Zulkefli, Muhammad Umar Bin; (2021) Displacement Compensation Units for Integral Abutment Bridges. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Integral abutment bridges (IAB) are increasingly popular in the past few decades due to their design simplicity. The IAB design is currently limited to 60m in the UK and 85m in Malaysia. Past research shows that the IAB’s performance is affected by the cyclic thermal actions that expand and contract the bridge deck, causing an escalation of stress behind the bridge abutment and irreversible settlements near the abutment surface due to the densification of the granular fill. This research attempts to mitigate the above issues by covering two (2) main parts. On one part, this research investigates the effect of the rubber-based displacement compensation unit (DCU) in the two main performance aspects of the IAB – the stress of the backfill (lateral earth pressure) and the settlement near the wall. The designed rubber DCU mechanism is in its ability to accommodate the change in the bridge’s deck displacement without excessive change of preload. This was achieved by designing the DCU in a hollow rubber cylinder (HRC), which exhibits nonlinear behaviour when axial compressive strains are applied. Laboratory tests were conducted by deploying the HRC and another type of DCU in the form of conical disc spring (CDS) in the scaled IAB wall with scenarios simulating two magnitudes of thermal displacements; the expected normal displacement for a 60m-long IAB and the displacement for a 120m-long bridge. The results showed that by using both types of DCUs in the wall model with two levels of displacements, the backfill stresses were shown to be approximately unchanged from their initial state, with almost no soil settlement. Whereby the IAB model with the normal thermal displacement showed a rapid backfill stress escalation at the first 50 cycles, with progressive settlements near the surface. The IAB wall with larger displacement showed similar stress escalation and settlement as the normal IAB, except at almost twice in magnitudes. The effect of the seasons in the IAB wall's initial directional movement was also studied to see how much it would impact the IAB’s performance, with and without the DCU. The results showed that, whether the initial wall position is at ‘winter’ or ‘summer’, there was no apparent difference in peak stress, with and without the DCUs. For the normal IAB walls (without DCUs), regardless of initial wall positions, the backfill stress and settlement values are similar after 20 thermal cycles. In another part, this research looked to understand the effect of strain and stress magnitudes on creep and stress relaxation of rubber, focusing on the simple shear and buckling deformations. An attempt to evaluate rubber's long-term properties was conducted, covering two main parts: 1) experimental and 2) numerical methods. The experimental covers the rubber's stress-strain and the long-term properties, while the numerical method covers the rubber's constitutive modelling by using the existing viscoelastic model applicable to rubber. Comparisons showed that there is fair agreement in the results between the two methods. Lastly, two accelerated methods – the Time-temperature superposition principle (TTSP) and the Stepped isothermal method (SIM) were used to predict the long-term stress relaxation of unfilled NR, both methods were used with time and temperature dependence and the results compared. The TTSP method was closer in predicting the stress relaxation of unfilled rubber, and the SIM method showed a low degree of confidence in doing so, partly due to the chemical relaxation at high temperature. The works conducted in both parts, the rubber engineering and the geotechnical engineering, complement each other, as it is important to understand how a rubber-based device behaves under permanent strain when deployed in the geotechnical problem, in this case, the IAB. The amalgamation of the research outcomes will help engineers and rubber technologists to anticipate the issues that present from having such rubber components in the IAB system and also will help to expand the scopes of the research related to these fields.

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