Cone Tip Apex Angle Effects on Cone Penetration Testing

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Title: Cone Tip Apex Angle Effects on Cone Penetration Testing
Author: Browning, John Adam
Advisors: M. Shamimur Rahman, Committee Member
Roy H. Borden, Committee Member
Mohammed A. Gabr, Committee Chair
Abstract: Research work in this thesis deals with the Cone Penetrometer Test's (CPT) cone tip apex angle effects on penetration resistance, and the soil parameters that can be derived from its value. Five tips with apex angles of 50, 60, 75, 90 and 120° were pushed in a controlled laboratory environment and at three separate field sites in North Carolina. The laboratory study was performed to investigate the relative effect, as compared to the standard 60o apex angle, of different angles on the thrust needed to push the cone. The field testing was completed to investigate the tip apex angle effect in naturally occurring conditions. In general, the results showed that the tip apex angle has an increasing effect on tip resistance with increasing relative density. This finding agrees with Nowatzki and Karafiath's (1972) suggestion that two tip geometries, one with an acute apex angle (2α< 90°) and one with an obtuse apex angle (2α&8805;> 90°), should be pushed at every site if a failure mechanism within a given soil is to be discerned. Following such approach, if the two tip resistances are relatively the same, the soils are loose (Dr ≥ 34%) and compression controls the failure mode and therefore the thrust magnitude. On the other hand, if the two tip resistances vary significantly, the soils are medium dense to dense (Dr > 34%) and failure is controlled by shear strength. Based on the field testing it is determined that as the tip angle increases in dense soils, the acute angle tips (50, 60, and 75°) provide higher tip resistances than the obtuse angle tips (90 and 120°). Since the non-standard, obtuse angle tips are more advantageous because they require less thrust to penetrate dense soils than the standard 60 degree tip, previously established relationships between tip resistance and soil parameters for the 60 degree tip were modified for non-standard tip geometries. Based on field testing and previously established relationships for the standard 60° tip angle, empirical relationships are developed for the five tips tested to relate normalized cone resistance to the soil behavior type index (as defined by Robertson 1990), relative density, friction angle, and normalized SPT N60.
Date: 2006-04-27
Degree: MS
Discipline: Civil Engineering

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