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Technical Services

Frequently Asked Questions on Underground Precast Concrete Utility Structures

  1. What type of underground utility construction are precast concrete utility vaults best suited for?
  2. What are the advantages of precast concrete construction over cast-in-place concrete construction?
  3. Can I use a cast-in-place design when specifying a precast concrete utility vault?
  4. What industry standards and specifications address precast concrete utility vaults?
  5. What engineering constraints must be taken into consideration when designing a utility vault?
  6. Are sealants required for precast concrete utility vaults?
  7. Do precast concrete manufactures have their own internal quality control program?
  8. What is the NPCA Plant Certification Program?
  9. What is the difference between a utility vault and a utility manhole?
  10. What is the difference between a utility manhole and a sanitary or stormwater manhole?
  11. How can I find a precast concrete utility vault producer in my area?
  12. How can I find an associate who supplies accessory products for precast concrete utility vaults?
  13. What are the Canadian Industry Standards for underground precast concrete utility structures?

1. Q. What type of underground utility construction are precast concrete utility vaults best suited for?
A. Precast concrete utility vaults provide an ideal solution for all underground utility projects, whether retrofitting old utilities or installing new ones. They have been utilized by the electrical, telecommunications, fiber-optics, water and gas industries for years.

2. Q. What are the advantages of precast concrete construction over cast-in-place concrete construction?
A. In general; reduced costs, time savings and superior quality and durability can all be expected when utilizing a precast concrete utility vault.

  • Because precast concrete utility vaults are produced in a controlled environment using higher strength concrete, the thickness of walls and area of steel can often be reduced when compared to cast-in-place designs.
  • Because precast concrete utility vaults are produced in a controlled environment, adverse weather conditions will not delay the construction or compromise the quality of the utility vault in comparison to cast-in-place construction.
  • Job site disorder and disruption to subcontractors and the local community can often be reduced, because excavation is not required until the structure is needed. There is no need to wait for the minimum curing time and the installation of accessory items when utilizing a value engineered precast concrete utility vault.

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3. Q. Can I use a cast-in-place design when specifying a precast concrete utility vault?
A. Cast-in-place designs are often over engineered to account for the unpredictable nature and quality of on site construction.

  • First, consult your local precast concrete manufacturer to review their standard utility vault designs.
  • If your project requires an odd shape or large vault, don’t worry. The flexibility of modular sectional and panel precast utility vaults make it possible to construct nearly any size and shape of utility vault.

4. Q. What industry standards and specifications address precast concrete utility vaults?
A. The following ASTM International standards address the design, fabrication, installation and inspection of precast concrete utility vaults.

  • ASTM C857, “Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures”
  • ASTM C858, “Standard Specification for Underground Precast Concrete Utility Structures”
  • ASTM C891, “Standard Practice for Installation of Underground Precast Concrete Utility Structures”
  • ASTM C1037, “Standard Practice for Inspection of Underground Precast Concrete Utility Structures”

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5. Q. What engineering constraints must be taken into consideration when designing a utility vault?
A. There are a number of constrains which must be taken into consideration when designing a utility vault, regardless of the material (i.e. concrete, fiberglass, steel):

  • Live Loads
    • Vehicles*
    • Pedestrians
  • Dead Loads
    • Roadbed
    • Walkways
    • Earth fill
    • Roof of the vault
    • Anything producing a static load above or near the vault
  • Handling Loads
  • Water Tables
  • Flotation
  • Soil properties
    • Dry
    • Saturated
    • Corrosive chemicals
  • Watertight Requirements
    • Joints
    • Penetrations

* Depending on the depth of the structure, live loads may need to be increased to account for impact

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6. Q. Are sealants required for precast concrete utility vaults?
A. Precast concrete utility vaults are fabricated utilizing a low water-cementitious ratio, creating a dense concrete with a low permeability. Therefore, concrete sealants are often not required, unless harsh conditions warrant the use of such protective coatings.

Concrete sealants and/or gaskets will be required for the joints of sectional precast concrete utility vaults which are required to be watertight.

7. Q. Do precast concrete manufactures have their own internal quality control program?
A. A majority of precast concrete manufactures have an internal plant specific quality control program. In addition, a number of precast concrete manufacturers are enrolled in third-party quality control programs such as the NPCA Plant Certification Program.

8. Q. What is the NPCA Plant Certification Program?
A. The NPCA Plant Certification Program is an exciting way for precast concrete plants to demonstrate their commitment to production of high-quality products. The basis of this voluntary certification program is the Quality Control Manual for Precast Plants published by NPCA. The program covers all aspects of a precast operation necessary for production of high-quality precast concrete products and quality assurance. Compliance with the Quality Control Manual is verified with inspections of the plant on an approximately annual basis. The program certifies that plants meeting the requirements set forth in the NPCA Quality Control Manual are capable of producing high-quality products. The program does not certify products themselves.

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9. Q. What is the difference between a utility vault and a utility manhole?
A. A utility vault houses or contains utilities and/or accompanying equipment, while a utility manhole provides access to the vault and/or utilities located underground.

10. Q. What is the difference between a utility manhole and a sanitary or stormwater manhole?
A. In regards to the structure its self, typically there is no difference. The only difference is in the application of the manhole section. One provides access to underground utilities while the other is used in the conveyance of sanitary waste or stormwater.

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11. Q. How can I find a precast concrete utility vault producer in my area?
A. Click here for the NPCA Producer Search, where you can search for a particular product. You can refine your search by company, state, and/or certified plants.

12. Q. How can I find an associate who supplies accessory products for precast concrete utility vaults?
A. Click here for the NPCA Associate Search where you can search for a particular accessory product.

13. Q. What are the Canadian Industry Standards for underground precast concrete utility structures?
A. The following CSA Standards address the fabrication, design and inspection of utility structures:

  • CSA 23.3, "Design of Concrete Structures"
  • CSA 23.4, "Precast Concrete Materials and Construction"

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