Badenhorst - Design of Concrete-Faced Rockfill Dams.pdf

The Des Design ign of Con Concre cretete-fac faced ed Roc Rockfi kfill ll Dams

Danie Badenhorst

Constructed Concrete-faced Rockfill Dams C oncret on crete-f e-faced aced R ockf oc kfiill D ams: ams : Trends T rends 220 22 0 200 20 0 180 18 0    ) 160 16 0   m 140 14 0    (    t 120 12 0    h100   g10 0    i   e 80    H 60 40 20 0

Uncompacted

Compacted

1 8 9 0 1 9 0 0 1 9 1 0 1 9 20 19 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 99 0 2 00 0 20 1 0

 Ye

Constructed Concrete-faced Rockfill Dams C oncret on crete-f e-faced aced R ockf oc kfiill D ams: ams : Trends T rends 220 22 0 200 20 0 180 18 0    ) 160 16 0   m 140 14 0    (    t 120 12 0    h100   g10 0    i   e 80    H 60 40 20 0

Uncompacted

Compacted

1 8 9 0 1 9 0 0 1 9 1 0 1 9 20 19 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 99 0 2 00 0 20 1 0

 Ye

The Le Lesotho Mohale Dam

The Lesotho Mohale Dam

Description of CFRD Parape t Wall Concrete Face Plinth

Rockfil l

Description of CFRD Zone designations: •

1 for soil materials

•

2 for processed granular materials

•

3 for rockfill zones

Section of CFRD

2B 3A

3A 3D 2B

2A

1B

3D 3B

1A

3E

3C

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.

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.

Section of CFRD

Materials Specification MATERIALS & COMPACTION DETAIL Zone

Classification

Lift Height

Type of Roller

Passes (Number  

Compaction by rubber tyre

4

(m)* 1A

Impervious earthfill

0,3

equipment 1B

Random earthfill

0,6

Compaction by rubber tyre equipment

2A

Fine filter

0,2

50 kN vibrator

2B

Crushed rock

0,4

> = 10 tonne vibratory roller  

4 4*** 4 + surface compaction **

Materials Specification MATERIALS & COMPACTION DETAIL Zone

Classification

Lift Height

Type of Roller

(m)*

Passes (Number 

3A

Selected small quarry run rock

0,4

>= 10 tonne vibratory roller

6

3B

Quarry run rockfill

1,0

>= 10 tonne vibratory

6

roller 3C

Quarry run rockfill

2,0

3D

Erosion protection, durable

NA

>= 10 tonne vibratory roller

Placed b

rock 3E

Drainage rock

6

backhoe 1,0/2,0

> = 10 tonne vibratory roller

6

Materials Specification

Maximum layer thickness after compaction.

**

Surface compaction by 130 kN vibratory plate on laser 

controlled backhoe. Similar

compaction shall be achieved

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

Materials Specification

Zone 2A material shall be thoroughly compacted with a backhoe mounted plate vibrator with

at

least

50

kN

vibratory force to a density similar as achieved by 4 passes using a 10 tonne

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

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

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.

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

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

Post Construction Crest Settlement

1200   m   m1000   n   e   m   e   e  ,   m   g   e

800 Height (m)

600

Settlement (mm)

400 200 0 1

2

3

4

5

6

7

8

9

10

Dam or rock type

11

12

13

14

15

16

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

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

Advantages of CFRD’s •

Multistage construction of the rockfill embankment is possible

•

Slipforming provides rapid and economical method of face slab construction

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

Foundation Preparation

Plinth Foundation Preparation

Plinth layout

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

Plinth section

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 dams respectively

•

Thickness for plinth on sound rock = face slab thickness. Minimum 300mm and 500mm for lower and higher dams respectively

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

Plinth Backfill Concrete

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.

Plinthline

Layout geometry in three dimensions

B V S

Plinth Control Point

A

Plinth concrete and reinforcement •

High durable, low permeability concrete

•

0,3% steel

•

anchored with grouted dowels

Face curb placing procedure

Face starter slab

Face thickness

•

H = 0,3 + 0,003H m

Slipform

Copper waterstop joint

Typical PVC Waterstop

Typical Stainless Steel waterstop joint

Copper waterstop joint

Plinth rubber and copper waterstop

Plinth rubber waterstop

Perimetric joint

Three dimensional perimeter joint meter 

Perimeter Joint Movement Damname Rock type Height ( Opening  Aguamilpa Gravel 187 Tianshengqestone and mudst 178 Foz do Arei Basalt 160 Salvajina Gravel 148  Alto Anchic Hornfels-Diorite 140 Xingo Granite 140 Golillas Gravel 130 Khao Laemestone and mudst 130 Shiroro Granite 125 Lower Piem Dolerite 122 Reece Dolerite 122 Cethana Quartzite 110 Kotmale Charnokite 97 Xibeikou Dolomite 95 Murchison Rhiolite 89 Sugarloaf Sandstone 85 MacIntosh Graywacke 75 Bastyan Graywacke 75 Chengbing Lava tuff 75

normal to joi Settlement normal to concrete Shear parallel to join 19 16 5 16 23 7 23 55 25 9 19 15 125 106 15 30 34 0 100 36 0 5 8 0 30 60 21 7 70 0 7 70 0 11 0 7 2 20 5 14 25 5 12 10 7 9 19 24 5 20 3 5 21 0 13 28 20

Perimeter Joint Movement 200

H e i g h t (m ) 175 150

Opening n o r m a l t o j o in t (mm)

125 100

Settlement n o rm a l t o c o n c re t e fa c e (mm) Shear parallel t o jo in t (m m )

75 50 25 0 P e r d a m m e n t i o n e d in t a b l e

Joint meter 

Face slab: temperature??

Plinth and face slab joints

Waterstops

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.

Rockfill and Cohesionless fines

2A 3A CF

2B 3B

Zone 2A compaction

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 /d854)

•

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

Functions of Zone 2B material

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

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

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

Segragated 3B Material

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 45 0 for low density poorly graded weak particles to 60 0 for high density, well graded strong particles

Seggregated 3B material

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

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

Plinth-cohesionless fines

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

Upstream Coffer Dam, Plinth

Downstream cofferdam

Parapet wall connected to face slab

Parapet Wall Functions •

Provide freeboard

•

act as wave wall

•

provide access during construction of face slab

•

to provide safety barrier on crest

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

Snowy conditions