Laboratory testing on an unsaturated soil: equipment, procedures, and first experimental results

This paper describes two new apparatuses recently developed at the Università di Napoli Federico II (Italy) in order to test soils under unsaturated conditions. The related experimental procedures and the first results obtained on a dynamically compacted silty sand are also discussed. The devices mentioned are a Bishop and Wesley stress-path cell and a Wissa oedometer, modified to control matric suction and to measure all the stress-strain variables relevant to unsaturated soil mechanics. Specific experimental procedures were established to perform tests under general conditions and were carefully verified during several tests. Using the triaxial cell, isotropic and anisotropic compression stages were carried out under constant suction levels of 0, 100, 200, and 300 kPa. Furthermore, two deviator stages were performed following different stress paths and water drainage conditions. Using the oedometer, an additional suction level (400 kPa) was investigated during compression tests driven up to 5 MPa of vertical net stress (sigmav - ua). This research is a part of a major project in progress at the Dipartimento di Ingegneria Geotecnica of Naples; it is aimed at the experimental analysis of the behaviour of several dynamically compacted soils and at the numerical modelling of boundary problems related to earth structures.Key words: unsaturated soils, equipment layout, silty sand, matric suction.

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Experimental behaviour and modelling of an unsaturated compacted soil

Canadian Geotechnical Journal ◽

10.1139/t00-004 ◽

2000 ◽

Vol 37 (4) ◽

pp. 748-763 ◽

Cited By ~ 53

Author(s):

Celestino Rampino ◽

Claudio Mancuso ◽

Filippo Vinale

Keyword(s):

Unsaturated Soils ◽

Mechanical Response ◽

Stress Path ◽

Silty Sand ◽

Triaxial Tests ◽

Experimental Program ◽

State Variables ◽

Soil Response ◽

Optimum Water Content ◽

Triaxial Cell

This paper reports the experimental study and modelling of the mechanical response of a silty sand used in the core of the Metramo dam, Italy. Specimens were prepared by compacting the soil at optimum water content conditions using the modified Proctor technique. Tests were performed under suction-controlled conditions by a stress path triaxial cell and an oedometer. The experimental program consists of 23 tests carried out in the suction range of 0-400 kPa. The findings indicate the strong influence of suction on compressibility, stiffness, and shear strength. The mechanical properties of the soil improve with suction following an exponential law with decreasing gradient. Furthermore, the soil exhibited collapsible behaviour upon wetting even at low stress levels. Interesting results were also achieved in elastoplastic modelling as well. The results led to characterization of soil behaviour with reference to widely accepted modelling criteria for unsaturated soils, providing noteworthy suggestions about their applicability for granular materials with a non-negligible fine component. Finally, some remarks are made for the extension under unsaturated conditions of the "Nor sand" model for saturated granular soils. The proposed approach yields improved predictions of deviator soil response of the tested soil when Cambridge-type frameworks prove invalid.Key words: unsaturated soils, stress state variables, triaxial tests, oedometer tests, constitutive model.

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The 1999 R.M. Hardy Lecture: The implementation of unsaturated soil mechanics into geotechnical engineering

Canadian Geotechnical Journal ◽

10.1139/t00-026 ◽

2000 ◽

Vol 37 (5) ◽

pp. 963-986 ◽

Cited By ~ 68

Author(s):

Delwyn G Fredlund

Keyword(s):

Soil Water ◽

Unsaturated Soil ◽

Unsaturated Soils ◽

Soil Property ◽

Soil Mechanics ◽

Geotechnical Engineering ◽

Characteristic Curve ◽

Engineering Practice ◽

Soil Water Characteristic Curve ◽

Unsaturated Soil Mechanics

The implementation of unsaturated soil mechanics into geotechnical engineering practice requires that there be a paradigm shift from classical soil mechanics methodology. The primary drawback to implementation has been the excessive costs required to experimentally measure unsaturated soil properties. The use of the soil-water characteristic curve has been shown to be the key to the implementation of unsaturated soil mechanics. Numerous techniques have been proposed and studied for the assessment of the soil-water characteristic curves. These techniques range from direct laboratory measurement to indirect estimation from grain-size curves and knowledge-based database systems. The soil-water characteristic curve can then be used for the estimation of unsaturated soil property functions. Theoretically based techniques have been proposed for the estimation of soil property functions such as (i) coefficient of permeability, (ii) water storage modulus, and (iii) shear strength. Gradually these estimations are producing acceptable procedures for geotechnical engineering practices for unsaturated soils. The moisture flux ground surface boundary condition is likewise becoming a part of the solution of most problems involving unsaturated soils. The implementation process for unsaturated soils will still require years of collaboration between researchers and practicing geotechnical engineers.Key words: unsaturated soil mechanics, soil suction, unsaturated soil property functions, negative pore-water pressure, matric suction, soil-water characteristic curve.

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Thirty-Ninth Canadian Geotechnical Colloquium: Unsaturated soil mechanics — bridging the gap between research and practice

Canadian Geotechnical Journal ◽

10.1139/cgj-2016-0709 ◽

2018 ◽

Vol 55 (7) ◽

pp. 909-927 ◽

Cited By ~ 3

Author(s):

Greg A. Siemens

Keyword(s):

Vadose Zone ◽

Unsaturated Soil ◽

Unsaturated Zone ◽

Unsaturated Soils ◽

Pore Water Pressure ◽

Soil Mechanics ◽

Water Pressure ◽

Coupled Processes ◽

Near Surface ◽

Unsaturated Soil Mechanics

The majority of geoengineering applications occur in the unsaturated (vadose) zone, which is the near-surface region forming the connection between meteorological phenomena above and saturated ground below. The key characteristic of the unsaturated zone is that water is in tension or, put another way, pore-water pressure is negative. Moisture content, as well as most material properties, vary spatially and temporally in the unsaturated zone and coupled processes are common. In geoengineering applications in the vadose zone, unsaturated soils may be present during part or all of their design lives. The question is how or when to consider the unsaturated soils’ principles in an analysis or design. Although most geoengineering applications have an unsaturated component, use of unsaturated soil mechanics in practice lingers behind the prolific number of publications due the uncertain benefit of accounting for unsaturated effects, complexity, and conservativeness among other reasons. The focus of this colloquium is to continue bridging the gap by illustrating unsaturated soils’ principles using application-driven examples in the areas of capillarity as well as flow, strength, and deformation phenomena. As principles of unsaturated soils become more understood and demand increases for incorporating climate change effects in design, use of unsaturated soils’ principles in practice will continue to increase.

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