1.2 Drain Layout, Junctions, Bends & Sweep Connections

Drain layout, junctions, bends, and access

A certifying drainlayer should treat layout as a combined hydraulic, maintenance, and compliance exercise. The relevant numbers in this section come from G13/AS2. AS/NZS 3500.2 may be used through G13/AS3, but it is a different compliance path. Requirements must not be mixed selectively.

Start at the outfall

Confirm the authorised outfall, boundary position, available invert, and any network-utility conditions before setting a private drain. Survey the connection physically where its level or location is uncertain. Work upstream from that fixed control, allowing for pipe diameter, fittings, access structures, required gradient, bedding, cover, and building penetrations. A mathematically adequate fall is not enough if the finished arrangement cannot be rodded, tested, protected, or connected lawfully.

Keep runs direct and use purpose-made drainage fittings. A branch junction under G13/AS2 must point in the direction of flow and enter at an angle not exceeding 45 degrees. Where a branch enters a graded drain, the branch is inclined upward at least 15 degrees. These controls reduce opposed flow and help place the branch flow into the receiving stream. A square tee in a horizontal graded drain is not an equivalent substitute.

Changes of direction can be made with bends and junctions permitted by the selected solution. The important exam habit is to consider the cumulative change and the access consequence, not just each fitting separately. Access is required where a change of direction or gradient exceeds 45 degrees. If several fittings create a larger effective change, evaluate the assembled layout and provide usable access.

Access schedule under G13/AS2

Prepare an access schedule while drawing the plan. G13/AS2 calls for access:

• immediately before the drain outfall;
• immediately inside the property boundary;
• at every junction, except a junction less than 2 m long that serves only a gully;
• at every change of direction greater than 45 degrees;
• at every change of gradient greater than 45 degrees;
• on a straight run at intervals no greater than 50 m where rodding points are used, or 100 m where access points, inspection points, or inspection chambers are used; and
• within 2 m outside the building.

“Immediately inside the boundary” should not be converted into an invented universal 1.5 m allowance. Likewise, the straight-run rule is not a universal 30 m spacing. The actual fitting or chamber must be accessible after paving, landscaping, and occupation. A buried cap that cannot be found or opened is not useful access.

Design review

Trace a cleaning tool from each access point through the downstream route. Check that tight compound bends, opposing branches, and abrupt level changes do not prevent rodding or camera passage. Where a significant level difference is necessary, use a documented detail appropriate to the chosen compliance path and network requirements. There is no safe basis for declaring that every drop above one remembered dimension requires the same proprietary arrangement.

Check the plan in three dimensions. Other services, foundation zones, traffic loading, groundwater, and trench stability may change an apparently simple route. Coordinate penetrations with the structural design and preserve pipe support on both sides of a wall or footing. At a public connection, follow the network utility operator’s approved connection method; do not core, saddle, or enter a live asset without authority.

Verification and records

Before cover, verify line, gradient, invert levels, socket engagement, junction orientation, access locations, bedding, and support. Test the installed drain by a method accepted by the selected compliance path. Record surveyed as-built positions and inverts. A camera survey can reveal a displaced joint or ponding, but it does not replace the specified leakage test and is not automatically required on every private project.

Worked check

A straight private drain is 72 m long, with a branch at 28 m and an outfall at the downstream end. Under G13/AS2, the branch normally needs access because it is not the short gully-only exception. Rodding points alone cannot be spaced more than 50 m, so the designer locates usable access to cover the full run. Access is also provided immediately before the outfall, immediately inside the boundary, and within 2 m outside the building where those are distinct locations. The final answer is a plotted access schedule, not merely “72 divided by 50.”