The effect of various loads on different surfaces is easily observable in our environment, and usually results in sagging of the surface on which it is applied. Depending on the support provided to the area under load, the degree of sag will vary. When it comes to pipes buried under the earth, the effect of load is not observable and is only evident at pipe failure that results in leakage, loss of flow or ground collapse. A failed pipe is costly to repair and can also cause significant environmental damage depending on what it is conveying.
There are different types of loads, and their effect on different pipe materials are different and therefore very important to study. An understanding of how the loads are transmitted and how these pipes react to loads will help designers and contractors to design and choose the right pipe material for the expected loading conditions.
Here we will briefly discuss loads, their transmission and effect on pipes. The reader is encouraged to study these factors in depth to gain better understanding of how loads work. (The type of soil you're working in can have a big impact on a project. Learn more in Ground Improvement Requirements When Working With Sandy Soil.)
Types of Loads
Buried pipes have two functions, namely, hydraulic i.e. conveyance of fluid or gas, and structural, i.e. supporting the weight of the surrounding soil and any dead or live load on it. An improperly installed pipe can damage itself and structures and pavements above it. Pipes are subjected to either dead or live loads. Both these loads have to be considered while designing pipes. Dead loads are the weight of the soil above the pipe and any static installation over it.
The higher the fill above the pipe, the higher the dead load and the greater the load the pipe has to carry. Live loads are experienced over a limited period such as when a vehicle passes over the region where the pipe is buried, and may be continuous on busy roads and intersections. A pipe that is buried deeper will experience a lesser intensity of live load compared to a pipe buried at shallow depth.
Types of Pipes and Pipe Materials
Pipes are either rigid, such as concrete, vitrified clay and cast iron pipes, or they are flexible, such as high-density polyethylene (HDPE), polyvinyl chloride (PVC) and glass-reinforced plastic (GRP) pipes. (To learn more about different pipe materials, see The Lifespan of Steel, Clay, Plastic & Composite Pipes.)
A pipe that can flex or bend when load is applied is a flexible pipe. The ability of these pipes to support load depends to a great degree on the support from the surrounding soil or backfill material. A flexible pipe will fail by deflecting to such an extent that it becomes incapable of keeping up with the flow rate or by causing a depression on the ground surface above it.
When the load above the pipe reaches a critical load or deflection, the pipe can become structurally damaged by buckling. Allowable decrease in vertical pipe diameter under a load is 5 percent, and before complete collapse a pipe may experience up to 20 percent change in vertical diameter.
Unlike flexible pipes, rigid pipes are capable of self-supporting loads with a little help from backfill. However, the capacity of the rigid pipe to carry load better can be increased by providing adequate support from bedding and backfill. A rigid pipe will fail by fracturing or cracking before considerable deflection has taken place.
Permissible crack width in reinforced concrete pipe is 0.01 inch. The ability of the pipe to convey fluids may not be affected by cracks, but the exposure of the steel to the surrounding soil can result in corrosion damage, tree root intrusion and infiltration.
A continuous soil mass transmits loads evenly and can be pictured as downward, regular, smoothly flowing lines. When a pipe is placed in this soil, the load is transmitted to the pipe through the soil over it. A pipe that has been placed properly with adequate support from bedding and backfill will not affect the load distribution pattern in any significant way.
A pipe more rigid than the surrounding soil will accept more load while lessening the load on the surrounding soil, changing the pattern of load transmission by deflecting the lines inward toward the pipe. A pipe more flexible than the surrounding soil, on the other hand, will deflect vertically and transfer some of its load to the surrounding soil, deflecting the load pattern outward from the pipe.
Pipe Reaction to Applied Load and Importance of Backfill
A properly supported and backfilled pipe will bear and transfer the load evenly along with the surrounding soil, resulting in increased service life of the pipe.
Reaction of Flexible Pipes
Flexible pipes, owing to their bending capability, can support loads very well when good side support is provided. It will deflect vertically along with the soil when a moving load passes over it. This deflection may be more for the pipe than for the soil, especially when the soil is less stiff than the pipe, eventually resulting in an increase in the horizontal diameter of the pipe by soil compression.
This problem can be avoided by providing adequate compaction to surrounding soil during installation.
Reaction of Rigid Pipes
Rigid pipes, on the other hand, when more stiff than the surrounding soil, will deflect less than the surrounding soil when a load passes over it, resulting in soil depressions on either side of the pipe.
This problem can also be prevented by providing adequate compaction to surrounding soil during installation. Rigid pipes have the inherent strength to carry loads without side support, but this can be greatly increased when adequate side support is provided.
Trenchless construction utilizes grout, pneumatically placed sand or excavated material to backfill the annular space between the pipe and the borehole. The necessity to backfill is subject to varied reasons such as method of excavation, level of groundwater, site geology and pipe material. All factors should be considered and appropriate backfill and side support should be provided to all pipe installations as priority.
It not only helps the pipeline complete its designed service life, it also considerably reduces maintenance and repair costs associated with improperly backfilled pipelines, including costs of damage to surface structures.