Long-Term Engineering Performance and In-Situ Assessment of Cement Bentonite Cut-Off Walls

Kaushal Joshi, Cambridge University
Geotechnical Engineering Group


Slurry trench cut-off walls, constructed using self-hardening slag cement-bentonite (CB) material, are the most common form of in-ground vertical contaminant containment barrier in the UK, Europe and Japan. However, little is known about their long-term performance in chemically aggressive ground conditions and there has been little in situ monitoring/testing of the walls. This thesis presents microstructural, hydraulic, mechanical and durability characteristics of up to 12 year old contaminated CB cut-off wall material. The research identifies factors affecting their long-term performance such as age, contamination (due to exposure to aggressive chemicals and mixing of surrounding soil during construction), confinement (or depth of wall), desiccation (due to groundwater fluctuation and weathering action), mix proportion of ingredients, and scale (or size) of test section.
A series of laboratory experiments was conducted on three types of CB samples having varying degree of contamination viz., the laboratory cured sample made from uncontaminated homogeneous CB slurry, the samples cast from contaminated heterogeneous slurry taken from excavation trench during the time of the cut-off walls’ construction at a test site in April 1996 and the samples obtained from the same test wall in form of block field specimens. The test site was a disused gasworks with high concentration of sulphates. The samples were tested at various ages which ranged from 1 day to 12.25 years.
The microstructural investigation revealed that all the ingredients fuse together to form an amorphous material with high porosity. The hydraulic conductivity and unconfined compressive strength (UCS) of CB samples tend to improve with time, especially during initial 90 days and the field samples have more variation in their results due to heterogeneity caused by contamination. Additionally, UCS and undrained triaxial tests on different mixes revealed that the mechanical property is affected more by the proportion of mix than by the type of bentonite or cement. Results from the undrained and drained triaxial tests, and shear wave velocity data from bender elements tests showed that the mechanical behaviour of the material depends upon aging and confining pressure. Depending upon age and confinement, the CB specimens fail in ‘tension’ or ‘shear’ modes under undrained loading and in ‘strain hardening’, ‘ductile’ or ‘brittle’ failure modes under drained loading. A conceptual framework for constitutive modelling is proposed based on the triaxial test results. Wetting-drying experiments showed that CB material can resist desiccation if surrounded by soil with residual water saturation. Although exposure to sulphates can degrade the material, the old and uncontaminated sample displayed better resistance.
Furthermore, in situ tests were conducted on the field wall in year 2003-04 using piezocone, self-boring pressuremeter and packers to evaluate hydraulic conductivity and undrained shear strength. The in situ data were analysed with newly developed shape factors by using FEMLAB package for 3D FEM analysis. The relative comparison of these three techniques highlighted that piezocone test is unsuitable for old and brittle CB wall. Also the comparison of in situ and laboratory tests results showed that their difference is because of variation in scale of measurement. Tests on large sized specimen are recommended. The overall behaviour of CB material is different than typical soils, rocks or cemented soils. The research outcome is useful for understanding future liability of CB walls and improving their design.