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Ground behavior and liquefaction analyses in central Taiwan-Wufeng
Engineering Geology 71 (2003) 119 – 139 www.elsevier.com/locate/enggeo
Ground behavior and liquefaction analyses in central Taiwan-Wufeng Bin-Lin Chu a,1, Sung-Chi Hsu b,*, Yi-Ming Chang a,1 b
a Department of Civil Engineering, National Chung-hsing University, Taichung, Taiwan, ROC Department of Construction Engineering, Chaoyang University of Technology, Wufeng, Taichung City, Taiwan, ROC
Abstract The 921 Chi-Chi earthquake (Mw = 7.6) in 1999 caused extensive ground failures in the central part of Taiwan. Severe damages due to soil liquefaction and ground softening occurred widespread in Wufeng. Various phenomena such as sand boils, settlement, horizontal translation of a building, and lateral spreading associated with liquefaction and ground softening were observed in Wufeng and documented. The locations of soil liquefaction, settlement, and no observed ground failures in Wufeng were also identified, investigated, and analyzed after the earthquake. The areas of sand boils and lateral spreading are mainly located beside the creeks and match the regions of present and old river channels pretty well. In situ soil profiles and material properties of Wufeng are obtained based on 25 borings and six cone penetrometer (CPT) soundings before and after the earthquake. Results of aftershocks and data obtained from Nantou Township are compiled in the analyses to delineate the boundary of liquefaction and nonliquefaction. Data of standard penetration tests (SPT), CPT, and shear wave velocity are selected and the empirical methods recommended by an NCEER workshop were employed to evaluate the liquefaction resistance of soils. The results are also compared with the performance functions obtained through artificial neural network modeling. The SPT-based performance function has better prediction on the cases of nonliquefaction than other empirical methods do. Impacts of fines content (FC) (both nonplastic and plastic) on liquefaction resistance are also found to be profound. For SPT cases, an empirical boundary is suggested for soils with fines content larger than 35%. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Liquefaction; Chi-Chi earthquake; Aftershock; SPT; CPT; Shear wave velocity
1. Introduction An earthquake with a magnitude of Mw 7.6 (ML 7.3) struck central Taiwan on September 21, 1999 at 01:47 a.m. local time. The energy released by the * Corresponding author. Tel.: +886-4-23323000 ext. 4242; fax: +886-4-23742325. E-mail addresses: [email protected] (B.-L. Chu), [email protected] (S.-C. Hsu). 1 Fax: +886-4-22852798x231.
Chelungpu fault caused the Chi-Chi earthquake, resulting in a rupture of the earth surface that extended for almost 100 km (Fig. 1). The synchronization of the earthquake’s intensity with long duration led to severe damages due to soil liquefaction in several counties and townships including Wufeng, Nantou, Taichung Harbor, and Changhua all located at the central part of Taiwan. Numerous major aftershocks followed the earthquake with one magnitude over 6.5. Wufeng is located only 26 km away from the epicenter in the north. The Chelungpu fault is found
0013-7952/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0013-7952(03)00129-7
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Fig. 1. Locations of downtown Wufeng, the Chelungpu fault, and the rivers.
to pass through Wufeng. The fault displacement on this section of the fault is up to 2 m. This strong earthquake resulted in the loss of 87 lives and the collapsing or severe damage of around 4500 buildings in Wufeng. Fig. 1 shows the locations of Wufeng, the epicenter, and the Chelungpu fault. According to the free-field strong-motion station (TCU065) at the Wufeng Elementary School in Taiwan, the recorded peak ground accelerations in the EW, NS, and V (vertical) directions were 774, 563, and 257 gals, respectively. Fig. 2 shows the recorded accelerograms. There were no signs of liquefaction observed at this station. The shapes of recorded accelerograms also indicated that the soils underneath the station did not liquefy during the earthquake, i.e., the measured accelerations did not reduce to a small and consistent value after major ground shaking.
The authors conducted reconnaissance immediately following the earthquake. The major disaster occurred in the prosperous area of Wufeng. Buildings, riverbanks, golf courses, rice fields, bridges, and roads suffered severe damage due to soil liquefaction and ground softening. Phenomena such as sand boils, settlement, horizontal translation of a building, and lateral spreading associated with liquefaction and ground softening are observed in Wufeng and well documented.
2. Geological conditions in Wufeng Wufeng Township is located on the alluvial plain next to the central Western foothills of Taiwan. This alluvial plain, consisting of Quaternary deposits, covers most of the geological formations near the surface.
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Fig. 2. Accelerograms at Wufeng Elementary School during 921 Chi-Chi earthquake (TCU 065; duration = 44 s).
The thickness of these Holocene sediments varies from place to place and may extend to 150 m below the surface at this area, estimated from the geological map and explanatory text published by Central Geological Survey of R.O.C. (Ho and Chen, 2000). Rock formations, including Chinshui Shale, Choulan, and Toukoshan Formations, are present in the foothills in the eastern regions of Wufeng. These formations were deposited during the Pliocene and Pleistocene ages. A
geology map of the studied area in Wufeng is shown in Fig. 3 (Land and Mineral Resources Division (LMRD), 1995). The Chelungpu fault is between the foothills and the floodplains. The major surface rupture of the fault passed through Wufeng along the edge of western foothill. The distance from the fault to Chung-Cheng Road, the prosperous area of Wufeng, is less than 300 m (Figs. 1 and 4). The alluvium on the west part of the fault is comprised of silts, sands,
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Fig. 3. Geology map of the studied area in Wufeng (LMRD, 1995).
gravels, and cobbles eroded from the foothills and transported by Kan Creek (Fig. 4).
3. Hydrological conditions in Wufeng 3.1. River locations Da-Lee Creek and its tributary, namely Tsao-Hu Creek, constitute the northern border of soil liquefaction of Wufeng while the Wu Creek is the southern border (Figs. 1 and 4). The major creeks inside Wufeng are Kan Creek and its tributaries, Ko-Niao-Keng and Lai-Yuan Creeks. Ko-Niao-Keng Creek travels from east to west and merges into Kan Creek and its zigzag
path consists of six 90j sharp curves within 600 m of its travel distance (Fig. 4). The major sediments along KoNiao-Keng Creek consist of brown-yellow fine sands. Lai-Yuan Creek meandered within the main territory of Wufeng and its travel path also includes many 90j sharp curves (Fig. 4). The major sediments along LaiYuan Creek consist of brown-gray fine sands. 3.2. Old river channels Areas of old river channels and floodplains could cause potential disaster geologically and hydrologically. Geologically, most soils in these areas are unconsolidated and have low bearing capacity. Hydrologically, drainage in these areas is poor
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Fig. 4. Locations of the fault, old river channels, and observed ground failures in Wufeng during the Chi-Chi earthquake.
because of high ground water table and flat topography. The aerial photos taken during 1973 to 1974 are compared with the recent aerial photos. Several liquefaction-related failures were located in the major ranges of old river channels at Taichung basin as shown in Fig. 4. The old river channels cover very wide areas and coincide roughly with the present river channels. The range of old river
channels in Wufeng located between Tsao-Hu Creek and Ko-Niao-Keng Creek is 50 to 100 m east and 200 to 300 m west of Kan Creek. Most of the sand boils with brown-gray color were found inside this range. Soils at shallow depths classified as ML and CL based on the site investigation conducted in these areas. The old river channels and overflow flooding deposited these fine grain sediments.
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4. Distribution of liquefaction failures Soil liquefaction and settling occurred along approximately 2.5-km stretch in a north – southerly direction and over a width of 1.5 km in the east –west direction in Wufeng (Fig. 4). The range of liquefied areas from east to west starts at Chung-Cheng Road through Lin-Sen Road to 600 m west of Kan Creek. From north to south, the range starts from the Wufeng Junior High School, which is beside Tsao-Hu Creek, and ends at 200 m south of Ko-Niao-Keng Creek. Ground failures associated with liquefaction were mainly located at Kan, Lai-Yuan, Ko-Niao-Keng Creeks, and the intersection of Tsao-Hu and Da-Lee Creeks (Fig. 4).
buildings or ground. Along Chung-Cheng Road in the downtown area of Wufeng many buildings collapsed, settled, or tilted (Figs. 1 and 4). The footings of several connected buildings settled severely and the floor slabs heaved and broke as shown in Fig. 5. The settling and tilting of buildings due to the loss or partial failure of bearing capacity during the earthquake are quite obvious. Song-Gi jewelry store at the First Market of Chung-Cheng Road (BH-5 in Fig. 1) tilted severely and its footing was lifted above the ground as shown in Fig. 6. The structure of the building is still intact. The site is underlain by a silty sand (SM) layer at a depth of 1.5 to 4.0 m, a lean clay with sand (CL) layer at a depth of 4.0 to 4.5 m, and a gravel layer at 4.5 m below the surface (BH5 in Fig. 1). The water table is 1.2 m below the
5. Failures and phenomenon associated with liquefaction The observed ground failures associated with soil liquefaction in Wufeng include sand boils, the losses or partial failures of bearing capacities (subsidence, tilting, or horizontal translation of a building or the ground), lateral spreading, and flow failure. The size of each symbol used in Fig. 4 indicates the extent of each failure. The major phenomenon and associated failures are described as follows. 5.1. Sand boils Signs of sand boils can be seen at many places which include local communities, parks, roads, basement, fences, farms, gardens, orchards, retaining walls, levees etc. (Fig. 4). The height of sand boils was as high as 2 m as witnessed by an owner in his backyard and by the soils splashed on the walls. So the magnitude of horizontal ground acceleration should be very high and the duration of the earthquake should be quite long to induce such high pore water pressure. 5.2. Loss of bearing capacity Soil liquefaction and ground softening are the main causes for the loss or the decrease of bearing capacity during the earthquake which will result in settlement, tilting, and horizontal translation of
Fig. 5. Foundation settled and floor heaved by 1 m at Chung Cheng Road.
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surface. The SPT N-value, fines content, and clay size content for the SM layer are 13%, 45%, and 14%, and for the CL layer are 7%, 90%, and 21%, respectively. Despite the medium low blow count, no signs of sand boils were observed along ChungCheng Road. Another case is a three-story building (BH-3 in Fig. 1) moved 70 cm westward horizontally and the movement was arrested by the adjoining building during the earthquake. 5.3. Lateral spreading Lateral spreadings generally develop on gentle slopes (between 0.3j and 3j) and move toward a free surface, such as river channels or coastal lines. The locations of lateral spreadings in Wufeng were found beside the creeks, especially at the deposition side (point bar) of a meandered creek. Lateral spreading failures were found at five locations along Ko-Niao-Keng Creek (Figs. 7 and 8), three of them at Lai-Yuan Creek, two at Kan creek, and one beside Tsao-Hu Creek (Fig. 4).
6. Index properties of sand boils
Fig. 6. Song-Gi jewelry store tilted severely and its footing was lifted above the ground.
Samples near the vents of sand boils were taken right after the earthquake and their colors and textures were examined visually. Index property tests, including
Fig. 7. Sand boils and lateral spreading of a parking lot next to Ko-Niao-Keng Creek.
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Fig. 8. Tilted buildings and lateral spreading of a rice field next to Ko-Niao-Keng Creek.
grain size analysis, specific gravity, and plasticity, were also conducted. Results of grain size analyses of sand boils at several places are compared with the proposed curves by the Japanese Society of Civil Engineers (JSCE, 1977) as shown in Fig. 9. The grain size
distributions of sand boils in Wufeng are within the suggested range of high possibility of liquefaction. The D50 of these sand boils is in the range of 0.08 to 0.28 mm, while the D10 is in the range of 0.002 to 0.023 mm. These sand boils are classified as nonplastic silty sands
Fig. 9. Grain size distributions of sand boils in Wufeng during Chi-Chi earthquake.
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(SM). Most of the fines contents (grain size smaller than #200) of these sand boils are larger than 20% and some of them are around 40% to 50%. Since there is a chance for the fine-grained portions of the sand boils being washed away, the study on the sand boil samples may underestimate the fines content of the in-place liquefiable soils. Sand boils with three different colors namely brown-gray, brown-yellow, and black were found in Wufeng, while the black one was found at only one place. At some locations, the other two colors of sand boils were seen together. Thus, the sand boils could have come from different soil layers and depth levels; otherwise, it may be due to major geological variation of soils in Wufeng. The color of sand boils and grain
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size distribution were used to identify the possible depth of liquefaction.
7. Subsurface investigation and testing Fourteen different places were drilled in Wufeng after the earthquake. The locations of borings were selected based on the observed liquefaction phenomenon during the Chi-Chi earthquake (Fig. 1). Field investigation methods included standard penetration tests (SPT), cone penetration tests (CPT), and shear wave velocity (Vs) measurements. These exploratory borings were funded by National Center for Research and Earthquake Engineering of Taiwan and performed
Table 1 Summary of SPT data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake SPT boring
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical N-value
Critical (N1)60-value
Fines content (%)
Clay size content (%)
D50 (mm)
CSReqa
BH-1 BH-2 BH-3 BH-4 BH-5 BH-6 BH-7 BH-8 BH-9 BH-10 BH-11 BH-12 BH-13 BH-14 B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 LBH-1 LBH-3 LBH-4
No Nob Yes Noc Marginal Nod Yes No Yes Yes Yes Marginal Marginal Yes Yes Yes Yes Yes Yes Yes Marginal Yes Yes No Yes
2.30 5.10 2.40 1.35 3.05 13.55 3.75 1.35 1.35 5.85 2.85 8.90 2.85 1.35 5.85 4.40 5.85 4.35 2.85 4.35 3.35 11.35 2.85 3.35 3.00
1.38 5.10 1.30 1.30 1.20 3.20 3.20 0.50 1.00 1.40 0.90 4.20 0.90 1.00 2.70 2.30 2.75 2.55 2.70 1.90 3.26 3.20 2.50 2.00 2.00
22 >100 2 4 13 7 12 2 5 11 7 13 3 11 12 7 6 8 4 7 15 11 4 13 6
34.1 – 3.2 4.1 19.3 7.1 14.4 2.3 7.6 14.4 11.1 14.0 4.8 16.7 14.1 8.8 7.2 9.7 5.2 9.2 17.5 12.2 5.6 17.5 8.4
10 – 22 76 45 87 20 65 38 22 45 40 38 21 49 27 27 34 33 40 39 77 47 71 30
4 – 6 26 14 44 4 23 9 4 9 14 11 7 12 5 5 7 9 10 0 0 14 35 15
0.24 – 0.14 0.02 0.09 0.01 0.41 0.06 0.10 0.12 0.08 0.10 0.10 0.21 0.08 0.19 0.20 0.20 0.19 0.13 0.20 0.20 0.18 0.19 0.26
0.56 – 0.57 0.45 0.65 0.58 0.48 0.64 0.51 0.70 0.67 0.57 0.67 0.51 0.61 0.58 0.58 0.56 0.45 0.61 0.45 0.63 0.47 0.56 0.53
Use Mw = 7.6 and amax = 0.67g in the liquefaction resistance calculation. Most of the soils are classified as SM or ML. a CSReq is the equivalent CSR for Mw = 7.5. b The soil is classified as GM. c The soil is classified as CL and its liquid limit and plasticity index are 32 and 9, respectively. d The soil is classified as CL – ML and its liquid limit and plasticity index are 28 and 6, respectively.
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by Moh and Associates (2000). SPT tests generally followed the guidelines recommended by Seed et al. (1985). The SPT N-value was measured at 1.5-m interval. However, the spatial variation of the soil layer in Wufeng was found to vary very much in vertical direction. Thus, the interval of 1 m is highly recommended for the use of SPT in order to obtain more details of soil information since the strata are complicated in Wufeng. Moh and Associates performed energy measurements of SPT in Yuanlin City being one of the extensively liquefied areas during the ChiChi earthquake. The energy ratio (ER) of SPT in the analyses is a function of depth as follows: ERð%Þ ¼ 30
Z þ 50 for Z < 10 m 11
ERð%Þ ¼ 80 for Zz10 m
ð1Þ
Yuanlin was applied for liquefaction analyses of borings in Wufeng. In addition, 11 borings performed by the Panku Engineering, the Sinotech Engineering Consultants (Building and City Development Council, 1995), and this study were also synthesized. A total of 25 borings were collected, with two drilled before the earthquake and 23 drilled after the earthquake. SPT data for the above borings are presented in Table 1 and shown in Figs. 10 and 11. Six seismic cone penetration tests (SCPT) were also conducted after the earthquake. The CPT tests conformed with ASTM D3441-79. Shear wave velocities of soils at 1-m interval were measured while doing cone penetration tests. The results of CPT tests and shear wave velocity are summarized in Tables 2 and 3, respectively.
ð2Þ 7.1. Subsurface soil conditions
where ER is the hammer energy ratio or hammer efficiency at the depth of Z (in meter). The energy ratio in Eqs. (1) and (2) includes both energy and rod length corrections. As the same drilling equipment was used in Wufeng, also the ER measurement obtained in
Subsurface conditions and results of SPT are illustrated by the two cross-sectional soil profiles shown in (Figs. 10 and 11). The soil strat in (Figs. 10 and 11) are all considered as Holocene sediments (Ho and
Fig. 10. Soil profiles and boring information in east – west direction in Wufeng.
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Fig. 11. Soil profiles and boring information in north – south direction in Wufeng.
Chen, 2000). The major geological materials in the liquefied areas namely east and west of Lin-Sen Road differ in consistency. The soils on the eastern side consist mainly of fine sediments of silts and clays, which could be eroded from weathered Chinshui Shale from the eastern side of the foothills. However, soils with interlayers of sand, gravel, cobble, and silt are present on the western side of the road and these sediments might be due to river deposits, which were transported and deposited by Kan Creek.
7.2. Depth of liquefaction The borings drilled after the earthquake at selected locations and their corresponding grain size distributions revealed shallow soil liquefaction. On the western side of Lin-Sen Road, near Kan Creek, mostly liquefaction occurred in a layer of silty sand (SM) of brown-yellow color at a shallow depth of under 5 m. SPT N-values for the soils are very low being around 3 to 5. However, on the eastern side of
Table 2 Summary of CPT data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake and 611 earthquake CPT Liquefaction Critical Water Critical Critical Friction Fines Clay size D50 (mm) amax sounding observed? depth (m) depth (m) qc (bars) qc1N (g) ratio, Rf (%) content (%) content (%) CPT-7 CPT-8 CPT-9 CPT-10 CPT-15 CPT-16 CPT-7 CPT-9 CPT-10 CPT-15 a
Yes No Yes Yes Yes No No No No No
3.75 1.35 3.00 5.85 11.35 3.60 3.75 3.00 5.85 11.35
3.0 0.5 2.5 4.0 2.5 2.0 3.0 2.5 4.0 2.5
50 4.6 10.8 11.5 25.4 19.8 50 10.8 11.5 25.4
CSReq is the equivalent CSR for Mw = 7.5. For 921 Chi-Chi earthquake, Mw = 7.6; amax = 0.67g. c For 611 earthquake, Mw = 6.9; amax = 0.22g. b
59.74 10.92 14.70 13.65 21.05 26.71 59.74 14.70 13.65 21.05
0.03 0.30 0.01 0.15 0.04 0.24 0.03 0.01 0.15 0.04
20 65 47 22 77 – 20 47 22 77
4 23 14 4 0 – 4 14 4 0
0.41 0.06 0.18 0.12 0.20 – 0.41 0.18 0.12 0.20
0.67b 0.67 0.67 0.67 0.67 0.67 0.22c 0.22 0.22 0.22
CSReqa 0.48 0.64 0.44 0.70 0.61 0.57 0.13 0.12 0.19 0.17
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Table 3 Summary of Vs data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake and 611 earthquake Seismic CPT Liquefaction Critical Water Fines Clay size Median grain Vs (m/s) Vsl (m/s) CSR CSReq amax sounding observed? depth (m) depth (m) content (%) content (%) diameter (mm) (g) CPT-7 CPT-8 CPT-10 CPT-15 CPT-16 CPT-7 CPT-10 CPT-15 a b
Yes No Yes Yes No No No No
3.75 1.35 5.85 11.35 3.60 3.75 5.85 11.35
3.0 0.5 4.0 2.5 2.0 3.0 4.0 2.5
20 65 22 77 – 20 22 77
4 23 4 0 – 4 4 0
0.41 0.06 0.12 0.20 – 0.41 0.12 0.20
167 98 130 164 158 167 130 164
183 151 142 149 184 183 142 149
0.46 0.62 0.67 0.61 0.54 0.15 0.22 0.20
0.47 0.64 0.69 0.63 0.56 0.12 0.18 0.16
0.67a 0.67 0.67 0.67 0.67 0.22b 0.22 0.22
For 921 Chi-Chi earthquake, Mw = 7.6; amax = 0.67g. For 611 earthquake, Mw = 6.9; amax = 0.22g.
Lin-Sen Road, which is the downtown area, liquefaction was deeper up to 10 m. The main soil type is brown-gray sandy silt (ML) and the SPT N-values are around 3 to 15.
8. Structure performance and subsurface conditions The main types of damage for buildings in liquefied areas of Wufeng were generally settling and tilting. The infrastructures of most buildings remained intact or suffered only minor damage. However, in areas without soil liquefaction, such as Wufeng Elementary School (BH-8 in Table 1 and shown in Figs. 1 and 11), the columns and beams of building structures either collapsed or faced severe damage and could not be easily repaired. Some observations can be concluded as follows based on structure performance and subsurface soil conditions in Wufeng. (1) Sand boils were observed with brown-yellow soils classified as silty sands (SM) at shallow depths. Differential settlement and tilting of a building might occur due to soil liquefaction. However, the structure of the building is usually intact unless the tilted angle is too high. (2) Brown-gray soils classified as sandy silts (ML) are located at deeper depths. The liquefaction potential of these soils is also very high, but the sand boils were not commonly seen because of
greater overburden depths and larger fines content. Most of the buildings in this area underwent settlement, but the degree of settlement was normally greater than that of the buildings in areas with sand boils. (3) Most of the buildings experienced structural damage and a little settlement provided most of the soil layers underneath the buildings were clay.
9. Evaluation of liquefaction potential Each in situ test location, i.e., SPT, CPT, or shear wave velocity (Vs) profile, was employed to obtain one critical combination of earthquake-induced cyclic stress ratio (CSReq) and normalized in situ measurement, i.e., (N1)60, qc1N, or Vs1. CSReq is the equivalent CSR for Mw = 7.5 earthquake, i.e., CSReq = CSR/ MSF. The parameter magnitude scaling factor (MSF) is calculated as (Youd et al., 2001).
MSF ¼ 102:24 =Mw2:56
ð3Þ
Each set of critical combinations was interpreted considering together the overall performance of the site, the subsurface soils, and the color and grain size distribution of sand soils. The cyclic stress ratio induced by an earthquake was calculated based on the formulation proposed by Seed and Idriss (1971) and the suggested modifications by Youd et al. (2001). Value of stress reduction coef-
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ficient (rd) for calculating the cyclic stress ratio (CSR) is estimated as (Liao and Whitman, 1986) rd ¼ 1:0 0:00765Z for ZV9:15 m
ð4Þ
rd ¼ 1:174 0:0267Z for 9:15 m < ZV23 m
ð5Þ
9.1. Evaluation of liquefaction based on SPT data The simplified procedure proposed by Seed and Idriss (1971) and Seed et al. (1985) was applied to evaluate the liquefaction resistance (CRR) based on the SPT data. Modifications suggested by Youd et al. (2001) were also taken into consideration. SPT penetration resistance was normalized to an equivalent overburden stress of 100 kPa (1 atm) using the correction factor (CN) proposed by Liao and Whitman (1986). The value of CN was limited to a maximum value of 1.7 as suggested by Youd et al. (2001). The energy ratio (ER) was calculated based on Eqs. (1) and (2). Because the borehole diameter is 86 mm and a standard sampler is used for the SPT, the correction
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factors for the borehole diameter and sampling method are both equal to 1.0. The depth of groundwater table for each boring was based on the field observation and Mw equals 7.6 for all the analyses. The peak horizontal ground acceleration (amax) for liquefaction analyses is 660 gals (0.67g), which is the geometric mean of the two maximum horizontal accelerations recorded in Wufeng. The determination of critical layer for SPT was first estimated by comparing the color and grain characteristics (such as grain-size distribution and fines content) of sand boils with the sample information from the corresponding borehole. Then the simplified procedure suggested by Seed et al. (1985) was used to calculate the liquefaction resistance and factor of safety at sampled depths of the borehole. For soil layers with the similar color texture and grain-size characteristics within a borehole, the depth with lowest factor of safety was chosen as the critical layer. Table 1 summarizes the field observation, the results of the SPT data, fines content, clay size content, and CSReq for selected borings. Fig. 12 illustrated a graph
Fig. 12. Critical combinations of cyclic stress ratio and SPT (N1)60 for soils in Wufeng with liquefaction resistance curves by Seed et al. (1985) and modified by Youd et al. (2001).
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of calculated CSReq and corresponding (N1)60 data obtained from various locations in Wufeng where liquefaction effects were either observed or not following the Chi-Chi earthquake. Four borings, where the buildings suffered settlement with no evidence of sand boils, are marked as marginal in the figure. The number beside each data point in the graph is the fines content of the data. The CRR curves proposed by Seed et al. (1985) and modified by Youd et al. (2001) for granular soils with the fines content of 5% or less, 15%, and 35% are also shown in Fig. 12. Many major aftershocks and earthquakes, several over magnitude 6.5, following the earthquake were encountered. The maximum peak ground acceleration (PGA) recorded at TCU065 after the Chi-Chi earthquake was due to the earthquake (Mw = 6.9) on June 11, 2000 (611 earthquake). The recorded peak ground accelerations in the EW, NS, and V (vertical) directions were 267, 173, and 71 gals, respectively, and thus the peak horizontal ground acceleration (amax) used for liquefaction analyses is 215 gals (0.22g). Since there were no evidences of liquefaction following this earthquake, CSR based on this and corresponding SPT data will be considered as nonliquefaction. The results of which are tabulated in Table 4 and also plotted in Fig. 12.
9.1.1. Effects of fines content and clay size content Most of the data from the Chi-Chi earthquake are plotted within the range of liquefaction zone since CSR was very large based on the recorded amax. However, there are four borings, which did not liquefy during the earthquake fall within the range of liquefaction. The soils at the critical depths of these four borings, BH-4, BH-6, BH-8, and LBH-3, have high percentage of fines content (FC) (percent passing the Number 200 sieve), 76%, 87%, 65 and 71%, and clay size content (CC) (percent finer than 0.002 mm by weight), 26%, 44%, 23% and 35%, respectively (Table 1). The BH-4 and BH-6 are classified as CL and CL – ML, respectively, as they have plastic fines and the liquid limits (plasticity indexes) for them are 32 (9)% and 28 (6)%, respectively. The other two soils, BH-8 and LBH-3, have nonplastic fines and are classified as ML. The data based on the aftershock (611 earthquake) are all considered as nonliquefaction. Only the locations that liquefied during the ChiChi earthquake and did not liquefy during the 611 earthquake are plotted and marked as open square in Fig. 12. However, most of the data still fall within the liquefaction zone. The fines contents for these data are also high as shown in Fig. 12 and in Tables 1 and 4. Based on the results of nonliquefaction cases in Fig.
Table 4 Summary of SPT data obtained in Wufeng, Taiwan after 611 earthquake in 2000 SPT boring
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical N-value
Critical (N1)60-value
Fines content (%)
Clay size content (%)
D50 (mm)
CSReq
BH-3 BH-5 BH-7 BH-9 BH-10 BH-11 BH-12 BH-13 BH-14 B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 LBH-1 LBH-4
No No No No No No No No No No No No No No No No No No No
2.40 3.05 3.75 1.35 5.85 2.85 8.90 2.85 1.35 5.85 4.40 5.85 4.35 2.85 4.35 3.35 11.35 2.85 3.00
1.30 1.20 3.20 1.00 1.40 0.90 4.20 0.90 1.00 2.70 2.30 2.75 2.55 2.70 1.90 3.26 3.20 2.50 2.00
2 13 12 5 11 7 13 3 11 12 7 6 8 4 7 15 11 4 6
3.2 19.3 14.4 7.6 14.4 11.1 14.0 4.8 16.7 14.1 8.8 7.2 9.7 5.2 9.2 17.5 12.2 5.6 8.4
22 45 20 38 22 45 40 38 21 49 27 27 34 33 40 39 77 47 30
6 14 4 9 4 9 14 11 7 12 5 5 7 9 10 0 0 14 15
0.14 0.09 0.41 0.10 0.12 0.08 0.10 0.10 0.21 0.08 0.19 0.20 0.20 0.19 0.13 0.20 0.20 0.18 0.26
0.15 0.17 0.12 0.13 0.18 0.17 0.15 0.17 0.13 0.16 0.15 0.15 0.14 0.12 0.16 0.11 0.16 0.12 0.13
Use Mw = 6.9 and amax = 0.22g in the liquefaction resistance calculation.
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12, the soils with higher fines content tend to have either higher CRR or lower penetration resistance. In the original development, Seed et al. (1985) noted an apparent increase of CRR with increase of fines content. Whether this increase is caused by an increase of liquefaction resistance or a decrease of penetration resistance is not clear yet (Youd et al., 2001). Therefore, the effects of fines content and clay size content are quite obvious which need further study and considered for the estimation of CRR. In order to take the effects of fines content (FC) and clay size content (CC) into consideration, CSR is decreased to some extent based on the percentage of FC and CC instead of increasing CRR. The data collected by Tokimatsu and Yoshimi (1983) and Hwang and Yang (2001) were compared with the data collected in this study as it also included clay
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size content. As CSReq is divided by the sum of FC and CC and plotted versus (N1)60 for data with fines content greater than 35%, it is found that there is a boundary curve which could delineate the boundary between liquefaction and nonliquefaction regions (shown in Fig. 13). This curve is drawn by connecting data points of liquefaction cases located at the lower right boundary in the figure empirically. There are only four nonliquefaction data located inside the liquefaction range and the percentage amount of FC (CC) for these four data are 76 (26)%, 87 (44)%, 65 (23)% and 38 (11)%, respectively, as indicated in Fig. 13. The percentage of clay size content (CC) for these data is high. The first two data, from BH-4 and BH-6, also have plasticity indexes of 9% and 6%, respectively, and there may exist the influence of CC and plasticity index on the liquefaction resistance (Toki-
Fig. 13. (CSR 10/(FC + CC)) and SPT (N1)60 for the liquefiable and nonliquefiable soils with fines content greater than 35%.
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matsu and Yoshimi, 1983). Tokimatsu and Yoshimi (1983) and Seed and Idriss (1981) indicated that none of the soils they studied containing more than 20% clay has suffered serious strength loss due to liquefaction. Based on this, if the first three data having fines content greater than 20% are excluded from Fig. 13, there is only one nonliquefaction data inside the liquefaction range. This point is plotted very close to the boundary curve. Thus, this empirical curve in Fig. 13 could be used to represent the boundary between liquefaction and nonliquefaction regions for soils with fines content greater than 35% but with CC < 20%. The other available liquefaction data, whether it contains clay size content or not, from Boulanger et al. (1997) and Goh (1994) are plotted with the previous data and shown in Fig. 14. For clean sand, only the
CSReq is used for the longitudinal axis since its FC is zero. For nonliquefaction cases only fines content greater than 35% are plotted in Fig. 14. All the liquefaction data are also plotted within the liquefaction range. 9.1.2. Prediction of occurrence/nonoccurrence of liquefaction Methods of Seed et al. (1985) and the limit state function based on Artificial Neural Network (ANN) (Juang et al., 2000c) were chosen to predict the occurrence or nonoccurrence of liquefaction for the case records in this study. This ANN model used 243 field liquefaction performance cases to establish a liquefaction limit state function (Juang et al., 2000c). The liquefaction resistance (CRR) can be determined
Fig. 14. (CSR 10/(FC + CC)) and SPT (N1)60 for the nonliquefiable soils with fines content greater than 35% and for all the liquefiable soils from previous cases.
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using this limit state function. Then the factor of safety (FS) can be calculated as FS = CRR/CSR. Liquefaction is said to occur if FS V 1, and no liquefaction occurs if FS>1. Both methods can predict the occurrence of liquefaction pretty well. The success rate for accurate predictions of the occurrence of liquefaction is 100%. But for 921 Chi-Chi earthquake (Table 1), however, the success rate for predictions of the nonoccurrence of liquefaction by both the methods is only 33% as four miss out of six case records. As for the data from the 611 earthquake, which are all no liquefaction as listed in Table 2, the success rates by Seed’s method and by ANN for predictions of the nonoccurrence of liquefaction are 28% (7 success in 25 cases) and 88% (22 success in 25 cases), respectively. Thus, the approach by ANN is more promising for predicting the nonoccurrence of liquefaction than Seed’s method, if the amax or CSReq is not too high such as the cases from 611 earthquake. For cases with large amax such as 921 Chi-Chi earthquake, however, the success rate for predictions of the nonoccurrence of liquefaction by both the methods is poor as there were only limited field performance cases for high FC and for CSReq>0.3 for both methods. Thus the data obtained from Chi-Chi earthquake will be of high value in implementing the data in these ranges for ANN model and for Seed’s method. 9.2. Evaluation of liquefaction based on CPT data The CPT-based method proposed by Robertson and Wride (1998) was utilized to evaluate the liquefaction resistance (CRR). Modifications suggested by Youd et al. (2001) were also considered. In selecting the critical value of normalized CPT tip resistance ( qc1) in each CPT sounding, the following guidelines suggested by Boulanger et al. (1997) were adopted to reduce potential biases that may be introduced by different individuals. If the critical depth occurred within a thick stratum of homogeneous material, the critical value was taken as the average qc1N over 0.5m interval. Second, if the critical interval was between 0.15 and 0.6 m, excluding zones (0.15 m) influenced by adjacent contacts, then the average value was taken over the full interval thickness. Therefore, the minimum sand layer thickness is about 0.45 m for obtaining a reasonably representative qc (Boulanger et al., 1997).
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There are interlayers of gravel or cobble of 5 to 15 m thick at shallow depths in Wufeng. Therefore, the actual tested depths using CPT were between 5 and 28 m. The results of qc1N, CSReq, and fines content during 921 Chi-Chi earthquake and 611 earthquake are tabulated in Table 2. The fines contents in Table 2 are estimated based on the adjacent borings, as shown in Fig. 1. Fig. 15 shows cyclic stress ratio versus corrected and normalized CPT resistance qc1N of the study at Wufeng. Data obtained from Nantou City (Table 5) are also plotted in Fig. 15 as there were only six CPT soundings performed in Wufeng. Fig. 15 also provides the clean-sand base curve formulated by Robertson and Wride (1998) for direct determination of CRR for clean sands (FC V 5%) from CPT data. The curve proposed by Stark and Olson (1995) for FC = 35% is also shown in Fig. 15 for reference. Fines content for the corresponding CPT sounding is also indicated in the figure. The liquefied data points are all plotted above the clean sand curve, i.e., liquefaction region, and are in good agreement with the curves proposed by Robertson and Wride (1998) and Stark and Olson (1995). The nonliquefied cases are also above the clean sand curve since most fines contents of the soils are higher than 20%. Thus, predictions of nonoccurrence of liquefaction by Robertson and Wride (1998) and the ANN model by Juang et al. (2000a,b) are poor. Both the success rates are only 11% (one success in seven cases). For the ANN model by Juang et al. (2000a), the obtained limit state function is used to calculate the liquefaction resistance (CRR) of a soil. 9.3. Evaluation of liquefaction based on shear wave velocity data Shear wave velocity (Vs) was measured at 1-m interval together with the CPT test using seismic cone at the tip and triggered source at the ground surface. Shear wave velocity measurements at 1.0-m intervals were usually ineffective in characterizing the critical strata at some locations because the measurement interval crossed over two or more thin strata with significantly different characteristics (Boulanger et al., 1997). So some boreholes were conducted to detect and delineate thin liquefiable strata in this study. Thus, the critical layer of Vs is selected based on the information from the nearby borehole with SPT, Vs
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Fig. 15. Critical combinations of cyclic stress ratio and qc1N for soils in Wufeng and Nantou (modified from Robertson and Wride, 1998).
values, and CPT results. Data for those locations where measurements of shear wave velocity coincided reasonably well with the suspected critical strata during 921 Chi-Chi earthquake and 611 earthquake are tabulated in Table 3. The measured shear wave velocities were usually less than 200 m/s in Wufeng. Data obtained from Nantou are also synthesized and listed in Table 6. The shear wave velocity method proposed by Andrus and Stokoe (1997, 2000) was applied to evaluate the liquefaction resistance (CRR) based on
the Vs data. The equation suggested in the paper by Youd et al. (2001) to account for overburden stress for Vs is also adopted. CRR versus Vs1 (overburdenstress corrected shear wave velocity) curves recommended by Andrus and Stokoe (2000) for magnitude 7.5 earthquake and soils with various fines contents, z 35%, 20% and V 5%, are shown in Fig. 16. The data comprising liquefied and nonliquefied cases from Wufeng and Nantou all fall within the liquefiable region as proposed by Andrus and Stokoe (2000). The nonliquefied cases are also above the
Table 5 Summary of CPT data obtained in Nantou, Taiwan after 921 Chi-Chi earthquake CPT sounding
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical qc (bars)
Critical qc1N
Friction ratio, Rf (%)
Fines content (%)
Clay size content (%)
D50
amax (g)
CSReq
CPT-2 CPT-3 CPT-7 CPT-8 CPT-11 CPT-13 CPT-15
Yes Yes Yes Yes Yes No Yes
5.0 6.7 5.8 5.0 4.9 7.0 5.8
5.0 6.7 5.0 5.0 1.9 7.0 1.5
85.0 26.0 75.0 52.0 15.0 20.0 50.0
85.0 20.6 72.4 46.3 18.6 14.9 62.6
0.15 3.80 0.40 2.30 1.80 3.70 2.00
– – 25 – 24 – 30
– – 7 – 8 – 7
– – 0.10 – 0.11 – 0.30
0.39 0.39 0.39 0.39 0.39 0.39 0.39
0.25 0.25 0.27 0.25 0.35 0.25 0.42
Mw = 7.6; amax = 0.39g.
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Table 6 Summary of Vs data obtained in Nantou, Taiwan after 921 Chi-Chi earthquake Seismic CPT sounding
Liquefaction observed?
Critical depth (m)
Water depth (m)
Fines content (%)
Clay size content (%)
Median grain diameter (mm)
Vs (m/s)
Vsl (m/s)
CSR
CSReq
CPT-2 CPT-3 CPT-7 CPT-8 CPT-11 CPT-13 CPT-15
Yes Yes Yes Yes Yes No Yes
5.0 6.7 5.8 5.0 4.9 7.0 5.8
5.0 6.7 5.0 5.0 1.9 7.0 1.5
– – 25 – 24 – 30
– – 7 – 8 – 7
– – 0.10 – 0.11 – 0.30
200 176 184 147 120 160 165
200 168 181 139 129 151 185
0.24 0.24 0.26 0.24 0.34 0.24 0.41
0.25 0.25 0.27 0.25 0.35 0.25 0.42
Mw = 7.6; amax = 0.39g.
clean sand curve since most fines contents of the soils are higher than 20%. Predictions of nonoccurrence of liquefaction by Andrus and Stokoe (2000) and the ANN model by Juang and Chen (2000) and Juang et al. (2001) (using the proposed limit state function) are not satisfactory. The success rates by Andrus and Stokoe (2000) and by ANN for predictions of the nonoccurrence of liquefaction are only 17% (one success in six cases) and 33% (two success in six cases), respectively.
10. Concluding remarks A case-history study of liquefaction-related failure and consequences in Wufeng, Taiwan during 921 ChiChi earthquake was presented. Subsurface data of SPT, CPT, and Vs from Wufeng and Nantou were selected and compiled in this paper. These data will provide the bases for further development of semi-empirical design procedures and the artificial neural network (ANN) modeling since the recorded peak horizontal ground
Fig. 16. Critical combinations of cyclic stress ratio and Vs1 for soils in Wufeng and Nantou (modified from Andrus and Stokoe, 2000).
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acceleration was high, amax = 0.67g, and fines contents of liquefiable and nonliquefiable soils were also very high, larger than 20%. Some findings and conclusions can be drawn as follows based on this study. 1. Liquefaction in Wufeng during Chi-Chi earthquake appears to have occurred primarily within Quaternary deposits at depths less than 12 m. The major regions of liquefaction are located along the creeks and basically matched with the old river channels. 2. The SPT-based performance function based on the ANN model provides better prediction on the cases of nonliquefaction than other empirical methods do. 3. Impacts of fines content (both nonplastic and plastic) on liquefaction resistance are also found to be profound. For SPT data, a new empirical boundary which separates liquefaction and nonliquefaction cases was obtained for soils with fines content larger than or equal to 35% based on this study. 4. The CRR curves for SPT, CPT, and Vs recommended by 1998 NCEER workshop for evaluating liquefaction resistance are conservative but acceptable in engineering practice for predictions of liquefaction occurrence. Gravel formations are common strata at the Western Foothills of Taiwan. The CPT is found not suitable to use in these formations since the CPT cone is unable to penetrate these formations. The interval of 1 m or less is highly recommended for the use of SPT in order to obtain more details of soil information since the strata are complicated in Wufeng. It is hoped that the data and results presented in this study will contribute to the further development of semi-empirical correlations for predicting liquefaction-related damage during earthquakes.
Appendix A The following symbols are used in this paper: Notation amax peak horizontal acceleration at ground surface CC clay size content (%), i.e., percent finer than 2 Am by weight CN correction factor for overburden pressure applied to SPT CPT cone penetration test CRR cyclic resistance ratio CSR cyclic stress ratio CSReq equivalent CSR for Mw = 7.5 D50 median grain diameter ER hammer energy ratio of standard penetration test (%) FC fines content (%), i.e., percent passing the Number 200 sieve MSF magnitude scaling factor Mw moment magnitude of earthquake N blow count of standard penetration test (N1)60 corrected standard penetration resistance qc field cone penetration resistance measured at the tip qc1N normalized cone penetration resistance rd stress reduction coefficient to account for flexibility in soil profile Rf calculated friction ratio (%) SPT standard penetration test Vs small-strain shear wave velocity Vs1 overburden stress-corrected Vs Z depth below ground surface (m)
References Acknowledgements The authors would like to thank National Center for Research and Earthquake Engineering (NCREE) and the National Science Council (NSC) of the Republic of China for financially supporting this research under Contract No. NSC89-2921-Z319-005-18. The authors also wish to thank the editor, Dr. C. Hsein Juang of Clemson University, and the reviewers for providing many helpful suggestions and comments.
Andrus, R.D., Stokoe II, K.H., 1997. Guidelines for evaluating of liquefaction resistance using shear wave velocity. Submission to the 1996 NCEER Workshops on Evaluation of Liquefaction Resistance (Youd, T.L., Chair), Salt Lake City, Utah, USA. Andrus, R.D., Stokoe II, K.H., 2000. Liquefaction resistance of soils from shear wave velocity. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 126 (11), 1015 – 1025. Boulanger, R.W., Mejia, L.H., Idriss, I.M., 1997. Liquefaction at Moss Landing during Loma Prieta earthquake. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 123 (5), 453 – 467. Building and City Development Council, Taiwan Province, 1995.
B.-L. Chu et al. / Engineering Geology 71 (2003) 119–139 Evaluation and investigation of geology and ground characteristics for Taichung transit system project (blue line) (in Chinese). Goh, A.T.C., 1994. Seismic liquefaction potential assessed by neural networks. Journal of Geotechnical Engineering, ASCE 120 (9), 1467 – 1480. Ho, H.-C., Chen, M.M., 2000. Explanatory text of the geologic map of Taiwan, scale 1:50,000, sheet 24: Taichung. Central Geological Survey of ROC. Hwang, J.H., Yang, C.W., 2001. Verification of critical cyclic strength curve by Taiwan Chi-Chi earthquake data. Soil Dynamics and Earthquake Engineering 21, 237 – 257. JSCE, 1977. Earthquake resistant design for civil engineering structure, earth structure and foundation in Japan. Japan Society of Civil Engineers. Juang, C.H., Chen, C.J., 2000. A rational method for development of limit state for liquefaction evaluation based on shear wave velocity measurements. International Journal for Numerical and Analytical Methods in Geomechanics 24 (1), 1 – 27. Juang, C.H., Chen, C.J., Tang, W.H., Rosowsky, D.V., 2000a. CPTbased liquefaction analysis: Part 1. Determination of limit state function. Geotechnique 50 (5), 583 – 592. Juang, C.H., Chen, C.J., Rosowsky, D.V., Tang, W.H., 2000b. CPTbased liquefaction analysis: Part 2. Reliability for design. Geotechnique 50 (5), 593 – 599. Juang, C.H., Chen, C.J., Jiang, T., Andrus, R.D., 2000c. Risk-based liquefaction potential evaluation using standard penetration tests. Canadian Geotechnical Journal 37, 1195 – 1208. Juang, C.H., Chen, C.J., Jiang, T., 2001. Probabilistic framework for liquefaction potential by shear wave velocity. Journal of the Geotechnical Engineering Division, ASCE 127 (8), 670 – 678. Liao, S.S.C., Whitman, R.V., 1986. Overburden correction factors for SPT in sand. Journal of Geotechnical Engineering, ASCE 112 (3), 373 – 377. LMRD (Land and Mineral Resources Division), 1995. Investigation and evaluation of geology and subsurface characteristics for
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Taichung Transit System (blue line). Final Report to Bureau of Housing and City Development of Taiwan Province, Report No. 06-3-84-0200, Industrial Technology Research Institute (in Chinese). Moh and Associates, 2000. Site investigation of liquefied soil at Nantou and Wufeng areas (in Chinese). Robertson, P.K., Wride, C.E., 1998. Evaluating cyclic liquefaction potential using the cone penetration test. Canadian Geotechnical Journal, Ottawa 35 (3), 442 – 459. Seed, H.B., Idriss, I.M., 1971. Simplified procedure for evaluating soil liquefaction potential. Journal of the Geotechnical Engineering Division, ASCE 97 (7), 1249 – 1273. Seed, H.B., Idriss, I.M., 1981. Evaluation of liquefaction potential of sand deposits based on observations of performance in previous earthquakes. Preprint 81-544, In Situ Testing to Evaluate Liquefaction Susceptibility, ASCE National Convention, St. Louis, Missouri. Seed, H.B., Tokimatsu, K., Harder, L.F., Chung, R.M., 1985. The influence of SPT procedures in soil liquefaction resistance evaluations. Journal of the Geotechnical Engineering Division, ASCE 111 (12), 1425 – 1445. Stark, T.D., Olson, S.M., 1995. Liquefaction resistance using CPT and field case histories. Journal of the Geotechnical Engineering Division, ASCE 121 (12), 856 – 869. Tokimatsu, K., Yoshimi, Y., 1983. Empirical correlation of soil liquefaction based on SPT N-value and fines contents. Soil and Foundation, JSSMFE 23 (4), 56 – 74. Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn Jr., W.D.L., Harder Jr., L.F., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson III, W.F., Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K., Seed, R.B., Stokoe II, K.H., 2001. Liquefaction resistance of soils: summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. Journal of the Geotechnical and Geoenvironmental Engineering, ASCE 127 (10), 817 – 833.
Ground behavior and liquefaction analyses in central Taiwan-Wufeng Bin-Lin Chu a,1, Sung-Chi Hsu b,*, Yi-Ming Chang a,1 b
a Department of Civil Engineering, National Chung-hsing University, Taichung, Taiwan, ROC Department of Construction Engineering, Chaoyang University of Technology, Wufeng, Taichung City, Taiwan, ROC
Abstract The 921 Chi-Chi earthquake (Mw = 7.6) in 1999 caused extensive ground failures in the central part of Taiwan. Severe damages due to soil liquefaction and ground softening occurred widespread in Wufeng. Various phenomena such as sand boils, settlement, horizontal translation of a building, and lateral spreading associated with liquefaction and ground softening were observed in Wufeng and documented. The locations of soil liquefaction, settlement, and no observed ground failures in Wufeng were also identified, investigated, and analyzed after the earthquake. The areas of sand boils and lateral spreading are mainly located beside the creeks and match the regions of present and old river channels pretty well. In situ soil profiles and material properties of Wufeng are obtained based on 25 borings and six cone penetrometer (CPT) soundings before and after the earthquake. Results of aftershocks and data obtained from Nantou Township are compiled in the analyses to delineate the boundary of liquefaction and nonliquefaction. Data of standard penetration tests (SPT), CPT, and shear wave velocity are selected and the empirical methods recommended by an NCEER workshop were employed to evaluate the liquefaction resistance of soils. The results are also compared with the performance functions obtained through artificial neural network modeling. The SPT-based performance function has better prediction on the cases of nonliquefaction than other empirical methods do. Impacts of fines content (FC) (both nonplastic and plastic) on liquefaction resistance are also found to be profound. For SPT cases, an empirical boundary is suggested for soils with fines content larger than 35%. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Liquefaction; Chi-Chi earthquake; Aftershock; SPT; CPT; Shear wave velocity
1. Introduction An earthquake with a magnitude of Mw 7.6 (ML 7.3) struck central Taiwan on September 21, 1999 at 01:47 a.m. local time. The energy released by the * Corresponding author. Tel.: +886-4-23323000 ext. 4242; fax: +886-4-23742325. E-mail addresses: [email protected] (B.-L. Chu), [email protected] (S.-C. Hsu). 1 Fax: +886-4-22852798x231.
Chelungpu fault caused the Chi-Chi earthquake, resulting in a rupture of the earth surface that extended for almost 100 km (Fig. 1). The synchronization of the earthquake’s intensity with long duration led to severe damages due to soil liquefaction in several counties and townships including Wufeng, Nantou, Taichung Harbor, and Changhua all located at the central part of Taiwan. Numerous major aftershocks followed the earthquake with one magnitude over 6.5. Wufeng is located only 26 km away from the epicenter in the north. The Chelungpu fault is found
0013-7952/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0013-7952(03)00129-7
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Fig. 1. Locations of downtown Wufeng, the Chelungpu fault, and the rivers.
to pass through Wufeng. The fault displacement on this section of the fault is up to 2 m. This strong earthquake resulted in the loss of 87 lives and the collapsing or severe damage of around 4500 buildings in Wufeng. Fig. 1 shows the locations of Wufeng, the epicenter, and the Chelungpu fault. According to the free-field strong-motion station (TCU065) at the Wufeng Elementary School in Taiwan, the recorded peak ground accelerations in the EW, NS, and V (vertical) directions were 774, 563, and 257 gals, respectively. Fig. 2 shows the recorded accelerograms. There were no signs of liquefaction observed at this station. The shapes of recorded accelerograms also indicated that the soils underneath the station did not liquefy during the earthquake, i.e., the measured accelerations did not reduce to a small and consistent value after major ground shaking.
The authors conducted reconnaissance immediately following the earthquake. The major disaster occurred in the prosperous area of Wufeng. Buildings, riverbanks, golf courses, rice fields, bridges, and roads suffered severe damage due to soil liquefaction and ground softening. Phenomena such as sand boils, settlement, horizontal translation of a building, and lateral spreading associated with liquefaction and ground softening are observed in Wufeng and well documented.
2. Geological conditions in Wufeng Wufeng Township is located on the alluvial plain next to the central Western foothills of Taiwan. This alluvial plain, consisting of Quaternary deposits, covers most of the geological formations near the surface.
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Fig. 2. Accelerograms at Wufeng Elementary School during 921 Chi-Chi earthquake (TCU 065; duration = 44 s).
The thickness of these Holocene sediments varies from place to place and may extend to 150 m below the surface at this area, estimated from the geological map and explanatory text published by Central Geological Survey of R.O.C. (Ho and Chen, 2000). Rock formations, including Chinshui Shale, Choulan, and Toukoshan Formations, are present in the foothills in the eastern regions of Wufeng. These formations were deposited during the Pliocene and Pleistocene ages. A
geology map of the studied area in Wufeng is shown in Fig. 3 (Land and Mineral Resources Division (LMRD), 1995). The Chelungpu fault is between the foothills and the floodplains. The major surface rupture of the fault passed through Wufeng along the edge of western foothill. The distance from the fault to Chung-Cheng Road, the prosperous area of Wufeng, is less than 300 m (Figs. 1 and 4). The alluvium on the west part of the fault is comprised of silts, sands,
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Fig. 3. Geology map of the studied area in Wufeng (LMRD, 1995).
gravels, and cobbles eroded from the foothills and transported by Kan Creek (Fig. 4).
3. Hydrological conditions in Wufeng 3.1. River locations Da-Lee Creek and its tributary, namely Tsao-Hu Creek, constitute the northern border of soil liquefaction of Wufeng while the Wu Creek is the southern border (Figs. 1 and 4). The major creeks inside Wufeng are Kan Creek and its tributaries, Ko-Niao-Keng and Lai-Yuan Creeks. Ko-Niao-Keng Creek travels from east to west and merges into Kan Creek and its zigzag
path consists of six 90j sharp curves within 600 m of its travel distance (Fig. 4). The major sediments along KoNiao-Keng Creek consist of brown-yellow fine sands. Lai-Yuan Creek meandered within the main territory of Wufeng and its travel path also includes many 90j sharp curves (Fig. 4). The major sediments along LaiYuan Creek consist of brown-gray fine sands. 3.2. Old river channels Areas of old river channels and floodplains could cause potential disaster geologically and hydrologically. Geologically, most soils in these areas are unconsolidated and have low bearing capacity. Hydrologically, drainage in these areas is poor
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Fig. 4. Locations of the fault, old river channels, and observed ground failures in Wufeng during the Chi-Chi earthquake.
because of high ground water table and flat topography. The aerial photos taken during 1973 to 1974 are compared with the recent aerial photos. Several liquefaction-related failures were located in the major ranges of old river channels at Taichung basin as shown in Fig. 4. The old river channels cover very wide areas and coincide roughly with the present river channels. The range of old river
channels in Wufeng located between Tsao-Hu Creek and Ko-Niao-Keng Creek is 50 to 100 m east and 200 to 300 m west of Kan Creek. Most of the sand boils with brown-gray color were found inside this range. Soils at shallow depths classified as ML and CL based on the site investigation conducted in these areas. The old river channels and overflow flooding deposited these fine grain sediments.
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4. Distribution of liquefaction failures Soil liquefaction and settling occurred along approximately 2.5-km stretch in a north – southerly direction and over a width of 1.5 km in the east –west direction in Wufeng (Fig. 4). The range of liquefied areas from east to west starts at Chung-Cheng Road through Lin-Sen Road to 600 m west of Kan Creek. From north to south, the range starts from the Wufeng Junior High School, which is beside Tsao-Hu Creek, and ends at 200 m south of Ko-Niao-Keng Creek. Ground failures associated with liquefaction were mainly located at Kan, Lai-Yuan, Ko-Niao-Keng Creeks, and the intersection of Tsao-Hu and Da-Lee Creeks (Fig. 4).
buildings or ground. Along Chung-Cheng Road in the downtown area of Wufeng many buildings collapsed, settled, or tilted (Figs. 1 and 4). The footings of several connected buildings settled severely and the floor slabs heaved and broke as shown in Fig. 5. The settling and tilting of buildings due to the loss or partial failure of bearing capacity during the earthquake are quite obvious. Song-Gi jewelry store at the First Market of Chung-Cheng Road (BH-5 in Fig. 1) tilted severely and its footing was lifted above the ground as shown in Fig. 6. The structure of the building is still intact. The site is underlain by a silty sand (SM) layer at a depth of 1.5 to 4.0 m, a lean clay with sand (CL) layer at a depth of 4.0 to 4.5 m, and a gravel layer at 4.5 m below the surface (BH5 in Fig. 1). The water table is 1.2 m below the
5. Failures and phenomenon associated with liquefaction The observed ground failures associated with soil liquefaction in Wufeng include sand boils, the losses or partial failures of bearing capacities (subsidence, tilting, or horizontal translation of a building or the ground), lateral spreading, and flow failure. The size of each symbol used in Fig. 4 indicates the extent of each failure. The major phenomenon and associated failures are described as follows. 5.1. Sand boils Signs of sand boils can be seen at many places which include local communities, parks, roads, basement, fences, farms, gardens, orchards, retaining walls, levees etc. (Fig. 4). The height of sand boils was as high as 2 m as witnessed by an owner in his backyard and by the soils splashed on the walls. So the magnitude of horizontal ground acceleration should be very high and the duration of the earthquake should be quite long to induce such high pore water pressure. 5.2. Loss of bearing capacity Soil liquefaction and ground softening are the main causes for the loss or the decrease of bearing capacity during the earthquake which will result in settlement, tilting, and horizontal translation of
Fig. 5. Foundation settled and floor heaved by 1 m at Chung Cheng Road.
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surface. The SPT N-value, fines content, and clay size content for the SM layer are 13%, 45%, and 14%, and for the CL layer are 7%, 90%, and 21%, respectively. Despite the medium low blow count, no signs of sand boils were observed along ChungCheng Road. Another case is a three-story building (BH-3 in Fig. 1) moved 70 cm westward horizontally and the movement was arrested by the adjoining building during the earthquake. 5.3. Lateral spreading Lateral spreadings generally develop on gentle slopes (between 0.3j and 3j) and move toward a free surface, such as river channels or coastal lines. The locations of lateral spreadings in Wufeng were found beside the creeks, especially at the deposition side (point bar) of a meandered creek. Lateral spreading failures were found at five locations along Ko-Niao-Keng Creek (Figs. 7 and 8), three of them at Lai-Yuan Creek, two at Kan creek, and one beside Tsao-Hu Creek (Fig. 4).
6. Index properties of sand boils
Fig. 6. Song-Gi jewelry store tilted severely and its footing was lifted above the ground.
Samples near the vents of sand boils were taken right after the earthquake and their colors and textures were examined visually. Index property tests, including
Fig. 7. Sand boils and lateral spreading of a parking lot next to Ko-Niao-Keng Creek.
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Fig. 8. Tilted buildings and lateral spreading of a rice field next to Ko-Niao-Keng Creek.
grain size analysis, specific gravity, and plasticity, were also conducted. Results of grain size analyses of sand boils at several places are compared with the proposed curves by the Japanese Society of Civil Engineers (JSCE, 1977) as shown in Fig. 9. The grain size
distributions of sand boils in Wufeng are within the suggested range of high possibility of liquefaction. The D50 of these sand boils is in the range of 0.08 to 0.28 mm, while the D10 is in the range of 0.002 to 0.023 mm. These sand boils are classified as nonplastic silty sands
Fig. 9. Grain size distributions of sand boils in Wufeng during Chi-Chi earthquake.
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(SM). Most of the fines contents (grain size smaller than #200) of these sand boils are larger than 20% and some of them are around 40% to 50%. Since there is a chance for the fine-grained portions of the sand boils being washed away, the study on the sand boil samples may underestimate the fines content of the in-place liquefiable soils. Sand boils with three different colors namely brown-gray, brown-yellow, and black were found in Wufeng, while the black one was found at only one place. At some locations, the other two colors of sand boils were seen together. Thus, the sand boils could have come from different soil layers and depth levels; otherwise, it may be due to major geological variation of soils in Wufeng. The color of sand boils and grain
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size distribution were used to identify the possible depth of liquefaction.
7. Subsurface investigation and testing Fourteen different places were drilled in Wufeng after the earthquake. The locations of borings were selected based on the observed liquefaction phenomenon during the Chi-Chi earthquake (Fig. 1). Field investigation methods included standard penetration tests (SPT), cone penetration tests (CPT), and shear wave velocity (Vs) measurements. These exploratory borings were funded by National Center for Research and Earthquake Engineering of Taiwan and performed
Table 1 Summary of SPT data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake SPT boring
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical N-value
Critical (N1)60-value
Fines content (%)
Clay size content (%)
D50 (mm)
CSReqa
BH-1 BH-2 BH-3 BH-4 BH-5 BH-6 BH-7 BH-8 BH-9 BH-10 BH-11 BH-12 BH-13 BH-14 B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 LBH-1 LBH-3 LBH-4
No Nob Yes Noc Marginal Nod Yes No Yes Yes Yes Marginal Marginal Yes Yes Yes Yes Yes Yes Yes Marginal Yes Yes No Yes
2.30 5.10 2.40 1.35 3.05 13.55 3.75 1.35 1.35 5.85 2.85 8.90 2.85 1.35 5.85 4.40 5.85 4.35 2.85 4.35 3.35 11.35 2.85 3.35 3.00
1.38 5.10 1.30 1.30 1.20 3.20 3.20 0.50 1.00 1.40 0.90 4.20 0.90 1.00 2.70 2.30 2.75 2.55 2.70 1.90 3.26 3.20 2.50 2.00 2.00
22 >100 2 4 13 7 12 2 5 11 7 13 3 11 12 7 6 8 4 7 15 11 4 13 6
34.1 – 3.2 4.1 19.3 7.1 14.4 2.3 7.6 14.4 11.1 14.0 4.8 16.7 14.1 8.8 7.2 9.7 5.2 9.2 17.5 12.2 5.6 17.5 8.4
10 – 22 76 45 87 20 65 38 22 45 40 38 21 49 27 27 34 33 40 39 77 47 71 30
4 – 6 26 14 44 4 23 9 4 9 14 11 7 12 5 5 7 9 10 0 0 14 35 15
0.24 – 0.14 0.02 0.09 0.01 0.41 0.06 0.10 0.12 0.08 0.10 0.10 0.21 0.08 0.19 0.20 0.20 0.19 0.13 0.20 0.20 0.18 0.19 0.26
0.56 – 0.57 0.45 0.65 0.58 0.48 0.64 0.51 0.70 0.67 0.57 0.67 0.51 0.61 0.58 0.58 0.56 0.45 0.61 0.45 0.63 0.47 0.56 0.53
Use Mw = 7.6 and amax = 0.67g in the liquefaction resistance calculation. Most of the soils are classified as SM or ML. a CSReq is the equivalent CSR for Mw = 7.5. b The soil is classified as GM. c The soil is classified as CL and its liquid limit and plasticity index are 32 and 9, respectively. d The soil is classified as CL – ML and its liquid limit and plasticity index are 28 and 6, respectively.
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by Moh and Associates (2000). SPT tests generally followed the guidelines recommended by Seed et al. (1985). The SPT N-value was measured at 1.5-m interval. However, the spatial variation of the soil layer in Wufeng was found to vary very much in vertical direction. Thus, the interval of 1 m is highly recommended for the use of SPT in order to obtain more details of soil information since the strata are complicated in Wufeng. Moh and Associates performed energy measurements of SPT in Yuanlin City being one of the extensively liquefied areas during the ChiChi earthquake. The energy ratio (ER) of SPT in the analyses is a function of depth as follows: ERð%Þ ¼ 30
Z þ 50 for Z < 10 m 11
ERð%Þ ¼ 80 for Zz10 m
ð1Þ
Yuanlin was applied for liquefaction analyses of borings in Wufeng. In addition, 11 borings performed by the Panku Engineering, the Sinotech Engineering Consultants (Building and City Development Council, 1995), and this study were also synthesized. A total of 25 borings were collected, with two drilled before the earthquake and 23 drilled after the earthquake. SPT data for the above borings are presented in Table 1 and shown in Figs. 10 and 11. Six seismic cone penetration tests (SCPT) were also conducted after the earthquake. The CPT tests conformed with ASTM D3441-79. Shear wave velocities of soils at 1-m interval were measured while doing cone penetration tests. The results of CPT tests and shear wave velocity are summarized in Tables 2 and 3, respectively.
ð2Þ 7.1. Subsurface soil conditions
where ER is the hammer energy ratio or hammer efficiency at the depth of Z (in meter). The energy ratio in Eqs. (1) and (2) includes both energy and rod length corrections. As the same drilling equipment was used in Wufeng, also the ER measurement obtained in
Subsurface conditions and results of SPT are illustrated by the two cross-sectional soil profiles shown in (Figs. 10 and 11). The soil strat in (Figs. 10 and 11) are all considered as Holocene sediments (Ho and
Fig. 10. Soil profiles and boring information in east – west direction in Wufeng.
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Fig. 11. Soil profiles and boring information in north – south direction in Wufeng.
Chen, 2000). The major geological materials in the liquefied areas namely east and west of Lin-Sen Road differ in consistency. The soils on the eastern side consist mainly of fine sediments of silts and clays, which could be eroded from weathered Chinshui Shale from the eastern side of the foothills. However, soils with interlayers of sand, gravel, cobble, and silt are present on the western side of the road and these sediments might be due to river deposits, which were transported and deposited by Kan Creek.
7.2. Depth of liquefaction The borings drilled after the earthquake at selected locations and their corresponding grain size distributions revealed shallow soil liquefaction. On the western side of Lin-Sen Road, near Kan Creek, mostly liquefaction occurred in a layer of silty sand (SM) of brown-yellow color at a shallow depth of under 5 m. SPT N-values for the soils are very low being around 3 to 5. However, on the eastern side of
Table 2 Summary of CPT data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake and 611 earthquake CPT Liquefaction Critical Water Critical Critical Friction Fines Clay size D50 (mm) amax sounding observed? depth (m) depth (m) qc (bars) qc1N (g) ratio, Rf (%) content (%) content (%) CPT-7 CPT-8 CPT-9 CPT-10 CPT-15 CPT-16 CPT-7 CPT-9 CPT-10 CPT-15 a
Yes No Yes Yes Yes No No No No No
3.75 1.35 3.00 5.85 11.35 3.60 3.75 3.00 5.85 11.35
3.0 0.5 2.5 4.0 2.5 2.0 3.0 2.5 4.0 2.5
50 4.6 10.8 11.5 25.4 19.8 50 10.8 11.5 25.4
CSReq is the equivalent CSR for Mw = 7.5. For 921 Chi-Chi earthquake, Mw = 7.6; amax = 0.67g. c For 611 earthquake, Mw = 6.9; amax = 0.22g. b
59.74 10.92 14.70 13.65 21.05 26.71 59.74 14.70 13.65 21.05
0.03 0.30 0.01 0.15 0.04 0.24 0.03 0.01 0.15 0.04
20 65 47 22 77 – 20 47 22 77
4 23 14 4 0 – 4 14 4 0
0.41 0.06 0.18 0.12 0.20 – 0.41 0.18 0.12 0.20
0.67b 0.67 0.67 0.67 0.67 0.67 0.22c 0.22 0.22 0.22
CSReqa 0.48 0.64 0.44 0.70 0.61 0.57 0.13 0.12 0.19 0.17
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Table 3 Summary of Vs data obtained in Wufeng, Taiwan after 921 Chi-Chi earthquake and 611 earthquake Seismic CPT Liquefaction Critical Water Fines Clay size Median grain Vs (m/s) Vsl (m/s) CSR CSReq amax sounding observed? depth (m) depth (m) content (%) content (%) diameter (mm) (g) CPT-7 CPT-8 CPT-10 CPT-15 CPT-16 CPT-7 CPT-10 CPT-15 a b
Yes No Yes Yes No No No No
3.75 1.35 5.85 11.35 3.60 3.75 5.85 11.35
3.0 0.5 4.0 2.5 2.0 3.0 4.0 2.5
20 65 22 77 – 20 22 77
4 23 4 0 – 4 4 0
0.41 0.06 0.12 0.20 – 0.41 0.12 0.20
167 98 130 164 158 167 130 164
183 151 142 149 184 183 142 149
0.46 0.62 0.67 0.61 0.54 0.15 0.22 0.20
0.47 0.64 0.69 0.63 0.56 0.12 0.18 0.16
0.67a 0.67 0.67 0.67 0.67 0.22b 0.22 0.22
For 921 Chi-Chi earthquake, Mw = 7.6; amax = 0.67g. For 611 earthquake, Mw = 6.9; amax = 0.22g.
Lin-Sen Road, which is the downtown area, liquefaction was deeper up to 10 m. The main soil type is brown-gray sandy silt (ML) and the SPT N-values are around 3 to 15.
8. Structure performance and subsurface conditions The main types of damage for buildings in liquefied areas of Wufeng were generally settling and tilting. The infrastructures of most buildings remained intact or suffered only minor damage. However, in areas without soil liquefaction, such as Wufeng Elementary School (BH-8 in Table 1 and shown in Figs. 1 and 11), the columns and beams of building structures either collapsed or faced severe damage and could not be easily repaired. Some observations can be concluded as follows based on structure performance and subsurface soil conditions in Wufeng. (1) Sand boils were observed with brown-yellow soils classified as silty sands (SM) at shallow depths. Differential settlement and tilting of a building might occur due to soil liquefaction. However, the structure of the building is usually intact unless the tilted angle is too high. (2) Brown-gray soils classified as sandy silts (ML) are located at deeper depths. The liquefaction potential of these soils is also very high, but the sand boils were not commonly seen because of
greater overburden depths and larger fines content. Most of the buildings in this area underwent settlement, but the degree of settlement was normally greater than that of the buildings in areas with sand boils. (3) Most of the buildings experienced structural damage and a little settlement provided most of the soil layers underneath the buildings were clay.
9. Evaluation of liquefaction potential Each in situ test location, i.e., SPT, CPT, or shear wave velocity (Vs) profile, was employed to obtain one critical combination of earthquake-induced cyclic stress ratio (CSReq) and normalized in situ measurement, i.e., (N1)60, qc1N, or Vs1. CSReq is the equivalent CSR for Mw = 7.5 earthquake, i.e., CSReq = CSR/ MSF. The parameter magnitude scaling factor (MSF) is calculated as (Youd et al., 2001).
MSF ¼ 102:24 =Mw2:56
ð3Þ
Each set of critical combinations was interpreted considering together the overall performance of the site, the subsurface soils, and the color and grain size distribution of sand soils. The cyclic stress ratio induced by an earthquake was calculated based on the formulation proposed by Seed and Idriss (1971) and the suggested modifications by Youd et al. (2001). Value of stress reduction coef-
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ficient (rd) for calculating the cyclic stress ratio (CSR) is estimated as (Liao and Whitman, 1986) rd ¼ 1:0 0:00765Z for ZV9:15 m
ð4Þ
rd ¼ 1:174 0:0267Z for 9:15 m < ZV23 m
ð5Þ
9.1. Evaluation of liquefaction based on SPT data The simplified procedure proposed by Seed and Idriss (1971) and Seed et al. (1985) was applied to evaluate the liquefaction resistance (CRR) based on the SPT data. Modifications suggested by Youd et al. (2001) were also taken into consideration. SPT penetration resistance was normalized to an equivalent overburden stress of 100 kPa (1 atm) using the correction factor (CN) proposed by Liao and Whitman (1986). The value of CN was limited to a maximum value of 1.7 as suggested by Youd et al. (2001). The energy ratio (ER) was calculated based on Eqs. (1) and (2). Because the borehole diameter is 86 mm and a standard sampler is used for the SPT, the correction
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factors for the borehole diameter and sampling method are both equal to 1.0. The depth of groundwater table for each boring was based on the field observation and Mw equals 7.6 for all the analyses. The peak horizontal ground acceleration (amax) for liquefaction analyses is 660 gals (0.67g), which is the geometric mean of the two maximum horizontal accelerations recorded in Wufeng. The determination of critical layer for SPT was first estimated by comparing the color and grain characteristics (such as grain-size distribution and fines content) of sand boils with the sample information from the corresponding borehole. Then the simplified procedure suggested by Seed et al. (1985) was used to calculate the liquefaction resistance and factor of safety at sampled depths of the borehole. For soil layers with the similar color texture and grain-size characteristics within a borehole, the depth with lowest factor of safety was chosen as the critical layer. Table 1 summarizes the field observation, the results of the SPT data, fines content, clay size content, and CSReq for selected borings. Fig. 12 illustrated a graph
Fig. 12. Critical combinations of cyclic stress ratio and SPT (N1)60 for soils in Wufeng with liquefaction resistance curves by Seed et al. (1985) and modified by Youd et al. (2001).
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of calculated CSReq and corresponding (N1)60 data obtained from various locations in Wufeng where liquefaction effects were either observed or not following the Chi-Chi earthquake. Four borings, where the buildings suffered settlement with no evidence of sand boils, are marked as marginal in the figure. The number beside each data point in the graph is the fines content of the data. The CRR curves proposed by Seed et al. (1985) and modified by Youd et al. (2001) for granular soils with the fines content of 5% or less, 15%, and 35% are also shown in Fig. 12. Many major aftershocks and earthquakes, several over magnitude 6.5, following the earthquake were encountered. The maximum peak ground acceleration (PGA) recorded at TCU065 after the Chi-Chi earthquake was due to the earthquake (Mw = 6.9) on June 11, 2000 (611 earthquake). The recorded peak ground accelerations in the EW, NS, and V (vertical) directions were 267, 173, and 71 gals, respectively, and thus the peak horizontal ground acceleration (amax) used for liquefaction analyses is 215 gals (0.22g). Since there were no evidences of liquefaction following this earthquake, CSR based on this and corresponding SPT data will be considered as nonliquefaction. The results of which are tabulated in Table 4 and also plotted in Fig. 12.
9.1.1. Effects of fines content and clay size content Most of the data from the Chi-Chi earthquake are plotted within the range of liquefaction zone since CSR was very large based on the recorded amax. However, there are four borings, which did not liquefy during the earthquake fall within the range of liquefaction. The soils at the critical depths of these four borings, BH-4, BH-6, BH-8, and LBH-3, have high percentage of fines content (FC) (percent passing the Number 200 sieve), 76%, 87%, 65 and 71%, and clay size content (CC) (percent finer than 0.002 mm by weight), 26%, 44%, 23% and 35%, respectively (Table 1). The BH-4 and BH-6 are classified as CL and CL – ML, respectively, as they have plastic fines and the liquid limits (plasticity indexes) for them are 32 (9)% and 28 (6)%, respectively. The other two soils, BH-8 and LBH-3, have nonplastic fines and are classified as ML. The data based on the aftershock (611 earthquake) are all considered as nonliquefaction. Only the locations that liquefied during the ChiChi earthquake and did not liquefy during the 611 earthquake are plotted and marked as open square in Fig. 12. However, most of the data still fall within the liquefaction zone. The fines contents for these data are also high as shown in Fig. 12 and in Tables 1 and 4. Based on the results of nonliquefaction cases in Fig.
Table 4 Summary of SPT data obtained in Wufeng, Taiwan after 611 earthquake in 2000 SPT boring
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical N-value
Critical (N1)60-value
Fines content (%)
Clay size content (%)
D50 (mm)
CSReq
BH-3 BH-5 BH-7 BH-9 BH-10 BH-11 BH-12 BH-13 BH-14 B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 LBH-1 LBH-4
No No No No No No No No No No No No No No No No No No No
2.40 3.05 3.75 1.35 5.85 2.85 8.90 2.85 1.35 5.85 4.40 5.85 4.35 2.85 4.35 3.35 11.35 2.85 3.00
1.30 1.20 3.20 1.00 1.40 0.90 4.20 0.90 1.00 2.70 2.30 2.75 2.55 2.70 1.90 3.26 3.20 2.50 2.00
2 13 12 5 11 7 13 3 11 12 7 6 8 4 7 15 11 4 6
3.2 19.3 14.4 7.6 14.4 11.1 14.0 4.8 16.7 14.1 8.8 7.2 9.7 5.2 9.2 17.5 12.2 5.6 8.4
22 45 20 38 22 45 40 38 21 49 27 27 34 33 40 39 77 47 30
6 14 4 9 4 9 14 11 7 12 5 5 7 9 10 0 0 14 15
0.14 0.09 0.41 0.10 0.12 0.08 0.10 0.10 0.21 0.08 0.19 0.20 0.20 0.19 0.13 0.20 0.20 0.18 0.26
0.15 0.17 0.12 0.13 0.18 0.17 0.15 0.17 0.13 0.16 0.15 0.15 0.14 0.12 0.16 0.11 0.16 0.12 0.13
Use Mw = 6.9 and amax = 0.22g in the liquefaction resistance calculation.
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12, the soils with higher fines content tend to have either higher CRR or lower penetration resistance. In the original development, Seed et al. (1985) noted an apparent increase of CRR with increase of fines content. Whether this increase is caused by an increase of liquefaction resistance or a decrease of penetration resistance is not clear yet (Youd et al., 2001). Therefore, the effects of fines content and clay size content are quite obvious which need further study and considered for the estimation of CRR. In order to take the effects of fines content (FC) and clay size content (CC) into consideration, CSR is decreased to some extent based on the percentage of FC and CC instead of increasing CRR. The data collected by Tokimatsu and Yoshimi (1983) and Hwang and Yang (2001) were compared with the data collected in this study as it also included clay
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size content. As CSReq is divided by the sum of FC and CC and plotted versus (N1)60 for data with fines content greater than 35%, it is found that there is a boundary curve which could delineate the boundary between liquefaction and nonliquefaction regions (shown in Fig. 13). This curve is drawn by connecting data points of liquefaction cases located at the lower right boundary in the figure empirically. There are only four nonliquefaction data located inside the liquefaction range and the percentage amount of FC (CC) for these four data are 76 (26)%, 87 (44)%, 65 (23)% and 38 (11)%, respectively, as indicated in Fig. 13. The percentage of clay size content (CC) for these data is high. The first two data, from BH-4 and BH-6, also have plasticity indexes of 9% and 6%, respectively, and there may exist the influence of CC and plasticity index on the liquefaction resistance (Toki-
Fig. 13. (CSR 10/(FC + CC)) and SPT (N1)60 for the liquefiable and nonliquefiable soils with fines content greater than 35%.
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matsu and Yoshimi, 1983). Tokimatsu and Yoshimi (1983) and Seed and Idriss (1981) indicated that none of the soils they studied containing more than 20% clay has suffered serious strength loss due to liquefaction. Based on this, if the first three data having fines content greater than 20% are excluded from Fig. 13, there is only one nonliquefaction data inside the liquefaction range. This point is plotted very close to the boundary curve. Thus, this empirical curve in Fig. 13 could be used to represent the boundary between liquefaction and nonliquefaction regions for soils with fines content greater than 35% but with CC < 20%. The other available liquefaction data, whether it contains clay size content or not, from Boulanger et al. (1997) and Goh (1994) are plotted with the previous data and shown in Fig. 14. For clean sand, only the
CSReq is used for the longitudinal axis since its FC is zero. For nonliquefaction cases only fines content greater than 35% are plotted in Fig. 14. All the liquefaction data are also plotted within the liquefaction range. 9.1.2. Prediction of occurrence/nonoccurrence of liquefaction Methods of Seed et al. (1985) and the limit state function based on Artificial Neural Network (ANN) (Juang et al., 2000c) were chosen to predict the occurrence or nonoccurrence of liquefaction for the case records in this study. This ANN model used 243 field liquefaction performance cases to establish a liquefaction limit state function (Juang et al., 2000c). The liquefaction resistance (CRR) can be determined
Fig. 14. (CSR 10/(FC + CC)) and SPT (N1)60 for the nonliquefiable soils with fines content greater than 35% and for all the liquefiable soils from previous cases.
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using this limit state function. Then the factor of safety (FS) can be calculated as FS = CRR/CSR. Liquefaction is said to occur if FS V 1, and no liquefaction occurs if FS>1. Both methods can predict the occurrence of liquefaction pretty well. The success rate for accurate predictions of the occurrence of liquefaction is 100%. But for 921 Chi-Chi earthquake (Table 1), however, the success rate for predictions of the nonoccurrence of liquefaction by both the methods is only 33% as four miss out of six case records. As for the data from the 611 earthquake, which are all no liquefaction as listed in Table 2, the success rates by Seed’s method and by ANN for predictions of the nonoccurrence of liquefaction are 28% (7 success in 25 cases) and 88% (22 success in 25 cases), respectively. Thus, the approach by ANN is more promising for predicting the nonoccurrence of liquefaction than Seed’s method, if the amax or CSReq is not too high such as the cases from 611 earthquake. For cases with large amax such as 921 Chi-Chi earthquake, however, the success rate for predictions of the nonoccurrence of liquefaction by both the methods is poor as there were only limited field performance cases for high FC and for CSReq>0.3 for both methods. Thus the data obtained from Chi-Chi earthquake will be of high value in implementing the data in these ranges for ANN model and for Seed’s method. 9.2. Evaluation of liquefaction based on CPT data The CPT-based method proposed by Robertson and Wride (1998) was utilized to evaluate the liquefaction resistance (CRR). Modifications suggested by Youd et al. (2001) were also considered. In selecting the critical value of normalized CPT tip resistance ( qc1) in each CPT sounding, the following guidelines suggested by Boulanger et al. (1997) were adopted to reduce potential biases that may be introduced by different individuals. If the critical depth occurred within a thick stratum of homogeneous material, the critical value was taken as the average qc1N over 0.5m interval. Second, if the critical interval was between 0.15 and 0.6 m, excluding zones (0.15 m) influenced by adjacent contacts, then the average value was taken over the full interval thickness. Therefore, the minimum sand layer thickness is about 0.45 m for obtaining a reasonably representative qc (Boulanger et al., 1997).
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There are interlayers of gravel or cobble of 5 to 15 m thick at shallow depths in Wufeng. Therefore, the actual tested depths using CPT were between 5 and 28 m. The results of qc1N, CSReq, and fines content during 921 Chi-Chi earthquake and 611 earthquake are tabulated in Table 2. The fines contents in Table 2 are estimated based on the adjacent borings, as shown in Fig. 1. Fig. 15 shows cyclic stress ratio versus corrected and normalized CPT resistance qc1N of the study at Wufeng. Data obtained from Nantou City (Table 5) are also plotted in Fig. 15 as there were only six CPT soundings performed in Wufeng. Fig. 15 also provides the clean-sand base curve formulated by Robertson and Wride (1998) for direct determination of CRR for clean sands (FC V 5%) from CPT data. The curve proposed by Stark and Olson (1995) for FC = 35% is also shown in Fig. 15 for reference. Fines content for the corresponding CPT sounding is also indicated in the figure. The liquefied data points are all plotted above the clean sand curve, i.e., liquefaction region, and are in good agreement with the curves proposed by Robertson and Wride (1998) and Stark and Olson (1995). The nonliquefied cases are also above the clean sand curve since most fines contents of the soils are higher than 20%. Thus, predictions of nonoccurrence of liquefaction by Robertson and Wride (1998) and the ANN model by Juang et al. (2000a,b) are poor. Both the success rates are only 11% (one success in seven cases). For the ANN model by Juang et al. (2000a), the obtained limit state function is used to calculate the liquefaction resistance (CRR) of a soil. 9.3. Evaluation of liquefaction based on shear wave velocity data Shear wave velocity (Vs) was measured at 1-m interval together with the CPT test using seismic cone at the tip and triggered source at the ground surface. Shear wave velocity measurements at 1.0-m intervals were usually ineffective in characterizing the critical strata at some locations because the measurement interval crossed over two or more thin strata with significantly different characteristics (Boulanger et al., 1997). So some boreholes were conducted to detect and delineate thin liquefiable strata in this study. Thus, the critical layer of Vs is selected based on the information from the nearby borehole with SPT, Vs
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Fig. 15. Critical combinations of cyclic stress ratio and qc1N for soils in Wufeng and Nantou (modified from Robertson and Wride, 1998).
values, and CPT results. Data for those locations where measurements of shear wave velocity coincided reasonably well with the suspected critical strata during 921 Chi-Chi earthquake and 611 earthquake are tabulated in Table 3. The measured shear wave velocities were usually less than 200 m/s in Wufeng. Data obtained from Nantou are also synthesized and listed in Table 6. The shear wave velocity method proposed by Andrus and Stokoe (1997, 2000) was applied to evaluate the liquefaction resistance (CRR) based on
the Vs data. The equation suggested in the paper by Youd et al. (2001) to account for overburden stress for Vs is also adopted. CRR versus Vs1 (overburdenstress corrected shear wave velocity) curves recommended by Andrus and Stokoe (2000) for magnitude 7.5 earthquake and soils with various fines contents, z 35%, 20% and V 5%, are shown in Fig. 16. The data comprising liquefied and nonliquefied cases from Wufeng and Nantou all fall within the liquefiable region as proposed by Andrus and Stokoe (2000). The nonliquefied cases are also above the
Table 5 Summary of CPT data obtained in Nantou, Taiwan after 921 Chi-Chi earthquake CPT sounding
Liquefaction observed?
Critical depth (m)
Water depth (m)
Critical qc (bars)
Critical qc1N
Friction ratio, Rf (%)
Fines content (%)
Clay size content (%)
D50
amax (g)
CSReq
CPT-2 CPT-3 CPT-7 CPT-8 CPT-11 CPT-13 CPT-15
Yes Yes Yes Yes Yes No Yes
5.0 6.7 5.8 5.0 4.9 7.0 5.8
5.0 6.7 5.0 5.0 1.9 7.0 1.5
85.0 26.0 75.0 52.0 15.0 20.0 50.0
85.0 20.6 72.4 46.3 18.6 14.9 62.6
0.15 3.80 0.40 2.30 1.80 3.70 2.00
– – 25 – 24 – 30
– – 7 – 8 – 7
– – 0.10 – 0.11 – 0.30
0.39 0.39 0.39 0.39 0.39 0.39 0.39
0.25 0.25 0.27 0.25 0.35 0.25 0.42
Mw = 7.6; amax = 0.39g.
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Table 6 Summary of Vs data obtained in Nantou, Taiwan after 921 Chi-Chi earthquake Seismic CPT sounding
Liquefaction observed?
Critical depth (m)
Water depth (m)
Fines content (%)
Clay size content (%)
Median grain diameter (mm)
Vs (m/s)
Vsl (m/s)
CSR
CSReq
CPT-2 CPT-3 CPT-7 CPT-8 CPT-11 CPT-13 CPT-15
Yes Yes Yes Yes Yes No Yes
5.0 6.7 5.8 5.0 4.9 7.0 5.8
5.0 6.7 5.0 5.0 1.9 7.0 1.5
– – 25 – 24 – 30
– – 7 – 8 – 7
– – 0.10 – 0.11 – 0.30
200 176 184 147 120 160 165
200 168 181 139 129 151 185
0.24 0.24 0.26 0.24 0.34 0.24 0.41
0.25 0.25 0.27 0.25 0.35 0.25 0.42
Mw = 7.6; amax = 0.39g.
clean sand curve since most fines contents of the soils are higher than 20%. Predictions of nonoccurrence of liquefaction by Andrus and Stokoe (2000) and the ANN model by Juang and Chen (2000) and Juang et al. (2001) (using the proposed limit state function) are not satisfactory. The success rates by Andrus and Stokoe (2000) and by ANN for predictions of the nonoccurrence of liquefaction are only 17% (one success in six cases) and 33% (two success in six cases), respectively.
10. Concluding remarks A case-history study of liquefaction-related failure and consequences in Wufeng, Taiwan during 921 ChiChi earthquake was presented. Subsurface data of SPT, CPT, and Vs from Wufeng and Nantou were selected and compiled in this paper. These data will provide the bases for further development of semi-empirical design procedures and the artificial neural network (ANN) modeling since the recorded peak horizontal ground
Fig. 16. Critical combinations of cyclic stress ratio and Vs1 for soils in Wufeng and Nantou (modified from Andrus and Stokoe, 2000).
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acceleration was high, amax = 0.67g, and fines contents of liquefiable and nonliquefiable soils were also very high, larger than 20%. Some findings and conclusions can be drawn as follows based on this study. 1. Liquefaction in Wufeng during Chi-Chi earthquake appears to have occurred primarily within Quaternary deposits at depths less than 12 m. The major regions of liquefaction are located along the creeks and basically matched with the old river channels. 2. The SPT-based performance function based on the ANN model provides better prediction on the cases of nonliquefaction than other empirical methods do. 3. Impacts of fines content (both nonplastic and plastic) on liquefaction resistance are also found to be profound. For SPT data, a new empirical boundary which separates liquefaction and nonliquefaction cases was obtained for soils with fines content larger than or equal to 35% based on this study. 4. The CRR curves for SPT, CPT, and Vs recommended by 1998 NCEER workshop for evaluating liquefaction resistance are conservative but acceptable in engineering practice for predictions of liquefaction occurrence. Gravel formations are common strata at the Western Foothills of Taiwan. The CPT is found not suitable to use in these formations since the CPT cone is unable to penetrate these formations. The interval of 1 m or less is highly recommended for the use of SPT in order to obtain more details of soil information since the strata are complicated in Wufeng. It is hoped that the data and results presented in this study will contribute to the further development of semi-empirical correlations for predicting liquefaction-related damage during earthquakes.
Appendix A The following symbols are used in this paper: Notation amax peak horizontal acceleration at ground surface CC clay size content (%), i.e., percent finer than 2 Am by weight CN correction factor for overburden pressure applied to SPT CPT cone penetration test CRR cyclic resistance ratio CSR cyclic stress ratio CSReq equivalent CSR for Mw = 7.5 D50 median grain diameter ER hammer energy ratio of standard penetration test (%) FC fines content (%), i.e., percent passing the Number 200 sieve MSF magnitude scaling factor Mw moment magnitude of earthquake N blow count of standard penetration test (N1)60 corrected standard penetration resistance qc field cone penetration resistance measured at the tip qc1N normalized cone penetration resistance rd stress reduction coefficient to account for flexibility in soil profile Rf calculated friction ratio (%) SPT standard penetration test Vs small-strain shear wave velocity Vs1 overburden stress-corrected Vs Z depth below ground surface (m)
References Acknowledgements The authors would like to thank National Center for Research and Earthquake Engineering (NCREE) and the National Science Council (NSC) of the Republic of China for financially supporting this research under Contract No. NSC89-2921-Z319-005-18. The authors also wish to thank the editor, Dr. C. Hsein Juang of Clemson University, and the reviewers for providing many helpful suggestions and comments.
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