advanced drainage system - Polypipe

General Structural Design 3.2
Vertical soil pressure (Pe)
The vertical dead load applied to the pipe system
is typically restricted to the soil pressure generated
by the pipe backfill material. The load is taken as
the pressure imposed by the prism of soil directly
overlying the pipe. No allowance is made within
the standard for the effect of shear between the
backfill material and the trench walls. Where the
density of the backfill material is not available,
19.6 kN/m 3 may be assumed for design purposes.
Surcharge pressure due to
trafficking (Ps)
The imposed pressure from vehicle trafficking is
largely dependant on the depth of cover above
the pipe. Consequently construction traffic may
pose the worst case load condition, particularly
if precautions are not taken on site, as cover
depths are commonly less than when construction
is complete. Surcharge loads, calculated using
Boussinesq’s theory, may be derived from figures
3.2.4 & 3.2.1.
Pipes laid near railway lines are also subjected to
dynamic loading. Two catagories of design loading
are generally adopted for design. Type RU loading
covers all current and projected rolling stock on
UK railways. Type RL loading covers only passenger
rapid transit systems. The vertical stress at the
appropriate depth for both types of loading is given
in fig 3.2.2 for single track loading. Where multiple
tracks occur, the vertical stress should be multiplied
by an appropriate factor taken from fig 3.2.3.
Deflection Co-efficient (KX)
A bedding factor used to represent the extent of lateral
support provided by the pipe bedding. Pipes receiving
support over the full 180° lower half of the pipe a value
of 0.083 should be used, whereas bedding providing
only a line load support a value of 0.100 would be more
appropriate. Please refer to table 3.2.3 for a deflection
co-efficient appropriate to the classification of bed &
surround used.
Soil modulus (E’, E’2 & E’3)
Soil modulus is the parameter with the most influence
on the structural calculation, as soil stiffness will
generally be significantly greater than the pipe stiffness.
Modulus values have been determined from empirical
measurements and are indicated in table 3.2.5 & 3.2.6
for the native soil and pipe bed & surround material
respectively.
Where the native soil forming the trench walls is a weak
material, the level of support provided by this material will
be significantly lower than an engineered pipe surround.
Therefore only considering the modulus of the engineered
pipe bed & surround would over estimate the overall
modulus of the pipe/soil system. The soil modulus
adjustment factor (CL), is used to take into account the
influence of native soil properties on the overall soil
modulus (E’).
Deflection Lag factor (DL)
An empirical factor used to account for relaxation,
or creep, of the pipe/soil system and other general
long-term settlement effects. A conservative design
approach is taken by assuming no beneficial effect
is derived from frictional forces between the trench
walls and backfill material, in addition to the use
of a long-term pipe stiffness parameter. Values
generally range from 1.0 to 1.5, dependant on
the type of pipe surround used and its level of
compaction, given in table 3.2.3.
A well installed gravity flow pipe, utilising a single
sized granular bed & surround, a value of 1.0 is
typically taken for the deflection lag factor.
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