Geotechnical characteristics of Belfast's estuarine clayey silt (sleech)

Bryan A McCabe; Barry M Lehane; Bryan A McCabe; Barry M Lehane

doi:10.3934/geosci.2025006

AIMS Geosciences

2025, Volume 11, Issue 1: 91-116. doi: 10.3934/geosci.2025006

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Geotechnical characteristics of Belfast's estuarine clayey silt (sleech)

Bryan A McCabe ^{1
,
,},
Barry M Lehane ²

1.
School of Engineering, University of Galway, Galway, Ireland
2.
School of Engineering, University of Western Australia, Perth, Australia

Received: 04 December 2024 Revised: 28 January 2025 Accepted: 26 February 2025 Published: 10 March 2025

Belfast, the capital city of Northern Ireland and the second largest city on the island of Ireland, has experienced significant construction activity in recent years, yet relatively little has been published on the problematic soft estuarine deposits known locally as sleech which underlie the city and its hinterland. Results of a detailed characterization of the sleech at a site near Holywood, Co. Down, 8 km northeast of the city centre, are presented in this paper. This characterization was carried out in conjunction with a unique suite of full-scale foundation load tests at the site, commencing in the late 1990s. In situ tests clearly identified distinct sandy and silty horizons within the deposit, the bulk of which is a high-plasticity lightly-overconsolidated organic clayey silt, with clay content increasing with depth and clay fraction dominated by illite and chlorite. Both the moderate organic content and presence of diatom microfossils explain the relatively high Atterberg limits, the high compression index and the high friction angle of this material. Undrained shear strengths in triaxial compression fall at the upper end of the expected range based on a widely-used correlation with overconsolidation ratio, but are broadly compatible with in situ shear vane strengths corrected for plasticity index. The constant volume friction angle is remarkably insensitive to the specimen's stress history and the particle size distribution. Despite the high silt content, permeabilities and coefficients of consolidation are more typical of a clay than a silt. While the sleech behaviour is shown to be broadly similar to the estuarine deposits at the well-characterised geotechnical test bed at Bothkennar in Scotland, the paper illustrates that low OCR clays can have their own distinguishing characteristics.
- estuarine silt,
- sleech,
- organic,
- diatom microfossils,
- compressibility,
- shear strength
Citation: Bryan A McCabe, Barry M Lehane. Geotechnical characteristics of Belfast's estuarine clayey silt (sleech)[J]. AIMS Geosciences, 2025, 11(1): 91-116. doi: 10.3934/geosci.2025006

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Abstract

Belfast, the capital city of Northern Ireland and the second largest city on the island of Ireland, has experienced significant construction activity in recent years, yet relatively little has been published on the problematic soft estuarine deposits known locally as sleech which underlie the city and its hinterland. Results of a detailed characterization of the sleech at a site near Holywood, Co. Down, 8 km northeast of the city centre, are presented in this paper. This characterization was carried out in conjunction with a unique suite of full-scale foundation load tests at the site, commencing in the late 1990s. In situ tests clearly identified distinct sandy and silty horizons within the deposit, the bulk of which is a high-plasticity lightly-overconsolidated organic clayey silt, with clay content increasing with depth and clay fraction dominated by illite and chlorite. Both the moderate organic content and presence of diatom microfossils explain the relatively high Atterberg limits, the high compression index and the high friction angle of this material. Undrained shear strengths in triaxial compression fall at the upper end of the expected range based on a widely-used correlation with overconsolidation ratio, but are broadly compatible with in situ shear vane strengths corrected for plasticity index. The constant volume friction angle is remarkably insensitive to the specimen's stress history and the particle size distribution. Despite the high silt content, permeabilities and coefficients of consolidation are more typical of a clay than a silt. While the sleech behaviour is shown to be broadly similar to the estuarine deposits at the well-characterised geotechnical test bed at Bothkennar in Scotland, the paper illustrates that low OCR clays can have their own distinguishing characteristics.

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