Interfacial Processes between Mineral and Tool Surfaces - Inprotunnel

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Introduction

During mechanical headings with Tunnel Boring Machines (TBM) in fine grained soils or soft rocks (clay stones, silt stones, etc.), the excavated material often sticks to the cutting tools, cutting wheel or conveying system. This may cause problems in its excavation, transport and reuse or dumping. Responsible are mainly adhesion processes that occur at the interfaces and at the surfaces of clay minerals and tools. During the excavation and transport of the material the mechanical wear causes a loss of strength which may even lead to a complete disintegration of the composite structure. This is a desired effect, for example, if one considers the slurrification of the excavated material within an Earth Pressure Balance (EPB) shield, which allows its advance and transport in the first place. In many cases, however the excavated material sticks to the machinery, particularly in combination with wetting, which leads to far-reaching obstructions in the course of the construction progress.

Scientific background

The mechanical properties of clay and clay suspensions are primarily determined by surface geochemistry and charge distribution at the interfaces, which in turn affect the arrangement of the clay minerals.
According to the DLVO theory, far-reaching repulsive forces at the mineral surfaces occur with highly charged surfaces in diluted electrolytes that prevent dispersed particles from approaching the so-called “primary minimum”. Densely coagulated particles, on the other hand, cannot disperse since they cannot leave the attractive maximum. For example an increase in the electrolyte content, change in pH value or charge distribution may shift this energy barrier and thus changes the dispersion behaviour. As adhesion and cohesion behaviour are related to this processes they have a large impact to the problem of stickiness in tunnelling.

Aims

The Department of Engineering Geology and Hydrogeology and the Institute for Clay and Interface Mineralogy of RWTH Aachen University analyze and characterize rock and soil samples of formations, which caused problems during tunnel driving.. Mineralogical analyses (XRD, CAC, ζ-potential etc.) will be carried out to observe which minerals and effects are mainly responsible for adherence problems.

Especially the interactions at the interfaces between tool and the often past-like excavated material and the interface processes between colloids in the paste have to be understood. A change in the structure of these minerals through an active modification of the mineral surfaces allows a temporary or permanent influence on the geotechnical properties of the excavated material. This may be reached through a selective modification of the particle surface charge distribution. The modification of the mineral surfaces can be achieved either chemically, using additives or changes of the fluid chemistry, or alternatively physically, e.g. by applying electricity (electrokinetic method). Subsequently, the results are used to scale up the active manipulation of the clay minerals to the relevant site conditions and industrial requirements.
Within the project the Chair of Geotechnical Engineering of the RWTH Aachen University will develop a standardised adhesion test, that quantifies the adhesion forces realistically. The above described clay mineralogical investigations facilitate calibration and evaluation of the standard test. This test benchmarks the success of chemical and electrokinetic manipulation methods to prove the desired effects of surface charge manipulation or concerted ion change. The aim of these studies will be the development of a classification scheme that characterizes the adhesion propensity and clogging potential of the traversed material. The transfer of the results and their assessment are then tested in laboratory scale in cooperation with the industrial partners. Final aim is the transfer of feasible (new) manipulation method into real scale of tunnel driving.

The interdisciplinary joint project ‘Interfacial Processes between Mineral and Tool Surfaces – Causes, Problems and Solutions in Mechanical Tunnel Driving’ provides an important contribution to improve the efficiency in tunnel construction projects. The interdisciplinary aspect of the project considers research from mineralogical/ colloidal, geoscientific/ mechanical and engineering/ constructural perspectives and brings together results from atomic scale to industrial application scale.