Research and education are the very foundation of NCPI. It’s what we do. We’ve led the industry in this area for over 100-years.

The information here is a very abbreviated version of some of our research through the years.  To see more, visit our Papers & Tech Notes page.

Contact us or one of our Member Companies to arrange training for your group.


Manufacturing Pipe Bodies

Founded in 1917, the organization pre-dates the wide-spread use of electric motors in the manufacturing of clay pipe.  With the introduction of these motors, improved pipe strength became the standard and NCPI led the research and development of Standard Strength Vitrified Clay Pipe (VCP).

In the 1940s, manufacturers migrated from the electric motor-driven augers of the early 1900s to high strength vacuum extrusion of pipe and the specification for Extra Strength VCP was introduced.  All pipe manufactured in the U.S. now meets or exceeds this standard.


Even decades after the first standards for VCP installations were accepted, pipe may have been installed without joints.  Dilution was the solution to pollution for over 99% of the U.S. population living in cities (as reported in the famous Metcalf & Eddy study of 1935).

Eventually field-applied joints became common.  The inconsistency of materials, methods and quality of installation made these joints problematic. VCP manufacturers anticipated the need for better, tighter, more reliable joints than the field-applied joints of the day.  Research into factory-applied joint design and materials began in the 1940s.

In the 1960s, NCPI conducted Accelerated Root Resistance Studies to thoroughly test the new factory-applied compression joints under extreme conditions.

When the Clean Water Act passed in 1972, the industry was ready and made leak-free joints the standard.

Bedding & Factors of Safety

In 1979, Ed Sikora (the Director of Research for NCPI at that time) presented a paper at the California Water Pollution Control Convention on NCPI’s review of Load Factors and Non-Destructive Testing of Clay Pipe. The paper was published in the Journal of the Water Pollution Control Federation (now known as the Water Environment Federation or WEF) in December of 1980.

The research that constituted the basis for this paper included evaluation of three-edge bearing tests, the performance of angular vs. rounded aggregates in bedding materials when used in Class D, C or B bedding, sand as a bedding material, sidefill loading and the influence of bedding size on load factor in a flooded condition.

This research called into question the then accepted testing methods of artificially applying uniform pressures to both the sidefills and the pipe.

Some of the findings of this research included:

  • “When excessive sidefill loads are applied, the pipe is placed in compression and normal response is inhibited. Provision must be made for differential loading and settlement of the soil mass directly above the pipe and the vertical column of soil adjacent to the pipe.”
  • “Shovel-slicing is the preferred method of consolidating the bedding material under the haunches of vitrified clay pipe,” compared to machine compaction.”
  • “Flooding decreased the load factor in every bedding tested [Classes D, C and B] although angular beddings were least affected.”

This research provided the first quantitative evaluation of the impact of shovel-slicing on load factors.

CLSM and Modified Marston

In 1995, Committee C04 approved the use of what was then known as Controlled Density Fill (now known as CLSM or Controlled Low Strength Material) as a bedding material, thus creating the first bedding class for sanitary sewers to include flowable fill.  Prior to adopting this new bedding class, NCPI conducted field tests.  A paper based on this study was presented to ASCE in 1996.

Marston theorized that the material at the sides of rigid pipe was so loose (compared to the rigidity of the pipe) that support of any backfill load by the sidefill would be negligible. However, the presumed inability of the sidefills to carry a significant share of the backfill load is not applicable when CLSM bedding is used since it does not settle, compact or shrink significantly.

When Professor Spangler studied the properties of a flexible pipe installation, he found that “For the case of a flexible pipe conduit and thoroughly tamped sidefills having essentially the same degree of stiffness as the pipe …” the load was distributed across the surface of the pipe and sidefills.

When considering a VCP line bedded in CLSM, combining the work of both Marston and Spangler resulted in a modification to the standard Marston Equation.  This new equation has the effect of shifting a portion of the soil prism load above the pipe to the adjacent side prisms above the sidefills.  The theory was tested in 2012 with the results published in a paper presented at ASCE Pipelines Conference in 2013.

In early 2013 (revised September 2022), NCPI also published Guidelines for Controlled Low Strength Material (CLSM) Mix Design, Placement and Testing …


Experiments were conducted in 2013 to demonstrate the significance of shovel slicing during installation of VCP. Resistance to pushing rods and pulling straps at the pipe-soil interface was used to show the relative stiffness of crushed rock placed in the pipe haunch. Significant differences in stiffness were seen between dumped rock and shovel sliced rock.

Compaction of the soil in the haunch area significantly increases the support for the pipe. Gravels and crushed rock dumped into a trench beside the pipe result in the minimum densities of the soil, which is about 80-85% of their maximum density (Howard, 2013). Compacting the soil to about 95% (D 4253) can increase the stiffness (modulus) of the soil 300 to 600% (Howard, 2013).

For vitrified clay pipe (VCP), increasing the percent compaction increases the load factor from 1.1 to 1.9, almost doubling the allowable load.  There were four main take-aways from this study listed in the paper:

  1. Good haunch support significantly increases the load carrying capacity of buried pipe.
  2. Good haunch support requires compacting the soil in the haunch area or using flowable fill.
  3. Good haunch support is not attained by dumping gravels and crushed rock beside the pipe.
  4. Good haunch support can be attained by pipe settling into uncompacted bedding and mobilizing the strength of the haunch soil.

Testing of Pipe and Installed Pipelines

Research conducted by NCPI created the foundation for development of pipe strength standards, compression joint testing and the Low-Pressure Air Test to name but a few.

The testing and recommendations developed by NCPI are designed to be practical in field applications. The Optimal Mix for CLSM was developed to facilitate rapid curing and allow for final backfill as quickly as possible.  The Low-Pressure Air Test is an inexpensive, quick and definitive test of not just a single joint, but the full pipeline installation.  And the various bedding classes for VCP have developed over the years to provide significant and reliable Factors of Safety. See them on our Design page.

ASTM specifications for VCP manufacturing, testing and installation have evolved for over 100-years.  As development work on pipe bodies, joint design, joint materials, installation practices and long-term maintenance all progress, NCPI will continue to build on the extensive research in our library.

Can’t find what you need?

Contact us at or 262-742-2904.

For information about purchasing pipe, please contact one of our member companies.
For complete technical information, download our Manual or one of our Handbooks.

*Qualifies for PDH Credits