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Geomorphology and Flooding
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Giovanni Barrocu, Saeid Eslamian
Physical or mechanical weathering consists of rock breaking, or disintegration, produced by: Thermal stresses, due to differential rock expansion and dilatation, caused by different dilatation coefficients of rock mineral components and temperature differences between the outer and inner rock mass, as rocks are poor heat conductors.Frost weathering, also named ice wedging or cryofracturing, occurring mainly in cracks, joints, and pores where infiltrated water freezes, thus increasing almost 10% of its volume so that rocks break up. Rock fragments of various shapes and sizes accumulate at the foot of reliefs forming scree slopes.Unloading or pressure release, where overlying materials are removed by different types of erosion so that the outer parts of rock masses tend to expand for stresses, producing fractures parallel to the rock surface with a process of exfoliation.Salt crystallization or haloclasty: especially in reliefs exposed to saltwater spray, saline solutions seep and evaporate in rock discontinuities, thus forming salt crystals, which, growing, exert pressure on the confining surfaces of cracks and pores up to rupture rocks into slabs.Plant roots, acting as wedges into the rock mass along the upper part of rifts and weak zones, favor water infiltration and chemical weathering.
Desertification and Land Degradation Processes
Published in Ajai, Rimjhim Bhatnagar, Desertification and Land Degradation, 2022
Frost shattering or frost weathering is the process of breaking down rocks due to freeze and thaw actions in cold mountainous regions (Figure 4.7). In the cold regions, water seeps into the cracks, fissures, joints and pores in the daytime and freezes during the night, which melts again in the morning. Water in the cracks and fissures expands when it freezes into ice and thus exerts pressure on the rock, thereby widening the gap of the cracks. When water freezes at 0°C in a closed system, its volume increases by 9% and, a pressure of as high as 2,100 kg/cm2 is exerted at −22°C. This is the maximum pressure that is achieved if the system is totally closed. In the situation being discussed here, a closed system may normally get created when the water freezes first at the surface of the rocks thereby sealing the surface opening of cracks, joints and pores. The above hydrostatic pressure exerted on the rocks, due to freezing of water inside the pores and cracks, can exceed the tensile strength of many rocks (Cooke and Doornkamp 1974) and may, therefore, lead to shattering/breaking of rocks or widening of the cracks (Figure 4.7a). In the daytime, water melts and as the crack has been widened, it accommodates more water, thereby increasing the volume of the water in the crack. During the next night, a higher volume of accumulated water will freeze and expand, making the crack even bigger. Over a period of time, with continuing freeze-thaw cycle, the rocks break into pieces which may further break into still smaller pieces (Figure 4.7b). These small pieces of rocks, produced through the frost shattering process, are called scree. In mountainous terrain, scree moves down the slope and may also spill over and cover the plain foothills. In Figure 4.7c, scree materials are seen on the mountain slopes. A very large number of freeze-thaw cycles can break even the hardest rocks. The rate of the breaking down of the rocks and the size and shape of the end product depend on the rock type and its characteristics, as well as the intensity, frequency and rate of freezing and thawing, and the quantity and availability of water. The broken rocks move down the slope under the force of gravity. The process of frost shattering also leads to the degradation of the land where it settles down after the downslope movement. Usually, the broken pieces of rocks, comprising boulders and scree fall down the slope and spread over creating a stony layer over the plain land at the foothills. The boulders and scree layer settled and deposited over croplands or pasturelands in the foothills and valley area cause the degradation of agricultural and pasturelands (Figure 4.7c).
Mechanical behaviour and constitutive model of tailing soils subjected to freeze-thaw cycles
Published in European Journal of Environmental and Civil Engineering, 2021
Youneng Liu, Runqiu Huang, Enlong Liu, Feng Hou
To further confirm the reasonability and applicability of model proposed here, experimental data for tailing soils from different scholar were verified. We use the CU test results for tailing sand at optimum moisture content by Jin, Song, Chen, and Zhang (2017), in which the experiment condition is quite similar to our study. The tested freezing and thawing temperatures range in −20 °C to −15 °C and 15 °C–20 °C, respectively, with N = 0, 1, 5 and 9. The computational parameters obtained from test results are list in Table 5, with the comparisons of tested and computed results shown in Figure 16. It can be seen that the new fractional order constitutive model proposed here can also predict the CU test results of tailings fine sand samples. However, the curves of low confining pressures of 50 and 100 kPa with N = 0 are not fitting well with the test results. This difference may arise from that the samples were unsaturated when exposed to FTC with a closed system, which makes the frost weathering effect less intense than it on the saturated soil in open system. Compared with the CD test studied, the values of the calculated volumetric parameters η and ζ in CU test are selected as 0, thus the parameter γ determined by γ0 and l can be arbitrary. Besides, the calculated parameter of CU test shows a variation in the modulus correlation coefficient ξ and the fractional parameter β, and other parameters can keep a relative stable value.