When drilled shaft capacity depends heavily on penetrating a specific hard layer, add a plan note instructing the contractor and field personnel of the penetration requirement. If no specific penetration into a hard layer is required, no plan note is necessary:

Hard founding layer at depth: When a hard founding layer is expected to be present more than three shaft diameters below the surface, specify a minimum penetration of one shaft diameter on the plans if the design load is reached at this location. Increase this minimum penetration if additional skin friction is required to fulfill the design requirements.
Typical notes on bridge layouts:
"Found drilled shafts a minimum of one shaft diameter into hard rock," or
"Found drilled shafts at the elevations (lengths) shown or deeper (longer) to obtain a minimum XX drilled shaft diameter penetration into hard rock," where XX is determined by the design.
The designer can use the control of elevation or length if elevations are not called out on the layout. Expand the words "hard rock" to distinguish the type of material anticipated. Although not a common practice, the first note allows a drilled shaft to be shortened if rock is encountered at higher than anticipated elevations, and it requires the shaft to be lengthened if rock is not encountered where expected.
Rock at surface: When rock is present at or near the surface, consider load-carrying capacity along with the stability of the superstructure on the foundation. For these shafts, a minimum shaft length of three shaft diameters is recommended. That is, a minimum three-diameter shaft length, not a three-diameter penetration into rock. The final length of the drilled shafts should be based on both axial and lateral loading (if required). If the potential scour extends down to the top of rock then the minimum embedment of the drilled shaft should be three shaft diameters or deeper to obtain the required axial and lateral capacity.

A typical note on bridge layouts reads, "Found drilled shafts at the elevation (length) shown or deeper (longer) as necessary to obtain a minimum of three shaft diameter penetration into hard rock."

The designer can use the control of elevation or length if elevations are not called out on the layout. Expand the words "hard rock" to distinguish the type of rock. This note does not allow a drilled shaft to be shortened from plan length, but it does require lengthening if rock is not encountered at the expected elevation.

Plan notes should be specific as to the type of material to be penetrated. If more than one material is likely to be encountered, it is acceptable to have multiple descriptions, such as “into dense sand, sandstone, and/or shale.” Avoid using vague terms such as “hard strata” or “founding material.” In stream or river environments, the channel flow line and estimated depth of scour should be considered in determining the final shaft length and necessary penetration.