GEOMECHANICS ABSTRACTS

Geology

Tectonic processes and structural geology

See also: 885054

885001 Tectonic subsidence in California's Sacramento-San Joaquin 'Delta': technical note Prokopovich, N P Bull Assoc" Engng Geol V25, NI, Feb 1988, P141-144

The Sacramento-San Joaquin 'Delta' is a reclaimed area of peaty marsh deposits intersected by sandy alluvial s t r e ~ channels, which has subsided due to oxidation of peat. How- ever an examination of inorganic alluvial deposits also indi- cates subsidence, which is explained by tectonic crustal movement, since the area is near major faults. Subsidence rates may not be obtained using available data, but extrapola- tion of the elevation above sea level to the boundary of flat marsh deposits shows subsidence between 3 and 7 feet.

885002 Stress field due to faulting in the presence of a high rigidity barrier (asperity) - plane strain models Rybicki, g R Bull Geoi Soc Am V78, NI. Feb 1988, P276-298

Two models, cracks separated by an inclusion and a crack crossing an inclusion, are used to represent faulting in a medium with a high strength barrier. Stress fields round the barrier are calculated and compared to homogeneous medium values. Stress intensity factors are influenced as the crack approaches the barrier, and distinct stress concentrations exist in intact barriers, Once the barrier is broken, stresses rise at the ends of the fracture zone. Stresses due to the faulting obtained with the two models are qualitatively very similar but quantitatively very different.

885003 Microstrnctural shear criteria associated with grain-boundary sliding during ductile deformation Drury, M R; Humphreys, F J J Struct Geol VIO, NI, 1988, P83-89

Experiments on the deformation of some rock analogues were undertaken to consider development of some microstructure types which may be used as shear criteria. The microstructures are defined by preferred grain-boundary orientations and grain shape. Results enable determination of whether the deformation was coaxial or non-coaxial, and the sense of shear in zones of non-coaxial deformation.

© 1988 Pergamon Press plc.

885O04 Fault breccia, displacement, and rock type Robertson, E C Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June-I July 1987 P65-72. Publ Rotterdam: A A Balkema, 1987

The amount of brecciation of faults in mines is greater in felsic igneous rocks and shale than in quartzite, allowing for differ- ences in localities and fault movement by normalising breccia thickness by a direct log-log relation with fault displacements. Chlorite-cummingtonite schist accommodates regional stress by small-scale folding and brecciation without any large faults. Auth.

Environmental effects, weathering and soil formation

See also: 885305

885005 Influence of stratigraphy and structure on knickpoint erosion Clemence, K T Bull Assoc Engng Geol V25, NI, Feb 1988. P11-15

Knickpoints are abrupt changes in the longitudinal profile of stream valleys, and their accelerated erosion may cause problems for engineering geologists. A model of a steep walled canyon (in an air flume tunnel) is used to study the influence of fracture spacing, continuities and bedding thickness on the mechanisms of caprock knickpoint erosion. Results indicate: (1) a caprock can resist cantilever failure if undercut length is less than half the fracture length; (2) cantilever failure of a discontinuously fractured caprock is controlled by fracture spacing of the bed in contact with the underlying erodible unit; and (3) the permeable erodible unit allows the development of significant fluid pressure and a decreased resistance to bound- ary shear within and below the caprock.

885006 Drought-related changes to geomorphologic processes in central Mall Jacobberger, P A Bull Geol Soc Am VIO0. N3, March 1988, t'351-361

Remote sensing data (Landsat MSS), field mapping and sam- pling results, and precipitation records were collected in 1976 and 1985. Changes in albedo (surface brightness), a composite indicator of vegetation cover, soil moisture, and other factors, are used as the primary indicator of desertification. Wind is significant in transport and redistribution of soil materials, but fluvial erosion is responsible for much primary topsoil loss and landform modification.

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