Backfilling & Compaction
Initial Backfill
The initial backfilling takes place after the pipeline has been installed and tested according to the engineering specifications. The initial backfill extend from the bedding material up to the sides of the pipe to a level approximately 30cm over the top of the pipe as shown.

The backfill material shall be of select soil or sand or gravel, free from larger stones or clods. The bedding or backfill material shall be compacted with shovel or other hand tools to fill the folds.

Placing side fill material
Side fill of either granular or selected backfill material, depending on the class of bedding, should be placed evenly on either side of a rigid pipe taking care not disturb the line and level. Bedding material should not be compacted in the socket holes.

The sidefill should be placed to the top of the pipe and hand tamped in 150 mm layers, ensuring that the line of laid pipeline is not disturbed. The initial backfill should be placed to 150 mm above the crown of the pipe and this layer hand tamped.

Placing backfill material
As soon as the pipes are correctly jointed, bedded, tested and approved, selected material and backfill should be placed. The minimum thickness of the initial backfill over the pipe barrel shall be 150 mm with a minimum of 100 mm over the joint.

For clay pipes with class D, N, F or B bedding, selected material from the trench excavation can be used as sidefill above the bedding, or for the layer directly above the pipes, provided that it is readily compactable. It should exclude stones retained on a 14 mm sieve, hard lumps of clay retained on a 100 mm sieve, timber, frozen material, and vegetable and foreign matter. For class D, N or F bedding the selected material should be placed under the haunches of the pipe, care being taken not to displace the pipe from its correct line and level.

Laying pipes with concrete beds or surrounds
Bedding or surrounding a pipe in concrete may be required in some cases. The indiscriminate use of concrete for pipe beddings can cause problems unless carefully specified procedures are adopted.

The trench formation should provide a firm foundation for the concrete bed or its value in strengthening the pipeline will be lost. It may therefore be necessary to seal or firm up the trench bottom before laying the concrete bedding using a blinding layer of weak concrete or granular material. It may also be necessary to excavate soft spots and compact in some more suitable material, such as granular bedding material or small hardcore.

It is important that the following minimum dimensions for concrete bedding or surround are used in order to ensure that the specified bedding factors are realized. Any concrete bed or surround should extend at least 150 mm either side of the pipe. The depth of concrete below the pipe, and above the pipe for a surround, should be at least 150 mm or one quarter of the outside diameter, whichever in the greater.

Where pipes are laid on blocks, at least two layers of bitumen damp-proof sheeting should be positioned between the blocks and the pipes. This will allow the pipes to be uniformly supported by the concrete after normal setting shrinkage has occurred. Any rigid supports in contact with the pipes must be removed as concrete is placed.

The flexibility of a pipeline bedded on or surrounded with concrete should normally be maintained by the provision of flexible construction joints through the concrete at each pipe joint. These should be made from bitumen impregnated insulating board or other equally compressible material such as expanded polystyrene. The board should be cut to fit the pipes, and placed at the face of sockets or at one end of sleeve joints. Where more uniform support of the pipeline is found, the construction joints may be less frequent.

Flexible construction joints through the concrete
Where large shear forces may be expected to occur at construction joints because of heavy imposed loads, it is preferable to omit flexible construction joints and to longitudinally reinforce the concrete bed to obviate possible excess shear forces causing pipeline failure. Examples are on shallow pipelines under main roads or on very deep pipelines. However, it is necessary to introduce one flexible construction joint at least every 5 m length, keeping the longitudinal reinforcement continuous, so as to avoid problems due to the expansion and shrinkage of the concrete. This construction joint should be positioned at the face of a pipe joint.

Final Backfill
Final backfill extend from the initial backfill to the top of the trench. Final backfill shall be placed in layers according to the engineering specifications and shall have no rocks or stones.

The compacting level must be in agreement with the determinations of the static calculation that was made for the pipeline in question. The required compacting level should be tested according to a machine-specific procedure (compaction machine) or if necessary be demonstrated by measurement. If stipulated, the compaction of the covering directly above the pipe has to be done manually. The mechanical compaction of the remainder of the trench backfill directly above the pipe may only be done once a layer of a minimum thickness of 30 cmm is filled in above the pipe crown. The required total thickness of the layer directly above the pipe before mechanical compaction can be done, depending on the nature of the compaction machine. The choice of the compaction machine, the number of times that compaction is carried out and the thickness of the layer to be compacted have to be matched with the material to be compacted and the pipeline to be installed. Compaction of subsequent trench backfill or of the sideways addition by watering in is only permitted in exceptional circumstances and then only in connection with suitable, non-cohesive soils.

A wide bearing angle (at least 90°) has to be maintained between the trench bottom and the side of the pipe. In particular in this range, the following have to be observed:

The use of good compactable materials (sand, sandy gravel, sand cement ...); careful compaction using hand stamper or very lightweight mechanical material without touching the pipes.

The thickness of the backfill layers may be min. 15 and max. 30 cm.

When there is insufficient cover over the pipes (min. 30 cm) the machines must not be moved over the pipes in vibrating mode.
2nd Industrial City, New Saniya,
New Al – Kharj Highway,
P.O. Box 17952,
Ar – Riyadh 11494,
Kingdom Of Saudi Arabia.
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