Which requirements of statics must be strictly observed?

The decisive factors are:

• Soil type

• Degree of compaction

• "Interface" for shoring (embedding condition B1-B4 as per [ATVA127])

• Layered placement and compaction (both sides evenly)

• Compaction of the side fill does not replace the haunch compaction.

• If necessary, ensure that the filtering capability towards the bedding layer beneath is always sufficiently stable.

The following table presents recommendations for installation and use of soils and aggregates for bedding and haunching in accordance with [[ASTM D2321–05]].

(Table: Recommendations for installation and use of soils and aggregates for haunching [ASTM D 2321-05])

(Image: Hand tamper)

What is meant by initial backfill?

Initial backfill (or cover fill) describes the area immediately above the pipe.

The thickness of the initial backfill in the compacted state is usually 12-in (30 cm). [[ASTM D2321–05]] recommends that the minimum initial backfill above the pipe crown should be 6-in (15 cm). The minimum initial backfill above the pipe connection should be at least 4-in (10 cm).

Only hand tools should be used in the …

The following table presents the use of soil and aggregates for the initial backfill, as well as embedment compaction recommendations, as per [[ASTM D2321–05]].

(Table: Recommendations for installation and use of soils and aggregates for initial backfill [ASTM D 2321-05])

What are decisive factors, which need to be considered during installation of the final (or main) backfill?

The requirements of statics must be strictly observed. The decisive factors are:

• Soil type (not to be "better" than in situ soil)

• Degree of compaction

• "Interface" for shoring

• Layered placement and compaction

• No use of heavy compaction equipment up to 3 ft (1 m compacted thickness) above the pipe crown

• Vibration energy of the compaction …

The following table presents the use of soil and aggregates for the final backfill as per [[ASTM D2321–05]].

(Table: Recommendations for installation and use of soils and aggregates for final backfill [ASTM D 2321-05])

The backfill of the embedment differs between four conditions [ATVA127]:

(Table: Cover Filling Conditions A1-A4 as per ATV-DVWK-A 127)

The table below provides information about soil compaction, layer thickness, and number of passes, depending on the heaviness of the compaction equipment.

(Table: Soil compaction, layer thickness, and number of passes [DWAA139])

(Table: Soil Classification (DWA-A 139))

The most common causes of pipe failures are a direct result of the inadequate compaction of the pipe bedding, backfill and cover.

Following are examples of possible pipe failures which are directly or indirectly attributed to improper compaction of the pipe zone.

(Image: Longitudinal failure) (Image: Inappropriate use of a heavy compactor) (Image: Failure to meet the minimum trench width) (Image: Failure to meet the minimum trench width) (Image: Inadequately …

Flowable fill is a self-compacting low strength material with a flowable consistency that is used as an economical fill or backfill material, as an alternative to compacted granular fill. Flowable fill is not concrete nor it is used to replace concrete [[ACI 229R]].

Terminology used by [[ACI 229R]] for this material are unshrinkable fill, controlled density fill, flowable mortar or lean-mix backfill.

On the following pages are presented the advantages …

Hydraulically bounded filling materials consist principally of the following raw materials:

• Filler material, such as native aggregates or recycled materials

• Plasticizer, for example, mixture of water swellable clay (bentonite), foaming agents, air pore-forming additives (surfactants) or other mineral, organic and / or inorganic additives,

• Stabilizer (binder), such as cement or lime.

The hydraulically bonded fill mixture is composed of about 95% …

After the trench has been excavated and shored, the pipeline is alligned and fixed to the trench base in accordance with the planning specifications either on wooden boards or sandbags (based on the calculation concept by [Prof. Dr.-Ing. Stein & Partner GmbH], the latter are to be preferred for structural reasons).

Prior to pouring of the hydraulic fill from the ready mix truck, a so-called anchor saddle of the same filler material, but in a firmer consistency, has to be poured on the pipe at every 6.5 to 8 ft (2 to 2.50 m) in order to prevent the pipe from shifting or floating. Flowable fill can be conveyed by pump, chutes or buckets to its final location.

For efficient pumping, some granular material is needed in the mixture. The ballast can also serve as a dam for backfilling individual sections of the pipe, in which case the dam height should be in accordance with the final fill height. Placement of the hydraulically bound backfill material in the liquid form, directly from the ready mix truck, ensures an even and complete embedment of the pipe section.

Costs savings are one of the most important aspects the contractor is looking for when selecting materials. The table below provides an average cost comparison between flowable fill and granular fill placement [[VancePool2008]].

(Table: Flowable fill vs. granular fill cost)