EPB Excavation and Conditioning of Cohesive Mixed Soils: Clogging and Flow Evaluation

Abstract

When shield tunnels are being excavated along varying geological conditions, it is essential to understand the behaviour of the material being excavated, as it influences excavation speed and ground stability, in addition to other operational issues. For an Earth Pressure Balance Machine (EPBM), this requirement is increased because the excavated material, combined with the rotation of the machine screw conveyor and the thrust of the hydraulic jacks, is responsible for holding the pressure ahead of the machine. For this system to function, this material needs to maintain certain engineering characteristics. In the absence of those key target properties, additives should be mixed with the excavated soil in front of the machine or in the conveyor system, a process called ground conditioning. There are numerous bodies of research concerning EPBM conditioning that mostly focus on either sandy or clayey soils, with relatively little published information regarding mixed ground. Terrains originating from the tropical weathering of rocks present these mixed ground characteristics. EPBM have excavated this type of ground in many areas around the world. The aim of this thesis is to investigate and characterize this type of terrain, exploring the particularities regarding its excavation by EPB shields, including the conditioning of those materials. The focus is on cohesive mixed soil, thus restraining the topic. This research includes two main subjects: the assessments of clogging behaviour and flow behaviour of cohesive mixed soils for EPBM excavation, drawing a parallel between them whenever possible. A laboratory routine was developed for investigating and characterising these mixed soils and their flow and clogging tendencies. Soils were assembled in the laboratory, allowing control of all the inspected elements. Several conclusions related to the clogging potential and flow behaviour could be achieved by changing certain variables: clay mineral, clay fraction, grain size of the sand fraction, water content, foam, and polymers. This laboratory routine and some of the achieved conclusions can be also applied to materials other than cohesive mixed soils. Some considerations for future work are provided, foreseeing the need for a future correlation and validation of the assumptions here made with real tunnel drives.