ISSMGE Bulletin
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ISSMGE Bulletin Volume 12, Issue 4 August 2018 International Society for Soil Mechanics and Geotechnical Engineering If the quality of the distributed file is not satisfactory for you, please access the ISSMGE website and download an electronic version. www.issmge.org T ABLE OF CONTENTS Research highlights Select all items below University of Texas Arlington 1 Research highlights – University of Texas Arlington General overview 9 TC Corner The University of Texas at Arlington (UTA) is located 14 Conference report – Annual meeting of Italian in the heart of the Dallas/Fort Worth metropolitan Researchers in Geotechnics area and is the second-largest component of The 15 Hot news University of Texas System (UT System), holding Engineering in Chalk Conference 2018, Carnegie Doctoral Research Extensive status. The London, U.K. geotechnical program of the Civil Engineering Department is led by Profs. Laureano Hoyos, Anand De Beer’s Award Puppala, Sahadat Hossain, Xinbao Yu and Stefan 17 Obituary - Prof. Iacint Manoliu Romanoschi, featuring two state-of-the-art teaching 22 ISSMGE Foundation reports and research geotechnical facilities: the Soil 26 Event Diary Mechanics Laboratory and the Advanced Geomechanics Laboratory. A few research highlights, 38 Corporate Associates particularly from the UTA group’s work on unsaturated 41 News from Corporate Associates soil mechanics, are summarized in the following. 43 Foundation Donors Suction-controlled resonant column and bender element testing A resonant column device, including self-contained bender elements, is used for suction-controlled testing of unsaturated soils, as shown in Figure 1. A model PCP-15U pressure panel allows for implementation of E D I T O R I A L B OARD the axis-translation technique. Figure 2 shows damped free-vibration cycles generated from two SM soil al-Damluji, Omar (Editor for Asia) specimens under matric suctions, s = 50 kPa (curve on Bouassida, Mounir (Editor for Africa) left) and 400 kPa (curve on right), respectively; both Cazzuffi, Daniele (Editor for Europe) under net confining pressure of 50 kPa. The fee- vibration cycles, regardless of the initial magnitude of Chang, Der-Wen (Editor for Asia) shear strain amplitude, are fully attenuated after 0.08 Davis, Heather (Editor for Africa) seconds. Since the initial shear strain amplitude is Gonzalez, Marcelo (Editor for South America) lowest under the highest suction state, s = 400 kPa, Jefferis, Stephan (Editor for Europe) this is strongly indicative of lower material damping at Leung, Anthony Kwan (Editor-in-Chief) higher suction values, which can be attributed to the Ng, Charles Wang Wai (Ex-officio) increase in stiffness with increasing suction. Rujikiatkamjorn, Cholachat (Editor for Australasia) . Sanchez, Marcelo (Editor for North America) Siemens, Greg (Editor for North America) Taylor, Neil (Ex-officio) Baser, Tugce (Editor for YMPG) ISSMGE Bulletin: Volume 12, Issue 4 Page 2 Research highlights (Con’t) University of Texas Arlington Figure 1. Suction-controlled RC device with bender elements (GCTS Testing Systems: https://www.gcts.com/) Figure 2. Damped/suction-controlled free-vibration cycles from SM soil Suction-controlled triaxial testing A servo/suction-controlled double-walled triaxial system is used for unsaturated soil testing via either axis- translation or vapor transfer technique, as shown in Figure 3. Main features of the triaxial system include: 1 – Auto-RH system; 2 – Gas bubbler, desiccant, and temperature probe; 3 – Core triaxial system; 4 – Automated volume-change device; and, 5 – Automated flushing device. Recent efforts focused on suction- controlled testing of a relatively dense soil with a natural tendency to experience dilatancy and post-peak softening as it approaches critical state. Statically compacted specimens of silty sand were tested in the low-to-medium suction range (50-750 kPa), via the axis-translation technique, as well as the higher suction range (20-300 MPa), via the vapor transfer technique. ISSMGE Bulletin: Volume 12, Issue 4 Page 3 Research highlights (Con’t) University of Texas Arlington Figure 3. Suction-controlled TX device (GCTS Testing Systems; Soil Water Retention, LLC). Figure 4 shows the volume-change response of compacted SM soil from suction-controlled axisymmetric shearing under constant matric suction, s = 50 kPa, and initial net mean stress, p = 100, 200, or 300 kPa. Ceramics with 1-bar air-entry values were used to induce a matric suction of 50 kPa. Additional tests were performed on identically prepared specimens using 3-bar ceramics for matric suction of 250 kPa, and 15- bar ceramics for matric suctions of 500 kPa and 750 kPa. Results show a clearly manifest change from compressive to dilatant behavior, regardless of initial net mean stress, as the test soil approaches critical state condition under a constant matric suction even as low as 50 kPa. The amount of dilatancy, as expected, decreases with increasing net mean stress. -4 s = 50 kPa -3 100 -2 200 300 -1 Volumetric strain (%) strain Volumetric 0 1 0 4 8 12 16 Axial strain (%) Figure 4. Volume-change response of SM soil from suction-controlled axisymmetric shearing ISSMGE Bulletin: Volume 12, Issue 4 Page 4 Research highlights (Con’t) University of Texas Arlington Suction-controlled true triaxial testing A servo/suction-controlled true triaxial apparatus is used for testing cubical soil samples, 7.62 cm (3 in) per side, via axis-translation technique, as shown in Figure 5. The apparatus is a mixed-boundary type of cell, with the sample seating on a 5-bar or 15-bar ceramic and between five flexible membranes on the remaining sides of the cube. It consists of a stainless-steel frame, featuring six pressure cavities to accommodate one top and four lateral latex membranes, and one cubical base aluminum piece at the bottom assembly to receive the HAE ceramic and four coarse porous stones. The sample is gently slid in through one of the lateral cavities of the frame, with the membranes acting as the pressure seals between the wall assemblies and the frame, thus transmitting the water-based hydraulic pressure to the top and four lateral faces of the soil. Figure 5. Suction-controlled TTX device (GCTS Testing Systems; UC-Boulder) External hydraulic pressure is generated by a computer-driven, model PCP-5000-UNSAT pressure control panel, with independent control of major, intermediate and minor principal stress. A computer driven, model PVC-100-UNSAT pressure-volume control system maintains a constant pore-air pressure through the porous stones at the bottom of the sample. In both panels, model DC-750-5000 Macro Sensors hydraulic pistons are used for measurements of positive (compression) and negative (extension) deformations of the sample in all three principal directions. Figure 6 shows results from a suction-controlled simple shear (SS) test conducted on SP-SC soil at matric suction, s = 200 kPa, and net mean stress of 100 kPa. During SS testing, the major principal stress is monotonically increased, the minor principal stress is simultaneously decreased in the same magnitude, and the intermediate principal stress is kept constant. Consequently, the major principal strain is observed to be compressive (+), the minor principal strain expansive (–), and the intermediate principal strain is virtually negligible. Peak strength values from a series of suction-controlled TC, TE, and SS tests, all conducted under same net mean stress of 100 kPa, were used to identify the failure envelopes in the octahedral plane. It can be readily observed the marked effect that matric suction has over the size and positioning of the failure loci, with a significant expansion for s = 200 kPa. ISSMGE Bulletin: Volume 12, Issue 4 Page 5 Research highlights (Con’t) University of Texas Arlington Figure 6. General response of SP-SC soil from suction-controlled true triaxial testing. Suction-controlled plane strain (biaxial) testing A servo/suction-controlled biaxial apparatus has been used for testing unsaturated soils under plane strain conditions via axis-translation technique, as shown in Figure 7. The design of its core system is based upon the original Vardoulakis type of apparatus, allowing for direct measurements of normal stresses generated between a soil specimen and two lateral rigid walls during suction-controlled plane strain shearing. Plane strain condition is imposed on the soil specimens by means of two 8-mm thick rigid walls made of Type 304 stainless steel. These walls prevent the specimen from deforming along the intermediate principal axis X2. The bottom pedestal receives an HAE ceramic and rests on a U-shaped stainless-steel base frame, which couples the pedestal and the rigid lateral walls to a model NKL 6-110 sliding table. Lateral displacements experienced by the specimen in the minor principal axis X3 during plane strain shearing is measured by four Microstrain type DVRTs (differential variable reluctance transducers). The intermediate principal stress along axis X2 is measured via Sensotec type pressure cells installed on each of the rigid lateral walls. The assembly is ultimately placed inside a Wykeham-Farrance pressure cell with a 1.7 MPa pressure capacity. Figure 8 shows a photograph of a typical failure induced on SM soil sample subjected to suction-controlled plane-strain shearing. A fully developed failure surface, making a 65o angle with the horizontal, can be readily identified from a sample failed under matric suction, s = 100 kPa. A relatively less inclined failure surface, making a 61o