FLOW AND FILTRATION BEHAVIOUR OF POLYMER BASED EXCAVATION SUPPORT FLUIDS IN COARSE SOILS

  • John Ejezie

Student thesis: Phd

Abstract

Partially hydrolysed polyacrylamide (PHPA) based polymers are used in a variety of civil engineering applications including as excavation support fluids in construction work such as piling, diaphragm walling and borehole drilling. For these applications, the role of the fluid is to keep the excavation open during digging. PHPA based polymers offer promising solutions to challenging problems such as inhibiting the dispersion of fine soils into the support fluid. They also require a smaller site plant set-up than conventional bentonite clay based support fluids because of their ability to inhibit soil dispersion so that there is no requirement for soil-fluid separation plant. They also may show reduced environmental impacts. However, there can be problems, for example, excessive loss of polymer from the excavation by permeation of the fluid into the adjacent ground. This is not only detrimental to excavation stability but also wastes polymer. To tailor polymers to meet the demands of specific engineering projects it is necessary to develop a fuller understanding of their flow and sorption behaviours in soils. The research presented in this thesis sets out the results of a study to develop the understanding of the underpinning science as related to the rheological and sorption behaviours of pure polymer fluids and polymer slurries (that is polymer fluids with suspended soil) in coarse-grained soils. The results of a series of laboratory studies of the flow of PHPA polymer fluids in sands over a range of grain sizes and pressure heads are presented. The data were obtained using a constant head permeability set-up modified to obtain data over a range of depths into the sand bed and also with the American Petroleum Institute standard filter press apparatus. In parallel, theoretical aspects of polymer fluid flow in porous media have been revisited and applied to obtain a better insight into the underpinning microscopic and macroscopic phenomena involved. Measurements made under steady-state flow conditions showed that depending on the concentration of active polymer in solution, the soil hydraulic conductivity (i.e. Darcy permeability rather than intrinsic permeability) to polymer fluid is much lower than that for water. The validity of modified porous media models to explain the observed behaviour was tested against the experimental data. The results indicate that for the polymer fluids, fluids which show power-law type behaviour, the flow behaviour indices obtained from the permeability tests differ significantly from those from rotational viscometer tests over nominally comparable shear rates. When the resistance to flow of pure polymer fluids over a range of depths into the sand bed was examined it was found that there was a zone of reduced hydraulic conductivity perhaps due to sorption which was observed in a related experiment. Extensive microfiltration experiments to characterise this phenomena further confirmed that indeed a significant proportion of pores invaded by polymer fluid can be unavailable for flow. The validity of a capillary bundle model to explain the observed behaviour was tested against the experimental data. The results indicate that such a model is inadequate to address the blocking behaviour of polymer solutions in soil pores. The model assumes that the blocking index, which is an indicator of the type of blocking occurring, is equal to 1 for intermediate blocking irrespective of the morphology of the porous media. However, in the experimental work it was found that the blocking index was significantly less than 1 and also varied (although marginally) with increasing pressure. After an analysis of the flow behaviour of pure polymer solutions in porous media, the latter part of this thesis describes the behaviour of their counterparts containing dispersed soil particles (slurries) which are a fuller indicator of in-excavation behaviour. From the results with slurries, the various stages of blocking leading up
Date of Award31 Dec 2018
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorAhmad Syed Mohd Syed (Supervisor) & Majid Sedighi (Supervisor)

Keywords

  • Capillary bundle models; Excavation; Polymer; Rheology; Permeability; Stability mechanism; Filtration; Polymer slurry; Viscosity membrane; Polymer filter cake

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