Q. Show the Purpose of granular bedding for concrete pipes?

When designing the bedding for concrete drainage pipes, granular materials are generally specified in place of soils comprising a wide range of different particle sizes. The major reason of adopting granular material free of fine particles is the ease of compaction as it needs very little tamping effort to achieve a substantial amount of compaction and crushed aggregates readily move to appropriate place around the pipes. Though the use of granular materials has the disadvantage that a stable support can barely be provided for the drainage pipes. Particularly it can't maintain an accurate slope and level for the bedding of concrete pipes. Most pipes are gravity pipes and accuracy in level is vital to maintain the flow capacity. 

Fig: Bedding of concrete pipes