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Sampling and strength testing an unbonded locked sand

Sampling and strength testing an unbonded locked sand
Sampling and strength testing an unbonded locked sand

This study examines a material on the sand/sandstone borderline where the behaviour is not dominated by bonding between grains but by the shape of the grains and how they are fitted together. This has been termed fabric structure and produces a locked sand. How these sands are formed and how they are classified is explored and the relevant areas of the large body of work which investigates the mechanics of sands is reviewed.

Fabric structure poses a problem in that it cannot be created artificially and so intact material must be tested. New sampling and specimen preparation techniques are described that allow the intact testing of locked sands with negligible grain bonding.

The high density of the intact material cannot be reproduced by recompacting the disaggregated grains, and any comparison has to be between the intact material and recompacted material at a lower density. This density should be as high as possible; an improved slow pouring method and an investigation into the fundamental mechanism by which the high densities are achieved are described.

As expected uniaxial testing showed that the material was stiffer and more brittle than a dense sand without fabric structure. In the direct shear apparatus the material produced stress induced structures, in the form of columns supporting the applied load, and large peak friction angles (φ'=65o) and dilation. At normal loads above 100kPa these were suppressed and a bilinear failure envelope formed. Normal loads of 10MPa and above produced more rock like behaviour; peak strength was not suppressed and plumose structures were formed.

The triaxial tests did not produce a bilinear failure envelope, although changes in the failure mechanism were noted. The peak friction angles were similar to those for direct shear above normal loads of 100kPa. The differences between the observed behaviour in the two forms of test are due to the boundary interactions of the stress induced structures and the final orientation of the very narrow ultimate shear band compared to the applied stresses. The results from both testing methods indicated that the peak strength of these materials is due to their fabric structure. Dilation occurred after the peak as a consequence of the disintegration of the dense fabric structure.

University of Southampton
Cresswell, Andrew Warrington
dfbbea39-46fa-4c77-82d5-fa955a03574d
Cresswell, Andrew Warrington
dfbbea39-46fa-4c77-82d5-fa955a03574d

Cresswell, Andrew Warrington (1999) Sampling and strength testing an unbonded locked sand. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This study examines a material on the sand/sandstone borderline where the behaviour is not dominated by bonding between grains but by the shape of the grains and how they are fitted together. This has been termed fabric structure and produces a locked sand. How these sands are formed and how they are classified is explored and the relevant areas of the large body of work which investigates the mechanics of sands is reviewed.

Fabric structure poses a problem in that it cannot be created artificially and so intact material must be tested. New sampling and specimen preparation techniques are described that allow the intact testing of locked sands with negligible grain bonding.

The high density of the intact material cannot be reproduced by recompacting the disaggregated grains, and any comparison has to be between the intact material and recompacted material at a lower density. This density should be as high as possible; an improved slow pouring method and an investigation into the fundamental mechanism by which the high densities are achieved are described.

As expected uniaxial testing showed that the material was stiffer and more brittle than a dense sand without fabric structure. In the direct shear apparatus the material produced stress induced structures, in the form of columns supporting the applied load, and large peak friction angles (φ'=65o) and dilation. At normal loads above 100kPa these were suppressed and a bilinear failure envelope formed. Normal loads of 10MPa and above produced more rock like behaviour; peak strength was not suppressed and plumose structures were formed.

The triaxial tests did not produce a bilinear failure envelope, although changes in the failure mechanism were noted. The peak friction angles were similar to those for direct shear above normal loads of 100kPa. The differences between the observed behaviour in the two forms of test are due to the boundary interactions of the stress induced structures and the final orientation of the very narrow ultimate shear band compared to the applied stresses. The results from both testing methods indicated that the peak strength of these materials is due to their fabric structure. Dilation occurred after the peak as a consequence of the disintegration of the dense fabric structure.

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Published date: 1999

Identifiers

Local EPrints ID: 463789
URI: http://eprints.soton.ac.uk/id/eprint/463789
PURE UUID: dc2dadfd-e96e-4703-8dc2-386cf05e4fb5

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Date deposited: 04 Jul 2022 20:57
Last modified: 16 Mar 2024 19:05

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Author: Andrew Warrington Cresswell

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