Frequently Asked Questions

There are many questions that come into the offices of National Clay Pipe Institute about the use, installation and physical properties of Vitrified Clay Pipe.  Many of them deal with misconceptions based on pipe that was produced over 100 years ago.

Today's pipe is considerably different from pipe made even 50 years ago.  Some of the most common questions are noted below.  As more questions are submitted we will continue to update this area.  Please submit your question if you don't find it below.  The answer may surprise you and there may be others who will benefit from the question you submit.

Table of Contents

Can clay pipe be used in trenchless applications?
Do they still make Clay Pipe?
Is there a way to predict the effect of additional soil loading on an existing system?
What is Microtunneling?
What is Pipe Bursting?
When did compression joints become the standard in the US?
Do clay pipe joints leak?
What is transition width?
What is the best way to get updated information on proper installation techniques and bedding classes?
Are there any new deep installation techniques available for Vitrified Clay Pipe?
Will Clay Pipe float when bedded with CLSM?
Request Materials
Do you have a handout of dimensions of Vitrified Clay Pipe?

Questions and Answers

  Can clay pipe be used in trenchless applications? ^TOP
  Vitrified Clay Jacking Pipe is used for slurry microtunneling, pilot tube microtunneling (GBM), static pipe bursting and sliplining casing pipe.
  Do they still make Clay Pipe? ^TOP
  YES!  Clay Pipe has significantly improved through the years.  Yesterday's Clay Pipe has a long history of solid performance and a majority of those are still in service and in great shape today.  Today's Clay Pipe is fired at 2000ºF (1100ºC).  At this temperature, vitrification occurs as the clay mineral particles become fused into a chemically inert and stable material.  Today's Clay pipe has high strength and is unique in its corrosion and abrasion resistance.  The high quality of vitrified clay pipe manufacture and performance is maintained in accordance with Standards issued by the American Society for Testing and Materials (ASTM).  Vitrified Clay Pipe is the ONLY piping material exclusively designed to convey the full range of materials that a community or an industry may discharge into it.  There are four manufacturers in the United States providing the greatest VCP ever: Building Products Company , Mission Clay Products, Gladding McBean, and Logan Clay. VCP is not "old"; VCP is an innovated product and is here to stay.... forever!
  Is there a way to predict the effect of additional soil loading on an existing system? ^TOP

The effect of surcharge loading can be estimated but detailed knowledge of the original construction is very important to accurately estimating the effect of additional loading.  NCPI has put together a set of guidelines for evaluation of surcharge conditions.  We can also help with predictive calculations.  For more information please read our Tech Note on Surcharge Fills .

  What is Microtunneling? ^TOP
  Microtunneling, a new installation methods, is remotely controlled, guided, pipejacking that provides continuous support to the excavation face.  Pipes of 12 inches to approximately 90 inches can be installed using microtunneling.  The process involves the jacking of a  microtunneling machine (MTBM) and pipe from a jacking shaft to a reception shaft.  The machine has a closed faced shield.  Excavated soil is removed using slurry, which also counterbalances groundwater and earth pressures.  The machine is guided by a laser or other survey device mounted in the jacking shaft, which projects a beam onto a target in the articulated steering section of the MTBM.  The MTBM is steered by extending or retracting remotely controlled steering jacks.
  What is Pipe Bursting? ^TOP
  Pipe Bursting is a rehabilitation method which involves replacing a host pipe by fragmenting or cutting the existing conduit and installing a completely new pipe of equal diameter or larger in its place.  The process involves initial cracking, followed by fragmenting the host pipe, creation of a new tunnel and simultaneous installation of new replacement pipe.  Pipelines of larger diameter and capacity than the host pipe can be installed with pipe bursting.
  When did compression joints become the standard in the US? ^TOP

Factory applied compression joints were developed in response to the desire in the water treatment industry to install water tight systems.  In 1958, NCPI began to develop leak-free joining systems for our manufacturers.  It took many years to implement across the industry.  It is commonly accepted that 1970 to 1972 is the time frame when all of the manufacturers had implemented the leak-free, factory applied, compression joint technology.  For more information please read ASTM and the National Clay Pipe Institute - 100 Years of Teamwork and Achievement.

  Do clay pipe joints leak? ^TOP
  NOModern Clay Pipe joints do not leak. Clay Pipe has durable, factory-applied compression joints and couplings that effectively eliminate water leakage.  All factory applied clay pipe jointing systems, whether on bell-and-spigot pipe or plain-end pipe, are designed to provide resilience and flexibility to accommodate minor pipe movement.  All compression type jointing methods meet the requirements of ASTM-C425, which requires that the joints not leak.  With the proper installation, a clay pipe sewer system can meet standard infiltration or exfiltration requirements. Furthermore, an independent study done by The University of Houston demonstrated that the joints of Vitrified Clay Pipe exceeded the industry standards. Vitrified Clay Pipe joints are leak free.  For more details about this study please click the following link: VCP IS TOPS.
  What is transition width? ^TOP
  Trench Widths is explained in the NCPI Engineering Manual chapter 4 and may be calculated using the NCPI computer program "Tool Box". Tool Box is a free downloadable program from the NCPI web site at  the Toolbox page. It can also be obtained through your local NCPI representative and/or Member Company representative. Transition width is the point in the trench where the ditch load is equal to the embankment load. Simply put, the load on rigid pipe increases as trench width increases.  The transition width is the point where increasing the trench width will not add any additional load to the pipe and the pipe load is at its maximum value.  The transition width is shown to the far right in the NCPI Engineering Manual Load Table and in the computer program "Tool Box" to the right in italics.
  Do you have a handout of dimensions of Vitrified Clay Pipe? ^TOP
  The current ASTM C700 provides manufacturing tolerances and dimensions for today's pipe.  Clay Pipe is manufactured to a strength specifications not overall dimensional criteria. Our plants maintain control over ID but are allowed to vary the wall thickness depending on the type of clay raw materials and the processes used in their plant in order to achieve the strength requirements.  It is best to contact the manufacturer of the pipe you use to get the information you are looking for.  Contact information for Today's manufacturers can be found on the NCPI website at

With older pipe it is more difficult to get that information due to the fact that some of the old manufacturers are no longer in business and several different classes or strengths of pipe have been made over the 150 years that clay has been used in the United States.

  What is the best way to get updated information on proper installation techniques and bedding classes? ^TOP
  NCPI offers many tools: The NCPI Engineering Manual, ASTM Standards, Installation Handbook and a computer program "Tool Box". All of this information can be acquired through your local NCPI representative and/or through the NCPI website at the Toolbox page. Additionally, we offer free workshops tailored to your needs and conduct classes on the design and installation of vitrified clay pipe.  We can also arrange tours of our manufacturers facilities.  Contact an NCPI representative to discuss the services listed above.
  Are there any new deep installation techniques available for Vitrified Clay Pipe? ^TOP
  NCPI is currently working on developing methods of installation for VCP in deep applications. Our Research on CLSM, Controlled Low Strength Material is a promising medium for VCP structural support in deep installations.  Our research into the application of CLSM as a bedding material has been ongoing for roughly 12 years.  Microtunneling is also an option for deep burial VCP (ASTM C1208) either using pilot tube or standard slurry units.
  Will Clay Pipe float when bedded with CLSM? ^TOP
  In actual practice in the ditch, NO. Technically and mathematically, YES. When you do the math the pipe should float. However, in actual practice, the CLSM when placed around the pipe forms an immediate friction between the pipe and CLSM. The friction plus the weight of the pipe prevents the pipe from floating. Tests in the field using two concrete trucks dumping their chutes simultaneously, over the top of the pipe did not cause the pipe to float. That being said, caution should be used if placement of CLSM is at an unusual high discharge rate.

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