International audienceThis paper aims to study the influence of train speed on the mechanical behaviour of track-bed materials based on field data recorded at a representative site of the conventional French network. Capacitive accelerometers soil pressure gauges were installed in track-bed layers. The Intercity train was selected to perform this study as it is the most frequent train running on this site. In total, 1790 records corresponding to Intercity train passages were taken into account, with train speeds ranging from 60 to 200 km/h. The vertical strains of different layers were estimated by integrating the signals of accelerometers installed at different depths. It is observed that when train speed increased in the considered range, the traffic loadings, in terms of dynamic stress transmitted to track-bed materials, were amplified about 10%. However, the vertical strains of track-bed materials were also amplified by 2 in the same range of speeds. These amplifications appear mainly in shallower layers. The stress strain amplitude ratios for all the recorded trains were calculated to analyse the evolution of resilient moduli (M-r) of track-bed materials. It is found that Mr of interlayer soil decreased by approximately 25% when train speed increased from 60 to 200 km/h. (C) 2016 The Japanese Geotechnical Society. Production hosting by Elsevier B.V

To better understand the overall hydromechanical behaviour of interlayer soil, the compaction behaviour of one of the two components — the portion of fines (<4 mm) that is sensitive to water content changes — was investigated. The standard Proctor compaction curves were first determined for the soils. Then, the maximum shear modulus, G_max, and suction were measured on samples statically compacted at an identical dry density, but different remoulding water contents. The changes in G_max reveal the existence of a characteristic water content corresponding to the maximum G_max. The results also show that this characteristic water content increases with the soil plasticity, being similar to the variation trend of optimum water content with soil plasticity. A bimodal pattern was observed from the plot of total suction ψ versus the slope of water content w–log(ψ) curve. The suction corresponding to the maximum G_max is close to the lowest point between the two peaks in the ψ–dw/dlog(ψ) curve. A reasonable explanation was attempted for the correspondence between the “optimum water content” defined by the maximum value of G_max and the corresponding suction. The difference between the static and dynamic compactions was also explained in terms of suction values.

A new steel slag-based composite material (SSC), containing the steel slag, metakaolin and cement, was proposed to replace the traditional cement for the stabilisation of soft clays in deep mixing projects. The results obtained from unconfined compression tests showed that the metakaolin agent is crucial to the strength growth of the SSC paste. At a preferable mass ratio (Cement:Metakaolin:Steel slag=50:15:85), the SSC paste can achieve a higher strength than that of pure cement.The strength development of SSC stabilised clay is similar to traditional cemented soils, and the updated water/SSC ratio can be employed to assess the strengths based on Abrams’ law. The pore size distributions indicated that the cumulative pore volume and the macro pore size both decreased with the cement proportion increasing. The main components, such as β-C₂S, γ-C₂S, CSH, CaSO₄·2H₂O, Aft and CASH, were identified in XRD patterns and SEM images. The mechanisms were summarized as: (1) a large amount of CSH formatted the reticulate and fibrous structures, and became the binder between the unhydrated aggregates; (2) metakaolin promoted the secondary hydration and pozzolanic reaction of cement and steel slag, resulting in the further enhancement of the binders.

Polyacrylamide (PAM) is a water-soluble polymer with the ability to enhance a soil’s stability. PAM is currently being used to prevent irrigation-induced erosion and enhance the infiltration in farmland soil. To improve the compaction properties of the saline-soil-based filling material that is used in highway subgrade and the cracking resistance capacity of a saline soil’s crust, the consistency limits, compactability, microstructure, and cracking morphology of untreated and PAM-treated saline soil were investigated. The saline soils were sampled from the soil crust and a depth of 2.0–3.0 m in Gansu Province, China. Two PAM concentrations (0.1% and 0.5% in mass ratio) were selected. The liquid limits and plastic limits of the saline soil samples from the surface (0–0.05 m) and a depth of 2.0–3.0 m noticeably increased as PAM concentration increased. The maximum dry densities decreased as PAM concentration and plasticity increased, and the optimum water contents of the two saline soil types did not significantly change. These results suggest that a higher shearing resistance between particles partially prevented compression from occurring during compaction. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) test results showed that the PAM agent dispersed the bulky pellets, and the soil’s structure was formed by flaky and acicular platelets that filled the micropores. A quantitative analysis of crack patterns showed that the cross-points of the crack network and the crack length decreased as the PAM concentration increased. These results indicate that an increase in PAM reduces the shrinkage strain and the flaws or pores within saline soils. Therefore, PAM’s stabilizing effect on saline soil under a wetting–drying cycle was proven.