OR/14/052 Conclusions

Boon, D, Kirkham, M, and Scheib, A. 2014. Physical properties of till deposits from Anglesey, north west Wales. British Geological Survey Internal Report, OR/14/052.

1. The till deposits from Anglesey are variable in texture and composition ranging from widely-graded ‘fine’ soils to ‘coarse’ soils. Some tills are fissured. These characteristics result in slight variability in geotechnical properties across island and are partly a function of the underlying bedrock geology and pre-existing quaternary deposits prior to the last ice advance. Even slight variability in geotechnical properties may have affected subglacial processes within the deforming bed and may have influenced the formation of glacial landforms and ice stream dynamics.
2. The tills are typically ‘very dense’ or ‘hard’ soils and generally require ‘hard digging’ with a pick axe to hand excavate. Their relatively high strength (hard) makes them difficult to sample and prepare for geotechnical testing and in-situ field tests or large samples are recommended to assess field scale geotechnical parameters for design purposes.
3. The tills are mostly traction tills and contain very strong cobbles and occasionally boulders which may present a ground engineering risk, particularly in excavations and for reuse of material for earthworks.
4. Drumlinoid landforms are widely distributed across Anglesey and are predominantly composed of very stiff to hard traction till or high strength (strong to extremely strong) bedrock that may require blasting to excavate. Soil drumlins may contain irregular shaped sand bodies, which will affect their mass geotechnical and hydrogeological characteristics.
5. Coastal slopes in till can maintain steep angles, probably due to development of large pore suctions and high compaction efforts from the overriding ice sheet. Stability of slopes in excavations and tunnels in till may be affected by sand bodies and boulders present within the till sequence and sediment bedrock interface may be irregular. Dissolution of carbonate cements, particularly in the sand units in drumlins, may reduce strength over time and may lead to progressive slope failure.
6. The thickness of till at the coast is typically around 4–6 m, but locally may vary, and will be thinner in low ground between drumlins (onshore).
7. The local bedrock material partly influences the mineralogy and engineering behaviour of tills.
8. The trial use of handheld XRFS analysis of tills offers a rapid, cost effective, method for geochemical mapping of tills and this method could be deployed in the field with a trained operator. Geochemical analysis may offer a means to discriminate till deposits for provenance studies, provided a representative number of samples is available.
9. The Palynological analysis of grey tills at Cemlyn Bay provides new evidence that this till type is derived from Irish Sea bedrock sources. Differentiation of red till from grey till using Palynoflora has been attempted but the results were inconclusive.

Other engineering geological considerations:

10. presence of low-strength ‘normally consolidated’ Holocene sediments and peats in depressions between drumlins creates locally poor ground conditions, including compressible ground, poor drainage, and may impact on ground engineering.

11. Sub-glacial deformation processes and periglacial activity may have altered the geotechnical rock mass properties of bedrock materials at rockhead in places, particularly on the south west (downstream) side of drumlins.