Pseudo-N-value: From Geophysicists to Engineers

Abstract

Summary Engineers estimate hardness of the ground by the standard penetration test (SPT) and the result is presented as N-value. The multichannel analysis of surface waves (MASW) is known to be an effective geophysical method to estimate the hardness by S-wave velocity. It can provide far higher data density than SPT. S-wave velocity is a physical property close to related to shear modulus and Young’s modulus. But it is not commonly recognised by engineers as an indicator of the hardness of the ground. Many formulae were proposed to relate S-wave velocity to SPT N-value in the past. All the formulae are subject to errors. Accepting the error range, the pseudo-N-value was proposed in 2011 to estimate N-value from S-wave velocity with simple conversion parameters. While the best conversion should use a site specific formula derived from the actual measurement, the generic formula for pseudo-N-values may be used to approximate N-values. This presentation delivers a consideration of the source of the errors, validity and practical use of the pseudo-N-values in expressing the result of MASW survey. This facilitates engineers’ understanding of the distribution of ground hardness.