Quantification of stability improvement of a dump through biological reclamation

An integrated study on biological stabilisation of a dump slope has indicated that biological reclamation with grass and tree species should be considered for long term stability of this coal mine dump in India. The grasses have greater soil binding capacity and help to control soil erosion and improve dump stability. Native grasses such as Bamboo (Dendrocalmus strictus) and Kashi (Saccharum spontaneum) are the important constituents of grass species which can stabilise the dump slopes. Field observation of growth performance of grasses have indicated that mean grass height, root depth and below-ground root biomass are 185 cm (±68), 45 cm (±5) and 467 g m^–2 (±170), respectively after three years of grass growth on Mudidih overburden dump slope in India. The growth performance of tree species, namely Sisum (Dalbergia sisoo) and Subabool (Leucena lecocephala), in terms of height, diameter increment, below-ground biomass and root depth have shown mean values of 219 cm (±94), 48 mm (±6), 4.0 kg m^–2 (±1.5) and 1 m (±0.1), respectively. This acts as biological fertility which helps in root proliferation and enhancement of dump stability. From the numerical modelling it is suggested that roots of these grass and tree species have significantly enhanced the factor of safety of dump from 1.4 to 1.8 and therefore have a positive role in maintaining long term stability.     

Nitrogen and carbon dioxide occurrence in UK coal seams

Conclusions Some bituminous coals, mostly of high volatile rank, sampled from various coalfields contained on average 0.5 m³ t^–1 nitrogen and 0.13 m³ t^–1.Nitrogen is released more readily than methane during coal winning but the volumes of nitrogen released are small compared with ventilation quantities and are only likely to be of significance for detailed oxygen deficiency studies.     

Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand

Geotechnical characterisation is undertaken for 3 broad units comprising the bulk of the stratigraphy identified on White Island Volcano, Bay of Plenty, New Zealand, an active island stratovolcano. Field and laboratory measurements were used to describe rock mass characteristics for jointed lava flow units, and ring shear tests were undertaken to derive residual strength parameters for joint infilling materials within the lavas. Rock Mass Rating (RMR) and Geological Strength Index (GSI) values were calculated and converted to Mohr-Coulomb strength parameters using the Hoek-Brown criterion. Backanalysis of known landslide scarps was used to derive strength parameters for brecciated rock masses and hydrothermally altered rock masses. Andesite lava flows have high intact strength (σ_ci = 184 ± 50 MN m^− 2; γ = 24.7 ± 0.3 kN m^− 3) and typically 3 wide, infilled joint sets, one parallel to flow direction and two steeply inclined, with spacings of 0.3-1.7 m. Joints are rough, with estimated friction angles for clean joints of ?_j = 42-47°. Joint infill materials are clayey silts derived from weathering of wall rocks and primary volcanic sources; they have low plastic (54%) and liquid (84%) limits and residual strength values of c_r = 0 kN m^− 2 and ?_r = 23.9 ± 3.1°. RMR values range from 70 to 73, giving calculated strength parameters of c′ = 1161-3391 kN m^− 2 and ?′ = 50.5-62.3°. Backanalysis suggests brecciated rock masses have c′ = 0 kN m^− 2 and ?′ = 35.4°, whereas GSI observations in the field suggest higher cohesion (c′ = 306-719 kN m^− 2) and a range of friction angles bracketing the backanalysed result (?′ = 30.6-41.7°). Hydrothermally altered rock masses have c′ = 369 kN m^− 2 and ?′ = 14.9°, indicating considerable loss of strength, especially frictional resistance, compared with the fresh lava units. Values measured at outcrop scale in this study are in keeping with other published values for similar volcanic edifices; backanalysed data suggest weaker rock mass properties than those determined at outcrop. This is interpreted as a scale issue, whereby rock mass characteristics of a large rock mass (crater wall scale) are weaker than those of small outcrops, due in part to the overestimation of friction angle from measurements on small exposures.     

Causes, mechanism and environmental impacts of instabilities at Himmetoğlu coal mine and possible remedial measures

 Since the commencement of mining at the Himmetoğlu coal mine, northwest Turkey, serious stability problems have led to interruptions in mining and some environmental effects. A geotechnical investigation was initiated in 1997 and the significant factors that influence the stability have been defined. This paper outlines the results of the field and laboratory studies associated with the causes and mechanism of the slope instabilities and their environmental impacts. The possible remedial measures to improve the stability and to minimize the environmental problems are also described. Back-analyses and data from long-term monitoring indicate that the failures occur along two or three planar surfaces by combination of faults and localized strata steepening adjacent to the faults. The stability is sensitive to changes in length of the lower part of the basal sliding surface and shear strength of the bedding surfaces in the overburden. Suitable remedial measures include slope flattening (i.e. staged bench stripping), proper drainage and spreading of a rock blanket on the pit floor to increase spoil pile stability. Received: 18 April 2000 · Accepted: 15 August 2000     

A study on the bio-hydro mechanical effects of the tree roots on the Agaclı coal reclaimed – forested land

The research area that is in the north of Istanbul (Turkey) and it is a land of open pit coal mining residuals reclaimed and turned to forest in 1988–1989. The materials that are open pit mine spoils are formed by sandy loam, sandy clay loam, heavy loam and clay (noncalcareous Pliocene I sediments). Pseudoacacia (Robinia pseudoacacia), Maritima Pine (Pinus pinaster) and Stone Pine (Pinus pinea) are planted on these materials. The aims of this research were to determine the strength of the material provided by tree roots and evaluate the performance of the forestation practices at lands especially where open pit coal mining spoil (residual) materials piled up. The research has been performed at two stages. The first one was hydro-mechanical effect concerned about material moisture depletion as a result of transpiration and the second one was bio-mechanical effects due to strength parameters of the roots and their growth pattern within the material. All results were evaluated in relation to tensile strength and displacement curves due to pre and post failure behaviors of materials and root systems. The results showed the increase of the displacement of the material was the significant indicator of the bio-mechanics tensile strength of the material provided by the tree roots. The fast growing type of tree roots have stabilized the material up to a depth for 80–100 cm by covering the coal residual materials within 12 years.     

挡墙基底换填对弃土场边坡稳定性影响分析

为了研究弃土场边坡挡墙加固中,挡墙基底在不同换填宽度条件下以及在同一宽度条件下不同换填深度对弃土场边坡稳定性的影响。基于有限元强度折减法理论,利用midas GTS有限元软件,依托工程实例建立二维有限元模型。分析了不同换填宽度及换填深度情况下弃土场边坡在暴雨工况下的稳定性系数。研究发现在同一换填宽度条件下,换填深度对边坡稳定性起决定性作用;只有在换填深度达到一定值时,增加换填宽度才对边坡稳定性有提升作用。     

Texture, mineralogy, and rock strength in horizontal stress-related coal mine roof falls

Geologic structures can represent planes of preferential weakness that, by dismembering the roof beam, may contribute to the failure of roof spans. However, beam deflection and roof failure also occur in rocks where no visible geologic discontinuities are present. This suggests that roof failure may depend on rock strength, which in turn depends on intrinsic textural properties inherent to the rock. In this study, rock samples were collected from horizontal stress-related roof fall material in coal mines for petrographic characterization and compressive strength testing. Brittle, stress failure-prone rock types include thinly interlaminated siltstone and shale, and black shale that had been lightly recrystallized. Samples exhibit a narrow range of density values between approximately 2.5–3.0 g/cm³ but exhibit a wide range of unconfined compressive strength values, between approximately 20–70 MPa. Results of laboratory observations suggest that for samples of coal mine immediate roof shale, compressive strength is not well correlated with density, grain size, sutured grain boundaries, or quartz content. These results for shale are generally at odds with the results of similar studies for sandstone. The great variability of strength, texture, and mineralogy documented in these samples may be an indication of their complexity and the need for specialized methodology in the study of shale strength.     

The geotechnical characteristics and stability of a spoil heap at a southwestern pennsylvania coal mine, U.S.A.

The mechanical and geotechnical properties of bulk samples and samples extracted from boreholes through a sliding spoil heap at a southwestern Pennsylvania coal mine are examined. Size gradation, moisture content, density and strength are found to vary considerably within the spoil. Gradation results suggest the material type to be more important than the age of spoil in degradation. There is also evidence from analysis that the heterogeneity of spoil and its associated density and strength do not significantly affect stability results. In other words, spoils can be sufficiently heterogeneous for purposes of stability analysis; they can be considered statistically homogeneous with regard to shear strength.     

Geotechnical properties of the Ptolemais lignite

Summary The paper describes an investigation of the geotechnical properties of the Ptolemais lignite (brown coal) and shows that the soil mechanics principles used in the study of natural and cemented soils are applicable. The material has a high and variable void ratio incompatible with its preconsolidation pressure, which may be attributed to the existence of structure (chemical bonding). Bond strength and degradation have been studied and are found to be isotropic, while the stress-strain response in the prevield domain is intensely cross-anisotropic. The shear strength determined from triaxial CU and CD tests is variable but strongly dependent on the in situ moisture content. Tests on high quality intact specimens having a wide range of moisture contents (100–150%) verify a hypothesis that the magnitude of bond strength (expressed by the peak shear strength) is inversely proportional to the moisture content. A Hvorslev-type normalization process shows that the peak shear strength of the material can be estimated from the in situ moisture content with reasonable accuracy, thus facilitating preliminary stability calculations of production cuts in lignite mines. The properties of the Ptolemais lignite are compared with the properties of a lignite from another location in Greece (Megalopolis) and an Australian lignite (Morwell). It is shown that the shear strength characteristics of the lignite types studied are similar and strongly dependent on the in situ moisture content, which seems to be a measure of the magnitude of inter-particle bonding.     

节理岩坡中注浆效应的FLAC3D数值分析

通过理论推导,确定了边坡的有效注浆范围;由三维拉格朗日元法(FLAC3D)建立数值计算模型,在有效注浆范围内对弱面进行注浆。对比节理边坡在注浆前后应力场和位移场的变化,结果表明:通过注浆提高了节理面的抗剪强度和抗剪刚度,使岩体的应力场和位移场变得均匀和连续;边坡上缘的拉应力区逐渐消失;坡体内各个部位的位移分叉现象逐渐减少,有利于边坡稳定。      

Theory of the collapsed zone at the front of a coal seam and its effect on translatory rock bursting

If the slope of the uniaxial stress–strain diagram in the post-failure regime is steep enough, it is shown, using a simplified (‘elementary’) theory, that the frontal zone of a coal seam close to a cavity may suddenly collapse. This collapse, well known from in situ, evidence, is also shown to reduce in most cases the danger of initiation or the strength of a subsequent coal mine bump.     

Heat flow in the Altai-Sayan Area: new data

Eleven new estimates of heat flow (q) from the southern Altai-Sayan Folded Area (ASFA) have provided update to the heat flow map of Gorny Altai. Measured heat flow in the area varies from 33 to 90 mW/m², with abnormal values of >70 mW/m^q at four sites. The anomalies may have a deep source only at the Aryskan site in the East Sayan (q = 77 mW/m²) while high heat flows of 75–90 mW/m² obtained for the Mesozoic Belokurikha and Kalguty plutons appear rather to result from high radiogenic heat production in granite, which adds a 25–30 W/m² radiogenic component to a deep component of 50–60 mW/m². The latter value is consistent with heat flow estimates derived from helium isotope ratios (54 mW/m² in both plutons). Heat flow variations at other sites are in the range from 33 to 60 mW/m². The new data support the earlier inferences of a generally low heat flow over most of ASFA (average of 45–50 mW/m²) and of a “cold” Cenozoic orogeny in the area (except for southeastern ASFA), possibly driven by shear stresses associated with India indentation into Eurasia.     

Access tunnel convergence prediction in longwall coal mining

Summary The paper describes theoretical andin situ studies of tunnel deformation in longwall coal mining. It develops a method to predict tunnel convergence profiles from the faceline in longwall mining. The method accounts for the effect of panel width, extracted seam height, deformation moduli of the goaf material and coal pillar, depth of cover,in situ structural defects, tunnel shape and tunnel size in addition to the strength characteristics of surrounding strata. The analytical technique has been validated by reference toin-situ deformation measurements in 26 face-access tunnels in Cape Breton Coalfield mines. Based on this method a series of vertical convergence profiles for different depths and extracted panel widths have been presented.     

Geotechnical Evaluation of Reddish Brown Tropical Soils

Laboratory investigations were carried out on reddish brown tropical soils from Moniya, Ibadan Southwestern Nigeria to determine the basic unconfined compressive strength of the soil samples which is an important factor to be considered when considering materials as liners in waste containment structure. Clay mineralogy, major element geochemical analyses were carried out by means of X-ray diffractometry and X-ray fluorescence spectrometry respectively. The engineering tests such as sieve size analyses, Atterberg limits, natural moisture contents, specific gravity and compaction using four different compactive efforts namely reduced proctor, standard proctor, West African standard and modified proctor. The tests were carried out in line with the procedures of the British standard 1377 of 1990 and Head of 1992. The soils were found to contain kaolinite as the major minerals with some mixtures of smectite, muscovite, halloysite, quartzite, biotite and aluminium phosphate. Values of the unconfined compressive strength obtained within 12.5 and 22.5% moulding water contents equal to or greater than 200 kN/m² which is the minimum acceptable value required for containment facilities. The maximum dry density, Mg/m³ ranged between 1.68 and 1.98 while Optimum moisture content, % ranged between 12.3 and 21.2. Hence, unconfined compressive strength values were found to be greater than 200 kN/m² at dry unit weight of 16.20 kN/m³ especially when WAS and modified proctor compactive efforts were used which met the minimum required unconfined compressive strength of 200 kN/m² for hydraulic barriers in waste containment facilities.     

Soft rock pillars

Summary Soft rock pillars can be designed by several methods available in the mining literature. All of these methods include the effect of shape, or geometry, on the average strength of specimens and pillars. All of the pillar design methods include some measurement of the strength of specimens of the pillar rock. The most common rock specimen strength property measured is the unconfined compressive strength. However, the average strength of triaxially confined rock specimens is much greater than the unconfined specimen strength, which can be more important to pillar strength. The estimation of the strength of a pillar is complicated by the decrease in rock specimen strength with increase in specimen size.Editor's note: In common with North American engineering practise, the paper uses English units throughout, where feasible conversions are included in the text. Where not, the following factors may be used: 1 inch=25.4 mm; 1 ft=0.3048 m; 1 lbf/in.^–2=6.895 kn/m^–2; 1Tonf.=8.896 kN.     

Geotechnical characterization of lignite-bearing horizons in the Afsin-Elbistan lignite basin, SE Turkey

The Afsin-Elbistan lignite deposit, with its 3.4 billion metric tons of reserves, is the biggest lignite basin and one of the most important resources for electrical energy production in Turkey. Kislakoy mining field was selected as the first opencast mine to feed four power station blocks of 300 MW each. Slope instability has been a continuing problem in the Kislakoy opencast mine. Particularly complex failures along a noncircular failure surface appearing at the final slope stage and covering large areas in the mine increase the importance of slope stability. This study outlines the geotechnical characteristics of the lignite-bearing horizons and describes the causes and mechanisms of slope instabilities, which threaten the safety of the mine. Quantitative X-ray diffraction (XRD) analyses were carried out using an interactive data processing system (SIROQUANT™) based on Rietveld interpretation methods. Parametric slope stability analysis and backanalysis were carried out for the failure that occurred at the northwestern final slope stage of the mine. The Spencer-Wright limiting state equilibrium method was used in order to determine with confidence the most representative values of regional shear strength parameters, to explain the failure mechanism, and to assess the conditions at the time of failure. In the analysis, phreatic and piezometric surfaces were considered.Site observations and numerous backanalyses of the slope failure reveal that a compound slide occurred where gyttja (contact zone) layers rest directly on the lignite. Gyttja (contact zone) contains the weakest material within the system. The analysis showed that the main cause of the northwestern slope instability was the presence of the groundwater flow within a Quaternary aquifer (through buried valleys), reducing the effective shear strength of the slope materials. It is also noted from backanalysis that the gyttja (contact zone) layer presents a shear strength at, or approaching, the residual value at the time of failure.     

Stability analysis of laterally confined slope lying on inclined bedding plane

A stability analysis of a laterally confined slope model, lying on an inclined bedding plane, was presented to evaluate the lateral shear resistance by considering the loading paths and failure envelopes. Two slope models were prepared on a bedding plane by compaction, one with and one without lateral confinement. The compacted models are related to the geological conditions at shallow depths where brittle deformation can occur and an excavation can induce horizontal field stress that significantly influences the stability of the slope. Three distinct loading paths, controlled by either tilting the angles or increasing the surcharge loads, were applied to achieve the failure of the slope models. Rankine’s passive earth pressure due to compaction was reduced by the shear strength reduction ratio. The shear strength reduction ratio was estimated through the least-squares fitting method based on the results of model tests at failure when the loading paths intersected the failure envelope. Provided that the effect of lateral confinement in a rock mass can be described by the shear strength reduction ratio, the proposed equations will be beneficial for slope stability analyses of laterally confined slopes on bedding planes. A case study of an undercut pit wall in an open-pit mine was demonstrated by showing that the unknown shear strength reduction ratio can be back-analyzed from the rainfall-induced landslide case. Therefore, the design of other undercut slopes with different geometries and groundwater conditions in the rock mass, which have undergone the same geological process as the back-analyzed case, is possible.

     

Behaviour of model footing on pond ash

This paper focuses on the effective utilization of pond ash, as foundation medium. A series of laboratory model tests have been carried out using square, rectangular and strip footings on pond ash. The effects of dry density, degree of saturation of pond ash, size and shape of footing on ultimate bearing capacity of shallow foundations are presented in this paper. Local shear failure of a square footing on pond ash at 37% moisture content (optimum moisture content) is observed up to the values of dry density 11.20 kN/m³ and general shear failure takes place at the values of dry density 11.48 kN/m³ and 11.70 kN/m³. Effects of degree of saturation on ultimate bearing capacity were studied. Experimental results show that degree of saturation significantly affects the ultimate bearing capacity of strip footing. The effect of footing length to width ratio (L/B), on increase in ultimate bearing capacity of pond ash, is insignificant for L/B ≥ 10 in case of rectangular footings. The effects of size of footing on ultimate bearing capacity for all shapes of footings viz., square, rectangular and strip footings are highlighted.     

Stress state analysis of a collapsed overhanging rock slab: A case study

This paper describes the geomechanical back-analysis of the flexural failure of an overhanging limestone slab (volume = 68 m³) that collapsed on 26/01/1999 on the abandoned Cellina Valley state road SS 251 (western Friuli, NE Italy). The survey carried out on site examining the contact surfaces between the block and the slope has ascertained that the rock slab was connected to the stable rock mass by means of a single great rock bridge. The detailed reconstruction of the slab geometry and of the actual restraint conditions acting before the rockfall permitted an estimation of the stress state by using a 3D finite element code (Strand7). The finite element numerical model, referred to the restrained cross-section measured on site (A_res = 2.82 m²), gives a maximum value of σ_t = 5.19 MPa for the tensile stress, which is in good agreement with the estimated value for the characteristic strength mobilized at rupture by the intact rock material (T₀ = 3.5–5.5 MPa). Considerably lower maximum tensile stress (σ_t = 2.25 MPa), smaller than one half, is obtained if the whole contact surface is assumed as resisting surface (A_res = 10.50 m²) and the rock bridge is neglected.     

The diel cycle of carbonyl sulfide in marine surface waters: Field study results and a simple model

The concentrations of atmospheric and dissolved carbonyl sulfide (COS) were measured during a Lagrangian study aboard the R/V Meteor in the northeast Atlantic Ocean, April/May 1992, and during a campaign on the research platform Nordsee in the German Bight (southeastern North Sea), September 1992. The arithmetic means and standard deviations of the COS saturation ratios were 1.27 ± 0.58 (northeast Atlantic) and 3.23 ± 0.73 (German Bight). Sea surface COS showed a pronounced diel cycle with highest concentrations in the late afternoon and a mean concentration amplitude of about 2. To account for this diel cycle, we analyze our results using a simple empirical model, which includes a zeroth order photoproduction constant, sea surface UV light intensity, and terms for hydrolysis removal and air-sea exchange. Fitted and observed COS concentrations agreed to within11 % (northeast Atlantic) and 14% (German Bight). The in situ COS photoproduction constants were (0.030 ± 0.008) fmol L^–1 s^–1 W^–1 m² in the northeast Atlantic (n = 8) and (0.17 ± 0.07) fmol L^–1 s^–1 W^–1 m² in the German Bight (n = 10). After normalization to the cloud cover corrected UV irradiance at 40 latitude, we obtained sea surface COS production rates of (0.034 ± 0.017) nmol L^–1 d^–1 in the northeast Atlantic and (1.62 ± 0.62) nmol L^–1 d^–1 in the German Bight. Currently available in situ photoproduction rates show a high degree of correlation with the UV absorbance (r ² = 0.98, n = 4) and fluorescence (r ² = 0.85, n = 4) of chromophoric dissolved organic matter (CDOM). The regional differences between the COS productivity in the northeast Atlantic Ocean and the German Bight is attributed to the distribution pattern of CDOM optical properties.