There are various types of flexible pipes used in of sewer sanitary system such as ductile iron pipe, acrylonitrile-butadiene styrene composite pipe and ABS solid wall pipe, polyvinyl chloride pipe, polyethylene pipe, fiberglass reinforced plastic, reinforced plastic mortar pipe and coated corrugated metal pipe.

A generic method for the design of flexible sewer sanitary system is discussed.

**Flexible Sewer Sanitary Pipe Design**

The capacity of flexible pipe to carry imposed loads is composed of pipe strength and passive soil resistance when the pipe is deformed laterally against the backfill materials and vertically.

So, compaction of backfill material on either side of the pipe would affect the performance of the pipe in the future. Flexile pipes fail due to considerable deflection, collapse, buckling, delamination and cracking.

Usually, the defection of flexible pipe measures the performance of the pipe and it is utilized as a base for the design. The allowable deflection of the pipe is based on not only the project limitations but also on the pipe material properties.

Pipe manufacturers usually provide empirical data regarding the long-term deflection of the pipe in various placement conditions.

If such data is not available, then the long-term deflection of flexible sewer sanitary pipe can be computed by employing the Modified Iowa expression:

**Where:**

: horizontal deflection of the pipe, mm

*D _{L}*: deflection lag factor

*K _{b}*: bedding factor

*W _{c}*: load, N/m

*r*: mean radius of flexible pipe, mm

*E*: modulus of tensile elasticity, N/m^{2}

*I*: moment of inertia per length, in mm^{2}/mm

*E*: modulus of soil reaction, N/mm^{2}

If the deflection of the flexible pipe is smaller, then horizontal deflection estimated using equation-1 is nearly equal to the vertical deflection. However, this is not the case if the ratio of pipe stiffness to soil stiffness is low.

When the flexible pipe is encased with concrete, the pipe manufacturer recommendation should be considered and the concrete encasement should be designed to carry to the entire vertical load otherwise the concrete encasement should be prevented.

The passive resistance of soil which greatly influence the deflection of the pipe is expressed as modulus of soil reaction. It is related to the type of soil and its degree of compaction.

The US Bureau of Reclamation have established a relationship between soil reaction modulus, type of bedding material and the degree of compaction of bedding. Soil reaction modulus values which may be used in the equation-1 are recommended and provided in Table-1 and soil classification along with typical soil names and symbols can be found in Table-2.

### Table-1: Average values of modulus of soil reaction (E’) for initial flexible pipe deflection

### Table-2: Soil Classification

Soil class | Typical names | Group symbol | Comments |

I | Crushed stone | Angular, 6-40mm | |

II | Well-graded gravels Poorly graded gravels Well-graded sands Poorly graded sands | GW GP SW SP | |

III | Silty gravels Clayey gravels Silty sands Clayey sands | GM GC SM SC | |

IV | Inorganic silts Inorganic clays | MH, ML CH, CL | Not recommended for bedding, haunching or initial backfill |

V | Organic silts and clays Peat | OL, OH PT |

As far as deflection lag factor is concerned, it is a rectification factor for time soil consolidation properties that may allow the deformation of flexible sewer pipe for a while after pipe placement. The lighter the degree of compaction the greater the deflection lag factor.

If the fill material on both sides of the pipe is compacted adequately, the initial deflation would be small and consequently the deflection lag factor would be larger. The recommended deflection lag factor ranges from 1.25 to 1.5.

Regarding bedding requirements for flexible sewer pipe installation, it is dependent on the breadth of sewer sanitary pipe bedding and recommended values for bedding constant can be seen Table-3.

**Table-3: Values of Bedding Factor**

Bedding angle, in degrees | Bedding factor, |

0 | 0.11 |

30 | 0.108 |

45 | 0.105 |

60 | 0.102 |

90 | 0.096 |

120 | 0.090 |

180 | 0.083 |

The stiffness factor (EI) in equation-1 represent the influence of sewer sanitary pipe inherent stiffness on deflection and the effect of passive pressure on the sides of sewer sanitary pipe is represented in the equation-1 by the term 0.061E’.

One should be aware that the soil reaction modulus values provided in Table-1 are average and hence there are fifty percent chance that calculated deflection is smaller than the actual deflection. So, it is recommended to use 75% of the values provided in Table-1 to compute maximum deflection.

It is mandatory to practice great care during bedding and initial backfill because the ability of the pipe to maintain its original shape and integrity are dependent on the determination, placement, and compaction of the soil around the sanitary sewer pipe.

It is recommended to choose cohesionless or granular material as a backfill material because it provides satisfactory shear property. In contrary, cohesive material should be prevented since it cannot be compacted properly due to space restrictions.

The occurrence of ground water table changes at soil-pipe envelope should be avoided, because it leads to fine grain soil movement into the granular material and probably sidewall supports would be endangered and possibly lost.

**Loads on Flexible Plastic Pipe**

Conservatively, the load on flexible pipes may be assumed to be the entire load of the prism over the pipe and the following equation can be used to compute it:

*W _{d}=HwB_{c}* -> Equation-2

The above design method for flexible sewer sanitary pipe is a general method. Different flexible pipes have different specific properties. Therefore, each flexible pipe type may require more design details which should be considered for the pipe under considerations.

So, design procedure for each sewer sanitary pipe type will be provided briefly in the following sections:

**Design of Thermoplastic Flexible Sewer Pipes **

There are three types of thermoplastic pipes namely ABS, PE, and PVC which are all influenced by temperature. Low temperature would increase thermoplastic pipe stiffness and brittleness as well but high temperature declines rigidity of the pipe and increase the resistance against impacts.

Another factor to be considered is the pipe stress cracking due to concentrated oxidizing agents, organic chemicals, and oils, fats, and waxes.

Therefore, thermoplastic pipes should be handled properly in low temperature environment and its applications may be reviewed when chemical concentration is high.

**Thermoplastic Pipe Stiffness**

Pipe stiffness may be estimated using parallel plate loading test. The following equation is used to calculate pipe stiffness:

**Where:**

*F*: force per unit length

: pipe deflection

*E*: elastic modulus

*r*: mean pipe radius

**I=t ^{3}?12 ->Equation-4**

Where: *t* is the pipe thickness

Pipe stiffness factor (SF), which is plugged into equation-1 to compute field deflection under earth load, may be calculated using the following expression:

**Thermoplastic Pipe Analysis Methods **

In the analysis of thermoplastic pipes, it is necessary to examine and check values that involve pipe deflection, pipe stiffness, hydrostatic wall buckling, wall crushing strength, wall strain cracking and ring buckling strength.

**Reinforced Thermosetting Resin or Fiberglass Pipe**

It is designed according to the American National Standards Institute. The basis of the design is that the pipe behavior conduit under internal pressure and external loading is like flexible.

There are certain conditions that should be specified prior to the structural design computations.

These conditions include pipe size, surge pressure, working pressure, soil conditions, pipe lying conditions, cover depth, vehicular traffic load.

**Read More:**

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**Direct Design of Concrete Pipes for Sewer Sanitary**

**Marston-Spangler Load Analysis Theory for Sewer Sanitary System**