This Write-up/ Article guides to the proper installation of underground 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 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.
SOIL- PIPE SYSTEM:
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.
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 to maintain 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.
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 these informations are missing, or are not available or are incomplete, an investigation of these soils will have to be carried out.
- The result of this investigation not only will give the informations that are 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 behaviour have a determinant influence on the dimensioning formulae and on all of the verifications that are necessary for buried PRFV pipes.
IN SITU SOILS:
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: 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
SOIL STIFFNESS CATEGORIES:
The soil Classification and the Soil Stiffness Categories are summarised in the following table (Fig. 3)
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 native soil. Likewise, one must prevent 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 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 next part….