What are the applications and challenges of unsaturated soil mechanics in geotechnical engineering practice?

1 Application 1: Slope Stability

One of the most common applications of unsaturated soil mechanics is the analysis and design of slopes, especially in arid and semi-arid regions, where rainfall and evaporation can cause significant changes in the soil moisture content and suction. Suction is the negative pressure that exists in the pore water due to the presence of air, and it can enhance the shear strength and stiffness of unsaturated soils. Therefore, the stability of slopes depends not only on the soil weight, cohesion, and friction angle, but also on the suction distribution and variation. Unsaturated soil mechanics can help geotechnical engineers to estimate the factor of safety and the critical slip surface of slopes, taking into account the effects of suction and its changes due to climatic factors or human activities.

2 Application 2: Retaining Walls

Another application of unsaturated soil mechanics is the design and construction of retaining walls, which are structures used to support vertical or near-vertical cuts in the ground. Retaining walls are subject to lateral earth pressures from the backfill soil, which depend on the soil properties and the deformation of the wall. For unsaturated soils, the lateral earth pressure is also influenced by the suction, which can reduce or increase the pressure depending on the direction of the wall movement. Unsaturated soil mechanics can help geotechnical engineers to determine the appropriate wall type, dimensions, and reinforcement, considering the effects of suction and its changes due to wetting and drying cycles or seepage.

3 Application 3: Foundations

A third application of unsaturated soil mechanics is the design and performance of foundations, which are structures that transfer the load from a superstructure to the underlying soil. Foundations are subject to settlement, which is the vertical displacement of the soil due to the applied load. For unsaturated soils, the settlement is also affected by the suction, which can cause shrinkage or swelling of the soil depending on the moisture content and the stress state. Unsaturated soil mechanics can help geotechnical engineers to evaluate the bearing capacity and the settlement of foundations, considering the effects of suction and its changes due to loading and unloading or environmental factors.

4 Challenge 1: Theoretical Models

One of the main challenges of unsaturated soil mechanics is the development and validation of theoretical models that can capture the complex behavior and interactions of unsaturated soils. Unlike saturated soils, which can be described by a single stress variable (the total stress), unsaturated soils require two stress variables (the net stress and the suction) to characterize their state. Moreover, unsaturated soils exhibit nonlinear and hysteretic responses to changes in stress and suction, which means that their behavior depends on their loading history and path. Therefore, unsaturated soil mechanics requires more advanced constitutive models, such as elasto-plastic, viscoplastic, or damage models, that can account for the effects of suction, hysteresis, and volume change on the stress-strain relationships of unsaturated soils.

5 Challenge 2: Experimental Methods

Another challenge of unsaturated soil mechanics is the development and application of experimental methods that can measure and control the relevant parameters and variables of unsaturated soils. Unlike saturated soils, which can be tested using conventional laboratory equipment, such as triaxial or direct shear devices, unsaturated soils require special devices, such as suction-controlled or osmotic devices, that can impose and maintain a desired suction level on the soil specimen. Moreover, unsaturated soils require more sophisticated measurement techniques, such as tensiometers, psychrometers, or dielectric sensors, that can monitor the suction and the moisture content of the soil during testing. Therefore, unsaturated soil mechanics requires more complex and expensive laboratory facilities, procedures, and instruments, that can provide reliable and accurate data on the properties and behavior of unsaturated soils.

6 Challenge 3: Field Conditions

A third challenge of unsaturated soil mechanics is the estimation and prediction of the field conditions that affect the performance and safety of geotechnical structures. Unlike saturated soils, which can be assumed to have a uniform and constant moisture content and suction, unsaturated soils are subject to spatial and temporal variations in their moisture content and suction due to natural or artificial factors, such as rainfall, evaporation, irrigation, drainage, or vegetation. Therefore, unsaturated soil mechanics requires more extensive and frequent field investigations, such as boreholes, samples, or in situ tests, that can provide information on the initial and boundary conditions of the soil. Moreover, unsaturated soil mechanics requires more sophisticated numerical models, such as finite element or finite difference models, that can simulate the coupled hydro-mechanical behavior of unsaturated soils under various loading and environmental scenarios.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?