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A laboratory investigation was conducted to assess the influence of density on the shear strength of unsaturated soil. To vary density, compacted specimens of an expansive clay soil were subjected to different stress histories prior to shear. Tests were performed at a constant water content. The Hvorslev "true friction-true cohesion" concept was used to normalize the effects of density differences on the shear strengths of both saturated and unsaturated specimens. It was determined that shear strengths of unsaturated soils were dependent on the stress state, density, and water content of specimens at failure. A modified Mohr-Coulomb failure model to predict the shear strength of unsaturated soils is proposed. The effect of suction is to increase the value of the cohesion intercept in this model. The actual measurement of soil suction is not required to apply the model.
"Although the triaxial compression test is presently the most widely used procedure for determining strength and stress-deformation properties of soils, there have been no books published on triaxial testing since the 1962 second edition of the landmark work The Measurement of Soil Properties in the Triaxial Test by Bishop and Henkel. It is apparent there is a need to document advances made in triaxial testing since publication of Bishop and Henkel's book and to examine the current state of the art in a forum devoted solely to triaxial testing. Because of increasing versatility brought about by recent developments in testing techniques and equipment, it is also important that the geotechnical profession be provided with an up-to-date awareness of potential uses for the triaxial test."--Overview.
Unsaturated soil mechanics is now increasingly recognized as an integral part of mainstream soil mechanics, and the importance and relevance of unsaturated soil mechanics for the broad field of geotechnical engineering no longer needs to be emphasized. The two volumes making up Unsaturated soils include papers from the 4th Asia Pacific Confere
Unsaturated materials comprise residua, collapsible and expansive naturally occurring soils, compacted soils and, more recently, residues of solid wastes. The engineering problems associated with unsaturated materials range from those related to conventional geotechnical works (e.g. foundations, pavements, slopes and excavations, retaining structures, earthdams, irrigation canals, tunnelling, compacted embankments) to those included in the environmental area (e.g. natural slope instability, erosion and subsidence processes, tailings, residues or solid waste disposal, contaminant transport, remediation of contaminant sites, engineered barriers for environmental protection, re-use of residues). This book, published in three separate volumes, comprises a selection of selected and invited papers presented at the Third International Conference on Unsaturated Soils – UNSAT ‘2002 – that took place in Recife, Brazil, form 10th to 13th March 2002. The book is of interest to consultants, researchers, practitioners, lecturers and students with a background in geotechnical engineering, environmental engineering and engineering geology.
Triaxial Testing of Soils explains how to carry out triaxial tests to demonstrate the effects of soil behaviour on engineering designs. An authoritative and comprehensive manual, it reflects current best practice and instrumentation.References are made throughout to easily accessible articles in the literature and the books focus is on how to obtain high quality experimental results.
This publication is an assemblage of selected papers that have been authored or co-authored by D.G. Fredlund. The substance of these papers documents the milestones of both the science of unsaturated soil mechanincs and the career of the author during his tenure as a faculty member in the Department of Civil Engineering at the University of Saskatchewan, Saskatoon, Canada.
Unsaturated Soils: Advances in Geo-Engineering comprises 136 contributions from leading international researchers and practitioners, presented at the First European Conference on Unsaturated Soils (Durham, UK, 2-4 July 2008). The papers report on the latest advances in geo-engineering aspects of unsaturated soils. It is the first collection to focu
The triaxial test has been extensively used to evaluate both saturated and unsaturated soil behaviors. The conventional triaxial test apparatus for saturated soils cannot be used to test unsaturated soils due to difficulties in soil volume and suction measurement. In 1961, a suction-controlled triaxial test apparatus was developed to investigate behavior of unsaturated soils. Since this development, the suction-controlled test has been widely used for unsaturated soil characterization. Most important concepts concerning unsaturated soil mechanics were developed based upon results from suction-controlled tests. However, the suction-controlled triaxial test on unsaturated soils, which is a drained test, is usually laborious, time-consuming, and costly, and may not be justifiable for routine engineering projects. The constant water content (undrained) test has been widely used to investigate saturated soil behaviors. However, for unsaturated soils, due to difficulties in direct, rapid, and reliable suction measurement, the constant water content test was rarely used for unsaturated soil behavior evaluation. In addition, accurate volume change measurement of unsaturated soils was a great challenge for researchers. Recently, the Modified State Surface Approach (MSSA) has been developed to calibrate unsaturated soil behaviors. According to MSSA, both results from suction-controlled and constant water content triaxial tests can be used for constitutive behavior calibration on unsaturated soils. In this study, a new triaxial test system was developed to investigate unsaturated soil behaviors through constant water content triaxial tests. To measure soil suction variation during testing, a new type of high-suction tensiometer was developed based on a commercial miniature pressure transducer. A 15 bar air-entry ceramic disc was used as the filter of the high-suction tensiometer. After saturation and calibration, this new type of high-suction tensiometer could be utilized for matric suction measurement on unsaturated soils with a maximum measurable suction up to 1100 kPa determined via a free evaporation test. To measure the volume change of unsaturated soils during triaxial testing, a photogrammetry-based method was developed by integrating photogrammetry, optical-ray tracing, and least-square estimation techniques. Through two validation tests on a stainless steel cylinder and a saturated sand specimen, the average point and total volume change measurement accuracy were determined to be approximately 0.065 mm and 0.05%, respectively. With this method, the conventional triaxial test apparatus for saturated soils can be used for triaxial testing on unsaturated soils without any modification. In addition to total volume change measurement, the newly developed photogrammetry-based method can also be used to investigate the deformation characteristics of soils during triaxial testing such as full-field deformation, volumetric strain non-uniformity, full-field strain distribution, and shear band evolution process. To evaluate the performance of the new triaxial testing system, a series of constant water content triaxial tests were carried out on unsaturated soils. New methods were proposed to characterize shear strength of the tested unsaturated soils. Also, an example was given to calibrate the constitutive behavior of an unsaturated soil based on results from the constant water content triaxial tests. Analysis results indicated that the proposed triaxial testing system is a cost effective and time efficient alternative to the suction-controlled triaxial testing system. In geotechnical and highway engineering, many projects involve unsaturated soils at shallow depths with low confining stresses (less than 100 kPa). To investigate the behavior of unsaturated soils at low confining stresses, the new triaxial testing system was simplified to a modified unconfined compression testing system. In this simplified system, negative air pressure (i.e., vacuum pressure) was used to provide the low confining stress for the triaxial tests. The high-suction tensiometers were used to monitor soil matrix suction variation during testing. A photogrammetric method was utilized for deformation measurements of unsaturated soils during triaxial testing. A series of undrained triaxial tests was also carried out to demonstrate the use of the modified unconfined compression testing system for unsaturated soil behavior evaluation under different confining stresses.