Rock catchment dam with self-closing watertap

8/9/2019 Rock catchment dam with self-closing watertap

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HRVESTING RAINWATER

SEMI-ARID AFRICA

ManuaL N o . 3Rock Catchment Damwith se[f-c[osing Watertap.


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Harvesting Rainwater in Semiarid Africa consists o f 6 Xanuals:

Manual N o. 1 . W at e r T a nk s w i th G u tt e ri n g an d Hand-pump.

Manual No . 2 . Small Earth Dam b ui lt b y Animal Traction.

Manual N o. 3 . R o c k C a t ch m e nt Dam with selfclosing Watertap.

Manual N o. 4 . Shallow Wells w it h B u c ke t l ift .

Manual N o. 5 . Subsurface and Sandstorage Dams.

Manual N o. 6 . Spring Protections.

Each I 4 a n u a l deals with siting criteria, standard designs an d bills of

quantities i n a simple text an d drawings.

The Manuals ar e based on p r a ct ic a l e xp e r ie nc e g a ined by building some

700 w a t er s t r uc t u re s fo r rainwater harvesting i n semiarid K en ya o ve rth e last 1 4 years.

Copyright

Permission i s hereby given fo r reproduction o f this material, i n whole

or p ar t, f or educational, scientific, or development related purposes,

except those invo l ving c o m me r c ial sale o r services, provided t h at f u ll

citation o f th e source i s given.


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ACKNOWLEDGMENT

The authors o f these manuals are qrateful f or t he qrants g ive n byDanida w hi ch mad e p oss ib le th e compiling and publishing o f their

e xp e rie nc e w it h r a inw at e r harvesting fo r selfhelp groups i n semiarid

areas o f Africa.

M uc h g ra ti t ud e i s also due t o the Ministry o f Agriculture i n Kenya,w h ic h t og e th e r w i th Danida afforded the opportunity o f developing low

technology and labourintensive methods o f harvesting rainwater and

thereby enabling people and livestock i n a semi-arid region o f th ecountry t o h a ve a c ce s s to a steady water supply.

Thanks ar e also due t o th e local inhabitants with and for whom these

t ec hn i qu es w er e developed an d i mp le men te d. T he ir understandable

skepticism i n starting u p t he se d e ma n d i n g a c ti v i t i es g a ve th e processa sound an d realistic foundation on which t o build.

Pe r so na l t h ank s ar e v e ry m uc h due to:

Preben Enhard o f Mutomo Soil and Water Conservation Project for hisconsiderable support.

Jan NissenPetersen fo r assisting in drawing m o re t ha n half o f th e

n i a n y drawings.

Kim NissenPetersen f or t he many proofreadings and useful comments.


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CONTENTS

PAGE

SURVEYORS MANUAL ON ROCK CATCHMENT DAMS 1

1 . Introduction 1

2 . Siting an d Selection Considerations 1

3 . Deciding on th e Siz e o f Dam t o Build 44 . Calculating th e Required C a t ch m e nt A r e a 5

5 . Designing the Dam 76 . Dam Wall Volume Calculations 8

7 . Standard Designs 1 1

8 . Materials, Labour and Transport Requirements 1 3

9 . Bills o f Quantities and Costing 1 6

10 . Site Preparations 1 711 . Setting Up the Templates 1 8

12 . Preparing th e Foundations 2 013 . Bringing Materials a nd t he Contractor t o th e Site 2 0

14 . Maintaining and I m p ro ving W a t er Quality 2 1

CONTRACTORS MANUAL ON ROCK CATCHMENT D A M 2 3

1 . Preparing the Foundations 2 4


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SURVEYORS M A N U A L

ON

ROCK CATCHNENT DANS

1. Introduction

The masonry gravity dam i s a ve r t ica l w a l l constructed around th e

outer edge o f a depression or hollow i n a rocksurface t o capture an d

retain water running o ff t he rocky surface. R o c k s u r fa c e s have th e

same e f f icie nt r u no f f production a s ir o n sh e e t r o o ft o p s without thecost t o the user therefore local people i n A S AL r eg i on s h av e taken a

great liking t o these water harvesting systems. This i s particularly

true where the materials are provided b y an outside agency or th egovernment and the people supply a ll t he manual labour fo r sitepreparation and system construction.

Hasonry gravity walls can b e b u il t i n all lengths an d sizes depending

on the shape o f the s i te w he r e t h ey are built and the size o f the

re ser voi r the y wi ll hold. There i s no standard design for thereservoir a s with tanks but there i s a standard design for the wallitself a nd t he method by which i t i s built. A set o f techniques ar e

explained that can be used t o bu il d a single wall, o r a dam withs ev e ra l s e ct i on s e ac h o f a different height o r length. Gravity walls

h av e b e en b u il t t ha t t he re ar e 2 metres high and 1 0 metres long and 6metres high an d 6 0 metres long. R e se r vo i rs h av e b e en created that

contain 2 0 ,0 00 li tr es and o ne s t ha t contain 4 million litres, allusing th e same construction design, methods an d m at e ri a ls a l th o ug h

obviously i n different sizes and quantities and w it h d i ff er en tcatchment areas


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Sometimes th e d ip s ar e filled with soil. I f trees and shrubs growt h er e t h is i s good because i t means there will be a deep dip which ca n

be excavated and store plenty o f water.

The rock ab ov e th e dip should preferably be w id e an d slope like af un n el t ow ar d s th e dip s o t ha t w at er w il l d ra in there. However,

straight slopes or rounded hills are s ui t ab l e c at c hm en t s because

simple stone and mortar raised gutters ca n b e b u il t ou t from th e endso f th e gravity dam sloping up and across th e rock. T h ey c a tc h run-offw at er f ro m th e rock surface and br ing i t down t o th e dam. Where th e

rock outcrop i s a steepsided, flattopped hill with th e dam built a t

the base o f t he c li ff t he se gutters can s ti ll b e used. I n t hi s c as e

they can b e built around the top o f th e rock, bringing water t o apoint a bo ve t he dam and letting i t flow ov er t he cl if f an d f al l i ntothe reservoir.

Positioning the Dam Wall

When such a site has been found i n the field th e best position t obuild th e dam must be decided upon. Looking down t he s l op e o f th e

rock, th e best site i s th e o u te r e d g e o f a dip, where th e angle o f th e

r o ck f a ce i s near th e horizontal before i t steepens again.

By having a horizontal base, reinforcement o f the dam i s not requiredbecause th e weight o f th e dam can withstand th e pressure o f th e stored

water. With no reinforcement th e construction and th e design becomes

simple. The dam w a l l m u st be built on a rock sloping down towards

th e reservoir because th e weight o f th e wall ca n push i t down t h e r o ckface and cause its destruction. M a ke s u re th e foundations are b ui lton almost flat r o ck s u rf a c es o r o ne s sloping s l igh t l y ba c k wa r d s t o th e

reservoir. The foundation must b e a c l ea n , h a rd r oc k surface and not


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Because t h es e d i p s i n t he r oc ks c om e i n a variety o f slopes a n d s i ze s ,

t he s iz e a nd shape of th e dam wall varies too. The number o f separatesections i n th e dam will depend o n t he s h ap e and th e curve o f the lip

o r l ed ge o n which t he d a ms foundations ar e laid.

Generally, fo r a crossslope depression forming a small valley i n th e

r oc k s ur fa ce , a single w al l c an b e b ui lt across th e dip that i sdeepest i n th e middle an d tapers t o both ends when i t joins th e valley

sides.

For a funnel shaped depression like a small basin, a Vshaped dam willb e built with th e p o i n t a t t he f r on t o f th e d ep re ss io n w it h tw otapering arms b a c k t o t he t wo sides o f th e funnel.


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3 . Deciding on tbe S i z e o f Dam t o B u il d

I n dec iding h ow b ig t o b u i l d a dam s ev er al t hi n gs need t o b e

c o n s i de r e d;

a . the water demand o f th e s e l f he l p b u i l de r s a n d u s er sb. th e amount o f labour th e s e l f h e l p g r o u p ca n supply

c . th e size o f th e c a t c h me n t a r e a a nd t he rainfall

d . th e volume o f th e reservoir created b y t he dam.

A1 1 th es e have t o meet somewhere i n th e middle for a n ef fec ti ve ,appropriate water harvesting system.

Th e best size o f s e l f h e l p g r o u p f or t he construction o f a masonrygravity dam i s about 6 0 families s o that each family ca n supply on e

adult member t o th e workforce fo r three o r f ou r d ay s per w ee k d ur in g

th e construction.

There i s a limit t o th e number o f w o rk d a y s a s e l f he l p g r o u p can be

expected t o provide i n a g i v e n s ea so n. T hi s depends o n th e length o f

th e dry season and th e commitments o f th e s e l f - he l p g r o u p t o othera c ti v i t ie s . G en er al ly , a t th e most a s e l f h e l p g r o u p ca n b e expected

t o work fo r 9 months or 1 80 work days a y ea r. During th e r a i n y s e a so nth e construction si te wi ll b e mostly underwater a s th e partially

finished dam retains w at er . S om e d am s h av e b e en b u i lt t oo big f or t he

c at ch me nt a re a and never f i l l ed c o m p l et e l y . Fo r that reason i t i snecessary t o b u i l d i n stages. The first stage should be a maximum o f

2 .5 metres. Th e dam ca n b e raised t o 3.5 metres i n th e next dryseason should th e catchment area prove big enough. Should the user

g ro up n ee d more water and b e willing t o continue with construction,

then th e dam wall ca n be r a i s ed a n o t he r 1 0 metre


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Water Demand

To calculate th e water demand for a group o f people i s simple.

Formula:

No. o f families x litres per family per day in dry season = demand.

Example:

6 0 families x 6 0 litres per family per day x 1 80 days dry season

= 648,00 litres

The amount o f water t h at n e ed s t o b e stored a t th e end o f th e r a i n yseason i s t he re fo re 6 4 8, 0 0 0 l it re s (648 c u b i c metres) t o satisfy

demand i t i s wise t o i n c l u de l o s se s t o evaporation o f about 33%.

The minimum size o f th e dam reservoir can then b e determined as :

Formula: demand + 3 3% i n evaporation losses

Example:

6 4 8 , 0 0 0 l i tr e s + ( 6 4 8 , 00 0 l i tr e s x 3 3 ) = 861,840 litres (862 cu.m.)1 0 0 r

4 . Calculating the Required Catchment Area

I f th e catchment area i s a ll r oc k outcrop w i t h little soil cover, i t

i s possible t o calculate th e size o f th e area that will supply this

volume i f th e s e a s o n al r a i n f a l l i s known.


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Since th e length o f a n average step i s a bo ut 0 . 60 m (6 0 c m ) followingexamples on measuring catchment a re as b y walking can b e shown:

a . e as y w al k on a s q ua r e r o ck sloping only a little, lengths

measuring 140 steps x 7 5 steps = (140 s te ps x length o f step

0 . 60 m ) x (7 5 s te ps x length o f s te p 0 . 6 0 m ) = 12 9 sq.m.

b. d i ff ic ul t w al k on a s q ua r e r o ck sloping much, lengths measuring

140 steps x 7 5 steps = (140 steps x length 0. 60 m x 0.75) x ( 7 5

s te ps x length 0 . 6 0 m x 0.75) = 9 7 sq.m.

c . c l i m p i n g o n a square rock sloping very much, lengths measuring

1 40 s te ps x 7 5 steps=

(140 steps x 0 . 6 0 m x 0 . 5 ) x ( 7 5 steps x0 .6 0 m x 0.5) = 6 5 sq.m.

Example o f calculating run-off volume:

Rainfall 300 mm x runoff coefficient 0.7 catchment area 9 7 s q .m

= 2 0 , 3 7 0 l i tr e s (2 0 cu.m.)

Enlarging a catchment area.

The catchment area o f a dam ca n be enlarged by b u i l di n g stone gutters

around th e rock. Such gutters will c o l le c t r u n of f w hi c h o th er wi s e

w ou ld h av e bypassed th e dam reservoir. Stone gutters ar e b u i l d o fflat stones sa t i n m or ta r. S ta rt building the g u tt e rs f r om the end s

o f th e dam wall and le t t h em s u rr o un d a s m u ch r o c k s u rf a ce a s possibleby b u i l di n g them with a g r a di e n t sloping 3 cm upwards per 100 cm.

this i s best measured by u s i n g a gutterlevel 1 00 c m l on g w i th 2 legs,

th e one b e i n g 3 cm shorter than th e other (see contractors manual)


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Having marked o ut t he area, measured it , and multiplied th e surface

area b y t he factor o f its steepness th e size c an b e compared againstth e size o f th e catchment n eed ed . T hi s w il l i nd i ca te whether th e

gutters are needed or i f th e catchment i s too small. I f it i s to o

small i t may s ti ll b e w or th g oi n g a he ad w it h th e construction i f thereare no alternative sites nearby. I f th e area i s bigger t ha n th at

required this i s fine because then a bigger reservoir can be b u i l t and

th e demand w i l l s t an d a greater chance o f b e i n g exceeded b y the supplya l mo s t e v e ry year.

5 . Designing the Dat

Having worked o ut t he demand and th e supply potential o f th e catchment

i t i s necessary to work out exactly where t o p ut t he d a m , how big i t

must b e an d how m u ch w a te r th e reservoir i t creates will hold . Withrooftop t a nk s t h es e things ar e easy because th e shape i s standard an d

th e volume c an b e calculated knowing a few simple m ea su re me nt s. W it hmasonry gravity dams, t he s h ap e o f th e reservoir i s not standard an d

i s uneven. T o work out volume speedily an d without u s i n g highlytechnical methods i n v o l v e s approximating th e complicated slopes byc o n ve n i en t s t an d a r d s h ap e s and taking a few s i m p le f i e l d m e a s u r e me n t s .

The method should b e simple enough t o take measurements in th e field

an d work out the approximate volumes on p ap er w it h a pocketcalculator. As an example o f th e methods t o b e adopted we use th e

singlewall gravity d am. To design m u l t i p le w a l l dams r eq ui re s m or e

e x pe ri e nc e w hi c h w il l b e gained from designing and building some o fth e less complex d a ms f i rs t .

The Single Gravity Dam

h h f th i b i d h b l


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To calculate th e volume o f th e reservoir, th e f o l lo w ing d im e ns io ns ar e

required:

a . th e proposed h eight o f th e dam a t its deepest point ( h ) .

b . th e length o f th e crest o f th e dam on either side o f th e deepest

point (Xa and Xb )

c . the l e ngt h f r o m t he t op o f th e proposed dam wall a t th e highestp oi nt b ac k a lo ng th e horizontal t o th e rock surface ( L ) . This

marks the surface o f the w a t er w h e n th e dam i s full.With these dimensions the volume o f the reservoir can b e found

considering t he t wo sides A and B separately;

Formula: Volume A = h x Xa x L + Volume B = h x Xb x L = Volume

Example:

4 4

A = h 2 .5 m x Xa 6.0 m x L 34. o m + B = h 2 . 5 m x Xb 8.0 x L 34.0 m

4 4

6 . Dam Wall Volume Calculations

= 2 9 7 . 5 cu.m.

by

There are two shapes that th e gravity w al l s e ct i on s can have, g ive n

th e standard design o f the wall. They depend on th e depth a t eitherend o f th e section and ar e illustrated a s Section 1 an d 2 below.


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Fran rneasurerient w e know t h e heights and lengths of the secticin. W e also know because oft h e standard design t h e width of t h e crest and t h e base. The crest is always 0 . 3 m n omatter what t h e dam height. Ho w ever, the base width varies according to t h e h e i g h t a s

show n in t h e r e l a t i c * i s h i p below.

6~h~gti~

5

4

3

2

00 1 2 3 4m.bcise

height 0.7 1.0 1.5

base 0 . 3 0 . 4 5 0 . 7

crest 0.3 0.3 0.3

2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0

1.1 1.4 1.8 2.1 2.5 2.8 3.2

0 . 3 0 . 3 0 . 3 0 . 3 0 . 3 0 . 3 0 . 3

This graph, which gives us t h e b a s e for each height allows us to calculate the volurries oft h e dam wa].ls.

For Sectic* 1 the volurne is equal to ha] f a rectangular bl k plus quarter f

,,

-

-

1111711 1

2 50-~-

~Sliding factor

~14ObaseOverturning i~ctor

~


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Thn~i1afor calculating volii of 4 a~plaster crest and frcnt wall:

Area to be plastered x 0.04 m thickness of plaster = vo lum e

Exa m ple: (AreaAh2.5xXaG.0) + (keaBh2.5xXb8.0) +2 2

(Crest 6 . 0 + 8 . 0 x 0 . 3 ) x 0.04 m plaster = volum e

A 7.5 sq.m. + B 10.0 sq.m. + crest 4.2 sq.m. x 0.04 = 0.87 cu.m. plaster = say 0 . 9 cu.m.

For secticn 2, the volum e is equal to a block with the average of the tw o heights ha and

hb plus half a block with the average of the t~heights ha and hb.

Secticn 2 therefore cctisists of block +half block.

Fbrninla f block: (h a + hb) x X a x 0.3 = vo lum e of block2

Fbrriilafhalfblock: (ha+hb)xXax(Baseb-Basec-0.6) = volumeofhalfblock2 2

2


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Standard Designs

Lay-out ptan1 : 1 0 0


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St a nd a rd D e s igns

o 30

L~Q~o~

O 45

Tapping station

Lay-out planV200


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7. Materia].s, labour aM transport requircments

The requireiients of materials, labcur aM transport are calculated as follows:

a. Voline of structure.

First calculate the total voltine o f t h e structure t o b e build by findir~the volinesof t h e varicius parts of t h e structure and thereafter adding it a l l t o ge t h e r . T h eresult will be th~total voline measured in cubic metres (cu.m.).

b. Type and weight of materials.

W h e n t h e total volum e of t h e structurevarious cccin-ients of that volu m e can b e

Weight per 1 cubic metre (cu.m.) material.

W a t e r 1 cu.m. weigbs 1000 kg = 1.00 tccneCeinen t 1 cu.m. weigbs 1350 kg = 1 . 3 5 tc*meS an d 1 cu.m. weigbs 1600 kg = 1.60 tccne

Stcnes 1 cu.m. weighs 2200 kg=

2.20 tccne1 cu.m. Stcce-iiiasccry weighs 2850 k g = 2 . 8 5

stcces 7 5 % 1650 kg = 1.65ixrtar 1 : 4 2 5 %ceinent 200 kg = 0.2sand 800kg=O.8water 200kg0.2

has been found, tbe type aM weigbt of t h eestiinated by using t h e following formula:

Cccverted measurcments.

1 tccne = apprax. 5 drums of water.1 tccne = 2 0 bags of cenent.1 tccne = apprax. 8 wheelbarrows o f s a n d .1 tccne = apprax. 8 wheelbarrows o f s t c c e s .

tccne aM cccsists o f :

tccne = 1 4 wheelbarrow o f stcces

tccne = 4 bags of cementtccne = 7 wheelbarrows of saMtccne = 1 d r um of water

l


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requireints = tc*mes = tlverted measurements.

Yor plastering: Multiply volum e with requirements for 1 cu.m. n~rtar1 : 3 wthnil.

V o l i i i e of p l a s t e r x 1 cu.m. requirements = tcmnes = atverted iasurements

O.9cu.m.xceinento.Sotccnes= O.5tccnesx2o= lObagsofcement

Fbr stcne gutters: 1 cu.m. n~rtar,

Required 1et~thx Requirements per

Ei l

1 . 5 t c t m e s x 8 = 12 wheelbarrows of sand

O.3tctmesx 5= 2drun~ofwater

d

1:4, gves100 metres of s t c c e gutters.

1 m = kg = tccnes = cccverted ineasurants

Eiaile:

Fbr walls: Multiply volui with requirements for 1 cu.m. o f stcce mascnry.

Volu of wall x 1 cu.m.

15.0 cu.m. x

15.0 cu.m. x

15.0 cu.m. x

15 .0

cement 0.20

stcces 1 . 6 5

sand 0.80

tcmne= 3.Otcnnesx20= 60bagsofcant

tcnne=24.8tccnesx 8=l98wheelbarrowsofstcne

tccne = 1 2 . 0 tctmes x 8 = 9 6 w h e e l b a r r o w s o f s a n d

cu.m. x water 0.20 tcnne = 3.0 tcnnes x 5 = 15 drums of w a t e r

0 . 9 c u . m . x s a n d 1 .6 0

0 . 9 c u . m . x w at er 0 .3 0

tcmnes =

tcnnes =


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Labour requirements:

O n experience i t is k n o w n that cne skilled artisan with the help of 1 5 s e l f - h e l p labourerscan prepare the site, carry material, inxm~rtaraM btiild 0.8 cubic metre of stcnemascnry per day. The estimate for labour requirements is therefore as follows:

Thrmula for wall: Vo lum e cu.m. = skilled days required. Unskilled days = x 150.8 cu.m. per day -

~mp1e: Vo lum e 15 cu.m. = 19 days of skilled labour. Uiskilled days 2850.8 cu.m. per day

Forimila for stcne gutters: m of gutters = skilled days. Unskilled days = x 151 5 m

E xa m p le : 2 0 0 m of gutters = 14 skilled days and 2 10 unskilled days15

&iter the requirements cc the Bil1s of ~antities

Transport requirements o f material

Transportaticc of materia1s is divided into t~categories:

a. Transport of la1 rnaterials, such as sand, stcnes and water, wi11 be transported tothe site by the self-help groups using oxen-donkey and hand carts given to then bythe project. The num ber of loads to be transported and the dstances involveddepends on local conditicns aM cannot be estimated here.

b. Transport of purchased materials, e.g. cement, reinforcenent wire aM tenplates isestimated accordngto tcnnage, distance aM ccst per k x n .


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Bi11 of Quantity fcr ite~to b e delivered free of tharge

Unskilled labour: 4 95 labour days . x

S and : 16.7 tcnnes (= 134 wheelbarrows) x

Stcces: 42.6 tcnnes (= 343 wheelbarrows) x

W 3 7 (

8. R~11~of QUantities and Cting.

T w o bills of quantities are needed, because about half the itans will be delivered by thedccor/ministry aM the other half will be delivered free of charge by the ccnamity

ccccerned. Since the ccinm unity is supposed to ccctribute about ha1f the st of theproject, a value of their inpit has to be calculated.

Eiaile.

Bills of QUantities for itai~to be delivered by the dcrsx/rriiriistzy.

Skilled labour: 1 ccctractor for 3 3 days x Shs = Shs

Cant 4 . 3 tonnes = 86 bags x Shs = Shs

G.I.piping,r:9ometres/6=lslengths xShs =Shs

Polythene sheeting f o r c u r i n g : 2 0 m e t r e s x S h s = S h s

Thansport o f ccntractor aM materials 8 tctmes x... k m x S hs = S h s

Thtal st S h s

I~the self-help group

Shs = Shs

Shs = Shs

Shs = Shs

Sh Sh


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9 . Site Preparations

Having decided on th e layout o f th e dam w a l l s e c tio ns and shape o f th edam and having calculated the size o f dam that can be built in one

work season th e construction site can be prepared. As indicated inth e p r ev i o us s ec ti o n, i f a large dam i s to b e built, but i n stages

then th e site should be prepared f or t he smaller dam (2.5 m firsts t ag e ) f i rs t (with the templates f or t he height o f th e smaller stage).

I n future work seasons the dam can be bu il t u p wa r d s and outward. I fthe base o f th e dam i s bu il t according t o larger templates a lot o f

effort i s needed t o create even a low structure and this effort will

be wasted i f th e f ut ur e w o rk p l an s are abandoned.

l s t p l

2nd ph

3n d


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Clearing a Track and th e Construction Camp:

a . The selfhelp group should be i s su e d w i t h t oo ls o n l o an s u ch a s

machetes, hoes and axes w i t h w h i ch they can clear a t ra ck wi deenough fo r a t r ac to r w i t h t r ai l er . They should avoid felling

l a rg e t r ee s t o prevent th e need fo r s t u m p r e m ov a l , and should not

cross deep gullies or steep slopes.

b. A t the end o f t he t r ac k, a s close t o th e construction site a s

possible, th e selfhelp group should clear a large enough areafo r vehicles t o park and turn, fo r materials to b e piled and for

th e selfhelp group t o si t and rest a nd f or meetings.

c . A t th e dam site, th e selfhelp g r ou p s h ou l d e x c av a t e any soilc on t ai n ed w he re th e r e se r v oi r w i l l b e an d sweep and c le ar s oi l

from th e catchment area where relevant. I f there i s a lo t o f

soil i t should be carried t o t he f o ot o f t he r oc k outcrop an d

b ui lt i n to a semicircular earth dam t o c a tc h w a te r an d provide alivestock watering point.

10. Setting Up the Templates

a . The templates should b e placed upright a t th e corners o f th e dam,

a t th e deepest point o f th e single wall an d can b e supported by

small stones and mortar. In the sketch this i s shown fo r adouble-walled dam.

b. The s it es f or t he t e mp l at e s s h ou l d h a v e already been marked i nth e s i t e a s s e s sm e n t b ut c an be doublechecked. Having confirmed

thi s then t he templates should b e positioned a t right angles t o


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c . The en d o f th e dam should be f i x ed w i t h a small cement ledge 3 0

cm w i de t o which th e b u i l de r s lines ca n be attached and whichmark th e p o i n t th e dam w il l b e b u i l t to.

Use a h os e p i pe f il le d w i th w at er t o measure t ha t t he top o f th etemplates are in a horizontal level, and t o locate the e n ds o fth e dam w al l o n th e rock.

d. F ou r b ui ld er s lines should b e f i x e d from th e l e dg e t o th etemplate (and between templates) t o show th e form o f th e masonry

w a ll a l on g its length.

e. Th e two top lines should p a ss i n si d e th e t e mp l at e f r am e and be

t ie d a ro un d t he t op corners o f th e frame. The bottom or side

lines m u st a l s o pass inside th e template and can be t ie d a ro un d

th e sides o f th e frame.

f. Once th e templates and dam e nd s a re definitely level and th elines f i x e d th e templates c an b e anchored in the upright by

applying mortar an d stones t o their base.

g . A s a double check, th e lengths o f th e section crests should b e

measured a g a i n and compared t o th e earlier measurements. I fthere i s any difference, th e materials calculations should b e

mad e agai n t o ensure th e c or re ct a mo un t o f materials are

delivered.


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11. Preparing the Foundations

Once th e builders lines h a ve b e en strung, th e foundation o f th e dam

walls can be traced exactly. Th e s e lf he l p g r ou p n e ed t o follow thesesteps t o ready th e rock surface a s a foundation f or t he dam wall;

a . Remove al l di r t and loose r o c k f r a g me n t s insid e t h es e l i n es which

w i ll f o rm th e foundation o f th e dam wall.

b . Thereafter, chisel this area s o t ha t i t has a rough enoughsurface t o ensure proper bonding with th e dam wall t o b e build

upon it . Any hollow sound from th e r o c k i n di c a t es a fracture o r

hollow space underneath. The rock must b e c hi se le d a wa y untilth e sound o f s o li d r o ck i s heard on s t ri k in g w i th th e hammer.

12 . Bringing Materials and th e Contractor t o t he S it e

a . On clearing th e reservoir area o f soil, many stones will b e

u ne ar th ed . T he se should be piled separately a t t he e d ge o f th es it e n ear th e foundations fo r use i n th e dam fill. I f th e rock

outcrop i s granite and ha s been folded an d fractured, then i t

will b e possible to lever o ff l ar ge f la t s la bs o f stones 310 cmthick like large roofing slates. these c an b e prized o ff f or u sei n facing th e outside o f th e dam and f or t he gutters.

b . Cement should be brought by tractor and trailer and the b ags

carefully counted and guarded b y a trained storekeeper from th e

local community. A c ar ef ul a cc o un t o f each bag used i n

construction should be kept t o m ak e s ur e n on e goes missing.


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13 . Maintaining aud Improving Water Quality

Water Quality

To b e able to obtain unpolluted w a t er f r o m a rock catchment, i t i simportant t o keep th e c a t ch m en t a r e a a nd t he r e s er v o i r c l e an and tidy.

This i s best i s d one by th e following methods;

a . Fen ce off the e n ti r e r o c k f o r ma t i on on which th e r oc k c at ch me nt

i s situated by planting a l iv e f en c e o f thorny bushes along it ssides and/or around it s entire foothill. Recommended plants

include;. Fig C a ct u s ( P ri c k ly Pear) which also provide fruit,

Sisal which provide p o le s f or roofing, an d any local thornyp lan ts w hi ch d et er li ves toc k, w il d a ni mal s an d people from

entering th e area.

b. Keep th e catchment a re a c le an b y removing al l soil, vegetation o ro th er l oo se debris from it . I f necessary, sweep th e catchment

area once o r t wi c e a y ea r.

c . Just before th e onset o f th e rainy season, i t i s recommended that

th e dam be emptied and cleaned thoroughly o f s i lt a nd a lg ae .

d. Dig a d itch a t th e tapping station t o lead w as te w a te r a wa y f ro mth e tapping poi nt. The d itch can e nd u p i n a pi t w he re b an an as ,

s ug ar c an e o r f ru it t re es ca n benefit from th e waste water.

e. T o avoid mosquito breeding w h i ch p r o mo t e s th e spread o f malaria,

Tilapia fish can be brought t o t he s it e a nd introd uced t o th e

reservoir. Besides eating mosquito larvae an d algae, the fi sh


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Water Quantity

Evaporation in ASAL regions can result i n two metres o f w ate r b ei ng

lost from a reservoir in a year. For a single dry season i n th e

tropics, this will be m or e t ha n one m e t re w h ic h amounts t o between 2 5 %and 5 0% o f th e volume o f most masonry gravity dam reservoirs.

Where a rock catchment dam cannot supply sufficient water, either

because i t i s too small, has too many c on su me rs , o r e ac h c on su me r

draws too m u ch t h en th e reservoir can be roofed to reduce evaporationb y up t o h al f.

In our experience, the most effective lowcost roof i s made by

building pillars o f concrete blocks within th e reservoir spaced 3 t o 4metres apart. Trunks cu t from nearby trees ar e t he n t ie d onto t he t op

o f the pillars with galvernized wire. S i s al p o le s are then ti ed

across these trunks with rope. Although roofs are expensive t o b ui l d,

i t i s often cheaper t o roof a reservoir t o c o ns er ve w a t er than i t i sto extend th e dam w a ll a n ot he r m et r e to compensate f or t he evaporation

lost.


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CONT~ACT0RSM A N U A L O N R O C K C AT C M ENT D A N

CONTENT S

PAGE

CONTRACTORS MANUAL ON ROCK CATCHMENT DA N 2 3

1 . Preparing th e Foundation 2 4

2 . Installing th e D r a w o ff P i pe 2 5

3 . Constructing the Dam wall 2 6

4 . Plastering th e Dam Wall 2 7

5 . Laying Ou t and Building Masonry Gutters 2 7

6 . Constructing the D r a w o ff P i pe and Tapping Station 3 0

7 . Quality Control and Maintenance 3 2

CONTRACTORS MANUAL ON SELF-CLOSING WATER TAP 3 4

1 . Selfclosing Water Tap 3 5

2 . Materials an d Cost 3 6


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Th e design o f th e r o c k c a t c h me n t dam will b e m a de by the Inspector.

The I n s pe c to r w i l l a l so se t up th e templates and ensure th e necessarysite preparation ha s been carried o ut b y t he community group. The

I n s pe c to r w i l l decide how m u ch m a te r i al i s needed f or t he constructionand supply th e sufficient cement, r o c k b a l l as t , sand and water f or t hej ob .

1 . Preparing th e Foundations

Once th e b u i l de r s lines h a v e b e e n strung, th e foundation o f th e d am

walls can b e traced exactly. The a r ti s a ns n e e d t o follow t he se s te ps

to r e a dy th e rock surface a s a foundation f or t he dam wall.

a . Remove al l di r t a nd l oo se r o ck f r ag m en t s i n si d e t h es e l i ne s which

w i l l f o rm th e foundation o f th e dam wall.

b . Thereafter, chisel thi s area so t hat i t ha s a rough enough

surface t o ensure proper bonding with th e dam wall t o be builtupon it . Any hollow s o un d f r om the r o ck i n d i c at e s a fracture o r

hollow space underneath. The rock m us t b e c hi se le d a wa y until

th e sound o f s o li d r o ck i s heard o n striking w i t h the hammer.

3cm f m o r t 1 :3


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2 . Installing th e D r a w o ff P i pe

a . Place a drawoff pipe, 3 metres long and made o f 3. 8 cm (1.5)

g al va ni ze d i ro n p ip e w it h threads a t both ends, a t th e lowestpoint o f th e planned dam wall and attach i t with mortar (1 :3 )

onto a clean surface of th e rock.

b. The dr a w of f p i p e should s lo pe i n order t o avoid air bubbles than

can block the passage o f water in the pipe.

c . When a rock catchment dam i s situated on a r o c k s h e l f or i n a

valley, the floor o f th e reservoir will be higher than th e bottom

o f th e dam wall. I n t hi s i n s ta n ce w a te r w i ll flow downwards bygravity without th e need o f syphon. Th e d ra w of f p ip e should,therefore, sl ope d own towards th e dam wall and th e plannedtapping station a t the foot o f the rock.

Gravitv flow / Upwards/ sloping

/ : reservoir

3mof 1 1/2G.l.pipe


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3 . Constructing the D am Wall

a . Carefully clean l ar ge f la t s t on e s ( p r e vi o u s ly prized off therock) w i th w a te r a nd s et them i n mortar (1:4), exactly along th e

inner sid e o f t he t wo buld ers lines marking each side o f th e

wall. Tap th e s t on e s w i t h a mason hammer t o ensure a firm

c on n ec ti on w it h th e mortar. The stones must not t ou ch e ac h

other. Mortar must be i n between them. Support th e stones i np lac e w it h small sticks until th e mortar i s hard enough t o hold

them securely. B u i l d b o t h i n ne r a nd o u t er w a l ls u p t o a height

o f about 5 0 cm.

S T O N E M A S O N R Y

Clean stones sa t inmortarl ~3cm.mortar t3.D ry cement or~w e t r o c k . -Glearock~

b. When these inner and outer walls h a v e c u r ed for a day o r tw o

und er polythene s he et , f i ll i n t h e s p ac e between t he m w i th clean

stones and mortar (1:4). A11 th e s to n es m us t h a ve mortar betweenthem. Reuse any mortar that has fallen outside th e wall an d

onto th e r o ck im me d ia te ly . K e ep al l mortar under shad e and usei t before i t i s one hour old. Compact th e stones and mortar

together by pushing d o wn w i th a stick. When this section o f th ewall i s filled in , leave i t with a very rough surface, withstones sticking u p s o a s to achieve a good bonding with th e next

section o f wall.

Build wall in c i p p r o x .5O cm high c o u r s e s .


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4 . Plastering th e Dam Wall

d. Moisten th e inner s id e o f th e wall facing th e r es er v oi r w i thwater Throw a layer o f plaster (1 : 3 ) onto the wal l un ti l i t

a . Some skilled artisans can bu il d a wall so watertight that no

plaster i s needed a t all. However, at most dams, the i nn er wal lfacing th e reservoir should be plastered.

b. Chisel th e stone masonry wall t o a smooth, clean surface free o f

loose chippings and wash any d ust away wi th water. I n

-particular, thoroughly clean th e corner between th e inner wall

and th e rock floor w it h c h ise l s and water.

c . Moisten th e c l ea n c or n er w i th w at er . Press compact mortar

int o it . This plaster should extend 1 0 cm u p th e wall and

out onto the rock floor. Smooth the mortar with a woodenuntil a 4 5 degree ba nk o f plaster i s created.

3 ) . P l a s t e r 3cmthick, 1:3.

4 ). Nl L,cementslurry. p r e s s e d

on t o freshm o r t a r .

(1 : 3 )

1 0 cm

float

PLASTERINO

1 ) .C lea n corner properly.2) Fill corner with

mortar 1 :3 .


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5 . Laying Out and Building Masonry Gutters

The catchment area draining int o th e reservoir created by th e dam ca n

be increased by constructing gutters ou t from th e ends o f th e dam wall

that c at ch r un o ff running down the rock and leads i t into th ereservoir. Without th e gutters i t w o u ld r u no f f e it he r side o f th e

dam. On c ir c ul a r r o c k outcrops, water can even be brought roun d from

th e ot her s id e of the rock. O n s t ee p , flattopped rock outcrops,

gutters can collect the water draining o ff t he flat to p and let i t ru ndown th e cliff directly above the site o f th e dam.

a . The gutters can b e l ai d ou t using a c h anne l l a you t frame, also

called a gutter level, and a s p ri t l e v el . Notice that one leg i s

3 cm shorter than th e other leg.

1 0 0~, ~ .,i

~=~nTflI ~0_~~3cms lo p e p e r lO O c m

G u t t e r - l e v e l


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d. Starting a t e , it h er end o f th e dam wall, clean flat stones about3 0 cm h i gh w i th water. Set t he m i n mortar (1 : 4 ) along the lines

marked out by th e layout frame. The s m o ot h e r s id e o f the stonesshould b e facing upslope s o that water passes by them freely.

Fill i n the space between t he se f la t s to nes w it h mortar andsmaller flat stones t make th e gutter wall a uniform height.

The flat s t one s s h ou l d be angled slightly downslope. Supportthem i n this position with a line o f s ma ll er r ou nd s to ne s also

se t in m or ta r. B ru sh a ll t he joints and mortar s u r fa c e s w it h awe t brush s o that a ll t he joints ar e smooth and closed an d run

of f water can flow smoothly int o th e r e s er vo ir w it h n o l e ak s .

G U T T E R S


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6 . Constructing the D r a w O ff P i p e and Tapping Station

a . Although some people may prefer t o f e t ch w a t er directly from thereservoir, they should be encouraged t o collect their w at er f ro m

a tapping p o int fo r reasons o f hygiene.

b . W he n w at er has t o be drawn by gravity from a r es e rv o i r s i tu at e d

a t a lower l ev e l t ha n th e dam wall, a s i n the example shown i nour diagrams, th e piping system must be equipped with a syphon

devce and a tapping point located at1 a level lower than the

floor o f th e reservoir. Attach th e syphon, w h ic h c on s i st s o f a

7 0 cm length o f 3 .7 5 c m (1.5) galvanized iron piping ( G I ) ,

vertically to th e drawoff pipe with a ( G I ) pipe tee a s shown.Close th e upper end o f the syphon with a removable plug.

c . From th e syphon tee, reduce the piping from 3. 8 cm (1.5) G I t o2 .5 cm (1) GI down to the tapping point which i s usually

situated a t th e foot o f th e rock outcrop on which the dam i sbuilt. Lay this piping o n th e rock s o t ha t i t bends t o fit its

p ro fi le . S up po rt the G I p pe e te r y 100 cm with a l ar ge s to ne


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d Build th e ta~pingstation o f stones o r blocks under a shady tree


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d. Build th e ta pingstation o f stones o r blocks under a shady tree.

Divide the 2 .5 cm G I p i pe w i th tees t o f ee d th re e water taps.Build th e tapping station s o there i s room fo r three jerrycans t o

stand under the three taps a t th e same time.

e . I n th e reservoir, reduce the drawoff pipe i n the dam wall from3. 8 cm (1.5) t o 2 .5 cm (1) an d extend i t down t o th e deepestpoint i n the reservoir. Connect a perforated PVC pipe, 10 0 cm

l on g t o t he 2 . 5 cm G I p i pe w i th a horizontal nonreturn valve.

Build a box o f f il te r bl o ck s (made o f a mixture o f cement and

s ma ll s to ne s) around the PvC pipe but with t he l id o f the non

r e tu r n va l ve just outside the box.

0 .40 ~O4 0020 . Q : - ? s t ~ 1 2 2

0 1 .5 ~ry1~ t?O _______9j9~

0 . 3 0 ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ______

//~/%~/)/)//)/~///,2~ //////////////, ~777

1 .2 0

f. Remember that a ll t he joints i n th e syphon pipe system must be


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7 Q lit C t l d M i t


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7 . Quality Control and Maintenance

I f th e instructions g ive n a re n ot followed closely then th e dam may

not prove watertight and th e takeoff system will not function. The

m o s t im p o rt a nt are a s follows;

a . I f th e foundations ar e not prepared p r o pe r l y, t h en th e dam will

leak a nd t he r e s er vo ir w il l rapidly empty. The rock surface mustbe cleared, washed and chiseled and a good base o f cementapplied.

b . The packing of mortar an d ballast within th e dam should b e tight,

w it h s to ne s and mortar pushed i n hard and th e stones wetted

beforehand. A piece o f wood should be used t o poke th e mortar i nbetween th e stones t o fill a ll t he airholes. I f this i s not d o ne

effectively, th e dam could leak.

c . The facing o f the wall n ee ds t o b e formed o f flat stones boundwith mortar. There should b e n o g ap s, a n d an even finish.

d . The d r aw o f f p i pe m u st be a t the l o we s t p o int i n the reservoir toallow a s m u c h w a t er a s possible t o be drained ou t through it . I f

sediment i s washed int o th e dam i t should be cleaned out i f th edam d r ie s u p s o t ha t th e filter box i s not buried and drawoff

prevented.

e. The syphon point, i f fitted, should be a t the highest p o int o fthe d r a w o f f p ip e and filter box because o t h er w is e w a t er may not

leave th e dam.

I t i s important that a ll t he a bo ve requirements are satisfied I f


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D o es w at er l ea k f ro m th e face o f th e dam?


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D o es w at er l ea k f ro m th e face o f th e dam?

I f it does there i s a weak s po t i n t he w a ll w h er e the facing, and/or

th e fill have not been properly constructed. I f th e dam i s t o b e

e n l a r ge d w i t h th e building o f another s t ag e , t h s f a ul t can b e ignoredbecause building the dam upward and outward will provide an effective

seal. I f i t is not t o b e e nl ar ge d, t h en o n ce th e water level ha sdropped below th e leak, th e facing should be chiseled away within a

large radius o f th e leaking area and a layer o f plaster a dd ed w it h

waterproofing.

D oe s w at er f lo w o ve r p ar t o f t he dam wall outside the spillover when

th e reservoir overfills?

I f i t does, the cre st o f th e dam i s not l ev el a ll t he way round.

Using th e hosepipe t he t op should be leveled with mortar and smallstones. This prevents concentrated overflow and erosion forming

downslope.

Do e s th e dam f ai l t o f i ll u p o v er m os t y e ar s e v en though your design

calculations say i t should?

I f so, c h ec k y o ur volumetric calculations again t o see i f you have notoverdesigned y o ur s to ra ge . I f they ar e c or re ct , c he ck a lo ng the

gutters fo r breaks o r leaks and i f possible, extend th e gutters tocatch r u n o ff f r om a large area.

I s th e plaster lining the dam cracked and flaking?

I f so, th e curing wa s not carried ou t properly, or else the plasteran d waterproofing mixtures were incorrect. I f th e dam d oe s n ot leak,


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C O N T R A C T O R S M A N U A L O N SELF CLOSING W A T E R T A P


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C O N T R A C T O R S M A N U A L O N SELF-CLOSING W A T E R T A P

CONTENTS

P A G E

C O N T R A C T O R S M A N U A L O N SELF-CLOSING W A T E R T A P 34

1. Selfclosing Water Tap 35

2. Materials and Cost 36

3. Making the Selfclosing Water Tap 37

4. Installing the Water Tap 39

5. Making a Fixed Spanner for the Caretaker 40


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1 Self Closing Water Tap


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1 . Self-Closing Water Tap

Below i s a drawing o f a selfclosing water tap. Fitting a ta p such as

this t o d r a w o ff p i p e f r o m a rockcatchment, spring p ro te ct io n , t an kor sanddam will provide a major saving in water use in ASAL

environments. Rural people will often le av e t aps running an dunattended, wasting l a r g e q u a n t i ti e s of water. The sight o f running

water g i ve s t he impression o f unlimited supply a n d j u st a s people i n

mo re h um id re gi on s leave a tap running when washing h a n ds or do i n g

dishes, ASAL rural people will do - t h e same.

By having a tap t h at t u rn s itself off b y the downward force o f th e

water and th e weight o f th e handle once th e upward pressure o n th e t aphead i s released b y t he user this problem i s successfully solved. I n

ad d ition, the use o f a rubber b a ll v a l ve i s a robust design with

little maintenance requirements and has few moving parts that can be

broken o r worn down.


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2 . Materials and Cost


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The materials required t o manufacture a selfclosing ta p are

illustrated in sequence and includ e;

a . Reducing bush, galvanized iron

(G.I.), 1.5 t o 3/4

b . Elbow pipe, G.I. 1.5

c . Rubber b a ll w i th 2 mm diametre

d . 7 5 mm len gth o f 8 mm diametre

iron bar

e . Reducing b ush, G. I. , 1.5 t o3/4

f . Nipple, G.I., 3/4

g . Round 2 nail

h . 4 cm length o f 1 G.I. pipe

wth one threade

i. Socket, G.I., 1 to 1/2

ab

c

d


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3. Making the Se],fClosing Water Tap


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The valve

a . W ind a

thread1.5 t o

few rounds o f sealing t a p e c l o ck w is e around one end o f a

3/4 G.I. nipple and screw i t into a reduction bush o f3/4 G.I.

b. Cu t two 5 mm w id e and 2 0 m m l on g s lo t i n th e nipple with a 4 mm

drill bit and file.

c . D r il l a seat fo r a 2 2 mm rubber ball i n th e joint o f the nipple

and reduction b u sh u si n g firstly a 1 9 mm d r il l bit, and then a 2 8mm d r il l b i t.

d . Removefile.

most o f the thread on the free end o f th e nipple with a


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The Eandle


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a . Cut a 4 0 mm l o ng p i ec e o f 1 G.I. p i pe w i th thread a t one end.

b. D r il l a 4 mm hole through the G . I. p ip e a t 1 3 mm from its

unthreaded end.

c . Wind a few rounds o f sealing tap clockwise around the threads

th e 1 G.I. pipe and screw i t int o a reducing socket o f 1

1/2 G.I.

_

Assembly o f the Water Tap

a . Place the rubber b a ll w i th its attached pin onto the seat o fv a lv e w i th the hole i n th e pin ponting towards the slits i n

nipple.

b . Place th e handle over th e nipple s o t ha t th e hole in th e handle

i s i n l in e with the hole i n th e pin an d slits.

on

t o

th e

the


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e. Wind sealing , t a p e around th e threads o f th e free end o f th e

r e du c ti o n b u s h an d screw i t into a G.I. 1.5 elbow.


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f. Wind s ea li n g t ap e o ve r th e reads o f a G.I. r e du ci n g b u s h o f 1.5t o 3/4 G.I. and screw i t into th e other end o f th e 1.5 elbow.

OEU V ERV

g . Th e selfclosing water ta p i s now ready

b f i d h


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5 . Naking a Tixed Spanner for th e Ca reta ker


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Th e inaterials needed to ma ke a fixed spanner in cl ude;

30 ci n of flat iron, 30 mm x 6 in m2 2 cm of flat iron, 25 m m x 6 m m

T ot a l C o s t o f Fixed Spanner

The fixed spanner is the tool the caretaker will use to takethe ta p o ff t he pipe i f there a re an y p r o b l e i n s .

a. Cut a 22 cm long piece of flat iron 2 5 mm x 6 inm . Slit th e ironbar at 5 places corresponding to the six sides of the hexagonalreducing b u s h o f 1.5 t o 3/4 used t o connect the tap t o th e

pipe. B e n d i t tightly around t h is s h a pe . W e l d the i r o n ba r a tea c h o f t he s l i ts an d a t th e join between t he t wo e nd s.

b. Cut a 300 m i n length of flat iron 30 n i i n x 6 m i n andi t o n t o th e hexagonal s p a n n e r en d a s a handle.

w e l d o ne en d o f


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1I

I

!


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4 ~

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Rock catchment dam with self-closing watertap

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