Buried GRP/FRP pipe Laying and Installation Procedure: Part -1 of 3

This Write-up/ Article guides to the proper installation of buried fiberglass piping systems. Designing a piping system to the latest engineering standards and techniques makes up for half of the job. The other half consists of the installation and the implementation of the design specifications. The requirements for installing buried fiberglass piping systems differ significantly from those of other conventional materials, for example from steel piping requirements.

GRP (glass reinforced plastic) pipes, possibly built with mortar siliceous aggregates, are classified as “flexible” pipes, since they can work in a deflected condition, up to 5% of the diameter (long term), fully in conformity with safety requirements.


The external loads (soil and traffic) above a GRP buried pipe cause a reduction of the vertical diameter and a consequent increase of the horizontal diameter (deflection).

This horizontal movement develops a passive soil resistance that enhances the pipe’s support by contrasting the deflection and increases its lift (fig.1). Please see the next picture.

Soil Pipe Interaction
Fig. 1: Figure showing Soil Pipe Interaction

Thanks to the flexibility of the pipe, all of the external loads, such as soil and traffic that are loaded on the pipe, are sustained by a combination of the pipe’s stiffness and the stiffness of the soil surrounding the pipe.


The amount of deflection depends on the soil load, on the alive load, on the native soil’s characteristics, on the pipe’s backfill material, on the trench width, on the filling and on the pipe’s stiffness.

Buried fiberglass pipes generally accommodate 4-5% of long term deformation without structural damage. An appropriate selection of the pipe’s stiffness class and its corresponding installation method allows maintaining the pipe deflection within acceptable values.


The figure (Fig. 2) below shows the meaning and the position of the elements that are used in this article, such as foundation, bed, primary backfilling, secondary backfilling etc.

Figure Showing Terminologies used for the article
Fig. 2: Figure Showing Terminologies used for the article

Following are listed a few terms and concepts that are used for soil description:

  • fines = particles passing through the ASTM No. 200 sieve (with an opening of 0.075 mm), made of silt and clay
  • fine-grained soils = soils where fine-grained particles are >50%
  • coarse-grained soils = soils where fine-grained particles are <50%; made of sand and gravel
  • sand = soil retained by the ASTM No.200 sieve but passing the ASTM No. 4 sieve (opening 4.5mm)


  • The conditions of the different soils crossed by pipelines to be laid should be determined before installation.
  • If this information are missing, or is not available or is incomplete, an investigation of these soils will have to be carried out.
  • The result of this investigation not only will give the information that is necessary to define the suitable backfilling and compaction procedures but will also define possible areas of unsuitable materials, in order to minimize the use of selected material
  • Fine-grained soils with a medium/high plasticity, as highly plastic clay and silts, or organic soils, generally are unsuitable for the backfilling area.
  • The parameters that define the soil’s behavior have a determinant influence on the dimensioning formulae and on all of the verifications that are necessary for buried PRFV pipes.


It is important to determine the in situ soil conditions prior to the installation and even prior to pipeline design.

Data to be collected are:

  • soil composition: the ratio between coarse-grained particles and fine-grained particles
  • compaction degree (for soils with a predominance of coarse-grained particles) or cohesive strength (for fine-grained soils), that can be ascertained by means of penetration and shear tests
  • groundwater conditions Investigations are addressed to evaluate the modulus of soil reaction (E’n) of the native soil at the pipe elevation and how it can affect the global reaction of the embedment.

Native soils with very low characteristics may reduce remarkably the stiffness of the embedment.

Since in most projects, the embedment materials and the rate of compaction are required to develop a modulus of soil reaction in the range 7-14 Mpa, any normally consolidated and undisturbed native soil, is able to produce a modulus of soil reaction of the same magnitude or higher.


The material used for the bedding and for the backfilling of the pipe is classified according to its composition and its compaction degree


The soil Classification and the Soil Stiffness Categories are summarised in the following table (Fig. 3)

Table showing Soil Stiffness Categories
Fig. 3: Table showing Soil Stiffness Categories

For further details regarding soil classification, please see ASTM-D2487.

  • Most coarse-grained soils (SC1, SC2, and SC3) make acceptable beddings and pipe zone backfill materials.
  • Fine-grained soils with medium to high plasticity, such as CH and MH, and organic soils such as OL, OH, and PT generally are proven to be unsuitable for pipe zone backfill materials. High plasticity and organic soils request special design considerations.
  • The maximum grain size for backfill materials is 18 mm.
  • Pipe zone backfill material must be compatible with the native trench so it will not wash away nor migrate into the native soil. Likewise, one must prevent the migration of the native soil into the pipe zone backfill area. Either of these events would result in a loss of side support for the pipe and consequently cause an excessive deflection.
  • Migration can only occur if there is a movement of water in the pipe zone. When using incompatible materials, they will have to be separated with a filter cloth.               
  • Refer Part-2 of this article for the next part….
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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