piping phenomena at ostrovul corbului dike - yrc 2011

8/19/2019 Piping Phenomena at Ostrovul Corbului Dike - YRC 2011

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SCIENTIFIC JOURNAL OF THE TECHNICAL UNIVERSITY

OF CIVIL ENGINEERING

Mathematical Modelling

in Civil Engineering

BUCHAREST

No. 4 -December- 2011


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CONTENTS

PREFATA

HARMONICS ANALYZER IN LABVIEW ......................................................................... 7

Florin Daniel Anton, Alexandru Mircea Iatan

MESH DEPENDENCE STUDY USING LARGE EDDY SIMULATION

OF A VERY LOW REYNOLDS CROSS-SHAPED JET .................................................. 16

Florin Bode, Ilinca Nastase, Cristiana Croitoru

GEOCOMPOSITE-REINFORCED GRANULAR WORKING PLATFORM -

IN SITU TESTING ................................................................................................................ 23

Natalia Butnarciuc

IMPROVING ENERGY CONSUMPTION AND INDOOR COMFORT OFAN

OLD PRIMARY SCHOOL ................................................................................................... 31

Tiberiu Catalina

CONSTITUTIVE LAWS AND NECESSARY GEOTECHNICAL PARAMETERS

FOR OPTIMUM DESIGN OF DEEP EXCAVATIONS ................................................... 38

Cătălin Că praru

COMPARATIVE STUDY FOR ESTABLISHING THE MOST APROPRIATE

FOUNDATION SOLUTION FOR LOW, FRAMED BUILDINGS ................................. 46

Sorina Constantinescu

THE INFLUENCE OF THE GEOMETRIC FORM OF THE VIRTUAL THERMAL

MANIKIN ON CONVECTIVE FLOW ............................................................................... 55

Cristiana Croitoru, Ilinca Nastase, Florin Bode

NONLINEAR MODELLING FEATURES OF REINFORCED CONCRETE SHEAR

WALLS.................................................................................................................................... 65

Ionut Damian

COMMAND AND CONTROL OF SHAPE MEMORY ALLOYS USED IN

RETROFITTED BUILDINGS ............................................................................................. 75

Radu Calin Donca, Mihai Petru Draghici

PIPING PHENOMENA AT OSTROVUL CORBULUI DIKE ......................................... 81

Daniel Andrei Gaftoi

THE INTRODUCTION OF GNSS METEOROLOGY IN ROMANIA –MONITORING

2D&3D WATER VAPOR DISTRIBUTION VIA GNSS ................................................... 88

Raluca Ianoschi, Alexandru Lepădatu

NUMERICAL MODELLING FOR IN-SEWER BED PROFILES .................................. 94

Elena Iiatan, Alexandru Iatan

SEISMIC RESPONSE OF DUAL STEEL ECCENTRICALLY BRACED

FRAMES WITH REMOVABLE LINKS .......................................................................... 101

Adriana Ioan

BEHAVIOUR OF REINFORCED CONCRETE COLUMNS UNDER NUMERICAL

VALIDATION OF A FREE-CONVECTIVE CELL SUBJECTED

TO AN AIR FLOW .............................................................................................................. 111

Claudia-Florentina Iorgoiu, R ăzvan-Silviu Ştefan


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Mathematical Modelling in Civil Engineering, no.4 - 2011 81

PIPING PHENOMENA AT OSTROVUL CORBULUI DIKE

DANIEL ANDREI GAFTOI - PhD Student, Technical University of Civil Engineering, Faculty ofHydrotechnics, e-mail: [email protected]

Abstract: Iron Gates II hydropower plant includes, for the protection of Ostrovul Corbului area,

an embankment dam founded on highly permeable and easily trained alluvial soils. The initial protection dike project did not supply a cutoff wall and the solution adopted to maintain acertain water level within the enclosure area was to pump the excessive water over the dike andout in the lake. Groundwater movement between the retention arrangement and lower levelscreated by pumped water lead to damages brought to the soil structure. Such a phenomenon isreported near the pumping station, which is emphasized by changes in water levels after thedrainage process. This study aims to identify the potential causes of the piping phenomenawhich have appeared at Ostrovul Corbului dike and the possible remedial solutions using the

available data and mathematical modeling. To confirm the assumptions regarding the pipingcauses and to establish the appropriate solutions, a transient 2D model was realized using thefinite elements method. Following the mathematical modeling some of the assumptions wereconfirmed and others were determined. To remedy the situation, two complementary solutionshave been proposed: interventions on the pumps in order to be able to adjust the flow and to

realize a cutoff wall which modifies the flow so that the current lines move away from thesensitive area near the pumping station.

Keywords: seepage analysis, finite element method, cutoff wall, embankment dike

1. Introduction

An increasing part of the hydro development in the world faces seepage phenomena through

the embankment or through the foundation. This phenomenon accompanied by hydrodynamic

picking up of the fine material is threatening dam stability and may cause unforeseen failure.

The ground conditions and the geological features of the dam site influence greatly the

amount of seepage and its relevant effects.

The seepage problem (prevention, determination of the causes and remedial solutions) has

been the main subject for many researchers in recent years. For example, Uromeihy and

Barsegari [1] evaluated the seepage problems at the Ghapar-Abad Dam and selected the

proper method for water-proofing prior to construction, Jin-Yong Lee et al. [2] investigated

the possible seepage paths and potential damaged areas within the Unmun Dam – a rockfill

dam with clay core from Korea, Stematiu and Teodorescu [3] studied the causes and proposed

the remedial solutions for the significant damages that occurred at Bilciuresti Dam, Ping Li et

al. [4] modeled seepage through fractured rock mass.

This study aims to identify the potential causes of the piping phenomena that have appeared at

Ostrovul Corbului dike and the possible remedial solutions using the available data andmathematical modeling.

2. Site Description

Iron Gates II hydropower plant includes, on the left side of the Danube, a 7 km long

embankment dike. The dike was executed by blocking the Old Danube branch at both ends in

order to protect the Ostrovul Corbului area (fig. 1). It was founded on highly permeable and

easily trained alluvial soils. The initial project of the dike did not supply a cutoff wall and the

adopted solution to maintain a certain water level into the protected area was to pump the

excessive water back into the lake. As part of this solution, the former Old Danube riverbed -

which drains water from adjacent terraces and from seepage – was designed as a drainage


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82 Mathematical Modelling in Civil Engineering, no. 4/2011

channel. A pumping station with 2 high capacity pumps (4 m3/s each) was built for pumping

the water over the dam.

Fig. 1 – Plan view of the Ostrovul Corbului dike

The pumping station was placed on a caisson founded on the bed rock at the base of the

coarse silt (sand and gravel) at approx. 18 – 20 m depth. The water access from the channel

into caisson is made through gaps within its wall.

The ground level for the pumping station area is about 4.00 m below the water level of the

storage lake (42.00 maSL) and the water level from the drainage channel is 1.00 m below the

ground level (fig. 2). Pumping water from the canal is intermittent – 2-3 rounds of pumping a

day. The total duration of a pumping round consists in about 3 – 4 hours during which thewater level from the channel lowers with 2.00 – 2.50 m. The pumping round is followed by a

break, during which the water level returns to its maximum level.

Fig. 2 – Plan view of the pumping station area

Dike

Drainage

Channel

FT and FATest holes

PumpingStation

TransformerStation

Danube


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The groundwater movement between the retention level and the lower levels created by water

pumping lead to soil structure damages. Such a phenomenon is reported near the pumping

station, which is exacerbated by changes in the water level after the drainage procedure.

Since the summer of 2003 repeatedly ground subsidence was observed near the pumping

station caisson - transformer station site. In order to bring the land field to its original level,

the hole was filled repeatedly with selected gravel but this measure has no influence on

stopping the piping phenomena. The ground subsidence continued over time, so now thetransformer station is affected.

Because these phenomena can be a real threat to the stability and the functionality of the

transformer station and the pumping station, including the risk of taking out of service the

pumping station and flooding of the Ostrovul Corbului area, the causes had to be identified

and corrective actions had to be established.

3. Site Geology

In order to determine the geologic characteristics of the dam site we used the existing

documentation realized by GEOTEC and INCERC in 1996 – 1997 and 2 new test holes (FTand FA represented in fig. 2). Based on the analysis and interpretation of the information

obtained from documentary sources and additional drilling, it has been found that:

- The route dam foundation soil consists of a discontinuous layer of silty – sandy earth,

with horizontally passes to silty – sandy clays. The thickness of this layer varies from

0 to 7-8 m.

- The lithological profile at the pumping station site is characterized by the following

lithological succession: from 0.00 to 3.30 m there is a layer of gravel and sand filling,

followed by a layer of gray silty sand to a depth of 10.20 m. Next, to a depth of 18.10

m only coarse sediments - gravel with boulders and gray-brown sand was found. Note

that this lithology corresponds to studies performed during the 1996 – 1997 period.

- The FT drilling positioned next to the transformer station is characterized by a layer

made of heterogeneous fillers (compensation settlements) followed by purple

micaceous sands and gravel with boulders. Lithological profile of the FA drilling

positioned near the antenna is characterized by a filling layer made of sand with

gravel. The lack of the “sand” fraction in the FT drilling highlights the hydrodynamic

effect involvement in this area with large gradients.

- The size analysis (fig. 3) showed that the silty sand present in the range of depth from

3.30 m to 10.20 m (noted with I in the figure) is characterized by a coefficient of

uniformity Un = 20 - 25 and for the coarse silt layer (depth interval from 10.20 to

18.10 m), noted with II and III in the picture, the coefficient of uniformity is Un = 50 -

150. According to the value of the coefficient of uniformity, the sands from the dam

foundation are classified as irregular.

- Hydro geological investigations carried out have shown that the values of the

permeability coefficient are high and very high, reaching hundreds of m/day and at the

pumping station site varies between 71 m/day and 982 m/day.

- According to the values of the coefficient of uniformity (Un = 20 - 150), based on the

ISTOMINA diagram (fig. 4), it results that the critic gradient for the sands from dam

foundation is greater than 0.18 – 0.30.


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Fig. 3 – Size analysis for the silty sand and for the coarse silt layer

Fig. 4 – ISTOMINA diagram

4. Causes of Piping Phenomena

4.1. Considered Assumptions

Based on the existing data, it is estimated that the piping phenomena of the sand from the

pumping station area has the following causes:

- Dam foundation soil that up to 18-20 m depth is composed of irregular sand and gravel,

unstable in terms of hydrodynamic picking up,

- Pumping station caisson which represents an obstacle in the flow net of the groundwater

flow from the dam to the drainage channel. This is materialized through the diversion andconcentration of flow lines to the sides of the pumping station caisson,

- Water pumping from the drainage channel into the storage lake takes place as a transient

flow with 2 high capacity pumps so there is no correlation between the pumped flow and

the water level from the channel, which favors hydrodynamic picking up.

4.2. Mathematical Model

To confirm the assumptions on the causes of the phenomenon, a transient 2D horizontal

model was realized using the finite element method. The meshed region includes a large zone

in order not to influence the results by domain limit boundary conditions. The mesh is finer in

the pumping station area and near its surroundings in order to have better results in the

interested area (fig. 5).

Coefficient of uniformity ( Un)

Safe zone

Dangerous zonefor piping

H y d r a u l i c g r a d i e n t ( i

)


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Fig 5 – Finite element model

The model was realized with Seep/w and the mesh had 2707 elements. The transmissivity

coefficient adopted for the permeability zone was 1200 m2/day, which means a permeability

coefficient equal to 0.0016 m/sec and a saturated volume water content of 20%.

4.3. Simulation to Verify the Assumption

To highlight the increase in hydraulic gradients induced by pumping, first the situation before

the pumps were starting was modeled – high water level in drainage channel, and then the

situation after the pumping round was modeled – low water level in drainage channel. The

initial head boundary conditions adopted were:

- H = 42.00 maSL in the storage lake;

- H = 37.00 maSL in the drainage channel.

Fig 6 and fig 7 illustrate the lines of constant head (left side), the hydraulic gradients and the

flow vectors (right side) for the analyzed situations. It has been discovered that geometrical

features of the drainage channel cause concentrations of large gradients when seepage water

exits within the channel. The barrier created by the pumping station caisson has a second role.These results explain why the ground subsidence appeared close to the transformer station and

near the end of the drainage channel.

Fig. 6 – Model results before pumping round (37.00 maSL - channel water level)

Fig. 7 – Model results after a pumping round (35.00 maSL - channel water level)

SP

3 7

3 7

. 5

3 8

3 8. 5

3 9

SP

0

. 0 6

0 .0 6

0. 0 8

0.08

0. 1 0

. 1

0 .12

0 . 1 8

SP

3 5

3 5

. 5

3 6

3 6. 5

3 7

3 7. 5

3 8

SP

0

. 0 8

0. 1

0 . 1

0 .12

0 . 1 2

0 . 1

4

0.14

0 .1 8

0 . 2 6

Drainage

Channel

Danube

Pumping station

area


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5. Corrective Surgery

Two types of interventions have been proposed as corrective surgery:

- Changes to the pumps so as to create the possibility of adjusting the flow in order to limit

the water level offset into the channel and the lowering speed of the water.

- Create a sheet pile cutoff wall on an alignment parallel to the canal bank and then rotated

to the limit of the drainage channel, behind the pumping station, in order to create a

hydraulic barrier to reduce the flow gradients below the critical values.

The effect of changing the pumping regime was highlighted by analyzing a transient model –

controlled pumping, which leads to lowering water levels from the drainage channel from

37.00 maSL to 36.00 maSL in 3 hours. After analyzing the results, it was found that this

intervention by itself does not change the seepage regime. The difference between the actual

pumping regime and the proposed one is insignificant – the value of the hydraulic gradient

decreases from 0.26 to 0.22 but the risk of hydrodynamic picking up still remains.

The cutoff wall leads to radical changes within the infiltration flow net. First we determine the

effect of the cutoff wall in case the current pumping scheme remains unchanged and fig. 8

represents the constant head lines (left side), the hydraulic gradients and the flow vectors(right side) for this analysis. It is noticed that the values of the hydraulic gradient are a lot

smaller than the actual conditions - without the cutoff wall. At the end of the pumping

process, the hydraulic gradient in the critical area is 0.15, about half that of current situation.

Certain levels of concentration of gradient lines appear at the extremities of the cutoff wall but

they are not dangerous since the flow path to the drainage channel is long and the gradients

decrease significantly. In the second phase we determine the combined effect of the

installation of the cutoff wall and the change of pumping regime (lowering the water level

from the drainage channel from 37.00 maSL to 36.00 maSL in 3 hours). In fig. 9 we present

the same representations for this analysis as in previous cases and it can be observed that this

corrective measure leads to stopping the hydrodynamic picking up. The maximum value for

the gradient is 0.1, providing a safety factor of about 1.8 to the critical hydraulic gradient.

Fig. 8 – The effect of the cutoff wall without changing the pumping regime

Fig. 9 – The combined effect of the cutoff wall and the change of pumping regime

SP

3 5

3 5

3 5

. 5

3 5. 5

3 6

3 6

3 6. 5

3 6

. 5

3 7

3 7

3 7. 5

3 8

SP 0 .0 5

0 . 0 5

0 . 1

0 . 1

0. 1 5

0. 1 5

0. 1 5

SP

3 6 . 5

3 6. 5

3 7

3 7

3 7. 5

3 7. 5

3 8

3 8

3 8. 5

3 9

SP 0 . 0 5

0 . 0 5

0 . 1

0. 1

0 . 1

0.15


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6. Conclusions

This paper aims to identify the causes of the subsidence phenomena observed near the

transformer station of the Ostrovul Corbului drain system and to define the remedial solution.

From the available hydrogeological and geological studies it appears that the coarse silt presented

in the dam foundation is characterized by high and very high values for the permeability

coefficient, reaching hundreds of m/day. According the values of the coefficient of uniformityresults that the critic gradient for the sands from the dam foundation is 0.18 – 0.30.

The analysis on the mathematical model showed that hydrodynamic picking up of the sand

from the dam foundation has 4 principal causes:

- the configuration of the terminal area of the drainage channel which ends close to the

transformer station and forms a concentration area of flow lines and gradients;

- the presence of the caisson which represents a barrier for the groundwater movement;

- the presence of easily trained alluvial soils;

-

the pumping regime characterized by an important oscillation of the channel waterlevel (2.00 – 2.50 m) in a very short time (3 hours) which favors the hydrodynamic

picking up of the dam foundation materials.

To control the seepage phenomena, which may jeopardize the integrity of the pumping

station, are proposed : the development of a sheet pile cutoff wall on an alignment parallel to

the canal bank and then rotated to the limit of the drainage channel, behind the pumping

station, in order to create a hydraulic barrier to reduce the flow gradients below the critical

values and changes to the pumps so as to create the possibility of adjusting the flow in order

to limit the water level offset into the channel and to lower the speed of the water.

A numerical simulation on a 2D horizontal model showed that the cutoff wall has the

maximum effect in stopping hydrodynamic picking up of the sand from the foundation. Thecombined solution – installation of the cutoff wall and change of pumping regime – is

recommended.

Acknowledgements

Thanks are expressed to the Institute of Hydroelectric Studies and Design for the opportunity

provided to the author to work on this project.

References

[1]

Uromeihy, A., Barzegari, G. – Evaluation and treatment of seepage problems at Chapar-Abad Dam, Iran,Science Direct, in Engineering Geology, No 91, 2007, pp. 219-228[2] Jin-Yong, L., Hyoung-Soo, K., Yea-Kwon, C., Jeong-Woo, K., Jeong-Yong, C., Myeong-Jae, Y. –

Sequential tracer tests for determining water seepage paths in a large rockfill dam, Nakdong River basin,Korea, Science Direct, in Engineering Geology, No. 89, 2007, pp. 300-315

[3] Stematiu, D., Teodorescu, D. – The damage of Bilciuresti diversion dam, in Hidrotehnica, Vol 51, 2006, pp.

12 - 22[4] Ping, L., Wenxi, L., Yuqiao, L., Zhongping, Y., Jun, L. – Seepage analysis in a fractured rock mass: The

upper reservoir of Pushihe pumped-storage power station in China, Science Direct, in Engineering Geology, No 97, 2008, pp. 53-62

[5] Krahn, J. – Seepage modeling with SEEP/W, User Guide, 2004

piping phenomena at ostrovul corbului dike - yrc 2011

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