Backfill placement

​Placement of backfill is as defined and designated in this section.

Trenches and shafts 6.9.1

Placement 1 - Mechanical compaction

For mechanical compaction, backfill must be placed and compacted using suitable mechanical equipment to achieve the minimum dry density ratios.

Trenches and shafts - backfill and surface reinstatement

Placement 2 - Water jetting and flooding compaction

For water jetting and flooding compaction, backfill must be compacted by complete saturation with water during and/or after backfilling, either by jetting with water under pressure or by flooding with water from the surface.

Drives, tunnels, shafts and bores 6.9.2

Placement 3 - Pneumatic placement for drives and tunnels only

For pneumatic placement, backfill must be blown into the excavation using compressed air.

Grouting 6.9.3

Grouting of tunnels, drives and bores must be carried out within 24 hours of pipe placement by one of the following methods:

Placement 4 - Slurry gravity grouting

For slurry gravity grouting, a slurry is allowed to flow under the force of gravity to fill the excavation. The slurry must be placed in shafts and allowed to flow into the drives to completely fill them.

Where the backfilling will be placed against a bulkhead, the Contractor must provide, in the uppermost of the bulkhead, a horizontal proving hole no less than 50 millimetre diameter. The Contractor must maintain the proving hole in a clear condition.

Placement 5 - Pressure grouting

For pressure grouting, a slurry is forced into the excavation using air or mechanical pressure.

Backfill method 6.9.4

The backfill method must be designated according to the appropriate backfill material type combined with the permissible backfill placement. For example, backfill method A3 - 1 is material A3 with placement 1.

Trenches and shafts

Backfill method for trenches and shafts must be as specified unless otherwise specified in road reserves by the responsible authority.

Drives, tunnels and shafts

Drives and tunnels must be refilled above the embedment zone using one of the following methods of backfill:

  1. Method A2 - 3 - 20 mm Class '2'crushed rock pneumatically placed

  2. Method C1 - 5 - Sand-cement slurry pressure grouted

  3. Method C1 - 4 - Sand-cement slurry gravity grouted

  4. Method C2 - 4 - Clay slurry gravity grouted

Gravity grouting should only be used for tunnels where there's sufficient head. Shafts may be refilled using Method C1-4 or C1-5 up to 600 mm below the surface, except if they're in road reserves or adjacent to structures. The remainder of the shaft must be backfilled in accordance with Trenches and shafts reinstatement, for the particular location of the pipeline.

Backfill within road reserves 6.9.5

Backfill material type A5-2 can't be used in road reserves.

Pipelines with insufficient cover 6.9.6

Backfilling over pipelines constructed in embankment conditions must be carried out in accordance with Drawing 7251/8/302.

Compaction testing 6.9.7

Where required, the dry density ratio of embedment and backfill must be determined. Testing must be carried out by a NATA registered tester in accordance with the current version of AS 1289 and/or ASTM D2922.

Backfill which fails to achieve the dry density ratio as shown on the Drawings or Trenches and shafts must be removed, replaced and re-compacted by the Contractor. All testing of the backfill will be at the Contractor's expense.


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