Performance of bridges during the 2010 darfield and 2011 Christchurch earthquakes. Liam Wotherspoon, Aaron Bradshaw, Russell Green, Clinton Wood, ...
Performance of Bridges during the 2010 Darfield and 2011 Christchurch Earthquakes Liam Wotherspoon, Aaron Bradshaw, Russell Green, Clinton Wood, Alessandro Palermo, Misko Cubrinovski, and Brendon Bradley
Liam Wotherspoon,1 Aaron Bradshaw, 2 Russell Green, 3 Clinton Wood,4 Alessandro Palermo,5 Misko Cubrinovski,5 and Brendon Bradley5
INTRODUCTION
LOCAL GEOLOGY
The region in and around Christchurch, encompassing Christchurch city and the Selwyn and Waimakariri districts, contains more than 800 road, rail, and pedestrian bridges. Most of these bridges are reinforced concrete, symmetric, and have small to moderate spans (15–25 m). The 22 February 2011 moment magnitude (Mw) 6.2 Christchurch earthquake induced high levels of localized ground shaking (Bradley and Cubrinovski 2011, page 853 of this issue; Guidotti et al. 2011, page 767 of this issue; Smyrou et al. 2011, page 882 of this issue), with damage to bridges mainly confined to the central and eastern parts of Christchurch. Liquefaction was evident over much of this part of the city, with lateral spreading affecting bridges spanning both the Avon and Heathcote rivers. The majority of bridge damage was a result of liquefaction-induced lateral spreading, with only four bridges suffering significant damage on non-liquefiable sites. Abutments, approaches, and piers suffered varying levels of damage, with very little damage observed in the bridge superstructure. However, bridges suffered only a moderate amount of damage compared to other structural systems. Because some bridges critical to the city infrastructure network sustained substantial damage, extensive traffic disruption occurred immediately following the event. This paper presents a summary of field observations and subsequent analyses on the damage to some of the bridges in the Canterbury region as a result of the Christchurch earthquake. Reference is also made to the performance of bridges following the 4 September 2010 Mw 7.1 Darfield earthquake (Gledhill et al. 2011), and details of damage progression are presented where applicable. The ground motion characteristics for both events and the regional soil conditions are first described. We provide descriptions of the damage at each selected bridge site and compare observations of liquefaction with predicted response using in situ test data.
The city of Christchurch, shown in Figure 1, is located along the central coast of the Canterbury Plains, an approximately 50-km-wide and 160-km-long region created by the overlapping alluvial fans of rivers flowing east from the Southern Alps. Interbedded marine and terrestrial sediments up to 40 m deep overlie 300 to 400 m of late Pleistocene sands and gravels (Brown and Weeber 1992). Much of the city was originally swampland, beach dune sand, estuaries, and lagoons, which were drained as part of the settlement and expansion of the city (Brown et al. 1995). A high water table, one to two meters below the ground surface in the east of the city, gradually increases in depth moving across the city to the west. To the south of the city are the Port Hills, formed from volcanic activity (Brown and Weeber 1992). Two spring-fed meandering rivers, the Avon and the Heathcote, cut through Christchurch (Figure 2). The Avon River passes through the city from west of the Christchurch
1. University of Auckland, New Zealand 2 . University of Rhode Island, Kingston, Rhode Island, U.S.A. 3. Virginia Tech, Blacksburg, Virginia, U.S.A. 4. University of Arkansas, Fayetteville, Arkansas, U.S.A. 5. University of Canterbury, Christchurch, New Zealand
▲▲ Figure 1. Overview of Christchurch city and its surroundings, with the epicenters of the Darfield and Christchurch earthquakes shown by stars. Boundaries of moderate bridge damage during the Darfield earthquake at Lincoln and Kaiapoi are represented by circles. The region of interest for Christchurch bridges presented in Figure 2 is bounded by the dashed white rectangle (Google Inc. 2011).
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doi: 10.1785/gssrl.82.6.950
▲▲ Figure 2. Horizontal peak ground accelerations recorded at strong motion sites in Christchurch during the Darfield and Christchurch earthquakes, and the locations of bridges highlighted in this paper (Google Inc. 2011).
central business district (CBD), through the CBD, and to the eastern edge of the city where it enters the Avon-Heathcote estuary. East of the CBD the Avon River widens as it nears the estuary. The Heathcote is a smaller river and runs from west to east in the southern part of the city before entering the estuary.
GROUND MOTION CHARACTERISTICS On 4 September 2010, the Mw 7.1 Darfield earthquake struck 40 km west of the Christchurch CBD at a focal depth of 11 km (Gledhill et al. 2011). The highest recorded ground motions were near the epicenter, having a maximum horizontal PGA of 0.76 g (geometric mean of the horizontal components, applies to all horizontal PGAs stated herein) and a maximum vertical PGA of 1.26 g. These large vertical accelerations are typical
of the near-source strong motion recordings for this event. A maximum horizontal PGA of 0.25 g and maximum vertical PGA of 0.22 g were recorded in the Christchurch CBD, and the PGA generally decreased with distance downstream along the Avon River. The largest vertical PGA in the central and eastern areas of Christchurch was 0.32 g at Pages Road pumping station. The Mw 6.2 2011 Christchurch earthquake was centered less than 10 km from the Christchurch CBD along the southeastern perimeter of the city in the Port Hills (Figure 1). The close proximity and shallow depth of this event caused higher intensity shaking in Christchurch as compared to the Darfield earthquake. In the city, ground motions were characterized by large vertical accelerations resulting from the close proximity to the fault plane, steeply dipping oblique thrust faulting mecha-
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(B) ▲▲ Figure 3. Response spectra of the geometric mean of the horizontal accelerations at strong motion station recordings in central and eastern Christchurch compared to NZS1170.5 design response spectrum for Christchurch, site subsoil class D for 500-year return period. A) Darfield earthquake, B) Christchurch earthquake. Four-letter symbols represent different strong motion stations, positions of which are indicated in Figure 2.
nism, and deep alluvial deposits (Beavan et al. 2011, page 789 of this issue; Bradley and Cubrinovski 2011, page 853 of this issue). The highest recorded ground motions were near the epicenter at the Heathcote Valley primary school, with the horizontal and vertical PGAs 1.41 g and 2.21 g, respectively. In the CBD, horizontal PGAs of between 0.37 g and 0.52 g and vertical PGAs of 0.35 g to 0.79 g were recorded. Horizontal PGAs ranging from 0.22 g to 0.67 g and vertical PGAs from 0.49 g to 1.88 g were recorded in the vicinity of the Avon River (Bradley and Cubrinovski 2011, page 853 of this issue). The horizontal PGAs for the Darfield and Christchurch earthquakes at the strong motion stations in central and east-
ern Christchurch are summarized in Figure 2. It is clear from the data in Figure 2 that the Christchurch event produced much higher ground motions than the Darfield event in the CBD and along the Avon and Heathcote rivers. While not shown in this figure, the same can be said for the level of vertical accelerations experienced in these areas. The horizontal acceleration response spectra from five of the strong motions stations in Figure 2 for the Darfield and Christchurch events are compared to the NZS1170.5 design response spectrum for a 500-year return period event in Christchurch (hazard factor Z = 0.22) on a site subsoil class D (Standards New Zealand 2004) in Figure 3. Because the bridges in the region are typically short to mid span, the natural period can reasonably be assumed as less than 0.8 seconds. Figure 3A shows that during the Darfield event, the spectral acceleration values in this range were generally less than the values that a bridge would have been designed for using current standards (although most bridges were designed according to older standards with lower design levels). Only the spectral accelerations of the ground motion recorded at Heathcote Valley primary school (HVSC) are above the design code values in this range, likely a result of basin wedge effects given its position at the head of the Heathcote Valley in the Port Hills. In general, the ground motion response spectra from the Christchurch earthquake in Figure 3B were higher than the 500-year-return-period design spectrum over the entire vibration period range. The periods of highest spectral response correspond to the expected natural periods of the bridge structures in the region. Even though bridges likely experienced shaking levels at or above their design levels throughout this region, the majority sustained minimal damage as a result of ground shaking alone. This can be attributed to the sturdy designs typical of bridges constructed in the 1950s and 1960s, which was a period of extensive bridge replacement in Christchurch.
OVERVIEW OF CANTERBURY BRIDGE PERFORMANCE Although liquefaction was widespread in central and eastern Christchurch, only five bridges suffered severe damage and ten developed moderate damage in the 22 February 2011 Christchurch earthquake. Most bridges were reopened within a week of the earthquake, with only one closed for a longer period of time. Because of the location of the earthquake on the southeastern edge of the city, most of the bridge damage was confined to central and eastern regions, where ground shaking was strongest and soil conditions weakest. This paper focuses on the performance of ten of these bridges, the locations of which are indicated in Figure 2. The majority of bridge damage was a result of lateral spreading of river banks, with only four bridges damaged on sites that did not experience liquefaction (locations 1, 8, 9, and 10 in Figure 2). The largest distance from the fault rupture to an affected bridge was 17 km (corresponding to the moderately damaged Chaney’s Overpass). Eleven of the 14 bridges along the Avon River within the CBD suffered
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only minor damage, mostly to their approaches. Outside the CBD, the two remaining bridges along the Avon that did not suffer moderate or severe damage had only minor approach damage. Compared to the Avon River, bridges crossing the Heathcote River sustained much less damage despite being close to the fault rupture, primarily due to the larger seismic resistance of the foundation soils of these bridges. Apart from three cases, all bridges along the Heathcote River were either undamaged or developed only minor approach damage. Eight road bridges suffered moderate damage following the 4 September 2010 Darfield earthquake, with five of these closed for five days or longer. Traffic weight limitations and/or restricted lane access was in place for a more extended period, all but one of which instances was due to approach damage as a result of lateral spreading. The Darfield earthquake had a larger magnitude, and thus resulting ground motions affected a much larger region, with bridge damage occurring from Lincoln, 15 km south of central Christchurch, to Kaiapoi, 16 km north. The most distant bridge damage, at the Williams Street Bridge in Kaiapoi due to lateral spreading, was approximately 30 km from the rupture of the Greendale fault. Within Christchurch city itself, Gayhurst Road Bridge and South Brighton Bridge both experienced moderate damage, principally as a consequence of lateral spreading (Allen et al. 2010; Palermo et al. 2010).
DAMAGE ASSOCIATED WITH LIQUEFACTION
The Christchurch CBD bridges crossing the Avon River generally performed well, with the most common damage being minor lateral spreading, compression or slight slumping of approach material, and minor cracking in abutments. All bridges were single span and were passable to recovery vehicles in the cordon soon after the event. (The cordon is the restricted-access area of the CBD, put in place due to the widespread earthquake damage in the area.) Compared to the Avon River, bridges crossing the Heathcote suffered much less damage. Apart from the Ferrymead Bridge at the mouth of the Heathcote, all bridges were either undamaged or experienced only minor damage. As previously noted, we infer that this is the result of more resistant foundation soils along the Heathcote River relative to the Avon River. Typical damage was minor approach settlement, with little impact on the bridge abutments and superstructure. Detailed descriptions of the bridges shown in Figure 4 with the most severe liquefaction-induced damage and the analyses of in situ test data at these sites follow. The PGAs used in the liquefaction evaluations were estimated using the ground motion prediction equations of Bradley (2010) and the spatial correlation model of Goda and Hong (2008). The estimated PGAs at the bridge sites are the geometric mean of the two horizontal components for site class D (Standards New Zealand 2004) and are summarized in Table 1. Further information on the calculation of these PGA values can be found in Green et al. (2011, page 927 of this issue).
Bridges along both the Avon and Heathcote rivers suffered varying levels of damage from lateral spreading due to the Darfield and Christchurch earthquakes, with ground conditions and distance from the epicenter influencing this response as described previously. Even at a given bridge location the level of damage varied significantly from one end of the bridge to the other, with more damage observed on the inner banks of the local river bends, likely a result of the low-energy depositional environment, as compared to the outer banks. In this section of the paper, we present an overview on the heavily damaged Ferrymead Bridge at the mouth of the Heathcote River and on the most affected bridges along the Avon River from the Christchurch earthquake. The type of bridge damage along the Avon was fairly consistent: settlement and lateral spreading of approaches, backrotation and cracking of the abutments, and some pier damage. In most cases bridge decks restrained movement of the top of the abutment, resulting in their back-rotation. There was little bridge superstructure damage, with only minor crushing and spalling as a result of pounding and relative movement. Unless otherwise noted, simply supported bridges discussed herein did not have any bearings. All the damaged bridges previously mentioned had pile foundations, with lateral spreading forces placing large demands on the abutment piles and likely resulting in plastic hinging below grade. The approach fill of several bridges subsided by up to a meter, resulting in the bridges being closed up to a week. In most cases, settlement and spreading of the approaches impacted bridge serviceability.
Ferrymead Bridge The Ferrymead Bridge (Figure 4A) was constructed in 1967, runs in the east-west direction, and spans the mouth of Heathcote River (Figure 2). The bridge is a three-span reinforced concrete bridge supported by wall abutments with wingwalls and two four-column bents connected to pile caps. The west abutment and bents are supported by floating pile foundations, while the eastern bent is supported by end-bearing pile foundations to bedrock, and the east abutment on shallow foundations on bedrock. Although the Ferrymead Bridge performed well during the 2010 Darfield earthquake, at the time of the Christchurch earthquake it was undergoing a major upgrade to include widening and underpinning of the deck with two reinforced concrete girders supported on two drilled shaft foundations. These upgrades had been planned before the occurrence of the Darfield earthquake. One of the girders at the east abutment had been completed and the girder at the west abutment was partly completed when the Christchurch earthquake struck. Also, to allow access for construction cranes and equipment, two temporary steel bridges were erected on both sides of the bridge and were in place at the time of the Christchurch earthquake. Each abutment consisted of two separate sections, one in front of the other (i.e., one section supporting the superstructure and the other abutment block behind it). Lateral spreading occurred at the east abutment, with the material overlying the bedrock moving both perpendicular and parallel to the bridge
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▲▲ Figure 4. Bridges damaged primarily as a result of liquefaction: A) Ferrymead, B) South Brighton, C) ANZAC Drive, D) Avondale Road, E) Gayhurst Road, F) Fitzgerald Avenue.
TABLE 1 Estimates of peak ground accelerations during Darfield and Christchurch earthquakes in the absence of liquefaction at bridges presented in Figure 2. Darfield Earthquake
Bridge Name Moorhouse Ave Bridge Fitzgerald Ave Bridge Gayhurst Rd Bridge Avondale Rd Bridge ANZAC Dr Bridge South Brighton Bridge Ferrymead Bridge Port Hills Overbridge Horotane Overbridge Railway Bridge 3
Christchurch Earthquake
Conditional Median PGA (g)
Conditional Standard Deviation (ln PGA)
Conditional Median PGA (g)
Conditional Standard Deviation (ln PGA)
0.208 0.214 0.206 0.183 0.180 0.188 0.247 0.284 0.292 0.364
0.259 0.293 0.293 0.360 0.379 0.392 0.371 0.350 0.344 0.266
0.412 0.448 0.495 0.344 0.276 0.618 0.673 0.677 0.682 0.814
0.284 0.323 0.319 0.339 0.168 0.404 0.400 0.379 0.373 0.288
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(C) ▲▲ Figure 5. Ferrymead Bridge field investigation data: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs data, comparing the cyclic resistance ratio CRR 7.5 for the site to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic stress ratios. C) Damage to western abutment and temporary stabilization works.
axis. The lateral spreading caused permanent rotation and cracking of the abutments and a number of the piers. Extensive flexural cracking was evident at the base of the piers at their connection to the pile cap. The rear section of the east abutment back-rotated 2.5° and the section supporting the bridge deck back-rotated 5°. Additionally, surveys showed the east abutment moved vertically upward 10 cm, but there was negligible movement of the eastern pier. Approximately 8-cm-wide lateral cracks were observed in the vicinity of the drilled shaft supporting the new girder, with the cracks running in both the longitudinal and transverse directions. This caused the top of the new concrete bridge girder to rotate about 2° toward the river and caused approximately 30 cm of ground settlement, measured relative to the bottom surface of the new girder, which was originally cast ongrade. Severe liquefaction, as evidenced by significant volumes of ejecta, and lateral spreading occurred in the area leading up to
the west abutment. Surveys showed that the west abutment and pier had settled 20 cm and shifted horizontally 20 cm toward the river. The soil in front of the abutment settled approximately 80 cm, but no appreciable rotation of the abutment was observed. The foundations supporting the west bridge pier in Figure 5C had shifted to the east, causing the support columns to be out of plumb. Remedial efforts have been completed to tie back the foundations supporting the western pier that experienced significant tilting to the west abutment using highstrength steel rods. Following the Christchurch event, Spectral Analysis of Surface Waves (SASW) was performed at a location 60 m to the west of the west abutment. The shear wave velocity (Vs) profile for the west end of the bridge is shown in Figure 5A. The Vs profile shows a soft soil layer between 1.5 and 4 m depth, overlying a much stiffer layer, and the water table at 1.75 m depth. Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the methodology outlined in Youd et al. (2001). The magnitude scaling factors (MSF) recommended by Andrus and Stokoe (2000) were used to scale the CSRs to an Mw 7.5 event (i.e., CSR7.5). Using the shear wave velocity data shown in Figure 5A, the cyclic resistance ratio (CRR7.5) for the profile was calculated following the Andrus and Stokoe (2000) procedure, also outlined in Youd et al. (2001). The overburden correction factor, Kσ, was further used to modify the CRR7.5 values (Hynes and Olsen 1999). This method allows for the direct comparison of the CSR7.5 induced by the two earthquakes with the CRR7.5 for the profile, as shown in Figure 5B. As may be observed from this figure, liquefaction is predicted to have occurred from ~1.5 to 4 m during both the Darfield and Christchurch earthquakes (i.e., CSR7.5 > CRR7.5), with the factor of safety against liquefaction being significantly lower during the Christchurch event. While evidence of severe liquefaction was observed following the Christchurch earthquake, no liquefaction was evident following the Darfield earthquake. South Brighton Bridge The South Brighton Bridge (Figure 4B) was constructed in 1980, runs in the east-west direction, and spans the Avon River just north of where the river empties into the Avon-Heathcote estuary (Figure 2). The bridge is a three-span skewed reinforced concrete structure with seat-type abutments on rubber bearings and single piers, all of which are supported by raked octagonal precast, prestressed concrete piles. The abutment rubber bearings were removed due to the permanent movements that developed during the Darfield earthquake (Palermo et al. 2010) and were replaced with temporary hardwood packers. The bridge site was a wide wetland prior to the bridge construction. To construct the bridge, two approach embankments approximately 4 m in height were extended out into the wetlands, with the bridge structure spanning the river channel. These embankments were constructed of uncontrolled fill material. Significant cracking of the approach embankments on both sides of the bridge occurred during the Darfield earth-
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spalling of the bottom flange of the deck. These displacements represent the cumulative effect of both seismic events. Minor flexural cracking at the base of the central pier from transverse movement due to ground shaking was evident following the Darfield event, with minimal additional damage following the Christchurch event.
▲▲ Figure 6. Settlement of approach material, exposure of raked piles, and cracking of the western abutment of South Brighton Bridge. Note the rotation of the abutment in relation to the girder and cracking at rear of abutment seat.
quake. Slumping of the material adjacent to the abutments developed as a result of movement toward the river, while the approaches developed lateral spreading perpendicular to the river (parallel to the sides of the approach embankment). Liquefaction ejecta was evident in the area surrounding the approaches, with lateral spreading parallel to the river extending to the north and south of the approaches on both sides. Similar damage occurred as a result of the Christchurch earthquake, with further severe lateral spreading. Lateral spreading due to both the Darfield and Christchurch earthquakes caused the east abutment to backrotate by approximately 7°, with spreading of the underlying soils exposing the abutment piles. The piles rotated along with the abutment structure, with evidence of plastic hinge development in both the front and rear rows of piles. The gabion mat used for erosion protection in front of the abutments had moved away from the abutment as the underlying soil spread. These soil movements were larger than those observed in the Darfield event. The west abutment back-rotated by approximately 5° and light cracking was observed on the tension face of the abutment piles after that event. The damage to the piles supporting the west abutment caused by the Darfield earthquake was exacerbated during the February earthquake where the abutment had back-rotated by an additional 3° for a total rotation of approximately 8° (Figure 6) and plastic hinging was clearly visible on the abutment piles. Soil beneath the abutment had settled significantly, exposing 80 cm of the supporting piles. Compared to the post-Darfield conditions, there had also been a significant increase in settlement and spreading at this abutment. Differential movement of the abutments relative to the bridge deck was evident, with the east abutment moving about 22 cm along the line of skew to the north and settled about 3 to 4.5 cm. The west abutment moved 20 cm along the line of skew to the south and settled 8.5 to 9.5 cm, with minor crushing and
ANZAC Drive Bridge ANZAC Drive Bridge was constructed in 2000, runs in the north-south direction on State Highway 74 and spans the Avon River (Figure 2). Shown in Figure 4C, the bridge is a triple-span precast concrete girder structure (hollow core deck) that is supported by two four-column bents and concrete abutment walls with wingwalls. The south approach and abutment were constructed on an embankment fill, while the north end of the bridge was constructed at surrounding grade. The bridge site experienced marginal liquefaction and minor lateral spreading during the Darfield earthquake, but the bridge and its functionality were not affected by this event. However, the bridge was damaged by the Christchurch earthquake, yet remained functional after regrading the approaches. Severe liquefaction, as evidenced by the large volumes of ejecta, and significant lateral spreading occurred in the areas surrounding the north and south abutments during the latter earthquake, with evidence of liquefaction being more pronounced on the south end of the bridge. There were a significant number of sand boils and ejecta observed in the low-lying areas adjacent to the embankment on the south end. Additionally, lateral spreading was observed on both the sides of the embankment with the cracks running parallel to the roadway and having widths of about 8 to 18 cm. A short section of the south approach roadway was repaved and showed an abrupt elevation change due to ground settlement in the vicinity of the bridge abutment. Liquefaction and lateral spreading were less evident on the north end of the bridge. However, a roundabout directly north of the approach possibly obscured some of the evidence. Cracking parallel to the river developed across the roadway leading up to the north abutment and extended to both sides of the bridge. The higher elevation of the area around the north approach likely resulted in smaller volumes of liquefaction ejecta as compared to the south approach area. The south abutment back-rotated 6°, as shown in Figure 7, and lateral spreading at the base of the abutment resulted in a 30 to 40 cm gap between the concrete abutment and backfill. Also, a large horizontal gap formed between the abutment and the edge of a walkway running along the riverbank, with the bridge superstructure restraining the horizontal abutment movement. The rotation of the south abutment exposed a row of steel H-piles supporting the abutment, which also appeared to have rotated along with the abutment. Numerous rubber tires were also exposed that had been placed between the abutment and a walkway running along the riverbank. These tires were designed to act as a lateral spreading buffer for the walkway. The north abutment showed similar rotational movements but had less rotation, of 3.5–4°. The lateral spreading along the
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▲▲ Figure 7. Damage to southern abutment of the ANZAC Drive Bridge, with back-rotation of approximately 6° and spreading between abutment and adjacent walkway.
▲▲ Figure 8. ANZAC Drive Bridge pier damage, with cracking and spalling of cover concrete.
base of the abutment was also less, resulting in an 18 to 24 cm gap between the abutment and the backfill. Additionally, the horizontal gap between the abutment and a walkway running along the riverbank was much less relative to the south end. Both of the bridge piers suffered extensive but superficial cracking to the concrete columns and bent as well as the beamcolumn joint region, with up to 2° of rotation (Figure 8). While the damage first appeared extensive, with apparent shear cracking, further inspection showed that in reality these cracks were limited to the concrete cover. Spalling of the cover concrete appeared to be primarily the result of rotation of the piles, causing stresses to be concentrated at the edges of the members. These rotations can be attributed to horizontal movement of the pile foundations toward the center of the river due to lateral spreading. Following the Christchurch event, SASW tests and Dynamic Cone Penetration tests (DCPTs) were carried out
▲▲ Figure 9. ANZAC Drive Bridge field investigation: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs , comparing the cyclic resistance ratio CRR 7.5 for the site to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratios; C) dynamic cone penetration test (DCPT) profile (i.e., NDCPT and equivalent N1,60cs); D) liquefaction assessment using equivalent N1,60cs, comparing the CRR 7.5 for the site to the CSR 7.5 DAR and CSR 7.5 CHC.
50 m southwest of the south abutment. The DCPT N-values (NDCPT) were converted to equivalent standard penetration test (SPT) N-values using a modified relationship to that proposed by Sowers and Hedges (1966). Then, the N-values were further corrected for rod length, hammer energy, effective confining stress, and fines content following the procedures outlined in Youd et al. (2001). The resulting profiles from the SASW tests and DCPTs are shown in Figures 9A and 9C. The Vs data from the SASW test indicates a soft soil layer between depths of 1 and 6 m, and the water table at a depth of 1.5 m. The CRR7.5 profiles for the site were determined using both the SASW and DCPT data, per Youd et al. (2001) and as outlined previously for the SASW. Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the
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methodology in Youd et al. (2001) and as outlined previously. Comparisons of the CRR7.5 and CSR7.5 for the Darfield and Christchurch earthquakes are presented in Figures 9B and 9D for the SASW and DCP tests, respectively. As may be observed from Figure 9B (Vs data), liquefaction is predicted to have occurred from ~1.75 to ~6 m during both the Darfield and Christchurch earthquakes, with the factor of safety against liquefaction being slightly lower during the Christchurch event. Similar trends are predicted in Figure 9D (DCPT data), but liquefaction is predicted to have occurred during both earthquakes in a slightly thinner layer, ~2.5 to ~3.25 m. These predictions are consistent with field observations (i.e., liquefaction occurred at the site during both earthquakes, with the liquefaction being more severe during the Christchurch earthquake). Avondale Road Bridge Avondale Road Bridge (Figure 4D) was constructed in 1962, runs approximately in the north-south direction, and spans the Avon River (Figure 2). The bridge consists of three spans of precast reinforced concrete girders that are supported on two three-column bents and seat-type abutment walls with wingwalls. Since its construction, the bridge has been seismically retrofitted using steel brackets, which are bolted to tie the elements of the bridge together. The bridge was not damaged during the Darfield earthquake, with the region north of the bridge showing no signs of liquefaction damage. However, just south of the bridge, along the inner bank of the river, there were minor to moderate levels of liquefaction ejecta, with the volume increasing toward the southwest. Liquefaction and lateral spreading were more severe during the Christchurch earthquake, with larger volumes of ejecta and significant lateral spreading adjacent to both sides of the south abutment. To the north, there was also increased volume of ejecta, and moderate spreading 30 m to the west. There was minimal roadway damage adjacent to the north abutment; however the north abutment back-rotated approximately 3°. At the south, the abutment has back-rotated 7°, with moderate settlement of the approach and damage to roadway and services (Figure 10C). Large lateral spreading cracks extended out from both sides of the abutment, transitioning from perpendicular to the riverbanks to parallel over a distance of approximately 15 m. The superstructure and piers showed no signs of damage after either earthquake. A cone penetration test (CPT) was performed after the Darfield earthquake, approximately 15 m to the west of the south abutment, with the results shown in Figure 10A (Tonkin and Taylor 2011a). The CRR7.5 profile for the site was determined using the CPT data, per Youd et al. (2001). Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the methodology outlined in Youd et al. (2001). Comparisons of the CRR7.5 and CSR7.5 for the Darfield and Christchurch earthquakes are presented in Figure 10B. As may be observed from this figure, the site is predicted to marginally liquefy during the Darfield earthquake, with the severity
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(C) ▲▲ Figure 10. Avondale Road Bridge field investigation: A) CPT profile; B) liquefaction assessment using CPT data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) damage to southern abutment and approach.
of the liquefaction increased for the Christchurch earthquake. These predictions are consistent with field observations (i.e., liquefaction occurred at the site during both earthquakes, with the liquefaction being more severe during the Christchurch earthquake). Gayhurst Road Bridge Gayhurst Road Bridge was constructed in 1954, runs in approximately the north-south direction, and spans the Avon River (Figure 2). This integral bridge, shown in Figure 4E, consists of three-spans of precast reinforced concrete girders supported by wall piers that were cast in place within the deck and seat-type concrete abutments with wingwalls. Both the piers and the abutments are founded on reinforced concrete piles. Prior to the earthquakes, both approaches were approximately level with the bridge deck as part of the natural level of the river banks. Severe liquefaction occurred during the Darfield earthquake, indicated by the significant volume of ejecta to the north
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▲▲ Figure 11. Settlement of the region surrounding the northern abutment of the Gayhurst Road Bridge.
of the bridge on the inner bank of the river, with lateral spreading and large settlements developing throughout the entire area The effects were more severe following the Christchurch earthquake, with an increased volume of ejecta and further lateral spreading and settlement (Figure 11). To the south on the outer bend of the river there was minimal spreading on either side of the bridge and only moderate ejecta volumes after the Christchurch earthquake. Both earthquakes caused significant damage to the north approach and abutment of this bridge, with their combined effects resulting in approximately one meter of settlement of the approach adjacent to the abutment. The wingwalls on both sides of the north abutment displaced toward the river by about 90 cm at their top and moved laterally between 10 and 15 cm away from the abutment perpendicular to the bridge axis (Figure 12C). Extensive cracking was evident, with total exposure of the reinforcement connecting the wingwalls to the abutment. The north abutment developed 5° of back-rotation, with a fraction of this being initiated during the Darfield earthquake, as were the wingwall movements. The base of the northern pier rotated toward the center of the river, with one face of the pier cracking horizontally along its length, approximately one meter from the deck soffit. This was initiated in the Darfield event, with crack widening and further rotation during the Christchurch earthquake. Lateral spreading was the cause of this damage, with the lateral force on the pier base developing a large moment at the stiff pier-deck interface and cracking the pier. At the south abutment there was little indication of settlement of the approach. The wingwalls did not show any appreciable displacement, nor did the abutment show any measureable rotation. The southern pier also did not show any obvious signs of distress. A CPT was performed after the Darfield earthquake, approximately 5 m east of the north abutment, with the results shown in Figure 12A (Tonkin and Taylor 2011b). Following the procedure outlined in the previous section, CRR7.5 and
(C) ▲▲ Figure 12. Gayhurst Road Bridge field investigation: A) CPT profile; B) liquefaction assessment using CPT data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) damage to northern approach, with approximately 1 m of slumping of the approach, with damage and movement of wingwalls.
CSR7.5 were developed for the Darfield and Christchurch earthquakes and are compared in Figure 12B. As may be observed from this figure, the site is predicted to marginally liquefy during the Darfield earthquake, with the severity of liquefaction increased for the Christchurch earthquake. While the prediction for the Christchurch earthquake is consistent with field observations (i.e., severe liquefaction), the prediction underestimated the severity of the liquefaction observed during the Darfield earthquake. Fitzgerald Avenue Bridges Fitzgerald Avenue Bridge, constructed in 1964, runs in the north-south direction and spans the Avon River (Figure 2). The bridge, shown in Figure 4F, consists of two structures supporting southbound traffic and northbound traffic, respectively. Each bridge consists of double-span precast concrete girders with a single wall pier and pile-supported concrete wall abutments. Retrofit had recently been carried out, involving steel
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brackets linking the piers and abutments to the deck. At the location of the bridge, the river undergoes a significant change of direction with the north abutment on the inner bank and the south abutment on the outer bank. This bridge was on the edge of the central city cordon setup after the Christchurch earthquake, and consequently, was inaccessible to the general public and was used only by vehicles with cordon access. As discussed in Bradley et al. (2010), the soil profile underlying the north end of the bridge can be approximated as four distinct layers: (1) sand, ~4.5 m thick, N1 = 10, Vs = 130 m/s; (2) sand with fines, ~6.5 m thick, N1=15, Vs = 160 m/s; (3) sand, ~6.5 m thick, N1 = 10, Vs = 130 m/s; and (4) sand, N1 = 30, Vs = 220 m/s. The soil profile underlying the south end of the bridge is similar to the north end, minus layer (3). The bridge was undamaged by the Darfield earthquake, with no evidence of liquefaction on either side of the bridge. However, during the Christchurch earthquake, significant lateral spreading developed on the east side of the north abutment, with cracks running parallel to the riverbank and material moving south toward the river. The north abutment of the western bridge was very near the bend in the river, with a free face both perpendicular and parallel to the bridge. Lateral spreading was noted with movement occurring both to the south and west. Settlements of approximately 0.5 m were observed on the north approach as well. Both north abutments showed back-rotation, which— combined with settlement of the river banks at the base of the abutments—exposed the abutment piles. The abutment rotation caused the easternmost pile on the north abutment (Figure 13A) to fail in tension, with the tension face opening up and crack widths measured up to 10 mm. Spalling of the cover concrete on the bottom flange of the deck girder (Figure 13B) developed as a result of relative movement of the superstructure and abutment. Minimal settlement of the approach was observed at the south abutments. Large cracks were noted, however, in the abutment and wingwalls. This bridge had been previously identified as critical to the bridge network, with an extensive field testing program performed in the late 1990s. The program included multiple CPTs and standard penetration tests (SPTs) performed at the abutments of both the twin bridges. The subsequent analyses showed that the north abutment of the eastern bridge was most vulnerable to liquefaction and structural damage (Bowen and Cubrinovski 2008a, 2008b; Bradley et al. 2010), with liquefaction predicted in the relatively loose sandy soil between 2.5 m and 17.5 m. These predictions are very consistent with the observed response on the bridge during the Christchurch earthquake.
DAMAGE NOT ASSOCIATED WITH LIQUEFACTION Moving away from the Avon and Heathcote rivers, where liquefaction-induced lateral spreading was the main cause of damage, four bridges suffered damage not related to the effects of liquefaction. One bridge, Railway Bridge 3, was damaged due to the seismically induced lateral earth pressures acting on
(A)
(B) ▲▲ Figure 13. Fitzgerald Avenue Bridge damage: A) tension failure of abutment pile and exposure of reinforcement, B) spalling of bottom flange of deck girder.
the abutments. Two bridges, Moorhouse Overbridge and Port Hills Overbridge, were damaged due to shaking effects that activated the transverse response of the structure. The final bridge, Horotane Overbridge, sustained damage as a result of shaking and slope stability issues. The final three bridges did not develop any significant superstructure damage in any of the earthquakes. Railway Bridge 3 The Railway Bridge 3 was constructed in 1950 and consists of a timber deck with brick masonry wingwall abutments, spanning a roadway between built-up railway embankments approximately 3 m in height (Figure 2). The bridge was not damaged by the Darfield earthquake, but extreme shaking during the Christchurch earthquake resulted in severe cracking and movement of the abutments. This caused deformation in the track ballast and tracks, result-
960 Seismological Research Letters Volume 82, Number 6 November/December 2011
(A)
(B)
(C) ▲▲ Figure 14. Railway Bridge 3 field investigation: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) remediation of bridge using steel frame structure to prop abutments and tracks.
ing in a train derailment soon after the event. The bridge was temporarily remediated in the days following the event using a steel frame between the abutments and stabilizing walls in front of the wingwalls as shown in Figure 14C. An SASW test was performed 20 m to the west of the bridge following the Christchurch event; the shear wave velocity profile at this site is shown in Figure 14A. As indicated by this Vs data, the profile consists of ~4 m of a medium dense layer overlying a denser stratum. Using this shear wave velocity data and the PGAs listed in Table 1, the CRR7.5 for the site and the CSR7.5 induced during the Darfield and Christchurch earthquakes were calculated as outlined above for the other bridges. The results are plotted in Figure 14B. As may be observed from this figure, liquefaction is not predicted to occur during either the Darfield and Christchurch earthquakes (i.e., CSR7.5
