The core problem
If you've designed a buried flexible pipe to AS/NZS 2566.1, you've used the effective soil modulus E' — the single most important input in the deflection calculation. What most engineers don't realise is that the standard uses the same E' symbol to refer to two conceptually different things.
Look & Cameron (2018) set out the problem clearly in Australian Geomechanics: AS2566.1 incorrectly implies that E'e (the modulus of soil reaction for the embedment material) and E'n (the Young's modulus of the native trench wall soil) are equivalent and interchangeable. They are not.
AS/NZS 2566.1 Table 3.2 values for E' were derived from back-calculation of horizontal pipe deflections in embedment material only. Applying these values to the native trench wall is physically incorrect and can lead to unconservative design — particularly for deep cover, large-diameter pipe, or soft native soils.
Where E' comes from
The modified Iowa formula — which forms the basis of the AS2566.1 deflection calculation — was developed by Spangler (1941) and refined by Howard (1977) at the US Bureau of Reclamation. Howard back-calculated a modulus of soil reaction from measured pipe deflections in controlled tests. This E' is an empirical parameter specific to the embedment zone.
Δy/D — deflection ratio (× 100 for %)K — bedding factor (0.1 default per Cl. 5.2)w — total vertical pressure at crown (kPa)SDL — ring bending stiffness (N/m/m)E′ — effective modulus of soil reaction (MPa) — the contested parameterFor most practical installations the denominator is dominated by E', not SDL. A typical SN4 HDPE pipe has SDL = 3,200 N/m/m. A moderately compacted sandy gravel gives E' = 5 MPa. The soil term (0.061 × 5 = 0.305) is roughly 10 times the pipe stiffness term (8×10⁻⁶ × 3,200 = 0.026). This is why AS2566.1 commentary states that pipe-soil stiffness is secondary to soil stiffness.
E'e vs E'n: the distinction that matters
The effective modulus that governs pipe deflection is a combination of two distinct stiffness contributions:
- E'e — embedment modulus. The modulus of soil reaction of the imported material you compact around the pipe. This is what Howard's research measured. AS2566.1 Table 3.2 values are valid for this parameter when applied correctly.
- E'n — native soil modulus. The Young's modulus of the trench wall soil. This is a conventional geotechnical stiffness parameter. It is not the same as E'e and should not be read from Table 3.2.
When you select a Table 3.2 value and apply it as E' in the deflection formula, you are — as the standard intends — using E'e for the embedment zone. That's correct. Where AS2566.1 fails is in not clearly distinguishing this from E'n, and in providing inadequate guidance on when and how to account for the native trench wall.
The Leonhardt correction factor
AS2566.1 Clause 3.4.3 does include a mechanism to account for native soil: the Leonhardt correction factor ζ (zeta). When a trench is narrow relative to pipe diameter, the native trench walls provide passive resistance and stiffen the system. When the trench is wide, the embedment material dominates.
E'n — native soil Young's modulus (MPa) — from geotechnical investigationE'e — embedment modulus from Table 3.2 (MPa)Bd — trench width at pipe centreline (m)De — outside diameter (m)The combined effective modulus becomes: E' = ζ × E'e
For wide trenches (Bd/De ≥ 3), ζ approaches values close to 1.0 and the native soil effect is reduced. For narrow trenches in stiff native soil, ζ can exceed 1.0 — the native soil strengthens the system. In soft native soil with any trench geometry, ζ less than 1.0 means your E' should be reduced below E'e. This is the case most engineers miss.
Which E' values to use
For E'e, use AS2566.1 Table 3.2. These values are appropriate for the embedment zone. The table below shows the most commonly used values.
For E'n, use values from your geotechnical investigation — SPT correlations, pressuremeter tests, or published correlations. Do not use Table 3.2 values for the native soil.
| Embedment material | Compaction | E'e (MPa) | Notes |
|---|---|---|---|
| Crushed rock / gravel | >95% std Proctor | 20 | High-value / deep cover installations |
| Crushed rock / gravel | 90–95% | 15 | Well-supervised standard practice |
| Sandy gravel / sand | >95% | 10 | |
| Sandy gravel / sand | 90–95% | 5 | Most commonly assumed in AU practice |
| Sand / sandy loam | 85–90% | 2.5 | Marginal compaction |
| Clay | >90% | 1.0 | Cohesive embedment, well compacted |
| Soft clay / peat | Any | 0.2 | Problem soils — avoid if possible |
Practical guidance
1. Always separate E'e and E'n in your design
Even if your final calculation collapses to a single E' value, document them separately. Obtain E'n for the native trench wall from your geotechnical engineer — don't assume it equals your embedment value.
2. Apply Leonhardt correction when Bd/De < 3
For typical urban water and sewer installations, trench widths are often less than 3× the pipe diameter for pipes larger than DN400. If native soil is soft (E'n < E'e), ignoring ζ is unconservative.
3. Conservative E' for preliminary design
For preliminary work, apply E' = E'e only and select E'e based on the minimum compaction you can realistically guarantee and inspect on site. Remember: if you can only guarantee 90% std Proctor compaction for sandy gravel, your E'e is 5 MPa — not 15 MPa.
4. Consider FEA for critical installations
At road and rail crossings, large-diameter installations (DN800+), deep cover (>6m), or difficult native soils, finite element analysis with properly calibrated soil stiffness parameters will give more reliable results than the Iowa formula.
The difference between well-compacted (E'e = 15 MPa) and marginally compacted (E'e = 5 MPa) sandy gravel is a factor of 3 in effective soil modulus. For SDL-dominated denominators, this translates to roughly a 2× change in calculated deflection. Compaction control is the most important design variable in flexible pipe installation.
Summary
The E' confusion in AS2566.1 affects real design outcomes across Australia. Key takeaways:
- E'e (embedment modulus) and E'n (native soil modulus) are different parameters. Don't conflate them.
- AS2566.1 Table 3.2 values are valid for E'e only. Source E'n from your geotechnical investigation.
- Apply the Leonhardt ζ factor whenever trench width is less than 3× pipe diameter.
- For critical or complex installations, finite element analysis is more appropriate than the Iowa formula.
- Compaction quality dominates pipe performance far more than pipe stiffness class selection.