Comparison of polyethylene and fiberglass pipes for installation in earthquake-prone areas

5-2-1- Behavior of Different Pipes Against Earthquakes

Enhancing and optimizing the design of structures against earthquakes, given the frequent changes and evolution of regulations over the past decades, is an inevitable subject. The first step towards optimizing or retrofitting structures is determining the extent and nature of their vulnerability to earthquakes. Earthquake vulnerability assessment involves evaluating equipment, communications, safety, and the functioning of a system during an earthquake. Using this method, the areas with the highest risk levels are identified quickly and cost-effectively, which are then prioritized for subsequent phases, including design, preparation of execution plans, and ultimately the retrofitting process. One method of assessing vulnerability is learning from past earthquakes and the behavior of equipment against seismic forces. This method is logical and defensible because it considers the actual conditions of the system and evaluates the seismic behavior of equipment in past earthquakes, integrating information and engineering judgment to assess the results.

This report specifically addresses pipelines. Initially, the definitions of pipeline vulnerability factors are presented, followed by examples of pipeline damages in various earthquakes, and finally, the probable modes of pipeline failure due to earthquakes are discussed.

It is necessary to mention that despite the numerous earthquakes in Iran, unfortunately, comprehensive documents and reports on the impact of earthquakes on vital arteries, especially water pipelines in Iran, are not available, or existing documents are not comprehensive.

5-2-1-1. Definition of Factors Causing Damage to Pipelines

Having a common understanding of the damage factors is essential for determining the solutions to mitigate them. Therefore, in this section, the factors that cause pipeline damage during an earthquake are listed and briefly defined.

5-2-1-1-1. Ground Shaking

Ground shaking occurs due to the propagation of waves through the earth. The period and amplitude of these waves depend on the amount of released energy, the distance from the energy source, the type of wave, geological conditions along the wave path, soil conditions, and the topography of the area. During wave propagation, energy is transferred to underground and surface structures. Buried structures like pipelines may experience transient displacements during wave propagation.

5-2-1-1-2. Permanent Ground Displacement

Seismic waves can permanently displace the foundation and support ground. Permanent ground displacements are significant factors in damaging buried pipelines. Types of permanent ground displacements include faulting, landslides, liquefaction, settlement, and uplift, which are further explained below.

a. Faulting

Fault ruptures usually occur at the earth's surface and can be horizontal, vertical, or a combination of both. Equipment located in the fault rupture zone is highly vulnerable.

b. Liquefaction

Liquefaction occurs when the soil temporarily behaves like a liquid. This can happen both underground and at the surface. The probability and severity of liquefaction increase with the intensity and duration of the earthquake. Liquefaction and lateral movements are major damaging factors that can push underground structures to the surface or submerge surface structures into the soil.

c. Settlement

Ground shaking can densify the soil and cause settlement. Although the permanent displacement due to settlement is less than that caused by liquefaction, the importance of settlement lies in creating non-uniform ground displacements.

d. Soil Lateral Spread

Even without liquefaction, ground shaking can move soil slopes. The severity of soil lateral spread depends on the slope angle, soil strength, water content, soil consolidation, and the earthquake's intensity and duration. Besides, soil lateral spread can block access routes and cause significant damage.

e. Uplift

This phenomenon, primarily caused by the vertical component of an earthquake, significantly affects entry and exit points (connections).

5-2-1-2. Failure Modes of Buried Pipelines

The main failure modes of buried pipelines due to ground displacement and failure include sliding, horizontal and vertical displacements, settlement due to soil consolidation, and most importantly, liquefaction.

a. Failure Due to Ground Sliding

As mentioned, pipeline construction should be avoided in areas where ground displacement due to earthquakes or other environmental factors is likely. If construction in these areas is unavoidable, appropriate measures (soil stabilization, soil reinforcement, etc.) should be taken to prevent pipeline failure due to ground sliding. Figure (5-2-1-2-1) illustrates the behavior of pipelines with various positions relative to ground sliding.

b. Failure Due to Displacement

Ground displacement around the pipeline due to earthquake forces exerts tensile and compressive stresses on the pipe wall. If the pipeline has sufficient strength, it can withstand these stresses. Also, if the pipeline or its connections are ductile, they can move along with the surrounding soil, thus reducing the stresses on the pipe wall. If the pipeline lacks either of these two features (strength or ductility), its likelihood of damage due to ground displacement increases. Figure (5-2-1-2-2) shows an example of pipeline failure due to adjacent ground displacement.

c. Failure at Connections

Similar to surface pipelines, the connection points in buried pipelines are highly vulnerable to displacement, settlement, or soil sliding, and liquefaction effects. Using flexible connections in buried pipelines also reduces the likelihood of damage during an earthquake. Figure (5-2-1-2-3) shows the behavior of a pipeline with flexible connections during an earthquake. Figure (5-2-1-2-4) illustrates the appropriate behavior of a buried pipeline with flexible connections during the Kobe earthquake in Japan.

d. Failure Due to Liquefaction

Liquefaction in non-cohesive soils is one of the main reasons for the failure of buried pipelines. In surface pipelines, the supporting structures are also prone to damage due to liquefaction. When the soil liquefies, its resistance to gravitational and lateral loads significantly decreases, putting the pipeline at risk of settlement. Increasing the ductility of the pipeline using flexible connections can prevent damage due to liquefaction-induced displacements. Identifying areas susceptible to liquefaction and avoiding pipeline construction in those areas or implementing special measures such as increasing soil cohesion are other ways to prevent pipeline damage due to liquefaction. Figure (5-2-1-2-5) shows a manhole cover damaged due to soil liquefaction. Figures (5-2-1-2-6) and (5-2-1-2-7) show examples of liquefaction along the Alaska pipeline route.

5-2-1-3. Impact of Earthquakes on Different Types of Pipes

This section presents the behavior of different types of buried water and sewer pipes and the major damages inflicted on them due to seismic waves (dynamic effects) and permanent ground displacements (static effects). All results are based on experiences from past earthquakes in various parts of the world, especially the United States and Japan.

To comparatively examine the behavior of buried pipes during earthquakes, one of the best methods is using the damage levels and repair rates from past earthquakes. To this end, the comparative behavior of pipes against the dynamic and static effects of earthquakes has been studied, and the results are presented in Figure (5-2-1-3-1).

Figure (5-2-1-3-1) shows the vulnerability of different pipes due to various factors.

According to the existing vulnerability curves, the damage level of pipes at the same seismic intensity from low to high is as follows:

1. Steel pipes with welded joints
2. Ductile iron pipes
3. Polyethylene pipes
4. PVC pipes
5. Ordinary cast iron pipes
6. Asbestos cement and concrete pipes

In terms of performance against liquefaction, landslides, and fault movement, the ranking from suitable to unsuitable is as follows:

1. Welded steel pipes and polyethylene pipes
2. Ductile iron pipes
3. PVC pipes
4. Ordinary cast iron pipes
5. Asbestos cement and concrete pipes

As observed, welded steel pipes (arc welding) perform very well against various earthquake effects, followed by ductile iron pipes.

Based on the studies and the presented charts, the seismic behavior of pipes is generally categorized in Table (5-2-1-3-1).

Table (5-2-1-3-1) shows the seismic resistance and behavior of different types of steel, cast iron, and asbestos pipes.

According to the table above, using welded steel pipes (arc welding) carries the least risk during an earthquake, while the risk of using cast iron pipes is moderate.

5-2-1-3-1. Seismic Behavior of Pipes

The following briefly describes the seismic behavior of cast iron (ordinary and ductile), asbestos cement, reinforced concrete, steel, plastic, polyethylene, and GRP pipes.

a. Cast Iron Pipes (Ordinary and Ductile)

Ordinary cast iron pipes are highly vulnerable to earthquakes. Their deformation and flexibility, especially in smaller diameters, are negligible, resulting in brittle behavior and damage under both dynamic and static earthquake movements. These pipes are not recommended for seismic regions.

However, earthquake-resistant ductile iron pipes are now available and can be used as appropriate. In terms of lower risk during earthquakes, these pipes rank second after welded steel pipes.

b. Asbestos Cement Pipes

Asbestos cement pipes are not only environmentally unsuitable but also highly vulnerable to earthquake movements due to their lack of flexibility. In classes C and D, due to the thick wall, they show less damage.

c. Concrete Pipes

Concrete pipes are more flexible than asbestos cement pipes but are still not recommended for seismic regions. These pipes suffer damage due to low bending behavior and deformation percentage, especially in larger diameters. Concrete pipes with a steel core have shown better performance in past earthquakes than other concrete pipes.

d. Steel Pipes

Steel pipes, especially if used in curved or broken paths, are resistant to earthquake movements. Damage to these pipes usually occurs perpendicular to the fault, but they