 0.410 m3/s which is merely 4z of its capacity. Finally, the end drainage channel of 600 mm in size collects water from three zones and the water amount of 0.680 m3/s is 68z of the capacity of 1.004 m3/s.
MURATA ET AL.
800
Fig. 3.
Time history of rainfall records
Embedded Drainage Facilities The failed embankment was situated on three small valleys as found in Fig. 5. Since spring water was detected at the time of construction as mentioned before, drainage of ground water which may seep into the embankment, was considered important. Hence, three types of underdrainage were embedded in the bottom of the valleys as indicated in Fig. 7. Figures 8(a)–(c) are drawn cross section of the three underdrainages. The ground water was initially collected by underdrainage A with perforat-
ed pipes of 150 mm in diameter which was backˆlled with crushed stone. Water from underdrainage A merged into underdrainage B with a perforated pipe of 200 mm in diameter. Thus, it may be said that the underdrainage channel for ground water had Y-shape conˆguration. Additional underdrainages C were made elsewhere by installing crushed stone without pipe. All the ground water was led into the same local drainage channel as the surface rain water at the bottom of the embankment.
SLOPE FAILURE OF EMBANKMENT
Photo 2.
Aerial photograph of damaged area Fig. 6.
Fig. 4.
801
Catchments for three drainage systems
Cross sections of failed expressway embankment Fig. 7.
Location of pipes for ground water drainage
SITE INVESTIGATION ON CAUSE OF EMBANKMENT FAILURE
Fig. 5.
Plan view of failed section of expressway
Figure 9 indicates the details of the damaged site. The investigation was initiated immediately after the disaster, consisting of topological survey, soil investigation, and inspection of drainage facilities etc. by excavating them. The early site investigation detected the following points; 1) East-bound part of the expressway embankment collapsed over a length of about 50 m. The height of the failed soil was 23 m. 2) The total volume of the failed soil mass was 13,800 m3. The soil mass moved along the former valley topography and further travelled over 80 m until it reached the other side of the local valley. The shape of the soil mass was of little distortion as indicated by standing trees.
MURATA ET AL.
802
for inspection as well. As mentioned earlier, these pipes were installed in the original ground, forming a Y-shape conˆguration through which water from two small valleys merged into one bigger pipe. Although the pipes appeared intact near the merging point, it disappeared over 1.5 m length at 15 m from merging. Since the state of damage appeared very old, as suggested by soil plugging with irony sedimentation, it was inferred that the pipe had been lost since the time of expressway construction. Moreover, the spring water stopped when the soil plug was removed from the drainage pipe. Therefore, the drainage of ground water was most probably insu‹cient at the time of embankment failure. Although gravel backˆll can still maintain the drainage function, the missing pipe reduced the drainage capacity to a signiˆcant extent, and it is reasonable to infer that water from the upstream ground seeped into the embankment during the heavy rainfall. It may further be stated that the conˆguration of the drainage system for both surface precipitation and ground water, merging ˆnally into a single trench (600 mm by 600 mm), may not have been of su‹cient capacity during the present heavy rainfall, leading to submergence of the embankment. Fig. 8.
Design of pipes for drainage of ground water
3) The shape of the slip surface was circular. 4) Three water springs were found after the failure and the one from the former small valley was the largest. 5) The failed soil mass was similar to ‰uid as implied by the long ‰ow distance. Probably it had high water content, and ground water table likely rose in the foundation of the embankment. Figure 9(b) shows three water springs at distances of 44 m and 57 m from the former bottom of the slope. For example, at 44 m on September 12th which was ˆve days after the end of rainfall, the amount of water ‰ow was 0.062 m3/min. To precisely understand the entire amount of ground water ‰ow, a study was made of water ‰ow in the local drainage channel. By taking the diŠerence of ‰ux in the upstream and downstream sides of the failure, the water spring under the slope failure was estimated to be of 0.0375 m3 per unit area per hour. This number was later used for further analysis. The function of drainage facilities was examined by excavating them. The Colgate pipe, which was responsible for drainage of A-3 zone, was found to be broken at its intersection with the slip plane. Although debris was found inside the pipe, it was very local and the pipe was still empty in the downstream area. Hence, it was considered that the pipe maintained its function at the time of the slope failure. Since the damaged embankment intersected with the active Hataki fault as shown in Photo 2 which often supplies substantial water into the surface soil, drainage from the ground water is very important. In this regard, drainage pipes for ground water (Fig. 7) were excavated
SOIL TESTS Bore holes were drilled at many places in the damaged area as indicated in Fig. 10 in order to collect information which was necessary for detailed analyses. According to bore hole logs in Fig. 11, 1) The surface of the original topography is covered by 2 to 5-meter thick colluvium which becomes thicker in lower part of the slope. 2) The colluvium is underlain by talus deposit which is 3.3 m thick and has mean SPT-N＝8. 3) Under talus there is the base slate rock which is fractured and is of artesian water pressure. 4) The remnant of the expressway embankment consists of silty gravel which is 7.85 m thick in the B-4 bore hole with SPT-N＝6 on average. 5) Sandy silt with gravel, which is a talus deposit, was detected under the embankment; its thickness being 0.55 m. 6) Under this sandy silt, there is a slate layer.
Hydraulic Conductivity In-situ permeability tests were conducted. The results are shown in Table 3. The soft slate rock was subjected to artesian water pressure that opens ˆssures and increases the permeability. However, the appearance of collected rock core samples suggested more intact nature of the material. Hence, the numerical analysis will consider this material to be less permeable and will employ k＝1.0× 10－8 cm/s. Mechanical Properties The materials collected from the embankment were classiˆed as sandy gravel with some ˆnes. Their ˆnes content varied between 14z to 39z and the gradation curves
SLOPE FAILURE OF EMBANKMENT
Fig. 9.
803
Illustration of results of site investigation
are presented in Fig. 12. The symbols in the ˆgure mean the sampling site and correspond to that in Fig. 10. The mass density of the remaining part of the embankment, which was measured by water replacement method, was in the range of 1.87 to 2.01 g/cm3 with the mean value being 1.92 g/cm3. The dry density on the other hand was between 1.68 and 1.89 g/cm3, and the mean value was 1.78 g/cm3. Moreover, the underlying Talus deposit revealed the wet mass density of 2.03 to 2.04 g/cm3, and its dry density was 1.77 to 1.79 g/cm3 whose mean value was 1.78 g/cm3. The measured wet density will be used in the numerical analyses.
Triaxial shear tests in consolidated-undrained manner (CU tests) were conducted by preparing samples at the above-mentioned dry density. The results of maximum stress are shown in Fig. 13 as Mohr-circle. The obtained strength parameters are presented in Table 4. NUMERICAL ANALYSES ON FAILURE MECHANISM OF EMBANKMENT Numerical analyses were performed in order to understand the causative mechanism of the embankment failure. The analyses consisted of ˆnite element hydraulic
MURATA ET AL.
804
Fig. 12.
Fig. 10.
Particle size distribution of embankment materials
Location of bore holes and other in-situ investigations
Fig. 13. Fig. 11.
Table 3. Materials Embankment Talus Weathered slate Intact soft slate
Mohr coulomb failure line
Examples of bore hole logs Table 4. Parameters of shear strength of soils obtained by CU triaxial shear tests
Permeability of tested materials Range of permeability, k (cm/s) －3
1.1×10 1.3×10－ 3 2.9×10－ 5 8.4×10－ 4
to to to to
－2
1.6×10 2.7×10－ 3 1.0×10－ 3 2.2×10－ 3
Mean value (cm/s)
Soil
Dry density g/cm3
Cohesion cCU kN/m2
Friction angle qCU degrees
Remarks
Embankment 1 2 3
1.76 1.78 1.78
25.2 25.6 26.2
13.55 13.45 13.62
Mean values: cCU＝26 kN/m2 qCU＝13 deg.
Talus
1.78
35.4
12.24
－3
1.8×10 1.9×10－ 3 6.5×10－ 4 1.4×10－ 3
analyses which aimed to determine the ground water condition upon the onset of failure and stability analyses to evaluate the factor of safety. The cross section in Fig. 14 corresponds to the broken line as shown in Fig. 10. The hydraulic analyses took into account the following issues; 1) Time history of rainfall as recorded at the nearby Terayama station was applied in Fig. 15. 2) Permeability of soils is set equal to those in Tables 3 and 5 which were obtained from saturated specimens. 3) The initial ground water table is set based on bore hole data which was obtained during the post-failure inves-
tigation in Fig. 14. 4) In accordance with the location of water springs, ground water seeps into the embankment within 57 m from the bottom of the embankment slope. 5) The rate of in‰ow of ground water is 0.0375 m3/m2/ hour. 6) The in‰ow occurs between 32 hours and 48 hours (time of embankment failure) when the intensity of rainfall was high. 7) Soil parameters for analyses are as shown in Table 5. The unit weight and strength parameter for weathered
SLOPE FAILURE OF EMBANKMENT Table 5.
Two-dimensional model of cross section for analyses
Soil parameters employed in analyses Friction angle degrees
Permeability
13.0
1.8×10－ 3
wet unit weight kN/m 3
Cohesion
Embankment
19.0
26.0
Talus
20.0
Weathered slate
19.0
122.0
21.0
6.5×10－ 4
Slate in base
20.0
302.0
21.0
1.0×10－ 8
Retaining wall and frame
23.5
380.0
0.0
1.0×10－ 8
Soil
Fig. 14.
805
kN/m
2
35.0 (33.0) 12.0 (15.0)
cm/s
1.9×10－ 3
Numbers in brackets stand for those when gravel content is high
Fig. 16. Failure mechanism obtained by the ˆrst analysis (Factor of safety＝0.949) Fig. 15.
Rainfall record at Terayama station
slate, intact slate, retaining wall and frame was decided according to the design guideline (JH, 1998). 8) Function of drainage is ignored due to the breakage of the perforated pipe in underdrainage B. The analyses were conducted ˆrstly by the hydraulic one by which the variation of eŠective stress was calculated, and secondly the limit equilibrium analyses were conducted by using the undrained strength parameters which assume quick and undrained progress of failure. Figure 16 illustrates the critical slip-failure mechanism which was obtained by the ˆrst analysis. This analysis employed c＝35.0 kPa and friction angle＝12.0 degrees as the strength parameters of Talus in Table 5. Although the factor of safety less than unity was obtained, the slip circle passes thorough the bottom of the Talus and does not reach the bottom of the slope. Since this feature is not consistent with the fact that the retaining wall at the bottom was distorted, the input data had to be improved. The in-situ Talus gravel included 6.9z of gravel particles greater than 37.5 mm. Since this amount of gravel was removed in triaxial tests, the obtained cohesion was overestimated and the friction angle was underestimated. Considering this point, the second analysis was run by using the revised values as shown in brackets in Table 5. Figure 17 shows the new slip mechanism which is consistent with the observation. Consequently, it was found that the present embankment failure was caused by the following process. 1) The extraordinary intensity of rainfall induced
Fig. 17. Failure mechanism obtained by the revised analysis (Factor of safety＝0.989)
seepage of ground water into the embankment. 2) This water seepage was facilitated by the existence of an active fault in which the fractured zone supplied a signiˆcant amount of ground water. 3) Since three small valleys merged at the place of the failed embankment, more ground water was accumulated. In spite of this topographic condition, the drainage was not so easy due to the narrow exit of the merged valley. 4) The perforated pipe for the ground water was missing over some distance. Although the crushed rock backˆll remained to facilitate the intended drainage, the drainage was not fully possible. 5) The combination of these issues led to the submergence of the embankment and the failure.
806
MURATA ET AL.
CONCLUSIONS
ACKNOWLEDGMENT
Heavy rainfall due to typhoon triggered a collapse of an expressway embankment. After the event, a detailed investigation was conducted of this failure in order to identify its causative mechanism. Field studies as well as laboratory tests and numerical analyses made it possible to draw the following conclusions: 1) The extent of rainfall in the particular area was the greatest one since the construction of the expressway. 2) The site of the embankment failure is located on a small valley where an active fault crosses the route of the expressway. 3) Signiˆcant amount of water ‰owed out of the base rock during the typhoon time, and this water submerged the expressway embankment. 4) The drainage system which was installed to mitigate the ground-water in‰ow was partially damaged, leading probably to the submergence of the embankment to some extent. 5) Seepage and equilibrium analyses suggest that the failure of the embankment was caused by the combined eŠects of the above issues.
The present paper describes the conclusion of a research committee which was established by the West Nippon Expressway Company Limited after the disaster in order to understand the causative mechanisms as well as the restoration measures. The members of the committee were Prof. H. Murata and Prof. K. Furukawa of Yamaguchi University, Prof. S. Okuzono of Kyushu Sangyo University, Prof. H. Nakamura of Tokyo University of Agriculture and Technology, Dr. H. Miki formerly at the Public Research Institute in Tsukuba, Dr. K. Fujisawa of the same institute, and Dr. H. Yoshimatsu of Sabo Technical Center. The content of the original o‹cial report was translated brie‰y into English by Prof. Ikuo Towhata, Professor of the University of Tokyo and the Editor-in-Chief of the Soils and Foundations Journal. The eŠorts made by these people are deeply appreciated. REFERENCE 1) Japan Highways Organization (1998): Design Guideline, 1–37.
Vol. 49, No. 5, 797–806, Oct. 2009
SLOPE FAILURE OF EMBANKMENT IN SANYO EXPRESSWAY DUE TO PASSAGE OF TYPHOON NO. 14 IN 2005 HIDEKAZU MURATAi), KAZUYA TAKEKUNIii) and YUKIO NAKATAiii) ABSTRACT Typhoon No. 14 in 2005 caused signiˆcant rainfall in Yamaguchi Prefecture of Japan, and, due to this, an embankment of Sanyo Expressway at Hataki in Iwakuni city failed. Detailed investigation was conducted after this event in order to identify the mechanism of this failure, which consisted of the ˆeld studies as well as laboratory tests and numerical analyses. It was consequently shown that the combined eŠects of heavy rainfall and the local geological and topographic conditions as well as the reduction of drainage capability due to the breakage of perforated pipe in the underdrainage system were the causes of the failure. Key words: case history, embankment, expressway, rainfall, seepage, slope stability (IGC: C7/E6) PM on 6th of September due to the rainfall, and no distortion was noticed during the inspection at around 11 PM on 6th of September. The concerned part of the Sanyo Expressway was opened to tra‹c on 25th June, 1992. During the 13-year period thereafter until the failure, it experienced heavy rainfall several times as listed in Table 2. However, neither slip failure nor any instability had occurred. Obviously, the never-experienced intensity of precipitation due to the present typhoon triggered the failure. This point is further evidenced by Fig. 3 in which the time history of accumulated rainfall is illustrated. It is noteworthy that the present rainfall lasted for more than 60 hours and the total precipitation exceeded 500 mm. Photo 2 illustrates the aerial view of the damaged area to depict three existing faults. The most important faults herein are called the Otake fault and the Iwakuni fault. Being sandwiched between these faults, the damage area has a small Hataki active fault which crosses the expressway embankment in the N-S direction. The preliminary geological study prior to the expressway construction reported spring water at the crossing of the fault on the expressway route. Furthermore, the site of the present embankment failure is identical with the location of debris ‰ow in the past.
TYPHOON NO. 14 IN 2005 Typhoon No. 14 of 2005 started on 29th of August in the Paciˆc Ocean and proceeded towards North West while getting greater in its size as shown in Fig. 1. When it landed at the Kyushu Island of Japan on September 6th, its maximum wind velocity was 35 meters/s and its range of strong wind (À15 m/s) reached 750 km on its eastern side and 700 km on the west. The eŠects of this typhoon were characterized by heavy rainfall; recording 1321 mm in the extreme case at Mikado in Miyazaki Prefecture of Kyushu. Yamaguchi Prefecture experienced the typhoon No. 14 from the morning of September 5th till the morning of 7th. Since this typhoon was large in size and low in velocity, its eŠects lasted for a long time. Table 1 indicates the rainfall records at diŠerent places in the prefecture. The locations of the sites in this table are depicted in Fig. 2 together with the place of embankment failure which is the topic in this paper. DETAILS OF RAINFALL-INDUCED FAILURE OF SANYO EXPRESSWAY EMBANKMENT The embankment failure as shown in Photo 1 occurred at around 0:50 AM on 7th September, 2005 during the typhoon. Half of the expressway, which was the eastbound lanes, collapsed over a length of 50 m. The depth of failure was 23 m and the total volume of the failed soil mass was 13,800 m3, which was deposited on the adjacent local road with a thickness of 4 m. Unfortunately 2 houses were buried under the soil resulting in the death of 3 residents. The expressway had been closed since 2:55 i) ii) iii)
DESIGN OF EXPRESSWAY EMBANKMENT
Body of Embankment Three typical cross sections of the failed embankment are illustrated in Fig. 4, and their locations are found in the plan view in Fig. 5. It is evident that the failed part of
Vice president, Yamaguchi University, Japan. Group Leader, West Nippon Expressway Company Limited, Engineering Department, Engineering Division, Japan. Professor, Yamaguchi University, Japan (nakata＠yamaguchi-u.ac.jp). The manuscript for this paper was received for review on December 3, 2007; approved on July 27, 2009. Written discussions on this paper should be submitted before May 1, 2010 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 797
MURATA ET AL.
798
Fig. 1.
Table 1.
Route of 2005 Typhoon No. 14 in the Paciˆc Ocean
AMEDAS rainfall records in Yamaguchi Prefecture during typhoon No. 14 (September 6th, Japan Meteorological Agency) Site
Terayama
Rakanzan
Hirose
Shinobu
Wada
Iwakuni
Kuga
Kudamatsu
Cumulative rainfall Daily precipitation Maximum hourly precipitation
524 449 68
540 472 59
397 352 55
313 244 31
357 337 47
347 295 47
444 382 45
322 293 31
Unit: mm
Fig. 2.
Location of AMEDAS precipitation monitoring stations in Yamaguchi Prefecture and place of expressway embankment failure
SLOPE FAILURE OF EMBANKMENT
Photo 1.
Failure of expressway embankment
Table 2. Heavy rainfall records at stations near embankment failure (June 1992–Sept. 2005) Site Terayama Kuga* No. 1 2 3 4 5 6 7 8 9 10
Daily precipitation mm (0AM-12PM) 449 382 219 204 182 182 171 163 157 156 150
2005/9/6# 2005/9/6# 1993/7/17 1993/7/27 1992/8/4 2004/8/30 2005/7/3 1993/7/2 1993/7/28 2001/6/19 1997/6/28
Site
Hourly precipitation mm
Terayama Kuga* No. 1 2 3 4 5 6 7 8 9 10 10 10
68 60 59 58 49 47 45 44 44 40 40 40 40
799
2005/9/6# 2002/9/16 1993/7/27 2004/7/8 2005/7/3 1992/8/8 2005/9/6 # 2004/8/30 1992/8/8 1996/8/14 2003/7/7 2003/7/23 2005/7/2
* Japan Meteorological Agency at Kuga, # Present typhoon event
the expressway was situated on a high embankment, while the adjacent parts on lower ˆll. The upslope part of the damaged section was cut in order to construct the embankment. The gradient of the embankment slope was H:V＝1.8:1 and its typical height was 23 m. The lower part of the earth ˆll had concrete blocks, while the long slope of the central part was covered by vegetation with a frame structure with anchorage at the bottom.
Drainage System Drainage from the embankment was considered to be very important at the time of construction. This is particularly true because of the evidence of past debris ‰ow.
Accordingly, the embankment and the surrounding area were divided into three zones as described in Fig. 6 which were named A-1, A-2, and A-3, respectively. Among them, A-1 was responsible for surface precipitation in the north (downstream side) slope of the embankment. This slope was divided into three smaller slopes and drainage trenches were installed in the transverse direction at their bottoms. The cross section of the trenches was 300 mm by 300 mm. The A-2 zone is located on an adjacent slope and its surface water ‰owed into another trench (300 mm by 300 mm) which was installed along the boundary with A-1. The A-3 catchment zone was responsible for the precipitation inside the expressway as well as the cut slope. The water ‰owed under the expressway through a concrete Colgate pipe which measured 1,500 mm in diameter and 75.3 m in length. Finally, all the water drained out of these three zones ‰owed into a bigger trench (600 mm by 600 mm) and then into a local drainage network. The capacity of the drainage channels was examined by using the rainfall record at Terayama in Table 2 which is the most signiˆcant based on experience. When the hourly precipitation is 68 mm, the analysis shows that the 300-mm trench for A-1 receives 0.145 m3/s which is 54z of the capacity of 0.269 m3/s. Similarly, the trench of the same size for A-2 receives 0.125 m3/s which is 46z of the capacity. Moreover, the Colgate pipe tunnel for A-3 receives 0.410 m3/s which is merely 4z of its capacity. Finally, the end drainage channel of 600 mm in size collects water from three zones and the water amount of 0.680 m3/s is 68z of the capacity of 1.004 m3/s.
MURATA ET AL.
800
Fig. 3.
Time history of rainfall records
Embedded Drainage Facilities The failed embankment was situated on three small valleys as found in Fig. 5. Since spring water was detected at the time of construction as mentioned before, drainage of ground water which may seep into the embankment, was considered important. Hence, three types of underdrainage were embedded in the bottom of the valleys as indicated in Fig. 7. Figures 8(a)–(c) are drawn cross section of the three underdrainages. The ground water was initially collected by underdrainage A with perforat-
ed pipes of 150 mm in diameter which was backˆlled with crushed stone. Water from underdrainage A merged into underdrainage B with a perforated pipe of 200 mm in diameter. Thus, it may be said that the underdrainage channel for ground water had Y-shape conˆguration. Additional underdrainages C were made elsewhere by installing crushed stone without pipe. All the ground water was led into the same local drainage channel as the surface rain water at the bottom of the embankment.
SLOPE FAILURE OF EMBANKMENT
Photo 2.
Aerial photograph of damaged area Fig. 6.
Fig. 4.
801
Catchments for three drainage systems
Cross sections of failed expressway embankment Fig. 7.
Location of pipes for ground water drainage
SITE INVESTIGATION ON CAUSE OF EMBANKMENT FAILURE
Fig. 5.
Plan view of failed section of expressway
Figure 9 indicates the details of the damaged site. The investigation was initiated immediately after the disaster, consisting of topological survey, soil investigation, and inspection of drainage facilities etc. by excavating them. The early site investigation detected the following points; 1) East-bound part of the expressway embankment collapsed over a length of about 50 m. The height of the failed soil was 23 m. 2) The total volume of the failed soil mass was 13,800 m3. The soil mass moved along the former valley topography and further travelled over 80 m until it reached the other side of the local valley. The shape of the soil mass was of little distortion as indicated by standing trees.
MURATA ET AL.
802
for inspection as well. As mentioned earlier, these pipes were installed in the original ground, forming a Y-shape conˆguration through which water from two small valleys merged into one bigger pipe. Although the pipes appeared intact near the merging point, it disappeared over 1.5 m length at 15 m from merging. Since the state of damage appeared very old, as suggested by soil plugging with irony sedimentation, it was inferred that the pipe had been lost since the time of expressway construction. Moreover, the spring water stopped when the soil plug was removed from the drainage pipe. Therefore, the drainage of ground water was most probably insu‹cient at the time of embankment failure. Although gravel backˆll can still maintain the drainage function, the missing pipe reduced the drainage capacity to a signiˆcant extent, and it is reasonable to infer that water from the upstream ground seeped into the embankment during the heavy rainfall. It may further be stated that the conˆguration of the drainage system for both surface precipitation and ground water, merging ˆnally into a single trench (600 mm by 600 mm), may not have been of su‹cient capacity during the present heavy rainfall, leading to submergence of the embankment. Fig. 8.
Design of pipes for drainage of ground water
3) The shape of the slip surface was circular. 4) Three water springs were found after the failure and the one from the former small valley was the largest. 5) The failed soil mass was similar to ‰uid as implied by the long ‰ow distance. Probably it had high water content, and ground water table likely rose in the foundation of the embankment. Figure 9(b) shows three water springs at distances of 44 m and 57 m from the former bottom of the slope. For example, at 44 m on September 12th which was ˆve days after the end of rainfall, the amount of water ‰ow was 0.062 m3/min. To precisely understand the entire amount of ground water ‰ow, a study was made of water ‰ow in the local drainage channel. By taking the diŠerence of ‰ux in the upstream and downstream sides of the failure, the water spring under the slope failure was estimated to be of 0.0375 m3 per unit area per hour. This number was later used for further analysis. The function of drainage facilities was examined by excavating them. The Colgate pipe, which was responsible for drainage of A-3 zone, was found to be broken at its intersection with the slip plane. Although debris was found inside the pipe, it was very local and the pipe was still empty in the downstream area. Hence, it was considered that the pipe maintained its function at the time of the slope failure. Since the damaged embankment intersected with the active Hataki fault as shown in Photo 2 which often supplies substantial water into the surface soil, drainage from the ground water is very important. In this regard, drainage pipes for ground water (Fig. 7) were excavated
SOIL TESTS Bore holes were drilled at many places in the damaged area as indicated in Fig. 10 in order to collect information which was necessary for detailed analyses. According to bore hole logs in Fig. 11, 1) The surface of the original topography is covered by 2 to 5-meter thick colluvium which becomes thicker in lower part of the slope. 2) The colluvium is underlain by talus deposit which is 3.3 m thick and has mean SPT-N＝8. 3) Under talus there is the base slate rock which is fractured and is of artesian water pressure. 4) The remnant of the expressway embankment consists of silty gravel which is 7.85 m thick in the B-4 bore hole with SPT-N＝6 on average. 5) Sandy silt with gravel, which is a talus deposit, was detected under the embankment; its thickness being 0.55 m. 6) Under this sandy silt, there is a slate layer.
Hydraulic Conductivity In-situ permeability tests were conducted. The results are shown in Table 3. The soft slate rock was subjected to artesian water pressure that opens ˆssures and increases the permeability. However, the appearance of collected rock core samples suggested more intact nature of the material. Hence, the numerical analysis will consider this material to be less permeable and will employ k＝1.0× 10－8 cm/s. Mechanical Properties The materials collected from the embankment were classiˆed as sandy gravel with some ˆnes. Their ˆnes content varied between 14z to 39z and the gradation curves
SLOPE FAILURE OF EMBANKMENT
Fig. 9.
803
Illustration of results of site investigation
are presented in Fig. 12. The symbols in the ˆgure mean the sampling site and correspond to that in Fig. 10. The mass density of the remaining part of the embankment, which was measured by water replacement method, was in the range of 1.87 to 2.01 g/cm3 with the mean value being 1.92 g/cm3. The dry density on the other hand was between 1.68 and 1.89 g/cm3, and the mean value was 1.78 g/cm3. Moreover, the underlying Talus deposit revealed the wet mass density of 2.03 to 2.04 g/cm3, and its dry density was 1.77 to 1.79 g/cm3 whose mean value was 1.78 g/cm3. The measured wet density will be used in the numerical analyses.
Triaxial shear tests in consolidated-undrained manner (CU tests) were conducted by preparing samples at the above-mentioned dry density. The results of maximum stress are shown in Fig. 13 as Mohr-circle. The obtained strength parameters are presented in Table 4. NUMERICAL ANALYSES ON FAILURE MECHANISM OF EMBANKMENT Numerical analyses were performed in order to understand the causative mechanism of the embankment failure. The analyses consisted of ˆnite element hydraulic
MURATA ET AL.
804
Fig. 12.
Fig. 10.
Particle size distribution of embankment materials
Location of bore holes and other in-situ investigations
Fig. 13. Fig. 11.
Table 3. Materials Embankment Talus Weathered slate Intact soft slate
Mohr coulomb failure line
Examples of bore hole logs Table 4. Parameters of shear strength of soils obtained by CU triaxial shear tests
Permeability of tested materials Range of permeability, k (cm/s) －3
1.1×10 1.3×10－ 3 2.9×10－ 5 8.4×10－ 4
to to to to
－2
1.6×10 2.7×10－ 3 1.0×10－ 3 2.2×10－ 3
Mean value (cm/s)
Soil
Dry density g/cm3
Cohesion cCU kN/m2
Friction angle qCU degrees
Remarks
Embankment 1 2 3
1.76 1.78 1.78
25.2 25.6 26.2
13.55 13.45 13.62
Mean values: cCU＝26 kN/m2 qCU＝13 deg.
Talus
1.78
35.4
12.24
－3
1.8×10 1.9×10－ 3 6.5×10－ 4 1.4×10－ 3
analyses which aimed to determine the ground water condition upon the onset of failure and stability analyses to evaluate the factor of safety. The cross section in Fig. 14 corresponds to the broken line as shown in Fig. 10. The hydraulic analyses took into account the following issues; 1) Time history of rainfall as recorded at the nearby Terayama station was applied in Fig. 15. 2) Permeability of soils is set equal to those in Tables 3 and 5 which were obtained from saturated specimens. 3) The initial ground water table is set based on bore hole data which was obtained during the post-failure inves-
tigation in Fig. 14. 4) In accordance with the location of water springs, ground water seeps into the embankment within 57 m from the bottom of the embankment slope. 5) The rate of in‰ow of ground water is 0.0375 m3/m2/ hour. 6) The in‰ow occurs between 32 hours and 48 hours (time of embankment failure) when the intensity of rainfall was high. 7) Soil parameters for analyses are as shown in Table 5. The unit weight and strength parameter for weathered
SLOPE FAILURE OF EMBANKMENT Table 5.
Two-dimensional model of cross section for analyses
Soil parameters employed in analyses Friction angle degrees
Permeability
13.0
1.8×10－ 3
wet unit weight kN/m 3
Cohesion
Embankment
19.0
26.0
Talus
20.0
Weathered slate
19.0
122.0
21.0
6.5×10－ 4
Slate in base
20.0
302.0
21.0
1.0×10－ 8
Retaining wall and frame
23.5
380.0
0.0
1.0×10－ 8
Soil
Fig. 14.
805
kN/m
2
35.0 (33.0) 12.0 (15.0)
cm/s
1.9×10－ 3
Numbers in brackets stand for those when gravel content is high
Fig. 16. Failure mechanism obtained by the ˆrst analysis (Factor of safety＝0.949) Fig. 15.
Rainfall record at Terayama station
slate, intact slate, retaining wall and frame was decided according to the design guideline (JH, 1998). 8) Function of drainage is ignored due to the breakage of the perforated pipe in underdrainage B. The analyses were conducted ˆrstly by the hydraulic one by which the variation of eŠective stress was calculated, and secondly the limit equilibrium analyses were conducted by using the undrained strength parameters which assume quick and undrained progress of failure. Figure 16 illustrates the critical slip-failure mechanism which was obtained by the ˆrst analysis. This analysis employed c＝35.0 kPa and friction angle＝12.0 degrees as the strength parameters of Talus in Table 5. Although the factor of safety less than unity was obtained, the slip circle passes thorough the bottom of the Talus and does not reach the bottom of the slope. Since this feature is not consistent with the fact that the retaining wall at the bottom was distorted, the input data had to be improved. The in-situ Talus gravel included 6.9z of gravel particles greater than 37.5 mm. Since this amount of gravel was removed in triaxial tests, the obtained cohesion was overestimated and the friction angle was underestimated. Considering this point, the second analysis was run by using the revised values as shown in brackets in Table 5. Figure 17 shows the new slip mechanism which is consistent with the observation. Consequently, it was found that the present embankment failure was caused by the following process. 1) The extraordinary intensity of rainfall induced
Fig. 17. Failure mechanism obtained by the revised analysis (Factor of safety＝0.989)
seepage of ground water into the embankment. 2) This water seepage was facilitated by the existence of an active fault in which the fractured zone supplied a signiˆcant amount of ground water. 3) Since three small valleys merged at the place of the failed embankment, more ground water was accumulated. In spite of this topographic condition, the drainage was not so easy due to the narrow exit of the merged valley. 4) The perforated pipe for the ground water was missing over some distance. Although the crushed rock backˆll remained to facilitate the intended drainage, the drainage was not fully possible. 5) The combination of these issues led to the submergence of the embankment and the failure.
806
MURATA ET AL.
CONCLUSIONS
ACKNOWLEDGMENT
Heavy rainfall due to typhoon triggered a collapse of an expressway embankment. After the event, a detailed investigation was conducted of this failure in order to identify its causative mechanism. Field studies as well as laboratory tests and numerical analyses made it possible to draw the following conclusions: 1) The extent of rainfall in the particular area was the greatest one since the construction of the expressway. 2) The site of the embankment failure is located on a small valley where an active fault crosses the route of the expressway. 3) Signiˆcant amount of water ‰owed out of the base rock during the typhoon time, and this water submerged the expressway embankment. 4) The drainage system which was installed to mitigate the ground-water in‰ow was partially damaged, leading probably to the submergence of the embankment to some extent. 5) Seepage and equilibrium analyses suggest that the failure of the embankment was caused by the combined eŠects of the above issues.
The present paper describes the conclusion of a research committee which was established by the West Nippon Expressway Company Limited after the disaster in order to understand the causative mechanisms as well as the restoration measures. The members of the committee were Prof. H. Murata and Prof. K. Furukawa of Yamaguchi University, Prof. S. Okuzono of Kyushu Sangyo University, Prof. H. Nakamura of Tokyo University of Agriculture and Technology, Dr. H. Miki formerly at the Public Research Institute in Tsukuba, Dr. K. Fujisawa of the same institute, and Dr. H. Yoshimatsu of Sabo Technical Center. The content of the original o‹cial report was translated brie‰y into English by Prof. Ikuo Towhata, Professor of the University of Tokyo and the Editor-in-Chief of the Soils and Foundations Journal. The eŠorts made by these people are deeply appreciated. REFERENCE 1) Japan Highways Organization (1998): Design Guideline, 1–37.