3.7 Utility Analysis & Infrastructure Capacity
Key Takeaways
- Utility analysis answers four separate questions — availability, capacity, location, and depth — and a utility that exists at the frontage may still be unable to serve the project.
- Utility locate marking follows the APWA uniform color code: red for electric, yellow for gas and petroleum, orange for communications, blue for potable water, green for sewer and drain, purple for reclaimed water, and white for proposed excavation.
- A one-call locate marks only the approximate horizontal position of a utility and never its depth, so vertical conflicts require potholing or subsurface utility engineering.
- Gravity sewer and storm systems cannot be rerouted around a conflict without changing invert elevations, so they control the layout while pressure utilities bend around them.
- Tree planting within a utility easement is generally prohibited or limited to a restricted species list, because maintenance excavation by the utility owner is an unrestricted right.
1. Four Questions, Not One
Perform Utility Analysis appears in the Physical Analysis content area with the parenthetical "capacity, availability, proximity, suitability." The exam treats these as four distinct questions, and confusing them is the most common error.
| Question | What it asks | How it is answered |
|---|---|---|
| Availability | Does the service physically exist within reach? | Utility atlas maps, will-serve inquiry |
| Capacity | Can the existing system accept this project's demand? | Written will-serve letter or capacity study from the utility owner |
| Proximity / location | Exactly where is it horizontally and vertically? | Record drawings, one-call locates, potholing, subsurface utility engineering |
| Suitability | Is it appropriate for the intended use? | Pressure, flow, voltage, pipe material and age, treatment quality |
A 12-inch water main at the frontage satisfies availability. It does not establish that the system can deliver the required fire flow at the required residual pressure, which is a capacity question answered only by the water purveyor. A project can be denied a certificate of occupancy for failing fire flow on a street with an apparently adequate main.
2. The Utility Families and What Each Constrains
Gravity systems (sanitary sewer, storm sewer). These are the controlling utilities. Their inverts are fixed by downstream elevations, so they cannot be raised, lowered, or rerouted without re-engineering the entire run. A gravity main sets the minimum achievable finished floor elevation for anything that must drain to it. Where a gravity connection is impossible, a pump station or grinder pump is required, which adds permanent operating cost and a failure mode.
Pressure systems (water, gas, force main). These can bend horizontally and vertically around obstacles, so in a conflict the pressure line moves and the gravity line stays.
Dry utilities (electric, communications, cable). Delivered overhead or in a duct bank. Duct banks are shallow, wide, and unforgiving of tree roots. Overhead lines impose canopy clearance limits that dictate species selection along the entire frontage.
Reclaimed and non-potable water. Marked purple, physically separated from potable systems, and subject to backflow prevention and signage requirements.
3. Locating: Color Code, Limits, and Potholing
The APWA uniform color code for temporary marking of underground facilities is directly testable:
| Color | Facility |
|---|---|
| Red | Electric power lines, cables, conduit, lighting cables |
| Yellow | Gas, oil, steam, petroleum, gaseous materials |
| Orange | Communications, alarm and signal lines, cable, conduit |
| Blue | Potable water |
| Purple | Reclaimed water, irrigation, slurry lines |
| Green | Sewers and drain lines |
| Pink | Temporary survey markings |
| White | Proposed excavation |
Critical limitation: a one-call ticket produces marks showing the approximate horizontal location of a facility — typically within a tolerance zone of 18 inches on each side of the marked centerline. It does not indicate depth. Any design that depends on vertical clearance requires potholing (vacuum excavation test holes) or a Subsurface Utility Engineering investigation delivered at an ASCE 38 quality level.
4. Separation, Cover, and Conflict Resolution
Standard requirements that recur on the exam:
- Water and sanitary sewer horizontal separation: commonly 10 feet edge to edge, with water above sewer at crossings and a minimum 18 inches of vertical separation.
- Minimum cover: water mains are typically buried below the local frost depth; storm and sanitary depth is set by the required slope and downstream invert, not by a fixed cover.
- Utility easement width: typically 10 to 20 feet for a single line, wider for a duct bank or multiple lines.
Resolution hierarchy in a conflict:
- Move the proposed element (tree, wall, structure) if it is not fixed by program.
- Move the pressure utility, which can deviate.
- Move the gravity utility, which requires re-establishing inverts and slope.
- Relocate the existing utility, which requires a relocation agreement, the utility owner's design, and usually the project's money.
Relocations are schedule risk, not just cost risk: utility owners work on their own design and construction calendars, and a relocation requested late can delay a project by a full season.
5. Utilities and Trees: The Recurring Conflict
Planting within a recorded utility easement is generally prohibited or restricted, because the easement holder has an unrestricted right to excavate for maintenance at any time. A tree planted over a duct bank is not protected; the utility may remove it without compensation and without notice.
Design responses:
- Horizontal offset. Shift the tree row outside the easement, even at the cost of a less regular streetscape rhythm.
- Species substitution. Where planting inside an easement is permitted, the utility's approved list typically limits mature height under overhead lines (commonly 25 feet or less) and root aggressiveness over buried lines.
- Structural soil or suspended pavement. Provides rootable volume outside the utility corridor.
- Root barriers. Used at the interface, though they redirect rather than eliminate root growth.
Overhead distribution lines impose a canopy clearance envelope along the entire corridor. Specifying a large-maturing shade tree beneath a distribution line guarantees a lifetime of topping, which destroys the tree's form and the streetscape's purpose.
6. Exam Traps & Pitfalls
- Treating availability as capacity. Only a will-serve letter or capacity study from the owner answers capacity.
- Relying on a one-call locate for depth. Locates are horizontal only; pothole for vertical conflicts.
- Moving the gravity line to solve a conflict. Move the proposed element or the pressure line first.
- Planting shade trees under overhead distribution. Use the utility's approved low-maturing species list.
- Assuming an easement permits planting. The easement holder's excavation right supersedes the planting.
A landscape architect confirms that a 12-inch potable water main runs along the frontage of a proposed 180-unit residential development. What does this confirmation establish, and what remains unresolved?
Under the APWA uniform color code for temporary marking of underground facilities, what do orange and purple markings respectively indicate?
A proposed specimen shade tree location conflicts with an existing 8-inch gravity sanitary sewer main and a parallel 6-inch pressurized water line. Neither the tree location nor either utility is fixed by an overriding program requirement. What is the correct conflict resolution sequence?
A landscape architect receives one-call locate markings showing an electric duct bank crossing a proposed plaza where a 36-inch-deep tree pit is planned. What additional investigation is required before the tree pit detail can be finalized?