Lecture 2 - Embankment Dams Extended

MINE 480 -‐ Mine Waste Management MINE 590Q – Advanced Mine Waste

Management

Term 2 -‐ 2013 Lecture 2 – Embankment Dams, Seepage, Filter Design and

Cri@cal Gradient

Dirk van Zyl Norman B. Keevil Ins@tute of Mining Engineering

[email protected] (604) 827-‐3462

Sta@s@cs on Dams.

-‐Dams worldwide – General descripBon A data base from ICOLD accoun@ng 33105 dams worldwide has been compiled. Results from this informa@on is presented as follows:

• Purpose of dams

Ref.-‐Web. (10) ICOLD (2011)

Sta@s@cs on Dams.

-‐Dams worldwide – General descripBon

• Single purpose dams.

• Mul@-‐purposes dams.

Ref.-‐Web. (10) ICOLD (2011)

Sta@s@cs on Dams.

-‐Dams worldwide – General descripBon

• Dams type.

Ref.-‐Web. (10) ICOLD (2011)

Sta@s@cs on Dams.

-‐Dams worldwide – Failure staBsBcs

• Earth and rockfill dams o General sta@s@c of failures

ü Percentages based on % of cases with known failure mode. ü Percentages do not sum 100% as dams were classified as mul@ple modes of failure.

Failure mode % Total failures Overtopping 34.2 Spillway 12.8 Piping through embankment 32.5 Piping from embankment to founda@on 1.7 Piping through founda@on 15.4 Downstream slide 3.4 Upstream slide 0.9 Earthquake 1.7 Totals 102.6

Ref. (5) M. Foster, R. Fell and M. Spannagle (2000)

Sta@s@cs on Dams.

-‐Dams worldwide – Failure staBsBcs

• Earth and rockfill dams o Sta@s@c of failure by zoning

Ref. (5) M. Foster, R. Fell and M. Spannagle (2000)

Sta@s@cs on Dams.

-‐Dams on BriBsh Columbia – Failure staBsBcs

• There are approximately 1985 dams opera@ng in Bri@sh Columbia.

• 287 dams are classified as high hazard.

• Purpose of these dams range from hydro electric to domes@c supply .

• Dams range in size from small privately-‐owned irriga@on dams to B.C. Hydro's 242 metre high Mica Dam.

Ref.-‐Web. (11) Ministry of Environment of BriBsh Columbia (2011)

Sta@s@cs on Dams.


• Some dam failures have caused serious damage too the province. The most relevant cases are summarized as follows:

o In 1912 a small water supply dam servicing the coal shipping port of Union Bay on Vancouver Island failed killing one man and causing extensive damage to property and the coal loading port facili@es.

o In 1941 a 10 metre high dam located above the town of Pen@cton failed resul@ng in severe damage to the downstream area. If the dam failed with today's popula@on living below, economic and environmental damage would be extensive and the probability of loss of life would be high.

o In May 1995, a 6 metre high earth-‐filled irriga@on dam failed causing approximately half a million dollars damage. The sudden release of storage killed 48 head of cafle, destroyed 1.5 km of a public road, damaged 100 acres of hay field and introduced 700,000 cubic metres of material into the Quesnel River. (Ministry of Environment BC., 2011).

Ref.-‐Web. (11) Ministry of Environment of BriBsh Columbia (2011)

Sta@s@cs on Dams.


• BC Hydro has developed a deficiency priori@za@on system to evaluate the level of performance of dams within the province. The actual and poten@al deficiencies are presented in the next matrix.

Ref. (11) from: BC Hydro, 2003. Ref.-‐Web. (11) Ministry of Environment of BriBsh Columbia (2011)

Sta@s@cs on Dams.

-‐Dams on United States – Failure staBsBcs

• Some of the most relevant dam failures in US history have been summarized by the Flood Hydrology and Meteorology, Technical Service Center, Bureau of Reclama@on, Denver Federal Center. Results from this inves@ga@on are presented as follows:

Ref. (6) W. Graham (2009) Ref.-‐Web. (12) AssociaBon of Dam Safety Officials (2011)

Types of Embankment Dams.

Embankments dams may be classified into different categories depending their purpose. Based on the United Sate Department of the Interior -‐ Bureau of Reclama@on (1977), three broad classifica@on may be considered. Use:

-‐Storage dams -‐Diversion dams -‐Deten@on dams

Hydraulic design:

-‐Overflow dams -‐Non-‐overflow dams

Materials:

-‐Concrete gravity dams -‐Concrete arch dams -‐Concrete bufress -‐Earthfill dams -‐Earth and rockfill dams

Ref. (1) United States Department of the Interior, Bureau of ReclamaBon (1977)


-‐Storage dams

•  Constructed to store water in periods of surplus supply for use in periods of deficient supply. • Periods may be seasonal, annual or longer.

• Water is mostly storage on winter and spring season for use on summer periods.

• Based on the purpose of the water storage (i.e. hydroelectric power, recrea@on, irriga@on) these dams may be sub-‐classified in different groups. • Construc@on design is most of the @me influenced by the purpose of the dam.



-‐Diversion dams

•  These dams do not generally impound water in a reservoir. Instead water is carried into ditches, canals and other conveyance systems.

• Diversion dam are generally used for:

§ Irriga@on § Return to the river aker passing for hydroelectric generators § Diversion from a live stream to an off-‐channel-‐loca@on storage reservoir § Municipal uses § Industrial uses



-‐DetenBon dams

• Constructed to retard flood runoff and minimize effects of sudden floods. • There are three main types of deten@on dams based on their use:

§  Type 1: Water is temporarily stored and released through an outlet structure without exceeding the capacity of the channel downstream. § Type 2: This type of dam is usually called water-‐spreading dam or dike, having its main used recharging underground water supply. § Type 3: Oken called debris dams, these are constructed to trap sediments.



-‐Overflow dams

• Constructed to carry discharge over their crest. • As discharge will tend to erode the design condi@on over the crest, no erodible materials must be used for construc@on. • Most common construc@on material include concrete, masonry, steel and wood.

-‐Non-‐overflow dams

• Constructed not to be overtopped.

• No erodible materials as well as earthfill and rockfill are commonly used for their construc@on.



-‐Concrete gravity dams

• Concrete gravity dams are most frequently constructed on sites with a reasonable firm and stable rock founda@on. • If adequate cutoff is provided, concrete gravity walls may be constructed on alluvial founda@ons. • May be adapted to be use as an overflow spillway crest for earthfill and rockfill dams. • Design of concrete gravity dams may be either curved or straight. Type of founda@on, use, safety and cost are some of the characteris@cs influencing the design to be used.



-‐Concrete arch dams

• Adaptable to sites where founda@on at the abutments is solid rock capable of resis@ng arch thrust. • It is most commonly designed where the ra@o of width between abutment and height is not appropriated, usually suitable for narrow canyons and gorges with steep walls. •  Required less material than most of the dams, making them economical and prac@cal. • Require high level of stress and force analysis.



-‐Concrete arch dams

• Basic design are defined by the curvature radius in two main categories, constant and variable radius. • For concrete arch dams with variable radius, upstream and downstream curves decrease in radius below the crest.



-‐Concrete bu_ress dams

• Comprise flat deck and mul@ple arch structures. • The solid water-‐@ght upstream site is supported at intervals in the downstream site. • Require about 60% less of concrete that solid gravity walls.

• Expensive due to formwork and reinforcement steel required for their construc@on.



-‐Earthfill dams -‐Homogeneous earthfill -‐Earthfill with toe drain -‐Zoned earthfill -‐Earthfill with horizontal drain -‐Earthfill with ver@cal and horizontal drain

Ref. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005)


-‐Earthfill dams -‐Homogeneous earthfill

-‐General layout

-‐Control for internal erosion and piping

• Seepage is likely to be evidenced on the downstream face through cracks and earthfill.

-‐Control for pore pressure • Pore pressure is not predictable.


Ref. (2) Fell et al., 2005


-‐Earthfill dams -‐Earthfill with toe drain -‐General layout -‐Control for internal erosion and piping

• Seepage is likely to be evidenced on the downstream face through cracks and earthfill. • Deficient control into rockfill if no filters provided.

-‐Control for pore pressure

• Pore pressure is not predictable. Ref. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005)



-‐Earthfill dams -‐Zoned earthfill -‐General layout -‐Control for internal erosion and piping

• Seepage will be intercepted by the earth and rockfill. • May act as a filter for the earthfill zone depending of grain size distribu@on.

-‐Control for pore pressure • Good control of pore pressure as earth and rockfill present higher permeability than earthfill zone.




-‐Earthfill dams -‐Earthfill with horizontal drain -‐General layout -‐Control for internal erosion and piping

•  Seepage is likely to be evidenced on the downstream face through cracks and earthfill.

-‐Control for pore pressure • Pore pressure is not predictable.




-‐Earthfill dams -‐Earthfill with verBcal and horizontal drain -‐General layout -‐Control for internal erosion and piping

• Seepage in earthfill and cracks is intercepted by ver@cal drain. • Drains have to be designed as filters with enough discharge capacity.

-‐Control for pore pressure • Good pore pressure control with filter drains controlling seepage through founda@on and dam.




-‐Earthfill dams q  Zone func@on

• Earthfill:

§ Controls seepage through the dam.

• Filter under rip rap: § Control erosion of earthfill through rip rap.

• Rip rap: § Upstream erosion control against wave ac@on. § Downstream erosion control from backwater flows from spillways.

• Fine filter: § Control erosion of earthfill by seepage water. § Dam founda@on erosion control (as horizontal drain). § Buildup of pore pressure (as ver@cal drain).



-‐Earthfill dams q  Zone func@on

• Coarse filter:

§ Erosion control of fine filter into rockfill. § Seepage water collected discharge.

• Earth rockfill: § Provides stability § Provides erosion control



-‐Earthfill dams q  Construc@on material for zone type

• Earthfill

§ Clay, sandy clay, clayey sand, silty sand possibly with gravel. § Greater than 15% passing 0.075mm. § Siltstones, shale and sandstones may give fine material if needed.

• Filter under rip rap § Sand gravel, gravelly sand. § Well graded with no more than 8% passing 0.075mm. § Usually obtained from gravel pit run or crusher run with minimum washing and screening.




• Rip rap

§ Selected dense rockfill size. § Prevent erosion by wave ac@on. § In earth and rockfill dams, oken constructed by sor@ng large rocks from rockfill and coarse fill zones.

• Fine filter § Sand or gravely sand. § Less than 5% fines passing 0.075mm § Non plas@c fines § Obtained from sand-‐gravel deposits by crushing, washing and screening.




• Coarse filter

§ Gravely sand or sandy gravel. § Obtained from sand-‐gravel deposits by crushing, washing and screening. § Design to strict par@cle size grading limits to act as filters. § Required to be dense with hard aggregates.



-‐Earth and rockfill dams -‐Earth and rockfill central core -‐Earth and rockfill sloping upstream core -‐Concrete face rockfill -‐Puddle core earthfill -‐Earthfill with concrete core wall -‐Hydraulic fill



-‐Earth and rockfill dams -‐Earth and rockfill central core -‐General layout -‐Control for internal erosion and piping

• Seepage in earthfill is discharge in the rockfill aker being intercept by filters.

-‐Control for pore pressure • Excellent control of pore pressure as rockfill provides free drainage.




-‐Earth and rockfill dams -‐Earth and rockfill sloping upstream core -‐General layout -‐Control for internal erosion and piping

• Seepage in earthfill is discharge in the rockfill aker being intercept by filters.

-‐Control for pore pressure • Excellent control of pore pressure as rockfill provides free drainage.




-‐Earth and rockfill dams -‐Concrete face rockfill -‐General layout -‐Control for internal erosion and piping

• Excellent for internal erosion as fine and coarse cushion layers act as filters. -‐Control for pore pressure

• Excellent pore pressure control as rockfill provides free drainage and fine and coarse cushion are effec@ve.




-‐Earth and rockfill dams -‐Puddle core earthfill -‐Hydraulic fill -‐General layout -‐Earthfill with concrete core wall


Ref. (2) Fell et al., 2005 Ref. (2) Fell et al., 2005



-‐Earth and rockfill dams q  Zone func@on

• Rockfill:

§ Erosion protec@on of coarse filter into coarse rockfill. § Provides stability § Allow seepage discharge through the dam providing free draining.

§ Coarse rockfill: § Provides stability § Allow seepage discharge through the dam providing free draining.

• Coarse cushion layer: § Provides concrete face layer support. § Erosion protec@on for fine cushion layer into rockfill.

• Fine cushion layer: § Provides concrete face layer support. § Prevent leakage for cracking or joints openings.



-‐Earth and rockfill dams q  Zone func@on

• Earthfill • Filter under rip rap • Rip rap • Fine filter • Coarse filter

Same func@on as earthfill dams



-‐Earth and rockfill dams q  Construc@on material for zone type

• Coarse rockfill

§ Quarry run rockfill. § Dense, strong and providing free drainage aker compac@on. § Compacted in 1.5 – 2.0m layers. § Maximum par@cle size equal to compacter layer thickness.

• Fine cushion layer: § Silty sandy gravel well graded. § From 2 to 12% passing 0.075mm. § Obtained from rock or gravel by crushing and screening. § Par@cles up to 200mm are allowed but internal instability may occur.

• Coarse cushion layer: § Well graded sand-‐gravel-‐cobbles mix. § Placed in 500mm layers. § Sa@sfy filter grading requirements.




• Rockfill

§ Quarry run rockfill. § Dense, strong and providing free drainage aker compac@on. § Compacted in 0.5 – 1.0m layers. § Maximum par@cle size equal to compacter layer thickness.

• Upstream filter § Sand gravel, gravelly sand. § Well graded with no more than 8% passing 0.075mm. § Usually obtained from gravel pit run or crusher run with minimum washing and screening.




• Earthfill • Rip rap

• Fine filter

• Coarse filter

Same func@on as earthfill dams


SelecBon of Embankment Dams

Selec@on of Embankment Dams.

-‐Availability of construc@on materials -‐Founda@on condi@ons -‐Climate -‐Topography -‐Saddle dam -‐Staged construc@on -‐Time for construc@on



-‐Availability of construcBon materials -‐Earthfill

• Availability and uniformity of earthfill will influence design and construc@on method.

v Example 1: Site condi@ons : Area underlain by sandstone which weather to produce a thinner sandy soil cover. Alterna@ve: Rockfill dam with concrete membrane or concrete gravity dam.

v Example 2: Site condi@ons: Two borrow areas with finer and coarser earthfill. Alterna@ve: Zoned dam (with ver@cal and horizontal drain) with coarser earthfill placed downstream of drain zones.

• In the presence of alluvial clayey soils, addi@onal zoning may be appropriate.



-‐Availability of construcBon materials -‐Earthfill

• When cobbles and boulders are present in clayey soil deposits, these have to be removed to prevent compac@on problems. • Rela@ve permeable soils can be used as earthfill with permeability not higher than the founda@on permeability.

• Blending of soils has to be avoided as it leads to increased cost and difficulty in quality control.



-‐Availability of construcBon materials -‐Rockfill

• Rock which can be quarried to yield free draining rockfill may leads to save in costs.

•  Igneous and low grade metamorphic rocks yield free draining rockfill. These rocks include granodorite, diorite, basalt, rhyolite, andersite, marble, greywacke quartzite, indurated siltstones and sandstones.

• Some metamorphic rocks, even though it may be dense with high modulus may break down due to compac@on to yield a poorly draining rockfill.

• Highly weathered igneous and metamorphic rocks will not yield free draining rockfill.

• Blas@ng may be use to get the required size of rockfill.

• On thick beds of sandstones and siltstones, oversize rockfill have to be break down and sor@ng when disposal.



-‐Availability of construcBon materials -‐Rockfill

• Rockfill may come from excava@ons of spillways, dam founda@on, inlet works, including others. These materials may not have the ideal proper@es, including shape and size, and changes in the embankment zoning may be required.

• Some sedimentary rocks tend to break down under compac@on. Under these circumstances, incorpora@on of free drainage rockfill zones may be required to warranty the embankment is capable of remaining free draining .



-‐Availability of construcBon materials -‐Filter and drains

• Filter aggregates may be obtained from alluvial sand , gravel deposits and quarries.

• Most aggregates comes from igneous rocks but may be found in metamorphic rocks.

• Not many filters use sedimentary rocks as these rocks present poor condi@ons considering durability and shape.

• It is common to locate screening and crushing plants to produce high quality aggregates for filters and drains.



-‐FoundaBon condiBons

Strength, compressibility and permeability on dam founda@ons will define the type of embankment design to be used.

• Soil founda@ons with low strength may require rela@vely flat embankment slopes to keep embankment stability.

• Permeable soil founda@on may lead to leakage and erosion, requiring some type of drain filters and cutoff protec@on.

• Low permeability rock founda@on is suitable for all types of dams, par@cularly for concrete gravity and concrete arch dams.

• In zones suscep@ble to ground mo@ons, removal or densifica@on of sandy soils, specially for loose to medium dense sandy soils, will be necessary to avoid liquefac@on effects.



-‐FoundaBon condiBons

• Grou@ng or special work may be required for dams on limestone founda@on. For this case earth and rockfill dams and concrete face rocks may give the best performance among all others.

•  For some sedimentary rocks subject to folding and faul@ng, including weak claystone and mudstone and strong sandstone, low effec@ve fric@on angles result for bedding plane shear condi@ons. Under this case, flat slopes may be required on design and earthfill dams with horizontal and ver@cal drain may propor@on one of the best op@ons.

• In most of the tropical areas, weathering of rock may lead to high permeability soil strength founda@on. Embankments with flafer slopes are generally adopted under this case.

• Embankments may present large amount of seflement when constructed on deep soils founda@ons. To prevent cracking induced by differen@al movement, construc@on of filters is required.



-‐Climate

• Construc@on of earthfill embankments turns difficult when wet weather or freezing temperatures are presented, especially when rain is con@nuous with low evapora@on levels.

• In some cases, concrete face rockfill or sloping upstream core construc@on may present advantage as the rockfill can be placed even in wet condi@ons.

• For very arid areas, concrete face rockfill may be preferable rather than earthfill due to the amount of water required.

• On tropical condi@ons, weathering products of igneous rocks (Saprolites) exists as residuals soils. This type of soils may induce high pore water pressure on embankments causing big displacements. Precau@on is always recommended if saprolites are used as embankment material.



-‐Climate

• Punchina cofferdam is a clear example of problems due to saprolite materials. The 45m high dam was constructed on sandy silt belonging to the soil group of saprolites. Construc@on progress had to be adjusted based on the precipita@on season leaving a short 2 months period for placing this material.

High pore water pressure leads to displacements up to 1.5m horizontal and 0.45m ver@cal , at pore pressure in the centre por@on of 60 and 70% of the dead weight.

Ref. (2) from: ChrisBan Kutzner, 1997. Ref. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005)


-‐Topography

Construc@on of embankments may have significant effects depending the type of topography presented in the area. Choosing op@mum embankment loca@on (not always possible) may lead to economic safes and more efficient structures. A couple of examples explaining this situa@on are given below.

• Changes in valley cross sec@on and curve of the river in plan, may favour upstream sloping core rather than central core to reduce amount of earthfill required for the design.

• Changes in slope of abutment may cause cracking on the embankment. Construc@on of face rockfill with extensive filter drains may lead to befer performance under this type of topography.

• Narrow steel sided valleys my cause problems for haul road construc@ons. Simple zone embankments such as concrete face rock may favouring this condi@on.



-‐Saddle dam

• Saddle dams are auxiliary dams confining the reservoir created by a primary dam in order to permit storage or to limit the extent of a reservoir for increasing efficiency. • Saddle dams have to be treated in the same way as main dams even if its loss might seen minor in rela@on to downstream consequences or loss in storage. Founda@on problems including erosion may arise jeopardizing the condi@on of the whole system. •  Issues may arise dealing with dam’s founda@on. Ridges may present different geological condi@ons than valleys so enough inves@ga@on has to be directed from a geologist perspec@ve for each par@cular dam site.



-‐Staged construcBon

• Economic safes is oken influenced when construc@on of a dam takes place in different stages.

•  For irriga@on, hydropower and water supply projects, lower dams and storage may comply with the demands in the early years.

• For tailings dams, storage may increase progressively as tailings are deposit in the dam.

• Concrete face rockfill, earth and rockfill with sloping upstream core or earthfill with ver@cal and horizontal drain are good op@ons when staging is carefully




v Boondoma Dam. Loca@on Boyne River in the South Burnef region of Queensland, Australia

View

Ref.-‐Web. (13) Sun Water Dam Porfolio (2011)



v Boondoma Dam. Use Supply water to the Tarong Power Sta@on

Water volume 204,200 ML of water

Surface Area 18.15 km²

Construc@on Stage one: -‐Final surface eleva@on: 295.55m -‐Concrete face dam with filter material enclosing rockfill up to 600mm DIA. at the upstream face. -‐Rockfill up to 900mm DIA at the downstream face. -‐Rockfill with no more than 5% passing through a 100mm aperture at the toe of the dam. Stage two: -‐Final surface eleva@on: 303.50m -‐Rockfill up to 900mm DIA.

Ref.-‐Web. (13) Sun Water Dam Porfolio (2011)

Embankment Dams Details.

Design considera@ons -‐Freeboard -‐Slope protec@on -‐Overtopping during construc@on Detail considera@ons -‐Embankment crest -‐Embankment dimensioning -‐Interface between concrete structures and earthfill



-‐Freeboard -‐DefiniBon

• Difference in eleva@on between the maximum water surface level in the reservoir and the dam crest.

• Normal freeboard: distance in eleva@on between the normal reservoir full supply level and the crest of the dam without considering camber effects.

• Minimum freeboard: distance in eleva@on between the maximum reservoir water level and the crest of the dam without considering camber effects.

• Camber is assumed as the extra height added to the crest to ensure detrimental condi@ons such as seflements and embankment consolida@on.




• Freeboard provides protec@on against overtopping resul@ng from: o Wind effects o Wave effects o Seismic effects o Seflement o Malfunc@on of structures o Uncertain@es in design

• Freeboard may be influenced by certain factors including: o Poten@al changes in design flood es@mates o Reliability of design flood es@mates o Assump@ons in flood rou@ng o Type of dam o Erosion suscep@bility




• Some recommenda@ons have been addressed by USBR (1992) when considering freeboard condi@ons.

o Freeboard at maximum reservoir water surface level § Minimum freeboard greater than

a) 0.9m b) Sum between wind set up and wave runup (During large flood condi@ons)

o Normal water surface level

§ Should be the wind set up and wave runup for the highest wind velocity that could occur.

o Intermediate water surface freeboard

§ Freeboard designed so that it has a remote probability of exceedance by any combina@on of wind set up, reservoir levels and wind generated waves.



-‐Freeboard -‐Wave and wind runup (preliminary design)

• USBR (1977) may be adopted based on: o Wind velocity =160km/hr (normal freeboard) o Wind velocity =80km/hr (minimum freeboard)

For rip rap slopes, freeboard requirements may be adopted as follows

Fetch Normal freeboard Minimum freeboard

>1.6km 1.2 m 0.9 m

1.6 1.5 m 1.2 m

4 1.8 m 1.5 m

8 2.4 m 1.8 m

16 3.0 m 2.1 m

Ref.(2) from: Foster et al., 1998



-‐Slope protecBon -‐Upstream slope protecBon -‐Requirements

• Currently, most of earth and rockfill dams are protected from erosion by dumping rockfill (rip rap).

• Characteris@c of rip rap may include: o Large to dissipate energy of wave effects. o Strong to avoid break down to small par@cles. o Durable to withstand long term effects.

• For earthfill dams, rip rap should be constructed under a filter layer to prevent erosion of earthfill material.

Ref. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005) Ref. (2) from: Fell et al., 2005


-‐Slope protecBon -‐Upstream slope protecBon -‐Requirements

• For earth and rockfill dams, rip rap is obtained by pushing large rocks from the coarse rockfill to the edge.

• For reservoirs maintaining a high water level, it may be possible to provide less or no rip rap protec@on on the lower part of the embankment. Rip rap should be placed up to MOL less 2 @mes the wave height.


Ref. (2) from: Fell et al., 2005

Ref. (2) from: Fell et al., 2005


-‐Embankment crest -‐Crest width

• The crest width has small influence in the overall stability of a dam and it is mainly determined by the minimum width required for construc@on (i.e. roadway).

• For larger dams, a 6 to 8 meters crest width is typically adopted.

• For small dams, 4 meter crest width is usually enough.

• Crest dimensioning is determined by arrangement of zoning under three principles.

1.  Filters have to be taken as close to the crest as possible to prevent internal erosion and piping control.

2.  Filter width can be narrowed at the crest under certain design condi@ons. 3.  Rockfill on the downstream face of the dam (if used) has to be taken as

close to the crest as possible to protect filter drains.



-‐Embankment crest -‐Crest width

• Arrangement of a road pavement on the crest for an earth and rockfill dam may give a befer perspec@ve about narrowing of filter zones.

• Crest is generally sloped towards the reservoir and cover with pavement to prevent erosion caused by traffic.

• Pavement reduce desicca@on cracking of the core when non plas@c materials are used



Founda@ons on Embankment Dams.

-‐Requirements -‐Founda@on prepara@on -‐Cut off founda@on -‐Slope modifica@on -‐Assessment



-‐Requirements

• Founda@on prepara@on for embankment dams depends mostly on: o Type of dam o Height of dam o Topography o Climate o Groundwater o Soil and rock proper@es

• For general founda@ons: o  low strength and compressive materials are removed. o Permeability is not a cri@cal factor. o Liquefiable materials have to be treated and/or removed.

• For cut off founda@ons: o  Highly permeable and erodible materials are removed. o Drains have to be consistent to create a non erodible low permeability condi@on.



-‐FoundaBon preparaBon

• Rock founda@on under earthfill

o Remove topsoil and depending of the topography, colluvial soil and rock to expose in situ rock founda@on. o Where weak seams in rock are evident, these may need to be removed and/or iden@fied in the design. o Slope modifica@on may be modified as required.

• Soil founda@on under earthfill

o Remove topsoil and weak compressible soils. To locate this weak soils, surface may be proof rolled with a tamping foot roller. o Where soils are fissured or landslide ruptures are present, these may need to be removed and/or iden@fied in the design. o Slope modifica@on may be modified as required.

Ref. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005) (3) R.C. Hirschfeld and S.J. Poulos (1973)



• Founda@on under rockfill

o Remove topsoil and weak compressible soils witch have a strength lower than the rockfill.

o Where landslide ruptures are present in the rock, these may need to be removed and/or iden@fied in the design.

o To ensure a op@mum support between rockfill and rock founda@on, cleaning of loose soil and rocks may be necessary with aids of bulldozer or grader.

o Slope modifica@ons seems unlikely for most condi@ons.




• Founda@on under horizontal filter drains

o For founda@ons with erodible soil and/or rock, horizontal drains are required.

o Remove topsoil and week compressible soils based on the assump@ons for design filter criteria.

o Where landslide ruptures are present in the soil and rock, these may need to be removed and/or iden@fied in the design.

o Slope modifica@ons are usually not required but only in especial cases (i.e. earthfill placed on top of filter drain).

o Surface must not be rolled prior to placing the filter as it will reduce permeability due to soil structure rupture.

o Clean up may be necessary if break up of surface is evident (par@cularly on low strength rock and soil).


Simplified Filter Criteria •  Piping criterion: (D85B represents the par@cles size that must

be retained; D15f representa@ve average pore size, filter to trap par@cles larger than about 0.1D15f)

D15f < 5 D85B •  Permeability criterion:

D15f > 5 D15B •  Grada@on control:

D50f < 25 D50B

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100 1000

Fine

r tha

n (%

)

Grain size [mm]

PSD Tailings - Sand Interface - Rockfill Relave Muestra 1

Relave Muestra 2

Relave Muestra 3

Relave Muestra 4

Relave Muestra 5

Relave Muestra 6

Relave Muestra 7

Relave Muestra 8

Relave Muestra 9

Relave Muestra 10

SAND fine envelope

SAND coarse envelope

ROCKFILL fine envelope

ROCKFILL coarse envelope

Darcy’s Equa@on

v = ki Q = kiA

v – Darcy velocity k – saturated hydraulic conduc@vity i – flow gradient i = h/l

Analy@cal Solu@ons for Two Dimensional Flow With Different

Boundary Condi@ons

Flow Nets

Confined Flow

Unconfined Flow

CriBcal Exit Gradient

Down Up

γ’ ioγw (γb -‐ γw)

Consider Stable Condi@ons

io γw ≤ γ’

or io ≈ γ’ γw

≈ 1.0 io ≈ 62.4 62.4

• Therefore, the cri@cal condi@on exists when the escape gradient exceeds unity.

• When this occurs, “piping” can happen.

Consider the forces on an element

= (120?-‐62.4)/62.4

References.

(1) United States Department of the Interior, Bureau of Reclama@on (1977). Design of Small Dams. Washington : United States Government Prin@ng Office. (2) R. Fell, D. Stapleton, P. MacGregor and G. Bell (2005). Geotechnical Engineering of Dams. Netherlands : A.A. Balkema Publishers Leiden. (3) R.C. Hirschfeld and S.J. Poulos (1973). Embankment Engineeing – Casagrande Volume. New York : Wiley Interscience

(4) S. D. Wylson and R. J. Marsal (1979). Current Trends in Design and Construc@on of Embankment Dams. New York : American Society of Civil Engineers. (5) M. Foster, R. Fell and M. Spannagle (2000). The sta@s@cs of embankment dam failures and accidents. Canadian Geotechnical Journal No 37. Canada : NRC Research Press.

References.

(6) W. Graham (2009). Major U.S. Dam Failures: Their Cause, Resultant Losses, and Impact on Dam -‐ Safety Programs and Engineering Prac@ce. World Environmental and Water Resources Congress 2009: Great Rivers History. (7) H.B. Seed and J.M. Duncan (1981). The Teton Dam Failure – A Retrospec@ve Review. University of California, Berkley, CA.

(8) Wallace Chadwick and Arthur Casagrande (1976). Report to US Department of the Interior and State of Idaho on Failure of Teton Dam. (9) Lessons from the Failure of the Teton Dam. Proceedings of the 3rd ASCE Forensics Congress, October 19 -‐ 21, 2003, San Diego, California.

References -‐ Websites.

(10) Interna@onal Commission of Large Dams ICOLD. hfp://www.icold-‐cigb.net/GB/World_register/general_synthesis.asp (11) Ministry of Environment of Bri@sh Columbia. hfp://www.env.gov.bc.ca/wsd/public_safety/dam_safety/responsible.html#failures (12) Associa@on of Dam Safety Officials. hfp://www.damsafety.org/news/?p=412f29c8-‐3fd8-‐4529-‐b5c9-‐8d47364c1f3e (13) Sun Water Dam Por�olio. hfp://www.sunwater.com.au/__data/assets/pdf_file/0020/2099/SunWater_Dams_2011.pdf

Useful References.

G. Gedeon (2004). Design and Construc@on of Earth and Rock-‐Fill Dams. New York : Con@nuing Educa@on and Development. ICOLD Bulle@n 91 (1993). Embankment Dams – Upstream Slope Protec@on.

ICOLD Bulle@n 99 (1999). Dam Failure – Sta@s@cal analysis. Failure of Teton Dam hfp://www.archive.org/stream/failureoketonda00teto#page/n57/mode/2up Teton Dam Failure Narra@ve. hfp://www.geol.ucsb.edu/faculty/sylvester/Teton_Dam/[email protected]

Date post:	13-Apr-2015
Category:	Documents
Upload:	werewaro
View:	126 times
Download:	10 times

Lecture 2 - Embankment Dams Extended

Documents