November 18, 2016
Engineering efforts in the past decade have focused on fixing our nation’s infrastructure. Fixing infrastructure means more utilities in the ground, and working with underground utilities means risk and expense. Inaccurate mapping of buried utilities can lead to several problems during construction, including delays, change orders, redesigns, service disruptions, and injuries. These issues are particularly apparent with water infrastructure.
More thorough utility research during the design of drinking water, wastewater, and stormwater pipelines can reduce utility conflicts and project costs.
Resources
With an unlimited budget, we could pothole every utility in our way and get exact measurements. Since that isn’t realistic, we must find the best data we can and exercise some judgment, using all available resources.
Some common utility research resources include:
- Utility location services (e.g., Blue Stakes)
- As-built or record drawings
- Data from nearby projects
- Institutional knowledge from utility personnel
- Site visit and search for visible clues (boxes, manholes, valves, etc.)
- County surveyor websites (third-party drawings)
- Google Earth (Street View and historic imagery of utility markings and construction)
- Potholing (physical exposure of utility)
We must often use many or all of these resources to ensure that we capture telephone, gas, water, sewer, cable, and other utilities installed at different times by different entities. Data provided by others should be verified before design.
Quality Levels
The American Society of Civil Engineers developed standard guidelines (ASCE 38-02) for subsurface utility mapping with four cumulative quality levels:
- Quality Level D includes information from existing records, oral recollections, utility maps, and record drawings.
- Quality Level C adds information from above-ground surveys of manholes, water valves, and other visible utility features.
- Quality Level B adds information from field surveys using detection methods (tracer wires, pipe locators, etc.) and vertical measurements of inverts, pipe depths, and manhole or vault features.
- Quality Level A adds horizontal and vertical location of buried utilities by exposure at the specific crossing point to observe elevations, pipe material, pipe diameter, pavement thickness, soil type, and other properties.
The most cost-effective process is to research utilities to Quality Level B, prepare a basemap with the results, complete preliminary design, and then proceed to Quality Level A for major utilities that could make or break the design.
Benefits
The Federal Highway Administration, New York State Department of Transportation, and the Society of American Value Engineers have investigated the cost benefits of proper utility mapping. They discovered average savings of $3 to $10 for every $1 spent on utility mapping before or during design, which in some cases reduced overall project costs by 10% to 15%. In general, urban and suburban projects realize greater savings than rural projects since more utilities are located in the roads. Other benefits include time savings during design and construction, reduction of unexpected utility conflicts, and safer working conditions. The extra investment in utility mapping pays off.
Buried utilities come with uncertainty. Engineers can help the contractor and owner manage the risk by collecting as much utility data as possible, using all available resources, and exercising good engineering judgment when interpreting the data. The extra effort in utility mapping contributes to a better project for everyone involved.