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Use of native bedding material is the most cost effective method of installation. In addition, it provides suitable support for clay pipes. However, special care must be taken to remove rock particles that are larger, as they may be the cause of point loads [[VCP Engineering Manual]]. |
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Oct 16, 2016 AE-02 Open Cut Method: Embedment Zone and Pipe Installation Creation of the Bedding for Various Pipe Materials in the US The following table presents the compaction recommendations for foundation and bedding as per [[ASTM D2321–05]]. (Table: Recommendations for installation and use of soils and aggregates for foundation and bedding [ASTM D 2321-05]) |
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This following information is extremely useful for engineers and contractors who work internationally. The German standard [ATVA127] "Static Calculation of Drains and Sewers", categorizes the placement of the bedding in the following four conditions (B1 to B4). (Image: Bedding condition B1) Bedding conditions B1 (Image: Bedding condition B2) Bedding conditions B2 (Image: Bedding condition B3) Bedding conditions B3 (Image: Bedding condition B4) Bedding conditions … |
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(Image: Bedding condition B1) B1 - Placed lengthwise or in layers against the natural soil or embankment (without evidence of degree of compaction); also applies to soldier pile walls. |
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B2 - Vertical shoring within the embedment using piles or light sheet profiles, which are removed only after backfilling using equipment which ensures proper compaction of the embedment. (Image: Bedding condition B2) (Image: Bedding condition B2) |
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(Image: Bedding condition B3) B3 - Vertical shoring within the embedment using sheet piling and compaction against the shoring extending below the trench base. (Vertical sheeting using wooden piles, shoring panels or devices, which are removed only after the completion of the backfilling and compaction can result in the formation of voids which are not detectable by any computing model.) |
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(Image: Bedding condition B4) B4 - Placed lengthwise or in layers against the natural soil or embankment with compaction proof required by the StB ZTVE. The embedding condition B4 is not applicable to soils of the group G4 (In the future, named as B0). |
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(Image: Influence of the embedding condition on the pipe stress) The graphic shows the impact of the embedding conditions on the pipeline stress/strain. As an initial starting point, the embedding condition B1 is selected. In contrast, the pipe stress may be reduced by 20% as a result of compaction testing (B4). The stress on the pipe is increased by a factor of 2.5 if the shoring is removed abruptly rather than gradually during the backfilling process (… |
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In Europe, three basic bedding types are differentiated, with the difference being the thickness of the upper bedding layer which provides the necessary support angle (2α) :
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The bedding “Type 1” represents the standard. It is distinguished between a lower bedding layer “a” and an upper bedding layer “b”. As a rule, the bedding has to reach across the entire trench width. (Image: Bedding type 1 according to DIN EN 1610) (Image: Schematic representation of the type 1 bedding) |
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For a bedding “Type 2” there is no lower bedding layer. The pipes are directly laid on the accurately pre-profiled (pre-formed) trench bottom, thus reducing the thickness of the upper bedding layer at the same time. For that purpose it is required that the natural (in situ) soil is relatively fine and can be compacted well. (Image: Bedding “Type 2” DIN EN 1610) (Image: Bedding “Type 2”) (Image: Example bedding type 2 for a GRP pipe - Attention: Minimum … |
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The bedding “Type 3” is also only applicable in especially suitable soils. This method does not have a lower bedding layer or a pre-formed trench bottom. The pipes are laid directly on the trench bottom, hence increasing the risk of point load damage especially for pipes without a base. (Image: Bedding type 3 according to EN 1610) (Image: Schematic representation of the type 3 bedding) |
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When is a concrete bedding useful? Concrete bedding (concrete support) is a “special type of bedding construction” which is used in the following circumstances:
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The placement of pipes in concrete bedding includes:
(Image: Installation of a reinforced concrete … |
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In the case of concrete pipes with an integrated base, the base takes over the function of the upper bedding layer. Therefore, they can usually be placed directly on a prepared, horizontally levelled and compacted bedding of suitable soil material (lower bedding layer), directly on the prepared trench base or on a concrete support. Even with these type of pipes, it is necessary to compact the haunch areas by hand to ensure proper compaction. For placement … |
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(Image: Installation of a reinforced concrete pipe on a concrete base in accordance with) After the placement of the lower bedding (concrete slab), the pipe is first placed on wedges and then the upper bedding layer is cast. This "concreting in two phases" is practiced in order to ensure a uniform embedment. Direct “placement" of the pipe on a concrete slab followed by concreting of the pipe haunch can lead to a linear bearing! Important: The wedges … |
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In the structural calculation of the pipe, the pipe bedding is defined by the "placement method" and the "bedding angle" [ATVA127].
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The bedding type is determined as part of the planning process. The default bedding should be type 1. (Table: Bedding and Placement type allocation as per DIN EN 1610 and ATV-DVWK-A 127) |
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For pipes without the integrated base, the bedding angle (2α) is usually set between 90 ° and 180 °. For pipes with the integrated base, the bedding angle is normally 60 °. (Image: Variable support angle (2α) in pipe without a base) (Image: Variable support angle in pipe with a base) |
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(Image: Influence of the support angle on the pipe stress) What is the impact of a decreasing bedding angle? Influence of the bedding angle on the pipe stress.
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Incorrect:
Result: load concentration Correct:
Result: good load distribution (Image: Load concentration due to an unfavorable compaction ratio between the trench base and subbase) (Image: Good load distribution as a result of a favourable compaction ratio between the trench base and subbase) |
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