11.1 Bedding Classes & Material Specifications under AS/NZS 3500.2

G13/AS2 note: Clean granular bedding has particles no larger than 20 mm and supports the barrel without hard spots. Selected fill is free of topsoil and rubbish with particles no larger than 20 mm. Do not substitute an invented universal 7–10 mm aggregate or lettered class.

Bedding Classes & Material Specifications under AS/NZS 3500.2

In gravity drainage engineering, a pipeline's structural integrity depends as much on the ground support surrounding it as on the pipe wall itself. When an underground drain collapses, cracks, or loses gradient, the primary culprit is rarely defective pipe manufacture; in over 90% of forensic investigations, the failure stems from substandard trench bedding, uncompacted haunches, or contaminated backfill.

For a certifying drainlayer in New Zealand, mastering the bedding classifications and material specifications governed by AS/NZS 3500.2:2021 Clause 5.4, NZBC Acceptable Solution G13/AS2 Section 5.0, and AS/NZS 2032 (Installation of PVC pipe systems) is a core statutory competency.

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1. Structural Soil-Pipe Interaction: Flexible vs Rigid Pipelines

Underground drainage pipes are categorized into two fundamental structural classes, each transferring earth and surface traffic loads in an entirely different manner:

Flexible Pipes (uPVC, Polyethylene, Polypropylene)

Modern sanitary and stormwater systems in New Zealand are predominantly constructed from unplasticised polyvinyl chloride (uPVC) complying with AS/NZS 1260, or high-density polyethylene (PE) complying with AS/NZS 4130.

Flexible pipes have relatively thin walls that cannot withstand heavy vertical soil and wheel loads through ring stiffness alone. Instead, they rely on soil-structure interaction (governed by Spangler's Iowa Formula):

1. Vertical earth load presses down on the pipe crown, causing slight vertical deflection.
2. As the vertical diameter compresses, the horizontal diameter expands outward into the trench sidewalls.
3. If the haunch and side support zones are packed with densely compacted granular aggregate, this outward movement mobilises passive lateral earth resistance.
4. The surrounding gravel pushes back against the pipe walls, creating a balanced arching mechanism that supports the overburden.
5. Under AS/NZS 2566.1 and AS/NZS 3500.2, the maximum permissible long-term vertical diametral deflection for uPVC drains is strictly 5% (or up to 7.5% depending on class and design life).

If the haunch zones contain voids, loose soil, or uncompacted clay, the pipe walls expand unchecked, leading to catastrophic ovalization, joint seal unseating, and wall buckling.

Rigid Pipes (Vitrified Clay, Reinforced Concrete)

Rigid pipes possess high inherent compressive ring strength and minimal flexural ductility. When vertical surcharge loads are applied, the pipe wall resists the load directly through shear and bending stresses without expanding laterally. Rigid pipes do not rely on side soil pressure; instead, they require a continuous, uniform bedding cradle across the lower quadrant to distribute the load evenly across the pipe base and avoid high point-load concentrations.

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2. Standard Bedding Classifications under AS/NZS 3500.2 & G13/AS2

AS/NZS 3500.2:2021 Clause 5.4 and NZBC G13/AS2 define four standardized bedding configurations:

Detailed Analysis of Bedding Types

Type A: Concrete Cradle & Encasement

Type A bedding utilizes concrete with a minimum 28-day compressive strength of 17.5 MPa (under NZBC G13/AS2 Paragraph 5.2) or 20 MPa (under AS/NZS 3500.2).

• 120° Concrete Cradle: Concrete is poured beneath the pipe to support the lower third of the circumference, providing a rigid foundation that prevents vertical settlement in poor soils.
• Full Concrete Encasement: The pipe is completely surrounded by a minimum of 100 mm of concrete on the bottom, sides, and top. This configuration is deployed where the drain has insufficient cover depth beneath roads or driveways.
• Critical Joint Isolation Requirement: Because concrete is unyielding and rigid while uPVC expands and contracts thermally, continuous concrete encasement will snap pipe sockets during structural movement. A flexible compressible wrap (6 mm to 10 mm closed-cell polyethylene foam or bitumen-impregnated fiberboard) must be wrapped around every rubber-ring joint, and transverse contraction joints must be provided across the encasement at maximum 3 m intervals.

Type B: Granular Bedding & Haunching (The Professional Standard)

Type B is the most critical bedding type for certifying drainlayers. It consists of two distinct zones:

1. Bedding Cushion: A layer of clean, compacted granular aggregate placed across the full width of the trench bottom before the pipe is laid. It must be minimum 75 mm thick in ordinary stable soil, and minimum 100 mm thick in rock, hard shale, or unyielding ground.
2. Haunching & Side Support: Granular aggregate placed simultaneously on both sides of the pipe barrel and compacted in layers up to the springline (the horizontal centerline of the pipe, representing 50% of the outside diameter). Under AS/NZS 3500.2 Figure 5.4.1, for flexible pipes in roadways, this granular embedment is extended to 75 mm to 100 mm above the pipe crown.

Type C: Minimal Granular Bedding

Type C uses a 75 mm granular bedding cushion beneath the barrel, but the haunching material consists of selected excavated soil compacted by hand. Because native soil rarely compacts to the uniform density of crushed aggregate, lateral support is compromised. This class is generally restricted to rigid pipes or non-trafficked landscape zones.

Type D: Shaped Trench Bottom

Type D requires the excavator operator or drainlayer to hand-trim the trench floor into a concave groove matching the outside curvature of the pipe barrel. While it eliminates imported aggregate costs, it is virtually impossible to execute accurately in modern trench conditions. If stones or hard spots remain on the trimmed surface, the pipe point-loads and fractures. Type D is almost never accepted by New Zealand territorial authorities for modern residential or commercial subdivisions.

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3. Granular Bedding Material Specifications

Bedding materials must satisfy strict grading, particle shape, and cleanliness criteria under AS/NZS 3500.2 Table 5.4.1 and NZBC G13/AS2 Section 5.1.

Approved Bedding Aggregates

1. Pea Gravel / Pea Metal (7 mm to 10 mm): Naturally rounded or crushed river stone screened to a uniform size. Rounded pea metal flows readily around the pipe barrel and achieves 90% to 95% compaction with minimal hand-tamping, making it the most reliable bedding aggregate in trade practice.
2. Crushed Rock / Basalt / Greywacke Chip (7 mm to 10 mm): Angular crushed quarry rock. Provides outstanding mechanical interlock and shear strength under heavy wheel loads, but requires deliberate hand-tamping into the haunches to eliminate bridging voids.
3. Coarse Washed Sand: Concrete sand or graded river sand free of clay, silt, and salts. Effective in dry trenches, but prone to washing out or migrating if the trench experiences subsurface water flows.

Material Grading Envelope & Particle Limits

Why Fines Must Be Excluded

Aggregate containing more than 5% clay and silt fines behaves unpredictably. When groundwater infiltrates the trench, clay fines soften, turning the bedding cushion into a fluid slurry. Under wheel loads, the pipe sinks into the mud, reversing the gradient and causing severe bellies (dips) that trap solids.

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4. Trench Dimensions, Haunching & Bell Hole Scooping

Trench Width Parameters

Trench width directly influences the earth load transmitted to the buried pipe. Under Marston's Trench Load Theory, as trench width increases, the weight of the backfill prism pressing down on the pipe increases exponentially.

• Minimum Trench Width: The trench must provide adequate clearance for a drainlayer to stand, lay the pipe, and operate hand-compaction tools. Under AS/NZS 3500.2, the minimum trench width is Pipe Outside Diameter (OD) + 200 mm (providing at least 100 mm clearance on each side of the barrel):
  • For DN100 uPVC (110 mm OD): Minimum trench width = 110 + 200 = 310 mm (typically dug with a 350 mm or 400 mm bucket).
  • For DN150 uPVC (160 mm OD): Minimum trench width = 160 + 200 = 360 mm (typically dug with a 450 mm bucket).
• Maximum Trench Width: The trench width at the top of the pipe should not exceed OD + 600 mm (or 600 mm total for DN100). If an excavator digs an excessively wide trench (e.g. 1.2 m wide for a DN100 pipe), the pipe is subjected to embankment loading conditions rather than trench loading, requiring an upgrade from Type B to Type A concrete encasement.

The Haunch Zone: Elimination of Bridging Voids

The haunch zone is the triangular space bounded by the lower curvature of the pipe barrel, the flat bedding cushion, and the trench sidewall.

• When aggregate is dumped into a trench from an excavator bucket, it cascades over the pipe crown and bridges across the gap, leaving large hollow voids directly under the lower shoulders of the pipe.
• The Hand-Haunching Rule: Aggregate must be shoveled in evenly on both sides in lifts not exceeding 100 mm. Drainlayers must use a blunt-ended wooden tool, shovel handle, or curved hand tamper to actively pack and slice the aggregate beneath the haunches until firm resistance is met. Mechanized vibrators must never be placed against the pipe barrel.

Bell Hole Scooping: Preventing Beam Loading

uPVC and concrete pipes feature push-fit rubber ring socket joints (collars) that have a significantly larger outside diameter than the pipe barrel.

• The Failure Mechanism: If a pipe is laid onto a flat, unyielding bed of gravel, the oversized bell collar rests on the bedding while the barrel floats suspended 15 mm to 25 mm above the trench floor.
• When backfill is dumped on top, the pipe acts as a supported beam. Tensile bending stresses concentrate at the socket junction, causing the collar to crack or the pipe barrel to shear clean through.
• The Correct Procedure: Before offering up each pipe length, the drainlayer must scoop out a localized pocket (a bell hole) in the bedding aggregate directly beneath the socket position. The bell hole must be deep enough and wide enough to ensure that the collar hangs completely free with zero contact pressure, allowing the entire length of the pipe barrel to rest uniformly on the bedding cushion.

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5. Worked Calculation: Trench Bedding and Haunching Volume for a DN100 Run

Project Scenario

A certifying drainlayer is preparing a material order for a new commercial foul water installation in Hamilton.

• Pipe specification: DN100 SN8 uPVC (Outside Diameter OD = 110 mm = 0.11 m).
• Total drainline length: 35 metres.
• Trench dimensions: 400 mm wide (0.40 m), excavated in stable clay.
• Bedding specification: AS/NZS 3500.2 Type B bedding (100 mm granular bedding cushion beneath barrel, granular haunching extending to the springline at 50% OD).
• Material: Screened 7 mm to 10 mm pea metal (bulk density = 1.6 tonnes/m3).

Step-by-Step Quantity Take-Off

Step 1: Calculate the Bedding Cushion Volume (Beneath Pipe Barrel)

The bedding layer extends across the full 0.40 m trench width for the entire 35 m length at a depth of 0.10 m: Bedding Volume = Length * Width * Thickness = 35 m * 0.40 m * 0.10 m = 1.40 m3

Step 2: Calculate the Gross Haunch Zone Volume (Up to Springline)

The springline represents half the pipe outside diameter: 0.11 m / 2 = 0.055 m. The gross volume of the trench from the top of the bedding cushion up to the pipe springline is: Gross Haunch Volume = Length * Width * Height = 35 m * 0.40 m * 0.055 m = 0.77 m3

Step 3: Calculate the Displaced Volume of the Lower Pipe Half

The bottom half of the circular uPVC pipe occupies space within the haunch zone. The cross-sectional area of half a cylinder is: Lower Pipe Half Area = 0.5 * pi * r^2 = 0.5 * 3.14159 * (0.055 m)^2 = 0.00475 m2 Pipe Displacement Volume = 0.00475 m2 * 35 m = 0.166 m3

Step 4: Calculate Net Haunch Volume

Net Haunch Volume = Gross Haunch Volume - Pipe Displacement Volume = 0.77 m3 - 0.166 m3 = 0.604 m3

Step 5: Total Theoretical Aggregate Volume

Total Theoretical Volume = Bedding Volume + Net Haunch Volume = 1.40 m3 + 0.604 m3 = 2.004 m3

Step 6: Factor for Compaction, Trench Irregularity & Wastage

In field practice, trench over-excavation, sidewall sloughing, and aggregate compaction account for a 15% allowance: Ordered Volume = 2.004 m3 * 1.15 = 2.305 m3 Rounding up to standard supplier increments: 2.4 m3 (or approx. 2.4 * 1.6 = 3.84 tonnes of pea metal, costing approx. NZ$380 delivered).

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6. Trade Traps & Common Certification Pitfalls

• The Flat Shovel Bedding Trap: Dumping pea metal into the trench, raking it flat, and pushing pipes together without digging bell holes. When the trench is backfilled, the pipe rests entirely on its joint collars. Within months, differential settlement causes circumferential cracking at the sockets, resulting in failed council water testing and costly excavation.
• The Dirty Aggregate Trap: Accepting a load of cheap "pit-run" gravel or unwashed quarry metal containing 15% clay fines. In dry weather, it appears firm; during the first winter storm, groundwater turns the clay into liquid mud, the aggregate loses all bearing capacity, and the pipeline slumps, creating a permanent reverse grade.
• The Bridging Haunch Void Trap: Backfilling the trench by dumping aggregate straight from an excavator bucket directly over the crown of the pipe without hand-tamping the sides. The aggregate bridges over the haunches, leaving continuous air voids beneath the pipe shoulders. When driveway traffic passes overhead, the pipe ovalizes by 12% to 15%, causing rubber rings to leak raw sewage into the subsoil.
• The Over-Excavated Rock Point-Loading Trap: Encountering hard basalt or greywacke bedrock and blasting or hammering the rock, but leaving sharp rock pinnacles within 20 mm of the pipe barrel while padding the rest with loose sand. Over time, dynamic surface vibrations drive the sharp rock pinnacle straight through the bottom of the uPVC pipe wall.