1T N G 15 0002 B Dewatering 3D Model

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Dewatering model

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    List of Contents

    1. Introduction ........................................................................................................................................................... 5

    2. References ............................................................................................................................................................ 6

    2.1 Tractebel Engineering/Coyne et Bellier design documents .................................................................................... 6

    2.2 EFNAB JV technical offer...................................................................................................................................... 6

    2.3 Other documents .................................................................................................................................................. 6

    3. Current geometry and Project details .................................................................................................................. 7

    3.1 Project Context ..................................................................................................................................................... 7

    3.2 The cofferdam ...................................................................................................................................................... 8

    3.3 Diaphragm walls geometry .................................................................................................................................... 8

    3.4 Dewatering and excavation stages ...................................................................................................................... 11

    4. Data inventory and hypothesis ........................................................................................................................... 12

    4.1 Ground surface elevation .................................................................................................................................... 12

    4.2 Geotechnical model ............................................................................................................................................ 12

    4.3 Water levels ........................................................................................................................................................ 15

    5. Modelling approach ............................................................................................................................................ 16

    5.1 Introduction......................................................................................................................................................... 16

    5.2 Modelling stages: ................................................................................................................................................ 16

    5.3 3D modelling ...................................................................................................................................................... 17

    5.3.1 Description of the model ................................................................................................................................... 17

    5.3.1.1 The model development ................................................................................................................................... 17

    5.3.1.2 The mesh of the different models...................................................................................................................... 20

    5.3.1.3 Boundary conditions ......................................................................................................................................... 20

    5.3.2 Model Validation by the initial state ................................................................................................................... 21

    5.3.3 Conclusion ....................................................................................................................................................... 24

    6. Results in steady state flow for construction stages ........................................................................................ 25

    6.1 Net flow .............................................................................................................................................................. 25

    6.2 Modelled pumping system description ................................................................................................................. 26

    6.3 Pumping rates .................................................................................................................................................... 29

    6.4 Influence of the project on the closest vicinity ...................................................................................................... 29

    7. Sensitivity assessment of the 3D modelling ...................................................................................................... 318. General conclusion ............................................................................................................................................. 32

    Appendix ....................................................................................................................................................................... 33

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    Attached documents

    List of figures Figure 1 : Project location .................................................................................................................................................. 7

    Figure 2 : Temporary cofferdam geometry ......................................................................................................................... 8

    Figure 3: Location of the diaphragm walls.......................................................................................................................... 9

    Figure 4 : Temporary slurry wall ........................................................................................................................................ 9

    Figure 5 : Power house diaphragm walls ......................................................................................................................... 10

    Figure 6 : Sluice way diaphragm walls ............................................................................................................................. 10

    Figure 7 : Permanent cut-off wall below the sluice way and powerhouse ......................................................................... 11

    Figure 8 : Geotechnical section ....................................................................................................................................... 13

    Figure 9 : EFNABJV suggestion for Deep Wells location ................................................................................................. 14

    Figure 10 : 3Dimensional model extent ............................................................................................................................ 17

    Figure 11 : 3D visualization of the project area ................................................................................................................ 18

    Figure 12 : Location of the waterproof organs .................................................................................................................. 19

    Figure 13 : Mesh Project stage 4 ..................................................................................................................................... 20

    Figure 14 : Fixed head cells limit conditions .................................................................................................................. 21

    Figure 15: Boundary conditions applied for the new 3D model ......................................................................................... 22

    Figure 16 : Initial state - calculated heads in longitudinal section (section location on fig 15) ............................................ 23

    Figure 17 : Initial state - calculated heads in transversal section (section location on fig 15) ............................................. 23

    Figure 18 : 3D view of Hydraulic head - Initial state ......................................................................................................... 24

    Figure 19: 3D view of flowlines - stage4 .......................................................................................................................... 25

    Figure 20 : stage 2 dewatering layout .............................................................................................................................. 26

    Figure 21 : stage 3 dewatering layout .............................................................................................................................. 27

    Figure 22 : Stage 4 dewatering layout ............................................................................................................................. 28

    Figure 23 : 3D model - hydraulic heads - Initail stage Vs construction stage 4 (optimized number of wells) ...................... 30

    List of tables Table 1: Soils permeability coefficients ............................................................................................................................ 13

    Table 2 : Geotechnical works characteristics ................................................................................................................... 13

    Table 3 : Piezometric data ............................................................................................................................................... 22

    Table 4 : 3D model Assessed flow................................................................................................................................... 29

    Table 5: Permeability assessment extracted from Ref.[1]................................................................................................. 31

    Table 6: Parametric study results .................................................................................................................................... 31

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    1. INTRODUCTION

    The following document aims at assessing the seepage flow to be pumped for the construction of the new Assiut barrage and hydropower plant. The project is located approximately at 450 m downstream of the existing Assiut barrage.

    The solution considered for construction works within Nile riverbed requires to set up a temporary closed cofferdam (ring dam) in order to carry out construction works in a dry pit.

    Considering the geometry of the project and the important issue of dewatering, it has been decided to perform a 3 Dimensional model using finite difference software MODFLOW.

    According to the construction pit dewatering design report, the top of the temporary cofferdam varies from +49.0 m.a.s.l. A temporary slurry wall will be set up within the cofferdam to reduce the seepage into the temporary excavation pit.

    The different excavation stages are expected to be achieved by using deep wells for groundwater lowering.

    A previous axisymmetric model has been carried out by EFNABJV and was based on a former geometry of the cofferdam. Some modifications of the cofferdam geometry have been made since the development of previous model. This modification of geometry resulted in a reduction of the temporary cofferdam. This new element has been integrated to this model.

    The first purpose of this calculation note is to verify in what extent the pumping system and the different measures foreseen can be used during the construction stage: new geometry of the cofferdam, integration of the environment of the project (banks of the river and existing Assiut barrage). In order to do so, steady state simulations have been performed to assess the global flows required to achieve the different excavation stages.

    Then the second goal to reach is to assess the impact of such work on the closer vicinity: influence of the dewatering on the water table compared with the initial stage.

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    2. REFERENCES

    2.1 Tractebel Engineering/Coyne et Bellier design documents

    [1] Geotechnical data - Soil Permeability interpretation - Tractebel engineering Coyne & Bellier -1T-N-G-00-0005-B;

    2.2 EFNAB JV technical offer

    [2] Preliminary designs, geotechnical evaluation , group I documents 12.1 Technical offer, April 2011;

    [3] Preliminary designs, numerical model for the temporary phase , group I document 12.4 technical offer, April 2011;

    [4] Construction pit dewatering, Design report , calculation note referenced HC-07-CNO-0P-001-1 rev0, March 2011;

    [5] Review of existing studies , draft note on hydraulic models referenced CB-01-ENO-00-001-0 rev0, January 2011;

    [6] Dewatering calculations sensitivity analysis and impact , engineering note referenced VT-03-ENO-0G-002-0, January 2011;

    [7] New Assiut barrage Groundwater Model Final report ABDC December 2010;

    [8] Dewatering scheme deep wells schedule, scheduled plan for the deep wells locations 1JDH-15-10002-A - EFNABJV - October 2012;

    [9] Preliminary guide drawings 03 - Power house Sections A-A & B-B 1JDK-03-0130-A - EFNABJV - July 2012;

    [10] Preliminary guide drawings 03 - Power house Sections 1-1 & 2-2 1JDK-03-0140-A - EFNABJV - July 2012;

    [11] Preliminary guide drawings 04 Sluiceway Sections A-A & B-B 1JDK-04-0130-A - EFNABJV - July 2012;

    [12] Construction pit dewatering Scheduled plan for dewatering stages Tender documents - EFNABJV - March 2011;

    2.3 Other documents

    [13] Processing Modflow Pro version 7.0 notice document.

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    3. CURRENT GEOMETRY AND PROJECT DETAILS

    3.1 Project Context

    The project is located on the Nile River south of Bani Murr Island in the city of Assiut. The old Assiut Barrage, made of masonry, is located 450 m upstream of the project site.

    In the vicinity of the project, the infrastructures are composed of build-up areas, agricultural lands and hydraulic structures (irrigation canals, piezometer locations, the existing Assiut Barrage, siphons). This kind of infrastructures could be influenced in term of settlements if the project has an important impact on the water table.

    Figure 1 : Project location

    The New Assiut Barrage & Hydropower Plant construction involves different geotechnical elements. Parts of them are temporary or permanent. A temporary cofferdam made of sand fill and protected by riprap, is used to delineate a dry area where construction works will be performed. To ease up the dewatering process, a temporary slurry wall is entrenched within this cofferdam.

    Then, three other permanent diaphragm-walls are part of the structure of the hydropower plant. Each lateral sluiceway and the hydropower pits are achieved between diaphragm walls. The last geotechnical element is the permanent cut off wall that is used to reduce hydraulic gradient. The details of these structural elements are described in the following section.

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    3.2 The cofferdam

    The current geometry of the cofferdam corresponds to the following plan:

    Figure 2 : Temporary cofferdam geometry

    This drawing shows that the surrounding ring dam is no longer attached to the Bani Mur Island. The current design, in green continuous line, downsizes the pit surface by the North face of the dam compared with the initial design in red dotted line. The North side of the ring dam is now straight and closer to the powerhouse of about 50.0 m.

    3.3 Diaphragm walls geometry

    The different geotechnical elements introduced previously described in this section are the following:

    - The temporary slurry wall; - The permanent sluiceway diaphragm walls; - The permanent hydropower plant diaphragm walls; - The permanent cut off wall.

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    A temporary slurry wall set up in the surrounding ring dam is present to reduce the incoming water seepage. This temporary slurry wall is done till after the ring dam constructions. Its bottom reaches +10.0 m.a.s.l.

    Figure 4 : Temporary slurry wall

    Figure 3: Location of the diaphragm walls

    Sluiceway pit

    Sluiceway pit

    Powerhouse pit

    Permanent sluiceway diaphragm walls

    Permanent powerhouse plant diaphragm walls

    Permanent powerhouse diaphragm walls

    Permanent sluiceway diaphragm walls

    Temporary slurry wall

    Cut off wall

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    Permanent diaphragm walls of the two sluiceways and of the powerhouse are realized when the excavation level reaches +38.0 m.a.s.l. Those structures will also be used as permanent deep foundations for the future hydropower plant.

    Nota: The thicknesses of the structural elements will be confirmed at later stage.

    Figure 5 : Power house diaphragm walls

    Figure 6 : Sluice way diaphragm walls

    The last diaphragm wall of the project is the permanent cut-off wall of the barrage. It is foreseen all across the river that is roughly 820 m long. A part of this wall is built within the temporary cofferdam from the excavation level +38.0 m.a.s.l.

    0.8 m

    11.0 m

    33.0 m.a.s.l

    26.5 m.a.s.l 26.5 m.a.s.l

    38.0 m.a.s.l

    0.8 m

    Reinforced concrete Diaphragm Walls

    16.8 m

    24.7 m.a.s.l

    17.5 m.a.s.l 17.5 m.a.s.l

    38.0 m.a.s.l

    0.8 m 0.8

    Reinforced concrete Diaphragm Walls

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    Figure 7 : Permanent cut-off wall below the sluice way and powerhouse

    3.4 Dewatering and excavation stages

    The proposed dewatering system is composed of sumps, trenches and wells. It is adapted and structured according to the different excavation stages performed during the project:

    - Construction stage 1: Construction of the sandy gravel fill ring dam.

    - Construction stage 2: Construction of the temporary slurry wall within the cofferdam.

    - Construction stage 3: The first excavation work is executed dredging the riverbed bottom. The excavation level reached is located at +39.0 m.a.s.l.

    - Construction stage 4: Water level is lowered down within the cofferdam area using a floating pumping system. Once the riverbed is reached some deep wells are added on the emerged platform within the cofferdam to lower the water table 1.0m below the dredge platform that is +38.0m.a.s.l.

    - Construction stage 5: Construction of the permanent sluiceway D-wall, powerhouse D-wall and of the permanent cut-off wall described in section 3.3 of this document.

    - Construction stage 6: Water table lowering in the sluiceway pit and in the powerhouse pit area. The targeted water table levels are respectively :

    +32.0 m.a.s.l for the two sluiceways pits; +26.5 m.a.s.l for the first powerhouse platform.

    - Construction stage 7: Performing of the excavation work for the two sluiceways and the central

    hydropower house. The targeted excavation levels of this stage are respectively +33.0 m.a.s.l(lowest elevation for the sluiceway slab) for the two sluiceways pits and +27.15 m.a.s.l for the powerhouse pit(lowest elevation for downstream powerhouse slab, beyond axis C-C).

    - Construction stage 9: Last pumping phase within the powerhouse chamber. The targeted level is +22.4 m.a.s.l.

    - Construction stage 9: Last excavation works within the powerhouse pit. A second platform is achieved at the level +24.70 m.a.s.l (lowest elevation of the upstream powerhouse slab) from where the central pit is carried out. The targeted excavation level is +22.9 m.a.s.l in the central powerhouse pit (lowest elevation of the underside slab in the central pit).

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    The excavation levels given in the former description are extracted from the guide drawings Ref.[9] to [11].

    4. DATA INVENTORY AND HYPOTHESIS

    4.1 Ground surface elevation

    The site topography is rather flat. As a consequence, outside of the riverbed, on the banks of the Nile, the elevation of the ground surface is assumed to be constant: roughly +51.0 m.a.s.l.

    The bathymetry of the riverbed has been generated from the data used to develop the hydraulic physical model. According to that information, the bathymetry varies between +39.5 m.a.s.l at the south of the ring dam and +44.0 m.a.s.l just before the existing barrage due to an accumulation of sediments.

    Regarding the elevation within the working area, the slopes and shapes of the excavations come from the tender documents (see Ref.[12]). Those data have been updated to take into account the last excavation levels known (see Ref.[9] to [11]).

    4.2 Geotechnical model

    The geology of the model has been assessed from the different investigation campaigns performed on site. The total amount of geological data comprises the geotechnical investigation performed from 1982 to 2012.

    According to the past investigations, the boreholes carried out till the depth of 50 m below the ground level or riverbed revealed that the soil was more or less made of silty sand or gravely sand.

    During the previous geotechnical investigations and even during the current investigations, the base of this aquifer has not been reached. Nevertheless for this study, we have assumed that the depth of the main aquifer is -200.0 m.a.s.l. A sensitivity study carried out during the tender stage revealed that considering a deeper substratum and lowering the base of the diaphragm walls had low impact on the calculations as long as the deepest excavation level is set at +22.9 m.a.s.l.

    The values of permeability used in this study are the ones defined in the Ref. [1]. These values have been updated from the results of the pumping tests, permeability tests and other geotechnical tests. In this context of fluvial deposits, the coefficient of permeability is ordinarily maximum in the direction of the stratification and minimum in the direction normal to that of the stratification. The ratio between horizontal and vertical permeability is set at 5 (mean value assumed in the Ref. [1]).

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    The figure hereafter shows the geotechnical section applied to the different models

    The adopted values of soil permeability coefficient and anisotropy are extracted from the geotechnical report Ref. [1].

    Soil Base level of the

    formation

    m.a.s.l

    Horizontal Permeability

    coefficient kh(m/s)

    Anisotropy Kh/kv

    U material +42.0 10-3

    5

    Silty sand +40.0 10-5

    Fine sand +30 4.2*10-4

    Gravely sand +0.0 7*10-4

    Fine to medium sand -200.0 2.1*10-4

    Table 1: Soils permeability coefficients

    - Geotechnical data The following table presents the geometrical characteristics of the geotechnical works set up for the construction:

    Geotechnical organ Top Bottom Thickness

    Powerhouse D-walls +38.0 m.a.s.l +17.0 m.a.s.l 0.80 m

    Sluiceway D-walls +38.0 m.a.s.l +26.5 m.a.s.l 0.80 m

    Cut-off wall +38.0 m.a.s.l +10.0 m.a.s.l 0.80 m

    Temporary cofferdam slurry wall

    +49.5 m.a.s.l +10.0 m.a.s.l 0.80 m

    Table 2 : Geotechnical works characteristics

    1-Silty sand

    U material cofferdam material

    +47.6 m.a.s.l +49.5 m.a.s.l

    +42.0 m.a.s.l

    -200.0 m.a.s.l

    4-Fine to medium sand

    +0.0 m.a.s.l

    +30.0 m.a.s.l

    +40.0 m.a.s.l 2-Fine sand

    3-Gravely sand

    Figure 8 : Geotechnical section

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    The concrete permeability used for modelling the diaphragm walls is assumed to be equal to 1.0x10-8 m/s. From this permeability an equivalent is calculated to take into account the size of the cells for the 3D model. The detail of this calculation is shown in appendix 1.

    - Assessed dewatering system

    Dewatering stage 1: The dewatering system proposed here is composed of floating pumps and deep wells to lower down the water level within the cofferdam.

    Dewatering stage 2: After a first excavation to the level +39.0 m.a.s.l, a system of relief wells is activated to carry out the diaphragm walls avoiding the gradient effect. The relief wells are vertical pits filled with a high permeability material. The dewatering of this stage is performed using part of the deep wells described in the following stage.

    The relief wells and their impact on the dewatering are not taken into account in this calculation note.

    Dewatering stage 3: Then the dewatering of the third excavation stages: first powerhouse platform and sluiceways pits, is fulfilled by deep wells. The location of the wells and the depth of the bottom come from EFNABJV layout for deep well location (see ref.[8]). The details are shown on the following figure.

    Figure 9 : EFNABJV suggestion for Deep Wells location

    The imposed flow on each well is the same: 120 m3/h. The total amount of the dewatering system is composed of 100 deep wells: 46 whose bottoms are located at +5.0 m.a.s.l (well groups W-A ; W-B ; W-C; W-H ;W-I ; W-J) and 54 whose bottoms are located at +10.0 m.a.s.l (well groups W-D ;W-E ; W-F ;W-G). Nevertheless only 65% of the system is supposed to be used to achieve the deep excavation works. The 35% remaining devices are foreseen as an additional safety.

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    Dewatering stage 4: The dewatering system proposed for the last excavation stage of the deep powerhouse platform and the central pit requires additional 5.0m bottom deep wells in comparison to the previous stage.

    In this calculation note an optimization of the foreseen layout is proposed. On the basis of the calculation results we got thanks to the proposed well layout, the goals of this optimization are the following:

    - To set up the deep wells outside of the working area that are still in progress; - To set up the deep wells on the platform and not in the slopes foreseen to reach the excavation levels; - To make the dewatering fit as well as possible with the excavation performed; - To limit the gradient effect in soils on the vicinity of the different diaphragm walls performed.

    4.3 Water levels

    According to the tender documents the Nile water level during construction is set at its maximum value: +47.6 m.a.s.l corresponding to a discharge of 2,200 m3/s.

    The model developed here required information about water levels upstream of the existing Assiut barrage. The final report on the groundwater model for the New Assiut Barrage (December 2010) developed by ABDC, shows in its APPENDIX K-7 a local Assiut focused view of the groundwater levels where the water levels of the Nile upstream of the existing Assiut barrage is roughly +49.5 m.a.s.l. This value and the general groundwater flow shape will be taken as reference to build the initial condition of the model.

    Still upstream of the existing barrage, the Ibrahimia Canal used for irrigation is also present at the west side of the project. Its water level is defined at +49.5 m.a.s.l because it is regulated by the existing Assiut barrage according to the groundwater model report for the New Assiut Barrage (December 2010) developed by ABDC ref [7].

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    5. MODELLING APPROACH

    5.1 Introduction

    The dewatering calculations are performed using finite difference based software named Processing MODFLOW Pro developed by USGS (U.S. Geological Survey). A 3 dimensional model is used to represent the construction pit. The post-treatment of the results has been performed on the software 3D Master.

    The finite difference method aims at discretizing a continuous middle in plane-parallel volumes. The whole cells gathered generate an orthogonal mesh made up of columns, rows and layers. In a same line or row, the thickness is constant but the height of the cells of a same layer can be modified. This last aspect will be used to generate the topography of the site and to integrate the geological formations.

    In the model construction, the following information must be provided for each cell:

    - Geometrical data: height, length and width, - Value of horizontal permeability coefficient, - Value of vertical permeability coefficient.

    Three types of cells are defined in the model: inactive cells where no flow is allowed, fixed head cells where an initial and constant head value is imposed and the calculated cells where the seepage equations are solved by the finite difference method.

    Using the different elements described previously, a complete three dimension numerical model has been performed. It allows assessing the seepage rates required to be pumped in order to have dry areas during different excavation stages. The major goal of this model is to integrate the current geometry of the temporary ring dam, the different diaphragm walls introduced in part 4.2 and assess the global seepage values required to achieve dry excavation works.

    5.2 Modelling stages:

    The modelled stages represent the major excavation work achieved during the construction. These stages are described in the following section:

    - Modelling stage 0: Scaling the model with piezometric data

    - Modelling stage 1: After the establishment of the cofferdam, the slurry wall and the dredged platform, the construction pit is dewatered by floating pumps till reaching the bottom of the riverbed. The targeted water level is +41.0 m.a.s.l. The dredged platform is still filled of water. The pumping system is simulated using a surface drainage for this stage where a fix hydraulic head is set up.

    - Modelling stage 2: Once the former water level is reached, deep wells are used to lower the water table till reaching the level +38.0 m.a.s.l that is one meter above the dredged platform. This stage introduced the wells location and number required to ensure the feasibility of the dry construction of the diaphragm walls.

    - Modelling stage 3: The following stage of construction consisted in performing the excavation work for

    the two sluiceways and the central hydropower plant. The whole diaphragm walls, as described in section 3.3, are already performed. The targeted excavation levels of this stage are respectively +33.0 m.a.s.l (lowest elevation for the sluiceway slab) for the two sluiceways pits and +27.15 m.a.s.l for the first powerhouse platform (lowest elevation for downstream powerhouse slab, beyond axis C-C). In those areas the water table level is lowered down 0.50 m under the excavation levels using deep wells. The three pits excavations are performed in the same time.

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    - Modelling stage 4: This last stage concerns the excavation of the local deep pit in the centre of the powerhouse axis and of the second powerhouse platform. The excavation level of the two works are respectively level of +22.9 m.a.s.l for the central pit (lowest elevation of the underside slab in the central pit) and +24.7 m.a.s.l for the second platform (lowest elevation of the upstream powerhouse slab) . To allow a dry excavation the water table will be lowered down to +22.4 m.a.s.l using wells located all around this deeper excavation pit.

    5.3 3D modelling

    5.3.1 Description of the model

    5.3.1.1 The model development

    The whole project and the different excavation stages have been modelled using 5 different models. All of those five have the same extents. The first one represents the actual conditions without the project and was used to set up the initial piezometric conditions of the model. The four remaining models represent the main stages of the project.

    The following figures show the 3 dimensional model limits as well as a visualisation of the project area:

    Existing Assiut barrage

    Ibrahimia canal

    Bani Murr Island Initial state

    N

    Model thickness

    Bottom -200 m.as.l

    Figure 10 : 3Dimensional model extent

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    The local topography and the bathymetry of the Nile River have been generated with the data gathered and used for the creation of the hydraulic physical model.

    The elements of geometry used in the four remaining models that represent the excavation works within the cofferdam have been generated using the current information described in section 3.2 of this document.

    The model extension has been chosen to include all important features of the current situation of the project. The south limit is located at about 200 m upstream of the Ibrahimia canal and the North one is roughly located at about 500 m downstream of the project location. The western limit has been extended on 1km to generate a boundary condition with the Ibrahimia canal that sets up a constant water level till the eastern limit. Eventually, the eastern boundary is far enough of the project to avoid boundary condition effects and also to be able to see the impact on the water table generated by the project construction compared with the initial stage.

    On this topographic base, the structural elements of the project described in appendix 1 have been added. Their location and representation are listed and represented next:

    - The existing Assiut barrage, - The surrounding ring dam, - The temporary slurry wall, - The sluiceway and powerhouse diaphragm walls, - The permanent cut-off wall.

    Project stage 4 Surrounding cofferdam

    Excavation pit

    N

    Sluiceway pits +32.5 m.a.s.l

    Powerhouse pits +21.4 m.a.s.l

    Figure 11 : 3D visualization of the project area

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    Figure 12 : Location of the waterproof organs

    N

    Powerhouse D-walls Cut-off wall

    Existing Assiut barrage

    Sluiceway D-walls

    Temporary slurry wall

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    5.3.1.2 The mesh of the different models

    The four previous models correspond to the following excavation stages:

    1st: Lowering down of the water level till reaching Nile River bed at +42.0m.a.s.l using the floating pumps

    2nd: Dewatering of the dredged platform using deep wells. Targeted water level of +39.0m.a.s.l.

    3rd: Excavation of the sluiceway pits (+33.0 m.a.s.l) and of the first powerhouse plateform (+27.15 m.a.s.l)

    4th: Last excavation stages of the local deep pit in the centre of the powerhouse (+22.4 m.a.s.l) and of the second powerhouse platform (+22.90 m.a.s.l)

    They are discretized using two kinds of cells: square cells of 5x5 m in the project area and of 15x15m far from the project. Using refined mesh allows improving the accuracy of the model. The layers are also discretized using the same principles in high gradients areas under the d-walls bases.

    The surface covered by the model is a rectangle 2220 m long (North to South) and 2565 m wide (West to East). The global mesh includes 226 rows and 251 columns which represents 56726 cells per layer.

    5.3.1.3 Boundary conditions

    The boundary conditions proposed by MODFLOW software influence the cells hydraulic activity. Three options given by the software are described in section 5.1 Introduction to the modelling approach:

    2220 m

    2565 m

    Figure 13 : Mesh Project stage 4

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    - Inactive cells; - Fixed head cells; - Calculated cells.

    The boundary conditions of this model are set by fixed head cells that represent the Nile water level and canals water level. Outside of the Nile riverbed, the cells are calculated even on the border the model. The base of the model located at -200.0 m.a.s.l is automatically considered by the software as a no flow boundary.

    The upstream of the project is located in the south direction, in the opposite direction of the river flow whereas the downstream is located at the North boundary of the model, following the River flow (see fig.17).

    The model boundary conditions are varying on the model and will be developed in a following section.

    5.3.2 Model Validation by the initial state

    The first model performed in this study represents the initial condition. It was performed to set up the boundary condition and also to compare the results of the calculations with observed data. The information we used to generate the model boundary conditions are extracted from the final report on the groundwater model dated December 2010 and elaborated by ABDC [7]. The results introduced in this report were based on piezometric measurements and water levels observations.

    Ibrahimia canal

    Bani Murr Island

    Works area

    Nile river

    N

    Figure 14 : Fixed head cells limit conditions

    Project upstream

    Project downstream

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    Figure 15: Boundary conditions applied for the new 3D model

    In the closer vicinity of the project, two piezometers have been used to check the model results in the initial state (see location fig 15). The imposed water level in the model corresponds to the one observed during summer time. The following table shows the measured and the calculated heads for the two piezometers placed on figure 15 (piezometers data, extracted from ref [7], have been gathered in appendix 3):

    Table 3 : Piezometric data

    The general assumption made here is that the groundwater is fully supplied by the Nile River and its different canals.

    The two following figures represent respectively the initial hydraulic heads in longitudinal and transversal section on the initial state of the project. The locations of the two following sections have been added to the previous figure 15:

    Piezometer Location Measured head during summer time

    m.a.s.l

    Calculated heads m.a.s.l

    66 Easting : 632003

    Northing : 499547 48.50 48.45

    3 Easting : 633194

    Northing : 499727 48.40 48.55

    Transversal section

    Longitudinal section

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    Figure 16 : Initial state - calculated heads in longitudinal section (section location on fig 15)

    Figure 17 : Initial state - calculated heads in transversal section (section location on fig 15)

    Those sections show the initial hydraulic head and will be compared to those made in the different stages of the project.

    At the North boundary the higher water level is 50.0 m.a.s.l whereas at the South it is lowered at 47.0 m.a.s.l. The general gradient generated by the topography and the existing Assiut barrage is roughly of 0.15%.

    Direction of Nile flow

    Direction of Nile flow

    Hydraulic heads m.a.s.l

    Hydraulic heads m.a.s.l

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    Figure 18 : 3D view of Hydraulic head - Initial state

    In the initial state, we saw that in the different layers, the hydraulic head follows the imposed head values in the Nile River. In a steady state the hydraulic head is roughly constant vertically in the initial state. Those observations are fitting with the natural behaviour observed of the system.

    5.3.3 Conclusion

    This previous step gave us the possibility to set up the boundary conditions for the new 3D model. We observed that the size of the model is sufficient to avoid boundary conditions effects and the results fit well with the reference data.

    Hydraulic heads m.a.s.l

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    6. RESULTS IN STEADY STATE FLOW FOR CONSTRUCTION STAGES

    The results are introduced in the following sections with tables and graphs. These results are explained in term of iso-pressure surfaces, current net and hydraulic flows for each construction stage.

    The results are generated from the steady-state seepage flow simulation for the four constructions stages. The models integrate the waterproof elements foreseen for the project.

    For each modelled stage, the flow is assessed either using surface drainage (stage 1) or deep wells (stage 2 to 4).

    6.1 Net flow

    The following section aims at analysing the flow path induced by the dewatering system. To do so, the following figures show on 3D view and in cross-sections current flow lines.

    Figure 19: 3D view of flowlines - stage4

    The presence of waterproof barriers forces the flow lines to go underneath and exit at the bottom of the excavation. The path of flow lines seems to be extended thanks to the waterproof barriers. As a result these barriers seem to play their roles efficiently by reducing hydraulic gradients.

    It can be noted that the influence of the project extends quite far from the work area in all layers on the right bank of the Nile River. The left bank in the area of the project is not significantly affected by the seepage flow since there is a recharge on the left bank water table by the Ibrahimia Canal and also by the Nile River.

    Longitudinal section of the project area

    Transversal section of the project area

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    6.2 Modelled pumping system description

    The first dewatering stage is performed using floating pumps. A dredging area is already performed at the level +39.0m.a.s.l. The targeted water level for this stage is +41.0 m.a.s.l. The dredged area is still filled of water after this stage.

    On the basis of the layout introduced by ENFABJV (cf. section 4.2) the optimized layout modeled in the successive stages are shown in the following sections. The modeled stage two requires all the wells that bottom are located at +10.0 m.a.s.l. Parts of the deep wells which bottoms are located at +5.0 m.a.s.l are also activated to achieve the targeted water levels in the vicinity of the powerhouse.

    Surface drainage. Fixed hydraulic head at +41.0m.a.s.l.

    12 Deep wells bottom +5.0m.a.s.l

    27 Deep wells bottom +10.0m.a.s.l

    Upstream

    Downstream

    Figure 20 : stage 2 dewatering layout

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    The deep wells used for this stage are all part of the design proposed by EFNABJV. A rearrangement compared to the initial layout has been adopted among the 5.0 m.a.s.l bottom wells located in the upstream part of the project. It aims at improving the system for the next excavation by moving those deep wells as close as possible of the powerhouse pit. Those wells have been implanted outside of the next excavation stage and sufficiently far away from the slurry wall (in the downstream direction) to avoid excessive gradient effects. In total, the modeled stage two appeal to 39 deep wells. The imposed flow for each well is 120m3/h.

    The modeled stage 3 corresponds to the excavation of the two sluiceways and of the first platform within the powerhouse. The number of 5.0 m.a.s.l deep wells has been increased upstream and downstream of the project

    The 29 additional 5.0 m.a.s.l bottom wells have been apportioned at the upstream and downstream part of the powerhouse pit, as closed as possible of the excavation to limit as much as possible excessive hydraulic gradients in the ring dam. This allows a dry excavation of the first platform avoiding seepage issues on the slopes. The additional number of wells allows the water table downsize for the lateral sluiceways. As a total, the modeled stage two requires 62 deep wells. The imposed flow of each well is 120m3/h.

    29 additional Deep wells bottom +5.0 m.a.s.l

    Downstream

    Upstream

    Figure 21 : stage 3 dewatering layout

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    The modeled stage 4 examines the excavation of the last platform in the powerhouse pit and of the central deep powerhouse pit.

    Figure 22 : Stage 4 dewatering layout

    This last stage deep wells configuration takes advantage of the work phasing to perform the dewatering. Setting up the deep wells within the powerhouse pit enhances the wells efficiency while limiting the gradients close to the slurry wall. In total this last stage requires 72 deep wells to fulfill the dewatering requirements.

    10 additional deep wells bottom at +5.0 m.a.s.l on the platform at +27.15m.a.s.l

    2 of the former deep wells are stopped (see on the former stage location)

    2 additional deep wells are needed at the upstream part of the cut-off wall

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    6.3 Pumping rates

    The following table sum up the flow assessed for the different excavation stages modelled.

    Stage Assessed flow

    (m3/day) Dewatering system established

    1: Lowering of the Nile water level to +41.0 m.a.s.l

    60480 Floatingpumps

    2: Excavation of the first platform to the

    level+39.0m.a.s.l. Water level lowered to +38.0 m.a.s.l and

    work of the d-walls.

    11232039wellspositionedwithintheworkingareaarerequiredtoperformthestage

    twodewatering

    3: Excavation of the two sluiceways and of the

    powerhouse pits. Water table downsized using an optimized

    number of wells.

    118850

    65wellsarerequiredtofulfillthedewateringneededforthe2

    sluicewaysandforthefirstplatforminthepowerhousepit.

    4: Excavation of the central deep powerhouse pit using

    the optimized number of deep wells.

    207360

    Additionaldeepwellspositionedonthefirstpowerhouseplatform.Partofthestage3wellscanbedeactivatedtominimizethegradienteffect.72wells

    arerequiredtofulfillthelastdewateringstage.

    Table 4 : 3D model Assessed flow

    The hydraulic heads computed (surface hydraulic head isovalues, longitudinal and transversal section) for each modelling stages are gathered in appendix 3 of this document.

    An additional appendix 4 gathered extended views of the piezometric lines generated by the dewatering.

    Concerning the piezometric lines shown in annex 3 & 4, the values printed outside of the ringdam are lowering during the different stages. This phenomenon results of a combined effect of:

    - The rapid hydraulic head loss generated in the first layer; - The graphical printout of the results that are given in the middle of each cell.

    The hydraulic head to consider at the top of the Nile riverbed is still +47.6m.a.s.l.

    6.4 Influence of the project on the closest vicinity

    Observations made on flow lines revealed that the project in the final stage could have an influence quite far from the project. The concerned area regarding the influenced zone of the project is the right bank of the Nile. The following figures shows the transversal sections of the hydraulic heads computed in the final stage of the model and in the initial state.

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    Figure 23 : 3D model - hydraulic heads - Initail stage Vs construction stage 4 (optimized number of wells)

    Hydraulic Heads in transversal section in the works area Initial state

    Hydraulic Heads in transversal section in the works area Stage 4

    In the initial state of the project the computed head on the right bank of the Nile River was roughly between 47.0 and 47.5 m.a.s.l. In the construction stage of the project, where the pumping rate is the highest, we can see that on the right bank the computed hydraulic head is reduced to a value in the range of 38.0 and 39.0 m.a.s.l. Thus, the impact of the project on the water table represents a water level reduction of 8.0 to 9.5 m. The area affected by this phenomenon goes from the cofferdam, to the eastern border of the model. On the transversal section this area is roughly 600.0 m long.

    Direction of Nile flow Direction of Nile flow

    Hydraulic head m.a.s.l

    Hydraulic head m.a.s.l

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    7. SENSITIVITY ASSESSMENT OF THE 3D MODELLING

    The model sensitivity is assessed on the basis of the modelling stage 4 whose design and hypothesis are described in the previous sections: wells layout, flow assessment and soil parameters.

    The parameters tested are the permeability coefficients and the anisotropy of the different layers. The geotechnical analysis, reported in Ref.[1], introduced a range of those parameters. The following table sums up the analysis:

    Soil Profile Horizontal Permeability Anisotropy (KH/KV)

    Soil Base level Min. Max. Adopted Value

    Deviation of the adopted

    value Min. Max. Adopted Value

    m.a.s.l (m/s) (m/s) (m/s) From Min. From Max. KH/KV KH/KV KH/KV

    Silt Sand 40 4,0E-06 1,4E-05 1,0E-05

    -60% +40%

    2 7 5

    Fine Sand 30 1,6E-04 5,6E-04 4,0E-04 2 7 5

    Sand with Gravel 0 2,8E-04 1,0E-03 7,0E-04 2 7 5

    Fine to Medium Sand, some Gravel -200 8,0E-05 2,8E-04 2,0E-04 2 7 5

    Table 5: Permeability assessment extracted from Ref.[1]

    The purpose of the calculation is to assess the flow required to perform the dewatering considering the limit values of the range gave in the previous table.

    Two calculations have been performed:

    - The maximum value of the data range is assessed taking the maximum value of the horizontal permeability and the minimum value of anisotropy in all the layers;

    - Conversely, the lower range boundary is assessed taking the minimum value of the horizontal permeability and the maximum value of anisotropy in all the layers.

    The flow value is assessed modifying the imposed flow in each wells of the modeling stage 4. The imposed flow is changed till reaching the same dewatering level as the one found in the design case (model stage 4 performed with the adopted values of the table above). The results in terms of flow variations are sums up in the following table.

    Case

    Horizontal permeability coefficients

    values

    Anisotropy kh/kv Computed

    flow (m3/day)

    Fixed flow /well (m3/h)

    1- Design case Adoptedvalues Adoptedvalues=5 207360 120

    2- lower limit estimation Min Max=7 65670 38

    3- upper limit estimation

    Max Min=2 380160 220

    Table 6: Parametric study results

    In as much as the well unit flow given in this section are either much lower or much greater than the well unit capability, the number of wells and their location will need to be updated according to the considered situation.

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    8. GENERAL CONCLUSION

    The model performed here confirmed the feasibility of the dewatering based on the last construction stages foreseen. The flow generated by the dewatering system is in the same order of magnitude of the one found in the previous stages of the project.

    Nevertheless several observations can be made on the results we get:

    - For the model stage two, the solution of relief wells will need to be verified to see in what extent they can downsize the gradient effect during the construction of the diaphragm walls

    - Based on the current parameters, the solution proposed to fulfil the requirements in terms of dewatering consists in using 27 wells whose bottoms are located at 10.0 m.a.s.l and respectively 35 and 45 wells whose bottoms are located at +5.0 m.a.s.l for the stages 3 and 4. The dewatering proposed for the stages 3 and 4 are relying on the works phasing to downsize the hydraulic head loss, close to slurry wall, and to improve the efficiency of the system.

    The wells location proposed will be updated to take into account the final site conditions. The layout for the deep wells locations will also need to take into account the gradient generated within cofferdam material, in the vicinity of the d-walls and also during the construction of the d-walls.

    The parametric analysis gave two limit flow values. It allows considering the uncertainties on the permeability coefficient and anisotropy estimations. The results in terms of flow are very sensitive to those parameters: the flow required for the design case (adopted value of permeability and anisotropy) is roughly three times superior to the flow computed for the lower parameter range limit(min horizontal permeability an max anisotropy) and is twice lower than the upper parameter range limit.

    Nevertheless, those two values are computed taking into account the maximum Nile water level known for a 50 years return period flow.

    Combination of the different parameters could be assessed to have a better definition of the influence of each soils specification on the calculations.

    The presence or not of the silty sand layer in the Nile riverbed has not been considered in this parametric analysis. This layer identified at the top of the geotechnical model has a lower permeability than the other and creates an important head loss under the Nile River. The influence of this parameter need to be discussed for the final layout for the detailed layout of wells.

    Regarding the influence of the project on neighbouring areas, the model shows that it is more on the right bank of the Nile River where the drop in hydraulic head is generated by the project pumping. This drop is in the range of 8.0 to 9.5 m. The affected area can exceed 450 m according to the simulation. A specific study will be needed to assess the consequences in term of settlement.

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    APPENDIX

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    Appendix 1: Assessment of the equivalent permeability for waterproof organs

    Inasmuch as the most little cells are 5.0 m width*5.0m length in the 3D model and 1.0 m width*1.0 m length in the 2D model , positioning diaphragm walls of 0.80 m width within this cells require to calculate an equivalent permeability. This permeability must take into account the proportion of soil and the proportion of slurry wall within a cell. For this type of analysis, the D-walls equivalent permeability can be estimated assuming that we have a flow normal to stratification.

    Several types of cells have been used to build this study: the cells dimensions are between squares of 15.0 m side and squares of 5.0 m side in the 3D model and square of 1.0 m side in the 2d model. The slurry wall permeability used for D-wall is set up at 1.0*10-8 m/s.

    The soils hydraulic parameters are the one chosen in the design criteria note:

    Soil Horizontal coefficient of permeability (m/s)

    Vertical coefficient of permeability (m/s)

    Sandy gravel fill cofferdam material

    1.0x10-4 1.0x10-4

    Subground sand 5.0x10-4 1.0x10-4

    The following graph represents the analysis carried out for the determination of the equivalent permeability of waterproof organs. It is based on the previous formula to determine equivalent permeability in a multi-layer soil under a normal flow. The thickness of D-walls is constant and equal to 0.80 m.

    Flow direction Q

    Layer 1 Layer 2 Layer 3

    K1

    L1

    K1

    L1

    K2

    L2

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    We can see on the previous graph that in our case where horizontal permeability is comprise between 1.0*10-3 and 1.0*10-5 m/s, till cells are smaller than 7.5 m width, the soil permeability has no impact on the equivalent permeability determined with the method exposed previously.

    As a consequence in the 3D model where refined mesh on the project area is composed of 5.0 m width cells, the D-walls are represented by a constant horizontal permeability of 6.1*10-8m/s.

    By the same token, in the 2D model where D-walls are represented in 1.0 m width cells, the equivalent permeability calculated is 1.0*10-8 m/s.

    0,00E+00

    2,00E08

    4,00E08

    6,00E08

    8,00E08

    1,00E07

    1,20E07

    1,40E07

    1,60E07

    1,80E07

    2,00E07

    Equivalent

    horizon

    talpermeabilitykeq(m

    /s)

    Horizontalpermeabilityofsoilkh(m/s)

    Equivalentpermabilityforthedifferenttypeofcellsusedinthemodellisatione=0,80m

    1mwidthcells

    2.5mwidthcells

    5mwidthcells

    7.5mwidthcells

    15mwidthcells

    Soil permeabilityrangekh [1.0*105;1.0*103]

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    Appendix 2: Piezometric data

    Groundwater level at piezometer 66 (extracted from ref [7])

    Groundwater level at piezometer 3 (extracted from ref [7])

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    Appendix 3: 3D model sections computed hydraulic heads

    - Modelling stage 1: After the establishment of the cofferdam and of the slurry wall, the construction pit is dewatered by floating pumps till reaching the bottom of the riverbed. The dredged platform at the level +39.0m.a.s.l is still filled of water. The targeted level is roughly +41.0 m.a.s.l.

    3D Visualization of the work area stage 1

    Longitudinal section

    Transversal section

    Surface drainage

    Surrounding ringdam

    First excavation stage

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    Computed hydraulic heads dewatering +41.0m.a.s.l (Vertical scale factor of 3)

    Longitudinal section of model stage 1

    Hydraulic heads m.a.s.l

    Hydraulic heads m.a.s.l

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    Transversal section of model stage 1

    The pumping rates assessed for the model stage 1 is 60480m3/day.

    Hydraulic heads m.a.s.l

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    - Modelling stage 2

    Once the riverbed is reached, trenches and sumps are lowered down within the cofferdam till the water table is lowered to the targeted level of +37.5 m.a.s.l. Then the first excavation work is executed from the riverbed top to +38.0 m.a.s.l. Once the excavation level is reached, a relief well system is set up around the location of the diaphragm walls to avoid the gradient effect. The whole diaphragm walls are assumed to be performed in the same time.

    Wells location model stage 2

    Diaphragm walls equivalent permeability is kh=6.1x10-8m/s. This equivalent permeability takes into account a 0.80 m width Diaphragm-wall within a 5.0 m side square cell.

    Longitudinal section

    Transversal section

    Deep wells

    wells

    Temporary slurry wall

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    Computed hydraulic heads dewatering +37.0m.a.s.l (Vertical scale factor of 3)

    Longitudinal section of model stage 2

    Hydraulic heads m.a.s.l

    Hydraulic heads m.a.s.l

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    Transversal section of model stage 2

    The pumping rates assessed for the model stage 2 is 112320m3/day.

    Hydraulic heads m.a.s.l

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    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    - Modelling stage 3:

    The following stage of construction consisted in performing the excavation work for the two sluiceways and the central hydropower plant. The targeted excavation levels of this stage are respectively +32.5 m.a.s.l for the two sluiceways pits and +27.15 m.a.s.l for the powerhouse first platform. In those areas the water table level is lowered down 0.50 m under the excavation levels using deep wells. The three pits excavations are assumed to be performed in the same time. For this third stage the foreseen configuration of 65 wells doesnt meet the requirement of the dewatering. The results shown here correspond to an adaptation of the wells location.

    The number of wells foreseen for this stage is 62 with 27 whose bottoms are located at +10.0 m.a.s.l and 35 whose bottom are located at +5.0 m.a.s.l. Deep wells location

    Diaphragm walls equivalent permeability is kh=6.1x10-8m/s.

    Longitudinal section

    Transversal section

    Deep well location

    Temporary slurry wall

    Diaphragm walls:

    Powerhouse

    Sluiceways

    Cut-off

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    TRACTEBEL ENGINEERING/COYNE ET BELLIER Color None A4 44/51

    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Computed hydraulic heads dewatering +26.50m.a.s.l (vertical scale factor of 3)

    Longitudinal section of model stage 3

    Hydraulic heads m.a.s.l

    Hydraulic heads m.a.s.l

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    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Transversal section of model stage 3

    The pumping rates assessed for the model stage 3 is 178850m3/day.

    Hydraulic heads m.a.s.l

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    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Modelling stage 4: This last stage concerns the excavation of the local deep pit in the centre of the powerhouse axis and of the last platform at. Their excavation levels are respectively +22.9m.a.s.l for the central deep pit and +24.7 m.a.s.l . To allow a dry excavation the water table will be lowered down to +22.4 m.a.s.l under the pit and to 24.2m.a.s.l under the second platform using wells located all around this deeper excavation pit and surface drainage.

    The dewatering system set up for the stage 3 doesnt fully fulfill the needs for the excavation of the deep powerhouse pit. Additional wells are added at the rear of the cut off wall and on the first powerhouse plateform. Deep wells location

    Diaphragm walls equivalent permeability is kh=6.1x10-8m/s.

    Longitudinal section

    Transversal section

    Powerhouse central deep pit

    Temporary slurry wall

    Diaphragm walls:

    Powerhouse

    Sluiceways

    Cut-off

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    TRACTEBEL ENGINEERING/COYNE ET BELLIER Color None A4 47/51

    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Computed hydraulic heads dewatering +22.40m.a.s.l (vertical scale factor of 3)

    Longitudinal section of model stage 4

    Hydraulic heads m.a.s.l

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    DEWATERING 3D MODELING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Transversal section of model stage 4

    The pumping rates assessed for the model stage 4 is 207360m3/day.

    Hydraulic heads m.a.s.l

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    Dewatering -3D modeling

    Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Dewatering stage 2Appendix 4 : Hydrualic heads isovalues

    +39.0m.a.s.l

    +39.0m.a.s.l

    +39.0m.a.s.l

    +39.0m.a.s.l

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    1 T N G 1 5 0 0 0 2 B

    Dewatering stage 3

    +38.0m.a.s.l +38.0m.a.s.l

    +38.0m.a.s.l

    +38.0m.a.s.l

    +38.0m.a.s.l

    +33.0m.a.s.l +33.0m.a.s.l

    +37.0m.a.s.l+37.0m.a.s.l

    +38 0m a s l

    +27.15 m.a.s.l

    +27.15 m.a.s.l

    +27.15 m.a.s.l

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    DEWATERING 3D MODELLING Originator Div Type Element Number Ind

    1 T N G 1 5 0 0 0 2 B

    Dewatering stage 4

    +38.0m.a.s.l

    +38.0m.a.s.l +38.0m.a.s.l

    +38.0m.a.s.l

    +38.0m.a.s.l

    +33.0m.a.s.l+33.0m.a.s.l +24.2m.a.s.l

    +22.9m

    +27.15m.a.s.l