British Coal Measures mudrocks are dominantly non-marine, mature sediments with a high average clay minerals content of over 75%. A geological classification based on quartz content does not differentiate the dominant durable mudrocks from the non-durable types and overconsolidated clays in the formation. A uniaxial compressive strength greater than 3.6 MN/m 2 with a three cycle slake durability value of over 60% is suggested as an appropi'iate division between durable and non-durable mudrocks of this age. Physical breakdown, which acts as a control on chemical weathering triggered by pyrite oxidation, is considered to be governed by incidence of sedimentary structures, slaking and expandable mixed-layer clay content.Shear strength reduction during post-glacial weathering can be significant. A tentative fully weathered effective stress peak strength for the weakest mudrocks and clays is deduced to be q~'= 22 ~ (c'=0). Colliery tip investigations indicate that chemical weathering effects on the shear strength of the mudrocks are small. A diagenetic rank factor applies both to peak shear strength and slake durability of mudrocks excavated from UK coal mines.

Swelling, shrinking and physical breakdown processes are reviewed with reference to well-known mudrock and overconsolidated clay formations in the UK and USA. Swelling results from two processes: the equilibration of depressed porewater pressures following stress relief, and the physico-chemical (osmotic) response of component clay minerals. Expansion in Na-smectite, and to a lesser extent Ca-smectite, clays is governed by double-layer swelling, whereas in kaolinites it is purely a mechanical unloading phenomenon; illites show an intermediate response. Intraparticle swelling in mudrocks older than the Silurian in the UK, or Upper Mississippian in the USA, can be expected to be reduced because of the removal of expandable layers by burial diagenesis. Shrinkage, like mudrock breakdown, is restricted to the partly saturated zone. Suction pressure-moisture content curves of indurated mudrocks are shown to be different from mudrocks and clays with high proportions of expandable clay minerals. Classification of expansion potential based on activity ratio poses problems with indurated types, but with some modification of method reasonable predictions can be made. Controls on physical disintegration are identified as: (i) incidence of sedimentary structures and discontinuities, (ii) slaking (air breakage), (iii) expandable clay mineral content, especially smectite, and (iv) clay mineral fabric orientation. Exceptionally high exchangeable sodium percentages have been measured in Coal Measures rocks susceptible to breakdown.

Summary The ‘rock’ and ‘soil-like’ properties of British mudrocks are shown to be influenced by: (a) their lithology; (b) their geological history of loading (especially during exhumation); (c) the type and method of testing; and (d) the degree of weathering. In particular, unloading and weathering leads ultimately to a normally-consolidated clay of much the same undrained shear strength, irrespective of age and origin of the parent material. For this reason, the engineering properties of the unweathered mudrocks are illustrated separately in terms of classification indices, undrained and effective shear strengths and deformability in relation to geological age.

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The nature of methylene blue (hydrochloride) adsorption is reviewed because the dye has recently been adopted for determining the presence of clay in quarry aggregates. Although it is demonstrated that methylene blue is not an ideal exchange reagent, details are given for overcoming most of the shortcomings, with particular reference to the aggregates test. It is demonstrated that methylene blue commonly underestimates the cation exchange capacity (CEC) of untreated expandable smectites. In contrast, it provides a fair reflection of the change in ratio of 7 Å/10 Å clay minerals in British mudrocks and overconsolidated clays. Plasticity indices of the latter show a high level of positive correlation with measured CEC.