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Design of cfrd dams


The Design of Concrete-faced Rockfill Dams

Danie Badenhorst
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Constructed Concrete-faced Rockfill Dams
Concrete-faced Rockfill Dams: Trends
220 Uncompacted Compacted 200 180 160 140 120 100 80 60 40 20 0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Height (m)

Year
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The Lesotho Mohale Dam

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The Lesotho Mohale Dam

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Description of CFRD
Parape t Wall
Concrete Face
Plinth

Rockfil l

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Description of CFRD
Zone designations: ? ? ? 1 for soil materials 2 for processed granular materials 3 for rockfill zones

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Section of CFRD

2B 3A 2B

3A 3D

2A

1B 3B 1A 3C

3D

3E

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Description of CFRD
Zone designations: ? ? ? Zone 1A, a silt or fine sand act as face slab or perimeter joint healer. Zone 1B supports zone1 materials Zone 2A is a processed fine filter < 20mm and will limit leakage when a waterstop fails and can act to heal.

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Description of CFRD
? ? Zone 2B, The face support zone, is crusher run < 75mm Zone 3 is quarry run rockfill. The differences in A,B and C are principally in layer thickness and size and type of rock. Zone 3A, provides compatibility and limit void size adjacent to Zone 2B. Zone 3B provides mass, resist the water load and helps in limiting face deflection. Zone 3C receives little water loading, and settlement is essentially during construction. The two-metre-thick layer in Zone 3C accepts large size rocks, is more economical to place and its lower density saves rock volume.
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? ? ?

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Section of CFRD

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Materials Specification
MATERIALS & COMPACTION DETAIL Zone 1A 1B 2A 2B Classification Impervious earthfill Random earthfill Fine filter Crushed rock Lift Height (m)* 0,3 0,6 0,2 0,4 Type of Roller Compaction by rubber tyre equipment Compaction by rubber tyre equipment 50 kN vibrator > = 10 tonne vibratory roller Passes (Number) 4 4 4*** 4 + surface compaction **

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Materials Specification
MATERIALS & COMPACTION DETAIL Zone 3A 3B 3C 3D 3E Classification Selected small quarry run rock Quarry run rockfill Quarry run rockfill Erosion protection, durable rock Drainage rock Lift Height (m)* 0,4 1,0 2,0 NA 1,0/2,0 > = 10 tonne vibratory roller Type of Roller >= 10 tonne vibratory roller >= 10 tonne vibratory roller >= 10 tonne vibratory roller Passes (Number 6 6 6 Placed by backhoe 6

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Materials Specification

Maximum layer thickness after compaction. ** Surface compaction by 130 kN vibratory plate on laser compaction shall be achieved

controlled backhoe. Similar

as using: 2 passes with the 10 tonne vibratory roller without and subsequently 4 passes downslope without and 4 passes upslope with vibration.

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Materials Specification

Zone 2A material shall be thoroughly compacted with a backhoe mounted plate vibrator with using a 10 tonne at least 50 kN vibratory force to a density similar as achieved by 4 passes vibratory roller. Particular care shall be taken to prevent damage to the concrete or to the waterstops embedded in the plinth concrete. The material shall be thoroughly wetted before compaction. and during

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Site Requirements
? CFRD’s are best suited to sites where: – rock foundation is close to plinth – nearby source of rockfill is available – lack of suitable core material – there is location for economical spillway

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Features of a CFRD
? ? ? Settlements of rockfill are small Parapet wall can be placed on crest Leakage is not a concern, face slab in biaxial compression, rockfill stable without face slab Water load transmitted into foundation upstream of dam axis Uplift under rockfill is not involved. The pressure on the foundation exceeds reservoir pressure over 3/4 of base width.

? ?

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Features of a CFRD
? ? Sliding factor for reservoir water and rockfill exceeds 7 High shear rockfill, no pore pressures in rockfill, small settlement of rockfill under seismic load make CFRD resistant to seismic loading Drainage galleries in abutments not required Overtopping can cause failure

? ?

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Post Construction Crest Settlement
Damname Rock type Height (m) Aguamilpa Gravel 187 Tianshengqiao Limestone and mudstone 178 Foz do Areia Basalt 160 Segeredo Basalt 145 Alto Anchicaya Hornfels-Diorite 140 Xingo Granite 140 Golillas Gravel 130 Khao Laem Limestone and mudstone 130 Ita Basalt 125 Turimiquire Limestone 115 R.D. Bailey Sandstone and shale 96 Sugarloaf Sandstone 85 Chengbing Lava tuff 75 Minase Liparite 76 Cabib Creek Gneiss 64 Kangaroo Creek Weak schist 60 Settlement (mm) 340 1060 210 160 170 49 50 150 450 270 420 40 100 400 110 180 Settlement (% of height) 0.18 0.6 0.13 0.11 0.12 0.33 0.04 0.16 0.36 0.23 0.44 0.04 0.13 0.6 0.22 0.3

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Post Construction Crest Settlement
1200 1000 800 600 400 200 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Dam or rock type

Height (m), Settlement (mm)

Height (m) Settlement (mm)

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Disadvantages of CFRD’s
Plinth to be designed well where rock is poor. Care to be taken in fabrication and construction of waterstops. Design movement on perimetric joint is 200 to 300mm. Further research required for joints and seals on joints for dams higher than 200m

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Advantages of CFRD’s
? ? Ancillary works shorter than for earthfill dams Diversion capacity can be reduced by allowing overtopping of rockfill Rockfill is suitable for wet weather placement Foundation clean-up required no hand-work except at plinth Grouting can be done parallel to placement of rockfill

? ? ?

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Advantages of CFRD’s
? ? Multistage construction of the rockfill embankment is possible Slipforming provides rapid and economical method of face slab construction

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Plinth Foundation Preparation

?

The foundation is subdivided into three main parts:
– The plinth foundation – The embankment foundation – The transition foundation between the two –
Last mentioned is a non-differential deformable base

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Foundation Preparation

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Plinth Foundation Preparation

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Plinth layout

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Plinth functions

? ? ?

Connects the face slab to the rigid rock Act as a grout cap during grouting provides a starting position for concrete face slab slipforming equipment

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Plinth section

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Dimensioning plinth

?

Standardization of the slope and dimensions will reduce cost and limit construction time minimum plinth widths of 2m and 3m for <25m high and >25m high dams respectively Thickness for plinth on sound rock = face slab thickness. Minimum 300mm and 500mm for lower and higher dams respectively

?

?

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Hydraulic gradient guide for plinth
? ? Ratio of reservoir head to plinth width = hydraulic gradient Foundation erodibility in terms of rock erosion/acceptable hydraulic gradient
– Fresh : 20 – Slightly to moderately weathered : 10 – Moderatelty to highly weathered : 5 – Highly weathered : 2

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Plinth Backfill Concrete

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Orientation of the plinth apron
? ? ? ? Apron contours normal to plinth line Apron contours normal to dam axis Apron contours to suit as-excavated contours The alignment should always allow the concrete face to pull away from the plinth and move downstream under water load.

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Plinthline

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Layout geometry in three dimensions

B
V S A

Plinth Control Point
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Plinth concrete and reinforcement
? ? ? High durable, low permeability concrete 0,3% steel anchored with grouted dowels

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Face curb placing procedure

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Face starter slab

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Face thickness

?

H = 0,3 + 0,003H m

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Slipform

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Copper waterstop joint

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Typical PVC Waterstop

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Typical Stainless Steel waterstop joint

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Copper waterstop joint

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Plinth rubber and copper waterstop

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Plinth rubber waterstop

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Perimetric joint

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Three dimensional perimeter joint meter

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Perimeter Joint Movement
Damname Rock type Height ( Opening normal to joinSettlement normal to concrete fShear parallel to joint Aguamilpa Gravel 187 19 16 5 16 23 7 Tianshengqestone and mudst 178 Foz do Arei Basalt 160 23 55 25 Salvajina Gravel 148 9 19 15 Alto AnchicaHornfels-Diorite 140 125 106 15 Xingo Granite 140 30 34 0 Golillas Gravel 130 100 36 0 5 8 0 Khao Laemestone and mudst 130 Shiroro Granite 125 30 60 21 Dolerite 122 7 70 0 Lower Piem Reece Dolerite 122 7 70 0 Cethana Quartzite 110 11 0 7 Kotmale Charnokite 97 2 20 5 Xibeikou Dolomite 95 14 25 5 Murchison Rhiolite 89 12 10 7 Sugarloaf Sandstone 85 9 19 24 MacIntosh Graywacke 75 5 20 3 Bastyan Graywacke 75 5 21 0 Chengbing Lava tuff 75 13 28 20 Gravel 40 2 12 1 Pichi-Picun-

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Perimeter Joint Movement
200

H e ig h t (m )
175 150 125 100 75 50 25 0 P e r d a m m e n t io n e d in t a b le

O p e n in g n o rm a l t o jo in t (m m ) S e t t le m e n t n o rm a l t o c o n c re t e fa c e (m m ) S h e a r p a ra lle l t o jo in t (m m )

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Joint meter

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Face slab: temperature??

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Plinth and face slab joints

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Waterstops

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Placement of rockfill
? Rockfill is end dumped on the edge of a placed layer and spread by dozer. There is inherent segregation in the dumping and intentional segregation in the spreading. The smooth surface on top of the layer is desirable for compaction and for reduced tyre and dozer track costs. The top half consists of smaller size rock and is well graded in comparison to the larger rocks in the bottom half. The upper half is of higher density. Energy is transmitted through the larger rocks providing strength and density by wedging and crushing of edges. Water is added during compaction to smooth the edges and to increase the density. The maximum size rock in a layer may be equal to the layer thickness. Immediately adjacent rockfill will not be fully compacted and does not need to be. The larger rock particles will attract load in the area.
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?

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Rockfill and Cohesionless fines

2A 3A CF 2B 3B

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Zone 2A compaction

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Requirements of Zone 2A material
? ? ? Non plastic mixture of rock fragments with hard durable particles Designed as a filter - must retail silt and fine sand, (D15/d85<4) Filter material must be more permeable than the clogged filter interface, (D15/d15>4) To achieve the above 2A material will
– – – – –
not segregate during placement, uniform -not change in gradation during processing not have cohesion or will cement be eternally stable have the ability to control and seal a concentrated leak
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?

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Functions of Zone 2B material

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Requirements of Zone 2B material
? ? It forms a cushion to uniformly support the face slab It must provide a smooth dense surface on which to place the face slab to reduce concrete quantities It restricts leakage flows that might result from damage to the concrete face Under draw down conditions is free draining through the downstream rockfill

?

?

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Requirements of Zone 2B material
? In case of weak rockfills, measures as flattening the upstream slope or providing drainage downstream of the transition zones may be a requirement Must be designed as self healing in case of a crack in the face slab Permeability>10-4 cm/s Must be protected against erosion during construction

?

? ?

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Requirements of Zone 3A material
? The transition between 2B and 3B consists of 3 to 4m wide zone 3A material it provides filter stability between the zones and ensure limiting differential movement of the face slab

? ?

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Segragated 3B Material

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Requirements of Zone 3B and 3C material
? ? ? ? ? Capable of carrying the imposed reservoir load 10-2 to 10-1 cm/s for 3B and 3C respectively 3B - 1m layers, water added during compaction 3C - 2 m layers, water not added during compaction Shear strength varies from about 450 for low density poorly graded weak particles to 600 for high density, well graded strong particles
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Seggregated 3B material

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Series of filters/seals of perimeter joint
? ? ? ? ? Zone 1A and Zone 2A material Cohesionless fines top stainless steel waterstop Central PVC waterstop Bottom copper waterstop

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Series of filters/seals of perimeter joint
? ? ? ? Zone 2A fine filter material Zone 2B transition filter material Zone 3A small quarry run material Zone 3B rockfill

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Plinth-cohesionless fines

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Placement of impervious 1B blanket on face slab and fly ash on perimeter joint

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Upstream Coffer Dam, Plinth

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Downstream cofferdam

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Parapet wall connected to face slab

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Parapet Wall Functions
? ? ? ? Provide freeboard act as wave wall provide access during construction of face slab to provide safety barrier on crest

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Parapet Wall
? ? ? Heights up to 8.5m have been successfully constructed Stability regarding shear and overturning important Seismic loads must be considered - the crest of an embankment dam is moving maximum during the occurrence of earthquakes CFRD’s can easily be raised by adding a parapet wall Finish of parapet wall is important - the wall is visible after completion and impoundment

? ?

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Snowy conditions

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CFRD

? ?

Simple Safe

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