standard penetration resistance
standard penetration resistance, Proctor penetration resistance
1. The unit load required to maintain a constant rate of penetration of a probe into a soil.
2. The unit load required to produce a specified penetration into a soil, at a specified rate of penetration; for a Proctor needle, the specified penetration is 2.5 in. (6.35 cm) and the penetration rate is 0.5 in. (1.27 cm) per second.