Clay membrane made of natural high plasticity clay: leachate migration due to advection and diffusion

摘要

Leachate containment in Denmark has thorughoutthe years been regulated by the DIF Recommendation for SanitaryLandfill Liners (DS/R 466). It states that natural clay depositsmay be used as membrane material provided the membrane anddrainage system contains at least 95% of all leachate createdthroughout the lifetime of the landfill. Presently, therecommendation is under revision partly because this clause wasconsidered to be insufficient for securing the drinking waterquality I underlying aquifers—a necessary containment of at least99% is foreseen. The revision should consequently take intoaccount advective ion transport as well as diffusion. Clayprospecting for clays rich in smectite has revealed large depositsof Tertiary clay of very high plasticity in the area aroundR?dbyhavn on the Danish island of Lolland. The natural claycontains 60-75% smectite, dominantly as a sodium-type. The claymaterial has been evaluated using the standardized methodsrelated to mineralogy, classification, compaction andpermeability, and initial studies of diffusion properties have beencarried out. Furthermore, at a test site the construction methodsfor establishing a 0.15-0.3m thick clay membrane have beentested successfully. At a natural water content of w=40-45% it ispossible to establish a homogeneous membrane with hydraulicconductivity k<5×10-12m s-1 giving an extremely low transport byadvection. The hydraulic conductivity measured in oedometertests used for establishing swell and deformation properties wereshown to be very dependent on the stress level.It varies fromk=10-2 m s-1 at vertical stresses from 5 to 4800 kPa and is clearlyrelated to a reduced effective porosity diminishing with stress.Preliminary diffusion tests indicate a similar influence on theeffective diffusion coefficient being much lower than anticipatedusing the total porosity. These properties are of major importanceto the future use of clay membranes for the containment ofhazardous waste. In order to explain these propertiesmicrostructural investigations were initiated to establish theboundary conditions for the evaluation of relevant transportprocesses. Consequently, this clay material may be used as rawmaterial to be compacted for membrane purposes in a natural wetcondition using high compaction effort comparable to themodified Proctor energy. It fulfils the requirements that arepresently proposed in the new Danish Recommendation forLandfills and a number of other applications that are foreseen forclay embedment of hazardous waste.