Effect of CO2 exposure on the mechanical strength of geopolymer-stabilized sandy soils

This is the final version. Available on open access from Elsevier via the DOI in this recordIn recent years, there has been growing interest in developing methods for mitigating greenhouse effect, as greenhouse gas emissions continue to contribute to global temperature rise. On the other hand, investigating geopolymers as environmentally friendly binders to mitigate the greenhouse effect using soil stabilization has been widely conducted. However, the effect of CO2 exposure on the mechanical properties of geopolymer-stabilized soils is rarely reported. In this context, the effect of CO2 exposure on the mechanical and microstructural features of sandy soil stabilized with volcanic ash-based geopolymer was investigated. Several factors were concerned, for example the binder content, relative density, CO2 pressure, curing condition, curing time, and carbonate content. The results showed that the compressive strength of the stabilized sandy soil specimens with 20% volcanic ash increased from 3 MPa to 11 MPa. It was also observed that 100 kPa CO2 pressure was the optimal pressure for strength development among the other pressures. The mechanical strength showed a direct relationship with binder content and carbonate content. Additionally, in the ambient curing (AC) condition, the mechanical strength and carbonate content increased with the curing time. However, the required water for carbonation evaporated after 7 d of oven curing (OC) condition and as a result, the 14-d cured samples showed lower mechanical strength and carbonate content in comparison with 7-d cured samples. Moreover, the rate of strength development was higher in OC cured samples than AC cured samples until 7 d due to higher geopolymerization and carbonation rate.MatSoil CompanyEuropean Union Horizon 202

Centrifuge model study on geogrid reinforced soil walls with marginal backfills with and without chimney sand drain

The objective of this paper is to investigate the performance of geogrid reinforced soil walls with panel facing using marginal backfill with and without chimney sand drain subjected to seepage. A series of centrifuge model tests were performed at 40 gravities using a 4.5 m radius large beam centrifuge facility available at IIT Bombay. The results revealed that a geogrid reinforced soil wall with low stiffness geogrid and without any chimney drain experienced a catastrophic failure due to excess pore water pressure that developed in the reinforced and backfill zones at the onset of seepage. In comparison, a soil wall reinforced with stiff geogrid layers was found to perform effectively even at the onset of seepage. Provision of chimney sand drain effectively decreased pore water pressure not only at the wall toe but also at mid distance from toe of the wall and thereby resulted in enhancing the wall performance under the effect of seepage forces. However, a local piping failure was observed near the toe region of the wall. The observed centrifuge test results were further analysed by performing seepage and stability analyses to evaluate the effect of thickness of sand layer in a chimney drain. An increase in thickness of sand layer in chimney drain was found to improve the discharge values and thereby enhancing the factor of safety against piping near the toe region. Based on the analysis and interpretation of centrifuge test results, it can be concluded that marginal soil can be used as a backfill in reinforced soil walls provided, it has geogrid layers of adequate stiffness and/or proper chimney drain configuration. (C) 2017 Elsevier Ltd. All rights reserved

Multivariate analysis of GPS position time series of JPL second reprocessing campaign

The second reprocessing of all GPS data gathered by the Analysis Centers of IGS was conducted in late 2013 using the latest models and methodologies. Improved models of antenna phase center variations and solar radiation pressure in JPL’s reanalysis are expected to significantly reduce errors. In an earlier work, JPL estimates of position time series, termed first reprocessing campaign, were examined in terms of their spatial and temporal correlation, power spectra, and draconitic signal. Similar analyses are applied to GPS time series at 89 and 66 sites of the second reanalysis with the time span of 7 and 21 years, respectively, to study possible improvements. Our results indicate that the spatial correlations are reduced on average by a factor of 1.25. While the white and flicker noise amplitudes for all components are reduced by 29–56 %, the random walk amplitude is enlarged. The white, flicker, and random walk noise amount to rate errors of, respectively, 0.01, 0.12, and 0.09 mm/yr in the horizontal and 0.04, 0.41 and 0.3 mm/yr in the vertical. Signals reported previously, such as those with periods of 13.63, 14.76, 5.5, and 351.4 / n for n=1,2,…,8 n=1,2,…,8 days, are identified in multivariate spectra of both data sets. The oscillation of the draconitic signal is reduced by factors of 1.87, 1.87, and 1.68 in the east, north and up components, respectively. Two other signals with Chandlerian period and a period of 380 days can also be detected.Aircraft Noise and Climate Effect