BRIDGING OVER A LANDSLIDE

Konstantinos Seferoglou, Civil (Geotechnical) Engineer, M.Sc.
Isabella Vassilopoulou, Dr. Civil (Structural) Engineer
Fragiskos Chrysohoidis, Engineering Geologist M.Sc., DIC
Petros Fortsakis, Dr. Civil (Geotechnical) Engineer
Georgios Prountzopoulos, Dr. Civil (Geotechnical) Engineer
Konstantinos Nikas, Civil (Geotechnical) Engineer, M.Sc.
Building a bridge over an active landslide
Geotechnical and Structural Challenges
ODOTECHNIKI Ltd, Athens, Greece

Building a bridge over an active landslide. Geotechnical and Structural Challenges
2
CONTENTS
• INTRODUCTION
• TEMPORARY INFRASTRUCTURE PROJECTS
• INSTRUMENTATION, MONITORING AND EARLY WARNING SYSTEM
• DESIGN OF TEMPORARY INFRASTRUCTURE PROJECTS
• THE BRIDGE UNDER CONSTRUCTION

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In 2003 a major landslide destroyed a
highway in southern Greece causing a
mass displacement of about 6.000.000m3.
INTRODUCTION
Major landslide event

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An arched steel bridge was designed
over the active landslide creating
geotechnical and structural challenges
during the construction.
2003
2013 2016
INTRODUCTION
Bridge project

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INTRODUCTION
Bridge project
The 390m long bridge consisted of an access part made of prestressed concrete between the
northern abutment and the pier with total length 130m and an arched part with suspended steel
deck of 260m between the pier and the southern abutment.

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Landslide limits
INTRODUCTION
Bridging an active landslide
The arch was designed to bridge over the landslide, which was active and moved downstream at an
average rate of about 1.5-2.0mm/month. The rate of movement increased dramatically during the
winter rainfalls, exceeding for short periods of time, 30mm/month.

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• INTRODUCTION
TEMPORARY INFRASTRUCTURE PROJECTS

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Construction of the pier and the concrete part of the superstructure
The V-shaped supporting struts, forming the pier, was constructed first. The 55m part of the deck
followed, ensuring the stability of the pier. The 130m access concrete bridge was completed, with
the construction of the 75m part of the deck between the pier and the abutment, using heavy duty
PERI bridge-type scaffolding and founded with piles on adverse surface morphology and soil
conditions.

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Landslide limits
Construction of the arch
During the construction period, it was required to use the landslide body as a foundation ground to
support 16 steel towers up to 60m high. The towers aimed at heavy lifting and final assembly at
height of the steel arch parts. At the same time, the heavy highway traffic was deviated to pass on
top of the slide body.

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Landslide limits
Foundation of the towers
The foundation of the towers consisted of piles connected with pile-caps. Between the pile-caps
concrete slabs on ground and small temporary bridges on pile systems were also constructed,
which were used for the assembling/welding of the arch and deck segments. Most of the deep
foundation and the supporting structures were built in the active landslide where the evolving
movements were observed.

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Infrastructure for the erection of the steel arches
Southern
Abutment
Temporary traffic
Pilewall to retain the
temporary road
Assembly
towers ΜΤ
Assembly piers
Π1Δ-Π7Δ
Assembly
slabs/bridges
Assembly piers
Π1Α-Π7Α
Assembly
slabs on
ground
Middle pier
strut
Landslide limits and
movement

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• INTRODUCTION
INSTRUMENTATION – MONITORING – EARLY WARNING SYSTEM

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4 optical targets for surface movements
26 optical targets for movements of pier foundation
6 optical targets for movements of the pile wall
4 automated permanent inclinometers
3 automated permanent piezometers
7 conventional inclinometers
conventional piezometers
Geotechnical instruments

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Automated Monitoring - Schematic Function
Data Logger & Modem
Scheduled Readings
(i.e. every 4 hours)
Readings
downloaded to
server in txt
format
Programmed
spreadsheet
collects
downloaded
data
Data elaborated,
analyzed and
graphs for each
instrument is
created
automatically
Alarm SMS is released in
case the movements exceed
the limits (TSAKONA
ALARM)
Extra readings and
evaluation performed,
instruction for contingency
measures provided.
The updated spreadsheet
is uploaded on a shared
cloud server and provides
the evaluation of the
developments at least
every 4 hours.
Monthly reports
submitted

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Main conclusions drawn from monitoring results
• A seasonal variation of the rate of movement of the slide was noted related to rainfall.

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• Differences of the rate of movement were observed among the various monitoring locations
(13 - 40mm/year). Factors influencing the rate are the depth of the sliding surface and the
inclination of the bedrock surface.

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Sliding surface
(zone)
Characteristic section
Bedrock surface
Bridge Axis
Temporary
highway traffic
Position of piers foundation
Pile wall
Sliding zone

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• Deep movements (inclinometric readings) and surface movements (optical targets movements)
were in good agreement.
Location of readings
Depth of
sliding zone
Mean total annual
movement
Sliding zone Τ4-ΚΝ1 and Τ4-ΚΝ2 15-20m 27mm
Optical targets on pile wall Μ2, Μ3, Μ4 18-22m 25mm
Optical targets on pile caps Π2Α, Π3Α, Π4Α 18-20m 29mm
Optical targets on pile caps Π2Δ, Π3Δ, Π4Δ 30-34m 34mm
Sliding zone ΚΛ5 and ΚΛ6 35-40m 32mm
Comparison of Deep Vs Surface movements (28/03/13÷7/03/14)
• For the design of the projects a two years construction period was assumed and the proposed
design movement (transverse to the bridge axis) was 30mm/year and for the longitudinal
direction a total differential movement between temporary piers of about 15mm/year was
considered.

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• INTRODUCTION
DESIGN OF TEMPORARY INFRASTRUCTURE PROJECTS

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Design of the towers foundation
The most challenging of the required temporary infrastructure, was the design of the foundation of
the 14 steel piers used for the heavy lifting final assembly at height, of the 36m parts of the arch
pair. The demanding stability within a moving mass, combined with the very tight accuracy
requirements of the arch welding activities and the significant loads transferred to the ground,
comprised the technical framework for the design of these works.

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• Two years of arch construction
• 30mm/year max horizontal movement
• Temporary structures – low cost
Assumptions
Design Criteria
• Not possible to stabilize the landslide
• Safety of the temporary traffic of the
highway
• Safe erection of the arch
• Stability and tight accuracy for the
welding activities
Aim
• Floating piles in the sliding mass
(depth 8m-22m)
• Piles in the rock for depth of sliding
mass smaller than 5m
• Deep foundation with pile groups

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Piers foundation design methodology
• As the major concern was the influence of the landslide movements on the bending moments
and shear forces on each pile group, it was decided to use initially a 3D (ABAQUS) model as a
guide for the geotechnical assessments. The excitation due to soil mass movements was
introduced using a real profile of the movements as it was depicted from the monitoring data.
External loading from the superstructure was also added.
• The 3D guide-model was then used to calibrate 2D geotechnical models (PLAXIS) for each one of
the 14 piers, which led to the assessment of bending and shear increase due to the soil
movement, considering in each case the local morphology of the sliding surface and the local
profile of the soil mass movement.
• Finally, the results of the geotechnical analyses provided the required data to feed more detailed
3D structural analysis (SOFiSTiK), used for the optimization of the dimensioning and required
reinforcement of each structural element of the foundation.
Phase 1: Analysis with the
detailed 3D FE «guide-
model»
Phase 2: Calibration of the
geotechnical 2D FE model
with the «guide-model»
Phase 3: Geotechnical analysis of 2D FE
models adjusted to the local morphology
and loadings for each pair of piers
Phase 4: Calibration of the
structural 3D FE model with
the «guide-model»
Phase 5: Structural design of 3D FE models and reinforcement
optimization for each pier. Extra calculations for contingency hypotheses
of shallow failure planes as well as anchoring of the pile groups
DESIGN OF INFRASTRUCTURE PROJECTS

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Phase 1: 3D geotechnical analysis (guide-model, ABAQUS)
Step 1 Step 2 Step 3 Step 5
Transverse slope movement
Step 4
The slope movement practically
led to a uniform movement of the
soil above the failure plane,
causing small increase of bending,
shear and axial internal forces of
the piles. (ΔN=+3.4%, ΔQ=4.2%,
ΔM=37.5%)

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Phase 2: 2D geotechnical analyses (PLAXIS)
Model calibration
PLAXIS ABAQUS

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Π1 Π2 Π3
Π4 Π5 Π6 Π7
Phase 3: 2D geotechnical analyses for each pair of piers (PLAXIS)
Earth pressure and
soil spring values
calculated to be
fed into structural
models in Phase 4.

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Phase 4: 3D structural analyses (SOFiSTiK)
Model calibration and design
SOFiSTiK ABAQUS

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• Local shallow failure plane developed introducing a 15mm transverse movement.
Anchoring is also activated in this case.
Contingency working hypotheses

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• Contingency anchoring of the pile caps in case the movements exceeded the
predicted values or the tolerance of the superstructure during the erection of the
steel arches
Location of contingency
anchoring on the pile cap
Contingency working hypotheses

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Design of the temporary towers foundation
• Self weight
• Loads from the temporary towers
• Loads from the slabs/bridges
• Earth pressures (static and seismic)
• Seismic action α=0.08g (temporary project)
• Soil movements 15mm (Local shallow failure planes at the piers Π2Δ – Π5Δ)
• Forces from anchoring
Comparison of piles internal forces due to static and seismic load combinations
(SOFiSTiK) with the corresponding ones due to the soil movement (PLAXIS)
Small differences were observed up to 3% for the axial forces and 6% for the
bending moments. Hence, for the piles of piers Π2Δ – Π5Δ an increase of the
required reinforcement was considered up to 10%.

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• INTRODUCTION

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THE BRIDGE UNDER CONSTRUCTION

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Contractor: TERNA S.A. (Initially J/V TERNA S.A. –
ALPINE BAU)
Structural design of the Tsakona bridge: DOMI S.A.
Geotechnical and foundation design of the Tsakona bridge: EDAFOS S.A.
Design of infrastructure projects and construction consulting: ODOTECHNIKI Ltd
Design of heavy lifting scaffolding: PERI Formwork Scaffolding Engineering
Steel fabrication and erection: EMEK S.A.
Consulting support on optimization of ABAQUS by Prof. I. Anastasopoulos and K. Tzivakos, Civil Engineers.
DESIGN AND CONSTRUCTION TEAM

BRIDGING OVER A LANDSLIDE

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