Slope stability

Last updated
Real life landslide on a slope Landslide Boschungsrutschung.jpg
Real life landslide on a slope

Slope stability refers to the condition of inclined soil or rock slopes to withstand or undergo movement; the opposite condition is called slope instability or slope failure. The stability condition of slopes is a subject of study and research in soil mechanics, geotechnical engineering and engineering geology. Analyses are generally aimed at understanding the causes of an occurred slope failure, or the factors that can potentially trigger a slope movement, resulting in a landslide, as well as at preventing the initiation of such movement, slowing it down or arresting it through mitigation countermeasures.

Contents

The stability of a slope is essentially controlled by the ratio between the available shear strength and the acting shear stress, which can be expressed in terms of a safety factor if these quantities are integrated over a potential (or actual) sliding surface. A slope can be globally stable if the safety factor, computed along any potential sliding surface running from the top of the slope to its toe, is always larger than 1. The smallest value of the safety factor will be taken as representing the global stability condition of the slope. Similarly, a slope can be locally stable if a safety factor larger than 1 is computed along any potential sliding surface running through a limited portion of the slope (for instance only within its toe). Values of the global or local safety factors close to 1 (typically comprised between 1 and 1.3, depending on regulations) indicate marginally stable slopes that require attention, monitoring and/or an engineering intervention (slope stabilization) to increase the safety factor and reduce the probability of a slope movement. A previously stable slope can be affected by a number of predisposing factors or processes that make the safety factor decrease - either by increasing the shear stress or by decreasing the shear strength - and can ultimately result in slope failure. Factors that can trigger slope failure include hydrologic events (such as intense or prolonged rainfall, rapid snowmelt, progressive soil saturation, increase of water pressure within the slope), earthquakes (including aftershocks), internal erosion (piping), surface or toe erosion, artificial slope loading (for instance due to the construction of a building), slope cutting (for instance to make space for roadways, railways or buildings), or slope flooding (for instance by filling an artificial lake after damming a river).

Examples

Simple slope slip section Slopslump2.jpg
Simple slope slip section

Earthen slopes can develop a cut-spherical weakness area. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package. [1] A primary difficulty with analysis is locating the most-probable slip plane for any given situation. [2] Many landslides have only been analyzed after the fact. More recently slope stability radar technology has been employed, particularly in the mining industry, to gather real time data and assist in determining the likelihood of slope failure.

Real life failures in naturally deposited mixed soils are not necessarily circular, but prior to computers, it was far easier to analyze such a simplified geometry. Nevertheless, failures in 'pure' clay can be quite close to circular. Such slips often occur after a period of heavy rain, when the pore water pressure at the slip surface increases, reducing the effective normal stress and thus diminishing the restraining friction along the slip line. This is combined with increased soil weight due to the added groundwater. A 'shrinkage' crack (formed during prior dry weather) at the top of the slip may also fill with rain water, pushing the slip forward. At the other extreme, slab-shaped slips on hillsides can remove a layer of soil from the top of the underlying bedrock. Again, this is usually initiated by heavy rain, sometimes combined with increased loading from new buildings or removal of support at the toe (resulting from road widening or other construction work). Stability can thus be significantly improved by installing drainage paths to reduce the destabilizing forces. Once the slip has occurred, however, a weakness along the slip circle remains, which may then recur at the next monsoon.

Angle of repose

The angle of repose is related to the shear strength of geologic materials, which is relevant in construction and engineering contexts. [3] For granular materials, the size and shape of grains can impact angle of repose significantly. As the roundness of materials increases, the angle of repose decreases since there is less friction between the soil grains. [4]

When the angle of repose is exceeded, mass wasting and rockfall can occur. It is important for many civil and geotechnical engineers to know the angle of repose to avoid structural and natural disasters. As a result, the application of retaining walls can help to retain soil so that the angle of repose is not exceeded. [5]

The angle of repose and the stability of a slope are impacted by climatic and non-climatic factors.

Water content

Water content is an important parameter that could change the angle of repose. Reportedly, a higher water content can stabilize a slope and increase the angle of repose. [5] However, water saturation can result in a decrease in the slope's stability since it acts as a lubricant and creates a detachment where mass wasting can occur. [6]

Water content is dependent on soil properties such as grain size, which can impact infiltration rate, runoff, and water retention. Generally, finer-grained soils rich in clay and silt retain more water than coarser sandy soils. This effect is mainly due to capillary action, where the adhesive forces between the fluid, particle, and the cohesive forces of the fluid itself counteract gravitational pull. Therefore, smaller grain size results in a smaller surface area on which gravitational forces can act. Smaller surface area also leads to more capillary action, more water retention, more infiltration, and less runoff. [7]

Vegetation

The presence of vegetation does not directly impact the angle of repose, but it acts as a stabilizing factor in a hillslope, where the tree roots anchor into deeper soil layers and form a fiber‐reinforced soil composite with a higher shear resistance (mechanical cohesion). [8]

Roundness of grains

The shape of the grain can have an impact on the angle of repose and the stability of the slope. The more rounded the grain is, the lower the angle of repose. A decrease in roundness, or an increase in angularity, results in interlocking via particle contact. This linear relationship between the angle of repose and the roundness of grain can also be used as a predictor of the angle of repose if the roundness of the grain is measured. [5]

Slope stabilization

Since the stability of the slope can be impacted by external events such as precipitation, an important concern in civil/geotechnical engineering is the stabilization of slopes.

Application of vegetation

The application of vegetation to increase the slope stability against erosion and landslide is a form of bioengineering that is widely used in areas where the landslide depth is shallow. Vegetation increases the stability of the slope mechanically, by reinforcing the soils through plant roots, which stabilize the upper part of the soil. Vegetation also stabilizes the slope via hydrologic processes, by the reduction of soil moisture content through the interception of precipitation and transpiration. This results in a drier soil that is less susceptible to mass wasting. [9]

Stability of slopes can also be improved by:

Figure 1: Rotational failure of slope on circular slip surface Slopslump2.jpg
Figure 1: Rotational failure of slope on circular slip surface

Analysis methods

This section is an excerpt from Slope stability analysis.[ edit ]
Method of slices Pett bish.gif
Method of slices

Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of slopes of soil- and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. It is performed to assess the safe design of a human-made or natural slopes (e.g. embankments, road cuts, open-pit mining, excavations, landfills etc.) and the equilibrium conditions. [10] [11] Slope stability is the resistance of inclined surface to failure by sliding or collapsing. [12] The main objectives of slope stability analysis are finding endangered areas, investigation of potential failure mechanisms, determination of the slope sensitivity to different triggering mechanisms, designing of optimal slopes with regard to safety, reliability and economics, designing possible remedial measures, e.g. barriers and stabilization. [10] [11]

Successful design of the slope requires geological information and site characteristics, e.g. properties of soil/rock mass, slope geometry, groundwater conditions, alternation of materials by faulting, joint or discontinuity systems, movements and tension in joints, earthquake activity etc. [13] [14] The presence of water has a detrimental effect on slope stability. Water pressure acting in the pore spaces, fractures or other discontinuities in the materials that make up the pit slope will reduce the strength of those materials. [15] Choice of correct analysis technique depends on both site conditions and the potential mode of failure, with careful consideration being given to the varying strengths, weaknesses and limitations inherent in each methodology. [16]

Before the computer age stability analysis was performed graphically or by using a hand-held calculator. Today engineers have a lot of possibilities to use analysis software, ranges from simple limit equilibrium techniques through to computational limit analysis approaches (e.g. Finite element limit analysis, Discontinuity layout optimization) to complex and sophisticated numerical solutions (finite-/distinct-element codes). [10] The engineer must fully understand limitations of each technique. For example, limit equilibrium is most commonly used and simple solution method, but it can become inadequate if the slope fails by complex mechanisms (e.g. internal deformation and brittle fracture, progressive creep, liquefaction of weaker soil layers, etc.). In these cases more sophisticated numerical modelling techniques should be utilised. Also, even for very simple slopes, the results obtained with typical limit equilibrium methods currently in use (Bishop, Spencer, etc.) may differ considerably. In addition, the use of the risk assessment concept is increasing today. Risk assessment is concerned with both the consequence of slope failure and the probability of failure (both require an understanding of the failure mechanism). [17] [18]

Mass classification and rating

Various classification and rating systems exist for the design of slopes and to assess the stability of slopes. The systems are based on empirical relations between rock mass parameters and various slope parameters such as height and slope dip.

Probability classification

The slope stability probability classification (SSPC) [19] [20] system is a rock mass classification system for slope engineering and slope stability assessment. The system is a three-step classification: ‘exposure’, ‘reference’, and ‘slope’ rock mass classification with conversion factors between the three steps depending on existing and future weathering and damage due to method of excavation. The stability of a slope is expressed as probability for different failure mechanisms.

A rock mass is classified following a standardized set of criteria in one or more exposures (‘exposure’ classification). These values are converted per exposure to a ‘reference’ rock mass by compensating for the degree of weathering in the exposure and the method of excavation that was used to make the exposure, i.e. the ‘reference’ rock mass values are not influenced by local influences such as weathering and method of excavation. A new slope can then be designed in the ‘reference’ rock mass with compensation for the damage due to the method of excavation to be used for making the new slope and compensation for deterioration of the rock mass due to future weathering (the ‘slope’ rock mass). If the stability of an already existing slope is assessed the ‘exposure’ and ‘slope’ rock mass values are the same.

The failure mechanisms are divided in orientation dependent and orientation independent. Orientation dependent failure mechanisms depend on the orientation of the slope with respect to the orientation of the discontinuities in the rock mass, i.e. sliding (plane and wedge sliding) and toppling failure. Orientation independent relates to the possibility that a slope fails independently from its orientation, e.g. circular failure completely through newly formed discontinuities in intact rock blocks, or failing partially following existing discontinuities and partially new discontinuities.

In addition the shear strength along a discontinuity ('sliding criterion') [19] [20] [21] and 'rock mass cohesion' and 'rock mass friction' can be determined. The system has been used directly or modified in various geology and climate environments throughout the world. [22] [23] [24] The system has been modified for slope stability assessment in open pit coal mining. [25]

See also

Related Research Articles

<span class="mw-page-title-main">Geotechnical engineering</span> Scientific study of earth materials in engineering problems

Geotechnical engineering, also known as geotechnics, is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics to solve its engineering problems. It also relies on knowledge of geology, hydrology, geophysics, and other related sciences.

<span class="mw-page-title-main">Landslide</span> Natural hazard involving ground movement

Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides occur in a variety of environments, characterized by either steep or gentle slope gradients, from mountain ranges to coastal cliffs or even underwater, in which case they are called submarine landslides.

<span class="mw-page-title-main">Angle of repose</span> Steepest angle at which granular materials can be piled before slumping

The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane on which the material can be piled without slumping. At this angle, the material on the slope face is on the verge of sliding. The angle of repose can range from 0° to 90°. The morphology of the material affects the angle of repose; smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands. The angle of repose can also be affected by additions of solvents. If a small amount of water is able to bridge the gaps between particles, electrostatic attraction of the water to mineral surfaces increase the angle of repose, and related quantities such as the soil strength.

<span class="mw-page-title-main">Retaining wall</span> Artificial wall used for supporting soil between two different elevations

Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to. They are used to bound soils between two different elevations often in areas of terrain possessing undesirable slopes or in areas where the landscape needs to be shaped severely and engineered for more specific purposes like hillside farming or roadway overpasses. A retaining wall that retains soil on the backside and water on the frontside is called a seawall or a bulkhead.

<span class="mw-page-title-main">Soil mechanics</span> Branch of soil physics and applied mechanics that describes the behavior of soils

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics.

Rock mass classification systems are used for various engineering design and stability analysis. These are based on empirical relations between rock mass parameters and engineering applications, such as tunnels, slopes, foundations, and excavatability. The first rock mass classification system in geotechnical engineering was proposed in 1946 for tunnels with steel set support.

<span class="mw-page-title-main">Rock mechanics</span> Study of the mechanical behavior of rocks

Rock mechanics is a theoretical and applied science of the mechanical behavior of rocks and rock masses.

Geomechanics is the study of the mechanical state of the Earth's crust and the processes occurring in it under the influence of natural physical factors. It involves the study of the mechanics of soil and rock.

A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses.

There have been known various classifications of landslides. Broad definitions include forms of mass movement that narrower definitions exclude. For example, the McGraw-Hill Encyclopedia of Science and Technology distinguishes the following types of landslides:

Landslide mitigation refers to several human-made activities on slopes with the goal of lessening the effect of landslides. Landslides can be triggered by many, sometimes concomitant causes. In addition to shallow erosion or reduction of shear strength caused by seasonal rainfall, landslides may be triggered by anthropic activities, such as adding excessive weight above the slope, digging at mid-slope or at the foot of the slope. Often, individual phenomena join together to generate instability over time, which often does not allow a reconstruction of the evolution of a particular landslide. Therefore, landslide hazard mitigation measures are not generally classified according to the phenomenon that might cause a landslide. Instead, they are classified by the sort of slope stabilization method used:

Slope mass rating (SMR) is a rock mass classification scheme developed by Manuel Romana to describe the strength of an individual rock outcrop or slope. The system is founded upon the more widely used RMR scheme, which is modified with quantitative guidelines to the rate the influence of adverse joint orientations.

<span class="mw-page-title-main">UTEXAS</span> Computer program

UTEXAS is a slope stability analysis program written by Stephen G. Wright of the University of Texas at Austin. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, and anywhere there is concern for mass wasting. UTEXAS finds the factor of safety for the slope and the critical failure surface. Recently the software was used to help determine the reasons behind the failure of I-walls during Hurricane Katrina.

<span class="mw-page-title-main">Slope stability analysis</span> Method for analyzing stability of slopes of soil or rock

Slope stability analysis is a static or dynamic, analytical or empirical method to evaluate the stability of slopes of soil- and rock-fill dams, embankments, excavated slopes, and natural slopes in soil and rock. It is performed to assess the safe design of a human-made or natural slopes and the equilibrium conditions. Slope stability is the resistance of inclined surface to failure by sliding or collapsing. The main objectives of slope stability analysis are finding endangered areas, investigation of potential failure mechanisms, determination of the slope sensitivity to different triggering mechanisms, designing of optimal slopes with regard to safety, reliability and economics, designing possible remedial measures, e.g. barriers and stabilization.

SVSLOPE is a slope stability analysis program developed by SoilVision Systems Ltd.. The software is designed to analyze slopes using both the classic "method of slices" as well as newer stress-based methods. The program is used in the field of civil engineering to analyze levees, earth dams, natural slopes, tailings dams, heap leach piles, waste rock piles, and anywhere there is concern for mass wasting. SVSLOPE finds the factor of safety or the probability of failure for the slope. The software makes use of advanced searching methods to determine the critical failure surface.

A discontinuity in geotechnical engineering is a plane or surface that marks a change in physical or chemical characteristics in a soil or rock mass. A discontinuity can be, for example, a bedding, schistosity, foliation, joint, cleavage, fracture, fissure, crack, or fault plane. A division is made between mechanical and integral discontinuities. Discontinuities may occur multiple times with broadly the same mechanical characteristics in a discontinuity set, or may be a single discontinuity. A discontinuity makes a soil or rock mass anisotropic.

The shear strength of a discontinuity in a soil or rock mass may have a strong impact on the mechanical behavior of a soil or rock mass. The shear strength of a discontinuity is often considerably lower than the shear strength of the blocks of intact material in between the discontinuities, and therefore influences, for example, tunnel, foundation, or slope engineering, but also the stability of natural slopes. Many slopes, natural and man-made, fail due to a low shear strength of discontinuities in the soil or rock mass in the slope. The deformation characteristics of a soil or rock mass are also influenced by the shear strength of the discontinuities. For example, the modulus of deformation is reduced, and the deformation becomes plastic rather than elastic. This may cause, for example, larger settlement of foundations, which is also permanent even if the load is only temporary. Furthermore, the shear strength of discontinuities influences the stress distribution in a soil or rock mass.

The sliding criterion (discontinuity) is a tool to estimate easily the shear strength properties of a discontinuity in a rock mass based on visual and tactile characterization of the discontinuity. The shear strength of a discontinuity is important in, for example, tunnel, foundation, or slope engineering, but also stability of natural slopes is often governed by the shear strength along discontinuities.

The Newmark's sliding block analysis method is an engineering that calculates permanent displacements of soil slopes during seismic loading. Newmark analysis does not calculate actual displacement, but rather is an index value that can be used to provide an indication of the structures likelihood of failure during a seismic event. It is also simply called Newmark's analysis or Sliding block method of slope stability analysis.

Soil sloughing is soil falling off banks and slopes due to a loss in cohesion. Soil sloughs off for the same reasons as landslides in general, with very wet soil being among the leading factors. Sloughing is a relatively shallow phenomenon involving the uppermost layers of the soil. Bare soils are more likely to slough than soils with plant cover in part because the roots help hold the surface against gravity. Unabated soil sloughing can end in massive bank or slope failure.

References

  1. ↑ "Slope Stability Calculator" . Retrieved 2006-12-14.
  2. ↑ Chugh, Ashok K. (2002). "A method for locating critical slip surfaces in slope stability analysis: Discussion". Canadian Geotechnical Journal. 39 (3): 765–770. doi:10.1139/t02-042.
  3. ↑ Kim, Donghwi; Nam, Boo Hyun; Youn, Heejung (December 2018). "Effect of clay content on the shear strength of clay–sand mixture". International Journal of Geo-Engineering. 9 (1): 19. doi: 10.1186/s40703-018-0087-x . ISSN   2092-9196. S2CID   139312055.
  4. ↑ Santamarina, J. Carlos (2003-01-13). "Soil Behavior at the Microscale: Particle Forces". Soil Behavior and Soft Ground Construction. Reston, VA: American Society of Civil Engineers: 25–56. doi:10.1061/40659(2003)2. ISBN   978-0-7844-0659-5.
  5. 1 2 3 Beakawi Al-Hashemi, Hamzah M.; Baghabra Al-Amoudi, Omar S. (May 2018). "A review on the angle of repose of granular materials". Powder Technology. 330: 397–417. doi: 10.1016/j.powtec.2018.02.003 .
  6. ↑ Balasubramanian A (2011). "MASS-WASTING". doi:10.13140/RG.2.2.10405.50407.{{cite journal}}: Cite journal requires |journal= (help)
  7. ↑ Kozicki, J.; Donzé, F.V. (2009-10-09). "YADE‐OPEN DEM: an open‐source software using a discrete element method to simulate granular material". Engineering Computations. 26 (7): 786–805. doi:10.1108/02644400910985170. ISSN   0264-4401.
  8. ↑ Kim, John H.; Fourcaud, Thierry; Jourdan, Christophe; Maeght, Jean-Luc; Mao, Zhun; Metayer, James; Meylan, Louise; Pierret, Alain; Rapidel, Bruno; Roupsard, Olivier; de Rouw, Anneke (2017-05-28). "Vegetation as a driver of temporal variations in slope stability: The impact of hydrological processes: Variable Stability of Vegetated Slopes". Geophysical Research Letters. 44 (10): 4897–4907. doi:10.1002/2017GL073174.
  9. ↑ Mulyono, A; Subardja, A; Ekasari, I; Lailati, M; Sudirja, R; Ningrum, W (February 2018). "The Hydromechanics of Vegetation for Slope Stabilization". IOP Conference Series: Earth and Environmental Science. 118 (1): 012038. Bibcode:2018E&ES..118a2038M. doi: 10.1088/1755-1315/118/1/012038 . ISSN   1755-1307. S2CID   134151880.
  10. 1 2 3 Eberhardt 2003, p. 4
  11. 1 2 Abramson 2002, p. 2
  12. ↑ Kliche 1999, p. 2
  13. ↑ USArmyCorps 2003, pp. 1–2
  14. ↑ Abramson 2002, p. 1
  15. ↑ Beale, Geoff; Read, John, eds. (2014). Guidelines for Evaluating Water in Pit Slope Stability. CSIRO Publishing. ISBN   9780643108356.
  16. ↑ Stead 2001, p. 615
  17. ↑ Cardenas, IC (2019). "On the use of Bayesian networks as a meta-modelling approach to analyse uncertainties in slope stability analysis". Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards. 13 (1): 53–65. Bibcode:2019GAMRE..13...53C. doi:10.1080/17499518.2018.1498524. S2CID   216590427.
  18. ↑ Liu, Xin; Wang, Yu (2023). "Analytical solutions for annual probability of slope failure induced by rainfall at a specific slope using bivariate distribution of rainfall intensity and duration". Engineering Geology. 313: 106969. Bibcode:2023EngGe.31306969L. doi:10.1016/j.enggeo.2022.106969. S2CID   254807263.
  19. 1 2 Hack, R. (1996). Slope Stability Probability Classification (SSPC) (PDF). ITC publication 43. Technical University Delft & Twente University - International Institute for Aerospace Survey and Earth Sciences (ITC Enschede), Netherlands. p. 258. ISBN   978-90-6164-154-4.
  20. 1 2 Hack, R.; Price, D.; Rengers, N. (2003). "A new approach to rock slope stability – a probability classification (SSPC)". Bulletin of Engineering Geology and the Environment. 62 (2): 167–184. doi:10.1007/s10064-002-0155-4. S2CID   140693335.
  21. ↑ Andrade, P.S.; Saraiva, A.A. (2008). "Estimating the joint roughness coefficient of discontinuities found in metamorphic rocks" (PDF). Bulletin of Engineering Geology and the Environment. 67 (3, number 3): 425–434. doi:10.1007/s10064-008-0151-4. hdl:10316/7611. S2CID   129119508.
  22. ↑ Filipello, A.; Giuliani, A.; Mandrone, G. (2010). "Rock Slopes Failure Susceptibility Analysis: From Remote Sensing Measurements to Geographic Information System Raster Modules". American Journal of Environmental Sciences. 6 (6): 489–494. doi: 10.3844/ajessp.2010.489.494 .
  23. ↑ Hailemariam, G.T.; Schneider, J.F. (May 2–7, 2010). "Rock Mass Classification of Karstic Terrain in the Reservoir Slopes of Tekeze Hydropower Project" (PDF). EGU General Assembly 2010. EGU2010-831, 2010. Vol. 12. Vienna, Austria. p. 831.
  24. ↑ Dhakal, S.; Upreti, B.N.; Yoshida, M.; Bhattarai, T.N.; Rai, S.M.; Gajurel, A.P.; Ulak, P.D.; Dahal, R.K. (2005). "Application of the SSPC system in some of the selected slopes along the trekking route from Jomsom to Kagbeni, central-west Nepal". In Yoshida, M.; Upreti, B.N.; Bhattarai, T.N.; Dhakal, S. (eds.). Natural disaster mitigation and issues on technology transfer in South and Southeast Asia; proceedings of the JICA Regional Seminar. Kathmandu, Nepal: Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal. pp. 79–82.
  25. ↑ Lindsay, P.; Campbellc, R.N.; Fergussonc, D.A.; Gillarda, G.R.; Moore, T.A. (2001). "Slope stability probability classification, Waikato Coal Measures, New Zealand". International Journal of Coal Geology. 45 (2–3): 127–145. Bibcode:2001IJCG...45..127L. doi:10.1016/S0166-5162(00)00028-8.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.