Frost weathering of chalk

The processes and factors that determine the heave and fracture of frost-susceptible bedrock exposed to temperature cycling above and below 0°C are little known but important to understanding of rock deformation, weathering and ground conditions. To investigate the early stages of heave, settlement and fracture of intact chalk, physical modelling experiments were performed on blocks of Saint Cyr Tuffeau and Totternhoe Clunch. Unidirectional (downward) freezing simulated seasonally frozen bedrock in non-permafrost regions, and bidirectional (upward from permafrost and downward from the surface) simulated an active layer above permafrost. Heave and settlement of the top of the blocks were monitored in relation to rock temperature and unfrozen water content. Heave and settlement showed complex behavior that varied with moisture content, freezing regime and time. Progressive heave of wet chalk during thaw periods (simulated summers) is attributed to microcracking in near-surface permafrost. Macrocracking was favoured near the rock top during unidirectional freezing and near the permafrost table during bidirectional freezing, producing extensive fracture networks. Four processes, operating singly or in combination, account for the heave and settlement behavior: (1) thermal expansion and contraction in dry chalk; (2) volumetric expansion of freezing water, causing bursts of heave; (3) ice segregation, causing sustained heave and rock fracture; and (4) freeze‒thaw cycling, causing initial consolidation and settling of wet chalk during unidirectional freezing. The experimental data and field observations of chalk weathering profiles elucidate the nature and origin of chalk brecciation. Type 1 brecciation (angular or subangular rock fragments separated by unfilled fractures with matched sides) is attributed primarily to ice segregation. Type 2 brecciation (subangular to rounded lumps of rock—lithorelicts—set in a fine-grained matrix of the same, but softer and remoulded material) probably resulted from frost weathering and limited ground movement, particularly beneath the sides and bottoms of wet (now dry) valleys

Long-term stress-strain response of chalk:a micro-mechanical interpretation

A long-term laboratory test programme of conventional compression and extension tests was carried out with test durations from 8 to 22-months, in a purpose built environmentally controlled facility, with specially designed loading frames and modified triaxial cells. In addition, Scanning Electron Microscope (SEM) techniques were employed in an effort to investigate the micro-mechanical res-ponse. Creep strains appeared to trigger an ageing process that produces elevated post-creep strength and stiffness irrespective of the ap-plied stress path

Engineering in chalk

This book provides guidance on engineering in chalk. It describes the chalk's geological setting, its origins, occurrence, its stratigraphy, weathering and geomorphological situations, the material and mechanical properties. The descriptions are supported by a comprehensive set of photographs. It explains recommended schemes for the engineering description and classification of chalk, building on the work presented in CIRIA PR011, 'Foundations in Chalk'. The publication looks at the mechanical and material properties of intact, in-situ and compacted chalk and considers their implications for the design and construction of earthworks, cuttings, retaining walls and anchorages. Major sections deal with the selection and design of shallow and piled foundations. Based on analysis of the results of pile testing, the book makes recommendations for the design and choice of bored, CFA, driven cast-in-place and pre-formed piles in chalk and for estimating shaft and base resistances)

Natural experiments in rock mechanics using high precision monitoring of chalk sea cliffs

Prediction of rock slide events remains a difficult task for geoscientists. Kinematic analysis provides an indication of possible modes of failure at a site. However, the highly variable nature of chalk compressive strength due to variations in water content and salt weathering is such that parameterizing models of slope stability can result in large variations in the resultant factor of safety. In this work, we use high-precision monitoring data of an eroding coastal cliff to characterize the geometry of a large wedge failure in chalk. We use these data in conjunction with published material properties to model the joint compressive strength of the chalk at the time of failure through back analysis. Results indicate a strength of 7.19 KPa for the chalk suggesting that the joint surface was close to saturation at the time of failure