Underground Glass Reinforced Pipe Laying & Installation: Part 2 of 3

Continued from Part 1…Click here to read part 1.

FRP PIPE BURIAL

STORAGE AND HANDLING

When storing fiberglass pipes, prior to their assembly and burial, use at least three supports (wooden beams). Separate each row of pipes with supports. When a pipe is stored directly on the ground, ensure that the surface is smooth and rock free. Do not roll the pipe on the ground.

  • Prior to installation, inspect each section of the pipe on both the internal and external surfaces in order to locate possible damages. Inspect also all joints, surfaces, and edges. Do not use damaged pipes without having first consulted our site supervisors.
  • Only lift pipe sections with fabric straps.

EXCAVATING THE TRENCH

  • On most construction sites it is best to execute the trench excavation, the pipe installation and the backfilling consecutively in order to minimize logistic problems and to reduce supervision costs.
  • The bottom of the trench must be flat, continuous, smooth and free from large rocks. The excavation of the trench bottom has to be deep enough to provide a minimum of 150 mm of bedding under the pipe.
  • Trench construction will vary according to the different types of soil encountered (stable or unstable, granular or soft). In any case, the bottom of the trench must be flat and continuous.

STABLE TRENCH WALLS OR ROCK TRENCH

  • Soils are considered stable only when it is possible to excavate the whole of the trench wall vertically, without the need for any supports or shores.

UNSTABLE TRENCH WALLS AND BOTTOM

  • With unstable soils, the trench wall may be excavated vertically with the insertion of sheet piles, bulkheads or shores (installation method 1).

Installation in unstable soil-Shoring should preferably be done using a small thickness and it will have to be removed in concomitance with the backfill. Every backfill layer should be placed and compacted as the sheeting is being pulled out of equivalent height, in order to avoid the occurrence of voids beneath the sheeting and maintain the requested compaction. Refer Fig. 4

Installation in unstable soils
Fig. 4: Installation in unstable soils
  • The second installation method is done with the trench wall angle at the natural slope of the soil.

GRANULAR SOIL TRENCH

In this case the trench walls will be at the natural slope (Fig. 4) of the native granular material.

SOFT SOIL TRENCH

When the native soil is composed of soils with high plasticity, very compressible, with a water content percentage on the dry soil weight exceeding 50%, as for soft clays, very melted muds, etc., the granular soil used for the laying bed and for the backfilling can be absorbed by the native soil. In this case, it is suitable to cover the bottom and the walls of the trench with a geotextile filter fabric, which has the function of separating the layers to prevent the granular materials composing the bed and the backfilling from being mixed or buried.

TRENCH WIDTH

The trench width must be wide enough to guarantee a minimum distance between the pipe and the trench wall that can allow the backfilling compaction, according to the type of material used and the compacting method.

Furthermore, in case of excavation in soils that are not able to guarantee the side support requested by the project, the trench will have to be widened, according to the designer’s prescriptions, in order to stabilize the trench wall.

Suggested values for the trench width (L) are the following:

DN ≤ 400 mm L= DN + 400 mm

400< DN ≤ 1000 mm L= DN + 600 mm

DN > 1000 mm L= DN + 800 mm

The depth of the trench must be executed in such a way to guarantee the dimensions of the bedding in accordance with the prescriptions of the following paragraphs. If the soil is not able to give the vertical support requested by the project, the trench will be deepened of 20cm or more, according to the prescriptions given by the designer, in order to obtain stabilization of the soil.

Moreover, should butt and strap joints be executed directly inside the trench, this must be widened and the trench bottom must be adequately lowered, allow the lamination operations being practical. These spaces will then be filled during the trench filling.

MULTIPLE PIPES

If several parallel pipes are laid in the same trench, all of the pipes should have the same bottom level. The clear spacing between adjacent pipes must be at least 0.2 m, in any case not less than the diameter of the smaller pipe, and can be limited to 1 m.

The same spacing will be used for crossing pipes at different heights.

TRENCH EXCAVATION BELOW WATER TABLE

When an unstable soil condition is encountered that is caused by a water table, the bottom of the trench must be drained before laying the pipes. This can usually be accomplished by lowering the water table to approx. 30 cm below pipe level by means of pumps and stabilizing the bottom as previously described.

To minimize the soil dewatering, only a length of trench to place one or two sections of pipe should be opened and then be backfilled.

FOUNDATION

The foundation shown in the above figures is required when the trench bottom is unstable, i.e. made of soils whose displacement, due to variation in stress or moisture content, is very high.

According to the different conditions of instability for the trench bottom, the installation contractor may require different types of foundations such as:

  1. stabilization and reclamation of the bottom, by removing an ulterior layer of at least 200 mm of depth and replacing it with stabilized gravel or sand, into which the unstable soil will not penetrate (ground capacity from 0.7 to 0.9 kg/cm2), that is able to redistribute the vertical pressures more regularly.
  2. pouring of lean concrete with a minimum depth of 150 mm (ground capacity from 0.5 to 0.7 kg/cm2);
  3. Foundation made with piles capped by concrete (ground capacity lower than 0.5 kg/cm2).

The above indications must be more closely followed as the diameter of the pipe to install is larger.

BEDDING

  • On the bottom of the trench, the bedding will be laid by using the materials that are described in the following sections. The bedding must have a thickness corresponding to 15% of the pipe’s diameter and in any case a minimum thickness of 150 mm that provides the pipe of a uniform and continuous support over its entire length.
  • The bedding surface must be even and recesses have to be projected in correspondence to every pipe joint. These recesses have to be backfilled after pipe installation and joining.
  • The use of pea gravel, or crushed stone or sand as bedding material, with fine content not exceeding 12% is recommended. Fines are considered the materials, which pass through the ASTM 200 sieve. The dimensions for the bed materials grain diameter should not be greater than 20mm.
  • The bed must be compacted until reaching 70% of its maximum density, before the pipe installation
  • Different kinds of materials and compaction ratios can be authorized by the Designer.

BACKFILLING

Backfilling is divided into two areas

  1. Primary backfilling, which extends vertically from the culvert of the pipe up to 70% of the diameter; it should preferably be composed by the same materials used for the bedding (maximum fines content 12% and maximum grain diameter 20mm), symmetrically laid by alternating layers of 20-25cm, compacted one by one, until reaching 70% of the maximum density for the specific material (90% Proctor Standard). Be sure that the areas under the pipe are filled up and compacted with accuracy, in order to grant valid support over an arc of at least 60% .
  2. Secondary backfilling, extending as far as 15 cm above the invert of the pipe; it can be made with the excavation resulting materials, even if it is preferable to use the same material as for the bed and the primary backfilling, symmetrically laid with alternating layers of 30-40 cm compacted one by one.
  3. Backfilling up to the ground level has to be completed with native material.
  4. During the backfilling one must avoid any pipe impact that may be caused by stones or by any other material that could damage it. If the backfilling is disposed of in a nonsymmetrical way or with non-uniform compaction, the alignment of the pipeline may be altered in such a way that it could influence the pipe’s seal.

USE OF DIFFERENT MATERIALS

Materials that are different from the ones above described, but approved by the Designer, can be used for the composition of the laying bed and the backfilling, as long as the content of the granular materials (retained by the ASTM 200 sieve) is greater than 30% and the liquid limit is lower than 50%.

Please take note that by increasing the fines content of the material, the energy required for the tamping of the material itself will increase. Furthermore, it’s useful to intensify quality controls and check the right laying of the backfilling.

Refer to Part 3 (last part) of the document to continue reading…

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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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