NHRP / Hazard themes / Resilient Buildings and Infrastructure / Engineering Highlights 2015-16

The Effect of the Underlying Soil on Earthquake Response

Bridge piers punching through the bridge deck following the 2010 Chile earthquake. Photo: Nawawi Chouw, University of Auckland

In conventional engineering practice, the seismic design of New Zealand’s buildings and bridges rarely considered the influence of the supporting ground. New research by the University of Auckland, incorporating observations of past earthquakes and laboratory experiments, shows that the ground plays a significant role in the intensity of seismic shaking. Civil engineers can now incorporate some of these findings into their design.

While seismic shaking travels up from the depths of the earth, the rock and soil along the way alter the characteristics of the spreading shaking patterns. This influence is particularly strong when the seismic waves approach the surface, with the geology of the site further affecting ground movement, sometimes to a significant extent.

In current design practice the movements predicted at the ground surface at a building or bridge site are usually used as the motion causing possible damage. In reality, the building or bridge will affect the earthquake ground movements through its own weight and movement. In some cases, the magnitude of the building movements will significantly change the shaking pattern initiated by the earthquake alone.

The weight of any construction applies load to the underlying soil and will modify how the soil responds to earthquake movements as the structure compresses the soil, increasing its toughness or capacity to sustain strong earthquake shaking.

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Influence of a structure on earthquake ground shaking. Data: N. Chouw, Univ Auckland.

The graph above shows an example of how a structure modifies ground movement. Without a structure (solid line) the soil movement can fluctuate faster, as illustrated within the red circle. With a structure present (dashed line) the movement is smoother and quite dissimilar in places (e.g., within the red circle). A structure designed for the conditions of the solid line will lead to different outcomes during an earthquake than that of the dashed line.

When buildings and bridges respond to earthquake ground movements, their foundations (where the concrete and steel is connected to the ground) cause the soil to deform and thus create further movement in the supporting soil. These additional movements in turn may affect the surrounding infrastructure and modify the earthquake-generated movements in the structure, as well as the soil in the vicinity. If designers ignore the soil effects described above, then the building or bridge may fall short in terms of structural safety.

The practice of excluding the soil effects is known as a ‘fixed base assumption.’ While using a ‘fixed base assumption’ is simpler, it is not in the best interests of New Zealand engineering practice. Civil engineers now have the ability to incorporate the effects of the supporting soil into their designs through the knowledge gained by research, for example that carried out in the new laboratories of the University of Auckland Centre for Earthquake Engineering Research (UACEER).

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Influence of soil-footing-structure interaction on structural responses. The footing is the structural component that distributes the weight of the structure to the soil. Data: N. Chouw, Univ Auckland.

The graph above shows an example of results from an experiment carried out at UACEER. The figure shows the distance moved by a model building during earthquake shaking where in one case the model is ‘fixed base’ and in the other resting on sand. The significant difference in the distance moved by the building is shown. These results can be reproduced using theoretical calculations, thus advancing the practice of earthquake-resistant design in New Zealand.

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Preparation of a large-scale model of a bridge on sand. Photo: Univ Auckland.

To pursue our mandate of developing safe and economic civil infrastructure, UACEER has been carrying out research with the support of the Natural Hazards Research Platform. The photo above shows preparation for a large-scale investigation of a bridge positioned on sand, using a platform that can shake the bridge in all directions. These studies will contribute to safer future design practices and improvements to existing structures needing rehabilitation, enabling infrastructure equal to the best in the world.

By Nawawi Chouw and Tam Larkin, University of Auckland  

Contact: Nawawi Chouw

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Last updated 22 Sept 2016