Strength performance of low-bearing-capacity clayey soils stabilized with ladle furnace slag

In this paper, the performance of ladle furnace slag (LFS), a by-product of secondary steel refining, is evaluated as a binder to stabilize clayey soils of low bearing capacity. The aim is to define whether additions of this by-product to clayey soil can stabilize the soil in accordance with the technical specifications of Spanish standards. To do so, three different soils stabilized with 5% LFS were compared with the same soils stabilized with 2% lime and with no stabilization, in order to investigate their different behaviors. The chemical and mineralogical characterizations of all the soil mixes were conducted using X-ray fluorescence, X-ray diffraction, and scanning electron microscopy. The Atterberg limit test was used to study the plastic behavior of the soils, and the results of compaction, bearing capacity, unconfined compressive strength, and direct shear strength (cohesion and friction angle) tests defined their strength characteristics. The analysis was completed with the pH monitoring of the mixes along the curing time in order to relate the pH changes with the strength evolution. The addition of LFS to the soils has resulted in an increase in the liquid limit and plastic limit, causing therefore a slight decrease in the plasticity index. All the soils showed increases between 30% and 70% in their California Bearing Ratios immediately after mixing with 5% LFS, and after 90 days of curing, improvements of 30-188% in their unconfined compressive strength were noted in comparison with untreated soil, which were higher than the lime-stabilized soils. The cohesion of soils stabilized with LFS at 28 days of curing obtained improvements ranging from 40 to 300% depending on the type of soil. However, the friction angle showed a slight increase of 10% in two of the soils and zero in another. The high initial pH in LFS-stabilized soils was maintained during the curing time, which favored the development of pozzolanic reactions that improve the soil strength. These results confirmed that the substitution of lime with LFS is a feasible option for soil stabilization.

Investigation on using magnetic water technology for leaching high saline-sodic soils

One of the most important factors considered in leaching salt-affected soils is reducing the amount of water and leaching time particularly when magnetizing water is used. In this study, soil column experiments were conducted to assess the rate of salt removal and estimate the amount of water required for leaching in order to reduce salinity (EC) to ≤ 4 dS m^-1 and sodicity (ESP) to < 10. Soil samples with EC = 216 dS m^-1 and ESP = 82 were taken from Basrah City, Iraq, for the conduction of laboratory experiments, and these samples were subjected to magnetized water (MW) with magnetic fields of 1, 3, 5, 7, and 9; different exposure time; and constant flow velocity. Experimental results were compared with the results of control soil columns leached with non-magnetized water (NMW), and the comparison shows that the leaching times for MW with magnetic fields of 9, 7, 5, 3, and 1 were less by 17.3%, 10.8%, 8.9%, 7.6%, and 3.9%, respectively. Also, less amount of MW was required for leaching, but when field magnetic was 9, the amount was increased by 20%. Predicted values of EC and ESP obtained from the proposed equations were found in agreement with experimental results.

Deformation of the aquifer system under groundwater level fluctuations and its implication for land subsidence control in the Tianjin coastal region

This study demonstrates characteristics and mechanisms of deformation of an aquifer system in response to seasonal fluctuations of groundwater level when groundwater pumping has been strictly regulated after experiencing longtime land subsidence. Two boreholes with depth of 1226 m (G2 site) and 905 m (G3 site) were drilled at the Tianjin coastal region where severe land subsidence had occurred since the 1950s. Extensometer/piezometer groups installed at the G2 site illustrate synchronized variations of compaction and groundwater level since 2010 in the aquifer system between depth of 100-400 m which contributes most groundwater pumpage. Monitored land subsidence demonstrates that the shallow aquifer has become the main contributor to the land subsidence, and inelastic compaction still occurred in the aquifers where groundwater level has recovered. Pre-consolidation stresses show that clayey soils in depth < 100 m are under-consolidated, and deep clayey soils show the state of normal- to over-consolidation. The effects of the cyclic groundwater level oscillation on deformation were investigated using repeated loading and unloading tests. Void ratio changes in loading/unloading cycles illustrate that inelastic deformation rate decreases gradually and elastic deformation rate remains almost unchanged with increases of cyclic numbers. The deformation of soil samples from 100 to 400 m is mostly elastic for loading stress in the over-consolidation stress range. These findings suggest that groundwater dewatering in the shallow (depth < 100 m) aquifer will be the primary target to control land subsidence. Groundwater level fluctuations higher than pre-consolidation value in 100-400 m only lead to elastic and recoverable deformation even small residual permanent deformation may continue for a long time. The results improve the understanding of deformation in complex urban aquifers affected by groundwater level fluctuations and highlight the importance of city planning management for controlling land subsidence in coastal cities.

Interaction between ash and soil microaggregates reduces runoff and soil loss

Areas subjected to fire have a two-layer system (i.e., ash and soil), which brings enormous complexities to hydrogeomorphic processes. In addition, the combinations of variables from the ash and the soil characteristics result in several possible two-layer system contexts. Here, the interactions among ash and microaggregates (i.e., ash placed over fine soil microaggregates) and their effects on hydro-erosional processes are explored. The ash was produced by an experimental fire and collected from a field managed by a slash-and-burn agricultural system. The design of the experiment included a strategy for considering combinations in which each of the various factors of interest, i.e., ash and microaggregates, was present or absent. In addition, the study searched for interactions between the two factors when both were present. In total, 600 g m² of fine ash mixture (<0.250 mm), obtained from fire at different temperatures, and 90 g m² of microaggregates was placed over a small splash pan (0.135 m²). Next, a rainfall of 56 mm h^-1 lasting for 30 min was applied in four replicates for each treatment: 1) bare soil, 2) bare soil + microaggregates, 3) ash, and 4) ash + microaggregates. The interaction between the ash and soil microaggregates changed the soil hydrology dynamics, reducing soil moisture by 28% and surface runoff by 78%. The ash-microaggregates combination reduced soil loss by sheetwash by 20% and by rainsplash by 25%. Overall, the ash treatment increased soil loss by 47% compared to the case of bare soil. On the contrary, the ash-microaggregates interaction decreased soil loss by 26% compared to the ash treatment.

Contaminant transport in the sub-surface soil of an uncontrolled landfill site in China: site investigation and two-dimensional numerical analysis

A field investigation of contaminant transport beneath and around an uncontrolled landfill site in Huainan in China is presented in this paper. The research aimed at studying the migration of some chemicals present in the landfill leachate into the surrounding clayey soils after 17 years of landfill operation. The concentrations of chloride and sodium ions in the pore water of soil samples collected at depths up to 15 m were obtained through an extensive site investigation. The contents of organic matter in the soil samples were also determined. A two-dimensional numerical study of the reactive transport of sodium and chloride ion in the soil strata beneath and outside the landfill is also presented. The numerical modelling approach adopted is based on finite element/finite difference techniques. The domain size of approximately 300 × 30 m has been analysed and major chemical transport parameters/mechanisms are established via a series of calibration exercises. Numerical simulations were then performed to predict the long-term behaviour of the landfill in relation to the chemicals studied. The lateral migration distance of the chloride ions was more than 40 m which indicates that the advection and mechanical dispersion are the dominant mechanism controlling the contaminant transport at this site. The results obtained from the analysis of chloride and sodium migration also indicated a non-uniform advective flow regime of ions with depth, which were localised in the first few metres of the soil beneath the disposal site. The results of long-term simulations of contaminant transport indicated that the concentrations of ions can be 10 to 30 times larger than that related to the allowable limit of concentration values. The results of this study may be of application and interest in the assessment of potential groundwater and soil contamination at this site with a late Pleistocene clayey soil. The obtained transport properties of the soils and the contaminant transport mechanisms can also be used for the design of engineered barriers for the control of the long-term pollution of the site.

Vertical migration of leachate pollutants in clayey soils beneath an uncontrolled landfill at Huainan, China: a field and theoretical investigation

To assess the extent of leachate migration, continuous samples of clayey soils (about 9m) were obtained beneath a 17-year old uncontrolled landfill in southeastern China. The soil samples were sub sectioned and analyzed to determine the concentrations of chloride, sodium and COD in the pore water. Total nitrogen and soil organic matter content of the soil samples were also determined. Leachate-derived chloride was detected in the clayey soil to a maximum depth of 9m. Sodium and COD were found to migrate into the soils to depths of 3-4m due to the attenuation of solutes by the soil organic matter and clay minerals at the shallow soils. The estimated migration depths for the chloride are 3m in the case of pure diffusion. Advection and mechanical dispersion were found to be more important than molecular diffusion for this site with an 8m high leachate mound. By comparing the results obtained by the mathematical modeling for layered advection-dispersion problem with the measured concentration profiles, the ranges of the effective diffusion coefficient, retardation factor and dispersivity of the soils were estimated. Better fits are obtained by employing an artificial effective interface about 1m above the observed interface. The clayey soils showed a relatively high attenuation capacity for COD with the estimated retardation factor of 5.