ABSTRACT Reliable estimation of critical shear stress (CSS or τc) is essential for accurate prediction of erosion and scour around bridge piers with cohesive soils. The CSS is the shear stress required to initiate erosion, typically assumed at a rate of 0.1 mm/h. This assumption, although convenient, oversimplifies the erosion process and introduces significant variability across different testing techniques and flow regimes. This study compares critical shear stress values obtained from two erosion testing methods: the erosion function apparatus (EFA) and a portable scour testing device (PSTD), using undisturbed CL soil samples from two different bridge sites in Illinois. Two analytical methods are proposed: Method A, which determines τc as the range between the onset of erosion and the 0.1 mm/h erosion rate threshold for individual tests; and Method B, which aggregates data across multiple sites to account for soil and site-specific variability. Results show that EFA generally reports higher τc values than PSTD, likely because of differences in flow regimes. Turbulent conditions in PSTD consistently produced lower τc values, emphasizing the importance of flow regime-specific assessments in scour depth design. Comparison of τc determined in this study with τc determined from an empirical correlation presented in previous literature revealed substantial scatter, with prior studies often underestimating τc. The proposed methods offer a more field-representative and standardized approach to determining τc, which is imperative for the development of realistic and site-appropriate scour prediction models.
ABSTRACT Ground-borne vibrations generated by metro train operations on elevated structures pose significant concerns related to human annoyance in urban environments. Although previous studies have primarily relied on theoretical approaches, limited field-based evidence exists to quantify vibration attenuation through the complete train–track–structure–soil system. This study addresses this gap through simultaneous in situ measurements conducted at both structural (rail track, plinth, slab deck, and pier) and ground levels using accelerometers and geophones, with uncertainties accounted for throughout the monitoring period. The results indicate a 23 % reduction in vibration acceleration from the rail track to the pier, a 31 % reduction in vibration velocity between 2 and 12 m from the pier centerline, and dominant vibration components higher than 40 Hz. Uncertainty analysis revealed variations of 2–19 VdB in Unit I and 0.5–5 VdB in Unit II, whereas peak responses showed reductions of 95 % in acceleration from the rail track to the pier and 96 % in peak velocity from 2 to 12 m from the pier centerline. Overall, vibration velocity decreased by nearly 78 % from the track to the ground. Comparison with existing empirical models indicated deviations within 1.5–10 %, and a simplified trendline equation is proposed for the preliminary assessment of ground-borne vibration.
ABSTRACT The cyclic direct simple shear (CDSS) test is increasingly being used to evaluate the cyclic resistance of clean sands. However, there are differences related to equipment manufacturers, sample preparation techniques, testing procedures, failure criteria, and quality control (QC) measures, all of which lead to uncertainties in CDSS results even for the same sand. In particular, system stiffness of the equipment has been shown to greatly affect test results. As a result, accurate evaluation of the cyclic behavior in clean sands using CDSS tests requires rigorous QC measures. In this study, several active control constant volume CDSS tests were conducted on samples of Monterey and Ottawa sand with different stress histories and relative densities. Various quality assurance/QC (QA/QC) measures published in the literature were applied and evaluated. The results of tests that passed the QA/QC checks compared well with recently published “high-quality” results despite differences in equipment and some testing procedures. Preconditioning (i.e., many cycles of low shear stress during the consolidation stage) was also evaluated. There appeared to be no significant effect of preconditioning on the cyclic resistance of dense Monterey sand, but it increased the cyclic resistance of medium-dense Ottawa sand. Unexpectedly, the cyclic resistance results for Monterey sand reveal a stress-dependent trend (i.e., Kσ) that does not follow the commonly accepted pattern in the literature, and more work is needed to better understand the role of confining stress on cyclic strength in simple shear. Improved communication and transparency regarding QA/QC procedures in CDSS testing is recommended to ensure comparability of results across different studies and ensure that CDSS test results are reproducible.
ABSTRACT Geosynthetic-reinforced pile-supported (GRPS) foundation, extensively constructed as a soil improvement beneath highways and high-speed railways, has recently been increasingly adopted in the flight areas of airports in coastal cities. However, considering the differences in the applied loads, especially aircraft landing, and the lack of instructions in the aviation industry to consider the cyclic effects after construction, it is a necessity to investigate the effects of repeated aircraft landings on the GRPS runway. A series of small-scale model tests were conducted considering different aircraft types, fill heights, and pile spacings, where the variations of pavement settlement, soil arching ratio, and influential zone dimensions within the pile–soil composite with landing times are focused. The weakening effect of repeated landing on soil arching is verified, and an empirical formula for the influential radius with depth is proposed to evaluate variations in pile response with different GRPS runway configurations. It is found that, under the test conditions, the lateral and vertical dimensions of the influential zone can be up to 7 and 20 times the wheel track (St), respectively, and increase with increasing pile spacing, decreasing fill height, and larger overall aircraft weight.
ABSTRACT A laboratory study was carried out on sandy silt gold tailings to investigate the effect of sample homogenization methods on the measured critical state line (CSL) in triaxial compression (TC) tests. Initially, 22 drained and undrained TC tests were performed on loose moist-tamped specimens prepared from the tailings that had been prepared using four different levels of homogenization: (a) without thorough mixing of the initial sample; (b) mixing during tamping of the specimens; (c) homogenization via sieving the material through 2.36 or 1.18 mm sieves; and (d) mixing the material as a slurry, air-drying it, and then sieving the material through 2.36 and 1.18 mm sieves. The results indicated fairly consistent CSL results within a void ratio of 0.03 for the first three sample homogenization methods employed, but showed a significant lowering of the CSL for the specimens homogenized by mixing as a slurry. An additional eight drained TC tests were carried out on dense moist-tamped and air-dried (AD) specimens to assess the effect of the lowering of the CSL on stress- and state-dilatancy behavior, indicating a lower state-dilatancy parameter, χtc, estimated from the AD tests than from the DMT tests.
