Foulwater drainage

Foulwater drainage

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Installation of sanitary plumbing and drainage systems in New Zealand must be in accordance with NZ Building Code clause G13 Foul water or AS/NZS 3500 Plumbing and drainage. The legislation sets out the minimum requirements, and your drainage plan will show details about the specific installation you are working on.

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​Foul Water Drainage System.

G13 V AS/NZ3500.2.2021

G13 is a document from the building code. It deals specify with foul water drainage systems in NZ. And set out the minimum standard to achieve compliance. This document was first published in July 1992.

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AS/NZ3500.2.2021 Sanitary plumbing and drainage. This is a combined standard designed for both Australian and NZ applications. It is now seen as the Compliance document of choice for designers of foul water drainage systems.

​Installing a drainage system.

​The first step towards installing a drainage system is to decide on its position. This includes where the drain outfall should go, the fixed points to connect to, and other reference points. Before drains can be installed the site must be prepared . Site preparation is making an evaluation of the site (working out the requirements )and taking steps to provide what is needed to install the drainage system.

Minimum Grades for 100mm Foulwater drains.

• According to G13 minimum grade for a 100mm pipe is 1:60. (Table 2 pg 40 )

• A drain can be laid to a lesser gradient however discharge units loadings must be met.

• According to AS/NZ3500.2.2021 the minimum grade is 1.65% for a 100mm drain.

• A reduced grade can be used 1% for drains that serve septic tanks.(table 3.4.1 pg21)

Overflow Relief Gully.

• The term gully trap is commonly used to 

  include both the gully trap, gully riser and 

  the gully dish.

• The gully riser should be no more than 

  600mm (an arm’s length)

• This water seal needs to be charged 

  (topped up) and this can be done by, for 

  example, discharging from a waste pipe, or 

  hose tap.

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​Backflow protection.

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If a surcharge (backflow) is likely to occur regularly in a gravity drainage system, a reflux valve may be installed. A reflux valve also referred to as a surcharge protection device or a backflow prevention device is designed to prevent any surcharge of sewage or stormwater. A surcharge is any reverse flow of sewage or stormwater, usually from a public sewer network utility operator’s system into a private property.

Self Check.1​

Answer the following ​

Multiple Select

What are 2x functions of a gully trap ?

• Overflow relief for sewer system

• For venting of sewer system.

• Access into the sewer drain.

• Receive discharge from wastewater fixtures

Multiple Select

What are the 2xcodes used for foul water drainage.

• G12,water

• AS/NZ3200

• G13/AS2

• AS/NZ3500.2.2021

• G14 trade waste.

Multiple Choice

According to G13/AS2 what height must a gully dish be set above unpaved ground?

• 100mm

• 50mm

• 150mm

• 75mm

Multiple Choice

What is the minimum level a gulley dish should be set below a floor level.

• 50mm

• 200mm

• 100mm

• 150mm

Multiple Choice

According to AS/NZ3500.2.2021 what is the minimum grade for a 100mm pipe.

• 1:80

• 1:40

• 1:60

• 1:120

​Anchor blocks are needed anywhere that water hammer occurs within a pipe.Water hammer occurs when a liquid moving through a pipe meets a change of direction, such as a gradient which causes the liquid to move faster. As the liquid speeds up the pressure inside the pipe increases, causing stress in the pipe walls, joints, junctions, and bends. Anchor blocks will provide extra strength and stability to these areas.

​Anchor blocks.

​Thrust blocks work a similar way to anchor blocks by supporting pipework and protecting it from stress when there is a horizontal change of direction, such as a bend or junction. They are very common in water supply systems, which are under constant pressure. They are also used in drain and stormwater systems where gravity can cause water flowing through long lengths of pipework to quickly gain speed and put pressure on the pipes.

​Thrust blocks.

​Anti-scour blocks are used to support pipework in unstable ground, or where surface water might erode this ground base. As well as propping up the pipework,anti-scour blocks form a series of partitions along the trench . These walls stop surface water from running along the trench and washing away the underlying soil, which is something that is a problem for pipework installed in loose sand or fine silty soil.

​Anti-scour block.

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A drain should be tested for soundness before it is backfilled.

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The test is to make sure the pipework does not leak and to make sure groundwater does not enter the drainage system. Groundwater entry is known as groundwater ingress.

The testing of foul water drainage pipework is set out in the Acceptable Solutions G13/AS2 and G13/AS3 (which is AS/NZS 3500.2 modified).

The testing of stormwater drainage pipework is set out in E1. Surface water drains shall be capable of passing one of the tests described in E1/VM1 Paragraph 8.0.

G13/AS2 requires water or air tests on drains and indicates where details of those tests can be found, for example, an air test is described in AS/NZS 3500.2.

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Access points

G13 provides the definition of access points as being a place where access may be made to a drain or discharge pipe for inspection, cleaning, or maintenance.

G13/AS2 section 5.7 shows methods of construction for access points, and where they must be positioned in a drainage system. These include inspection points, rodding points, inspection chambers, and access chambers.

​Self check 2

Answer the following

Multiple Choice

According to G13 When must you use Verifiable leveling equipment?

• When testing a drain

• When compacting backfill

• When laying a drain below minimum gradient.

• When installing a manhole.

Multiple Choice

What is the minimum width for a 100mm drain?

• 400mm

• 250

• 350

• 300

Fill in the Blanks

Multiple Choice

What is the maximum Head you can place on the drain when water testing?

• 50M

• 45M

• 10m

• 3M

Fill in the Blanks

Well Done, You are ready for your formative assessment.