The glacial and periglacial history of a Middle Pleistocene ice-margin of the British Ice Sheet (BIS) in north Buckinghamshire, England and its influence on geotechnical variability
Resilience is an important factor in the design and operation of new and existing linear infrastructure. Temporal and spatial variability in Quaternary, cold-climate processes, including deposition and weathering of glacigenic sediments and bedrock, subglacial drainage and permafrost development, has resulted in abrupt lateral and vertical anisotropy in the natural geotechnical state of the ground. The ability to anticipate vertical and lateral changes in the physical properties and structure of the shallow subsurface remains a major challenge for geotechnical design. This research uses a combination of field and laboratory techniques to investigate the Middle Pleistocene history of part of north Buckinghamshire and its potential influence on geotechnical variability of till and Oxford Clay mudrocks. The project area was chosen because of its position relative to the proposed route of Great Britain’s second highspeed railway (HS2) and its coincidence with a major Quaternary domain divide between lowland glaciated and non-glaciated, landscape assemblages. A regional database of geotechnical properties and behaviour was constructed. The database was used to describe variability in geotechnical property and behaviour parameters, against which the results of the laboratory investigations were compared. Geotechnical laboratory analyses included single- and multi-stage triaxial tests to determine undrained shear strength parameters, stress paths and strength envelopes. Small-strain stiffness was analysed using Hall effect sensors in triaxial compression. Compressibility behaviour was examined using 1D consolidation tests. This was supplemented by index testing, clay mineralogy analysis, geological field logging and numerical dating of glacigenic sediments using optically stimulated luminescence (OSL) techniques. The results of the laboratory investigation, and database parameters, were interpreted against a national framework of spatial Quaternary Domains which relate similar geological and geomorphological landscape assemblages to their interpreted Quaternary history. The research has demonstrated that there were at least two advances of glacier ice into the area, separated by the development of a large, proglacial ice-dammed lake. OSL dating has determined minimum ages of glacigenic sediment deposition between 145±10 ka and 205 ±13 ka. This implies that the glacial events are at least late Wolstonian in age but the OSL ages do not unequivocally discriminate between Middle Pleistocene glacial events. Direct field evidence for periglaciation occurs in the form of involutions in limestone bedrock and river terrace gravels, sub-horizontal shears in till and Oxford Clay bedrock and frost-cracking within till-derived chalk clasts and micritic limestone bedrock. There are no appreciable differences in the undrained behaviour of the Oxford Clay related to lithology or clay mineralogy. Geotechnical analyses suggest that the Oxford Clay is weaker and more compressible in areas that have not been previously glaciated, compared to those that have. Variability in undrained shear strength and secant shear modulus, G, is seen to be dependent on depth below ground level, regardless of Quaternary Domain. Variability in compressibility and secant effective strength parameters of the Oxford Clay are interpreted to be a function of proximity to a former Middle Pleistocene margin of the British Ice Sheet (BIS). Subglacial and periglacial shearing are interpreted to result in an increase in initial void ratio, increase in compressibility with a corresponding reduction in the secant angle of shearing resistance. Chalk-rich tills are stiff, have low compressibility and have mobilised shear strengths up to 477 kPa. The strength and compressibility variability in the ground between Quaternary Domains and between Oxford Clay and till exposed at the ground surface, has important implications for the design of railway infrastructure which requires geological subgrade stiffness and strength for its long-term performance.