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 ODOTECHNIKI Ltd, Athens, Greece

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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

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BRIDGING OVER A LANDSLIDE