Discontinuity (geotechnical engineering)

Last updated October 13, 2024

In geotechnical engineering, a discontinuity (often referred to as a joint) 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.^[1]^[2]^[3]^[4]^[5]

Mechanical

A mechanical discontinuity is a plane of physical weakness where the tensile strength perpendicular to the discontinuity or the shear strength along the discontinuity is lower than that of the surrounding soil or rock material.

Integral

An integral discontinuity is a discontinuity that is as strong as the surrounding soil or rock material. Integral discontinuities can change into mechanical discontinuities due to physical or chemical processes (e.g. weathering) that change the mechanical characteristics of the discontinuity.

Set or family

Various geological processes create discontinuities at a broadly regular spacing. For example, bedding planes are the result of a repeated sedimentation cycle with a change of sedimentation material or change in structure and texture of the sediment at regular intervals, folding creates joints at regular separations to allow for shrinkage or expansion of the rock material, etc. Normally discontinuities with the same origin have broadly the same characteristics in terms of shear strength,^[6]^[7] spacing between discontinuities,^[8] roughness, infill, etc. The orientations of discontinuities with the same origin are related to the process that has created them and to the geological history of the rock mass. A discontinuity set or family denotes a series of discontinuities for which the geological origin (history, etc.), the orientation, spacing, and the mechanical characteristics (shear strength, roughness, infill material, etc.) are broadly the same.

Single

A discontinuity may exist as a single feature (e.g. fault, isolated joint or fracture) and in some circumstances, a discontinuity is treated as a single discontinuity although it belongs to a discontinuity set, in particular if the spacing is very wide compared to the size of the engineering application or to the size of the geotechnical unit.

Characterization

Various international standards exist to describe and characterize discontinuities in geomechanical terms, such as ISO 14689-1:2003^[9] and ISRM.^[1]

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The rock mass rating (RMR) is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and 1973. Since then it has undergone multiple modifications out of which, RMR89 is commonly used. Recently RMR14 has been proposed to improve the RMR performance by incorporating new experiences from tunnel practices. Continuous functions and a software "QuickRMR" for RMR89 and RMR14 have also been proposed by Kundu. RMR combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality, which is used for the design and construction of excavations in rock, such as tunnels, mines, slopes, and foundations.

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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.

Persistence determines the possibilities of relative movement along a discontinuity in a soil or rock mass in geotechnical engineering. Discontinuities are usually differentiated in persistent, non-persistent, and abutting discontinuities (figure).

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 Q-slope method for rock slope engineering and rock mass classification is developed by Barton and Bar. It expresses the quality of the rock mass for slope stability using the Q-slope value, from which long-term stable, reinforcement-free slope angles can be derived.

References

1 2 ISRM (2007). Ulusay, R.; Hudson, J.A. (eds.). The Blue Book - The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974-2006. Ankara: ISRM & ISRM Turkish National Group. p. 628. ISBN 978-975-93675-4-1. Archived from the original on 2014-11-05. Retrieved 2011-03-08.
↑ Price, D.G. (2008). De Freitas, M.H. (ed.). Engineering Geology: Principles and Practice. Springer. p. 450. ISBN 978-3-540-29249-4.
↑ Laubscher, D.H. (1990). "A geomechanics classification system for rating of rock mass in mine design". Journal South African Institute of Mining and Metallurgy. 90 (10): 257–273. ISSN 0038-223X.
↑ Hack, H.R.G.K. (1998) [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 90-6164-154-3.
↑ Hack, H.R.G.K.; 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.
↑ Barton, N.R.; Choubey, V. (1977). "The shear strength of rock joints in theory and in practice". International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts. 10 (1–2): 1–54. Bibcode:1977RMFMR..10....1B. doi:10.1007/BF01261801. S2CID 137141629.
↑ Hack, H.R.G.K.; Price, D.G. (September 25–29, 1995). Fujii, T. (ed.). Determination of discontinuity friction by rock mass classification (PDF). Proceedings 8th International Society for Rock Mechanics (ISRM) congress. Vol. 3. Tokyo, Japan: Balkema, Rotterdam, Taylor & Francis. pp. 23–27. ISBN 978-90-5410-576-3.
↑ Priest, S.D.; Hudson, J.A. (1976). "Discontinuity spacings in rock". International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts. 13 (5): 135–148. doi:10.1016/0148-9062(76)90818-4.
↑ ISO 14689-1:2003 (2003). Geotechnical investigation and testing. Identification and classification of rock. Part 1: Identification and description. International Organization for Standardization (ISO). p. 21. doi:10.1007/s10064-002-0155-4. S2CID 140693335.{{cite book}}: CS1 maint: numeric names: authors list (link)