1、GEOTECHNICAL SPECIAL PUBLICATION NO. 300 PANAM UNSATURATED SOILS 2017 PLENARY PAPERS SELECTED PAPERS FROM SESSIONS OF THE SECOND PAN-AMERICAN CONFERENCE ON UNSATURATED SOILS November 1215, 2017 Dallas, Texas SPONSORED BY International Society of Soil Mechanics and Geotechnical Engineering The Geo-In
2、stitute of the American Society of Civil Engineers EDITED BY Laureano R. Hoyos, Ph.D., P.E. John S. McCartney, Ph.D., P.E. Sandra L. Houston, Ph.D., D.GE William J. Likos, Ph.D. Published by the American Society of Civil Engineers Published by American Society of Civil Engineers 1801 Alexander Bell
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9、s Reserved. ISBN 978-0-7844-8167-7 (PDF) Manufactured in the United States of America. Preface The Second Pan-American Conference on Unsaturated Soils (PanAm-UNSAT 2017) was held in Dallas, Texas, November 12-15, 2017, featuring the latest research advances and engineeringpractice innovations in the
10、 area of Unsaturated Geotechnics, with a focus on characterization, modeling, design, construction, field performance and sustainability. PanAm-UNSAT 2017 follows a now well-established series of regional and international conferences on Unsaturated Soils, bringing together researchers, practitioner
11、s, students and policy makers from around the world, particularly the Americas. The conference built upon the success of PanAm-UNSAT 2013 (First Pan-American Conference on Unsaturated Soils, Cartagena, Colombia), as well as that of previous conferences on unsaturated soils hosted in the United State
12、s, including UNSAT 2006 (Fourth International Conference on Unsaturated Soils, Carefree, Arizona) and EXPANSIVE92 (Seventh International Conference on Expansive Soils, Dallas, Texas, 1992). Proceedings of PanAm-UNSAT 2017 have been documented in four Geotechnical Special Publications (GSP) of ASCE i
13、ncluding Volume 1: Plenary Session Papers; Volume 2: Fundamentals; Volume 3: Applications; and Volume 4: Swell-Shrink and Tropical Soils. Current Volume 1 (Plenary Session Papers) consists of three sections: Section I includes 4 papers documenting the invited Keynote Lectures delivered by Profs. Del
14、wyn Fredlund, Ning Lu, Bernardo Caicedo and Tacio de Campos, respectively; and 2 more companion papers documenting the First Distinguished Pan American Lecture on Unsaturated Soils delivered by Prof. Sandra L. Houston. Section II includes 6 papers documenting the invited Fredlund Symposium Lectures
15、delivered by distinguished scholars in honor to the decades-long contribution of Prof. Delwyn G. Fredlund to the discipline of Unsaturated Geotechnics. Section III includes 6 more papers documenting the invited State-of-the-Art and State-of-the-Practice Lectures delivered by distinguished researcher
16、s and experienced practitioners from the region. Each paper was subject to rigorous technical review and received a minimum of two positive peer reviews before final acceptance by the conference technical committee. 3DQ$P8QVDWXUDWHG6RLOV*63 LLL$6 and Feixia (Cherry) Zhang2 1Golder Associates Ltd., 1
17、721 8th St. East, Saskatoon, SK, Canada S7H 0T4. 2Ph.D. Student, Dept. of Civil and Environmental Engineering, Univ. of Alberta, Edmonton, AB, Canada T6G 2W2. Abstract The test procedure for measuring the soil-water characteristic curve, SWCC, have been largely established within the soil physics di
18、scipline over a period of several decades. More recently, these test procedures have been incorporated into geotechnical engineering practice. While there are benefits associated with utilizing past experience, further refinement of the analytical procedures used in soil physics are required when es
19、timating unsaturated soil property functions. The primary assumption that has historically been made is that insignificant volume change occurs as soil suction is increased during the drying process. This assumption may be satisfactory for certain soil types and conditions, but in general, the effec
20、ts of volume change are significant for many soils and must be taken into consideration when estimating USPFs. The shrinkage curve (SC) of a soil can be effectively used to take volume changes upon drying into consideration. The shrinkage curve can readily be measured but there are also a number of
21、means whereby the curve can be estimated with sufficient accuracy. This paper outlines the steps involved in independently assessing the effects of volume change and desaturation on the calculation of unsaturated soil property functions. The shrinkage curve is used to separate the various volume-mas
22、s SWCCs required when dealing with various unsaturated soil mechanics problems. The proposed analytical procedure is described for estimation of hydraulic properties for low to high compressibility soils. INTRODUCTION Research studies on unsaturated soils have repeatedly shown that the relationship
23、between the amount of water in a soil and soil suction (referred to as the soil-water characteristic curve, SWCC, or the water retention curve), is pivotal to the application of unsaturated soil mechanics principles in geotechnical engineering practice. It is unfortunate, however, that there has bee
24、n numerous inconsistencies with regard to the test and analytical protocols associated with the measured gravimetric water content based SWCC (referred to as w-SWCC). The primary use for the SWCC is for purposes of estimating unsaturated soil property functions, (USPFs), for various geotechnical eng
25、ineering applications (Fredlund et al, 2012). The measurement of the w-SWCC has become the most common laboratory test performed to obtain insight into unsaturated soil behaviour. A common area of application of SWCCs in geotechnical engineering is the assessment of hydraulic properties required for
26、 modeling water movement through unsaturated soils. 3DQ$P8QVDWXUDWHG6RLOV*63 $6 however, there are also disadvantages and limitations. The primary limitation is related to an assumption related to the amount of volume change that might occur as soil suction is increased during the drying process. Wh
27、ile there may be insignificant volume changes that occur for low compressibility soils such as sands, it is clear that large volume changes can occur when testing other materials such as mine waste tailings and other slurries. Geotechnical engineers have attempted to adopt somewhat similar soil test
28、ing protocols and analytical data reduction procedures to those historically used in soil physics. It has become apparent, however, that refinements are necessary with regard to the interpretation of the SWCC for some soils. The primary improvement needed to more accurately utilize the SWCC for the
29、estimation of USPFs is an understanding of the effect of volume change as soil suction is increased. The required additional information can be obtained through use of the shrinkage curve, SC, (i.e., the relationship between void ratio change and gravimetric water content change during drying). The
30、objective of this paper is to provide a theoretical basis and associated analytical protocols for the refinements needed when solving unsaturated soil mechanics problems in geotechnical engineering. The refinements focus on the interpretation of laboratory measured soil-water characteristic curves f
31、or purposes of estimating unsaturated soil property functions, USPFs. A couple of data sets are presented to illustrate how a measured (or estimated) shrinkage curve can be used to advantage in the interpretation of laboratory measured w-SWCCs. The scope is limited to consideration of the effects of
32、 the refined analysis for unsaturated hydraulic property functions. THEORETICAL FRAMEWORK FOR ESTIMATING UNSATURATED SOIL PROPERTY FUNCTIONS There are two independent stress state variables involved in describing unsaturated soil behaviour; namely, i.) net total (along with shear stress) variables w
33、ith components in three orthogonal directions, (1 - ua), (2 - ua), and (3 - ua), and ii.) an isotropic matric suction (or soil suction) component, (ua uw). The theoretical justification for using independent stress state variables is based on equilibrium considerations of a multiphase system within
34、the context of continuum mechanics (Fredlund and Morgenstern, 1977; Fredlund, 2016). There are also two possible volume-mass properties that can change in response to a change in stress state. The soil state changes are: i.) a potential change in volume recorded as a change in void ratio, (de), and
35、ii.) a potential change in degree of 3DQ$P8QVDWXUDWHG6RLOV*63 $6 namely, i.) structural shrinkage where a few large pores are initially emptied, ii.) normal shrinkage where the volume of the soil decreases by an amount equal to the water lost, and iii.) residual shrinkage where essentially no volume
36、 change occurs as the soil is dried to zero water content. All of the above phases of drying may or may not be of interest in a particular geotechnical engineering application. Leong and Wijaya (2015) provided a summary of equations that can be used for various shrinkage paths. While there are a var
37、iety of possible shrinkage paths, the commonly used pathway for measuring the SWCC involves first bringing the soil to near saturated conditions. There may be hysteresis effects between drying and wetting that need to be taken into consideration. 3DQ$P8QVDWXUDWHG6RLOV*63 $6 however, that is not alwa
38、ys the case (Fredlund and Xing, 1994). A nest of two sigmodal SWCC curves can be used in cases where high volume changes occur when measuring the drying SWCC (Zhang et al., 2014). Likewise, nested sigmodal SWCCs can be used when bimodal behaviour is encountered (Stianson and Fredlund, 2014). In some
39、 cases of bimodal behaviour it may also be necessary to use a modified form of the shrinkage curve when analyzing the test results. LINKAGE OF VOLUME-MASS VARIABLES THROUGH SHRINKAGE CURVES A triaxial test cell can be modified such that volume change is measured while suction is applied to a soil. T
40、he costs associated with converting a triaxial apparatus for this purpose might be justifiable for research purposes; however, the costs are prohibitive for commercial, consulting engineering practice. For this reason, it becomes more acceptable in geotechnical engineering practice to perform two te
41、sts; namely the w-SWCC test and the SC test and combine the results in a theoretically 3DQ$P8QVDWXUDWHG6RLOV*63 $6 however, the use of mercury in laboratories is now prohibited and the volume of shrinkage curve soil specimens is more commonly measured using micrometer calipers. Shrinkage curve equat
42、ion for gravimetric water content versus void ratio M. Fredlund et al., (2002) proposed a mathematical equation to best-fit measured shrinkage curve data. The emphasis is on characterizing the drying SC with the suggestion that the effects of hysteresis can be handled independently once the drying b
43、ehaviour is quantified. The M. Fredlund et al., (2002) equation has the form of a hyperbole which can be used to best-fit the SC. 2 where: ash = minimum void ratio upon complete drying, bsh = variable related to the slope of the drying curve calculated as: bsh = (ash So)/Gs, and csh = variable relat
44、ed to the sharpness of curvature as the soil desaturates, and So = initial conditioned degree of saturation. Equation 2 can be applied for shrinkage from any initial degree of saturation and the equation is based on the assumption that the shrinkage curve starts asymptotic to a degree of saturation
45、line on the shrinkage curve plot. Since the measurement of the SWCC starts near the saturation of the soil specimen, the initial degree of saturation will be near 100% (e.g., S = 98%). 11shsh ccshshwe w ab3DQ$P8QVDWXUDWHG6RLOV*63 $6 ash = 0.40; bsh = 0.146 and the csh = 3.0. The specific gravity of
46、the soil is 2.68. FIG. 4. Typical shrinkage curve for a compressible soil There are alternate graphical representations that can be used to present SC data, an example of which is shown in Figures 5. Each type of representation of the volume-mass SC data assists in presenting various details of shri
47、nkage soil behavior; however, the most commonly used representation shown in Figure 4 will be used in this paper. 11A di gi tal micrometer for dia met er and thickne ss measu rementsSp ecimen size :12 mm thick37 mm d ia met er0.00.20.40.60.81.01.21.40 10 20 30 40 50 60Void ratioGra vi metric wa ter
48、co ntent, %Co mpre ssible so il s hr ink ag e c ur veS = 20%S = 40%S = 60%S = 80%Gs = 2. 680S = 98.0 %as h = 0.40bs h = 0.146csh = 3.003DQ$P8QVDWXUDWHG6RLOV*63 $6 Marinho, 1994; Tripathy, 2000). A further fitting study of shrinkage curve test results would provide useful correlations between the plasticity characteristics of a soil and the csh fitting parameter for the shrinkage curve. EQUATION FOR THE SWCC The drying SWCC (w-SWCC) can extend from a frac
