Bedding-plane Shear Research Articles

Experimental investigation of the interaction between water and shear-zone materials of a bedding landslide in the Three Gorges Reservoir Area, China

Most landslides induced by reservoir impoundment and rainfall in the Three Gorges Reservoir area of China are usually characterized by bedding-plane shear zones. This paper presents an experimental study of the interaction between water and shear-zone materials. A series of tests, including wetting-drying cycles and soaking tests, were carried out. The results show that the fluid-solid interaction not only reduces the shear strength of shear-zone soil but also accelerates the disintegration process of clastic rock from shear zone. After the fluid-solid interaction at different pH values, the clastic rock samples produce the most clay minerals in acidic solution. Similarly, the clay minerals of shear-zone soil samples increase dramatically after 45 days of soaking in acidic environment. According to the tests, it is found that the shear-zone soil in eluvium may originate from the weathered clastic rock near the shear zones under long-term fluid-solid interaction.

Experimental Investigation into the Bedding Plane Slip Effect on the Overlying Strata Behavior in Longwall Top Coal Caving of Soft Coal Seam

Bedding plane shear slip becomes more obvious in rock strata with quite different mechanical properties. A composite beam model considering the behaviors of the main roof and the immediate roof in the “three‐soft” coal seam is established based on physical similarity simulation experiments and the slip theory in this paper. The movement and failure of the overlying strata and the mechanism of the bedding plane slip at the 2211 working face are studied by experimental and theoretical analysis. The results suggest that the front abutment stress distribution occurs 50 m ahead of the working face, the initial caving interval of the main roof is 55 m, and the peak stress appears at a distance 20 to 32 m ahead of the working face. The bedding plane slip areas can be divided into the obvious slip area and the slight slip area along the mining direction. The range of the obvious slip area becomes wider and the range of the slight slip area grows to be narrower as the working face advances. The bedding plane slip becomes steady after gradual increase and leads to the subsidence of the overlying strata in the “three‐soft” coal seam. The observed initial caving interval of the main roof by field measurement is 51 m, which is consistent with the results of physical similarity simulation experiments and theoretical analysis. The results demonstrate that the beam slip model proposed in this paper is reasonable and able to describe the behaviors of overlying strata and bedding plane slip.

Time-dependent postseismic slip following the 1978 Mw 7.3 Tabas-e-Golshan, Iran earthquake revealed by over 20 years of ESA InSAR observations

We use over 20 yrs (1996–2017) of the European Space Agency's (ESA) radar interferometry (InSAR) observations to investigate the postseismic deformation of the Tabas fold segment following the 1978 Mw 7.3 Tabas-e-Golshan earthquake in eastern Iran. We generated maps of satellite line-of-sight (LOS) velocity using two ERS descending tracks (1996–1999), one Envisat descending track (2003–2010), one Sentinel-1A descending track (2014–2017) and one Sentinel-1A ascending track (2014–2017). The LOS velocity shows afterslip continuing for at least 40 yrs after the earthquake. Elastic dislocation modelling based on the InSAR measurements reveals a decrease in postseismic velocities from 5.0 ± 0.8 mm/yr in 1996–1999 to 3.9 ± 0.6 mm/yr in 2003–2005, 3.0 ± 0.4 mm/yr in 2006–2010, and a present rate of 2.3 ± 0.6 mm/yr in 2014–2017. The rates decay with time, t, as 1/t, consistent with the predictions of a simple block-slider model. We then combine the InSAR rates and our previous estimates of the total earthquake slip derived from optical image matching and DEM differencing to explore the frictional behaviour of the Tabas fold. We obtained a rate-and-state parameter a−b≈0.003, indicating rate-strengthening frictional behaviour of the Tabas fault. We also inferred a minimum coseismic slip of 4.7 m, which might have driven bedding-plane shear at shallow depth, resulting in distributed fold growth and secondary faulting observed in the field. The results imply that both coseismic slip and afterslip have occurred in the same location. One possible mechanism to explain such a phenomenon is that the frictional parameter a−b is small enough to allow dynamic ruptures to propagate into rate-strengthening regions.

Reply to discussion on ‘Reactivation of landsliding following partial cliff stabilization at Barton-on-Sea, Hampshire’ by M. E. Barton & P. Garvey Quarterly Journal of Engineering Geology and Hydrogeology , 44, 233–248

We thank the discussers for their contribution and the interesting questions they raise regarding our paper (Barton & Garvey 2011). We will divide their comments and questions into four sets and answer each of these in turn. A well-developed bedding plane shear surface is developed at 0.2 m above the ferruginous concretionary layer marking the top of Zone F1 in the cliffs fronting the Naish Farm estate. It forms a very prominent and unmistakable feature in a scarp face in this area, which is well seen to the west of the area being considered in the paper. We note that the discussers have been to see this but they make no comment on the problems encountered when trying to trace this surface eastwards towards the Cliff House Landslide and Barton-on-Sea. The scarp becomes a much less discernible feature, being dissected by a number of mudslides, and the area is extensively covered with talus and scrub vegetation. Exposures become very rare, which is not what one expects when a bedding plane shear surface is controlling the morphology of the undercliffs. By dint of struggling through the mud and scrub, we managed to find an exposure [NGR 423038 093065], which is about 50 m west of ‘Tom’s Garden’ (see fig. 1 of the paper), in the correct part of the stratigraphy but were unable to find either polishing or slickensiding along bedding planes above the concretionary layer. We presume, therefore, that shearing along the F2 surface is not continuous from east to west along this portion of the outcrop. We have commented upon this is our paper (on p. 235) and noted that it is a curious feature and an interesting subject for future research. Movement along the F2 surface is resumed further east and was clearly involved in …

The July 14, 2003 Qianjiangping landslide, Three Gorges Reservoir, China

The Qianjiangping landslide occurred after the first impoundment of the Three Gorges Reservoir in July 2003. Field investigation revealed that failure occurred when the reservoir reached 135 m, but the stability of the affected slope was already reduced by pre-existing bedding-plane shears, quarrying of mudstone from the landslide toe, and previous heavy rain. A possible explanation of the rapid and long runout mechanism of the landslide is that movement on a bedding-plane shear ruptured the calcite cement and rapidly reduced the sandstone strength to residual shear strength.

Determination of the Oldman River Dam foundation shear strength

The paper describes the results of site investigation and laboratory testing and the analysis performed for the determination of foundation shear strength at the Oldman River Dam site in southwestern Alberta, Canada. Horizontally bedded claystones, siltstones, and sandstones at the site contain relatively weak bedding-plane shears that adversely affect foundation stability. Data on the bedding-plane shear characteristics were collected by mapping, borehole coring, shaft exploration, and large-diameter sampling. Shear planes of structure-wide continuity were identified. Numerous laboratory direct shear tests were done to measure in situ and residual shear strengths. The design angle of shearing resistance of selected continuous bedding-plane shears was evaluated by summing the representative residual angle of shearing resistance and components of the angle of shearing resistance due to in situ state, roughness, and thickness of the bedding-plane shears. Relatively flat dam slopes were found to be required for stability. The methods used at the Oldman River Dam should be applicable at other sites located in flat-lying mudrock sequences. Key words: Oldman River Dam, foundation shear strength, sedimentary rocks, bedding-plane shear, residual angle of shearing resistance, in situ state, roughness, thickness.

Influence of bedding plane shears on slope instability in sedimentary rocks

Influence of bedding plane shears on slope instability in sedimentary rocks

Estimation of coal measures rock strength using sonic and neutron logs

Downhole geophysical logs, particularly neutron-neutron and sonic, have been used for estimating overburden strength at ten mine sites in Australia. The results are used in formulating specifications for numerical and physical models of coal extraction, which are in turn used for selecting pillar and roadway dimensions. Empirical relationships for estimating uniaxial compressive strength (UCS ) from neutron and sonic log response have been derived, based on several hundred test results from 24 drillholes. Other geotechnical applications of downhole logs include defining strata units with homogeneous mechanical properties, tracing bedding plane shears, detecting weaker and stronger zones in visually uniform massive rock, mapping lateral variations in strength and thickness of the seam's immediate roof and floor strata, and assessing bulk strength of coal seams and overlying strata. High resolution sonic logs have proved the most useful, despite problems in accurately locating test specimen depths on rapidly fluctuating log traces. Neutron logs may, however, indicate strength variations better in massive strata. Neither log is very good for identifying joints, but fractures sometimes show up as cycle skips on the sonic log.

The formation of planes of weakness in the highwall at Goonyella Mine, Queensland, Australia

Large highwail failures at Goonyella Mine, Queensland occur by the sliding of blocks of overburden along thin, soft, clayey bands near the base of the highwall. These bands, called black clay planes, are conformable with the lithological sequence but have a distinctive outcrop pattern, being softer and blacker than the enclosing sediments. Chips from the black clay planes disperse on immersion in water whereas the enclosing materials do not. Despite these differences, the black clay planes are shown to have almost identical chemical and mineralogical properties to the enclosing mudstones, indicating a common sedimentary origin. Bedding-plane shear is suggested as a mechanism for the formation of the black clay planes from typical mudstones. By breaking particle/particle bonds, the shearing has resulted in low strength and allowed the dispersion of the clay. The darker colour of the black clay planes is mainly due to their higher in-situ water content. The suggested mode of formation of the black clay planes is supported by laboratory modelling of the shearing process.

Controlled displacement-rate in situ shear tests with pore pressure measurements

Two large-scale in situ shear tests were conducted at a damsite in western Canada. The rock at the site is a Cretaceous shale containing a thin clay seam tentatively identified as a bedding-plane shear zone. The material in this seam is significantly weaker than the intact rock and influences the design of certain features of the dam and structures. In order to provide shear strength data for design, two large blocks of shale were sheared along the clay seam. These tests differed from conventional in situ shear tests in two significant ways: the rate of shear displacement was strictly controlled, and pore pressures (both positive and negative) in the shear zone were carefully monitored throughout the test. This note presents the material properties of the shear zone, describes the test equipment and techniques, and discusses the results obtained. Key words: shear strength, in situ testing, pore pressure, shale.

The characteristics and rates of the various slope degradation processes in the Barton Clay Cliffs of Hampshire

Summary Research is in progress to determine the nature and pattern of the degradation processes in an actively eroding stretch of the Barton Clay cliffs in the Naish Farm area of Highcliffe, Hampshire. In response to loss of beach material the rate of recession has accelerated and is currently averaging 1.9 m/year, resulting in steepening of the overall cliff angle from an average of 13° in 1947 to a maximum recorded average of 19° in 1976. The presence of three preferred bedding plane shear surfaces at various elevations within the cliff produce a benched type of cliff profile, comparable with that seen in the Gault and Lias Clay controlled cliffs of Fairy Dell, Dorset but in contrast with the published descriptions of London Clay cliffs. The degradation processes include scarp slumping, spalling (including toppling and soil falls), bench sliding (involving movement of colluvium over a preferred bedding plane), debris sliding (including movement of screes over clay scarps), mud sliding (i.e., the traditional lobate ‘mudflows’), mud runs (true flows), stream (or gully) erosion and man-related processes. Regular surveying and monitoring using a variety of techniques and having to surmount difficult field conditions, has begun to elucidate the characteristics, rates and inter-relationships of these processes. A flow chart representing the systematic transfer of soil from the in situ state via various colluvial modes (including the three bench levels) en route to the sea and an accompanying colluvial soil budget for the year July 1981 to July 1982 has been drawn up. It is shown that bench sliding is by far the most significant process in terms of volume of colluvium moved through the under-cliff, accounting for 930f the total volume of colluvium contained in the area. Mud and debris sliding, although important processes, are of relatively minor significance.

The Gundahl Complex of the New England fold belt, Eastern Australia: a tectonic mélange formed in a palaeozoic subduction complex

The New England Fold Belt of eastern Australia preserves a Palaeozoic fore-arc terrain with a magmatic arc, fore-arc basin and a subduction complex. The Gundahl Complex is a tectonic mélange of regional extent in the subduction complex. The matrix and slabs of the Gundahl Complex have six mappable lithofacies: argillite, greywacke-argillite, greywacke, argillite-tuff, bedded chert and greenstone. The argillite matrix is pervasively sheared with many slickensided shear fractures. Locally the matrix is formed by highly sheared greenstone. Greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell. Folds, in places with axial-surface spaced cleavage, are common within those slabs comprised of well-bedded sequences. Bedding-plane shear and faulting at a high angle to bedding also occur in these slabs. On a map-scale much of the Gundahl Complex comprises slabs up to 10 km long in imbricate fault-bounded slices which repeat the disrupted pre-mélange stratigraphic sequence. Elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the Gundahl Complex. The unit is believed to have formed by accretion, imbrication and subsequent tectonic disruption of arc-derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex.

A statistical study of the cliff top slumps in part of the christchurch bay coastal cliffs

AbstractA distinction is made between slumping and spalling. Slumping involves a compound slide with rotation and translation: the latter along one of the bedding plane shear surfaces in the Barton Beds. Spalling is an isolated failure of the exposed cliff face due to weathering. Where slumping is the dominant mode of cliff top recession, the latter is an incremental process with the increments being equal to the breadth of the slumps. A statistical study has been made of the breadth, length and plan area of 42 slumps observed over a 2 km stretch of the Christchurch Bay coastal cliffs and significant trends deduced.

On a Mechanism for Seam Splitting and Implications for Exploration, Evaluation, and Exploitation of Coal Resources: ABSTRACT

Exploitation of coal resources to meet both economic and community (environmental) expectations is commensurate with the scale of exploration, the comprehensiveness of data evaluation, and the wisdom shown in mine design. As accepted criteria, all three requirements rely on appropriate knowledge of the geology of the orebody and its associated sediments. End_Page 959------------------------------ The Permian Coal Measures of eastern Australia are characterized by multiple seams within the measures which may coalesce or split over the entire field in a seemingly complex manner. Associated structures in the sediment pile include non-conformable attitudes of bedding, and seam displacement by low-angled overthrust faulting or bedding plane shears along stone bands. The origin of such structures is explained by a process of differential consolidation of the primeval peat, associated with intermittent sediment influx, and capture of local drainage. Consolidation, i.e., early lithogenesis of peats, relative to the consistency of associated sediments at the perimeters of areas undergoing differential compaction provided a mechanism for lateral compression and yielding of the seam by simple shear. The degree of splitting can be associated with variations in split quality, while associated deformations of the sediment may give rise to difficult surface and underground support conditions. Thus much information on important resource parameters is obtained from thorough analysis of exploration results which should include an interpreparation of the sedimentology of the coal measure section under evaluation. End_of_Article - Last_Page 960------------

Phanerozoic thrusting in Proterozoic belt rocks, northwestern United States

Newly identified listric thrust faults show eastward translation north of the Lewis and Clark line across all 800 km of Belt terrane from Spokane, Washington, on the west to Glacier National Park on the east. Right-lateral slippage on the line was accompanied by clockwise rotation on the thrusts. These movements were probably in response to the complex plate interactions that began about 200 m.y. ago along the western continental margin. Gravity and magnetic data suggest that basement rock is involved in the thrusting. Reconcentration of strata-bound copper sulfides along bedding-plane shears may help form ore near one thrust. West of the Rocky Mountain Trench, Phanerozoic strata that elsewhere have oil and gas potential may have been stepped up by the thrusting, and eroded, rather than extending beneath most of the thrust belt.

Engineering properties and slope stability of lower coal measures rocks from monk wood cutting, Unstone-Dronfield by-pass, Derbyshire

This 30 m deep cutting has been the subject of a detailed geotechnical analysis, in view of mass-sliding wedge failures which had occurred in cuttings in similar geological situations in other parts of Britain. The sequence of rocks includes two coal seams, the upper being thin (0.3 m) and the lower thick (2 m), the latter with a recognisable seatearth (underclay). There is a thick sandstone at the top of the succession, with siltstone, mudstone and black mudstone also present. Both field and laboratory testing was carried out. The most probable failure mechanism within the more competent sandstone sequence to the south of the site would probably have been in the form of wedge failures on pre-existing discontinuities within the rock mass. It was necessary to evaluate the shear strength which could be mobilised along these planes and a field rock shear box was used. Drained-soil shear tests were also made on potential failure-surface materials, namely seatearth and clay shear zones. Weathered mudstone samples were tested to provide design parameters for the analysis of rotational slips. Additional laboratory testing carried out comprised bulk density, classification tests, slake durability and point-load indices. X-ray diffraction analyses were made on a range of materials such as seatearth, mudstone, shear zone and sandstone bedding-plane shear surface. This extensive test programme enabled parameters to be accurately determined for use in stability analyses. The latter show the critical effects of water conditions on the factor of safety. The performance of the cut has been monitored since early 1974 and no recordable movement has been noted.

Structural Petrology of the Baraboo Quartzite

The microstructures of the Baraboo quartzite of Columbia and Sauk counties, Wisconsin, have been studied from oriented hand specimens, polished sections, and thin sections. Fabric axes, determined from the pole patterns of the deformation lamellae, vary consistently over the region and are not related to the bedding or the macrofoliation. Because of this variation, the deformation lamellae have been assigned to a deformation later than the major folding of the region. In addition, the character of the deformation lamellae themselves and the type of pole pattern have been used as a qualitative index of the intensity of the deformation. Microfractures are common features of the quartzite; they exhibit a strong preferred orientation and are geometrically related to the deformation lamellae. Quartz deformation bands are described, named, and correlated with bedding-plane shear. Quartz optic-axes fabrics are weak and difficult to interpret.