Technical requirements for sewerage

1. General design considerations

1.1 Objectives

The sewerage objectives are seen as being achieved when:

1. the planning, design and construction of new facilities are adequate in servicing new and future developments in accordance with the requirements of the ACT Government;
2. there is compatibility with the existing facilities, methods of operation, and maintenance techniques; and
3. the facilities provide public health, environmental, and asset protection consistent with the accepted design and construction requirements set out in this document and with developments in technology as approved from time to time.

ACTEW's sewerage system is to be designed to ensure the efficient and effective transfer of water-borne wastes. It should be able to be adequately operated and maintained to achieve minimum risk of failure through blockage, collapse or equipment failure, to ensure acceptable asset life. It must meet environmental protection requirements consistent with personal, public and asset safety, public health, economic operations and minimum total cost.

The pipe system may, on occasions, be subject to "surcharge" (where the hydraulic grade line is higher than the pipe obvert) or "overflows" (where sewage overflows out of maintenance holes). These situations may be the result of blockages and/or flows in excess of the design flows. In establishing the layout of the pipe network, designers should take care to ensure that any overflows are likely to cause only minimal nuisance or damage. The sewerage system is provided for "domestic" sewage. Non-domestic discharges will only be accepted if quality and quantity conform to the requirements set by ACTEW, as defined in the Trade Waste Acceptance Criteria. Designers of sewers for industrial blocks should seek early advice on these requirements from ACTEW.

1.2 Maintenance aspects

1.2.1 General

The sewerage system is to be designed with due regard to the continuing maintenance requirements after the works have been constructed. A system that can be easily and economically maintained is essential.

Maintenance holes located in readily identifiable locations (e.g. opposite a building line), and not within leased properties, are an aid to rapid clearance of sewer blockages. By far the greatest maintenance problem is tree root penetration of sewers less than DN450. Designers shall pay attention to this problem particularly in areas of high risks where the sewers are shallow or extensive planting of trees is likely to occur. Particular care in selecting pipe materials and jointing methods should be made in these areas. Refer to the document entitled The Invasion of Sanitary Drains by Plant Roots: Prevention and Cure (Reference 10.9).

1.2.2 Special equipment

The purchase of special maintenance equipment and plant requires considerable lead times, special approvals and funding. As a consequence, no design incorporating the need for special or unusual equipment should be prepared without the prior written approval of ACTEW.

This requirement also extends to the need to use special techniques or hired equipment. To ensure that maintenance personnel can respond and overcome operational problems consistent with service objectives, it is essential that maintenance of the system is not dependent on non-standard techniques or equipment.

1.3 Safety, corrosion and odour aspects

Hydrogen sulphide poses a potential safety threat to sewer workers, can generate a corrosive atmosphere that corrodes sewerage assets, and is responsible for many odour problems. Odour is a particular problem in the ACT due to the regular occurrence of temperature inversions and nondispersive wind velocities. These problems, which increase the risk of sulphide generation, can be overcome with careful attention.

In this regard it is important to:

• use adequate grades for slime control;
• minimise detention periods by avoiding the use of pump stations where possible;
• design for pump station reliability;
• ensure adequate ventilation for trunk sewers;
• avoid any unnecessary turbulence of stale sewage at junctions and changes in grades (drop junctions, vortex drops and particularly where rising mains enter the gravity system).

1.4 Discharges from stormwater systems to sewers

Unless approved otherwise, under the specific Trade Waste Agreement, no stormwater discharge will be accepted into sewers.

1.5 Non-domestic liquid waste disposal to sewer

Non-domestic liquid waste may NOT be discharged to sewer without prior approval from ACTEW Corporation. Acceptance criteria and advice on the discharge of non-domestic waste can be obtained from ACTEW Corporation's Customer Services Branch in Mitchell, telephone number 6242 1111.

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2. Location of sewers

2.1 Sewer locations

2.1.1 Sewer layout

1. Sewers located outside leased lands

   The design of a sewer system should take into account the fact that there is a significant increase in the risk of tree root blockages after a period of about 20 years. Further, the access to sewers for maintenance is a major problem in the ACT despite the use of sewerage reserves for this purpose. Therefore minimising the use of sewer alignments and reserves in leased land is an important feature of good sewer design. Where there is public land at the rear or the side of a leased block the sewer should be located within the public land rather than within the leased block.

2. Diversion of principal carrier sewers around leased lands

   Blockages in the sewer system have the potential to result in sewage overflows into leased properties.

   To minimise problems caused by blockages, wherever practicable, sewers, particularly main carriers, shall be located in public areas rather than within leases.

3. Other situations

   Where a sewer is to be constructed across open areas it is to be sited to (1) maximise its use for future development, and (2) minimise its impact on possible future use of the site.

   Wherever possible sewers under playing fields are to be sited so that maintenance holes are not located within the playing area.

   To lessen the risk of overflows into houses the design should include (1) careful placement of maintenance holes to limit the number of connections into small near-maximum loaded sewers, and (2) the location of connection ties in accordance with Clause 6.2.2.

2.1.2 Standard alignments

The aim of standardised locations for sewers is to limit the construction clashes with other services and permit ready location by maintenance crews.

1. Roadways

   Sewers are usually located on the high side of road reserves, which permits relatively short connections from adjacent high-side properties.

   The standard locations for sewers are shown on Road Verge Drawings - Drawing Nos. SEP4 - 01 to 06, Chapter 4 (Road Verges) (Reference 10.15). The usual location for sewers is 1.6 metres outside the property boundary.

   Where there is a significant advantage in placing a sewer on an alignment reserved for another service, it may be so placed provided that the relevant authority agrees in writing to release the reservation. A copy of the agreement is to form part of Design Submission 1.

   On curved roadways curved alignments are permitted as per Clause 2.4 and Clause 2.5.

   In selecting pipe alignments it is necessary to carefully consider maintenance hole and maintenance shaft locations. Maintenance hole and shaft location preferences are outlined in Clause 5.1.3 and Clause 5.2 respectively.

2. Residential land

   When sewers are required between adjacent properties (either for leases back-to-back or side-to-side) they are usually located along the low side of the uphill property. Sewers are usually constructed parallel to, and uphill of stormwater drains. Pipes connected to sewer service ties shall normally not cross stormwater drains.

   Alignments shall be offset at sufficient distances from property boundaries to allow working room for excavation equipment.

   Acceptable offset alignments from property boundaries are in accordance with Table 3-1.

Table 3-1
Sewer alignments - leased blocks




Size (DN)	Rear of property (m)	Side of property (m)
DNl00 to DN225	1.2	1.2 1.8 (if power poles involved)
DN300	1.8	1.8 (see note 2)

Notes

1. For reserve (easement) requirements see Clause 2.2.

2. DN300 sewers or larger are normally not permitted within leased residential land.

3. Commercial and industrial land

   These sewers should also be located along the low side of the uphill property as suggested for residential land (see Clause 2.1.2 (ii) above). However, restrictions on land utilisation for these categories of land are generally less acceptable and development is likely to occur right up to the edge of a service reserve. Consequently, larger sewers are located centrally within a reserve to provide working space on either side of a sewer.

   DN450 sewers or larger are normally not permitted within leased commercial or industrial land.

   Acceptable offset alignments from property boundaries are in accordance with Table 3-1. For reserve requirements see Clause 2.2.

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2.2 Service reserves (easements)

A sewerage reserve should be wide enough to contain the service and shall provide working space for future maintenance activities on each side of the service. For general information on easements refer to Part 1, General Design Standards (Clause 3.2).

Acceptable sewerage reserve widths will normally be in accordance with Table 3-2.

Table 3-2
Sewerage reserve (easement) widths for sewers up to 5m deep

Sewer size (DN)	Block type	Minimum width of reserve (m)
100 to 225	Residential	2.5
300	Residential	3.5
100 to 225	Commercial and industrial	2.5
300 and 375	Commercial and industrial	3.5

 

Notes

1. See Clause 2.1.2 for standard alignments.
2. See Clause 2.3 for building restrictions.
3. The minimum distance from the centre line of a sewer pipe to either edge of the reserve is 1.2m.
4. In exceptional circumstances common easements for sewers with other hydraulic services may be approved subject to the following conditions (which apply in addition to notes 1, 2 and 3 above):
   • the minimum clearance between pipes shall be the greater of 600mm or the required clearance to satisfy the conditions set in the diagram below;
   • the minimum common easement width is 3.5 meters, but may be wider if the minimum required spacing between pipes is greater than 600mm;
   • in all cases the MH's on the shallower main shall be founded sufficiently deep to prevent loads from being transferred into a nominal trench of the deeper main.

   The design drawings shall nominate the easement width, spacing between services, and if required minimum depth of footings of MH's on the shallower main.

   Controls on common easements for sewers with other hydraulic services

   

5. Reserve widths greater than 3.5 metres are to be specially noted and brought to the attention of ACTEW.
6. For sewers deeper than 5.0 metres or of size DN525 and larger, a proposed easement width shall be submitted to ACTEW for approval.

2.3 Building restrictions

For a diagrammatic representation of controls on building set back and depth of footing refer to the diagram below.

1. Restrictions due to nearby sewer

   The presence of a deep sewer (either within or outside a leased property) may result in a building, on or near the property line (or edge of reserve), needing its foundations deepened so that the building will not impose loads on the sewer.

   The "zone of influence" from building foundations is taken as being within an angle of 45° to the horizontal below the foundation. Foundations are to be deep enough for this zone to remain beneath the sewer pipe. As detailed below, the minimum permissible depth of foundation is to be calculated and the information provided to the Land Development Branch of the ACT Government for inclusion as a building restriction on lease documents.

2. Building restrictions over or near sewerage reserves (easements)

   The foundations are designed so that the sewer is not affected structurally or with respect to access (Clause 2.3 (i) above);

   At ground level the building is entirely out of the ACTEW reserve and at least 2.0 metres from the actual centre line of the sewer;

   Building eaves and awnings lower than 3.0 metres above the final surface level must not overhang the easement;

   Overhang of the easement is permitted if the easement width is less than 3.0 metres, and a clear height of 3.0 metres exists between the final surface level and the underside of any permanent part of the building. Minimum clear heights over easements wider than 3.0 metres will be determined by ACTEW on a case by case basis;

   In all cases the extent of the overhang is limited. The furthest edge from the building must not be closer than 600mm horizontal distance to the centre line of the sewer.

   Controls on building setback and depth of footing

   

2.4 Vertical curves in sewers

Vertical curves are permitted if substantial cost savings can be achieved through their use. Vertical curves must not be used within horizontal curves.

The following limitations apply:

• vertical curves are only permitted on solvent welded DN150 sewers;
• grade limitations apply as per Table 3-4 and Clause 3.1.3;
• the vertical curve length shall not be shorter than 20 metres;
• vertical curves will start and end at structures;
• the maximum rate of change in grade shall not exceed 0.1% per metre;
• the rate in change of grade shall be constant along the whole length of the vertical curve.

The following will be shown on drawings:

• rate of change in grade (%/m);
• chainage and grade at start of curve (%);
• chainage and grade at end of curve (%);
• to assure vertical curves are set out correctly, design drawings shall show invert levels and chainage at intermediate points with intervals not exceeding 5.0 metres.

2.5 Horizontal curves in sewers

Horizontally curved sewers may be utilised where there are significant advantages in their use. Ad hoc curving of sewers to avoid an obstacle such as a power pole, gas main etc. is not permitted.

Curves in sewers between successive maintenance holes are to be in one direction only (i.e. no reverse curves and combinations of vertical and horizontal curves). In constructing a curved sewer the angles at each joint shall be uniform (i.e. within an accuracy of ±0.25°).

Preference should be given to solvent welded pipe materials on curved alignments.

Curve radii are limited in order to minimise obstructions in the pipe bore or failure of jointing, and to avoid other significant maintenance problems. Minimum curve radii are shown in Table 3-3.

Table 3-3
Minimum radii

Pipe material and DN	Maximum pipe length (m)	Method of jointing	Minimum radii (m)
VC DN100-300 (Spigot - socket)	0.6	Rubber ring	30
VC DN100-300 (Spigot - socket)	1.2	Rubber ring	45
VC DN100-300 (Spigot - spigot) + polypropylene sleeve coupler	1.6	Twin rubber ring	60
PVC DN100 and DN150	-	Solvent welded	50
PVC DN225	3.0	Solvent welded	100

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1. Alignment

   Curved sewers should be located so that they follow easily identifiable surface features such as property lines, curb lines, fence lines and building setbacks.

2. Contributing properties

   The load contributing upstream of a curved sewer shall be at least 10 EP or 3 residential dwellings.

3. Minimum grades

   It is considered that curved sewers will marginally increase operation and maintenance requirements compared with sewers on straight alignments. Further, accuracy in constructing curved sewers is more difficult to achieve.

   To allow for these factors steeper sewer grades are required.
   

Table 3-4
Minimum grades For curved sewers

Sewer size (DN)	Minimum grade permitted (%)
DN150 (refer to Clause 2.5 (ii))	1.50
DN225	1.25
DN300	1.00

 

2.6 Clearance to other services

Minimum clearances have been established to reduce the likelihood of damage to sewers or other services, and to protect personnel during construction or maintenance work.

Under no circumstances are:

• sewers to be cranked to avoid other services or obstacles;
• sewers permitted to be set longitudinally above or below other underground services in the same trench.

Clearance provided to other services should be maximised. The design should be based on the actual location of those services, where proving of service depths during the design phase is recommended. Where a sewer crosses another service, the design drawings shall direct the contractor to prove the location of the service by hand excavation.

Acceptable minimum clearances are as follows:

1. Parallel services

   The minimum clear horizontal distance permitted between the sewer and another service is 600mm.

2. Subsurface structures

   Horizontal clearance to structures such as valve pits, stormwater sumps, maintenance holes, hydrants etc. shall be 150mm.

3. Crossing srvices
   • Stormwater drains: The normal minimum vertical clearance when crossing stormwater drains is 150mm.
   • Water mains: The normal minimum vertical clearance when crossing water mains is 150mm.
   • Telstra: When crossing major and residential Telstra services the minimum vertical clearance shall be 150mm. Increased clearance may be required when crossing coaxial cables.
   • Gas: The minimum vertical clearance to low and medium pressure gas mains shall be 150mm. For high pressure mains the minimum clearance shall be 300mm.
   • Electrical cables: When crossing low voltage cables the minimum vertical clearance shall be 150mm. The minimum clearance to high voltage cables is 300mm.

2.7 Sewer crossings of roads, creeks and playing fields

• Roadway crossings - The sewers are to be designed for highway loadings.
• Creek crossings - These sections are particularly vulnerable to damage by scouring action during flood flows. Depth allowance should be made for scour together with adequate protection measures. If the pipe cover is less than 1.0 metre, concrete encasing of the pipe with rock gabions lining the creek bed will be required.

  Creeks are often lined with disturbed sediments. Prior to pipe laying, unstable material under the creek must be removed to sound ground and replaced with single size crushed rock aggregate. Refer to Standard Drawing No. WSS 056.

• Playing fields - see Clause 5.1.17.

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3. Hydraulic design

3.1 General hydraulic aspects

It is required that ACTEW's sewerage system be designed using methods and data that ensure compatibility of all system elements. The design of the sewerage system shall be based on the following flows:

• Peak wet weather flow (PWWF): used to determine the minimum sewer pipe size;
• Most probable peak dry weather flow (Qdmp): used to ensure that the proposed pipe grade is sufficient for self-cleansing and sulphide-slime control.

These flows are based upon the contributing "equivalent population" (EP), and, in the case of the PWWF, a further component based on contributing net sewered area (NSA).

3.1.1 Flow estimation

1. Equivalent population

   Design EP's for standard, low, and medium density residential housing developments, and for some commercial, industrial and institutional areas, can be obtained from Table 3-5. The residential population for each district and division of the ACT, present and future, can be obtained from the Urban Economics Branch of the Chief Minister's Department. Design EP's for land use types not included in Table 3-5 can be obtained from the document titled Design of Separate Sewerage Systems (Reference 10.2). However, the applicability of these EP's to Canberra should be checked with ACTEW.

   Table 3-5
   Design equivalent populations

   
   

   Land use	EP	Unit
   Residential: - Low density (1) - Medium density - High density (2)	3.6 2.5 2.0	per dwelling unit, per dwelling unit, per dwelling unit
   Commercial facilities: - Shops and offices - Public visitor buildings or sport spectator facilities - Restaurants and clubs - Tourist or hospital accommodation	0.3 0.05 0.1 0.5	per employee, per short stay visitor, per seat, per bed.
   Industrial: - Dry trades - Wet trades	0.3 *	per employee, assess on a case by case basis
   Institutional: - Schools and educational facilities	0.2	per student or staff

    

   Notes

   1. Less than 15 dwellings per hectare.

   2. More than 80 dwellings per hectare.
   

2. Design flow rates

   Table 3-6
   Definition of terms - input parameters

   
   

   Input parameters	Abbreviated	Unit	Used to calculate
   Total equivalent population, Table 3-5	TEP	EP	ADWF, Qdmp ,PDWF
   Equivalent population from residential catchment	REP	EP	TEP
   Equivalent population from commercial or industrial catchment	CEP or IEP	EP	TEP
   Net sewered area	NSA	ha	PII
   Residential net sewered area (1)	RNSA	ha	NSA
   Industrial net sewered area(1)	INSA	ha	NSA
   Commercial net sewered area(1)	CNSA	ha	NSA

    

   Notes

   1. Sewered areas exclude arterial roads, major floodways, and parkland.

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   Table 3-7
   Definition of terms - flow components

   
   

   Flow component	Abbrev	Unit	Used to calculate
   Daily per capita sewage contribution	PCC	L/EP/Day	ADWFs
   Average dry weather flow from gravity source only	ADWFG	L/s	Sewage age, degree of septicity, PDWFs
   Average dry weather flow including pumped flows	ADWFT	L/s	Sewage age, degree of septicity, PDWFs
   Peak dry weather flows (as per ADWFs)	PDWFG PDWFT	L/s L/s	PWWF, PDWFT Qdmp
   Total pumped flow	TPF	L/s	PDWF, PWWF, cleansing flows
   Most probable peak dry weather flow	Qdmp	L/s	Sewer cleansing grades, sewer slime stripping grades
   Capacity	Qf	L/s	Maximum pipe capacity
   Peak infiltration and inflow	PII	L/s	PWWF
   Peak wet weather flow	PWWF	L/s	Pipe capacity, pump capacity

    

3. Flow algorithms
   1. Residential areas
      • PCC = 300 L/EP/day
      • ADWF_G = TEP x PCC / 86 400
        = TEP/288
      • ADWF_T = ADWF_G + 1/3 TPF
      • PDWF_G = 5.83 ADWF_G /TEP^0.1
      • PDWF_T = PDWF_G + 2/3 TPF
      • PII = 1.43 NSA^0.81
      • PWWF = PII + PDWF_G + TPF
      • Qdmp = 0.75 PDWF_T

        To assist in the calculation of design flows, values of PDWF_G and PII can be obtained from Appendix 3-1 and Appendix 3-2 respectively.

   2. Industrial or commercial areas

      Guidance for the estimation of Equivalent Populations (EP"s) for various types of land uses is given in Table 3-5 and the document entitled Design of Separate Sewerage Systems (Reference 10.2). The EP's estimated from these sources shall be used to estimate the ADWF and PDWF from non-residential areas using the same formulae as given in the above section for residential areas.

      For non-residential areas however, the area used to estimate the PII shall be adjusted using the factored method to reflect reduced infiltration/inflow from the greater impervious area, and to reflect fewer service connections typically encountered in non-residential areas:

      • PII = 0.944 (INSA + CNSA) ^0.81

   3. Mixed landuse catchment

      The following method of flow estimation shall be used for catchments containing areas of different types of land use. In such cases the peak flows from different areas are nonsynchronous, e.g. the peak morning flow from residential areas precedes the peak flow from commercial areas. For mixed development, the critical design flow may coincide with the peak flow originating from any of the various land use types, depending on the relative multitudes of the contributing EP"s. The TEP is taken as the greater of the following formulae:

      • TEP = REP + 0.67 IEP or
      • TEP = 0.36 REP + IEP
        Use whichever value of "TEP" is higher.
      • ADWF_G = TEP/288
      • ADWF_T = ADWF_G + 1/3 TPF
      • PDWF_G = 5.83 ADWF_G / TEP^0.1
      • PDWF_T = PDWF_G + 2/3 TPF
      • PII = 1.43 [RNSA + 0.6(INSA + CNSA)]^0.81
      • PWWF = PDWF_G + PII + TPF
      • Qdmp = 0.75 PDWF_T

      For EP's from industrial or commercial areas refer to Table 3-5.

      For land use other than residential, commercial and industrial, ACTEW should be consulted for advice.

      Qdmp can also be derived from Figure 3.3 or Appendix 3-3.

3.1.2 Pipe capacity

Surcharge shall be defined as the flow depth exceeding the pipe obvert.

The principles outlined below reflect the design assumption that the probability of surcharge shall not exceed 1 in 10 years ARI.

The capacity of the sewer pipe (Qf) shall normally be equal to, or exceed, the PWWF. Pipe capacities can be determined using either the Manning or Colebrook-White equations, with the roughness factors being obtained from Table 3-8. To simplify calculations, pipe capacities can be obtained from Appendix 3-4 or AS 2200 (Appendix 3-4 is based on guideline pipe diameters). Where an actual internal diameter is used for calculations, it shall be taken as the average internal diameter for the representative pipe length including joints. Surcharging of sewers at flows up to PWWF is not normally permitted, and the designer may be required to demonstrate to ACTEW that head losses through maintenance holes when added to pipe friction effects do not result in surcharging.

Table 3-8
Pipe roughness factors

Sewer	Manning "n"	Colebrook white "k"
Gravity mains: < DN600, - VC or Concrete - PVC, PE, GRP	0.012 0.011	1.0mm 0.6mm
>= DN600, - Concrete, PE, GRP	0.013 - 0.015*	1.5 - 3.0mm
Rising mains:	0.011	0.6mm

 

* In the case of sewers >= DN600, ACTEW should be consulted for appropriate roughness values.

Where the above factors conflict with AS 2200, Table 3-8 takes precedence.

3.1.3 Grades for sewers

In designing sewer systems it is critical to provide self-cleansing and, in most situations, sulphideslime control grades. This is to minimise operations and maintenance problems.

Minimum pipe grades shall be in accordance with the following criteria:

1. Minimum grades - general

   Where physically and economically practicable, all gravity sewers shall be designed with grades exceeding the sulphide-slime control grade (Sss). For sewers less than DN300, where large cost penalties are involved in achieving Sss, some relaxation of requirements may be permitted. Cases where Sss can not be achieved should be referred with supporting data to ACTEW for a decision. For DN300 and larger sewers this requirement will be stringently applied. All sewers shall be laid on grades exceeding the self-cleansing grade (Ssc).

   Relevant design submittals shall show the proposed grades, minimum slime control grades and any septicity control measures considered to be necessary. These measures may include limitations on pipe materials, special internal coatings or linings, forced ventilation, flushing flows etc. Normally unprotected concrete will not be permitted in systems where grades are less than those required for slime control.

   The designs shall give consideration to load build up within the catchment. Where it is unlikely that self-cleansing flows will be achieved within 2 years of the first connections to the system, or slime control grades within 5 years, the designer shall develop proposals for self-cleansing and slime control and refer these to ACTEW for approval.

   The minimum grades for self-cleansing and sulphide-slime control in sewers larger than DN150 can be derived as follows:

   • Ssc = 0.0135/Rp
   • Sss = 0.0338/Rp

     where:

     Ssc = minimum grade for self-cleansing (%)
     Sss = minimum grade for sulphide slime control (%)
     Rp = hydraulic radius at Qdmp (m)

     The absolute minimum grade (Smin) for sewers of DN225 or larger shall be obtained from the following equation:

   • Smin = 80/ID

     where:

     Smin = absolute minimum sewer grade (%)
     ID = pipe internal diameter (mm)

     Appendix 3-5 may be used to assist in determining the required minimum grades. Note that the Qf shall be used to calculate the minimum grades, not the PWWF.

2. Minimum grades for DN150 pipes

   Minimum permissible grades of the uppermost reaches of sewers are dependent on the number of connections at the upstream end of the reticulation line and are given in Table 3-9 below.

   Table 3-9
   Minimum grades for DN150 pipes

   
   

   Ultimate number of residential properties draining to sewer	Minimum grade (%)
   1 house	l.25
   2 houses	1.2
   7 houses	1.1
   12 houses	l.0
   18 houses	0.9
   28 houses	0.8
   35 houses or thereafter	0.7

    

   These are the absolute minimum grades that shall be used. In general, it is not considered good practice to use a minimum grade on a short intermediate section of sewer when the upstream and downstream sections are laid at steeper grades.

3. Maximum grades for sewers

   Restrictions are placed on the maximum grades of sewers to limit internal erosion of pipe material, and/or pipe movement (due to trench flows causing loss of bedding).

   The maximum pipe grade for sewers larger than DN150 is 15%. Where grades steeper than 15% are planned the circumstances are to be referred to ACTEW prior to Design Submission 1.

   To limit the scouring effect arising from water flow within the pipe bedding material, and also to anchor the pipe, special bedding, scour stops or trench stops may be required in accordance with Standard Drawing Nos. WSS 056 and WSS 012. To enable easy location, scour and trench stops shall be placed at intervals of equal length with spacing not exceeding that which is specified on Standard Drawing No. WSS 012. The actual spacing and number of stops shall be nominated on layout drawings.

4. Grade changes between pipe reaches

   It is essential in the lower reaches of the sewerage system, where sewage may have low dissolved oxygen levels, that turbulence leading to the release of hydrogen sulphide from solution be avoided. In these areas, conditions such as a rapid change from steep to flat pipe slope, which favours the formation of a hydraulic jump at dry weather flows, must be avoided.

   For sewers larger than DN375 the potential for hydraulic jump formation must be checked wherever, at full development, a change occurs at Qdmp (refer Clause 3.1.1 (iii)) from a Froude number greater than 1.5 to less than 1.0 between adjacent sewer reaches. Designs shall be arranged so as to avoid formation of a jump at Qdmp, possibly by the introduction of a "neutralising length" of intermediate grade and a suitable length between the steep upstream and flat downstream sections. In relevant cases, calculations shall be submitted with Design Submission 2 justifying the neutralising slope and length adopted.

3.2 Minimum diameters

• Service tie for one residential block: DN100;
• Sewer mains: DN150;
• For sewers up to DN600: the downstream sewer shall not be smaller than the upstream sewer.

3.3 Dead-end pipelines

Sewers should normally terminate with a maintenance hole located 2.0 metres uphill of the low point of the most upstream property. A "dead-end" shall be defined as the section of sewer upstream of the last maintenance hole. Prior to using a "dead end" pipeline, all reasonable measures should be taken to achieve termination of a sewer with a maintenance hole by readjustment of maintenance hole locations, and/or slightly increased maintenance hole spacing. However, a "dead end" may be utilised if its use results in significant cost saving without creating undue additional maintenance problems.

"Dead end" pipelines shall terminate with a rodding point as per Standard Drawing No. WSS 061.

Acceptable "dead end" pipelines will have:

• a diameter not less than DN100 where serving a single property;
• a diameter not less than DN150 where serving two or more properties;
• a maximum length of 20 metres from the nearest downstream maintenance hole;
• no curved alignment (vertical or horizontal).

3.4 Sewer main junctions

Within a sewerage system it is mandatory that all sewer main junctions occur within maintenance holes (refer Clause 5.1.13). However, DN100 sewer tie connections can be connected by means of maintenance holes or sloped junctions. For connection of service ties see Clause 6.2.5.

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4. Structural design

4.1 Sewer pipe materials and construction methods

4.1.1 Types of pipe

Sewers shall be constructed from materials proven to be structurally sound and durable, and shall have satisfactory jointing systems. The use of two or more types of pipe material on a single run of pipe between adjacent maintenance holes is not acceptable.

Materials approved for use in ACTEW sewers are:

• Vitrified Clay - VC
• Reinforced Concrete - RC, see notes 1, 2 and 3
• Ductile Iron - DICL, see notes 1, 2
• Polyvinyl Chloride - PVC (Equivalent to class SEH, solid wall or approved structured wall), see note l
• Glass Reinforced Plastics - GRP, see note 4 (Polyester Based)
• Polyethylene - PE, see note 4
• Acrylonitrile Butadiene Styrene - ABS, see note 4

Notes

1. Not to be used within, nor up to 1 km downstream of industrial areas or hospitals.
2. Concrete shall be made with Type "SR" sulphate resisting cement with a tri-calcium aluminate content not greater than 5%, or Type "LH" low heat cement. Concrete pipes intended for other than trunk sewers shall be manufactured with a minimum 10mm sacrificial layer on the inside of the pipe. Thickness of sacrificial layers in sewers larger than DN375 shall be as shown in Table 3-10. The sacrificial layer thickness shall be the thickness required over and above that required for minimum cover to reinforcement and structural integrity.

   TABLE 3-10
   Sacrificial Layer Thickness - Sewers Larger than DN375

   
   

   DN	Thickness of sacrificial layer (measured on radius )
   450	20mm
   525	25mm
   600	30mm
   675	40mm
   >= 750	50mm

3. Concrete pipes are not acceptable for DN150 and DN225 sewers.
4. Subject to special conditions and only with written approval of ACTEW.

Proposals for the use of other materials will be considered if supported by adequate technical and performance data.

Where the pipe material is known it shall be shown on the drawings. Where the pipe material is not known prior to submission for detailed design acceptance, the drawings are to contain notes ensuring that the above requirements are satisfied.

4.1.2 Class of pipes

• Sewerage pipes must be of adequate strength to meet overburden and traffic loads. Loads are to include loads created from likely construction and maintenance activities;
• VC pipes shall be Class 4 or stronger;
• Class 2 (X), 3 (Y) and 4 (Z) reinforced concrete pipes manufactured in accordance with the latest version of AS 4058 are acceptable if used in accordance with the requirements of AS 3725;
• PVC pipes shall be of grade Sewer Extra Heavy (SEH) or of equivalent SN grade in accordance with AS/NZS 1260;
• Classes for Ductile Iron, Glass Reinforced Plastics, Polyethylene, or ABS pipes shall be approved by ACTEW prior to use.

Notes

1. Where load limits apply the locations shall be clearly designated on drawings.
2. During the construction phase specific load provision shall be made for heavy construction equipment where required.
3. No more than one type of pipe material will be used between successive maintenance holes or sewer maintenance shafts.

4.1.3 Pipe jointing

The sewer pipes are to be capable of excluding groundwater, resisting root intrusion, and withstanding pressure loading, both internal and external. Sewer systems must also have some flexibility, either through controlled deflection at joints (rigid materials) or pipe bending (flexible materials).

Acceptable pipe jointing systems are:

1. VC pipes with rubber ring jointing comprising:
   • Spigot - Socket system;
   • Spigot - Spigot system utilising approved Socket-Socket coupler.
2. Reinforced Concrete Pipes, Spigot-Socket, with rubber ring jointing.
3. PVC pipes:
   • DN100: solvent welded;
   • DN150: rubber ring jointed or solvent welded; *
   • Larger than DN150: rubber ring jointed. *

* All PVC pipes laid on horizontal or vertical curvatures shall be solvent welded if <DN300.

4.2 Depth of sewer and cover

1. Minimum depth for sewers outside of leased blocks

   The minimum cover, measured from the top of the pipe to the final surface level shall be the greater of:

   • The depth needed to provide 600mm of clear cover;
   • The depth needed to serve the whole of the adjacent block (see Clause 6.2.1);
   • In road verges, the depth needed to provide 750mm of clear cover;
   • Under minor sealed roads, the required cover is 900mm;
   • Under unsealed roads, proposed future roads or sealed arterials, cover is 1.2 metres.

2. Minimum Depth Within Leased Blocks

   The minimum sewer depth shall be the greater of:

   • the depth needed to provide 600mm of cover;
   • the depth needed to serve the whole of the block;
   • the depth prescribed by upstream sewers.

   Where the whole block cannot be serviced, advice must be provided to ACTEW and the Land Development Group of the ACT Government to assure relevant constraints are noted as a building restriction on lease documents.

3. Maximum Depth

   Sewer mains are to be designed for a maximum depth to invert of 5.0 metres. In special cases (e.g. to avoid a pump station or for a short length of line through a ridge) specific approval may be sought from ACTEW to exceed this limit.

4.3 Pipe bedding and backfill

Bedding and backfill of pipes shall be in accordance with Standard Drawing No. WSS 056, the Basic Specification (Reference 10.12), and any relevant Australian Standard. Bedding shall provide continuous support for joins and shall be well compacted and not disturbed by groundwater or other conditions.

The materials used for bedding shall be durable stone or washed sand, and shall have low permeability and high stability when saturated. Bedding shall be free of organic matter.

For sewers on steep grades the bedding requirements are set out in Clause 3.1.3 (iii).

4.4 Pipes at interfaces with rigid structures

Where pipes are connected to rigid structures or are embedded in concrete it is necessary to provide flexibility so that any differential settlement does not result in pipe damage.

An acceptable design will incorporate:

• at maintenance hole inverts, a flexible joint at the maintenance hole wall with a second in close proximity achieved through the use of a short pipe length (300mm to 600mm);
• for concrete encasement, flexible joints in close proximity to the face of the encasement achieved through the use of a short pipe length (300mm to 600mm).

Where PVC pipes are used, the section through a maintenance hole wall shall be either epoxy primed and coated with coarse sand, or an approved manufacturer's maintenance hole entry-fitting is to be used. The external interface between the pipe and the maintenance hole wall should be sealed with a 12mm fillet of bitumastic sealant.

4.5 Ventilation

Sewers are subject to corrosion induced by biochemical processes related to the formation and release of hydrogen sulphide gas.

There are three basic controls in the management of the release of hydrogen sulphide gas:

1. delay the production of the gas for as long as possible;
2. properly manage its release from the sewage once its production becomes inevitable;
3. remove the gas from the sewerage system in a manner that minimises impact.

The production of gas is minimal in fresh, aerobic sewage. Therefore, at upper parts of each catchment or sub-catchment, this condition should be maintained for as long as possible. Flows can be turbulent and sewers vented through house vents. Boundary traps and special vents will generally not be required.

The need to manage release of hydrogen sulphide gas usually relates to sewage age. Typically this occurs when the size of the sewer pipe is greater than DN300 and as soon as "stale" pumped sewage is discharged into the system. Each particular catchment will have its own characteristics that require investigation and judgement to determine where this condition commences.

Corrosion is coupled to the release of hydrogen sulphide gas which occurs in parallel with moisture uptake from the sewage. Release is controlled primarily by ensuring that surface turbulence is minimal under most flow conditions. Moisture uptake from the sewage surface depends on:

• the surface area of sewage;
• the relevant velocity between the sewage surface and sewer air;
• the temperature of the sewage;
• the relative humidity of the sewer air;
• the presence of surface modifying substances such as grease, oil, surfactants; and
• the degree of surface turbulence.

The management of sewer air must be based on the consideration of alternatives of natural ventilation and forced ventilation. Both involve provision of inducts, educt vent stacks, and curtains, in various combinations, to control the airflow in separate sections of the sewer. A forced ventilation option utilises fans to provide a greater level of control of airflow.

Once the release of hydrogen sulphide gas occurs, it is essential that it be managed to minimise damage to the sewerage system. This again is done by controlling the sewer air volumes and velocities. A balance between the needs for control of moisture uptake from the sewage surface, and extraction of gas from the sewerage system must be made.

Each catchment has its own characteristics that affect sewer ventilation. This includes topography, microclimate, land uses generating the sewage, and land uses affected by the ventilation works. It is essential that sewerage system designs, for catchments which require collector or trunk sewers and sewage pump stations, be assessed for corrosion control by an engineer experienced in this field.

In particular, the need for sewer ventilation must be carefully assessed for sewers larger than DN450, and for sewers conveying pumped sewage. For works of these types, proposals for sewer ventilation shall be submitted to ACTEW for consideration and approval. Ventilation works shall generally be designed in accordance with the latest edition of the document entitled Hydrogen Sulphide Control Manual - Septicity, Corrosion and Odour Control in Sewerage Systems (Reference 10.13), and any specific requirements of ACTEW for that location.

Large sewers and pump stations are a high cost asset where protective measures are very cost effective in terms of life cycle costs.

4.6 Alternative pipelaying methods

ACTEW may at times approve the use of alternative pipelaying technology such as thrust boring, directional boring, or pipe bursting. Where such methods are envisaged to be used ACTEW should be consulted for appropriate standards and specifications.

 

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