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Soil loads

Technical solutions

Soil loads

Depth of cover diagrams

The following diagrams show the maximum and minimum depths of cover for ductile iron pipes, with or without wheel loads.
Four laying cases, corresponding to common practice are presented. In all other cases, please consult Saint-Gobain PAM.
They have been established on the basis of the following:
– pipe resistance and deformation criteria compliant with Standard EN 545 (wall stress and vertical ovality),
– calculation model without an aquifer

The four laying cases

The cases defined below are understood without an aquifer, and not in reinforced trenches.
For other cases of pipe laying (under earthworks, reinforcements, etc.), please consult Saint-Gobain PAM.

 

4 laying cases
 Case #1Case #2Case #3Case #4

-

Caption: 

DN ≤ 600

Caption: 

DN 60 to 2000

Caption: 

DN ≤ 1 400

Caption: 

DN ≤ 600

Caption: 

DN > 1 400

Caption: 

DN > 600

Caption: 

Trench bottomFlat bottomFlat bottom Bed in selected materialBed in selected material
Backfill zone (2)
– Soil group *
– Compacting
– Es (bar)
– 2 α (°)
4
Non compacted
< 3
30
3
Controlled compaction
7
30
3
Controlled compaction
10
90
1
Controlled compaction
20
90
Choice of materialsThe backfill materials used (selected or not) directly in contact with the pipe must be exempt of stones or
corrosive elements.

* See table 1.

Elements from Fascicule 70

The calculation method used takes into account:
– 6 soil groups, see table 1,
– 3 levels of compaction quality, see table 2 and (if applicable) the influence of:
• the aquifer on soil parameters,
• reinforcement contraction as a function of trench width,
• wheel loads (Bc system: two 30-tonne triple axle trucks crossing).
 

Table 1
Soil groupbrief description
1Clean or slightly silty sand and gravel (elements < 50 mm)
2Silty or medium clay sand and gravel
3Flint or millstone clay. Rubble, moraine, eroded rock, coarse alluvium with high percentage of fines
4Loam, fine sand, gravel, clay, more or less plastic marl (Ip < 50)
5 a (*)Very plastic clay and marl (Ip > 50).
Soluble or polluting organic matter
5 b (**)Unstable rock: chalk, sandstone, schist. Composite soils (flint and millstone clay,
moraine, eroded rock, coarse alluvium with elements that may exceed 250 mm.
Clean gravel, stable rock with elements > 50 mm.

(*) This material cannot be used either for the surround or the backfill zone.
(**) This material cannot be used for the surround but may be used for the backfill zone.

Table 2
 Non compactedControlled compactionControlled an checked compaction
Soil groupEs2aEs2aEs2a
 MPadegreeMPadegreeMPadegree
1  (+)0.7602905120
2  (+)0.6601.2903120
3  (+)0.5601902.5120
4  (+)<0.3600.6600.660
5b  (++)0.7-2-5-

(+) Surround or backfill zone.
(++) Only backfill zone.

 

Soil loads (pipe performance)

The various types of pipes can be divided into three categories, depending on their performance:

  • rigid pipes,
  • flexible pipes,
  • semi-rigid pipes.

Ductile iron pipes are classed as semi-rigid. They provide a good compromise between resistance to top loading and vertical deflection, thus providing long term operational security.

Pipe/soil system

The mechanical performance of a buried pipe can only be understood by considering the pipe/soil system: the interaction of the pipes with the surrounding soils depends on their stiffness or flexibility, which induces stresses under different laying conditions.

Rigid pipes

  • Examples: Prestressed concrete.
  • Performance: Rigid pipes only accept a very small amount of ovality before they fail. The deformation is insufficient to bring the side support resistance of the backfill into play. All the soil top load is supported by the pipe, inducing high bending stresses in the walls.
  • Design criteria: Usually maximum crushing load.
  • Consequences: Rigid pipes favour loads concentration at the pipe crown and invert. The performance of the rigid pipe/soil system is highly dependent on the bedding angle (α) and therefore on good bed preparation, particularly if there is any vehicleloading.

Flexible pipes

  • Examples: Plastics, steel...
  • Performance: Flexible pipes withstand high vertical deflection without failure. The soil top load is therefore simply balanced by the pipe side support provided by the surrounding backfill.
  • Design criteria: Maximum permissible ovality, or maximum permissible bending stress; also resistance to buckling.
  • Consequences: The stability of the flexible pipe/soil system is directly dependent on the capacity of the backfill to develop passive side support resistance, therefore on its modulus of passive soil resistance E’ and consequently on the quality of the backfill and its compaction.

Semi-rigid pipes

  • Examples: Ductile iron.
  • Performance: Semi-rigid pipes sustain sufficient ovality for part of the soil top load to mobilize backfill side support. The forces brought into play are therefore passive sidefill support and internal bending stresses in the pipe wall. The resistance to top loading is therefore distributed between the resistance of the pipe itself and that of the soil surround, the contributions of each being a function of the ratio of pipe and soil stiffness.
  • Design criteria: Maximum permissible bending stress (in small diameters) or maximum permissible ovality (for large diameters).
  • Consequences: By distributing the forces between the pipe and backfill, the semi-rigid pipe/soil system provides security against any changes in mechanical stressing with time, or of the support conditions themselves.