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One of the traditional tasks belonging to civil engineering is to build underground sealing systems. Vertical diaphragm walls are thereby placed underground in order to reduce the loss of water from storage areas and canals and to increase the level of stability in hydraulic engineering constructions. They are also employed to reduce the possibility of water entering excavations, that is an extensive lowering of the water table. Since it is necessary to enclose not only the storage area used for old industrial waste but also places of production which could endanger the ground water as well as create a means for storing drinking water and thermal energy, there has been an emergance of new fields in recent years which require diaphragm walls to be built. Diaphragm walls are a means of solving these tasks, since as a rule they bind in low-permeation soils or in artificially produced sealing bases.
The construction of diaphragm walls has gained increasing importance in recent years as part of the slurry cut-off-wall method. This can be ascribed to the fact that the economical self-hardening wall procedure (single-phase-method) has been further developed and that new areas in the field of environmental conservation have been found for their employment. Consequently, this paper will be concentrating mainly on diaphragm walls, which are produced in the single-phase-method.
In the above-mentioned fields where diaphragm walls are used, their main function is to seal off water. When dealing with the sanitation of old waste, the sealing wall materials are in addition often exposed to chemical reactions caused by polluted seepage water. Just how successful a diaphragm wall is in fulfilling its function is determined decisively by the chemical and physical properties of the sealing materials employed. Mineral sealing materials generally comprise bentonite, cement, loading material and water. In special cases a chemical additive is also employed. In determining the composition and features of mineral sealing material, experiments are carried out individually for each project to test whether the substance is suitable and this is in turn monitored by means of soundness tests on the building site. As a result, it is generally possible to determine basic characteristics of the new suspension from the hardening sealing material as well as singleľaxial cylindrical compression strength and permeability. Since each diaphragm wall project means different raw materials are used depending on what is available locally and are also in many cases not standardized, it is difficult to transfer, or rather compare the results derived from qualification tests of suitability carried out for different sealing walls. In addition, the specifications for diaphragm walls, and thus the quality standard that could so be achieved, are not set in recommendations or standardizations. The few publications existing on the mechanical behaviour of sealing materials describe specific projects only or deal with especially selected problems. A technology of concrete, comparable with "the technology of sealing materials" does not exist.
Hermanns, Rita and Meseck, Holger
Clay; ClayGroup; remediation; encapsulation; slurry walls; contaminated sites; barriers; Ton; Einkapselung; Dichtwande; Barrieren; Sickerung; Sanierung