Laboratory of Geodesy
Dept. of Civil Engineering
Aristotle University of Thessaloniki
Greece

Monitoring of the behavior of large technical structures
Monitoring of the behavior of large technical structures has been a reality for the last few decades.
Deformations that occur may cause severe damages to structures or even loss of life and injury to people.
These deformations may be attributed to several reasons:
 incomplete investigation of foundation soil properties
 improper construction of the foundation system
 insufficient knowledge of the operating conditions
 earthquakes etc.
The continuous monitoring of the behavior of a structure can detect deformations and displacements authenticating study and construction theories and proving the safety of the operation of the technical work.

The 50,000 t Sugar Storage Tank - 1
A heavy storage tank for storing
50,000 t of sugar founded on a large group of bored piles.
The tank is located in the installations of the Hellenic Sugar
Industry S.A. at Plati, about 50
Km from the City of Thessaloniki.
It is the largest structure of this category in Europe and its construction was completed in
1995 with a cost of more than
2,500,000 USD.

The 50,000 t Sugar Storage Tank - 2
The complex consists of a cylindrical shell of a 350 mm thick circular, prestressed concrete wall. The concrete shell has 46 m external diameter, it is 33.36 m high and it is covered by a wooden roof.
The superstructure is founded on a 1200 mm thick circular concrete raft, 2.50 m above the ground surface, which is supported by 124 bored piles.
The dead weight of the whole structure is approximately
12,000 t.

Foundations consideration and design - 1
Storage tanks are relatively flexible structures and they can tolerate greater settlements than other engineering structures. However, there is a limit to the settlements expected to take without distress.
The most important undesirable effects of settlements to avoid in designing tank foundations are:

Overall settlement of the tank.

Differential settlement across the diameter, which may overstress internal piping connections.

Differential settlement along the periphery, which may overstress the superstructure.

Differential settlement between the tank and the external connection pipework.

Foundations consideration and design - 2
Finally, the rotary, cast in place, large bored piles with bentonite slurry were selected as the most efficient pile type.

A group of 124 piles was designed and constructed.

Each pile has a diameter of 1.20 m and it is 37.17 m long.
The theoretical mean total settlement of the group of piles was evaluated equal to 12 cm.

A survey of similar structures
A heavy storage tank capable of storing 15,000 t of liquid ammonia located at the industrial area of the city of Thessaloniki near the seaside. It is founded on a circular concrete raft, which is supported by 112 bored piles.
Deformation measurements for vertical displacements.
1985-today
A heavy storage tank capable of storing 40,000 t of sugar at
Forlipopoli, Italy. The circular concrete raft is founded on 396 piles.
Deformation measurements for both horizontal and vertical displacements.
1994-1996

Measurement of deformation - 1

Measurement of deformation - 2
Loading history
60000
50000
40000
30000
20000
10000
0
0 100 200 300
Days from start
400 500
Measurement epochs:
6
The deformations of the tank were observed after the completion of its construction during the first loading with sugar. The tank was gradually filled with 10,000 t of sugar. Then the sugar was removed, the tank remained empty for about five months and, finally, it was again filled with sugar to the maximum load of 50,000 t.

Measurement of horizontal deformation - 1
In order to monitor the horizontal deformation , of the cylindrical shell concrete wall:
Two lines of control points, each of 12 cone-shaped targets, were installed on the wall of the tank from bottom to top, at the diameter NW-SE, the lower 6 at distance of 3 m from each other and the rest at distance of 6 m.

Measurement of horizontal deformation - 2

A control network consisting of a number of reference points
(pillars) was established around the tank.

The network was remeasured with GPS receivers at each epoch.

The coordinates of the targets on the wall were computed due to the measurement of intersections from the points of the control network.

Measurement of horizontal deformation - 3

A free network adjustment of the complete network (both reference and control points included) was done for each epoch.

A similarity transformation was used to obtain a common reference system between the coordinates of the points of the specific epoch and the coordinates of the points of the zero measurement.

The horizontal displacement of each control point was computed as the difference between the zero measurement coordinates and the corresponding transformed last epoch coordinates.

Measurement of vertical deformation - 1
In order to monitor the vertical deformation ,16 benchmarks were installed on the upper free part of certain piles.
The benchmarks can be divided into three groups:

P1, P2, P3, and P4 located on the central piles

P5, P6, P7, P8, P9, and P10 located on piles at an intermediate periphery

P11, P12, P13, P14, P15, and
P16 located on piles at the external periphery of the slab.

Measurement of vertical deformation - 2
High precision geodetic levelling

Measurement of vertical deformation - 3
Vertical movements of piles during the first loading of the tank with sugar
800
780
760
740
720
700
680
660
0 37 93 190 353 408
Days from start
P1, P2, P3, and P4
850
800
750
700
650
0 37 93 190 353 408
Days from start
P5, P6, P7, P8, P9 and P10
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
820
800
780
760
740
720
700
680
0 37 93 190 353 408
Days from start
P11
P12
P13
P14
P15
P16
P11, P12, P13, P14, P15 and P16

The vertical displacements of pile benchmarks generally follow the loading and unloading of the tank

Measurement of vertical deformation - 4

The maximum settlements of the slab occurred at central piles P1, P2, P3, and
P4 with a mean value of approximately 63.5 mm.

The settlements observed were smaller with increasing distance from the center of the circular slab.
Slab deformation under maximum load during the first loading of the tank with sugar (curved lines are lines of equal vertical displacement, in mm)

Conclusions – 1
Comparison to the behaviour of similar structures
The model of vertical deformation of the sugar tank at Plati was very similar to the behavior observed in the other two tank structures mentioned above (the ammonia storage tank in Thessaloniki and the sugar storage tank in Forlipopoli).
Thessaloniki
Plati

Conclusions – 2
During the first loading of the tank with 10,000 t of sugar, the mean value of the observed settlement was equal to the 25% of the predicted mean final settlement.
After sugar removal the mean remaining settlement was 9% of the final predicted value.
The differential settlements between the center of the foundation slab and the peripheral points were fractions of the overall slab settlement, quite close to the ratio estimated in the computations.
During the loading test with the maximum load of 50,000 t, the mean observed settlement seems to be approximately the 50% of the expected final settlements.
The observed horizontal displacements of the tank wall were insignificant, proving the effectiveness of the design and construction method used.