Rigid and Flexible Pipe Systems

System evaluation
A buried pipeline is a system that integrates the properties of the soil surrounding the pipe and the physical properties of the pipe itself. The pipe is a critical structure that conveys water or other fluid and shall be designed hydraulically to be capable of transporting the content within its walls but also needs to be capable of withstanding any considerable loads (live loads, dead loads, surcharge loads, construction loads) that can apply to the underground infrastructure. The structural analysis of a pipeline is based on the soil conditions and the construction inspections, which is extremely important for the project’s success. Construction inspection ensures that all these factors are met on-site.

To analyze rigid and flexible pipe systems, it is necessary to differentiate their behaviour and their apparent differences. As per various textbooks and international standards such as ASTM (International Standards Organization) and CSA (National/International Standards Organization).

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Generally, rigid systems must be supported at the bottom (bedding) of the pipe, and flexible systems, in addition to bedding support, also require soil support on the sides which ultimately controls the pipe deflection. This ensures the pipe will not over-deflect beyond its limits. The inspection of soil support for both systems involves different indicators that should be considered in the design phase, which include but are not limited to the following:

• Distribution of soil around the pipe
• Type of soil for bedding and backfill
• Interaction of the soil in the trench wall and the foundation
• Water movement within and outside the trench
• Type of trench

For the designer, the type of trench selection depends on the material’s disposition, the soil condition, and the area where the project is taking place. There are some limitations, especially when the designer is battling to get extra “space” to adapt the installation (zoning requirements) or perhaps; limitations due to the existence of other “locates,” as some contractors call the existing utility lines. Thereby, the installation and the design should be based on the conditions of the native soil and its interaction with the properties of the pipe.

Water movement and migration are another piece of the jigsaw puzzle that needs to be addressed carefully.

Migration
This term could be wrongfully disregarded. It is a term that designers and inspectors should thoroughly analyze. This problem refers to the particles from the backfill material or trench walls transferring into the bedding material and the embedment material, which ultimately transforms the supporting soil under the pipe and around it into a much weaker and less stiff ground. It is essential to analyze the installation procedure and the components of such installation. (Howard, 2015).

Analyzing the soil and the used compaction can be extremely important (95% of compaction). Hence determining the percent compaction from the laboratory could play an important role (ASTM D 698, standard proctor compaction test for cohesive soils).

When the soil is not compacted correctly, these particles make their way into the voids of the coarser material, degrading the support of the pipe due to the groundwater flow. It is crucial to determine how the water is behaving. Based on that analysis, the designer should decide how to mitigate any possible installation problems. Some designers recommend using geotextiles for migration issues (non-woven or woven geotextile), and others use “French drains” as a water passageway.

Other methodologies can be used to contain migration issues, such as:
• Trench plugs
• Bentonite clay plugs
• Trench breakers

Rigid and Flexible Pipe Systems
A pipe is a conduit, but it is also a structure. Under this concept, rigid and flexible pipe systems, regardless of their material, can fail if installed correctly. A rigid pipe is designed to transmit any loads (live load, dead load, surcharge load, construction loads) applied to the pipe contained by the concrete walls and aided by the steel reinforcement. A flexible pipe is designed to support the loads by transferring the loads, as mentioned above, to the surrounding soil.

Flexible Pipe
One of the primary ways to control any possible issues attached to the installation of a plastic pipe is by conducting a deflection test known as the Mandrel Test (ASTM D3034), which consists of introducing a go-/no-go device through the pipe. Using this methodology can help determine if the soil surrounding the pipe has been compacted correctly and if the maximum allowable deflection value of the pipe was met. Values between one and five percent are considered standardized (Howard, 1977).

Flexible pipe deflection is directly proportional to the soil pipe’s stiffness and the trench wall’s stiffness, which is comprised of native soil or other backfill material. Buried flexible pipe changes its geometry during installation (elongation, load lag, deflection lag, deformation). Determining any local deformation that could trigger more complicated issues (stress/strain in the pipe wall) is essential.

A flexible pipe, by design, tends to deflect according to the ratio of the stiffness of its composition and the pressure induced by the soil-pipe system. The stiffness of the pipe can be determined by using a parallel plate test (ASTM F4212). Thus, the expression can calculate the product of EI:

EI = 0.146pr³/Δy

Where:

E= Modulus of elasticity of pipe wall material

I= Moment of inertia
P= load
r= pipe radius
Δy= vertical deflection

Typically, some equations calculate the deflection of the flexible pipe. For instance, the most common ones are the U.S. Bureau of Reclamation Equation or the Modified Iowa Formula (Spangler, 1941).

Δx/d (%) = 100DI + KP/ 0.149 (PS) +0.061 Edesign

Where: Δx/d (%) = Max.Hz deflection

Dl= deflection lag factor
K= angle of contact with the bedding material
P= external load (dead load + live load / pipe diameter)
PS= pipe stiffness
E’design= modulus of soil reaction

The previous equation only calculates the horizontal deflection; therefore, the vertical deflection must be considered to estimate the overall deflection inside the pipe correctly. Note that the denominator (E’design) refers to the backfill stiffness but also includes the interaction with the native soil material.

In other words, the combination of the soil surrounding the flexible pipe and the maximum allowable deflection are the primary components when designing and installing a flexible system.

Proper installation standards must be followed thoroughly. The ASTM D2321 or CSA 182.11 are the perfect guidelines for adequately installing flexible pipe material.

Rigid Pipe
Reinforced Concrete Pipe or RCP is manufactured according to the standard ASTM C76, CSA A257 and should be installed per ASTM C 1479 or Concrete Pipe & Precast Installation guidelines (OCPA, 2019).

The design of RCP depends on multiple factors. The selection of a pipe-soil system is strong enough to mitigate extreme loading conditions.

The pipe walls must be sufficiently strong, and the supporting bedding below the pipe.

Since most of the load would be carried by the pipe as opposed to the soil, the rigid concrete pipe needs to be designed and installed with the correct strength classification. The pipe strength is determined in a controlled environment by a test called the “three-edge bearing test” (ASTM C76, ASTM C497); this test consists of three loading point conditions (a worst-case scenario in real life).

The test seeks to obtain a 0.3mm width by 300mm long crack, which is not a structural problem; it is the practical way to realize the classification according to the standards (CSA A257.2 & ASTM C76).

RCP can be designed by using the following:
• Direct design – limit states design*
• Indirect Design – empirical method to determine a bedding factor.
*not covered in this paper

Some design engineers have used first principles that are highly accurate but somehow tedious, or height of fill tables, which can be practical as long as the notes of those particular tables are similar to the condition of the site where the project is taking place.

Another alternative could be the usage of PipePac, a free web-based software that allows the designer to determine the proper strength of the pipe under any given soil condition (CCPPA, 2020).

The installation also plays an important role. The designer should first evaluate the type of trench:
• Confined trench
• Embankment (positive projecting)
• Tunnel

The type of trench selection influences the pipe’s strength classification. A confined trench would support the load acting on top of the pipe due to the proximity of the trench walls. However, having a more extensive excavation as the embankment condition, the soil prisms columns acting on top of the pipe are now supported by the pipe itself.

The selection of the bedding factor and the bedding as per standard installations (Canadian Highway Bridge Design Code – CHBDC) offer four types of installations for RCP. This classification depends on the quality effort of the soil compaction and its quality.

Pipe strength= We + WI/Bf – Fs/Diam
Where:
We= Earth load
Wl= Live load
Bf= Bedding factor
Fs= Factor of safety
Diam= Diameter of the pipe

As expressed in the previous equation, the bedding factor is in the denominator; therefore, selecting a good bedding factor can reduce the need for a high-strength classification pipe. It is essential to reaffirm that the design of the pipe and the design installation for both systems are extremely important. As a designer and practicing engineer is our responsibility to follow suitable methodologies and proper standards.

Camilo Marquez, M.Eng., P.Eng.
Region Engineer MB – SK
Canadian Concrete Pipe & Precast Association