If Quality Level (QLC) is the most debated, then QLB should be known as the work horse of Quality Levels. For the most part, we try to elevate utilities to QLB in critical areas where key construction activities will take place.
Quality Level ‘C’ (QLB) is commonly defined as using geophysical methods to designate the horizontal alignment of undergound utilities. Common geophysical techniques may include pipe and cable locators (PCL’s), ground penetrating radar, sondes, and more. Ideally, we have all utilities (with the expectation of sewers) to QLB in critical areas. Once we have QLB data, we can then pick required test holes (QLA) based on the QLB information.
The SUE technicians using the geophysical methods will paint or flag the designated location in the field so that they can be surveyed and tied into project control information. It is up to the field technicians and the engineer responsible for the SUE investigation to determine which geophysical techniques are most suitable to provide a reliable understanding of the horizontal location. They may be choosing between PCL’s or GPR or even choosing between the appropriate frequencies and methodologies to use for the PCL’s. The engineer must also look at all the background information like records, and configuration of adjacent utilities to finalize the location that will be shown on the drawings.
There are several situations where the geophysics can “lie”. Bends in pipes are one of those situations. The electromagnetic signal will not bend as sharply as the actual utility bends, therefore a sharp 90 degree bend, may show up as a gentle sweeping bend. It is up to the engineer to review that information and adjust the data to best reflect all the information available when depicting the line at QLB.
There are no major changes to QLB coming in the new ASCE 38-21 standard.