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The Effect of Continuous Flight Auger Pile Installation on the Soil-Pile Interface in the Mercia Mudstone Group

Seward, L (2009). The Effect of Continuous Flight Auger Pile Installation on the Soil-Pile Interface in the Mercia Mudstone Group. (Unpublished Doctoral thesis, City, University of London)


The research reported in this dissertation examines the physical and chemical changes that occur to in situ soil at the soil-pile interface for continuous flight
auger piles installed in the Mercia Mudstone Group. Four Continuous Flight Auger (CFA) piles were installed in the Gunthorpe Member of the Mercia Mudstone Group, central England. The effect on the soil-pile interface of overrotation of the auger during installation, and the addition of water during installation were investigated.

Once the piles had been left to cure, they were excavated and returned to City University, London, with the surrounding soil. The excavated piles and soil were
examined using a variety of microscopic and macroscopic techniques including inductively coupled plasma spectroscopy (ICP) and X-ray diffraction (XRD);
with water contents, chemical content (ICP) and mineralogical content (XRD) tested. Plastic index and particle size distribution tests were used to show the
physical effects of piling on the host soil and preliminary strength testing was carried out to provide insight into the strength characteristics of the soil
surrounding the pile.

In all four piles a distinct zone of remoulding was observed around the pile shaft. In each case the remoulded zone was a brown to red, clay rich layer varying between 0mm and 55mm in thickness. In almost all cases this remoulded zone had a structure and fabric which was not related to the in situ soil. Around all
piles it was further noted that vertical fissures were present, and fanned out from the pile shaft in a clockwise direction.

Two of the piles were installed with the addition of water. Around these piles it was noted that the remoulded layer often split into two or three distinct layers, with one of these layers often containing millimetre scale aggregations of green silt.

Tests showed a higher percentage of clays present within this remoulded zone, and indicated that SiO2 (a major rock forming element and considered by some
authors to be an aggregating agent within the Mercia Mudstone Group) was more abundant within remoulded than undisturbed soil. The clay fraction showed a low abundance of high swelling clays in all cases.

It was concluded that installing piles within the Mercia Mudstone Group causes remoulding of the soil directly adjacent to the pile shaft. The least remoulding
occurred when the pile was augered normally with no added water. All four remoulded zones contained fissures, fanning clockwise from the pile, however, these were more pronounced in the dry piles, while the wet piles had a more massive, granular texture to the remoulded zone. For all piles, except the pile which was over-rotated and installed with no added water, the percentage of
clays within the remoulded zone was greater than outside the remoulded zone. This indicates that the aggregates of clays found naturally within the Mercia Mudstone Group may be split into their constituent clays during the piling process.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > Engineering
Doctoral Theses
School of Science & Technology > School of Science & Technology Doctoral Theses
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