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Minimum crushing strength is a commonly adopted parameter for describing the strength of rigid pipes like concrete pipes. This value is determined in laboratory by subjecting the test concrete pipe to a line load diametrically along the pipe length while the pipe invert is supported on two bearers for stability reason. This test is called three-edge bearing test and the load at failure of pipes is expressed in terms of kN per length of test pipes (called minimum crushing strength).
Granular bedding is commonly used as bedding for flexible pipes. Granular material is self-compacting and it helps to reduce the pressure acting on the wall of pipe trench. Granular materials have higher modulus of soil reaction than adjacent in-situ soils and this allows it to carry higher loads than in-situ soils without deformation.
Sunlight contains ultraviolet which transform PVC particles to a complex structures with brownish discoloration. The effect of discoloration can be reduced with addition of UV absorbers such as titanium dioxide.
A typical pipe trench for flexible pipes is divided into the following layers:
(i) Final backfill
This region has little influence on the performance of pipes. However, as it is close to existing road surface, it highly affects functioning of roads and structures.
Flexible pipes are pipes that can deflect at least 2% of the pipe diameter without any damage. When compared with rigid pipes such as precast concrete pipes, flexible pipes are comparatively weak and they count predominantly on the composite action between pipe deflection and backfilled soils to achieve structural stability. On the other hand, rigid pipes rely mainly on their inherent structural strength to carry imposed earth and traffic loads. Hence, improper installation of flexible pipes would compromise structural performance and results in risk failure.
The structural capacity of flexible pipes (e.g. plastics and metals) is derived from ring bending stiffness. Owing to creep or relaxation the ring bending stiffness decreases with time. Flexible pipes are liable to failure by excessive vertical deflections, ring bending strain and buckling.
In designing the bedding for concrete drainage pipes, granular materials are normally specified instead of soils containing a wide range of different particle sizes. The main reason of adopting granular material free of fine particles is the ease of compaction as it requires very little tamping effort to achieve a substantial amount of compaction and the crushed aggregates readily move to suitable place around the pipes.
Bedding, which are normally made of granular materials or concrete, serves four main functions as suggested by O. C. Young and J. J. Trott:
(i) To enhance a uniform support under pipes in order to reduce the bending moment longitudinally;
(ii) To increase the load-supporting strength of the pipes;
It is obvious that pipes should lifted up and laid with “TOP” up. Otherwise, cracking may occur in the portion labeled “TOP” because they are not supposed to take up significant loads on the sides of the pipe.
Horizontal elliptical pipes are commonly used where vertical clearance is hindered by some existing structures. Moreover, horizontal elliptical pipes possess higher flow capacity for the same flow depth than most other structures with equivalent full capacity.
