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WEATHERING INFLUENCE ON ENGINEERING STRUCTURES
ROBERT HACK FACULTY OF GEO-INFORMATION SCIENCE AND EARTH OBSERVATION (ITC), UNIVERSITY OF TWENTE, THE NETHERLANDS. PHONE:+31 (0)6 24505442; EMAIL: [email protected]
ITC, The Netherlands; 30 October 2012
WEATHERING INFLUENCE ON ENGINEERING STRUCTURES Robert Hack Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, The Netherlands, [email protected] ABSTRACT An overview is given of the influence of weathering on engineering structures in and on natural materials. Some recently developed new ideas to incorporate weathering in design are discussed. All natural materials near and sometimes also deeper below surface of the Earth are subject to weathering. Notoriously difficult to forecast is the geotechnical behaviour due to future deterioration of the ground mass under influence of weathering. Weathering is a strongly local feature influenced by local circumstances such as local climate and weather, groundwater, and humans. Calculation methods and parameters used in design of engineering structures often do not properly anticipate the degradation of natural materials due to weathering. Preliminary failure, i.e. failure of the engineering structure before the end of the planned lifetime, may be the consequence. Slopes and tunnels start to raffle or may fail completely, and even buried foundations may be affected.
what is weathering in engineering implications for engineering forecasting weathering weathering rate
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WEATHERING
Physical, chemical, and biological alteration of rock and soil over time;
Influenced by the atmosphere and the hydrosphere, and
Cause rock or soil material to fall apart into smaller pieces or to dissolve in water.
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STRESS RELIEF
Due to change in stress field (orientation or magnitude) discontinuities may form or existing discontinuities may open (discontinuities are bedding planes, joints, fractures, etc.)
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WEATHERING & STRESS RELIEF
Difference between effects of weathering and stress relief difficult to distinguish:
Therefore in engineering taken together as being “weathering”
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EFFECTS OF WEATHERING
• weakening of intact rock blocks and soil grains • integral become mechanical discontinuities (i.e. bedding planes
not being yet a mechanical plane of weakness become a mechanical plane of weakness)
• material between rock blocks becomes weaker (remains of weathered material)
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WHAT GIVES ROCK AND SOIL MASSES STRENGTH
Rock and soil masses consist of: • intact blocks (rock) with discontinuities in-between (e.g. bedding
planes, joints, fractures, etc) • grains (in soil) with grain contacts in-between
Strength of a mass is given by:
• strength (shear & tensile) of intact rock blocks or soil grains • shear strength between blocks (rock) or grains (soil)
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ENGINEERING EFFECTS
weaker rock blocks and soil grains smaller rock blocks and grains shear strength between blocks and grains less
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reduction intact rock strength due to solution of cement (2)
blocks falling from disintegrating wall
Children playground made around 1995 Vandellos, Spain (photos: Hack, 2002)
REDUCTION BLOCK SIZE
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SAME MATERIAL THE MORE EXPOSED THE SMALLER THE BLOCK SIZE
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CINDARTO SLOPE: VARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING
bedding planes
Slightly higher clay content on bedding plane
April 1990
shear strength reduction between blocks and grains (1)
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CINDARTO SLOPE VARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING
April 1992
mass slid
shear strength reduction between blocks and grains (2)
carbonate is dissolved at bedding plane – clay remains
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shear strength reduction between blocks and grains (3)
DEGREES OF WEATHERING
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Degrees of weathering
IMPACT OF WEATHERING
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From: De Mulder, E.J.F., Hack, H.R.G.K., Van Ree, C.C.D.F., 2012. Sustainable Development and Management of the Shallow Subsurface. The Geological Society, London. ISBN: 978-1-86239-343-1. p. 192.
WEATHERING IN TIME
weathering rate depends on: local climate & weather (e.g. wind direction) groundwater land-use (fertilizer) does it stay exposed or is it covered by weathered material
(erosion) method of excavation
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WEATHERING & TIME Weathering based on rock & soil dissolution back analyses:
Intro Weathering - Hack 28/09/2012 18
rate [mm/y]
average world 0.006
(Olvmo, 2010) cold climate < 0.001
warmer climate > 0.1
tropics metamorphic rock mass 0.04 Brazil (Moreira-Nordemann, 1980)
tropics silica rock mass 0.058 Puerto Rico (White et al., 1998)
mountainous areas with high erosion rates mm's
WEATHERING & TIME
Dissolution in 50 years thus around ½ to 5 mm Dissolution in particular around discontinuities Dissolution in the order of fraction of mm Fraction of mm enough to reduce (shear) strength
Hence, even in relative short time spans of 50 to 100 years, weathering may have
a substantial negative influence on properties of soil and rock masses
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Cleopatra’s needle Central Park, New York
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1881
2011 copyright: michael martine, 2011
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Poor blasting of slope caused increased permeability and consequently allowed for loss of structure and rapid weathering of thin clay inter-bedded softer layers
POORLY BLASTED SLOPE
1990: new cut: slope height 13.8 m high, dip 70°
2002: slope dip about 55°
2005: slope dip about 52°
SSPC forecast final stability: slope dip about 45° (SSPC after Hack et al, 2003)
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BLASTING DAMAGE CAUSES RAPID RAVELLING AND LOSS OF STRUCTURE AND CONSEQUENTLY WEATHERING
Diabase (subvolcanic rock equivalent to volcanic basalt) made in: 1995 photos: 2011
Silver Creek Cliff Tunnel, Route 61, Duluth, MN, USA
(photos: Hack, 2011)
Road protection wall built in 2010 to prevent stones on the road
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debris
Fine grained calcareous mudstone falling apart within a couple of years after excavation South Korea
(photos Hack, 2008)
debris
FUTURE DEGRADATION
Reduction in slope angle due to weathering, erosion and ravelling (after Huisman et al, 2006)
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1.0
1.5
2.0
2.5
3.0
3.5
7.0 7.5 8.0 8.5 9.0 9.5
y [m]
z [m
]
Excavated 1999 May 2001 May 2002
FUTURE DEGRADATION
Main processes involved in degradation: Loss of structure due to stress release Weathering (In-situ change by inside or outside influences) Erosion (Material transport with no chemical or structural
changes)
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WEATHERING RATE
The susceptibility to weathering is a concept that is frequently addressed by “the” weathering rate of a rock material or mass.
Weathering rates may be expected to decrease with time, as the state of the rock mass becomes more and more in equilibrium with its surroundings.
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( ) ( )log 1appinit WEWE t WE R t= − +
WE(t) = degree of weathering at time t WEinit = (initial) degree of weathering at time t = 0 Rapp
WE = weathering intensity rate WE as function of time, initial weathering and the weathering intensity rate
GYPSUM CLAYSTONE SLOPE IN VANDELLOS
SSPC system with applying weathering intensity rate: - original slope cut about 50º (1998) - in 15 years decrease to 35º
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KOTA KINABALU, MALAYSIA
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10 years old
(after Tating, Hack, & Jetten, 2011)
KOTA KINABALU
Side road (dip 45°, 5 years old) sandstone: slightly weathered SSPC stability: Sandstone: stable (92%) Shale: unstable (< 5%) (after Tating et al., 2012)
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KOTA KINABALU
Main road (dip 30°, 10 years old): sandstone: moderately weathered SSPC stability: Sandstone: stable (95%) Shale: ravelling (<5%) (after Tating et al., 2012)
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10 years old
KOTA KINABALU
time [years]
dip [degrees]
SSPC visual
unit RM friction RM cohesion
[degrees] [kPa]
shale
slightly 5 45 4 2.4 instable
moderately 10 30 2 1.1 instable
sandstone
slightly 5 45 20 10.0 stable
moderately 10 30 11 6.3 stable
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SSPC system in combination with degradation forecasts gives: reasonable design for slope stability with minimum of work and in a short time (likely a reasonable tool to forecast susceptibility to weathering)
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REFERENCES De Mulder, E.J.F., Hack, H.R.G.K., Van Ree, C.C.D.F., 2012. Sustainable Development and Management of the Shallow Subsurface. The Geological Society,
London. ISBN: 978-1-86239-343-1. p. 192. Hack, H.R.G.K., 2002. An evaluation of slope stability classification; Keynote lecture. In: Dinis Da Gama, C., Ribeira E Sousa, L. (Eds) ISRM EUROCK 2002, Funchal,
Madeira, Portugal. Sociedade Portuguesa de Geotecnia, Av. do Brasil, 101, 1700-066 Lisboa, Portugal, pp. 3–32. Hack, H.R.G.K., Price, D.G., Rengers, N., 2003. A new approach to rock slope stability : a probability classification SSPC. Bulletin of Engineering Geology and the
Environment. 62 (2). DOI: 10.1007/s10064-002-0155-4. pp. 167-184. Hack, H.R.G.K., Price, D., Rengers, N., 2005. Una nueva aproximación a la clasificación probabilística de estabilidad de taludes (SSPC). In: Proyectos, U.D., Minas,
E.T.S.I. (Eds), Ingeniería del terreno : ingeoter 5 : capítulo 6. Universidad Politécnica de Madrid, Madrid. ISBN: 84-96140-14-8. p. 418. (in Spanish) Huisman, M., Hack, H.R.G.K., Nieuwenhuis, J.D., 2006. Predicting Rock Mass Decay in Engineering Lifetimes: The Influence of Slope Aspect and Climate.
Environmental & Engineering Geoscience. 12 (1). DOI: 10.2113/12.1.39. pp. 39-51. Moreira-Nordemann, L.M., 1980. Use of 234U/238U disequilibrium in measuring chemical weathering rate of rocks. Geochimica et Cosmochimica Acta. 44 (1). DOI:
10.1016/0016-7037(80)90180-5. pp. 103-108. Olvmo, M., 2010. Review of denudation processes and quantification of weathering and erosion rates at a 0.1 to 1 Ma time scale; Technical Report TR-09-18. Svensk
Kärnbränslehantering AB; Swedish Nuclear Fuel and Waste Management Co, Stockholm, Sweden. p. 56. Price, D.G., De Freitas, M.H., Hack, H.R.G.K., Higginbottom, I.E., Knill, J.L., Maurenbrecher, M., 2009. Engineering geology : principles and practice. De Freitas, M.H.
(Ed.). Springer-Verlag, Berlin, Heidelberg. ISBN: 978-3-540-29249-4. p. 450. White, A.F., Blum, A.E., Schulz, M.S., Vivit, D.V., Stonestrom, D.A., Larsen, M., Murphy, S.F., Eberl, D., 1998. Chemical Weathering in a Tropical Watershed, Luquillo
Mountains, Puerto Rico: I. Long-Term Versus Short-Term Weathering Fluxes. Geochimica et Cosmochimica Acta. 62 (2). DOI: 10.1016/s0016-7037(97)00335-9. pp. 209-226.
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