NTU-CEE Distinguished Seminar Series: Professor LU Ning

Topic

1. Unified Effective Stress Principle for Soil
Time: 11:00 am  - 12:00 PM

2. General Soil Phase Relationships: Errors in Conventional Formulations and Calculations
Time: 2:00 pm to 3:00 pm

Host

Professor Chu Jian

About the Speaker

Professor Ning Lu has been working on fundamental concepts of soil water potential and effective stress in the past two decades and made some breakthroughs. He has unified soil’s effective stress under both saturated and unsaturated conditions, moving beyond the classical Terzaghi’s and Bishop’s effective stress representations. For his seminal work on unification of effective stress, he received ASCE Norman Medal twice; first in 2007 for conceptualization of suction stress, and second in 2022 for formulation of a practical closed form effective stress equation. He further defines a general thermodynamics-based pore water pressure in soil under both saturated and unsaturated conditions. He recently received ASCE’s Karl Terzaghi Award for seminal contributions to the understanding of the fundamental mechanisms governing the behavior of unsaturated soils and their implications in slope stability and other geotechnical problems.

In the past two decades, he has formulated a new paradigm for slope stability under variably saturated conditions, which has been bestowed by ASCE for its 2017 R. B. Peck Award, and M. A. Biot Medal. He is a fellow of ASCE, Geological Society of America, and Engineering Mechanics Institute. He has published two widely used textbooks: Unsaturated Soil Mechanics (Lu and Likos, 2004, John Wiley and Sons), and Hillslope Hydrology and Stability (Lu and Godt, 2013, Cambridge University Press).

About the Seminar

1. Unified Effective Stress Principle for Soil

Since the early 2000s, suction stress has been conceptualized as a unitary way to quantify effective stress in soil, i.e., effective stress equals to total stress minus suction stress. Suction stress is the part of effective stress purely due to soil-water interaction. When soil is saturated, suction stress is the pore water pressure, whereas when soil is unsaturated, suction stress is a characteristic function of soil called the suction stress characteristic curve (SSCC). Two physicochemical soil-water retention mechanisms are responsible for the SSCC: capillarity and adsorption.

These two mechanisms are explicitly considered to develop a closed-form equation for the SSCC and effective stress. The SSCC data from the literature for a variety of soils ranging from clean sand to silty and clayey soils are used to validate the equation, indicating that the equation can well represent the data. Additional validation is achieved using experimental data of the apparent elastic modulus and the SSCC to predict the soil shrinkage curves. The equation can be reduced to Lu et al.’s previous closed-form equation for the SSCC when capillarity dominates soil-water retention, can be reduced to the Bishop’s effective stress equation when capillarity is the sole soil-water retention mechanism, and can be reduced to the Terzaghi’s classical effective stress equation when soil is saturated. Suction stress-based effective stress equation provides a cornerstone for effectively describing soil’s strength and deformation under all saturation conditions.

2. General Soil Phase Relationships: Errors in Conventional Formulations and Calculations

The definitions of the conventional basic soil physical properties of void ratio, saturation, water content and soil unit weights are established based on the assumption that the volumes of three soil phases, i.e., solid, water, and air, do not depend on each other. However, soil phases do physically interact each other in the presence of water, leading to possibly significant volume variation.

Two physical mechanisms are responsible for volume variations: soil water unit weight due to water adsorption, and soil void due to soil skeleton swelling or shrinking. The practical importance of the influence of soil-water interaction on basic soil physical properties are examined through variable-volume phase formulation and experimental data illustration for consolidation and soil shrinkage curves of various silty, non-swelling, and swelling clays. It is shown that for the conventional saturated consolidation tests, errors in calculating void ratio and saturated soil unit weight by the conventional phase relationships can be significant; up to 0.2 in void ratio (48% in relative error) and 3.5 kN/m3 (16% in relative error) in saturated soil unit weight for swelling clay. For soil shrinkage tests, errors in calculating moisture ratio, saturation, and soil shrinkage curve by the conventional phase relationships are also significant; up to 0.18 in moisture ratio (53% in relative error), and 20% (53% in relative error) in saturation for swelling clay.

Examples are provided for how to calculate basic soil physical properties with basic information of liquid limit or specific surface area. It is concluded that for fine-grained soils, it is practically necessary to use general variable-volume phase relationships to define basic soil properties in lieu of the conventional phase relationships.

Registration

• Click here to register for Topic 1 (registration close on 29 Feb 2024, 11:00am)
• Click here to register for Topic 2 (registration close on 29 Feb 2024, 2:00pm)