Cement-bentonite Cut-off Walls Research Articles

Effects of curing on the hydro-mechanical behaviour of cement-bentonite mixtures for cut-off walls

Cement-bentonite cut-off walls are commonly employed in geoenvironmental applications to limit ground water flow and pollutant transport. The wide diffusion of this artificial material in the current practice is not only due to its low permeability, but also to its simplicity of use. In this paper, experimental evidences about the role of curing on the hydro-mechanical behaviour of cement-bentonite mixtures are presented. Different curing times and curing conditions (representative for either water saturated or hydrocarbon polluted soils) have been considered, and their effects on both hydraulic conductivity and mechanical response in oedometer and triaxial conditions have been assessed. A unified hydro-mechanical framework, accounting for the changes of material fabric occurring with curing time and environment, is formulated. The hydraulic conductivity is very well predicted by a Kozeny-Carman like equation, whereas the mechanical behaviour is finely reproduced via an enhanced elastic–plastic constitutive model.

Effects of natural zeolite and sulfate environment on mechanical properties and permeability of cement–bentonite cutoff wall

Cement–bentonite (CB) slurry cutoff walls are extensively utilized to restrict the horizontal flow of fluids. In this study, cement was replaced by four various contents of 0%, 10%, 20%, and 30% by natural zeolite to evaluate its effects on the unconfined compressive strength, flexural strength, tensile strength, and the coefficient of permeability of cement–bentonite–zeolite (CBZ) specimens. Since underground cutoff walls are always in danger of sulfate attack, the CBZ specimens were immersed in three different containers with three different pH values of 2, 5, and 8 to study the sulfate resistance of this material. Results have indicated that the strength of specimens decreases with an increase in zeolite content at early ages; however, increasing the amount of zeolite up to 20% improves the mechanical properties and permeability of cutoff wall materials at late ages. The bentonite/cement (B/C) ratio equal to 0.2 shows a relatively better response than B/C ratios equal to 0.14 and 0.3. Curing specimens in the sulfate environment increased the mechanical strength of specimens and decreased the coefficient of permeability at first ages. At the late ages, curing in sulfate solutions manifested disadvantageous effects on specimens; however, specimens containing zeolite indicated higher sulfate resistance than specimens without zeolite.

The first microcapsule-based self-healing cement–bentonite cut-off wall materials

Despite the extensive use of cement–bentonite in contaminated land containment applications, there are still many challenges related to durability. The development of self-healing cement–bentonite materials could provide more resilient, sustainable and reliable cut-off walls with significantly enhanced durability, reduced maintenance costs, enhanced safety and protection against sudden or undetected failure. The objective of this study was to develop self-healing cement–bentonite cut-off wall materials incorporating microcapsules. Microencapsulated sodium silicate, as a healing agent, would be released into cracks when the microcapsules rupture as a result of any damage incurred, and would react with the cement–bentonite matrix to fill and heal the cracks. Novel microcapsules with switchable mechanical properties developed for self-healing cement applications were employed here. The results demonstrated the enhanced average crack mouth healing and recovered permeability performance provided by the microcapsules compared to control cement–bentonite samples. X-ray microcomputed tomography and scanning electron microscopy were applied to investigate the self-healing process of the microcapsule-containing cement–bentonite system. The microstructural analysis confirmed the survivability, uniform dispersion and crack-triggered rupture of the microcapsules, as well as the release of the healing agent and the generation of hydration products within the cracks. These are promising results for the application of the microcapsule-based system for self-healing cement–bentonite cut-off wall materials.

Crack-resistant cement–bentonite cut-off wall materials incorporating superabsorbent polymers

Preserving the integrity of cement–bentonite cut-off walls, particularly in aggressive environments, is critical to their serviceability in polluted sites. The hardened cement–bentonite material in cut-off walls is highly susceptible to desiccation and wet–dry cycles, commonly leading to cracking. The objective of the work presented in this paper was to develop crack-resistant cement–bentonite cut-off wall materials subject to wet–dry cycles. Superabsorbent polymers (SAPs), which are cross-linked polymers that can absorb and retain a large amount of water and swell as a result, were employed for this purpose. It is found that the added SAPs increased the compressive strength by decreasing the water-to-cement ratio and that the strain at failure also increased due to energy dissipative and reinforcement effects. In addition, crack resistance was greatly improved under the imposed wet–dry cycles as the matrix suction was reduced as a result of the reduction of the contact between the free pore water and cement–bentonite particles and the increase of the pore size in the matrix. The morphology and microstructure of the interconnected foam network formed by the SAP films in the matrix were identified with SEM-EDX and micro-CT scan analyses. The results demonstrated the significant potential for SAPs in the development of crack-resistant cement–bentonite cut-off wall materials.

Modelling evaporation processes of cement-bentonite mixtures

Upon drying, matter and energy are exchanged between the atmosphere and porous media through evaporation, which is a coupled process that involves the simultaneous transport of liquid water, water vapour and heat. At shallow depths, evaporation controls the water content and suction of both natural soils and earthworks, affecting their hydraulic response. This impact is particularly relevant when the earthworks are aimed at the containment of aqueous or non-aqueous pollutants, as in the case of cement bentonite cut-off walls. A coupled model for the transport of liquid water, water vapour and heat through cement bentonite mixtures upon evaporation was formulated. The model considers flow of water driven by both total suction and temperature gradients. Model predictions were compared to experimental results obtained in the laboratory on samples having different sizes and imposed boundary conditions. A good agreement between predicted and measured volumetric water contents was obtained, once defined a suitable dependency of the relative permeability of the mixture on degree of saturation. The results suggest that the proposed formulation correctly accounts for the underlying physical processes, and that it might be used to model the real scale behaviour of cut-off walls.

Laboratory and In Situ Tests for Long-Term Hydraulic Conductivity of a Cement-Bentonite Cutoff Wall

Slurry trench cutoff walls, constructed using self-hardening slag-cement-bentonite (Slag-CB), are the most common form of in-ground vertical contaminant barrier in the U.K., Europe, and Japan, and are increasingly being used in the United States. This paper presents a case study of the hydraulic conductivity evaluation of an 11-year-old Slag-CB wall material at a sulfate-contaminated site, using different in situ techniques and laboratory tests. The laboratory results suggest that the hydraulic conductivity of the samples, which vary in age from 4 weeks to 11 years, decreases with time for the first 3 years but then remains constant. The results indicate that the long-term performance of these containment walls is influenced by various parameters such as aging, the type/duration of contaminant exposure, mixing of surrounding soil during construction, and wall depth. Piezocone tests, packer tests, and self-boring permeameter tests were carried out in the field to determine the suitability of different in s...

The chemical compatibility of cement–bentonite cut-off wall material

Containment techniques are amongst the most common methods of remediating land contaminated by previous industrial use. An important part of the containment process is the placing of vertical in-ground barriers to minimise the movement of contamination from site. Self-hardening slurry trench cut-off walls of cement–bentonite are barriers that are increasingly being used in the United Kingdom. The use of cement–bentonite slurry trench cut-off walls, particularly in highly aggressive environments, raises concerns over durability and long-term performance. The relatively recent use of such barriers means that there is little information on their long-term performance. This paper describes research being undertaken to investigate the properties of cement–bentonite cut-off walls and to examine potential durability problems. Laboratory immersion tests have been used to assess the chemical resistance of typical cement–bentonite mixes containing ordinary Portland cement, ground granulated blast-furnace slag and pulverised fuel ash. These mixes showed varying degrees of resistance to chemical attack with pulverised fuel ash mixes more resistant than those with ground granulated blast-furnace slag. The advantages and limitations of such tests are discussed.

Design and construction of cement-bentonite cutoff wall for S.A. Murray Jr. hydroelectric station : Ramachandra, K; Swan, C H Proc Symposium on Slurry Walls, Construction and Quality Control, Atlantic City, 27–28 June 1991P140–150. Publ Philadelphia: ASTM, 1992 (ASTM Special Technical Publication N1129)

Design and construction of cement-bentonite cutoff wall for S.A. Murray Jr. hydroelectric station : Ramachandra, K; Swan, C H Proc Symposium on Slurry Walls, Construction and Quality Control, Atlantic City, 27–28 June 1991P140–150. Publ Philadelphia: ASTM, 1992 (ASTM Special Technical Publication N1129)

Use of cement-bentonite cut-off wall to contain possible pollutants from a landfill containing highly polluted dredging spoils : Loxham, M; Westrate, F A Engng GeolV21, N3/4, June 1985, P359–365

Use of cement-bentonite cut-off wall to contain possible pollutants from a landfill containing highly polluted dredging spoils : Loxham, M; Westrate, F A Engng GeolV21, N3/4, June 1985, P359–365

The use of a cement-bentonite cut-off wall to contain possible pollutants from a landfill containing highly polluted dredging spoils

The dredging spoil from the sea canal at Ghent in Belgium has been landfilled at a specially engineered site at Zelzate. In order to prevent the spread of pollutants from the site to the surroundings a cement-bentonite screen has been constructed down to a thick clay layer 25 m under the surface. Calculations show that this barrier is adequate for the environmental objectives stipulated.