By: Jerome O’Connor
Photos and reports from Sichuan, China, inform us of the destruction of the May 12 7.9 earthquake and serve as yet another reminder of the risk societies face from natural hazards. The people of Peru are still rebuilding from a similar event that struck off the coast of South America in August 2007.
Peru has a long history of earthquakes, but on this date the ground motion was unusual. The ground shook for almost three minutes, registering a moment magnitude of 8.0. Peak ground accelerations (PGAs) approached 0.5g (i.e., half of the acceleration due to gravity) and displayed two distinct phases of motion during the same event.
Soon after the earthquake, the author, representing MCEER, University at Buffalo, led an investigation team comprising ASCE investigators Lucero Mesa, P.E., a seismic bridge design specialist with the South Carolina DOT, and Monique Nykamp, P.E., a geotechnical engineer with Shannon & Wilson, Seattle. The purpose of the reconnaissance mission was to document the performance of the Pan-American Highway and other components of the national road network. This is beneficial to U.S. engineers because Peru uses the same AASHTO bridge design specifications and obtains a wealth of valuable performance data with each earthquake.
Roads of ruin
The Pan-American Highway was temporarily severed, primarily due to geotechnical issues. The geotechnical impacts of the earthquake occurred because of liquefaction, which takes place during ground shaking in loose, saturated, sandy soil when the water pressure in the pore spaces increases to a level that is sufficient to separate the soil grains from each other. This phenomenon results in a reduction of the shear strength of the soil, creating a quicksand-like condition. Liquefaction can result in ground settlement, lateral spreading, landsliding, localized ground disruptions from sand boils (ejection of sand and water at the ground surface) and reduced vertical and lateral capacity for structure foundations. Buildings, bridges and other structures founded on or in the liquefied soils may settle, tilt, move laterally or collapse.
Liquefaction occurred along the coast, where the soil is sandy and the groundwater level was high. This resulted in embankment failures, lateral spreading, shifting of the pavement and failure of buried culverts. Fifteen significant rockfalls blocked roads, and the team observed at least two instances of failed retaining walls. At least two large bridges were hit by falling boulders, causing serious damage. Of the approximately 24 sites inspected, 62% had evidence of damage.
Saving a lifeline
The Huamani Bridge is a significant link that was rendered out of service by the earthquake. It is located on the Pan-American Highway, 224 km south of the capital, Lima, a short distance east of the epicenter. This concrete structure was built in 1950. It is 446 ft long and two lanes wide. The bridge was designed to an H-15 AASHTO loading and had been posted with a 36-ton weight limit. Local authorities devised a ford to carry traffic while the bridge was out.
Although all five spans of the bridge remained standing, there was evidence of liquefaction in the sandy soils and evidence of strong shaking. The bridge is not supported on piles, but the 25-ft-deep concrete foundation successfully prevented the piers from tipping or settling. Lateral loading on the superstructure caused cracking of the pier caps, concrete diaphragms and shear blocks that were designed to restrict lateral movement of the superstructure. The fact that the superstructure was only moderately displaced is evidence of the successful use of shear blocks.
The south abutment tipped toward the stream as a result of liquefaction of the sandy soil at that end of the bridge. During the inspection, the superstructure was temporarily supported and the south abutment was in the process of being replaced. The approach embankment and pavement also needed repair.
Bridge damage can be summarized as:
- Severe cracking of one pier stem;
- Severe cracking of transverse concrete diaphragms;
- Minor to severe cracking and spalling of horizontal shear blocks and pier caps;
- Tipping of the south abutment toward the stream bed (approximately 4 in. at the top);
- Lateral displacement of the superstructure (approximately 4 in.); and
- Severe cracking of approach pavement.
Since the bridge did not collapse, it was successful in preventing loss of life. The damage it suffered, though severe, is repairable. Had the bridge not been part of a lifeline transportation route, this performance would be considered acceptable.
Not taken lightly
Just as in the U.S., structures on lightly traveled roads are often older and receive less attention because resources are directed to more important bridges. One example is the Los Molinos Bridge. This is a five-span, one-lane bridge that was built in 1932. The superstructure is structurally nonredundant, consisting of two cast-in-place concrete girders.
The bridge was more vulnerable to earthquake damage because of unrepaired damage stemming from extreme flooding that accompanied El Niño in 1998. There was over 6 ft of scour at one pier and another pier was tipped. There is a noticeable kink in the alignment of the bridge, partly because of the tipped pier but also because of a lateral shift from the recent earthquake. According to a local resident, “It was like that before but worse now.”
At three different locations, there are shear cracks that appear to be new in the girders. Although there was not excessive displacement, this type of damage is consistent with earthquake-induced lateral loading.
Several streets and bridges were inspected within the city of Ica. Although the team saw numerous adobe buildings that had failed, the bridges did not show any evidence of damage. The bridges inspected were cast-in-place concrete, built within the past 20 years, so the type of construction and newer design criteria probably played an important part in their good performance.
A stable education
A few lessons that we can learn from this earthquake are:
- Recognize the criticality of understanding subsurface conditions. Not only can soft layers of soil amplify the effect that an earthquake has on underlying rock, there also can be localized failures resulting from liquefaction, as vulnerable soils are frequently found at bridge sites near coastal areas;
- There are some relatively inexpensive design features that can make bridges more resistant to the potential horizontal loads and uplift forces. For instance, if a multiple-span bridge is built with structural continuity, there is innate protection against spans dropping in the event that pier support is lost. This is true in the event of earthquakes but also is applicable to vehicular or vessel collision, hydraulic scour and inundation from hurricane storm surge. There should be multiple load paths to provide structural redundancy to avoid collapse in the event of failure of a member. In Peru, fixity at abutments restrained bridge superstructures, as did the deliberate addition of cast-in-place concrete shear blocks and diaphragms. These features absorb energy as well as restrict lateral movement to acceptable levels;
- Older bridges are at much greater risk. In order for transportation agencies to be prepared, they need to understand that all bridges are not created equal. Their population of existing bridges are more vulnerable to natural hazards because:
- Many existing bridges were built before there was any appreciable understanding of seismic loadings;
- Bridges that are approaching or are beyond their intended service life may have accumulated deterioration such as section loss due to rust. Accumulated deterioration will have a detrimental effect on performance during an extreme event; and
- Over the life of a bridge, there may have been an exposure to other hazards, resulting in damage such as scour, or flaws from impacts, that has gone unrepaired;
- The possibility of multiple hazards needs to be considered;
- Some administrative lessons are just as important as the technical ones. For instance, a catastrophic event such as an earthquake illustrates the need to have a strong asset management system. In order to mobilize and respond quickly, it is necessary to have clear lines of authority and a response plan in place. Maps and up-to-date data should be readily available to the incident command center. Peru’s Ministry of Transportation (MTC) prudently utilized technology by posting daily damage reports online. Although most of the affected area was without Internet access, MTC’s daily postings made timely and accurate data available to the domestic and international media;
- Having maintenance agreements in place prior to the emergency saved Peruvian authorities vital response time. Instead of reacting to circumstances, they already had a mechanism in place so repair and reconstruction could commence immediately; and
- To encourage further dialogue with Spanish-speaking nations, an English-Spanish glossary of technical terms would be useful to transportation engineers. The investigators have compiled one and it is included in the MCEER report.
About The Author: O’Connor is senior program officer, Transportation Research at MCEER, University at Buffalo.
