1、GEOTECHNICAL SPECIAL PUBLICATION NO. 303 PANAM UNSATURATED SOILS 2017 SWELL-SHRINK AND TROPICAL SOILS 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 Engin
2、eering The Geo-Institute 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 180
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9、ineers. All Rights Reserved. ISBN 978-0-7844-8170-7 (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 i
10、nnovations in the 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 research
11、ers, practitioners, 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
12、 the United States, 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 Publication
13、s (GSP) of ASCE including Volume 1: Plenary Session Papers; Volume 2: Fundamentals; Volume 3: Applications; and Volume 4: Swell-Shrink and Tropical Soils. Current Volume 4 (Swell-Shrink and Tropical Soils) consists of five sections: Section I, Expansive Soils: Volume Change, includes 12 papers deali
14、ng with the measurement of the volume change of expansive soils during both shrinkage and swelling. This includes evaluations of different testing approaches, as well as the characterization of expansive soils under different conditions. Section II, Expansive Soils: Mitigation and Modeling, includes
15、 14 papers dealing with different strategies to treat expansive soils, using new and established additives and methodologies. Further, several papers on the modeling of expansive soils are included that focus on climate interaction and volume change progression over time. Section III, Foundations on
16、 Expansive Soils, includes 5 papers dealing with foundations on unsaturated soils, including both shallow footings and deep foundations. The papers focus on field observations, design approaches, and finite element simulations to consider the impact of unsaturated conditions on the deformation and b
17、earing capacity of the foundations. Section IV, Modeling of Cracked Soils, includes 6 papers dealing with the observation and theoretical modeling of cracking in unsaturated soils. Two papers 3DQ$P8QVDWXUDWHG6RLOV*63 LLL$6 Andrew J. Whittle, Sc.D., M.ASCE2; and John T. Germaine, Sc.D., M.ASCE3 1Dept
18、. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Room 1-353, Cambridge, MA 02139. E-mail: ivormmit.edu 2Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Room 1-346, Cambridge, MA 02139. E-
19、mail: ajwhittlmit.edu 3Dept. of Civil and Environmental Engineering, Tufts Univ., 113 Anderson Hall, Medford, MA 02155. E-mail: john.germainetufts.edu Abstract This paper summarizes the design, installation, and data collection from an autonomous field station located close to a new toll highway sou
20、th of Austin, TX, that is underlain by more than 10 m of expansive clay. Given the remoteness of the site, the system was designed to be fully automated, requiring very low maintenance. The field station combines a novel string pot system for measuring vertical deformations at five depths in a centr
21、al borehole, together with volumetric water content, and temperature data from multi-parameter reflectometer probes installed at similar depths in a series of surrounding boreholes. In addition, there is a conventional surface weather station, which records air pressure, temperature, rainfall, relat
22、ive humidity, and solar radiation. The system has been active for more than 1 year, and the data collected over this period shows its robustness. INTRODUCTION Many parts of the continental US are underlain by expansive clays that undergo significant changes in volume (swelling and shrinkage) due to
23、seasonal changes in moisture content. Perennial problems associated with the performance of foundations and pavements in these areas highlight the need for long-term monitoring data to understand and validate predictive models, ultimately leading to more reliable methods of design (Manosuthkij et al
24、., 2007). These problems are well illustrated by the performance of State Highway SH130 (Austin to Seguin, TX), which was constructed as a design-build-operate-transfer toll road that was completed in 2012. Sections of the roadway have required extensive maintenance due to cracking associated with u
25、nderlying ground movements. The driving mechanisms of ground displacements are believed to be related to changes in subsurface moisture contents. This research has established a remote field station for long-term monitoring the subsurface conditions (ground deformations and water contents) in relati
26、on to local atmospheric conditions (precipitation, temperature humidity, etc.). There are only a few instrumented test sites that measure water content and ground deformations within expansive soils (e.g., Fityus et al., 2004; Karunarathne et al., 2014; Fernandes et al., 2015; Denis et al., 2016), a
27、nd there remains much uncertainty in the relationships between the key variables. 3DQ$P8QVDWXUDWHG6RLOV*63 $6 Robinson et al., 2005). Today, the commercial TDR (or WCR) probes include a generator and oscilloscope at the head of the device that is connected to the rod probes, eliminating this issue.
28、Several authors have reported successful use of TDR measurements for obtaining soil water content (Topp et al., 1982; Topp and Davis, 1985; Roth et al., 1990). SITE DESCRIPTION Monitoring the behavior of expansive clays in the field comprised of a spatial analysis process that considered the thickne
29、ss of the fat (high plasticity) clay layer, the swell potential of the clay, and the presence of cracks on the existing pavement surface. This was done using geotechnical data available from the studies carried prior to the highway construction. The area of interest is located in the south corner of
30、 Travis county, which is underlain by a formation from the upper Cretaceous period. More specifically, the formations are from the Navarro and Taylor groups. Clays from the Navarro group are typically brown to grey calcareous soils, with thin layers of sand and silt. This group is also known for bei
31、ng a highly plastic formation, with high shrink-swell potential given their mineral composition. 3DQ$P8QVDWXUDWHG6RLOV*63 $6 (b) VDMS with string pot array The TDR probes are installed around the central borehole at different depths, as shown in Figure 4b. These sensors allow the measurement of soil
32、 temperature and electrical conductivity. The reported preliminary water content data are currently estimated from generic correlation relations and will later be updated using a site-specific calibration data using soil collected during the drilling process. An interesting aspect observed during th
33、e initial inspection of the tubes was the presence of layers of a mineral at several depths (Figure 5a). This made necessary the investigation of its composition in order to understand if it affects the swelling behavior of the clay. X-ray diffraction and electrical conductivity measurements have co
34、nfirmed the mineral to be gypsum. The presence of this mineral throughout the clay layer affects the reading of the TDR probes by altering the electrical conductivity of the medium. This reinforces the need for a site-specific calibration curve to interpret the readings from the probes. 3DQ$P8QVDWXU
35、DWHG6RLOV*63 $6 (b) depths of installed TDR probes Figure 5. Gypsum in the clay: (a) soil extracted from one of the Shelby tubes; (b) recrystallized mineral DATA COLLECTED The system collects four data points every hour, and transmits the data via cellular modem twice per day. Figures 7 and 8 summar
36、ize the subsurface measurements of vertical deformations, soil temperatures and volumetric water content recorded to date. The VDMS system shows that the ground surface and the upper ring (Ring 1 at 1.1m depth) exhibit very similar movements. The site has undergone heave movements up to 17mm (relati
37、ve to time of installation in Sept. 2015), and then settled more than 24mm (net settlement of 7mm in May 2016) followed by another 6-month period of net heave. Data from the deeper rings show much smaller movements and a general accumulation of heave over the same time period. These results suggest
38、that most of the water content changes have occurred in the upper 1-2m of the soil profile. The water content reflectometer (WCR) data (Figure 7.a) also shows that only the upper instrument (WCR at 1.5m) experiences significant cyclic fluctuations over the same time period. 3DQ$P8QVDWXUDWHG6RLOV*63
39、$6 (b) Precipitation at the local station Figure 7. WCR data: (a) Volumetric water content; (b) Subsurface ground temperatures Figure 7.b shows that ground temperatures below about 5m are approximately constant (22.5C), but fluctuate seasonally by up to 11.5C near to the ground surface (1.5m deep).
40、(a) (b) (a) (b) 3DQ$P8QVDWXUDWHG6RLOV*63 $6 topsoil, weathered clay, and intact clay layers, to monitor the crack development during a 35-day period of continuous evaporation. The desiccation cracks were observed in the intact and weathered clay layers; however, the topsoil layer remained without an
41、y cracks due to the lack of cohesive particles in the soil. The size of polygon shaped cracks in the intact clay was twice those created in the weathered clay layer. Li and Zhang (2010) focused on the crack development on the surface of a slope composed of silty clay through digital images of the cr
42、ack network. The average length and width of crack polygons were reported at 28.7 mm and 0.49 mm at the steady-state condition where the soil moisture content was at the lowest level. These studies focused more on the crack geometric characteristics manifested on the soil surface; however, some fiel
43、d studies have resulted in reports of crack propagation in the vertical direction depending on the soil properties. For example, Zhan et al. (2007) observed numerous cracks and fissures with maximum depth of 1.2 m and the maximum width of 10 mm in a well-instrumented slope. Ng et al. (2008) monitore
44、d a slope composed of expansive soil and found that desiccation cracks propagated to a depth of 1.5 m. The presence of desiccation cracks was also reported by Bordoni et al. (2015) as a cause of rapid rewetting from the ground surface to a depth of 0.6-0.7 m for a shallow slope. The objective of the
45、 study described in this paper was to measure the crack depth and the corresponding suction changes in a clayey soil bed prepared in the laboratory in order to verify the simple analytical procedure for predicting crack depths. An equation for predicting the change in suction needed to produce a spe
46、cific crack depth (Miller et al. 2015) was utilized in combination with measured suction profiles to predict crack depths in the bench scale model. Predicted and measured crack depths compared favorably. The bench scale model results were also used for validating a finite-element based model for pre
47、dicting crack depth, although this modeling is not presented in this paper. EXPERIMENTAL SET-UP The validation of the analytical model was addressed by comparing predictions of crack depth to measured crack depths in the bench-scale experiment. For this purpose, an experimental test was designed in
48、order to monitor and record the changes of suction associated with crack depth development in a clayey soil. A box with 570 x 895 x 100 mm length, height, and width was fabricated to monitor the desiccation crack depths in a soil bed. The material used for the front and back of the box was clear acrylic to observe the desiccation crack propagation within the depth of the compacted soil. Two sides of the box were made of wood. Wood screws were 3DQ$P8QVDWXUDWHG6RLOV*63 $6&(inst
