Influence of microfabric on geomechanical behaviour of tertiary finegrained sedimentary rocks from Central North Island, New Zealand: Fid-Bau Portal

Influence of microfabric on geomechanical behaviour of tertiary finegrained sedimentary rocks from Central North Island, New Zealand

Huppert, F.

Abstract

Abstract Tertiary to Early Quaternary fine-grained marine sedimentary rocks (soft rocks) of the Central North Island, New Zealand, suffer from high erosion rates and slope instability. In geomechanical laboratory tests they exhibit a range in behaviour which straddles the division between overconsolidated soils and rocks. Direct application of published classification systems to predict performance has been unsuccessful for New Zealand soft rocks so far. Mineralogical composition does not vary greatly between ages and locations, in contrast to geomechanical parameters. Most constituents are detrital, no cement has been precipitated and post-depositional diagenetic alterations are negligible. Mineralogy does not comprehensively account for the geomechanical performance of these rocks. More direct influence is exerted by their microfabric. Basic building blocks of the fabric are detrital grains, clay microaggregates, microfossils and larger clay aggregates. Grains, microaggregates and fossils are mechanically stable, but larger aggregates exhibit varying degrees of stability. The geometric arrangement of structural elements can be described within a framework of three fabric types, the skeletal, matrix and turbostratic microfabrics, which form an evolutionary sequence related to the loading history of the deposits. Some geomechanical characteristics of the rocks, particularly strength, cohesion and durability, correlate well with the fabric type classification. In all fabrics, contact between structural elements involve clay, and its physico-chemical properties are important for their performance under stress. The fabrics are dynamic and adjust to changing stress conditions by re-arranging their constituents and contact configurations. Their response to loading and slow unloading during regional uplift is quasielastic, resulting in a loss of stress memory. During rapid unloading by erosion or excavation stress relief fractures parallel to topography can develop. Aggregates and connector assemblages form weak links in the fabrics. They are less competent in their response to stress and wetting and drying than other structural elements. Their breakdown decreases effective grainsize, average pore size and permeability, and initiates further disintegration of the fabric.