Soil-bentonite Walls Research Articles

Analytical solution for contaminant transport through a GCL-enhanced composite cutoff wall system

The geosynthetic clay liners (GCLs) have been recently shown to be effective in cutoff wall applications. Yet, its influence on contaminant transport through cutoff wall system is rarely reported. This study presents an analytical solution for the transport of organic contaminant through a composite cutoff wall system comprising a soil-bentonite wall, a GCL, and an aquifer. The analytical solution was obtained by variable separation method and verified against an existing analytical solution and a numerical solution. Parametric analysis was conducted to elucidate the effects of GCL, soil-bentonite (SB) wall thickness (Lw), aquifer thickness (La), and contaminant half-life (t1/2) on contaminant transport. The results show that by adding a GCL layer, the contaminant concentration and the maximum flux can be significantly decreased. When the GCL retardation factor increases 1 to 300, the required time to reach the maximum flux is delayed from about 62 years to 1500 years. Larger aquifer thickness sufficiently weakens the contaminant transport process. Under the same breakthrough criteria, the required thickness of SB wall decreases by about 20% when the GCL retardation factor increases from 1 to 50. The proposed analytical solution provides a useful tool for the design of the SB-GCL cutoff wall system.

Performances of the Soil–Bentonite Cutoff Wall Composited with Geosynthetic Clay Liners: Large-Scale Model Tests and Numerical Simulations

The geosynthetic clay liner (GCL) overlap plays a key role in maintaining hydraulic performance of the barrier systems, e.g., the bottom liner and cover systems. However, its influences on the behavior of the vertical barrier have been rarely investigated. This paper aims to address this issue using the large-scale model test and 3-dimensional finite element (FE) modeling. In the model test, the GCL overlap at the width of 500 mm was tested under a constant hydraulic head of 1 m and confining stress ranging from 10 to 150 kPa using a newly developed large-scale apparatus. Compared with the flexible wall permeameter, this apparatus could guarantee full field-scale GCL overlap to be tested. Results showed that the effective hydraulic conductivity of the GCL overlap decreased from 10−8 to 10−9 cm/s as the confining stress increased from 10 to 150 kPa. The addition of supplemental bentonite paste in between the overlap with a water-to-bentonite ratio of 19:1 contributed to reducing the effective hydraulic conductivity by 60% compared with that for a GCL overlap without bentonite paste. The breakthrough time for the soil-bentonite (SB) cutoff wall composited with GCLs was 64% longer in comparison with that for the single SB wall. Additionally, the breakthrough after 50 years is made for the entire depth of the single SB wall while at the depth no more than 0.9 m for the composite wall with bentonite paste at the GCL overlap. With consideration that the depth of the groundwater table is generally greater than 1 m, the GCL–SB composite cutoff wall will exhibit a good performance in containing groundwater contaminants in the field, especially when applying bentonite paste at the GCL overlap.

Centrifuge modelling of lead retardation in soil–bentonite cut-off walls

Soil–bentonite (SB) walls are commonly used as barriers for in situ containments at landfills. SB walls are designed to control the movement of persistent pollutants such as heavy metals in the long term. This paper presents centrifuge and numerical modelling of 50 years of performance of a loess-amended SB (LSB) wall in comparison with a conventional SB wall. Both walls were 28 m high and subjected to a hydraulic head difference of 2 m in the prototype. A purposely designed apparatus was developed to impose the hydraulic head and a pollutant loading of 200 mg/l lead(II) on two barriers simultaneously. The apparatus was loaded on the ZJU400 geotechnical centrifuge and spun to 100g. Similitudes of dispersion and adsorption of lead(II) in a centrifuge are discussed. Stress redistribution was observed in small-scale walls due to the enhanced soil arching effect. The lead(II) migration distance in the LSB wall after 50 years was 44.3% shorter than that in the SB wall. The predicted breakthrough time for the LSB wall was 149.1% longer than that for the SB wall when the thickness was 0.6 m. Retardation effects could be attributed to the significant ability of loess to absorb lead(II).

Fate of bentonite in slag–cement–bentonite slurry trench cut-off walls for polluted sites

Soil–bentonite (SB) and slag–cement–bentonite (slag-CB) slurry trench cut-off walls are widely used low-permeability subsurface barriers against groundwater or contaminants. The bentonite in SB walls is instrumental to the performance of the barrier. However, when mixed with Portland cement and slag in slag-CB walls, bentonite participates in ‘secondary’ chemical reactions throughout cement hydration. Using mineralogical methods, the goals of this paper are to examine the fate of bentonite in slag-CB mixtures and to discuss the implications of the associated chemical reactions for the slag-CB wall performance. Laboratory-prepared mixtures of dry and hydrated slag-CB were analysed using X-ray diffraction over 3 months to investigate the evolution of mixtures during curing. Results confirm that bentonite clay minerals dissolve in slag-CB slurries and that bentonite is virtually absent in the hardened slag-CB material. Furthermore, carbonation and pozzolanic chemical reactions also take place on slag-CB hydration and curing, which form secondary minerals over time. Hence, unlike SB mixtures, the bentonite in slag-CB mixtures no longer retains its original crystalline structure, as it participates chemically in hydration processes, although it becomes a precursor of pozzolanic products. Nonetheless, bentonite is instrumental to the workability and initial set of the mixture. It is concluded that the concepts of compatibility traditionally accepted for SB walls are inappropriate for slag-CB walls. Possible implications include that due to the onset of carbonation, the barrier could degrade if the contaminants are acidic; also, the formation of pozzolanic hydrates may improve the contaminant sorption properties of the barrier.

Lead adsorption and transport in loess-amended soil-bentonite cut-off wall

Soil-bentonite slurry-trench cutoff walls using backfill consisting on-site sandy or silty soil and bentonite are extensively used as engineered barriers for the containment of groundwater and soil pollution. Chinese loess has been shown to have large adsorption capacity regarding heavy metals. There is a great potential to use Chinese loess as amendments to improve the adsorption capacity of the wall. Batch and column tests were carried out to investigate the adsorption and transport of Pb(II) in loess modified soil-bentonite (LSB). Batch tests were conducted to study the effects of contact time, initial Pb(II) concentration, loess proportion in LSB, and pH on Pb(II) adsorption. The rate constant of pseudo-second-order kinetic adsorption model of LSB is two times greater than that of soil-bentonite (SB), which indicates that the adsorption rate of LSB is much faster than that of SB. The mean free energy of adsorption evaluated by D-R model is 15.56kJ/mol and 18.89kJ/mol for SB and LSB, respectively. This indicates the adsorption mechanisms of SB and LSB are mainly ion exchange and chemical reaction. The adsorption capacity of LSB increases linearly with the increase of the loess amount in LSB. The maximum adsorption amount of LSB containing 20% loess can be 2 times greater than that of SB. Results also indicate that Pb(II) is first adsorbed onto loess until the adsorption amount Pb(II) onto loess reaches its adsorption maximum. An equation considering this effect was proposed for prediction of the adsorption capacity of LSB. A set of column tests were carried out to evaluate the tortuosity, mechanical dispersion and retardation factor regarding lead transport in SB and LSB vertical engineered barrier. The retardation factor of LSB and SB are determined to be 38 and 15, respectively, by the column tests. The Kd values obtained from batch tests are significantly larger than those obtained from column tests due to the difference of soil-water ratio used for these two types of tests. Using the obtained parameter values, the breakthrough time of Pb(II) through LSB wall is evaluated to be about 2 times greater than that of the SB wall when the hydraulic head difference is <3m. This may be due to the increase of adsorption capacity of the soil-bentonite wall amended with the loess.

Mitigation of reverse faulting deformation using a soil bentonite wall: Dimensional analysis, parametric study, design implications

Recent major seismic events, such as the Chi-Chi (1999) and the Wenchuan (2008) earthquakes occurred in Taiwan and China, have offered a variety of case histories on the performance of structures subjected to reverse faulting–induced deformation. A novel faulting mitigation method has recently been proposed, introducing a soft deformable wall barrier in order to divert the fault rupture away from the structure. This can be materialized by constructing a thick diaphragm-type soil bentonite wall (SBW) between the structure and the fault rupture path. The paper investigates the key parameters in designing such a SBW, aiming to mitigate the fault rupture hazard on shallow foundations. The paper employs a thoroughly validated finite element analysis methodology to explore the efficiency of a weak SBW barrier in protecting slab foundations from large tectonic deformation due to reverse faulting. A dimensional analysis is conducted in order to generalize the validity of the derived conclusions. The dimensionless formulation is then used to conduct a detailed parametric study, exploring the effect of SBW thickness w/H, depth HSBWl/H, and shear strength τsoil/τSBW, as well as the bedrock fault offset h/H, foundation surcharge load q/ρgB, and fault outcrop location s/B. It is shown that the wall thickness, depth, and shear strength should be designed on the basis of the magnitude of the bedrock fault offset, the location of the fault relative to the structure, and the shear strength of the soil. The efficiency of the weak barrier is improved using lower strength and stiffness material compared to the alluvium. A simplified preliminary design methodology is proposed, and presented in the form of a flowchart.

Soil bentonite wall protects foundation from thrust faulting: analyses and experiment

When seismic thrust faults emerge on the ground surface, they are particularly damaging to buildings, bridges and lifelines that lie on the rupture path. To protect a structure founded on a rigid raft, a thick diaphragm-type soil bentonite wall (SBW) is installed in front of and near the foundation, at sufficient depth to intercept the propagating fault rupture. Extensive numerical analyses, verified against reduced-scale (1 g) split box physical model tests, reveal that such a wall, thanks to its high deformability and low shear resistance, “absorbs” the compressive thrust of the fault and forces the rupture to deviate upwards along its length. As a consequence, the foundation is left essentially intact. The effectiveness of SBW is demonstrated to depend on the exact location of the emerging fault and the magnitude of the fault offset. When the latter is large, the unprotected foundation experiences intolerable rigid-body rotation even if the foundation structural distress is not substantial.

Flow past Bench-Scale Vertical Ground-Water Cutoff Walls

An experimental study was conducted to evaluate factors affecting flow rates past soil-bentonite (SB), geomembrane (GM), and composite geomembrane-soil (CGS) vertical cutoff walls. Intact walls and walls containing defects (inadequate keys, windows, and poor joint seals) were studied. For intact cutoff walls, CGS walls had the lowest flow rate, followed by SB and GM walls. CGS walls typically had flow rates as much as 100 times lower than comparable GM walls. Flow rates increased by a factor of 2--160 when the GM walls contained defective joints or the SB walls contained permeable windows. For all wall types, an effective key was required to achieve low flow rate past the wall. For GM walls, under seepage was not effectively controlled when the GM was placed in direct contact with the aquitard. Flow rates for GM walls placed in direct contact with the aquitard were nearly equal to the aquifer flow rate without a wall. Better control of underseepage was possible with SB and CGS walls placed in direct contact with the aquitard because the soft bentonitic backfill conformed to the surface of the aquitard. Hydraulic conductivity of unsealed joints in GM walls was estimated from flow rates past GM walls wheremore » a portion of the joint was unsealed. These hydraulic conductivities ranged between 1.8 x 10{sup {minus}4} and 5.6 x 10{sup {minus}3} cm/s.« less