mwmh fact sheet 5

8/3/2019 MWMH Fact Sheet 5

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1051 9 9 7 M I N E S I T E W A T E R M A N A G E M E N T H A N D B O O K

The intent o this act sheet is to allow the mine

operator to build small earth dams (arm dams)

or minor or temporary water supply or to orm

part o a diversion drainage scheme. I the dam is

an important water supply or food mitigation tool

then input rom civil engineers and hydrologists

is vital. The calculation o expected catchment

yields and food fows are covered elsewhere in

this handbook; hence this act sheet will cover

the selection o a dam site, dam design and

dam construction. The inormation in this act

sheet is collated rom the text Nelson (1991).

Selecting a Dam Site

The easiest and most ecient dams involve

constructing an earth embankment across a small

valley. These are commonly known as gully dams and

will be the ocus here. Other types o small dams,

including hillside dams, turkeys nest ponds andexcavated tanks, are easible alternatives i a suitable

gully is not available, and involve many o the same

principles to be discussed. The important points to

consider when selecting a dam site are as ollows:

minesitelicenceconditionsshouldbechecked

or local water resources authorities contacted to

ensure a dam is allowable under environmental,

water use and dam saety restrictions;

thestoragevolumeshouldbeselected

to suit the expected catchment runo

volumes. This will prevent excessive

earthworks or an eroded spillway;

unlessthedamisforsedimentcapturepurposes,

the upstream (u/s) catchment should not be

excessively disturbed. I this is unavoidable,

an u/s silt trap will have to be installed and

constantly maintained (ie. emptied);

anideal site is on a fat gradient watercourse

in a wide fat-bottomed valley immediately

upstream o a narrow gorge. Sides o the

valley must remain stable when saturated

to avoid land slips into the dam;

theoundations or the dam must be

suciently strong to support the embankment

without excessive settlement and must

be impervious to seepage. Sti inorganic

clay is ideal while sedimentary rock can be

acceptable. Fractured igneous rock or deep

layers o sand and gravel should be avoided;

theavailability o suitable material nearby

is vital. Available quantities will determine thetype o embankment used as illustrated in the

attached table (Figure FS 5.1). Impervious

material or embankment construction should

contain 20%-30% clay with sand, silt and some

gravel. No rocks greater than 75 mm size should

be present. As a saety actor, two to three times

the expected quantities should be available; and

a subsurace geotechnical investigation should

be carried out on avoured sites to assess the

above actors as well as groundwater levels,

cuto trench depths and borrow pit boundaries.

The investigation should include excavated pits

along the dam centreline, spillway and in borrow

areas ollowed by geotechnical testing o samples.

Dam Design

Good design o the dam and spillway is vital to ensure

a stable embankment and to prevent ailure due to

erosion or excessive seepage leading to piping ailure.

Piping ailure results rom seepage water transporting

materialoutoftheembankmentcausingapipe

which rapidly expands leading to massive ailure. The

basic geometric design principles or a stable dam

are illustrated in Figure FS 5.1. The ollowing points

should also be accounted or in the dam design. Cuto excavations are used to prevent

seepage under the embankment by providing a

impervious barrier linking the embankment to

Construction of Small Earth Embankment Dams

F A C T SH EET NO. 5


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106 1 9 9 7 M I N E S I T E W A T E R M A N A G E M E N T H A N D B O O K

impervious oundation material. It must be connected

directly to the impervious embankment material

and must be keyed into suitable oundation material

as shown in the table below. I a cuto trench is

impractical due to excessive depths, an eective

alternative where oundations are moderately

pervious is to use a clay blanket 0.6 m thick (approx.)

extended 35 m (approx.) u/s rom the embankment.

Spillway fows must be diverted away rom the

downstream (d/s) toe o the embankment to

avoid erosion. A small return wall at the spillway

may be required, as shown in the gure. I

continuous small fows are expected over thespillway it is advisable to install a trickle pipe

or a small fow channel just below the main

spillway level. This will prevent scour erosion.

Outlet pipes are sometimes necessary to create

a gravity supply, supply a pump, drain water

or dam maintenance, satisy legal requirements

or to allow the dam to be used as a food fow

detention storage. I these requirements are

not applicable it is best to avoid outlet pipes.

Freeboard is required on dams to allow

or uncertainties in food fow estimation,

inaccuracies in construction and wave action.

The heights shown on the gure assume

a maximum 500 mm fow depth over the

spillway. I an alternative spillway arrangement

is used, the reeboard must be altered to suit.

Dam Construction

Good control o construction methods and

material condition is vital to achieve a water

tight dam. The ollowing construction phases

and guidelines should be adopted:

priortocommencingconstructionofthe

dam a surveyor should identiy the extent o

inundation, the embankment centre line and

batter toe lines, the spillway and borrow pits;

ensuretheproperequipment is available.

This should include scrapers and dozers or

small embankments while larger projects will

also require graders, rollers and water carts;

ifthedamislocatedinagullyorstream

which fows regularly it will be necessary

to dewater the site. This is best achieved

using an upstream weir and a gravity drain

which bypasses the dam. Groundwater in

trenches will need to be pumped out;

areliablewater supply is important i

the material used in the embankmentneeds conditioning (ie. addition o

water to allow proper compaction);

theareatobeinundatedbythedamwater

must generally be cleared and grubbed. This

includes removing all trees, shrubs, rocks and

any debris. This can be modied i aquatic

habitat is to be an ancillary unction o the

storage. This should be burnt or pushed

downstream o the embankment. At the same

time the area under the embankment should

be cleared and have all topsoil stripped

(100 mm minimum) and stockpiled;



Suitable oundation

material (SFM)

Required penetration

depth into SFM

Width o cuto

trench at base

Batter slopes or

excavated trenchClay 0.6 m 2.5 m minimum 1 :1

Rock 0.3 m 0.3 m vertical


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thecuto excavation should then be carried

out and impervious material placed and

compacted to bring the level back up to that o

the stripped oundation. The whole oundation

area is then lightly scarifed (50 mm deep) in

preparation or construction o the embankment;

borrow pits should ideally be within the area

covered by the stored water. They should

have side slopes o 3H:1V and should be

positioned a minimum o 6 m away rom the

upstream toe o the dam embankment;

embankment construction requires control o:

the moisture content o the embankment

material when placed must generally be within

the range 3% dry to 2% wet o optimum

moisture content. This is the moisture content

which allows the maximum density to be

achieved by the compaction equipment used;

the loose thickness o layers placed

should not exceed 100 mm i dozers

and scrapers are used or compaction

or 200 mm or sheepsoot rollers;

the degree o compaction achieved should be

95% Standard Compaction or 90% Modied

Compaction. This will usually require between

our and eight passes with a sheepsoot roller;

batter slopes should be controlled using

a template (timber triangle with therequired horizontal and vertical length

ratios ie. 3H:1V) and spirit level;

thespillway must be constructed absolutely

level to ensure there are no preerential

fow paths which will erode. When

cutting is complete the surace should be

topsoiled, grassed and compacted;

outlet pipes, i required:

must only be placed in a trench cut into

natural ground or compacted embankment.

The trench should be at least 100 mm

deeper than the pipe diameter;

between three and six cuto collars (1.2m x

1.2m) shall be evenly spaced along the pipe

to prevent seepage o water along the pipe;

do not place pipes at the very base o the

dam i sediment is likely to be a problem;

it is advisable to include a trash

rack at the inlet to the pipe;

valves should be placed at the

discharge end or in a pit on the d/s

slope o the embankment; and

topsoil to a depth o 100 to 150 mm

minimum and good holding grass such

as kikuyu or couch should be placed over

the entire embankment (u/s and d/s) andspillway. This should be ertilised and

irrigated i necessary to ensure rapid growth

and hence immediate erosion prevention.




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Dam Element

GEOMETRIC DESIGN CRITERIA

Homogenous Zoned Dam Diaphragm Dam

HEIGHT OF DAM (m) 0-3 3-6 6-9 0-3 3-6 6-9 0-3 3-6 6-9CREST WIDTH (m) 2.8 3.5 4 2.8 3.5 4 2.8 3.5 4

UPSTREAM BATTER SLOPE (H : V) 3:1 3:1 3.5:1 2:1 2.5:1 3:1 3:1 3:1 3.5:1

DOWNSTREAM BATTER SLOPE (H : V) 2.5:1 3:1 3:1 2:1 2.5:1 3:1 2.5:1 3:1 3:1

DIAPHRAGMTHICKNESSD(m)

(Perpendicular to dam ace)

0.6 0.85 1.1

FREEBOARD (m) : FETCH < 1000 m 1.0 m - Assuming 0.5 m maximum spillover depth

FETCH > 1000 m 1.5 m - Assuming 0.5 m maximum spillover depth

SETTLEMENT ALLOWANCE (mm)

(Construction level above required

crest level)

150 300 500 150 300 500 150 300 500

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F I G U R E F S 5 . 1


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SPILLWAY DESIGN

FLOOD FLOW MINIMUM

INLET WIDTH

MINIMUM OUTLET WIDTH (m)

(Various Return Slopes.)

(m3 /s) (m)

mwmh fact sheet 5

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