Experiments were conducted to investigate the odometric swell behavior of expansive soil specimens mixed independently with two different granular additives: silica sand and granulated tire rubber (GTR). All specimens were prepared with the same global water content. However, specimens that contained sand swelled less than did analogous specimens that contained GTR, regardless of the surcharge stress imposed before inundation. Phase relationship analyses based on “oversize correction” equations derived for mixtures with three solid phases indicate that the specimens possessed clay portions with different dry density and initial water content. Such differences are attributed mainly to the role of additive grain stiffness during specimen preparation. Results from complementary discrete element method simulations indicate that the stiffness of the additive grains may also play a role in mitigating the swell via contact force alterations upon swell.

Experimental results from a comprehensive series of suction-controlled, consolidated drained (CD) triaxial tests, conducted on statically compacted specimens of unsaturated silty sand, are presented. The experimental program was accomplished in a newly implemented, fully automated double-walled triaxial test system via the axis-translation technique. Results were thoroughly analyzed to gain critical insight into some of the essential elastoplastic features of compacted intermediate geomaterials under controlled suction states, including the effect of suction on yield stress, apparent cohesion, tensile strength, and critical state line, as well as postpeak softening and strain-induced dilatancy under suction-controlled monotonic shearing. Constitutive parameters postulated by the Barcelona basic model (BBM) were then experimentally calibrated and used for prediction of compacted silty sand response at matric suction states that varied from 50 to 750 kPa. Predicted values of deviatoric stress at cri...

Assessing the stability of rock slopes is essential to ensuring the typical performance of adjacent transportation infrastructures but simulating the existing field topography and boundary conditions for stability analysis is challenging. This paper describes a case study in which highly weathered rock slopes adjacent to a railroad track in Texas were analyzed using close-range photogrammetry data obtained from an unmanned aerial vehicle with close-range photogrammetry (UAV-CRP). A highly weathered rock mass with low intact strength was considered as a single continuum that undergoes circular failure and was analyzed using Spencer’s two-dimensional (2D) and three-dimensional (3D) limit equilibrium method (LEM) analyses. A 3D critical slip surface was obtained to identify the critical sections on which to conduct the individual 2D stability analysis and obtain a safe resloping angle. These angles were further used to reslope the whole rock slopes and conduct a 3D analysis to arrive at a safe and economic slope angle for each side of the rock-cut. The realistic geometrical slopes developed from the aerial imagery were used to develop a framework for comprehensively analyzing the stability of a rock slope undergoing circular failure.

A fully automated relative-humidity (auto-RH) control unit has been adapted to a newly implemented double-walled triaxial cell to test unsaturated soils under higher total suction states via the vapor-pressure technique, allowing for direct measurement and control of the relative humidity inside the pores of the test soil. The triaxial system is also suitable for implementing the axis-translation technique. With the operational and fully integrated servo-controlled triaxial system, a series of conventional triaxial compression (CTC) tests were conducted on identically prepared specimens of compacted silty sand under constant total suction states of 20 MPa and 300 MPa, induced and controlled via the automated auto-RH control unit. The suitability and reliability of the integrated system was demonstrated by closely repeatable results obtained from the series of suction-controlled CTC tests. Suitable shearing rates for suction-controlled testing of compacted silty sand, via both axis-translation and relative-humidity-based techniques, were also empirically assessed through a series of strain-/suction-controlled tests. The latter were conducted at different axial loading rates (% axial strain per unit time) under either constant matric suction (0.5 MPa) or constant total suction (300 MPa). In both cases, the most suitable shearing rate was identified as the maximum rate for which the test soil continued to be subjected to a constant matric or total suction throughout the entire shearing stage.

This paper presents test results of comprehensive laboratory and field-testing program efforts for the development of bioengineering solutions such as growing vegetation for protection of slopes from erosion and landslides in a tropical environmental setting. Saturated shear strength of soil was determined using direct shear tests and unsaturated soil properties, such as soil water retention curve (SWRC), were obtained using a computer-controlled hydraulic property analyzer (HYROP) system as well as a WP4C instrument. Climate data were obtained via field instrumentation and appropriate vegetation data were assumed to perform a finite element method-based transient seepage analysis and coupled slope stability analysis to test the potential of tropical hillslope to fail with and without vegetation over a period of one month. Results show that the factor of safety (FOS) for test slope considering case (a) the rainfall and bare ground, case (b) no rainfall with vegetation, and case (c) rainfall with vegetation were found to be 1.630, 1.763, and 1.650, respectively. Although FOS is marginally improved during storm events due to consideration of vegetation as compared to bare slope, this improvement in FOS is much pronounced during antecedent rainfall (i.e., long duration and small intensity) up to the first 26 days of analysis before the storm event (i.e., high intensity and short duration rainfall), which occurs on 27th day and can be instrumental in preventing slope failures. Similarly, the negative pore water pressure (i.e., matric suction) in the top layer is reduced for case (a) from −260 kPa to −40 kPa, increased for case (b) from −260 kPa to −320 kPa, and decreased for case (c) from −260 kPa to −60 kPa. The practical application of these findings is more applicable to the engineered slopes with vegetation during the dry season when the slope is more stable due to high FOS which, however, will need careful watering just to keep them healthy but prevent complete loss of developed matric suction resulting from root water uptake (RWU). In addition, the small improvement in FOS due to matric suction induced from RWU could play a key role in keeping the slope just stable during extreme storm events especially, when FOS of the bare slope is close to 1. To the best knowledge of the authors this is the first documented geotechnical study, using the tropical soil of Guam, which considers the hydro-mechanical effect of RWU-induced matric suction in slope stability analysis in a tropical setting.