Limitations of Suction-Controlled Triaxial Tests in the Characterization of Unsaturated Soils


Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction-controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto-plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide-and-conquer approach in which an implicit assumption is used: soil behavior is stress-path independent. However, it is well-established that unsaturated soil behavior is elasto-plastic and stress-path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions.


Civil, Architectural and Environmental Engineering


This research was supported by the National Science Foundation of the United States (Project Number:1054532) and the '100 Gifted People Plan of Hubei Province, China'.

Keywords and Phrases

Atmospheric pressure; Constitutive models; Elastoplasticity; Soil mechanics; Soil testing; Soils; Stress analysis; Constitutive behaviors; Divide-and-conquer approach; Elasto-plastic; Elastoplastic behavior; State boundary surface; Unsaturated soils; Unsaturated soil mechanics; Yield curve; Unsaturated polymers; Atmospheric pressure; Porewater; Soil mechanics; Stress measurement; Suction; Triaxial test; Unsaturated medium; Loading/collapse

International Standard Serial Number (ISSN)

0363-9061; 1096-9853

Document Type

Article - Journal

Document Version


File Type





© 2016 John Wiley & Sons, All rights reserved.

Publication Date

01 Feb 2016