Post-earthquake investigation on several geosynthetic-reinforced soil retaining walls and slopes during the Ji-Ji earthquake of Taiwan Hoe I. Ling a, * , Dov Leshchinsky b , Nelson N.S. Chou c a Department of Civil Engineering and Engineering Mechanics, Columbia University, 500 West 120th Street, New York, NY 10027, USA b Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA c Chung Hwa University, and Genesis Group/Taiwan, 11-1 Fl., No. 268, Kuang Fu S. Road, Taipei, Taiwan 106, R.O.C. Accepted 17 January 2001 Abstract This paper gives an overview on the application of geosynthetic-reinforced soil structures in Taiwan. Taiwan has an unique topography and geotechnical conditions that rendered a less conservative and more challenging design compared to that of North America, Europe and Japan. The Ji-Ji (Chi-Chi) earthquake of 1999 gave an opportunity to examine the behavior of reinforced soil structures. The performance of several modular-block reinforced soil retaining walls and reinforced slopes at the vicinity of the fault was evaluated. Reinforced structures performed better than unreinforced soil retaining walls. The failure cases were highlighted and the cause of failure was identified. The lack of seismic design consideration could be a major cause of failure. The compound failure mode, the inertia force of the blocks, and the connection stiffness and strength relative to the large dynamic earth pressure, were among major items that would warrant further design consideration. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Seismic performance; Geosynthetic; Reinforced soil retaining wall; Reinforced slope; Modular block; Connection strength; Compound failure; Ji- Ji/Chi-Chi earthquake 1. Reinforced soil applications in Taiwan Taiwan is an island country of 360,000 km 2 with over 21.9 million people [22]. The main island is densely popu- lated (606 persons/km 2 ), ranked second in the world. More than 70% of the island is composed of slopes and moun- tains. Reinforced soil retaining walls and reinforced slopes have gained wide popularity in Taiwan over recent years because of the many large-scale housing and industrial development sites located at the slopes and hillsides. Chou [8] gave an overview of the recent development of geosynthetic-reinforced soil structures (GRSS) in Taiwan. There are several unique features for GRSS constructed in Taiwan compared to the technology that has been developed and established in North America, Europe and Japan: 1. The topography and geotechnical conditions of Taiwan are quite different from the rest of the world. Many recent constructions are located along the slopes and mountains. While GRSS constructed in US, Europe and Japan are mostly near vertical and for a height of less than 10 m, some of the reinforced slopes in Taiwan are over 30– 40 m, usually with a series of walls stacking over each other (multiple walls). 2. The on-site soil is usually used as backfill material. The cost of granular sand is relatively high at its scarcity. Disposal of on-site soils and transportation of granular soils to the construction site, typically in the mountains, are difficult and costly. 3. Wrap-around facing structure is commonly used for rein- forcing slopes. The wall face is usually finished with a vegetated facing. 4. For reinforced soil retaining walls, the modular block facing structure is most popular. The height is typically between 2 and 10 m. 5. Geogrids comprised more than 95% of the applications in reinforced soil structures for economic reason. There are several local geogrid manufacturers from Taiwan. The geotextiles and metallic reinforcements are not popular. 6. The designs of GRSS are typically provided by the manu- facturers. There is a lack of geotechnical consideration for certain specific applications. The reinforced soil technology has not been adopted widely by the public sectors compared to the private developers, such Soil Dynamics and Earthquake Engineering 21 (2001) 297–313 0267-7261/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0267-7261(01)00011-2 www.elsevier.com/locate/soildyn * Corresponding author. E-mail address: [email protected] (H.I. Ling).
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Post-earthquake investigation on several geosynthetic-reinforced soilretaining walls and slopes during the Ji-Ji earthquake of Taiwan
Hoe I. Linga,*, Dov Leshchinskyb, Nelson N.S. Chouc
aDepartment of Civil Engineering and Engineering Mechanics, Columbia University, 500 West 120th Street, New York, NY 10027, USAbDepartment of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA
cChung Hwa University, and Genesis Group/Taiwan, 11-1 Fl., No. 268, Kuang Fu S. Road, Taipei, Taiwan 106, R.O.C.
Accepted 17 January 2001
Abstract
This paper gives an overview on the application of geosynthetic-reinforced soil structures in Taiwan. Taiwan has an unique topography
and geotechnical conditions that rendered a less conservative and more challenging design compared to that of North America, Europe and
Japan. The Ji-Ji (Chi-Chi) earthquake of 1999 gave an opportunity to examine the behavior of reinforced soil structures. The performance of
several modular-block reinforced soil retaining walls and reinforced slopes at the vicinity of the fault was evaluated. Reinforced structures
performed better than unreinforced soil retaining walls. The failure cases were highlighted and the cause of failure was identi®ed. The lack of
seismic design consideration could be a major cause of failure. The compound failure mode, the inertia force of the blocks, and the
connection stiffness and strength relative to the large dynamic earth pressure, were among major items that would warrant further design
consideration. q 2001 Elsevier Science Ltd. All rights reserved.
retaining walls, with a modular-block facing, were
constructed at several locations along Roadway 129. At
one location, failure of a 3.4 m high modular-block geosyn-
thetic-reinforced retaining walls was found (Fig. 7). The
wall was constructed with a four-block reinforcement
spacing. Fig. 7(b) shows the largely deformed portion of
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313300
Fig. 3. Major sites of investigation.
the wall. The modular blocks were buried under the back®ll
soil (Fig. 7(c)). The back®ll material was a silty sand. A
polyester geogrid was used (Fig. 7(d)). Fig. 7(e) shows the
block that was used as facing for the reinforced soil retain-
ing wall.
Fig. 7(b) shows that the largest horizontal displace-
ment was at a height of eight blocks (1.6 m) from the
bottom of the wall. This point of maximum displacement
varied along the length of the wall. However, failure
could be initiated from the bottom of the wall, at the
region where the blocks totally collapsed, because of
excessive displacement. A major crack was observed at
a distance 5.6 m behind the wall. A minor crack was also
formed at about 2.5 m behind the wall, which corre-
sponded to the length of geogrid reinforcement. In
Taiwan, the length of geosynthetic reinforcement is typi-
cally selected as 70% the wall height for modular-block
reinforced soil retaining walls.
Note also that the transverse rib of the geogrid reinforce-
ment was torn at the location of the connection pins (Fig.
7(d)). Some of the pins were bent and yielded because of the
movement of the blocks. The results indicated that the
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 301
Fig. 4. Ta Kung housing development site: (a) failure and slabs and foundation, (b) rupture surfaces along the slope.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313302
Fig. 5. Failure of reinforced concrete retaining walls in Ta Kung housing development site: (a) horizontal crack, (b) separation of walls, (c) structural
failure of wall.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 303
Fig. 6. Ta Kung housing development site: (a) geosynthetic-reinforced soil retaining wall, (b) large settlement and failure surface behind GRS-RW, (c)
deformation of modular blocks along the top of the wall, (d) collapse of GRS-RW.
transverse stiffness and strength of geogrid, as well as that of
the pins, are required to keep the modular blocks in place
under large dynamic earth pressure induced by the earth-
quake.
4.3. Chung-Hsin Stadium, Chung Hsin New Village
Modular-block reinforced soil retaining walls were used
extensively around Chung-Hsin Stadium. Two walls were
affected by the earthquake.
The ®rst wall was located along the side of the Stadium,
of height 2 m or less (Fig. 8(a)). A series of lampposts were
installed very close to the wall. It was observed that the
blocks dislocated around the location of the lampposts
(Fig. 8(b)). The connection pins were seen through the
spacing between the blocks. The deformations were due to
the movement of the foundation of the lamppost. The post
de¯ected inward and thus pushing the foundation outward to
the wall. The problem could be avoided by installing the
post at a distance away from the wall, or with a deeper
foundation.
The second wall was located behind the Stadium, 3 m
high (Fig. 9(a)). At the crest of the wall, two cracks
were observed. The ®rst crack was about 0.5 m from the
blocks whereas the second crack was more than 2 m
from the block. The blocks moved away from the back-
®ll for over 30 cm. This wall collapsed at the lower
corner. A close view of the bottom corner is given in
Fig. 9(b). Note that the length of reinforcement at the
corner is likely less than normal because of the limited
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313304
Fig. 6. (continued)
Fig. 7. Ta Kung Roadway 129 geosynthetic-reinforced soil retaining wall: (a) front view of collapsed section, (b) side view, (c) back®ll soil, (d) geogrid
reinforcement, (e) block with the connection pins.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 305
Fig. 7. (continued)
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313306
Fig. 8. Chung Hsin Stadium: (a) geosynthetic-reinforced soil retaining wall along the side of stadium, (b) the gap exposing the connection pins at the location
of lamp post.
space available behind the wall. The reinforcement used
was a polyester geogrid, with a vertical spacing of three
blocks. Thus, the top layer had a spacing of ®ve blocks
or 1 m. The ®rst crack should correspond to the sliding
surface of the top back®ll soil layer.
4.4. Chung Hsin Nai Lu Shi Park, Chung Hsin New Village
The Park is located near to the Stadium. There were two
reinforced soil structures constructed opposite to each other
in this Park. Both structures were composed of three stacked
walls (Fig. 10(a)). Part of the structure facing west collapsed
whereas the structure facing east was stable. The collapsed
portion of the wall was unreinforced and was supported at
the back by the H-steel piles. Note that some of the blocks
were also damaged structurally (Fig. 10(b)). The portion of
the wall at the second level, which was reinforced, remained
stable (Fig. 10(c)). In this stable wall, the ®rst reinforcement
layer was placed two blocks from the base, followed by
four-block, three-block, and ®ve-block spacings, as marked
by the dry leaves in the picture.
This case history demonstrated the earthquake resistance
of reinforced soil retaining wall compared to the unrein-
forced wall. The difference in performance between the
walls facing east and west could be related to the accelera-
tions of the earthquake.
5. Reinforced slopes
5.1. Chi-Nan University, Pu Li
Pu Li is the town that was most severely damaged by
the earthquake. It is located at about 25 km from the
epicenter. The reinforced slope, 40 m tall, was located
at the front gate of National Chi-Nan University, facing
east. The geogrids were used as reinforcement and the
slope was back®lled by on-site soil, which was a silty
clay. The slope had a wrap-around facing. The reinforced
structure was constructed by stacking a series of rein-
forced slopes, with a reinforcement spacing of 1 m. The
reinforced slopes, after failure, is shown in Fig. 11(a)
(side view) and (c) (front view).
The back®ll soils and concrete structures from the slope
moved for more than 10 m and buried the road. The security
of®ce was damaged (Fig. 11(b)). A close view of the slope is
shown in Fig. 11(d), where the reinforcements are seen to
pull out of the slope. Note that the concrete pavement
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 307
Fig. 9. Chung Hsin Stadium: (a) geosynthetic-reinforced retaining wall behind the stadium, (b) closer view of the failure section.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313308
Fig. 10. Chung Hsin Lu Shi Park: (a) overall view of geosynthetic-reinforced soil retaining wall, (b) collapse of unreinforced section of the wall, (c) the
reinforced section (the leaves indicating location of reinforcement layers).
around the site, at the foot and crest of the slope, deformed
excessively (Fig. 11(e)).
It is, however, not certain if the failure of this reinforced
structure was attributed to the seismic excitation alone.
Excessive deformation of this reinforced slope was reported
previously following an excavation at the foot of the slope
in 1994 [8]. The original con®guration of this reinforced
slope and the con®guration after failure in 1994 are shown
in Fig. 12 [14].
5.2. Nai Lu housing development site, Chung Hsin New
Village
A 35 m high reinforced structure, located near Chung
Hsin New Village, remains stable after the earthquake.
The structure was composed of six multiple reinforced
slopes, facing south±west. The slope has a wrap-around
facing and was fully vegetated (Fig. 13(a)). The details of
this reinforced slope were given by Chou et al. [5]. It was the
tallest reinforced soil structures at the time of completion of
construction. Note that the road pavement along the slope
suffered signi®cant damage (Fig. 13(b)).
Fig. 14 shows the con®guration of this structure. The
slope was constructed on a V-shaped valley having an incli-
nation of 2(V):1(H) back®lled with on-site soils. The slope
was designed for seismic stability with a seismic coef®cient
of 0.15. A HDPE geogrid was used. The spacing of rein-
forced slope was 50 cm and the reinforcement was 18.5 m
long with an overlapping length of 2.5 m.
Note that the width of this slope was less than that of Chi-
Nan University and the orientation was different as well.
This reinforced slope behaved as an arch-like structure (see
[5]). The end effects could have improved the stability.
6. Conclusions
The Ji-Ji earthquake caused some damages to the geosyn-
thetic-reinforced soil structures in Taiwan. A few modular-
block geosynthetic-reinforced soil retaining walls and rein-
forced slopes were damaged. Some of the lessons learned
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 309
Fig. 11. Chi-Nan University geosynthetic-reinforced slope: (a) side view of failure, (b) damaged security of®ce, (c) front view of failure, (d) close view of
failure showing the reinforcement and back®ll soil, (e) settlement of concrete pavement along the foot of slope.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313310
Fig. 11. (continued)
from the post-earthquake investigation are:
² Taiwan is located in a seismically active region, but it is not
clear if seismic design was conducted for most reinforced
soil structures. While the design of geosynthetic-reinforced
soil structures is believed to be very conservative in other
parts of the world, the design conditions are more severe in
Taiwan because of the topography and economic reasons,
such as the use of on-site back®ll soil, above normal height,
stacked walls and slopes.
² The seismic design of reinforced soil structures has gained
attention world wide only in recent years. However, most of
the seismic design procedures do not incorporate
compound failure analysis. The cracks behind the wall indi-
cated that a few of the structures suffered compound failure
or did not have adequate global stability.
² The failure of modular-block reinforced soil retaining walls
could be attributed to a lack of professional design as seen
by arbitrary spacings used in several of the reinforced soil
retaining walls, and with a mixture of unreinforced and
reinforced retaining walls within a common structure.
² The connection between the modular blocks and reinforce-
ment is vital for a satisfactory performance of the structure
under high seismic load. The strength and stiffness of the
pins, and that of the reinforcement in the transverse direc-
tion, should be large enough to sustain the dynamic earth
pressure.
² The inertia of the modular blocks led to excessive
deformation under seismic excitation. The structures,
such as the lampposts, should not be installed at the vicinity
of the modular-block walls.
² For the sites where reinforced and unreinforced soil retain-
ing structures were found, a better performance was
achieved for the reinforced soil structures.
The information obtained from post-earthquake investiga-
tion is invaluable for the veri®cation and improvement of
seismic design procedure.
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313 311
Fig. 11. (continued)
Fig. 12. Con®guration of Chi-Nan University before and failure of 1994 (after Huang, 2000).
H.I. Ling et al. / Soil Dynamics and Earthquake Engineering 21 (2001) 297±313312
Fig. 13. Nai Lu housing development site: (a) stable geosynthetic-reinforced slope with vegetated facing, (b) severely cracked pavement along the road to the
slope.
Fig. 14. Cross-section of Nai Lu housing development site (Chou et al., 1995).
Acknowledgements
The material is based upon work supported by the
National Science Foundation (Grant No. CMS-0084449)
with Dr Richard J. Fragaszy as the Program Director. Any
opinions, ®ndings, and conclusions or recommendations
expressed in this material are those of the authors and do
not necessarily re¯ect the views of the National Science
Foundation. The source of all seismic information included
in this paper was from the Seismology Center, Central
Weather Bureau (CWB), Taipei, Taiwan.
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