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Design CriteriaEastern Province Water Supply Development Project

Ceywater/IDC/Infotec Consultants, May 2011 Page 1

DESIGN CRITERIA

-RURAL PIPE WATER SYSTEMS

CEYWATER CONSULTANT (PVT .) L TD

INTEGRATED DEVELOPMENT CONSULTANTS

INFOTECHS IDEAS (PVT .) L TD

IN ASSOCIATION WITH

&

MAY - 2011

Eastern Province Water Supply

Development Project (Rural Water Component)

Submitted by





TABLE OF CONTENTS

1.0 Demand Projections1.1 Design Horizon

1.2 Water serving Population1.3 Level of service1.4 Per capita consumption ( Domestic)1.5 Non-domestic consumption1.6 Non Revenue Water(NRW)

2.0 Pipe Network Characteristics2.1 Transmission Networks2.2 Reservoirs2.3 Distribution Network

2.3.1 Peak Factor2.3.2 Residual Pressure

2.4 Pipe Material2.4.1 Transmission2.4.2 Distribution

2.5 Design Procedure for pipeline designs

3.0 Pumps & Pumping Stations3.1 Pump Types3.2 Hours of operation3.3 Power Supply to pumping stations

4.0 Selection of water treatment options4.1 Treatment options for surface water sources

4.1.1 Aeration4.1.2 Removal of Algae4.1.3 Pre-settling4.1.4 Filtration4.1.5 Activated Carbon Galleries4.1.6 Disinfection

5.0 Structural Design Features5.1 Structural Design Standards5.2 Foundation Design of Structures

5.2.1 Geo-Technical Investigations(soil)5.2.2 Lateral soil & Ground water loads

5.2.3 Foundation Design Principles5.3 Analysis and design principles of water retaining structures5.3.1 Reinforced concrete structures

5.3.1.2 Joints in Water Retaining Structures5.3.2 Ferro-Cement Structures

5.3.2.1 Selection of construction Material5.3.2.2 Design of a Cylindrical Ferro-Cement tank





Design Criteria (Rural Pipe Water Systems)

1.0 Demand Projections

The basic element considered for assessing water demands under the proposed rural

water supply schemes are the GNDs. In the case of small towns, the boundary declaredby UDA shall be considered in population forecasts.

1.1 Design horizon

In general, design life of the rural water systems is taken as 15 years and that of smalltowns or rapid developing areas as 20 years. Hence in this case, in most of the rural sub-projects, Design horizon will be taken as year 2027, assuming that certain elements ofthe proposed projects could be started in year 2011 with the design & tendering phasesof the project. Hence the yearly demand projections shall be based on the starting year of2011. Accordingly the design horizon of small towns would be the year 2031.

The population of year 2006 shall be used as the base population for most of thepopulation forecasts since the official records of population are only available up to 2006in most of the project areas. Nevertheless population projections made by respective DSoffices could be made use of whenever possible.

In consideration of history of population growth in the project areas, many abnormalvariations could be observed, especially at the war affected areas. Hence it is not realisticto project the population throughout the design life straight from such historical values.Thereby a detailed analysis of growth rates should be made to decide on a realisticgrowth rate to be used in population projections in order to avoid the unreasonably highor low predicted populations at the sub-project areas.

1.2 Water Serving Population

The actual population served with domestic water supply as a percentage of totalpopulation in the area at each year is designated as “water serving population”. It isobvious that the 100% of the population at a particular sub-project area need not becovered under the proposed pipe networks, other than in very particular instances suchas densely populated areas with absolutely vulnerable status of present water supplyfacilities. In general, provision of piped water services to households may depend upontheir real need and social expectations. Hence, the water serving population by theprovision of new facilities would have to be analyzed accordingly. Water servingpopulation percentages shall be derived for the proposed sub-project areas from the

findings of the socio-economic surveys using following principle parameters.

o Status of residence-ship and the stability of re-settlemento Status of available water supply facilities and it’s user perception ( both

reliability & acceptability of their available water sources or services)o Development trends in the sub-project areas and current level of other

infrastructure facilitieso Affordability & willingness on improvement of present water facility





1.3 Level of service

Individually metered house connections & 24 hours pipe water supply is the fundamental level of service which shall be planned to achieve but at least 12hrs of supply shall bemaintained as the minimum where there is a scarcity of water at the water sources in aparticular period of a year. Sometimes this may be further lowered in cases where the

water sources have acute constraints. Such reduction of hours of supply shall be donewith the consent of the user groups and the other relevant authorities such as respectivePSS.

Minimum of 7m or maximum of 40m residual pressure at the service outlets will be the basic pressure criterion for distribution network designs. Stand posts are not encouragedas domestic water supply and would be confined only to the public places where therespective PS shall take the responsibility for the cost of metered water and the minimumof wastage by its use. Extra concessions are recommended to provide serviceconnections for the households lie below national poverty levels or badly affected by thewar with displacement. In such cases, project shall assist with the supply of water metersfree of charge and the O&M authority of the respective sub-project WSS shall provide the

service connection at the basic cost of the balance materials such as ferrule, service lineand the meter chamber. In providing all service connections, it is recommended to obtainservices from the respective household for excavation and backfilling of service line toreduce the cost of providing service connections. All in all to have a substantialamortization of service delivery at the very first years of the system soon aftercommissioning.

1.4 Per capita consumption (domestic)

Per-capita consumption of domestic supplies is a relative factor which varies with manysocial features and the water habits of the user groups in the sub-project areas. Also theconsumption patterns are linked with the topographical and climatic features of the sub-

project areas as well and in most instances vary with the seasonal climatic effects suchas wet/dry weather patterns of the year. Besides, densely populated areas like smalltowns with the limited land availability which restricts the ability of having own facility shallhave high expectation from the pipe supply to cater for their entire domestic requirement.In rural areas with availability of secondary water sources for domestic use (e.g.: bathing& washing) per capita consumption from the pipe service connection would be low.Hence per-capita consumption in rural pipe systems shall not be considered as a uniquevalue and shall consider to be varied from place to place. The following annual averageranges were the proven per-capita consumption for the rural pipe systems, according tothe past sector experiences.

Small Towns in Semi-urban nature (Dry Zone) - 100 to 120 l.p.c.d

Small Towns/Village Centres (Dry Zone) - 90 to 100 l.p.c.dVillages in developing nature (Dry Zone) - 80 to 100 l.p.c.dVillages in rural nature (Dry Zone) - 60 to 80 l.p.c.d

Thereby the most appropriate yearly average or seasonal per-capita consumption valueshave to be decided, considering all above facts. None of the water systems should bedesigned only for drinking or basic domestic water needs with a consumption rate lowerthan 25 l.p.c.d which was practiced in the sector during past as result of limitation at thewater sources. In such cases, where the consumers have agreed at the beginning for low





rate of l.p.c.d they subsequently demand for more supply and express theirdissatisfaction about the service. Then the designed system elements will be of under-capacity to deliver the additional quantity even with incorporation of supplementary watersources to the system at a later stage. On the other hand, assuming bogusly high percapita consumption values for designs would create uneconomical system designs andthe water system will have redundant capacity throughout the design horizon. As a whole,

making a proper judgement on realistic per-capita consumption would facilitatedeveloping a feasible and economical rural pipe water system to serve the intendedpurpose.

1.5 Non-domestic consumptions

Non domestic demands including Institutional and Commercial demands shall becomputed as a fraction of the respective domestic demand because Institutions andCommercial establishments serve the population in that area and their expansion orreduction can be considered proportionate to the population growth of the area. It is alogical fact and proven in the past.

In general, the total non-domestic demand of rural pipe water systems is at the range of8% to 12% of the domestic demand in many instances. Since the commercialestablishments and government institutions are somewhat higher in the village centers orsmall towns, that fraction would be in the range of 10% to 18% depending upon thenature.

1.6 Non-revenue water (NRW)

Keeping NRW within a lower range is economically advantageous to the schemeoperation and beneficial to the project as in conservation of valuable water resources.Quality construction and proper commissioning of the scheme shall minimize the losses

at the inception but the enthusiasm of O&M body on minimizing NRW is also an importantfact to maintain the NRW of a scheme at a lower range in long-term operation.

In general design practise of rural pipe systems, NRW shall be computed as 15% fromthe total production for distribution, soon after commissioning of the water system andthereafter shall take as gradually increasing up to 20% of the production at the 10th yearonwards of operation. When the systems are getting old there will be unavoidable naturallosses at the system components but in a properly maintained system, the NRW could bekept within 20% even at the end of design horizon.

2.0 Pipe Network Characteristics

2.1 Transmission Networks

Transmission pipes either raw water or treated water shall be modelled to suit transmitwater from one operational point to the other. It could be either gravity or pumping mainsdepend upon the ground profile of the connecting points. Peak factor of 1.1 shall be usedto keep the operational flexibility at daily operations of the production units. But if thewater sources have limitations of pumping then the peak factor will be changed to suit the





required storage in advance. In order to economise the pipe sizes and at the same timeavoid the siltation at the pipe lines, velocity at the water conveyances has to be kept lessthan 1.0 m/s where possible. “Water Cad “version 6.0 shall be used for the hydraulicmodelling of both transmission & distribution systems.

2.2 Reservoirs

Storage reservoirs feeding distribution networks shall also been modelled with “WaterCAD” version 6.0 computer software. Storage reservoirs would be placed at the groundlevel or will be elevated depending upon the ground morphology and the residual headsat the distribution ends. However, the storage reservoirs shall be placed at reasonabledistance to all decided distribution zones in order to minimize long distribution trunkmains and also to facilitate minimum disturbances of the water system during repairs andbreakdowns. Wherever possible, storage reservoirs are designed to operate atreasonably high water levels at the peak hours also and level cut-offs shall be installed toavoid overflows and have better control at the pumping operations. In general, zonemeters are proposed to install at the outlet pipes of storage reservoirs for effective flowmeasurements. Basic accessories such as overflow controls, washout facilities, access

ladders, flow control valves etc shall be incorporated with the structures. The storagereservoirs will be constructed of watertight concrete, following the standard structuraldesigns to BS practice for aqua retaining structures ( BS 8007 & BS 8001) or in line withthe structural features of Ferro-cement techniques which is described in a latter section.

2.3 Distribution Network

2.3.1 Peak factors

A peak factor of 2.0 is assumed in general to accommodate the morning & eveningdemand peaks of the respective areas. However hydraulic analysis shall made for the

distribution round the clock to compare the variations at 2 hr intervals.

2.3.2 Residual Pressures

Where geographical conditions permit, the distribution system shall be divided intodifferent supply zones. This will facilitate easy operation and maintenance andmaintaining the NRW at desired levels.

In a pipe network, Residual pressure is an important feature to consider with respect to itsmaximum & minimum values. Maximum residual pressure value will govern the safetyand durability of the appliances of the networks especially at the service connections.Minimum residual pressure is to maintain the continuity of supply at the service

connection with a reasonable head and velocity.Residual pressures within the supply zones shall be limited as follows assuming that thevillages and small towns do not have high rise buildings.

Minimum 0.7 bar (at ground level)Maximum 4.0 bar (at ground level)

Minimum pipe diameter for primary distribution network is uPVC 63 mm but the brancheswith dead ends could go even below that and could reduce up to 40mm depend upon the





design flows. However the systems which could later be possibly amalgamated withNWSDB pipe networks or the areas in rapid developing nature the minimum diametercriteria could vary and shall be as 90mm uPVC .

2.4 Pipe material

2.4.1 Transmission

To be consistent with working pressure considerations & the topography of the pipetraces either HDPE (High density Polyethylene)/ DI (internally cement coated DuctileIron) / u PVC (Type 1000) could be used.However HDPE pipes are very rarely put in place at the rural pipe systems, consideringthe technical barriers faced during O&M stage.

Roughness coefficient for friction loss calculations shall be based on the Hazen Williamroughness coefficient factors.For new pipes the following roughness coefficients shall be applied assuming that the

pipes are either from Sri Lankan Manufacturers and or from Asian suppliers

Type of pipe Roughness co-efficientuPVC 130

DI 120GI 100

2.4.2 Distribution

It is proposed to use uPVC type 600/1000 pipes in general for all distribution networksdepending upon the pressures. However the areas where hard rock formations exist orat special crossings the pipe material shall change either to GI or DI depend upon thesuitability.

Pipes for both transmission and distribution shall be laid with a minimum cover of 1.0 mfrom the ground surface. Where the pipelines are laid in a common trench thetransmission pipe shall be laid deeper and staggered from the distribution line. All theabove ground pipes need to be restrained for uplift pressures and the buoyancy.

If the types of soil along the pipe trace permits, the pipe bed will be designed from thesame graded soil with proper compaction in order to economize the construction cost.Sand bedding shall be compulsory be at the pipe traces where clay formation, soft or

blasted rock formations and or at lime stone belts. Pipe laying at marshy areas and or atwater logged areas shall be refrained from as much as possible to minimizecomplications at the O&M stage. However, under unavoidable circumstances, specialpipe beddings or concrete block support will be designed to lay pipes at such difficultterrains.





2.5 Design procedure for pipeline designs

The following are the key design steps of the standard pipe line designs.

Establish the pipe line conditions (trench, wide trench, embankment etc)

Establish the approximate classification of the natural soil (and imported beddingmaterial) based on geo-technical surveys

Establish the total external vertical load on the pipe (i.e. Trench fill loading plussuperimposed loading in KN/m2

Determine the appropriate deflection lag factor (DL) according to soil type.

With the above information select the relevant embedment for diameter deflectionand to suit ring bending stress.

Degree of compaction depending upon the backfill soil type is required toevaluate. It depends upon the modulus of passive resistance of pipe surroundingas well. In general 90% degree of compaction is to be achieved to satisfy the roadauthorities.

Anchorage to resist the thrust shall be designed taking full account of the

maximum pressure of the water main in service or on test and the safe bearingpressure of the surrounding soil. Could use grade 25 concrete blocks either pre-

cast or cast in situ for the anchor blocks.

3.0 Pumps and pumping stations

3.1 Pump Types

For taking in raw water from rivers, impounding reservoirs, Deep BHs, etc., submersiblepumps shall be used. This is to overcome the operation & maintenance difficulties at theseasonal water level fluctuations at the water sources. In borehole intakes submersiblepumps are solicited to avoid the constraints at the suction. The protection of such pumpsfrom external substances at the surface flows and the damages at dry running at lowwater flows shall be considered. However, for small systems having shallow suctiondepths, Centrifugal pumps are recommended with positive suction where feasible.

Pumping units shall be designed to operate without cavitations or damaging vibration atthe specified speed, flow and head conditions. The system design and pump curves shallillustrate the design conditions with operational efficiencies and power demandsillustrated on the systems curve or envelopes. The pump base shall be designed for

installing at minimum vibration to the surrounding elements and the motor shall becoupled to the pump by means of standard coupling specified for the operation. Thecomplete pumping unit shall be designed to operate without overloading any componentat any point along the pump curve.

All submerged moving parts of the equipment, pins, spindles, shall be of non-corrodiblemetals. All parts in direct contact with chemicals shall be completely resistant to corrosionor abrasion by those chemicals, and shall also maintain their properties without ageingdue to the passage of time, exposure to light or any other cause.





3.2 Hours of Operation of Pumps

Pumping stations in rural operation conditions could be limited to operational hours from10 to 16 hrs/day. This will facilitate more operational flexibility of the pump operationwithin maximum of two operational shifts. In general, BH pumping stations are partiallyautomated and thereby the operator has more flexibility to work on part-time basis. Also

the limitations at water sources or the recommendations of safe yield abstraction of BHsare the governing features to decide the operational hours of pumping stations.

3.3 Power Supply to Pumping Stations

Wherever feasible, Power supply for pump stations will be from National Grid but in theother instances the power supply shall be from Solar Power. The power supply could beeither single phase or 3-phase depending upon the capacity of the pumps. In general, thepumps having rated motor capacity of less than 3 KW could be operated with singlephase power in all economical operational context.

In the events of Solar power, pumping hours will be restricted to suit intensity of sunlightand tracking units shall be installed to optimise power generation of the solar panels.System coupled with batteries are not recommended having due consideration of costand operational difficulties at the rural context in its replacement & regular maintenance.

4.0 Selection of Water Treatment Options

In accordance with the variations in the water quality parameters, different watertreatment scenarios could be formulated. The principal issue would be whether thetreatment procedure is to be decided based on average water quality of the sources(average water quality throughout the year) or for peak variations (few days or hours peryear). It is clear that the capital cost involvement would be high if the process is to bedecided for rare peak quality variations with optimum value of certain quality parameters.

The treatment process proposed would be based on average values of water qualityvariations. In any case it is important to avoid complex treatment processes which areinappropriate to the local conditions and the capacity of the O&M bodies. There thetreatment process should suit the Village Level Operation & Maintenance (VLOM) withminimum backup support. Availability of chemicals, filter material etc and the operatortechnical knowhow are the other factors to be considered in sustainable operation oftreatment units.

Treatment options could broadly be classified as full treatment or partial treatment and in

some instances limiting to disinfections only. Conventional treatment process is thepreferable option in many instances by the O&M authorities due to their simplicity inoperation and maintenance. However, low-cost Package treatment units are nowavailable especially to use as domestic units. Domestic fluoride filters, Domestic filters forHardness removal etc are some of the examples. In addition, package treatment units inmedium scale are available to use in RWS, especially for Iron removal and improvementsto turbidity and colour. Despite of the high production cost, solar operated desalinationplants are also readily available in the market now for application in small to medium





scale water supplies with brackish or lagoon water where the instances the other types ofwater sources are remote.

4.1 Treatment options for Surface water sources

The following sections indicate general treatment options to consider depending upon the

raw water quality. The sequences of the treatment process with its selected elements ofthe treatment process should carefully be decided reviewing the past records of the rawwater quality of the propose water sources.

Conventional Treatment Options for surface water sources shall be as follows.

4.1.1 Aeration

Aeration would be necessary when raw water extraction points are located whereanaerobic conditions exist. And also to remove the iron and manganese and harmfulgases dissolved in water. The type of aerator shall be decided depend upon the effective

time required for aeration but the cascades types are very commonly use due to its lesscomplexity and the structural beauty.

The basic design parameters of cascade type aerator shall be as follows.

Number of Drops 2 to 3

Height of drops 0.4 m

Over flow rate 0.015 cum/sec

Depth of water in each pool 0.3 m

Cascade Area 2 m2/m3/ min of flow

Height of Aerator 2 to 3 m

4.1.2 Removal of Algae

Depend upon the existence of active environmental features favouring for algal blooms. Ifthe upstream inflows to the water sources are enriched with Nitrate & Phosphates andthe day time temperatures are favourable on algal growths then the alert is needed foralgal removals. Pre chlorination shall be proposed soon after aeration at a well designedchamber for the purpose. This would be done with a dosage range of 3.0 to 5.0 mg/l.Nevertheless the catchment preservation with a planned participatory environmentalmanagement program will give the sustainable long term results than pre-Chlorination

through out

4.1.3 Pre-settling

During rainy weather the sources could turn turbid. Hence it is recommended to includefor pre-settling facilities in such circumstances. However, this could be bypassed duringnormal weather flows. River bed filtration galleries are widely used in this respect whenthe selected water body is a stream or a river. Such galleries have added advantages





through its natural cleaning by the surface flow itself. However pre-settling is not muchapplicable if the selected water source is an impounding body.

Number of units 2

Filtration rate 1.0 to 1.5 m3/m2/ hr in normal weather

Depth of Filter media 1.0 to 1.25 m with gravel & coarse sand layers

4.1.4 Filtration

Upward flow roughing filter has high efficiency to act as pre-settler.

It has 70% removal efficiency for colour and turbidity

The operation is less complex

suitable for application in rural conditions

Average rate of filtration could be taken as 1.5 m3 per hour per square meter of effectivefiltration.

If the land availability is not a constraint and the turbidity variations could be lowereddown to < 10 mg/l with pre-settling, then Slow sand filters are the ideal treatmentelement applicable for RWS. Rate of filtration will be at the range of 0.15 to 0.2 m3 perhour per square meter of the effective filter area. Slow sand filters have an addedadvantage of working as a biological filter. However, pressure filters are common filtertypes in package treatment units and which has high filtration capacity up to 10 m3 perhour per square meter but shall have effective inbuilt backwash system.

4.1.5 Activated Carbon Galleries

If the water is extracted from an ancient irrigation tank at anaerobic status then the badtaste & smell could be expected even after a conventional treatment process. Introducingactivated carbon galleries after filtration would facilitate a better post-treatment option.This will also be suitable to absorb harmful substances added to water by contaminationthrough agro-chemicals. Since the activated carbon is expensive at its replacement, thegallery could be bypassed at favourable water quality instances.

As an alternative, locally available burned coconut husks could be used but the efficiencyof purification might be somewhat less and the replacement interval is also higher thanthat of activated carbon.

4.1.6 Disinfection

Chlorine in liquid form is recommended for disinfection since it is more versatile inoperation at rural nature. Bleaching powder arrangement (with constant head solutiontanks and regulators) shall be provided at the storage points as a provision foremergency operations and for sterilization of pipelines after repairs.

The other alternative is to promote domestic level disinfection mechanisms such asboiling water, Solar disinfection etc





4.2 Treatment options - Ground water sources

In general, Ground water extraction from production wells are at a depth greater than20m where the water is at anaerobic status and enriched with dissolved gasses andsubstances such as Iron, Fluoride or Manganese. In the project area, the predominantfeatures are basically excessive Iron or Manganese and calcium hardness is the main

critical quality issue. Besides, excessive hardness is not an easy element to removeunder less complicated treatment options other than dilution with soft water.

Effective aeration within a sufficient duration would facilitate the oxidation of Iron &Manganese to its precipitation forms. Type of aerator shall depend upon theconcentration of the Iron or Manganese and thereby the selection of types from Cascade,Multiple tray or drop bubble would be appropriate. Effective settling basins are needed forsettling precipitated substances. As mentioned before, package treatment units are moreeffective in removal of iron and manganese and the operation would be lesscumbersome.

Large scale removal of fluoride is not appropriate and domestic fluoride filters shall be

introduced instead. In case of domestic filters usage with proper awareness is essentialand it has to be monitored.

The domestic fluoride filter is easy to make, affordable and pieces of burned bricks canbe used as the filter media. The filter has specific dimensions of 1000 mm height and 200mm diameter cross sectional area and act as a upward flow filter. The filter media shouldbe changed at least every 3 months or more frequently depending on the fluoride contentof water..

5.0 Structural Design Features

5.1 Structural Design Standards

The following standards shall be used as applicable for designing of RC Concreteelements.

BS 8110 : Part 1 ;1997BS 8007BS8004BS 6399 Part 1

Reinforced Concrete Framed building structuresWater retaining structuresFoundationsDesign loading for building - Live Loads

BS6312“Design of Buildings for High

Winds –Sri Lanka –Ministry ofLocal Govt. Housing andConstruction -1980” and BSCP 3 –Chapter V.

Basic data for the design of buildings - Wind Loads





5.2 Foundation Designs of Structures

5.2.1 Geo-technical Investigations (Soil)Geo technical investigations are a prime requirement to assess the ground condition todecide the type of foundation to suit the structure. The main objectives of the geo-

technical investigations are to determine the following basic features.

Soil parameters, nature and characteristics of soils.

Ground improvements, if necessary.

Information required for excavations.

Soil bearing pressures for different foundations.

Suitable type of foundations for different structures.

Surface levels of rock formations

5.2.2 Lateral Soil and Groundwater Loads

The structures shall be designed for loadings based on the interpretation of the data

contained in the geotechnical report for each site, with the following minimum values:

1. Compacted soil density 20.0 KN/m2

2. Active lateral soil pressurecoefficient.

0.333

3. Passive earth pressure

coefficient.

3.0

4. At-rest earth pressurecoefficient.

0.50

5. Lateral surcharge fromvehicles

20.0 KN/m2

6. Water head at bottom of walls Groundwater level, if present oras stated below

7. Density of water 10.0 kN/m3

8. Allowable bearing pressure Based on soil investigationreports

5.2.3 Foundation Design Principles

The width and the depth of foundation should be decided upon the stresses to betransferred into the sub soil and the allowable bearing capacity of the subsoil at therespective depths.Long term settlements of the foundations could be ruled out and only immediatesettlements will take place within the period of construction. In general, raft foundationsare used in Water towers to avoid individual settlement of column footings of the towers.





Since the land availability is not a constraint in rural context, it is advisable to selectsuitable lands where less complex foundations could be adopted avoiding piling andother expensive foundation types.

Foundation designs are in compliance with BS 8004 for RC structures but simple

masonry foundations are suggested for utility buildings and staff quarters where theycould be placed at reasonably stabilized grounds.

5.3 Analysis and design principles of Water Retaining Structures

The analysis and design shall be carried out in accordance with the limit state designphilosophy of BS 8110 and BS8007. The structures will be checked for compliance withrequirements for strength at the ultimate limit state using factored loading with maximumliquid levels. To ensure satisfactory serviceability, the limit states of crack-width anddeflection shall then be checked with un-factored loading at normal working levels.

The structural analysis and design shall be carried out using manual calculations and theresults could be verified using software SAP2000. However, medium scale rural pipesystems have less complex concrete structures and manual calculation would be theeffective tool. Further the application of structural design principles will be as follows.

The structural designs will be carried out according to Limit State of Serviceabilityand Ultimate limit state.

The partial safety factor for retained water, surcharge and earth pressure shall be1.4 for most at Ultimate limit state (ULS) and 1.0 at Serviceability limit state (SLS).

The structures shall be designed with a factor of safety of at least 1.1 against

flotation.

The maximum crack widths shall be:

Reinforced concrete where all faces of liquid containing – 0.2 mm maxWhere aesthetic appearance is critical at water retaining– 0.1 mm max.

Minimum wall thickness shall be 150mm for walls with a single layer ofreinforcement except for low walls. Walls with two layers of reinforcement shallgenerally be a minimum of 250 mm thickness. The technical decision for decidingthe minimum wall thickness will also be governed by considering the requirementof inserting a concrete vibrator of diameter not less than 40 mm freely through the

reinforcement.

Structures will be designed for a basic wind pressure due to a wind speed of 125

metres per second (mps) and the calculations shall be based on ‘Design of

Buildings for High Winds –Sri Lanka –Ministry of Local Govt. Housing and

Construction -1980’ or BS CP 3 –Chapter V.





Wind forces on structures shall be based on the basic wind pressure and

modifying factors based on the shape of a structure and its exposure.

Structures which contain liquid, extend below grade, or both, shall be designed for thefollowing load combinations:

Liquid-containing compartments full, with no backfill for liquid

containing compartments. No relief will be given for any passive soil

pressure on a structure on the face remote from a contained liquid

unless otherwise approved.

Backfill and groundwater, with liquid-containing compartments empty

and full.

5.3.1 Reinforced Concrete Structures

5.3.1.1 Concrete Grade

The following table specifies the type of concrete depends on the applications.

Application ConcreteGrade

All water retaining structures comprising reinforced non-pre-stressed,cast in place construction or pre-cast elements

35A

Blinding concrete (screed) under footings and slabs of water retainingstructures

15

The structural concrete for all building structures 25

The structural concrete for All ground reservoir roofs 35AThe structural concrete for general foundations 25

The structural concrete for All other ancillary structures such as valvechambers, pavements etc

20

Infill Mass concrete 15

CementThe cement to be used should be ordinary Portland Cement; however, sulphate resistingPortland Cement shall be used for foundations at locations where soluble sulphatecontent of ground water is excessive if confirmed by soil investigations.

Reinforcement

Non-pre-stressed reinforcement will be high strength deformed bars (Tor steel) with aspecified characteristic strength of 460 N/mm2 or mild steel bars with a characteristicstrength of 250 N/mm2. Steel shall be in compliance with BS 4449 for the structural use.

5.3.1.2 Joints in Water Retaining Structures

Joints in water retaining structure comprise the following:

Movement joints - Expansion joints





-Complete contraction joints

-Partial contraction joints

Construction joints

The location of joints in the structure and the minimum reinforcement shall be in

accordance with Appendix A of BS 8007 which takes into account the effect of jointspacing.Water-stops shall be placed in every construction joint and expansion joint in waterretaining structures where applicable.

5.3.2 Ferro-Cement Structures

Ferro-cement is a composite material and consists of a cement rich mortar reinforced withlayers of wire mesh, sometimes with additional plain wire reinforcements for additionalstrength.

5.3.2.1 Selection of construction material

CementIn selecting a type of cement the quality of cement and the setting time of cement shouldbe considered. Masonry cement is not recommended for use in Ferro-cementconstruction. A type of cement having a setting time of ½ hr is good for Ferro-cement.Since the water storage tanks should be free of cracks, the cement should be a powderfree of clumps.

SandThe sand should be well graded fine sand with sharp and strong impermeable grains andshould be free of organic or foreign matter. The effective size range of 0.4mm-1.4mm anda Coefficient of uniformity of the range 1.5 – 2.5are recommended.

Wire meshFollowing types are commonly used in Ferro-cement construction

Hexagonal wire mesh/ chicken wire mesh – can use two meshes tied together to get

more strength when required.

Welded wire mesh- This is higher in stiffness than chicken mesh but should be checked

for quality of welding.

Woven mesh – can be woven according to the strength required. Needs more labour

than other types.

Expanded metal mesh – This mesh is made by cutting squares off a metal sheet and

would be more expensive than the others

Watson mesh

Skeletal SteelSkeletal steel is required to form the basic shape of the structure using the selectedmesh. 5mm wire is commonly used for this purpose. The amount needed depend on thesize and the shape of the structure.





5.3.2.2 Design of a Cylindrical Ferro- cement tank

The governing design criterion of a Ferro-cement tank is to design for the limitation ofcrack width. This can be done by

Maintaining an appropriate cement sand ratio in the cement mortar. Maintaining an appropriate workability (water/cement ratio) in cement mortar.

It needs to provide adequate reinforcements to withstand tensile stress on the cylindricalwall and horizontal steel to withstand the surface tension. Finishing the surfaces shouldbe well enough.

The components of a Ferro-cement tank are;

FoundationBy adopting a raft type foundation the differential settlement of the structure can becontrolled. The foundation should be designed to carry the weight of the tank and thewater that is going to be stored in it.

Tank FloorIn small tanks the walls and the floor of the tank would be constructed monolithicallywhere in large tanks these are constructed separately and the joints are made water tightusing tar and sand or an appropriate chemical sealant.Tank floor is constructed similar to a normal concrete floor. If the tank diameter is greaterthan 6m expansion joints will be required.

Tank wallUsually the walls of a Ferro-cement tank are about 5cm thick. The forces and momentsacting on the curved surface of the wall are as follows.

Water pressure acting perpendicular to the wall surface

Surface tension acting along the wall surface (horizontally)

The moment acting at the joint of the floor and the wall

Structural Analysis of Ferro-cement water tanks

Assumptions:

The tank is of cylindrical shape The Tank is constructed of a homogeneous elastic material The tank will be filled only with water

When the tank is filled with water the water pressure acts on the tank wall. The waterpressure depends on the water height and given by the equation;

P= hpgWhere,h= height of the water column,p=density of water,g= acceleration due to gravity





This pressure acts uniformly throughout the wall. The force acts at the centre of pressurei.e. 1/3 height from the floor of the tank.

A bending moment is developed due to this pressure force at the joint of floor and thewall which would try to deflect the joint. Therefore the joint has to be made water tightusing a water resistant material.

Water pressure acts perpendicular to the wall while the surface tension acts tangential tothe wall.

The horizontal hoop stress created by surface tension N = p. h. d/2Where p= density of water

h= height of water columnd= diameter of the tank

DesignDeciding on area of hoop wire (horizontal) required

As = N/ fsWhere, As= cross-sectional area of steel

N = Horizontal hoop stress (KN/m2)fs = tensile strength of steel

When As is known the number of wires that need to be placed per 1m height could becalculated.

Calculated stresses in thin walled cylindrical tanks of height 2m

Tank

diameter

in m

Wall

thickness

in cm

Tank

capacity in

m3

Max.

Hoop

stress

N/mm2

Position of

max hoop

stress h/H

Max

bending

stress on

inside faceN/mm2

Shear

stress at

base

N/mm2

2.5 3.0 9.0 0.73 0.2 1.32 0.09

5.0 3.0 40.0 1.26 0.24 2.34 0.16

5.0 5.0 40.0 0.65 0.29 1.42 0.11

10.0 6.5 150.0 0.88 0.42 2.06 0.13

20.0 8.0 630.0 0.88 0.55 2.26 0.13

There are no other tensile stresses acting on the tank. The mesh provided would onlyhelp form the skeleton and placing of cement mortar on the structure.

- The End-

Documents

Design Criteria 31st May