Piping Phenomena at O.C. Dike (Presentation)

7/30/2019 Piping Phenomena at O.C. Dike (Presentation)

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PIPING PHENOMENA ATOSTROVUL CORBULUI DIKE

Case Study

ing. Gaftoi Daniel Andrei

PhD student at TUCEB


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Presentation Content

I. Brief description of the site, of its geology and of the encountered phenomena

II. Considered assumptions causes of the piping phenomena

III. Confirmations of the considered assumptions using a mathematical model

IV. Proposal of corrective measures

V. Confirmation of the efficiency of the solutions using mathematical model


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Site description

Danube

TransformerStation

Pumping

Station Drainage

Channel

Ostrovul Corbului embankment dike :

part of the Iron Gates II hydropower plant

founded on highly permeable and easilytrained alluvial soils

initial project didnt supply a cutoff wall

the excessive water from the protected area is

pumped back into the storage lake

the pumping station is equipped with 2

high capacity pumps

it was placed on a caisson founded on the

bed rock at approximately 18 m depth

pumping water from the channel isintermittent ( 2- 3 rounds / day)

the duration of a pumping round 3 h

during which the water level lowers with

2 - 2.5 m

the water level difference between storage

lake and drainage channel is approx. 5 m


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Site geology

I silty sand layer (depth range : 3.3 m 10.2 m ) Un = 20 - 25

II, III coarse silt layer (depth range : 10.2 m 18.1 m ) Un = 50 - 150

icr = 0.18 0.30

Values of the permeability coefficient are high and very high k = 71 982 m/day

Size analysis for soil layers presented in the foundation of the dike

Hydraulicgradient(i)

Coefficient of uniformity ( Un)

Dangerous zonefor piping

Safe zone


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Aim of the study

to identify the potential causes of the piping phenomena

to identify the possible remedial solutions

Groundwater movement between the retention level and the lower levels created by waterpumping lead to soil structure damage - repeatedly ground subsidence near the

transformer station (observed since 2003)

Repeatedly filling of the hole withselected gravel no influence onstopping the piping phenomena

Affects the PS stability and

functionality


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Considered assumptions

Causes of the piping phenomena :

dam foundation soil that up to 18-20 m depth is composed of irregularsand and gravel, unstable in terms of hydrodynamic picking up

pumping station foundation which represents an obstacle in the flownet of the groundwater flow from the dam to the drainage channel.

no correlation between the pumped flow and the water level from thechannel


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Mathematical model 2D horizontal FEM model

Pumping

stationarea

Danube

Meshed region includes a large zone 2707 elements

The results are not influenced by domain limit boundary conditions

Finer mesh in the pumping station area better results in the interested area

T = 1200 m2/day k = 0.0016 m/sec

saturated volume water content - 20%


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Verify the assumption

Initial boundary conditions :

H = 42.00 maSL in storage lake

H = 37.00 maSL in thedrainage channel

Situation modeled a pumping round (first step before the pumping round)

Value of hydraulic gradient:

i = 0.18 0.20 i icr

lines of constant head

hydraulic gradients and flow vectors


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Verify the assumption

Boundary conditions :



Situation modeled a pumping round (last step after the pumping round)


i = 0.26 i > icr

Geometry of the drainage channel

Cause of the hydraulic gradientsconcentrations


hydraulic gradients and f low vectors


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Corrective measures proposed

1. Changes to the pumping rate - limit the water level offset into thechannel and the lowering speed of the water (1 m in 3 hours)

2. A sheet pile cutoff wall - create a hydraulic barrier to reduce the flowgradients below the critical values

3. The combined effect of the two changes to the pumping rate and asheet pile cutoff wall


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Corrective measures



H = 36.00 maSL in the drainagechannel

Situation modeled changes to the pumping rate 1 m in 3 hours


i = 0.22 i > icr


hydraulic gradients and f low vectors


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Corrective measures





i = 0.15

Situation modeled sheet pile cutoff wall




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Corrective measures





i = 0.1

Safety factor 1.8

Situation modeled sheet pile cutoff wall and changes to the pumping rate




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Conclusions

The paper aims to identify the causes of the subsidence phenomenaobserved at Ostrovul Corbului pumping station and to define theremedial solution.

The analysis on the mathematical model showed that hydrodynamic

picking up of the sand from the dam foundation has 4 principalcauses:

the configuration of the terminal area of the drainage channel the presence of the caisson the presence of easily trained alluvial soils the pumping regime - 2.00 2.50 m in 3 hours

To control the seepage phenomena we proposed :

the development of a sheet pile cutoff wall

changes to the pumping rate


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Thank you for your attention !

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Piping Phenomena at O.C. Dike (Presentation)