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ABSTRACT

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					         Geodetic Monitoring of the Deformation of a 50,000 t
        Sugar Storage Tank Founded on 124 Long Bored Piles

                                  P. Savvaidis; I. Ifadis
                                     Thessaloniki, Greece


                                            Abstract
        A large storage tank capable of storing 50,000 t of sugar was constructed in the
installations of the Hellenic Sugar Industry S.A. at Plati, about 50 Km from the City of
Thessaloniki. The tank, which is the largest structure of this category in Europe, is founded
on 124 long bored piles. Due to the fact that possible deformation of the tank may result into
major damage, geodetic measurements were used for monitoring the displacements of the
structure both in the vertical and the horizontal sense. These measurements included a GPS
high precision control network, 3D intersections and leveling. The recorded values of
deformation gave important information about the pile group settlement distribution and
horizontal displacements of the tank walls with time and tank loading history. Finally, a
comparison of the behavior of the tank to the behavior of similar structures and theoretical
deformation analysis was done and the corresponding conclusions were derived.

1. Introduction

        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, such as
incomplete investigation of foundation soil properties, improper construction of the
foundation system, insufficient knowledge of the operating conditions, earthquakes etc. In all
cases, the continuous monitoring of the behavior of the structure can detect deformations and
displacements authenticating study and construction theories and proving the safety of the
operation of the technical work. So, the observations and recordings of deformation donnot
present only scientific interest for the geotechnical engineers, but they are also indications of
the long-term behavior of the construction [1].
        In this paper, the deformation observations of a heavy storage tank for storing 50,000
t of sugar founded on a large group of bored piles are presented. 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 about 2,500,000 USD (fig. 1).
        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.
        Although numerous articles have been published about piles during the last years and
several design methods have been proposed, based on theoretical considerations or results
from model tests, knowledge about the bearing capacity and settlements of pile groups is
limited. This is due to lack of data from full-scale field tests. So, any new case history related
to pile foundation settlements offers useful information on the understanding of the real
mechanism and the role of the factors affecting pile group behavior [2]. The same applies
also for the detection of horizontal deformations of the superstructure walls, where even less
information from field tests has been collected since now. As a result, horizontal and vertical
deformation monitoring of the sugar tank can significantly contribute to the knowledge of the
behavior of structures of this category.




                            Fig. 1. The sugar storage tank at Plati


2. Foundations consideration and design

        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 as follows [2]:

   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.
       Due to the fact that the deposit of weak soils was found very deep (aprox. 23 m) and
foundation conditions could not be effectively improved by strengthening techniques such as
replacement with engineering fills, vibration, compaction, stone columns, line columns etc., a
piled foundation system was the only viable foundation solution. Furthermore, the loading
conditions, the existence of soft clayey strata and the available large amount of surrounding
plant were the main reasons for the decision to avoid the displacement piles and to choose the
rotary, cast in place, large bored piles with bentonite slurry as the most efficient pile type.
       Finally the pictured in fig. 2 group of 124 piles was designed and constructed. Each
pile has a diameter of 1.20 m and it is 37.17 m long. Taking into account the interaction
between piles in a semi-infinite mass, the mean total settlement of the group was evaluated
equal to 12 cm by applying Poulos method for piles group settlement computation [3].




Fig. 2. Group of piles layout. Black dots show benchmark locations for settlement monitoring


3. A survey of similar structures

       Some structures where similar construction techniques were used are:
      A heavy storage tank capable of storing 15,000 t of liquid ammonia [2]. It is 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. The design and
       construction of the foundation system is very similar to that used in the sugar storage
       tank under consideration in this paper. The ammonia tank has been monitored for pile
       settlements for many years. The measurement techniques were similar to the ones
       used for the sugar storage tank. In this structure, the maximum vertical displacement
       occurred at the central piles, with settlements getting smaller values when moving to
       the periphery of the tank.
      A heavy storage tank capable of storing 40,000 t of sugar at Forlipopoli, Italy [4], [5].
       The circular concrete raft is founded on 396 piles. It is a structure identical to the one
       at Plati. Both horizontal and vertical deformation monitoring of the tank have been
       carried out. The observed vertical displacements obtained maximum values again at
       the central piles of the foundation system. Also, horizontal deformations were
       detected at the lower part of the circular, prestressed concrete wall of the cylindrical
       shell of the tank.


3. Measurement of horizontal deformation

        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. The loading history of the tank is
shown in fig. 3.


                                        60000
                    Sugar Load [tons]




                                        50000
                                        40000
                                        30000
                                        20000
                                        10000
                                            0
                                                0     100     200      300     400      500
                                                             Days from start



                                        Fig. 3. Loading history of the sugar storage tank


        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 (profiles A and B), at the diameter NW-SE, the lower 6 at distance of
3 m from each other and the rest at distance of 6 m. The determination of horizontal
displacements of the superstructure of the tank was very important because, following the
experience gained during the construction and monitoring of the similar sugar tank at
Forlipopoli, a new design for the strengthening of the lower part of the wall of the circular
shell of the tank was used and had to be validated.
        A control network consisting of 6 reference points (pillars) was established around the
tank. The network was remeasured with GPS receivers 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 [4]. A free network adjustment of the complete network (both reference
and control points included) was done for each epoch and 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 (as computed also
by a free network adjustment) with the use of the NetS software package [6]. Then, the
horizontal displacement of each control point was computed as the difference between the
zero measurement coordinates and the corresponding transformed last epoch coordinates.
       The results of these measurements showed no significant horizontal deformation of
the wall of the tank, proving the effectiveness of the new design used.


4. Measurement of vertical deformation

         In order to monitor the vertical deformation of the tank, 16 benchmarks (P1, P2, …,
P16) were installed on the upper free part of certain piles (fig.2). 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 and P11, P12, P13, P14, P15, and P16
located on piles at the external periphery of the slab.
         High precision geodetic levelling was used for the determination of the possible
settlements of the benchmarks with time and load [5]. Three reference elevation points were
established at distances of 50, 100 and 400 m from the tank respectively. The measurements
were carried out with a Topcon automatic level with optical micrometer. In each epoch, the
same levelling network consisting of the reference and the control points was measured. The
network was then adjusted [6]. In this way, the elevation of the control points was computed
for each epoch. The vertical movement of the benchmarks could then be computed by
comparing their elevations between epochs.
         The vertical movements of the pile benchmarks during the measurements can be seen
in fig. 4 (P1, P2, P3, and P4), fig. 5 (P5, P6, P7, P8, P9, and P10) and fig. 6 (P11, P12, P13,
P14, P15, and P16). For a more thorough investigation, settlements of piles P1, P2, …, P16
were also taken into consideration, resulting in a clear deformation of the slab (fig.7).



                                     800
                                     780
                    Elevation [mm]




                                     760                                          P1
                                     740                                          P2
                                     720                                          P3
                                     700                                          P4
                                     680
                                     660
                                           0   37      93    190      353   408
                                                    Days from start



  Fig. 4. Vertical movements of piles P1, P2, P3, and P4 during the first loading of the tank
                                         with sugar


       As it can be seen from the above figures, the vertical displacements of pile
benchmarks generally follow the loading and unloading of the tank. 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.
                                    820
                                    800



                   Elevation [mm]
                                    780                                      P5
                                    760                                      P6
                                    740
                                    720                                      P7
                                    700                                      P8
                                    680
                                                                             P9
                                    660
                                          0   37      93   190   353   408   P10

                                                   Days from start



Fig. 5. Vertical movements of piles P5, P6, P7, P8, P9 and P10 during the first loading of the
                                      tank with sugar



                                    820
                                    800
                   Elevation [mm]




                                    780                                      P11
                                    760                                      P12
                                    740                                      P13
                                    720
                                                                             P14
                                    700
                                                                             P15
                                    680
                                          0   37      93   190   353   408   P16

                                                   Days from start



Fig. 6. Vertical movements of piles P11, P12, P13, P14, P15 and P16 during the first loading
                                   of the tank with sugar


5. Conclusion

       In this paper, the recorded values of the actual deformations of a sugar storage tank
capable of storing up to 50,000 t of sugar were presented, along with the measurement
methods used. From the results of the measurements and the analysis of displacements
following conclusions could be drawn:

   1. 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.
Fig. 7. Slab deformation under maximum load during the first loading of the tank with sugar
                (curved lines are lines of equal vertical displacement, in mm)


  2. 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.
  3. The differential settlements between successive piles, which may be attributed to soil
     profile variability and to small differentiations in pile construction, were kept less
     than several mm.
  4. The observed horizontal displacements of the tank wall were insignificant, proving
     the effectiveness of the design and construction method used.
  5. 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 Thessalonici and the sugar storage tank in Forlipopoli).
References

   1. Badellas A., Savvaidis P. Monitoring of Deformation of Technical Works and Ground
      Landslides with Geodetic Methods. Papageorgiou Publ. Co., 1990, p. 257 + XV,
      Thessaloniki, 1990.
   2. Badellas A., Savvaidis P., Tsotsos S. Settlements Measurement of a Liquid Storage
      Tank Founded on 112 Long Bored Piles. Proc. of 2nd Inter. Symp. on Field
      Measurements in Geomechanics, Kobe, Japan, Balkema Publ., pp. 435-442, 1988.
   3. Poulos H.G. Group Factors for Pile Deflection Estimation. Journal GE, ASCE, Vol.
      105:1489-1509, 1979.
   4. Vosdou S., Michaelidis P., Savvaidis Ch. Measurement of horizontal displacements of
      a Sugar Storage Tank at Plati. Diploma Thesis, Dept. of Civil Engineering, AUTH,
      1997.
   5. Bakavelou D., Panda E., Stefoulis A. Measurement of Settlements of a Sugar Storage
      Tank at Plati. Diploma Thesis, Dept. of Civil Engineering, AUTH, 1997.
   6. Savvaidis P. Program NetS for the Adjustment of Geodetic Networks. Scientific
      Papers from the School of Technology, Department of Civil Engineering, A.U.Th.,
      No. 13, Thessaloniki 1995.




Dr. Eng. Paraskevas D. Savvaidis is Professor in Engineering Surveying and Director of the
Laboratory of Geodesy, Department of Civil Engineering, Aristotle University of
Thessaloniki (AUTH), Univ. Box 465, 54006 Thessaloniki, Greece. Tel. +30 310 995724,
Fax +30 310 996159, e-mail: psav@civil.auth.gr.
Dr. Eng. Ioannis M. Ifadis is Associate Professor in the Laboratory of Geodesy, Department
of Civil Engineering, AUTH, Univ. Box 465, 54006 Thessaloniki, Greece. Tel. +30 310
995745, Fax +30 310 996159, e-mail: ifadis@civil.auth.gr.

				
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