advanced drainage system - Polypipe

General Structural Design 3.2
Pipe structural performance
Pipes are typically categorised as rigid or flexible, depending upon the material from which they are manufactured.
Rigid pipes, such as concrete, have a high inherent strength and resist applied loading by a bending action within
the pipe walls. They are generally stiffer than Compressible Board the pipe surround material, Compressible in Board particular the sidefill, and consequently
Trench
Trench
Support
Support
support a higher load than the sidefill material. For design purposes, they are generally assumed to support the entire
Compressible Board
Compressible Board
vertical load transmitted through the backfill material placed above the level of the pipe crown (Figure 1B)*.
Failure of rigid pipes occurs when the vertical
100mm
loading exceeds its load capacity and causes
fracture of the pipe wall 150mm (Figure 150mm 2B*). In order to
perform satisfactorily the pipe must therefore be
strong enough to support the design loading, in
addition to being laid on a stiff bedding material,
150mm
150mm
150mm
which must ultimately support the
200mm
200mm
200mm
loads transmitted through the pipes.
150mm
150mm
The bedding must not allow differential
settlement of the pipeline to occur, since this
100mm
would result in stress concentrations in the pipe
150mm 150mm
150mm
wall and result in failure.
1ess than 1.2m
Minimum 150mm
150mm
1ess than 1.2m
1ess than 1.2m
Minimum 150mm
150mm
150mm
150mm
150mm
150mm
Compared with rigid pipes, flexible pipes
100mm
are versatile and have important structural
performance advantages. Unlike rigid pipes, flexible
pipes have excellent resistance to differential
settlement. Plastic pipes, when overloaded, will
simply deform (Figure 2A) further to generate
150mm
greater passive earth pressures until the system
200mm
regains equilibrium. In contrast, overloaded rigid
Concrete
Concrete
Grade
150mm
C20
150mm
Grade C20
pipes are subject to fracture that can result in
catastrophic failure of the system.
100mm 150mm
1ess than 1.2m
150mm
150mm
150mm
A. Flexible B. Rigid
A. A. Flexible B. B. Rigid Rigid
A. Flexible
B. Rigid
Figure 1* - Illustration of performance mechanisms
Figure 2* - Illustration of overloading effect
As a consequence of the differing performance mechanisms, flexible pipes have structural performance advantages
over rigid pipe systems.
• Flexible pipes offer excellent resistance to differential settlement and ground movement.
•
When overloaded, rigid pipes are subject to fracture and failure of the system. Plastic pipes, when overloaded,
will deflect further to generate greater passive earth pressures until the system regains equilibrium.
48
Pipe deformation
The deformation of flexible pipes under load
results in the ovalisation of the pipe (a reduction
in the vertical diameter and an increase in the
horizontal diameter). As the horizontal diameter
of the pipe increases, it derives support from
the sidefill and trench wall. This passive earth
pressure increases as the pipe deforms further
until the pipe-soil system comes into equilibrium.
Further deformation will not occur thereafter
unless a higher vertical load is applied to the
pipe-soil system or consolidation (or creep) of the
materials occurs over a long period of time. It
is internationally recognised that when a pipe is
installed in accordance with an appropriate code
of practice, increases in deflection virtually stops
after a short period of time. The duration of time
is dependent on soil and installation conditions
but generally does not exceed two years.
Serviceability limits
Deformation of flexible pipes must occur if the
pipe-soil system is to reach equilibrium. Therefore
deformation is not detrimental, but a natural
action allowed for in flexible pipe design. European
research on flexible pipes has shown that pipe
deformations of more than 30% have occurred in
practice without signs of structural failure.
However, it is accepted that a limit on vertical
deformation is necessary to ensure adequate long
term pipe performance. Appropriate deflection
(serviceability) limits should be set on a case by case
basis. For example, greater limits may be allowable
in a deep landfill installation compared to a pipe
buried at a shallow depth under a road.
Deflection limits within the UK varies, depending
on the relevant adopting authority. For design
purposes the Highways Agency specifies a maximum
allowable deformation of 5% for thermoplastic
structured walled pipes, while the water industry
tends to specify 6%.