Underground Service Alert (USA), accessed by dialing 811 nationwide, is the system that notifies utility owners when excavation is planned near their facilities. It is the first step in damage prevention, and it is legally required before any excavation. But USA marks are approximate — they indicate the general location of underground utilities, not the exact position. Potholing is the verification step that determines the precise horizontal and vertical location of each utility before you start digging.

How USA / 811 Works

1. The excavator (or their contractor) calls 811 or submits a request online at least 2 working days before excavation begins (the minimum notice varies by state; California requires 2 working days).
2. USA notifies all member utility owners that have facilities in the area — PG&E (gas and electric), the telephone company, cable TV, water district, sewer district, and any other utilities with underground facilities.
3. Each utility owner sends a locator to the site to mark the approximate location of their facilities using paint or flags. The standard color code is:
   • Red = electric
   • Yellow = gas, oil, steam
   • Orange = communications (phone, cable, fiber)
   • Blue = water
   • Green = sewer
   • Purple = reclaimed water
   • White = proposed excavation (marked by the excavator)
4. The excavator verifies the marks and proceeds with excavation, maintaining the required clearances from the marked utilities.

The Accuracy Problem

USA locators use electromagnetic locating equipment that traces the signal along a metallic pipe or cable. The accuracy of this method depends on the utility type, depth, and surrounding conditions:

• Metallic utilities (steel gas mains, copper phone cables, metallic water mains) can be traced directly. Typical accuracy: plus or minus 12 to 24 inches horizontally. Depth accuracy is less reliable.
• Non-metallic utilities (PVC water and sewer pipes, HDPE gas pipes, fiber optic conduit) cannot be directly traced by electromagnetic methods. If a tracer wire was installed with the utility, the wire can be traced. If no tracer wire exists, the utility may not be marked at all, or the mark may be based on record drawings that are inaccurate.
• Congested areas with multiple utilities in close proximity can produce signal interference, reducing the accuracy of all marks.

Potholing: The Verification Step

Potholing (also called daylighting, soft-dig, or vacuum excavation) is the process of exposing underground utilities at specific points to verify their exact location, depth, size, and material. It is done before construction begins, typically during the design phase or early in the construction phase.

Methods

Vacuum excavation (preferred): A vacuum truck uses high-pressure air or water to loosen the soil around the utility, and a powerful vacuum extracts the soil into a debris tank on the truck. The utility is exposed without mechanical contact, virtually eliminating the risk of damage. Air vacuum is preferred for areas near fiber optic cables and gas lines because water can damage some utility types.

Hand excavation: Digging with hand tools (shovel, trowel) within the tolerance zone. Slower than vacuum excavation but does not require specialized equipment. Acceptable for shallow utilities in soft soils.

Mechanical excavation with hand finish: A backhoe or excavator removes soil to within 24 inches of the expected utility location, then hand tools or vacuum excavation exposes the utility for the last 24 inches. This is the standard practice for large-scale potholing operations.

When Potholing Is Required

• Design phase: When the civil engineer needs to verify existing utility locations for design purposes — setting proposed pipe alignments, determining crossing clearances, and identifying conflicts. This is called SUE (Subsurface Utility Engineering) and is categorized by ASCE 38 quality levels (A = potholed, B = surface-located, C = surveyed from records, D = record search only).
• Pre-construction: Before any excavation within the tolerance zone of marked utilities. This is a legal requirement in most states.
• Design-build projects: Where construction begins before the full design is complete and utility conflicts must be identified and resolved in real-time.

What You Learn

A pothole reveals:

• The exact horizontal position of the utility (to within 1 inch)
• The depth from finish grade to the top of the utility
• The pipe or cable size and material
• The condition of the utility (corroded, damaged, encased in concrete, etc.)
• Whether there are multiple utilities in the same trench (common for joint trench installations)

This information is recorded on the survey and transferred to the civil plans as a "verified utility" with a quality level of A. It is far more reliable than record drawings, which may be decades old and inaccurate.

Cost and Schedule

Vacuum excavation potholing typically costs $500 to $1,500 per pothole, including mobilization, excavation, survey of the exposed utility, and backfill. A typical site development project might require 10 to 30 potholes at critical crossing and conflict points, totaling $5,000 to $45,000.

This cost is small compared to the cost of hitting a utility during construction. A single gas main strike can result in evacuation, fire department response, emergency repair by the gas company, a stop-work order, and damages that easily exceed $50,000 to $100,000. A fiber optic cable strike can result in service interruption penalties of $10,000 or more per hour.

Pothole early and often. It is the cheapest insurance on a construction project.