Clay - The Permanent Solution

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Frequently Asked Questions


Thanks to our users for submitting your questions.  If there’s anything you’d like to see addressed here, please email us. If you have specific, technical questions, please contact us at 262-742-2904.

  • Can clay pipe be used in trenchless applications?

    Vitrified Clay Jacking Pipe has been used in trenchless installation in the U.S. for over 20 years. VCP is the preferred pipe material for the Pilot Tube Method (PTM) and is commonly used for slurry microtunneling, static pipe bursting and sliplining both casing and existing pipelines.


    For more information:  See Vitrified Clay Pipe Engineering Manual, Chapter 8.

  • Do they still make Clay Pipe?

    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 2000°F (1100°C).  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 bearing and compressive 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 "old," but it’s also innovated over the years.  That means it’s a proven material and is here to stay...forever!

  • Is there a way to predict the effect of additional soil loading on an existing system?

    The effect of adding surcharge loading or live loading can be computed but the age as well as detailed knowledge of the original construction is very important to accurately estimate the effect of the additional load. NCPI has put together a set of guidelines for evaluation of surcharge conditions.  We can also help with predictive calculations.   


    For more information: See Surcharge Fills Tech Note and Vitrified Clay Pipe Engineering Manual, Chapter 4.

  • How can VCP be used in trenchless installations?

    Vitrified Clay Jacking Pipe has been the predominant pipe material used in the Pilot Tube Method (PTM) process due to its high compressive strength, a low-profile zero-leakage joint, availability in the typical 1 or 2 meter pipe lengths and elimination of the external casing pipe.  Vitrified Clay Pipe has an extremely high compressive strength (18,000 psi average), a feature that makes it uniquely suited to endure the high jacking forces generated as the pipe is pushed through the ground. In addition to the Pilot Tube Method, Vitrified Clay Pipe has been used for Slurry Microtunneling, Pipe Bursting, and Sliplining.


    For more information: Vitrified Clay Pipe Engineering Manual, Chapter 8.

  • What is Pipe Bursting?

    This is a rehabilitation method which uses an expander, which can be either pulled or pushed through an existing pipeline. The existing pipe is fractured and displaced with the broken pieces being pushed into the surrounding soil while a pipe of the same diameter or larger is pulled or pushed into place. This technology offers the benefit of increasing hydraulic capacity without disrupting busy metropolitan areas.


    For more information: Vitrified Clay Pipe Engineering Manual, Chapter 8, page 8-9

  • When did flexible compression joints become the standard in the US?

    Factory applied flexible 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 believed that all U.S. manufacturers were using the factory applied, leak-free, flexible compression joint by 1970.  


    For more information: ASTM and the National Clay Pipe Institute - 100 Years of Teamwork and Achievement.

  • Do clay pipe joints leak?

    NO.  Modern Clay Pipe joints do not leak. Clay Pipe has durable, factory-applied flexible compression joints and couplings that effectively eliminate water leakage while providing the needed flexibility.  The misinformation about leaking joints is attributed to the long service life of clay pipe installations of well over a century in service.  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 while maintaining a strong, leak-free joint.  All compression type jointing methods meet the requirements of ASTM-C425, which requires that the joints not leak when subjected to both shear load and deflection.  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 the University of Houston study: VCP IS TOPS.

  • What is transition width?

    Transition width is the point in the trench where the trench 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 beyond which increasing the trench width will not add any additional load to the pipe and the pipe load is at its maximum value.  Trench width for any rigid conduit is measured at the top of the pipe barrel.


    Trench Width is explained in the NCPI’s Vitrified Clay Pipe Engineering Manual, chapter 4 and may be calculated using the NCPI computer program "Toolbox". To find out more and to use Toolbox, click here.  For assistance using the program or with any special concerns, please contact the NCPI office at 262-742-2904 or contact a member company for local representation. 

  • What are the dimensions of Vitrified Clay Pipe?

    Clay Pipe is manufactured to a three-edge bearing strength specification not overall dimensional criteria. ASTM C700 provides manufacturing tolerances and dimensions. Wall thicknesses vary by manufacturer.  Please contact the pipe manufacturer to get specific information.  Links to NCPI member companies’ websites and contact information can be found on our member’s page.


    It is difficult to get dimensional information for older pipe.  Over the 200+ years that clay pipe has been manufactured in the U.S., several different classes or strengths of pipe have been used, a number of manufacturers are no longer in business and it can be very difficult to identify which manufacturer made a given pipe.

  • What is the best way to get updated information on proper installation techniques and bedding classes?

    NCPI’s Vitrified Clay Pipe Engineering Manual, ASTM C12 and NCPI’s Installation and Inspection Handbook are the best sources of current information on installation techniques and bedding classes. The NCPI publications are available online or through our member companies.


    For information about safety factors of various bedding classes, a least cost analysis or flow calculations, visit the Toolbox page of the NCPI website. We also offer a variety of free workshops on the design and installation of VCP.  We can also arrange tours of our manufacturers’ facilities. Contact us to discuss your specific areas of interest.

  • Are there any new deep installation techniques available for Vitrified Clay Pipe?

    NCPI has completed many research projects resulting in methods of installation for VCP in deep applications. Our Research specifically on CLSM, Controlled Low Strength Material is a solution for structurally supporting open trench VCP in deep installations. 


    Trenchless installation is also an option for deep burial VCP.  Vitrified Clay Jacking Pipe can be installed by either using pilot tube or slurry microtunneling methods.


    For more information:  ASCE paper on Modified Marston and Vitrified Clay Pipe Engineering Manual, Chapter 4.


    For more information on trenchless installation methods: Vitrified Clay Pipe Engineering Manual, Chapter 8,

    page 8-4.

  • Will Clay Pipe float when bedded with CLSM?

    In actual practice in the ditch, NO. Theoretically 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, NCPI recommends against placement of CLSM at extremely high discharge rates.


    For more information on CLSM: Flotation, Optimal CLSM Mix Design, Guidelines for CLSM

  • Why should I specify clay pipe?

    Clay pipe is the only pipe that will not change over time in a sanitary sewer application.  Plastics lose strength over time while concrete and iron pipes are susceptible to corrosion from hydrogen sulfide.  The physical properties of clay pipe however do not change.


    VCP has been in service globally for more than 4,500 years.  It’s been in service in U.S. sewers for over 200-years.  In even the most corrosive soils and brownfields, VCP is inert.  In trenchless applications, the unmatched compressive strength of VCP allows for installation methods other pipe materials cannot support.


    There are a multitude of reasons for specifying VCP.  In short, you never have to apologize to a councilmember or citizen for specifying the best.  Contact us for more reasons to specify clay.

  • Why should I install VCP instead of plastic pipe?

    VCP is a fired ceramic with the longest service life of any pipe material available.  The corrosion resistant thermoplastic competitors are all made from a material that degrades with age.  Each project is unique.  The reasons for specifying VCP for one project may be wildly different than the reasons for specifying VCP for the next project.  But no matter the nature of the project, the inert nature of Vitrified Clay, the compressive strength of the pipe manufactured by NCPI member companies and the leak-free joints all lead to the longest proven service life of any pipe material.

  • How do I schedule educational programs for my group that will qualify for CEUs or PDHs?

    NCPI and its member companies offer a few different educational courses that qualify for CEUs.  We start by asking a few questions about your specific challenges so that we can present the material that is most useful to you and your group. To find out about scheduling, please contact one of our member companies to discuss the options and determine which program(s) will best meet the needs of your group or request more information here.

  • How do I get a hydraulic calculator?

    You can request a hydraulic calculator on our materials request form. If you are a registered user, log-in to request a calculator. If you have not registered, visit our registration page. Single units of the hydraulic calculator are provided with the compliments of NCPI member companies. For additional units, please contact Michelle at

  • How do I get a printed version of the Engineering Manual?

    Printed versions of NCPI’s Vitrified Clay Pipe Engineering Manual are available from our member companies. The most up-to-date version of the Manual can be downloaded from this web site. If you are already signed up at this site, click the Log-In button. If you have not registered, please do so by clicking the Register Here button.

  • What’s the difference between vitrified clay pipe and terra cotta?

    Terra cotta is a non-technical term that can be used to describe a color, a type of wall cladding, pots and other ceramic products that can vary greatly in strength and composition, but are generally manufactured to have a high porosity. Terra cotta, when describing a pipe refers to a material fired to approximately 1200°F.  Vitrified Clay Pipe is fired to over 2000°F to fuse the molecules resulting in a ceramic with high strength and low porosity.

  • Why is VCP better for the environment?

    VCP is made from soil which is extremely abundant and contains no harmful additives in the manufacture. VCP has achieved a Heath Product Declaration (HPD).  Energy consumption during manufacturing for both clay and concrete pipes are drastically lower than all other pipe materials. The thermoplastic products are all made from fossil fuels. Dioxin is an unavoidable byproduct of the manufacture of all PVC products.

  • How has clay pipe changed?

    In the more than 200-years since clay pipe was introduced in the sewers of major U.S. cities, ASTM standards have been adopted that require the pipe bodies to meet stringent strength metrics.  Leak-free compression joints have been developed and a multitude of installation practices have been tried, tested and adjusted or discarded.


    NCPI is proud of its history as a research institute.  As such, the findings of various research projects conducted by the institute have led to the continuous improvement of VCP, the joints used and the installation methods employed.


    ASTM was created and the clay pipe industry formed the C04 committee that eventually created the first specification for sewer design and construction.  Prior to the acceptance of this standard, thousands of miles of clay sewer pipe of radically varying quality were laid using dramatically varying methods.


    In the 100-years since the first standard was accepted, additional standards for pipe strength, testing procedures and joint performance have all been proposed and accepted, so that today’s VCP joints “shall not leak.”


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Information provided on this website is informational only and should not be substituted for the guidance of a qualified professional.