Update on a Normalized Soil Parameter Procedure (SP-SPW Method) for Evaluating In-Situ Undrained Shear Strength
Authors: Quiros Gerardo W. , Peters Patricia M. , Gan Kuat C. 
Published: April 2023

The authors provide an update on the normalized soil parameter (NSP) procedure for in-situ undrained shear strength evaluation in clay soils known as the SP (Strength- Pressure) - SPW (Strength- Pressure-Water) method. A modification is recommended for the SPW aspect of the method that will improve results for undrained shear strength interpretation. The SP-SPW method was initially developed in the year 2000 from a database of 172 CKoU direct simple shear tests and has proven to be very useful to practitioners in undrained shear strength interpretation over the past 23 years. The current investigators now have a database of over 2500 CKoUDSS tests. Analyses of this larger database of laboratory tests has led them to modify the SPW aspect of the shear strength interpretation method to yield more consistent and reliable results of in-situ undrained shear strength. Additionally, the current authors will present lessons gleaned from employing this NSP procedure over the past 23 years in a variety of clay soil types and stress histories. Analysis of the large database of CKoUDSS tests requires that practitioners keep in mind important aspects of soil behavior regarding consolidation pressure, particularly at the lower pressure range and at pressures exceeding a relatively high pressure threshold. Additionally, the recommended modification to the SPW aspect of shear strength evaluation leads to more consistent results. In the original 2000 paper, examples of application of the SP-SPW method were limited to a normally consolidated clay site and a second site that was normally consolidated to lightly overconsolidated. In this paper, the authors present examples of application in the following clay profiles: (1) very underconsolidated; (2) normally consolidated to lightly overconsolidated; (3) lightly to heavily overconsolidated; and (4) highly plastic, lightly overconsolidated due to aging. The approach presented here has proven to be priceless in assisting the practitioner in evaluating in-situ undrained shear strength, particularly when standard laboratory strength tests yield disturbed strength measurements. The examples of application of the method in the variety of soil profiles presented in the paper will demonstrate the reasonableness of the strength values obtained and allow the practitioner to avoid being unnecessarily conservative in evaluating in-situ undrained shear strength.

When the SP-SPW method was first proposed almost 23 years ago (Quiros et al, 2000), the authors noted the following significant findings:

No reliable cu/σ’vc – Ip correlation was discernible for the database of soils studied. Consolidation pressure clearly has a profound effect on the ratio and should be considered when planning SHANSEP-type laboratory programs as well as when evaluating test results. A remarkably good correlation was found to exist between laboratory shear strength, cu and consolidation pressure, σ’vc. The SPW method, which introduced water content into the correlation between shear strength and consolidation pressure, proved to be a useful tool for evaluating in-situ undrained shear strength.

Subsequently, this method has been employed in hundreds of geotechnical site investigations, and the results have corroborated the forementioned conclusions of the original study. In following sections of this paper, we will briefly discuss the SP- SPW method, present the updated database, discuss modification to the SPW aspect of the procedure, and then demonstrate results of the method at four different sites.