By Jim Noll, P.E.
| Sunday, May 01, 2016 | 4693 Views |
I get asked quite often a seemingly benign question related to the design and installation of corrugated metal (and plastic) pipes - a question that’s worded basically as – “Can a design engineer build in additional safety factor into the design to compensate for poor installation practices and techniques?”. This isn’t as simple or straightforward of a question as it might seem, and it’s actually fraught with potential pitfalls. The following discussion hopefully points out the relevant factors, issues and concerns regarding this topic.
The design procedures for buried flexible metal and plastic pipes are well-documented, proven and established. Adequate safety factors are included in those procedures. A buried flexible pipe system is actually a soil-structure interaction system, meaning that the design is based on a combination of pipe strength and the adequacy and performance of the surrounding soil envelope. Said a different way, the support provided by the surrounding soil envelope and the underlying foundation is a key component of the design and the overall performance of the pipe system. Proper installation is critical to good performance - and here is a simple but very real fact - you can't design a pipe for poor quality installation. Incorporating higher safety factors on pipe wall strength in your design is no guarantee you will overcome poor installation practices.
Proper installation of a buried pipe includes two important considerations:
- The quality of the backfill material used in the select fill zone
- The placement and compaction methods for that fill material
The key is to educate the contractors and field installation teams re: proper backfill, and proper installation methods - including the lift thickness, balanced placement methods, the type of equipment used to place and compact the fill, and adequate compaction density. These factors - combined with adequate minimum cover height for vehicle / equipment loads - are crucial to long term pipe performance.
On site or native soils may be satisfactory for use in the select backfill envelope, but the properties of those soils must be evaluated by a competent geotechnical engineer and approved by the project engineer for use as the select backfill material.
Balanced and controlled placement of the select backfill material is essential. This means the fill must be placed in thin lifts and the level of the fill kept in reasonable balance on both sides of the pipe (think 1 ft. to 2 ft. maximum differential from side to side of the pipe). This is especially important for multiple barrel, closely spaced pipes. Fill level must be kept balanced across the multiple barrels to avoid distortion and displacement of the pipes.
Here’s something else to consider – the soil envelope and the underlying foundation for the pipe and the adjacent side fill zones provide support for not only the pipe system, but provide support for whatever is installed above the pipe – such as a roadway, parking lot, railroad track, etc. Substandard installation of the backfill or lack of attention to the foundation will not only affect the performance of the pipe system, it will affect the long term stability and performance of the surface structure. This applies to both flexible and rigid pipe systems. Rigid pipe systems also rely on the surrounding and underlying soil for proper performance. A rigid pipe system that is installed on a substandard foundation would be subject to settlement and possible disjointing. Suspect side fill zones could lead to pipe movements and displacement, along with subsidence of the surface above.
Adequate minimum cover must be in place to protect the pipe system from live load effects and influences – cover that is of competent quality, that has been properly placed and compacted, and that is well maintained. Heavier vehicles such as construction traffic will require additional minimum cover over the pipe as compared to standard highway live loads. Also, since this construction traffic will often occur prior to surface paving, added cover will likely be needed to compensate for the fact that no pavement is in place – and that wearing surface needs to be kept level and free of ruts.
There are installation standards and guidelines pertaining to proper pipe installation available in AASHTO, ASTM, AREMA and other industry sources. Pipe manufacturers and suppliers also publish such installation guidelines and other collateral relative to proper pipe installation. Project specifications should incorporate such installation guidelines and references. Those responsible for installing and backfilling the pipe systems, whether they be outside installation contractors specifically hired for that purpose or in-house assembly / installation crews, need to be educated and trained in the key aspects of proper installation of such systems. Ignoring this fundamental component of pipe design and performance is quite simply asking for problems.