Effects of Near-Fault and Far-Fault Ground Motions on Pile Sections of Integral Abutment Bridges Considering Soil Liquefaction

When the ground shakes during an earthquake, the soil beneath our feet can sometimes lose its strength and turn into a thick, fluid-like slurry. This phenomenon, known as liquefaction, is a silent threat to the foundations of bridges. Unlike solid ground that holds a structure firmly in place, liquefied soil offers little resistance, allowing piles—the deep columns that anchor a bridge—to sink, tilt, or snap under the weight above. This is particularly dangerous for a specific type of bridge called an integral abutment bridge. These structures are built as a single, continuous frame without the expansion joints found on older bridges, which allows them to move more naturally with temperature changes. However, this seamless design means the bridge deck is rigidly connected to the piles, transferring every bit of seismic energy directly into the foundation. If the soil liquefies, the bridge loses its grip, and the piles must absorb the full force of the shaking without breaking.

The question facing engineers is not just how to build these bridges, but how to build them to survive different kinds of earthquakes. Earthquakes are not all the same; some occur close to the fault line, sending powerful, sudden pulses of energy that can catch structures off guard, while others happen far away, delivering a longer, more rolling shake. Researchers wanted to know how the choice of pile material and shape affects a bridge's survival when the soil turns to liquid under both types of shaking. To find the answer, a team of engineers from universities in China and Afghanistan created a detailed computer simulation of a single-span bridge. They modeled the bridge sitting on six different types of piles: large and small concrete cylinders, steel pipes, steel beams oriented in different directions, and a hybrid design where concrete is poured inside a steel tube. They then subjected these virtual bridges to ten recorded earthquake motions, half from near-fault events and half from far-fault events, while simulating the soil turning into a fluid.

The results revealed that the best choice for a pile depends heavily on the type of earthquake and the material used. When the ground shakes with the sharp, violent pulses of a near-fault earthquake, the rigid concrete piles, especially the larger ones, tended to bend and crack more than the steel options. The concrete, being stiff, could not absorb the sudden jolt as well, leading to higher stress in the structure. In contrast, when the shaking came from a distant source, the steel piles experienced greater movement and stress than the concrete ones. This suggests that steel's natural flexibility, which usually helps it survive, can sometimes work against it during the slower, rolling waves of a far-fault quake. However, one design stood out as the most resilient across the board: the concrete-filled steel tube. This hybrid pile combined the strength of concrete with the flexibility of steel, allowing it to handle the stress of both earthquake types with remarkable consistency.

The study also looked at how much the bridge itself moved. The simulations showed that bridges supported by the concrete-filled steel tubes moved less than those on pure steel piles, and the movement was nearly the same whether the earthquake was near or far. This stability is crucial because excessive movement can damage the road surface or the connections to the highway. The researchers found that while the steel piles were strong, they were more prone to buckling—a kind of structural collapse where the column bends out of shape—especially when the soil lost its grip. The concrete-filled tubes, by contrast, managed to bend and stretch without failing, effectively dissipating the energy of the earthquake.

For engineers designing bridges in areas prone to liquefaction, these findings offer a clear path forward. The study suggests that using concrete-filled steel tubes is the most reliable option for integral abutment bridges, as they provide a balance of strength and flexibility that protects the structure from both sudden, close-range shocks and distant, rolling tremors. If steel beams must be used instead, the research indicates that the high-stress areas where the pile meets the soil need extra reinforcement to prevent failure. By choosing the right foundation material, engineers can ensure that even when the ground beneath turns to liquid, the bridge above remains standing.