AS2566.2 Table 5.6 Explained: GRP Allowable Deflection

If you have searched for "AS2566.2 Table 5.6 GRP deflection", you are almost certainly looking for the allowable deflection of a GRP (glass-reinforced plastic) buried pipe — the limit an installed pipe is checked against. The single most useful thing to know up front is which part of the standard that limit lives in: it is Part 2 (Installation), not Part 1 (Design). Getting that straight is the difference between a clean acceptance test and an argument in the trench.

What Table 5.6 is

AS/NZS 2566.2 is the installation half of the buried flexible pipe standard. Where Part 1 tells you how to calculate a pipe's structural performance, Part 2 governs how the pipe is put in the ground and how the finished installation is verified. Its deflection provisions — including Table 5.6 — set the allowable deflection of the installed pipe: the field acceptance limit.

Because a flexible pipe carries load by mobilising the soil around it, its final deflection is largely a product of how well it was bedded and compacted. The installation acceptance limit exists to confirm that the soil support the design relied on was actually delivered on site.

Where it applies

Table 5.6 comes into play at the installation acceptance stage — after the pipe is bedded, embedded and backfilled, when deflection is measured (typically by a mandrel/proof gauge or a survey) and compared against the allowable limit. In practice that means:

• Material: GRP and other buried flexible pipe covered by Part 2.
• Stage: post-installation field verification, not design.
• Basis: measured diametral deflection, expressed as a percentage of the pipe's diameter.

How it differs from the Part 1 design check

In our deflection worked example, the modified Iowa formula predicts a design deflection from load and soil modulus. Table 5.6 sits at the other end of the process — the installed limit you verify against:

How the acceptance check works

The installed deflection check is simple in principle:

1. Install the pipe to the Part 2 requirements — bedding, embedment and compaction as specified.
2. Measure the vertical diameter reduction after backfill, as a percentage of the pipe diameter.
3. Compare the measured value against the Table 5.6 allowable for the pipe and conditions.
4. Accept or investigate — a measured deflection within the limit passes; one that exceeds it triggers investigation and remediation before acceptance.

If the installed deflection is comfortably inside the limit, it confirms the soil support assumed in the Part 1 design was achieved. If it is close to or over the limit, the embedment or compaction likely fell short of what the design assumed.

Common mistakes

• Using the design deflection as the acceptance limit (or vice versa). They come from different parts of the standard and serve different purposes.
• Reading the limit from the wrong part. The installed deflection limit is in AS/NZS 2566.2 (Installation), not AS2566.1 (Design).
• Quoting a remembered percentage. Confirm the current allowable from AS/NZS 2566.2:2002 for your specific pipe and conditions rather than relying on a number from memory or a forum.

Summary

• Table 5.6 concerns the allowable installed deflection of buried flexible pipe, from AS/NZS 2566.2 (Part 2, Installation).
• It is the field acceptance limit — distinct from the Part 1 design deflection you calculate.
• Design to AS2566.1; verify the installed deflection against Table 5.6. See AS2566.1 vs AS2566.2 for the full split between the two parts.
• Read the specific allowable value from AS/NZS 2566.2:2002 for your pipe and conditions.