Second Canadian Geotechnical Colloquium: Appropriate concepts and technology for unsaturated soils

1979 ◽  
Vol 16 (1) ◽  
pp. 121-139 ◽  
Author(s):  
D. G. Fredlund
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Three Dimensional ◽  
Earth Pressure ◽  
Saturated Soil ◽  
Engineering Practice ◽  
The Matrix ◽  
Matrix Suction

A practical science has not been fully developed for unsaturated soils for two main reasons. First, there has been the lack of an appropriate science with a theoretical base. Second, there has been the lack of an appropriate technology to render engineering practice financially viable.This paper presents concepts that can be used to develop an appropriate engineering practice for unsaturated soils. The nature of an unsaturated soil is first described along with the accompanying stress conditions. The basic equations related to mechanical properties are then proposed. These are applied to practical problems such as earth pressure, limiting equilibrium, and volume change.An attempt is made to demonstrate the manner in which saturated soil mechanics must be extended when a soil is unsaturated. Two variables are required to describe the stress state of an unsaturated soil (e.g., (σ – ua) and (ua – uW). There is a smooth transition from the unsaturated case to the saturated case since the pore-air pressure becomes equal to the pore-water pressure as the degree of saturation approaches 100%. Therefore, the matrix suction (i.e., (ua – uW) goes to 0 and the pore-water pressure can be substituted for the pore-air pressure (i.e., (σ – uW)).The complete volumetric deformation of an unsaturated soil requires two three-dimensional constitutive surfaces. These converge to one two-dimensional relationship for a saturated soil. The shear strength for an unsaturated soil is a three-dimensional surface that reduces to the conventional Mohr–Coulomb envelope for a saturated soil.The manner of applying the volumetric deformation equations and the shear strength equation to practical problems is demonstrated. For earth pressure and limiting equilibrium problems, the unsaturated soil can be viewed as a saturated soil with an increased cohesion. The increase in cohesion is proportional to the matrix suction of the soil. For volume change problems it is necessary to have an indication of the relationship between the various soil moduli.There is a need for further experimental studies and case histories to substantiate the proposed concepts and theories.

scholarly journals Numerical Evaluation of The Nonlinear Behavior of Soil-Pile Interaction Under The Effect of Coupled Static-Dynamic Loads

2021 ◽  
Author(s):  
Duaa Al-Jeznawi ◽  
ISMACAHYADI Mohamed Jais ◽  
Bushra S. Albusoda
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Three Dimensional ◽  
Ground Surface ◽  
Frictional Resistance ◽  
Saturated Soil ◽  
Shaking Table ◽  
Physical Models ◽  
Soil Layers

Abstract Liquefaction of saturated soil layers is one of the most common causes of structural failure during earthquakes. Liquefaction occurs as a result of increasing pore water pressure, whereby the rise in water pressure occurs due to unexpected change in stress state under short-term loading, i.e., shaking during an earthquake. Thus, general failure occurs when the soil softens and eliminates its stiffness against the uplift pressure from the stability of the subsurface structure. In this case, the condition of soil strata is considered undrained because there is not enough time for the excess pore water pressure to dissipate when a sudden load is applied. To represent the non-linear characteristics of saturated sand under seismic motions in Kobe and Ali Algharbi earthquakes, the computational model was simulated using the UBCSAND model. The current study was carried out by adopting three-dimensional-based finite element models that were evaluated by shaking table tests of a single pile model erected in the saturated soil layers. The experimental data were utilized to estimate the liquefaction and seismicity of soil deposits. According to the results obtained from the physical models and simulations, this proposed model accurately simulates the liquefaction phenomenon and soil-pile response. However, there are some differences between the experiment and the computational analyses. Nonetheless, the results showed good agreement with the general trend in terms of deformation, acceleration, and liquefaction ratio. Moreover, the displacement of liquefied soil around the pile was captured by the directions of vectors generated by numerical analysis, which resembled a worldwide circular flow pattern. The results revealed that during the dynamic excitation, increased pore water pressure and subsequent liquefaction caused a significant reduction in pile frictional resistance. Despite this, positive frictional resistance was noticed through the loose sand layer (near the ground surface) until the soil softened completely. It is worth mentioning that the pile exhibited excessive settlement which may attribute to the considerable reduction, in the end, bearing forces which in turn mobilizing extra end resistance.

Use of thermal conductivity sensors to measure matric suction in the laboratory

1989 ◽  
Vol 26 (3) ◽  
pp. 491-498 ◽  
Author(s):  
Pamela Sattler ◽  
D. G. Fredlund
Keyword(s):  
Thermal Conductivity ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Test Procedure ◽  
Matric Suction ◽  
Engineering Practice ◽  
Equilibration Time ◽  
Overburden Pressure ◽  
Conductivity Sensor

The measurement of soil suction is pivotal to the application of soil mechanics principles in geotechnical engineering practice related to unsaturated soils. Volume change, shear strength, and seepage analyses all require an understanding of the matric suction in the soil. This note summarizes the use of thermal conductivity sensors to measure matric suction in the laboratory. The thermal conductivity sensor is described along with its mode of operation. A brief description is given of the procedure for calibrating thermal conductivity sensors using a pressure plate apparatus. The measurement of matric suction can be performed in the laboratory on Shelby tube samples. The laboratory measurements of matric suction can be adjusted for the effect of overburden pressure in the field. The required equilibration time for suction measurements is discussed along with details of the test procedure. The applications of the measured suction values to design are briefly discussed.Key words: matric suction, negative pore-water pressure, thermal conductivity sensor, laboratory, undisturbed samples.

scholarly journals Experimental Study on Response Law and Failure Process of Slopes in Fully Weathered Granites Under Precipitation Infiltration

2021 ◽  
Author(s):  
Fei Tan
Keyword(s):  
Experimental Study ◽  
Physical Simulation ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Failure Process ◽  
Similarity Criterion ◽  
Weathered Granite ◽  
The Matrix ◽  
Soil Flow ◽  
Matrix Suction

Abstract To investigate the response law and failure process of slopes in fully weathered granites under precipitation infiltration, a typical fully weathered granite slope is selected for sampling in Fengkai, Guangdong. The physical simulation experimental study of rainfall-induced landslide is conducted, in which Weber criterion is used as the similarity criterion for precipitation. The research results reveal that under precipitation infiltration, the fully weathered granite slope responds quickly. Further, the water content increases sharply, and the matrix suction quickly dissipates. After dissipation, the matrix suction transforms into pore water pressure, which accelerates the deformation of the slope. The wet peak has a large infiltration depth in the slope, and the acceleration of deep part is lower than that of the shallow part. Under the action of precipitation, the fully weathered granite model undergoes four stages of failure. Firstly, gullies and cracks appear. Secondly, cracks propagate and link up. Then, the soil on the slope surface swells and ruptures. Finally, the slope slides locally until the entire slope creeps, collapses, and transforms into a "soil flow." Based on the analysis of precipitation similarity, the landslide will be triggered in fully weathered granite slope by precipitation when the precipitation intensity comes up to 155 mm/d, and the landslide occurs at an accumulated precipitation of 304 mm. Overall, the results can provide a reliable theoretical basis and abundant experimental data for the prevention, monitoring, and forecasting of geological disasters in granitic areas.

The shear strength of unsaturated soils

1978 ◽  
Vol 15 (3) ◽  
pp. 313-321 ◽  
Author(s):  
D. G. Fredlund ◽  
N. R. Morgenstern ◽  
R. A. Widger
Keyword(s):  
Shear Strength ◽  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Research Literature ◽  
Test Results ◽  
State Variables ◽  
Limited Data ◽  
Laboratory Test Results

The shear strength of an unsaturated soil is written in terms of two independent stress state variables. One form of the shear strength equation is[Formula: see text]The transition from a saturated soil to an unsaturated soil is readily visible. A second form of the shear strength equation is[Formula: see text]Here the independent roles of changes in total stress σ and changes in pore-water pressure uw are easily visualized.Published research literature provides limited data. However, the data substantiate that the shear strength can be described by a planar surface of the forms proposed. A procedure is also outlined to evaluate the pertinent shear strength parameters from laboratory test results.

scholarly journals The Effects of Rainfall, Soil Type and Slope on the Processes and Mechanisms of Rainfall-Induced Shallow Landslides

2021 ◽  
Vol 11 (24) ◽  
pp. 11652
Author(s):  
Yan Liu ◽  
Zhiyuan Deng ◽  
Xiekang Wang
Keyword(s):  
Water Content ◽  
Sediment Yield ◽  
Soil Type ◽  
Rainfall Intensity ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Shallow Landslides ◽  
Total Stress ◽  
The Matrix ◽  
Matrix Suction

Landslides are a serious geohazard worldwide, causing many casualties and considerable economic losses every year. Rainfall-induced shallow landslides commonly occur in mountainous regions. Many factors affect an area’s susceptibility, such as rainfall, the soil, and the slope. In this paper, the effects of rainfall intensity, rainfall pattern, slope gradient, and soil type on landslide susceptibility are studied. Variables including soil volumetric water content, matrix suction, pore water pressure, and the total stress throughout the rainfall were measured. The results show that, under the experimental conditions of this paper, no landslides occurred on a 5° slope. On a 15° slope, when the rainfall intensity was equal to or less than 80 mm/h with a 1 h duration, landslides also did not happen. With a rainfall intensity of 120 mm/h, the rainfall pattern in which the intensity gradually diminishes could not induce landslides. Compared with fine soils, coarser soils with gravels were found to be prone to landslides. As the volumetric water content rose, the matrix suction declined from the time that the level of infiltration reached the position of the matrix. The pore water pressure and the total stress both changed drastically either immediately before or after the landslide. In addition, the sediment yield depended on the above factors. Steeper slopes, stronger rainfall, and coarser soils were all found to increase the amount of sediment yield.

scholarly journals Evaluation on slope stability of unsaturated soils

2000 ◽  
Vol 22 ◽  
Author(s):  
L. J. Wang ◽  
M. Zhang
Keyword(s):  
Unsaturated Soil ◽  
Unsaturated Soils ◽  
Pore Water Pressure ◽  
Limit Equilibrium ◽  
Water Pressure ◽  
Matric Suction ◽  
Groundwater Table ◽  
Degree Of Saturation ◽  
Engineering Properties ◽  
Soil Slopes

In drought-prone and semiarid areas, the groundwater table is deep and the soils are at an unsaturated state because of evaporation or transpiration. The negative pore water pressure or matric suction (ua-uw) is an important property of unsaturated soils that are situated above the groundwater table. In the conditions of rainfall, ground seepage, or drainpipe leakage, the matric suction will decrease with the increase of the degree of saturation, and the soils will lose their part of shear strength, which is the main reason why many unsaturated soil slopes become unstable. This paper discusses the engineering properties of unsaturated soils. Following the limit equilibrium principle, the unsaturated soil slopes are evaluated by applying the slice method.

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

2018 ◽  
Vol 55 (7) ◽  
pp. 909-927 ◽  
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.

scholarly journals The Hydromechanical Interplay in the Simplified Three-Dimensional Limit Equilibrium Analyses of Unsaturated Slope Stability

Geosciences ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 73
Author(s):  
Panagiotis Sitarenios ◽  
Francesca Casini
Keyword(s):  
Analytical Solution ◽  
Slope Stability ◽  
Slope Failure ◽  
Failure Modes ◽  
Pore Water Pressure ◽  
Limit Equilibrium ◽  
Water Pressure ◽  
Three Dimensional ◽  
Stability Limit ◽  
Smooth Transition

This paper presents a three-dimensional slope stability limit equilibrium solution for translational planar failure modes. The proposed solution uses Bishop’s average skeleton stress combined with the Mohr–Coulomb failure criterion to describe soil strength evolution under unsaturated conditions while its formulation ensures a natural and smooth transition from the unsaturated to the saturated regime and vice versa. The proposed analytical solution is evaluated by comparing its predictions with the results of the Ruedlingen slope failure experiment. The comparison suggests that, despite its relative simplicity, the analytical solution can capture the experimentally observed behaviour well and highlights the importance of considering lateral resistance together with a realistic interplay between mechanical parameters (cohesion) and hydraulic (pore water pressure) conditions.

Three‐Dimensional Numerical Investigation of Pore Water Pressure and Deformation of Pumped Aquifer Systems

Ground Water ◽  
2019 ◽  
Vol 58 (2) ◽  
pp. 278-290 ◽  
Author(s):  
Yun Zhang ◽  
Xuexin Yan ◽  
Tianliang Yang ◽  
Jichun Wu ◽  
Jianzhong Wu
Keyword(s):  
Numerical Investigation ◽  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Three Dimensional ◽  
Aquifer Systems

Analyses of groundwater flow and plant evapotranspiration in a vegetated soil slope

2013 ◽  
Vol 50 (12) ◽  
pp. 1204-1218 ◽  
Author(s):  
A.K. Leung ◽  
C.W.W. Ng
Keyword(s):  
Groundwater Flow ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Three Dimensional ◽  
Root Water Uptake ◽  
Dry Season ◽  
Climatic Data ◽  
Wet Season ◽  
Soil Slopes ◽  
Transient Seepage

Understanding seasonal hydrogeological responses of vegetated soil slopes is vital to slope stability because pore-water pressure (PWP) varies from positive values upon rainfall in wet seasons to negative values upon plant evapotranspiration (ET) in dry seasons. There are, however, few case histories that report seasonal performance of vegetated soil slopes. In this study, a vegetated slope situated in Hong Kong was instrumented to analyse (i) groundwater flow during rainfall in the wet season and (ii) effects of plant ET on PWP in the dry season. Two- and three-dimensional anisotropic transient seepage analyses are conducted to identify groundwater flow mechanism(s) during a heavy rainstorm. Through water and energy balance calculations, measured plant-induced suction is interpreted with plant characteristic and climatic data. During the rainstorm, substantial recharge of the groundwater table was recorded, likely due to preferential water flow along relict joints and three-dimensional cross-slope groundwater flow. During the dry season, the peak suction induced by plant ET is up to 200 kPa and the depth of influence is shallower than 200% of the root depth. For the range of suctions monitored, root-water uptake is revealed to have been restricted by suction not very significantly and was driven mainly by the climatic variation.

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