field monitoring of a motorway viaduct moving on an ......inclinometer casing direct pendulum total...

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1 Field monitoring of a motorway viaduct moving on an extremely slow landslide Tombolato Sara, Pedrotti Matteo, Simeoni Lucia, Mongiovì Luigi University of Trento, Italy Introduction The A22 Modena-Brenner motorway runs in Northern Italy from the Alps to the Po plain, and connects Italy with Austria. In the mountainous area more than one third of the A22 motorway was built on viaducts, and their monitoring is of vital, economic and strategic importance for the whole network. Regularly, a team from the A22 company monitors the viaducts and carries out works for the maintenance of the structures and for preserving their functionality and safety. In particular, the relative pier-deck bridge displacements are checked by visual inspections and measurements to find out if shear deformations have occurred. At the late 1980s anomalous displacements were surveyed at the Micheletti viaduct (Figure 1), and since then the A22 office have been monitoring the piers and the slope. In 2007 the A22 board requested that the University of Trento study the stability of the viaduct by defining the causes of the displacements. This paper describes the instrumentation used for the monitoring and how the measurements were processed for assessing their reliability. In fact, dealing with an extremely slow movements, the changes of the measurements could be as small as their precision or accuracy. The reliability assessment is therefore fundamental to use the measurements for interpreting the failure mechanism. Different instruments for measuring the pier viaduct movements and the slope movements were used. Piers were monitored using a Total Station, pendula and clinometers; slope movements were measured with inclinometers. The reliability was assessed by analysing the redundancy of the measurements, and results were used for identifying the failure mechanism.

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Page 1: Field monitoring of a motorway viaduct moving on an ......Inclinometer casing Direct pendulum Total station prism Biaxial clinometer Cropping out rock Isarco river SS12 road pier 20

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Field monitoring of a motorway viaduct moving on an extremely slow landslide Tombolato Sara, Pedrotti Matteo, Simeoni Lucia, Mongiovì Luigi

University of Trento, Italy

Introduction The A22 Modena-Brenner motorway runs in Northern Italy from the Alps to the

Po plain, and connects Italy with Austria. In the mountainous area more than

one third of the A22 motorway was built on viaducts, and their monitoring is of

vital, economic and strategic importance for the whole network.

Regularly, a team from the A22 company monitors the viaducts and carries out

works for the maintenance of the structures and for preserving their functionality

and safety. In particular, the relative pier-deck bridge displacements are

checked by visual inspections and measurements to find out if shear

deformations have occurred. At the late 1980s anomalous displacements were

surveyed at the Micheletti viaduct (Figure 1), and since then the A22 office have

been monitoring the piers and the slope. In 2007 the A22 board requested that

the University of Trento study the stability of the viaduct by defining the causes

of the displacements.

This paper describes the instrumentation used for the monitoring and how the

measurements were processed for assessing their reliability. In fact, dealing

with an extremely slow movements, the changes of the measurements could be

as small as their precision or accuracy. The reliability assessment is therefore

fundamental to use the measurements for interpreting the failure mechanism.

Different instruments for measuring the pier viaduct movements and the slope

movements were used. Piers were monitored using a Total Station, pendula

and clinometers; slope movements were measured with inclinometers. The

reliability was assessed by analysing the redundancy of the measurements, and

results were used for identifying the failure mechanism.

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a)

b)

Figure 1. a) Piers 23 (right) through 25 (left) of the Micheletti viaduct; b) deformed

elastomeric pad revealing the relative displacement pier-deck.

Geology The viaduct Micheletti lies on the western slope of the Isarco Valley, about

10 km North of Bolzano in Northern Italy (Figure 2).

Figure 2. A22 Highway map and Micheletti Viaduct location

The Isarco Valley is a U-shaped glacial valley, with current terrace formations

ascribed to alternation of glacial and post-glacial (fluvial) quaternary deeping

processes.

Figure 3 shows a geological cross section of the western valley flank where the

viaduct is located.

PIER

DECK

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The profile exhibits different gradients corresponding to different geological

deposits. The lower valley flank is 40° dipping and covered by debris. These

debris consists of soil of gravitational origin (scree slope, rockfall deposit) mixed

with glacial deposits and alluvional lens. Debris spread out to a quasi vertical

ignimbrite rock slope outcropping from 410 to 475 metres above sea level. Up

here the slope becomes smoothly as a typical post-glacial landscape. Till and

morenic deposits covers these area, which ends in vertical outcropping rock

(not in figure).

Figure 3. Geological cross section of the studied valley flank.

The viaduct structure The highway viaduct is supported by a row of 8 concrete piers 35 meters apart.

The piers are 35 meters height and are founded on footing foundations in the

steeply inclined slope (about 40°). The depth of foundations is similar in all piers

and it is about 10 m from the ground surface. The piers were isolated from the

surrounding soil by elliptical and empty caissons, that are structurally jointed to

the piers foundations. At the top, each pier has six elastomeric bearings

arranged in 2 arrows which allow the deck to rotate and translate by distorting

the elastomeric pads.

336

386

436

536

met

ers

abov

e se

a le

vel

viaduct pier n.24

SS 12 street

Isarco river bed

486

Till and morainicdeposit

40°

Tuff rock 70°

Ignimbriti rock

cropping outrock

SS12 way

? ??

?

?

??

?

30°

Gravitationalalluvionalglacial deposit

0 50 m

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The monitoring The monitoring was planned to measure both piers and slope displacements.

Instrumentation for the piers monitoring consisted of 6 biaxial clinometers, 4

direct pendula and a Total Station with 37 prisms. 13 inclinometers were used

for the slope monitoring.

The monitoring covered an area of about 150 m long and includes 8 piers,

numbered from 20 to 27 as showed in Figure 4.

Inclinometer casingDirect pendulumTotal station prismBiaxial clinometer

Cropping out rock

Isarco river

SS12 road pier 20pier 21pier 22pier 23pier 26 pier 25 pier 24pier 27

35 mT6I

T10I

T3I

T4I I4T7I

I3 I2

T5I T2I

I1

TI1I6

Figure 4. Map of the viaduct piers and instrument location.

Monitoring has been performed discontinuously since 1988 and the collected

data are referred to different setups and operators. As an example, the

measurements performed at the pier 25 are shown in Figure 5.

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Jun-

88

Oct

-93

Jun-

94

Jun-

95

Mar

-96

Oct

-97

Aug

-98

Jun-

00

Jun-

08

Nov

-04

Jul-1

0

BIAXIALCLINOMETERGeoingeneria(avaible only plot)

BIAXIAL CLINOMETERGeoingegneria

INCLINOMETERGeoingegneria

INCLINOMETERSepi

INCLINOMETERUniTn

THEODOLITEAND

STADIA RODSing.Polluzzi

TOTAL STATIONtechn.office A22

DIRECT PENDULUMtechn.office A22

GIRDER-PULVINO DISPLACEMENTS

techn.office A22(different strumentation)

DIRECTPENDULUMing.Polluzzi

Figure 5. Monitoring at pier 25 since June 1988: instruments (capital letters) and

operators (small letters). Pier monitoring Pier monitoring included a Total Station (from pier 21 trough pier 26), 4 direct

pendula (fixed to piers 22, 23, 24 and 25) and 6 biaxial clinometers (from pier

21 trough pier 26).

Pier total displacements were measured by using a Total Station. Three targets

at different heights were measured on each pier. For each target the Total

Station provided dip direction (azimuth angle), dip (zenith angle) and distance

as an average of a set of measures. Having redundant data, each coordinate

was estimated using the method of least squares.

Figure 6 shows the map of the total station network which was adopted in the

piers monitoring.

For each measurement the maximum amount of standard deviation of easting,

northing and height was 1.8, 1.4 and 0.7 mm respectively. Hence the maximum

amount of standard deviation of planimetric coordinates was 1 mm. Ultimately

the monitoring system allowed to measure 2-3 mm as minimum displacements.

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Measurements were processed assuming the piers to rotate and slide as a rigid

bodies. In fact, the differences in the horizontal plan (x,y) between the real

displacements and the hypothesis of rigid movement were of the same order of

magnitude of measurement errors.

Once the coordinates of each targets were calculated, the absolute horizontal

displacements at the pier foundation, those at the pier tops and the relative

horizontal displacements of the foundations respect to the tops were estimated

(Figure 6). Finally, the vertical displacements respect the pier 22 are shown in

Figure 7.

Figure 6 Horizontal Displacements at the piers (November 2004-April 2010)

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verti

cal d

ispl

acem

ents

-z

(mm

)

Pier 21Pier 22Pier 23Pier 24Pier 25Pier 26

Figure 7 Pier vertical displacements ∆z respect to pier 22 (November 2004-April 2010)

Direct pendulum consisted of an iron wire supplied with a reading table. The

upper point of the wire was firmly fixed to the piers 22, 23, 24 and 25, the lower

point was linked with a mass working as a counterweight. To avoid oscillations,

which could be caused by external factors, the wire was protected by a metallic

case fixed on the pier and the counterweight was immersed in a damper oil

tank. Measurements were performed manually using a graduated square with

an order of precision of 1 mm.

Figure 8 shows the relative horizontal displacements top-foundation of the piers

measured by the direct pendula since 1997 .

Biaxial clinometers were permanently installed at the piers to provide automatic

long-term monitoring. They were equipped with two orthogonal force-balanced

servo-accelerometer sensors. The clinometer line was housed in a rugged

stainless steel cylinder. The instruments included spirit level, adjustable

mounting bracket and anchor plate.

Figure 9 shows the pier tilt vectors resulting in the horizontal plane from the

biaxial clinometer data.

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SS12 roadpier 20

pier 21pier 22pier 23pier 26 pier 25 pier 24pier 27

50 m

Monitoring period:July 1997 - April 2010July 1997 - May 2007

Isarco river

77 mm

135 mm

99 mm

9 mm

Figure 8. Relative horizontal displacement pier top- pier foundation measured by the

direct pendula

Monitoring period:September 1993 – Dicember 1998April 1995 – Dicember 1998Dicember 2000 – February 2002

0.29 °

0.23 °

0.04 °0.17 ° 0.13 °

0.21 °

50 m

SS12 road

pier 21pier 22

pier 23

pier 26 pier 25pier 24

pier 27

Isarco river

Figure 9. Pier tilts measured by the biaxial clinometers

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Slope monitoring Soil displacements were measured using a mobile biaxial SisGeo inclinometer

probe with servoaccelerometer sensors. Measurements were carried out four

times by lowering the probe in every groove of the casing. Consequently, the

measurements were redundant and could be processed in four different ways:

two ways by assuming the probe biaxial (elaboration 1-3 and elaboration 2-4),

and two ways by assuming the probe monoaxial (elaboration A-A and

elaboration B-B). Figure 10 shows the cumulative displacements calculated at

the inclinometer T5 with the four elaborations. It is worth noticing that the

displacements differ considerably.

Figure 10. Inclinometer T5. Cumulative displacements (October 2009 – April 2010) By observing Figure 10, it is clear that difference increases topwards from the

bottom of the casing as it happens for the propagation of systematic errors.

None of the errors studied by Mikkelsen, 2003 was recognized and corrected.

Hence, attempts were made to find any other type of systematic errors, such as

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those due to no-perpendicular grooves, or empirical relationships with

measurements, but none of them resulted successful.

In order to reduce the effects of the propagation of the systematic errors, soil

displacements were estimated by integrating the displacements just in the

intervals of depth where the local displacements exhibited both a magnitude

significantly larger than the precision (Simeoni et al., 2007) and an azimuth

coherent with the slope. Figure 11 shows two examples of local displacements

in inclinometer I3 and inclinometer T5. In the latter it could be seen that there is

not any significant local displacements. Therefore, in this inclinometer the local

displacements were not integrated and the inclinometer was assumed fixed,

despite the conventional cumulative displacement calculation had provided

displacements different form zero at the ground surface (Figure 10).

a) Inclinometer I3 b) Inclinometer T5

Figure 11. Inclinometers I3 and T5: local displacements (October 2009 – April 2010).

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Reliability analysis In order to assess the reliability of the measurements different sets of data were

compared. The redundancy of the horizontal relative displacements was studied

by comparing the data from the total station, the biaxial clinometers and the

direct pendula.

The redundancy of the absolute horizontal displacements was studied by

comparing the structure displacements with the soil displacements at the pier

foundation level.

Redundancy of total station and pendulum measurements

Both Total Station and direct pendulum measurements were performed in the

same period, from November 2004 to April 2010. It was therefore easy to study

the redundancy by comparing the two type of measurements in terms of

magnitude and orientation. The agreement between the two different

instruments was good for each pier. As an example, Figure 12 compares the

pier relative horizontal displacements top-foundation calculated at the pier 25.

Even though the simplicity of the instrument, it may be concluded that the direct

pendulum provided very reliable measurements.

10-N

ov-0

414

-Dec

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5

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-Jun

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2

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mag

nitu

de (m

m)

Direct pendulumTotal station

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60

120

180

240

300

360

orie

ntat

ion-

north

dire

ctio

n (°

)

Direct PendulumTotal station

(a) magnitude (mm) (b) orientation-north direction (°)

Figure 12. Redundancy of total station and pendulum measurements (pier 25)

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Redundancy of biaxial clinometer and pendulum

As showed in Figure 13 the pier inclinations calculated from the clinometers

data resulted completely different from the ones calculated from the direct

pendula data. Therefore the clinometer measurements were rejected.

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-Jun

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0

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0.01

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ango

lo α

risp

etto

alla

ver

tical

e (°

)

Pila 22Pila 23Pila 24Pila 25

clinometri biassiali

filo a piombo

Pier

incl

inat

ion

(°)

Biaxial clinometers

Direct pendula

Pier 22Pier 23Pier 24Pier 25

Figure 13. Redundancy of the biaxial clinometer and pendulum measurements.

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Redundancy of total station and inclinometers

For studying the redundancy between the inclinometer and the total station data, the

absolute soil displacements at the foundation (inclinometer cumulative displacement)

were compared to the absolute pier displacements (Total Station displacement).

Because the measurements were neither contemporary nor carried out on the same

date, the comparison was made in term of displacement rates. Soil displacements

were measured from October 2009 to July 2010, whereas pier displacements were

measured from November 2004 to April 2010.

All the inclinometers, except inclinometer I2, were drilled next to a pier (Figure 4).

Therefore the comparison was made between each inclinometer and the pier next to

it (Table 1). The soil displacements were estimated by integrating the local

displacements in the way explained before and only below the foundation level.

In Table 1 It is worth noticing that the soil displacements rates were very similar to the

pier ones, and ranged between 7 and 10 mm/year. Accordingly to Cruden and

Varnes, 1994 the slope movement was classified as extremely slow.

inclinometer pier soil rate [mm/year]

pier rate [mm/year]

I6 22 7 8

T1 23 8 9

I2 23/24 6 -

I3 24 8 10

T4 25 7 9

T3 25 10 9

T6 26 9 7 Table 1. Soil and structure displacement rate

The orientation of the displacements calculated from either the inclinometers or the

Total Station data were very similar and close to 110°clockwise from the North

direction). This value was coherent with the slope morphology.

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Mechanism of failure Sliding surface limits Despite the presence of discontinuities in the outcropping ignimbrite, their orientation

did not identify any sliding surface. Moreover, the inclinometers located at the base of

the slope, close to the road SS12 (inclinometers T2 and T5), did not detect significant

local displacements. Accordingly, it seemed reasonable to assume that the sliding

surface developed in the gravitational-alluvional-glacial deposit immediately above the

road SS12.

Kinematic of the soil-viaduct structure The soil displacement analysis identified two different sliding surfaces (Figure 14).

One developed in the northern zone (pier 22 trough pier 24) and it was located at the

level of the foundations. It was called upper sliding surface. The other developed in

the southern zone (pier 24 trough pier 26) and it was located 3-4 meters below the

foundations. It was called deeer sliding surface. In the middle zone (pier 24) both

surfaces were identified; each of them exhibited a smaller displacement rate than on

the sides, but the sum was similar (Table 1).

In the deep cinematism close to pier 26 and 25 the magnitude of displacement rate is

included between 7 and 10 millimetres per year. Whereas the magnitude of

displacement rate of superficial cinematism, close to pier 22 and 23, is included

between 6 and 8 millimetres per year. Close to the pier 24 both the superficial

cinematism and the deep one have a magnitude of displacement rate approximately

3-4 millimetres per year.

Due to the redundancy between soil and structure displacement rates (Table 1) it

seemed reasonable to assume that any other deep surface was not relevant to the

movement of the viaduct. In fact, if another deeper sliding surface, developing under

the bottom of inclinometers T2 and T5, existed it would move extremely slowly and

could not affect the stability of the viaduct. Furthermore, if the sliding surface is

considered circular, the small vertical displacements, measured from total station

analysis (Figure 5), show that there is no deeper sliding surface which includes the

total station base or not.

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Figure 14. Location of the two sliding surfaces.

Causes of the viaduct movement Because of the extremely slow movements, there was no a clear evidence on the

ground of the existence of one or more landslides. Therefore the viaduct piers had

moved either for a problem of bearing capacity of their foundations or for a problem of

global stability or both. The answer came from the analysis of the inclinometer

measurements. In fact, given that the inclinometer measurements were reliable

because redundant with the Total Station measurements, there were firm evidences

confirming that the failure mechanism was due to the slope instability. The first

evidence was the extraordinary repeatability of the deeper sliding surface location

beneath the foundations (3-4 meters). The second one was the uniformity of the

displacement rates and their directions in the whole area examined. Furthermore, the

inclinometer T6 was drilled 16 meters upslope pier 26 and detected the sliding

surface at the same elevation of the pier’s foundation (Figure 15). If the mechanism

was a bearing capacity problem, the sliding surface should intersect the inclinometer

at an higher elevation.

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PIER 26

T6I

T10I

ISARCO RIVER

10 m

LOCALDISPLACEMENTS

Figure 15. Cross section at Pier 26.

10 m

LOCALDISPLACEMENTS

T4I

T5I

PIER 25

ISARCO RIVER

Figure 16. Cross section at Pier 25.

Likewise, the two inclinometers T3 and T4 installed upslope and downslope the pier

25, respectively, detected the sliding surface at approximately the same elevation

(Figure 16). This showed that the sliding surface was sub-horizontal.

Hence, a bearing capacity mechanism seemed to be incompatible with the shape of

the sliding surfaces derived from the inclinometer measurements.

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Conclusions

The system composed by a Total Station and inclinometers has proved to be suitable

for monitoring extremely slow movements and for defining the failure mechanism

causing the viaduct to move. The reliability of measurements was assessed by

analysing the redundancy between the Total Station displacement rates and the

inclinometer displacement rates integrated only where the major displacements were

identified. The comparison between the Total Station and the direct pendula

measurements revealed that the pendulum can be assumed a reliable instrument

despite its simplicity. On the contrary, the clinometer measurements were not

redundant with those of the direct pendula and were rejected.

Given that the inclinometer measurements resulted reliable, they were used for

defining the shape of the sliding surfaces and to identify the failure mechanism. It was

proved that the viaduct movement was due to a slope instability instead of a bearing

capacity problem of the pier foundations.

References Cruden D.M. and Varnes D.J.; Landslides Types and Processes, Transportation

Research Board. National Academy of Sciences. "Landslides: Investigation and

Mitigation", pp. 36-75, 1994.

Mikkelsen P.E.; Advances in inclinometer data analysis. Proc., 6th International

Symposium on Field Measurements in Geomechanics, Oslo, Norway, September, 15-

18 2003, 555-567, 2003.

Simeoni L. and Mongiovì L.; Inclinometer monitoring of the Castelrotto landslide in

Italy. Journal of Geotechnical and Geoenvironmental Engineering, Vol. 133, No. 6,

June 2007, pp. 653-666, 2007.

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Authors

Dr Sara Tombolato, Ph.D.* [email protected] Dr Matteo Pedrotti** [email protected] Dr Lucia Simeoni, Ph.D.* [email protected] Prof. Luigi Mongiovì* [email protected] *Department of Mechanical and Structural Engineering University of Trento Via Mesiano 77, 38123 Trento (Italy) www.unitn.it/ingegneria **Strathclyde University, Glasgow