advanced drainage system - Polypipe

General Structural Design 3.2
3.2.3 Design Example
Product
Application
Burial Depth
Native Soil
Ovalization,
600mm Ø Ridgisewer (SN4)
Adoptable sewer beneath an A-road
3.0m to pipe crown
Very loose gravel
Kx [(DLPe)+Ps]
=
D (8EI/D 3 )+ (0.061E’)
Calculation of soil modulus adjustment factor, CL
Pipe O.D., BC
= 672mm
Trench width, Bd
= BC + 300mm
= 972mm
(Table 3.2.6)
Assuming native soil is a Very Loose Gravel
Native Soil Modulus, E’3 = 3 MN/m 2 ,
Pe = 19.6 x 3.0
= 58.8 kN/m 2
CL =
0.985+[0.544(972/672)]
[1.985-0.456(972/672)](10/3)-[1-(972/672)]
No soil density data available, therefore 19.6 kN/m 3
assumed. (Clause NA. 6.3 BS EN 1295-1)
Ps = 26 kN/m 2 (Figure 3.2.4)
DL = 1.0 (Table 3.2.5)
Kx = 0.083 (Table 3.2.5)
EI/D 3 = 1.74 kN/m 2 (Polypipe Civils Technical Data)
(Table 2.4.2)
Overall Soil Modulus, E’ = E’2 CL
CL = 0.3642
Overall Soil Modulus, E’ = 10 MN/m 2 x 0.3642
= 3.642 MN/m 2
0.083[(1.0 x 58.8)+26.0]
Ovalization, =
D (8 x 1.74)+(0.061 x 3642)
= 0.0298
Deflection, = 0.0298 x D
= 0.0298 x 0.672m = 0.020
(Table 3.2.5)
(Class S1 - Gravel (single size) )
Bed & surround soil modulus, E’2 = 10 MN/m 2
Percentage Deflection = (0.020 / 0.587) x 100
= 3.41% ≤ 6.0
Therefore acceptable
Cover depth is >1.5m, therefore buckling check with
soil support only is required.
3.2.4 Buckling check with soil support
Factor of Safety, FS =
1
(Pe/Pcrl)+[(Ps+Pv)/Pcrs]
Where: Pcrl = 0.6(1.74) 0.33 .(3642) 0.67 kN/m 2
= 0.6 x 1.201 x 243.275 kN/m 2
= 175.30 kN/m 2
Pcrs = 0.6(7.83) 0.33 .(3642) 0.67 kN/m 2
= 0.6 x 1.972 x 243.275 kN/m 2
= 287.84 kN/m 2
Factor of Safety, FS = 1/0.426
= 2.34 ≥ 2.0 Therefore adequate
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