BRIDGE-ABUTMENT-BACKFILL INTERACTION: BENEFICIAL OR

DETRIMENTAL FOR INTEGRAL ABUTMENT BRIDGES?

Hassan Ibrahim1, Arjun Baladas2 and Stergios A. Mitoulis3

1 BSc, MSc, MIStructE, CEng, PE, P.Eng

Principal Engineer, Parsons, Canada

hassan.yassin@parsons.com

2 GM.ICE, GHM.IStructE

Undergraduate Student, Department of Civil and Environmental

Engineering, FEPS, University of Surrey, Guildford, UK

ab00870@surrey.ac.uk

3 Dipl Eng, MSc, PhD, MASCE, EAEE

Associate Professor, Department of Civil and Environmental

Engineering, FEPS, University of Surrey, Guildford, UK

s.mitoulis@surrey.ac.uk, www.infrastructuResilience.com

Abstract

Integral Abutment Bridges (IAB) are becoming the structural system of choice for many bridge

jurisdictions in North America and Europe, because of their superior durability over conventional

bridges. IABs design out the movement joints and bearings, eliminating the need for costly maintenance

throughout the life of the bridge and improving the durability of the bridge structure. IAB bridges may

appear as simple structural systems, but the interaction of the many structural and non-structural

elements comprising the bridge structure during its lifetime make this type of bridges unique in their

structural analysis and behaviour prediction under the different loads imposed on the bridge during its

service lifetime. Thermal effects and vehicular load effect are well-studied areas in bridge engineering,

and their effects on integral abutment bridges have been the subject of many published literatures, but

little research is available for the effect of earthquake loading on the behaviour of the IAB bridge

systems and its interaction with the backfill soil. This paper aims at answering a critical question in IAB

seismic design, i.e. whether the consideration of the backfill soil and the abutment is beneficial or

detrimental in earthquake-resistant integral abutment bridges.

Keywords: integral abutment bridge, seismic response, design, interaction, SSI, beneficial, detrimental

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COMPDYN 2019 7th ECCOMAS Thematic Conference on

Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, M. Fragiadakis (eds.)

Crete, Greece, 24–26 June 2019

Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2019) 3673-3686

ISSN:2623-3347 © 2019 The Authors. Published by Eccomas Proceedia.Peer-review under responsibility of the organizing committee of COMPDYN 2019. doi: 10.7712/120119.7178.19334

1 INTRODUCTION

Conventional bridge systems have experienced maintenance challenges related to bridge deck joints

and bearings. Joints and bearings are expensive to buy, install, maintain, repair and replace and

maintenance activities can cause significant traffic disruptions. These challenges drove the engineering

community into finding innovative solutions to come up with jointless bridge systems. Integral

Abutment Bridges (IAB) have proven themselves as viable solution to these issues, and it has been used

since in Canada, the US and Europe to develop a “get-in, get-out, stay-out” sustainable, low maintenance

bridges. [1]

IABs are unique in terms of their structural analysis and behaviour under diverse actions due to the

interaction between the structural and non-structural, e.g. backfill, elements comprising the bridge

structure through the life of the bridge. Thermal movements of the bridge will result in movements in

the backfill soil, which in-turn affects the soil pressure acting on the abutment walls. The interaction

between the backfill soil and the abutment affects the design of the abutment foundation. The

configuration of the abutment foundation, in turn, affects the stiffness of the whole system, and that

influences the forces attracted to the abutments and piers, which requires many iterations from the

designer to reach an optimal design.

This research paper aimed at answering the question: is the backfill soil of Integral Abutment Bridges

(IABs) a factor that can reduce the seismic response of the bridge or is it a factor that can increase the

seismic response of bridges? For example, if the backfill is a source of damping and acts as an additional

external support to the bridge, then it is safer to consider this as a second line of defence and ignore it

when analysing the bridge. Thus, we can use the backfill soil as a potential retrofitting measure for

existing bridges and enhance its earthquake resistance. On the other hand, if the backfill is mainly a

source of inertia mass, then the effect of this additional mass should be considered in the analysis and

design of the bridge, as this might lead to larger displacements and bending moment in the structural

components of the bridge. This question can be broken down to smaller questions, such as, what is

the damping ratio of IABs [2, 3]; what is the stiffness of the backfill soil; what are the soil properties of

the IAB; what is the geometry and the typology of the bridge and the type of the abutment and how the

above factors may affect the response of the bridge. There are also other factors that may seem irrelevant,

but can still influence IAB responses drastically, for example, the thermal expansion and contraction of

the deck and the consequent movement of the abutment during the bridge service or prior to the seismic

action, which imposes backfill deterioration, i.e. soil flow, compaction and ratcheting. Other factors

might be the deterioration and/or fatigue of the abutment and the foundation of the abutment, for

example, when abutments are supported on piles. There are also other factors that may seem irrelevant

but can still influence IAB responses drastically, for example, the thermal expansion and contraction of

the deck and the consequent movement of the abutment during the bridge service, i.e. before the seismic

action, which imposes backfill deterioration, soil flow, compaction and ratcheting [4]. Other factors

might be the deterioration/fatigue of the abutment or the foundation of the abutment (e.g. the piles if

any). Currently, appears to be no agreement in the engineering community on the above question, and

the following are few variables that can influence the bridge behaviour.

With regard to the length of IABs, they are usually short bridges, and that is what the codes prescribe,

e.g. there are length limitations in each State in the USA. However, practical designs resulted in much

longer bridges. The research by Zhang and Makris [2] suggests that the seismic movement of the bridge

increases by a factor of two as the backfill almost drives the motion of the bridge, which refers to the

case where the bridge is of relatively small length and moves together with the backfill soil. Potentially

this is the source of the q=1 in Eurocode 8-2 [5,6]. However, for a longer bridge length, it is expected

that a different bridge behaviours will be obtained. The key here is to identify the effective length of the

backfill soil as this defines the amount of dissipation that occurs due to the plastification of the backfill

soil, which is beneficial for seismic design, but causes settlements and the need for backfill replacement.

On the other hand, the backfill is not only a source of dissipation and stiffness, but also a source of inertia

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mass, which potentially may increase the seismic response of the bridge, yet may as well increase the

period of the bridge.

Figure 1 Schematic diagrams of the ratcheting flow (Mitoulis, 2014)

In respect of the type and height of the abutment, the taller the abutment, the more significant the

interaction with the backfill soil, because a substantial part of the backfill soil is mobilised. However,

this might mean considerable energy dissipation and larger backfill inertia mass. On the other hand, a

short abutment has less interaction with the backfill, yet the abutment is seating on the backfill soil, as

opposed to retaining it, in which case the backfill is the foundation soil for the abutment, i.e. the backfill

soil may induce larger movements to the bridge as it is softer than original foundation soil, thus is

expected to magnify the movements. In the latter case, the abutment movement is strongly dependent

on the stem movement of the backfill soil.

The foundation type can as well affect the behaviour of the bridge, whether the bridge is supported

on shallow foundation or piled foundation. For IAB supported on piles, the flexibility of the piles will

play a role in determining the displacement capacity of the bridge during an earthquake. The interaction

between the soil and the piles is a factor in the overall behaviour of the bridge. This foundation type was

not considered in this research but is suggested for future work on the subject.

With regard to the condition of IAB components (structural and non-structural), concerns the

properties of soil, abutment and bridge components and how they might have been deteriorated during

the bridge service due to several factors, such as thermal movements, environmental conditions,

corrosion, and fatigue. This has not been examined in this research but is suggested for future studies of

the problem [4].

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2 FE ANALYSIS MODEL

For the purpose of this study, the complex soil-structure interaction for Integral Abutment Bridges

(IAB) under seismic load was analysed using the plain strain finite element code PLAXIS 2D (2017).

Multiple models were developed to consider the effects of backfill on the dynamic response in different

geometric scenarios such as length of bridge (spans) and height of abutments.

Dynamic absorbent boundaries were used to stimulate the far field behaviour of the medium, while

at the base the boundary conditions were fully restrained in translation and rotation [7]. The width

adopted for the model is sufficient to minimise the boundary effects, without significant increase in

computational cost verified by preliminary sensitivity analysis.

Grade C30/37 concrete is used for the abutment, piers and foundation. Typically for this grade of

concrete, the Young’s modulus can be taken as E=33GPa (Eurocode 2) but to account for creep,

shrinkage and cracking in the concrete cross section it has been reduced it to 10.5GPa.

Plate elements were used to model the concrete box deck to which axial and bending stiffness are

directly applied. Rotational restraints, to account for high flexural stiffness of the deck, were applied at

the top of pier and abutment cluster elements.

The IAB backfill soil was modelled with 14 layers behind the abutments at 500mm each. To better

model the interaction between the abutment and backfill material, interface elements have been input.

The bearing soil/ foundation consists of undrained very stiff clay which has a depth of 24m. This has

been modelled as 10 layers to have a realistic idealisation of how the soil would be present in reality.

However, for the multilayer model the layers of backfill have been specified differently which can be

seen further on. The interaction between different structural elements and the soil can be detailed and

captured by using the “interface elements” PLAXIS feature. This allows soil structure interaction

idealisation internally within the software for the chosen elements. Interface elements are defined by a

material type as well a virtual thickness. Earthquake induced ground motion in the analysis of earthquake

effects is one of the many possible sources of uncertainty and previous research has shown this to have

the highest effect on variability in the observed structural response [8]. Five dynamic motions were

identified for the study. They were chosen to reach the most unfavourable dynamic response of the

bridge system based on the expected natural period of the structure.

Figure 2 - Analysis Model

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3 PARAMETRIC STUDY

Six different variations of integral abutment bridge (IAB) geometries were chosen to represent the

variation of the bridge length, the abutment height and the type of fill retained by the bridge abutment.

The analysis models represent IABs of a single-span, two-span and three-span bridges, representing

bridges of 34m, 68m and 102m lengths respectively. The three-span structure considered three variations

for the abutment height of 3.5m and 5.5m and 8.0m. The sixth model represent a three-span bridge with

the full abutment height of 8.0m, but with varying shear wave velocity and other soil properties of the

backfill layers (Table 2). Also, models without fill were analysed.

Model Designation Model Description

REF/WF base reference model, 3-span bridge, 8m high abutment

REF/MLF 3-span bridge, 8m high abutment, backfill consists of multiple layers

REF/NF 3-span bridge, 8m high abutment, without backfill soil

REF/3.5m-WF 3-span bridge, 3.5m abutment

REF/5.5m-WF 3-span bridge, 5.5m abutment

1S/WF single-span bridge, 8m high abutment

2S/WF two-span bridge, 8m high abutment

Table 1 - Analysis Models Description

Figure 3 - General Arrangement of Base Model (REF/WF)

Figure 4 – Abutment of REF/3.5m-WF model Figure 5 – Abutment of REF/5.5m-WF

This enabled the study of the soil mass effect on the bridge response to determine whether it

influences the overall bridge system behaviour, or the integral bridge structure is driving the behaviour

of the system. Research showed that one of the many possible sources of uncertainty in the analysis for

earthquake effects is the assumed earthquake-induced ground motions used in the analysis model, and

research has shown this to have the highest effect on the variability observed in the structural response

[8].

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The models are subjected to five earthquake excitation records, which were carefully selected based

on real earthquakes to obtain the bridges responses to these applied motion records, see Figure 6 below.

These records were chosen based on the expected natural period of the structure under investigation, to

reach the most unfavourable dynamic response of the bridge system.

The signals obtained from seismic motion databases available are all recorded at 0.15g, therefore, to

investigate the structural system response in the range of moderate to high strains, these signals were

scaled to 0.3g and 0.6g, with maintaining their frequency content. The selection of 0.3g represents the

upper limit for Performance Zone 2 (AASHTO 2010), and it appears to be a typical range of PGA in

European Earthquake prone areas [7].

Figure 6. The response spectra of the five seismic motions and the elastic RS of Eurocode 8-1 soil A, PGA of 0.3g

Through time history analysis, the outputs from these models were used to determine the effect of

earthquake loading on the bending moment and shear forces of the bridge deck as well as the variations

in Pier drifts due to the earthquake loading on the structure. These models were analysed under

earthquake motions scaled at 0.3g and 0.6g to compare the earthquake effects. The above models were

re-analysed precluding the backfill soil (NF models) to determine the effect of the soil mass, stiffness

and damping on the bridge system behaviour. The results of the models that include the fill and the

models that do not include fill were compared to determine the contribution of the backfill soil on the

overall behaviour of the bridge system. The study looked at bending moments and shear forces at deck-

to-abutment and pier connections, bending moments at the deck mid-span, and compared relative

displacements, i.e. drift values, at the abutments and piers.

3.1 Backfill Soil Properties

The soil properties for the backfill were chosen for a soil above the phreatic line, hence the drained

soil assumption, the properties were assumed for sandy soil. The properties of the backfill soil are

assumed to be constant along the depth of the abutment. This model will be considered as the benchmark

for subsequent analyses.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

Sa (

T) g

T (s)

Elastic Response Spectra for Input Motions (scaled at 0.3g)EC-8, A(0.3g)

Kypseli 0.3g

Gebze 0.3g

Duzce 0.3g

Hectormine 0.3g

Umbria 0.3g

Mean Spectrum (0.3g)

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Figure 7 - Backfill Soil Layers Definition (Typical Backfill and Multiple-layer backfill)

The following properties are assumed for the backfill soil used for the different models:

Layer

Designation

Thickness

(m)

Undraine

d Shear

Strength

(kPa)

Shear

Wave

Velocity

(m/s)

Effective

Shear

Modulus

(kPa)

Poisson’

s Ratio

Young’s

Modulus

(kPa)

MC 7.0 0.01 271 138900 0.40 833,600

BF1 1.0 0.01 174 40892 0.40 245,300

BF2 1.0 0.01 206 57034 0.40 342,200

BF3 1.0 0.01 227 69288 0.40 415,700

BF4 1.0 0.01 243 79548 0.40 477,300

BF5 1.0 0.01 256 88541 0.40 531,200

BF6 1.0 0.01 268 96639 0.40 579,800

BF7 1.0 0.01 278 104061 0.40 624,400

Table 2 Backfill Soil Layer Properties (Applicable for REF/MLF)

The base soil is assumed to have average damping of 6%. In PLAXIS 2D this is calculated by the

software by defining Rayleigh Damping Coefficients. To reach the target damping of 6%, the

frequencies used are 1Hz and 3Hz.

4 RESULTS

The time history analysis results of the different motion records were compared to the values from

running the static analysis model, representing the bridge self-weight. The self-weight bending moments

and shear forces at the critical locations shown in Figure 8 were considered as the reference datum for

the comparisons to follow. The ratio of bending moment and shear force resulting from each motion

input scaled to the self-weight effects represent the change in the deck forces due to earthquake effect.

This ratio indicates the amplification or reduction in the bending moment/shear forces due to the seismic

loading above the bending moment or shear forces resulting from the dead loads of the structure. This

is important from the design engineer’s perspective, since the bending moment and shear forces

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ultimately affect the selection of the different structural element sizes, which subsequently affects the

mass and stiffness of the bridge structure.

4.1 Bending Moments and Shear Forces

Results extracted from REF/WF and REF/NF analysis models showed that the backfill soil had a

surcharge effect on the bridge behaviour. An average increase in shear forces at abutment face of 8.1%

in the case of REF/WF compared to 5.6% increase in the shear forces for the case of REF/NF was

noticed. The bending moment at the abutment face increased by an average of 9.2% for REF/WF

compared to 6.9% in REF/NF case. This suggests that the backfill soil increased the earthquake actions

on the structure and worsened the demand on the structural elements. Table 5 below shows the ratio of

outputs from REF/WF to outputs from REF/NF.

Figure 8 - Critical Sections Locations

Msupport-

1

Qsupport-

1

Mmid-

span

Msupport-

2/Left Qsupport-2/Left

Msupport-

2/Right Qsupport-2/Right

Kypseli/VL 7.40% 5.30% 10.00% 7.20% 5.60% 1.40% 2.00%

Gebze/VL 8.30% 7.10% 10.00% 8.00% 7.30% 1.40% 2.00%

Duzce/VL 9.70% 8.70% 12.10% 9.60% 8.30% 4.10% 3.00%

Hector/VL 8.80% 7.40% 11.60% 8.60% 8.10% 2.20% 2.00%

Umbria/VL 11.90% 11.90% 14.40% 11.90% 9.30% 2.80% 3.00%

Average 9.22% 8.08% 11.62% 9.06% 7.72% 2.38% 2.40%

Table 3 - Ratio of Bending and Shear from Dynamic analysis to the Dead load – Model REF/WF (Base Model REF/WF

Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge).

Msupport-

1

Qsupport-

1

Mmid-

span Msupport-2/Left Qsupport-2/Left Msupport-2/Right Qsupport-2/Right

Kypseli/VL 5.83% 4.63% 7.11% 5.64% 4.46% 0.72% 0.93%

Gebze/VL 5.36% 4.36% 6.35% 5.15% 4.16% 0.50% 0.99%

Duzce/VL 8.84% 7.59% 10.68% 8.36% 6.55% 2.73% 2.75%

Hector/VL 6.50% 5.13% 8.52% 6.76% 5.51% 0.46% 0.86%

Umbria/VL 7.83% 6.13% 9.73% 8.23% 6.44% 0.80% 1.31%

Average 6.87% 5.57% 8.48% 6.83% 5.42% 1.04% 1.37%

Table 4 - Ratio of Bending and Shear from Dynamic analysis to the Dead load – Model REF/NF (No Fill Model REF/NF

Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge).

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Support 1 Mid-Span1 Support 2-L Support 2-R

MwFill/MnoFill QwFill/QnoFill MwFill/MnoFill MwFill/MnoFill QwFill/QnoFill MwFill/MnoFill QwFill/QnoFill

Kypseli -0.08% -0.03% 0.00% 0.08% 0.03% 0.00% 0.00%

Gebze 1.44% 0.62% 2.66% 1.58% 1.17% 0.65% 1.31%

Duzce 2.69% 2.55% 3.46% 2.79% 3.02% 0.85% 1.48%

Hector 0.70% 1.04% 1.26% 1.24% 1.65% 1.38% 0.57%

Umbria 2.06% 2.12% 2.84% 1.81% 2.45% 1.74% 1.50%

Average 1.36% 1.26% 2.05% 1.50% 1.66% 0.92% 0.97%

Table 5 – Ratio of M and Q from No Fill Model (three span) to the Reference Model (REF/NF vs. REF/WF) (Base Model

REF/WF Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge)

Figure 9 - Variation of Bending Moment (mid-span) and Shear due to variation of the fill type

The effect of the backfill soil considering the varying properties of soil layers, i.e. the multi-layer fill

(MLF) showed that the backfill properties had a burdensome effect on the bridge response as it increased

the demand on the structural elements of the bridge. The reference model (REF/WF) utilised a constant

set of soil properties for the backfill mass, and the shear velocity was fixed at 271 m/seconds for this

model, as compared to the multi-layer model (REF/MLF) which utilised varying soil properties for each

1m thickness of the backfill soil; the lowest shear velocity was taken at 174 m/s for the top layer and

increases to 278m/s for the lowermost layer, demonstrating that the softer the backfill soil the worse for

the seismic behaviour of the IAB. Table 6 below shows the bending moment and shear from REF/MLF

compared to the self-weight effects. An average increase in shear at abutment of 16%, and a 21%

increase in bending moment at face of abutment was recorded, the mid-span moment showed and

average increase of 28% to the datum self-weight effects.

Msupport-

1 Qsupport-1

Mmid-

span

Msupport-

2/Left

Qsupport-

2/Left

Msupport-

2/Right

Qsupport-

2/Right

Kypseli/VL 15.37% 12.02% 18.35% 14.38% 11.21% 1.92% 3.06%

Gebze/VL 16.03% 13.46% 17.66% 13.55% 11.23% 1.87% 2.51%

Duzce/VL 31.82% 22.40% 42.40% 35.69% 27.26% 6.83% 7.33%

Hector/VL 17.30% 14.34% 21.02% 16.79% 12.09% 3.39% 3.49%

Umbria/VL 22.10% 20.31% 40.87% 22.07% 18.69% 4.65% 5.65%

Average 20.53% 16.51% 28.06% 20.50% 16.10% 3.74% 4.41% Table 6 - Ratio of Bending and Shear from Dynamic analysis to the Self-weight effect - Model REF/MLF (Multi-Layer

Model REF/MLF Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge)

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Figure 10 Variation of support moment (left) and mid-span moment (right) due to variation of abutment height.

The results showed that the abutment height had a beneficial effect on the bending moments and

shears on the bridge system, i.e. the higher the abutment the better for the earthquake resistance of the

bridge. This is clear form Figure 10 above, which showed a reduction in bending moment as the

abutment height increases. This may be attributed to the backfill acting as an energy dissipation medium.

Figure 11 - The bending moments at abutment and at mid-span for variable bridge lengths (spans varying from 1 to 3

spans) - BM correspond to a strip of 1m width of the 13.5 m wide deck)

Single Span Bridge (1S/WF) Single Span Bridge (1S/NF)

Flexure (kN.m) and Shear (kN) Flexure (kN.m) and Shear (kN)

Msupport-1 Mmid-span Qsupport Msupport-1 Mmid-span Qsupport

Kypseli/VL 4.09% 3.12% 4.68% Kypseli/VL -0.02% 2.41% 4.20%

Gebze/VL 2.38% 2.37% 5.67% Gebze/VL -0.16% 5.50% 9.37%

Duzce/VL 5.70% 5.44% 5.84% Duzce/VL -0.31% 6.51% 9.46%

Hector/VL 1.64% 1.45% 5.15% Hector/VL -0.21% 3.94% 7.01%

Umbria/VL 5.08% 4.93% 6.07% Umbria/VL -0.17% 5.79% 14.47%

Average 3.78% 3.46% 5.48% Average -0.17% 4.83% 8.90%

Table 7 - Single Span Bridge (With Fill vs. No Fill)

Analysis showed that increasing the length of the bridge had worsened the behaviour, since the

bending moments and shear forces increased with increasing the number of spans of the bridge. Figure

11 above shows that increasing the bridge length, i.e. the number of spans, resulted in an increase in the

bending moments and shear force values at the abutment face. FEM results showed that for the single

span bridge, the existence of backfill had a beneficial effect on the bending moments and shears at the

abutment face and the mid-span. On average, there was 9% increase in shear at abutment for the case of

no fill (1S/NF) versus a 5.5% increase in shear when the fill existed. This, however, was not the same

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for bending moment at the abutment face, and the moment at the abutment showed an average increase

of 3.78% for case with fill (WF) versus a negligible reduction in moment from the case of no fill (NF).

These results show that the existence of backfill tends to increase the bending moments and shears

on longer IABs, which should be accounted for in the design of the superstructure elements. On the

contrary, structures that are relatively short showed that the backfill is beneficial to its behaviour under

seismic loading.

4.2 Pier and abutment drifts

The following results represent the recorded displacements at the top of the abutment or pier and the

recorded displacements at the abutment or pier bottom. These results represent the maximum

displacements due to the earthquake loading. The difference between the top displacement and the

bottom displacement, occurring simultaneously, i.e. at the same time instance, represents the differential

movement of the bridge abutment or pier during the earthquake excitation and hence it is a measure of

the expected bending moment for given boundary conditions. Residual displacements, after earthquake

loading, represents the displacements recorded at the last step of the time history analysis, i.e. after the

completion of the seismic motion as per the equation below:

𝐷𝑟𝑖𝑓𝑡 = 𝑈𝑡𝑜𝑝 − 𝑈𝑏𝑜𝑡𝑡𝑜𝑚 𝐻𝑒𝑖𝑔ℎ𝑡⁄ (mm/m) (1)

Figure 12 - Drift Calculation

The drift values were summarised for both models including backfill and models without backfill.

The results show that the backfill has a significant effect on the displacement of the bridge as expected.

Comparing the drift values of models with backfill to those without backfill, show that the backfill

limited the displacements of the bridges significantly.

Comparing the outputs from the 3.5m and 5.5m models to the base model, the results showed that

the backfill acted as a mass driving the motion of the bridge, this behaviour tends to be very obvious

when comparing the base model to the 3.5m and 5.5m abutment height models. The lower mass of

backfill, behind the abutments of the 3.5m and 5.5m models, driving the motion resulted in reduced

displacements as compared to the base model, which had a larger mass of soil retained by the abutment.

This can also be attributed to the change in the bridge stiffness, which in turn affects its response to the

input seismic motion.

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REF/WF REF/NF

Kypseli 1.31 2.67

Gebze 1.53 2.87

Duzce 2.81 6.05

Hector mine 1.98 2.17

Umbria 2.56 3.38

Table 8 - Drift ‰ (mm/m) for 0.3g Input Motion – Bridge with fill (REF/WF) vs No fill (REF/NF)

The following tables show a comparison between the drift values at the abutments of REF/WF and

both REF/1S-WF and REF/2S-WF for input motions scaled at 0.3g and 0.6g. Single span and two span

bridges showed low stiffness compared to the three span bridge, and this resulted in the backfill soil

acting as the driving mass of the system, and therefore the drift values showed an increase in drift values

of both 1S and 2S bridges compared to the base model.

REF/1S-WF REF/2S-WF REF/WF

Kypseli 1.32 1.84 1.31

Gebze 1.57 2.28 1.53

Duzce 3.31 4.53 2.81

Hector mine 1.15 1.72 1.98

Umbria 1.89 2.54 2.56

Table 9 – Drift ‰ (mm/m) for 0.3g Input Motion – With Fill

REF/1S-WF REF/2S-WF REF/WF

Kypseli 2.40 2.90 1.31

Gebze 4.60 5.80 1.53

Duzce 5.00 7.50 2.81

Hector mine 2.20 3.00 1.98

Umbria 4.50 5.90 2.55

Table 10 – Drift ‰ (mm/m) for 0.6g Input Motion - With Fill

The analysis results using 0.6g input motions confirm the behaviour and conclusions drawn from the

0.3g analysis results. The results show that the backfill acted as a mass driving the motion of the bridge,

and this behaviour tends to be obvious when comparing the base model (REF/WF) to the 3.5m and 5.5m

abutment height models. The shorter height of the abutment meant a lower mass of backfill which

reduced the bridge displacements compared to the full height base model.

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REF/3.5m-WF REF/5.5m-WF REF/WF

Kypseli 1.00 1.90 1.30

Gebze 1.00 1.60 1.50

Duzce 1.20 2.60 2.80

Hector mine 1.20 1.70 2.00

Umbria 1.20 2.60 2.60

Table 11 - Drift ‰ (mm/m) for 0.6g Input Motion - With Fill (Abutment Height Effect)

5 CONCLUSIONS AND FURTHER RESEARCH

The present work aimed at establishing the effect of backfill soil on Integral Abutment Bridges (IAB)

seismic response and to answer the question as to whether it is a governing factor in the design of these

types of bridges. The effort comprised the development of finite element models using PLAXIS 2D for

the study of soil-structure interaction effects on a number of variations of integral abutment bridges

under seismic ground motion. The study covered in detail the seismic behaviour of six IAB bridge

configurations that varies in length, abutment height and type of backfill soil.

The study showed that the presence of backfill soil had affected the dynamic behaviour of the bridges

in multiple ways, and it also showed that the interaction between the backfill soil and the bridge structure

is complex enough to warrant soil-structure interaction sophisticated modelling to obtain reliable

responses with regard to the behaviour of the bridge under seismic loading. Although more research is

required on the topic, the present study confirmed that the interaction between the bridge and the backfill

is of importance to be scrutinized by the design engineers and to be considered using sophisticated

analysis models, rather than using simplified design formulas. The results to date do not lead to clear

conclusions, i.e. whether the backfill soil is beneficial or detrimental for the seismic response of IABs,

however it is evident that this is a strongly case-dependent effect and this hence this is an ongoing

research project.

The effects of thermal cycles, creep and shrinkage, and prestressing forces in concrete bridges and

the long-term effects’ evolution over time is neglected in the present research, but such effects on the

behaviour of the bridge is expected to have significant effects on the behaviour of the bridge and should

be considered in any future studies.

Future studies should research into establishing design guidelines or design formulas to be used in

the design office as access to an FE sophisticated software is not always available for design offices, in

addition to the computational cost of running such complicated analysis software.

The study also considered the case of supporting the abutments and piers on shallow foundation. This

system may be standard for European countries, but the more common system in North America is to

use piled foundation. The piles have some flexibility and would allow movement of the abutment and

piers which would affect the natural period of the structure, and this effect should be considered in

further studies.

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