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IMPACT OF SOIL-STRUCTURE INTERACTION ON SEISMIC BEHAVIOUR OF INTEGRAL ABUTMENT BRIDGES https://doi.org/10.17863/CAM.121173 Asia, Yazan Madabhushi, Gopal In recent decades, world populations have faced various climate emergencies, including prolonged droughts, heavy rains, and catastrophic flooding. In addition, strong earthquakes, such as the 2023 Turkey-Syria earthquake and the 2024 Noto earthquake in Japan, have shed light on the consequences of these natural events for both livelihoods and infrastructure. Although the occurrence time of these earthquakes and events cannot be predicted, understanding the behaviour of infrastructure, such as integral bridges, during seismic loading will allow bridge and geotechnical engineers to design these bridges to operate safely during and after earthquake loading and, therefore, reduce the risk of loss of lives and infrastructure damage. Renovating, retrofitting, and reducing structural damage will make our environment safe and also reduce its carbon impact by removing the necessity of reconstruction. Following the enormous and costly efforts required for the continuous maintenance of bridge bearings and the expansion/contraction joints utilised in traditional bridges, a recent structural scheme has been increasingly used in the transport networks, where the bridge deck and abutments are rigidly connected (i.e., monolithic connection). The rigid connection between the main structural elements of the integral abutment bridges (IAB) (i.e., the deckabutments- footings) has increased the interaction of the bridge with the backfill and foundation soils. Therefore, the bridge deforms more of the backfill and foundation soil. This mechanism results in increased stresses induced on the bridge due to the deformation of the backfill soil and, therefore, can compromise its structural integrity. In addition, more relative settlements between the bridge deck and the backfill soil cause discomfort when driving. However, research on the backfill-IAB interaction under earthquake loading is scarce. Very limited numerical studies are available, and only one experimental study on the bridge-soil interaction. Therefore, an experimental campaign consisting of nine centrifuge experiments has been conducted as part of this research to understand the seismic interaction between the backfill and the IAB and to lay the basis for studying the interaction between the soil and the bridge abutments during earthquakes. Three configurations of bridges were considered: a semi-integral bridge abutment, a single-span fully integral abutment bridge supported by spread footings, and an integral abutment bridge model supported by a single row of piles under each abutment. The general objectives of all these models considered are to understand the effects of the water table level, earthquake-induced liquefaction, and the use of geofoam material behind bridge abutments in the overall dynamic performance of the soil-bridge system. In addition, a numerical analysis using a 2D finite element program, Swandyne, is used to understand the effect of backfill-abutment relative stiffness on soil and structural response. The results of different instrumentation techniques are collated and synchronised while describing the observed soil deformation and bridge behaviour. This has allowed the proposal of simple soil-bridge deformation mechanisms for some centrifuge experiments. The key findings of this research are: • Integral abutment bridge behaviour is strongly affected by foundation soil stiffness, especially when groundwater levels rise and earthquakes occur. Earthquake-induced liquefaction in the foundation soil causes backfill deformation and flow toward abutments, increasing bending stresses and settlement in the bridge. • Climate change, along with the associated increase in flood frequency and groundwater levels, has a detrimental effect on the seismic response of integral abutment bridges. In particular, earthquake-induced liquefaction of the backfill soil significantly reduces the kinematic interaction between the soil and the abutment. As a result, the backfill loses its ability to dissipate the vibrations of the bridge, leading to a substantial increase in the bending stresses within the bridge structure. • Incorporating geofoam behind abutments negatively impacts seismic performance, as its flexibility and compressibility fail to absorb inertial energy, transferring additional forces to the bridge frame. • The use of geofoam as a backfill material increases horizontal bridge displacements while reducing vertical settlements. Due to its flexibility and compressibility, geofoam allows the bridge to experience greater lateral movement compared to conventional soil-abutment interaction systems. However, the kinematic interaction between the backfill and the abutment is diminished when geofoam is used, leading to reduced backfill deformation. Consequently, this results in lower lateral earth pressures on the abutments and their foundations, thereby decreasing overall bridge settlements. • Using a single row of piles to support integral bridges provides better serviceability than spread footings. However, this configuration also increases the overall stiffness of the bridge system, which can lead to higher seismic demands and greater internal stresses during earthquakes. University of Cambridge 2025-03-19 Thesis Doctoral Doctor of Philosophy (PhD) eng Jameel Education Foundation Cambridge Scholarship https://www.repository.cam.ac.uk/handle/1810/389137 embargo 2026-09-09 https://www.repository.cam.ac.uk/bitstreams/ae24ddd4-1751-438c-a7c3-e7ae3b87aab2/download 30b330124f955995c5e152e61932fcf5 https://www.repository.cam.ac.uk/bitstreams/b3f04e90-3b65-429f-a733-06eddd510ed7/download 87eda9de84448d1f82354d60eee3eb5f http://purl.org/NET/rdflicense/allrightsreserved CENTRIFUGE MODELLING EARTHQUAKES INTEGRAL ABUTMENT BRIDGES NUMERICAL MODELLING SOIL LIQUEFACTION SOIL-STRUCTURE INTERACTION