Microstructural controls on the geomechanics of coarse grained soft rocks; Waitemata Group, Auckland

The geomechanical behaviour of a variety of soft rocks from the Waitemata Group, Auckland, is characterised, and the influence of microstructure in determining the geomechanics examined. Field geotechnical measurements of joint spacing and Schmidt Hammer rebound are related to geomechanical properties to determine controls on joint development and applicability of field index tests for soft materials. Samples studied are dominantly sandstones, with one conglomerate; all of which have a significant component of swelling clay minerals. Low bulk densities, high porosities and large void ratios are characteristic of the samples. They vary from extremely weak, very low durability rocks to moderately: strong and durable materials. Extremely weak sandstones are highly deformable and undergo plastic deformation when loaded in compression. The stronger samples undergo strain softening and respond elastically to applied stresses prior to brittle failure. High softening factors indicate that compressive and tensile strengths are greatly reduced upon saturation. All the study rocks have high proportions of sand and silt size material (≤ 95 %), and up to 10 % of < 2 μ m proportions; significant gravel size material only occurs in the conglomerate. Volcanic and detrital rock fragments are the dominant constituents in all samples, with abundant quartz, minor calcic plagioclase, and a variety of accessory minerals. The dominant clay is smectite with lesser amounts of illite, kaolinite, and mixed-layer clays. Individual sand and silt grains are dominantly subrounded to subangular quartz, regular bricks of feldspar, and rods of rutile, and are often clay coated. Microaggregates and grains are combined into aggregates and assemblages, and in some cases macroassemblages, to form granular arrangements, which produce a network of interconnecting pores. The fabric of the rocks is characterised by discontinuous matrices. Fabric types range from skeletal to turbostratic, and arrangements range from tightly interlocking grains to loose granular structures. Clay micro aggregates are either flocculated or form welded FF honeycomb arrangements. Strength and durability are not influenced by quantity, size, or type of clasts or minerals present; rather, it is the arrangement of individual components (and associated pore spaces) in the rock fabric which directly determines the geomechanical behaviour. Strong, durable samples have an abundance of clean grain to grain contacts, and few large clay aggregates. Clay microaggregates are arranged in welded FF arrangements, and pore space is restricted to micropores which resist water infiltration. Progressively weaker rocks are characterised by an increase in clay coated grains, clay aggregates, and clay connectors (which form weak links for stress transmission), and interconnecting pores at all levels in the fabric. In the weakest rock macroassemblages of clay coated grains, and open flocculated smectitic aggregates connected by point contacts, produce a network of macropores, allowing easy access of water which flocculates clays into weaker states by reducing bonding and cohesion. Schmidt rebound values in the field do not provide a useful indication of either compressive or tensile strength of these rocks. The rebound values do, however, correlate well with laboratory determined dynamic elasticity values, suggesting that the instrument should be used in the field to predict rock elasticity and not strength. Joint spacing within the rock units is related to elasticity with units capable of storing stress through plastic deformation having reasonably wide, uniformly spaced joints. Units which release stress through brittle fracture have closely spaced, and more complex joint systems