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Seepage effects of groundwater and its make-up water on triggering ground subsidence
Mineral
Journal of University of Science and Technology Beijing Volume 13, Number 1, February 2006, Page 11
Seepage effects of groundwater and its make-up water on triggering ground subsidence Zhenhua Ouyang, Meijeng Cai, Changhong Li, and Mowen Xie Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing 100083, China (Received 2004-12-03)
Abstract: The functioning mechanism of groundwater and its make-up water in the process of ground subsidence was studied from such three aspects as osmotic corrasion, osmotic pressure effect and concretion effect. As to osmotic corrasion, its forming conditions, mechanical mechanism and process were analyzed. As to osmotic pressure effect, it was mainly studied from hydrostatic pressurizing effect, sop softening effect and negative pressure sealing effect. Through concretion and saturation of soil, the factors of concretion settlement were analyzed. The results showed that both groundwater and its make-up water are important triggering factors to ground subsidence.
Key words: groundwater; make-up water; ground subsidence; seepage effect
[This work was financially supported by the National Nature Science Foundation of China (N0.50490271). and the National Key Technologies R & D Program of China (N0.2004BA615A-18).]
1. Introduction Beiming River Iron Mine is 8 km away from Wu'an city, Hebei Province of China, and the geographical coordinates of this mine is 114"7'30" in east longitude and 36'45'0" in north latitude. The iron ore is embedding in the riverbed of Beiming River. At the end of February 2003, two ground collapse-pits and many collapse-fissures appeared on the earth surface around the measure-well, which made the well discarded as useless. The bigger one of the two ground collapse-pits is about 20 m long from east to west, and about 15 m wide from south to north. And the biggest collapse-fissure is about 400 mm in width. Now, the ground collapse becomes more and more severe, which menaces the mine production security and destroys the ground surface's environment. The reasons that arouse the ground settlement and collapse of Beiming Rive Iron Mine are not only the underground mining but also the co-function of many factors, in which the groundwater and its make-up water play an important role. There are mainly five kinds of replenishment approaches of groundwater: (1) atmospheric precipitation; (2) the drainage of mill plants on the ground; (3) water leakage of Beiming river; (4) water leakage of agricultural irrigation; (5) Correspondingauthor: Zhenhua Ouyang, E-mail:[email protected]
replenishment of side direction. In Beiming River Iron Mine, both the rock layer above the ore body and the wall rock are Ordovician metaphase calcareous rock which contains a lot of water. The rock body can be classified as dissolved rock cave cracks ore body with complicated conditions on hydrologic geology. Above the ore body is quaternary system loess and gravel abroad, which is about 150 m in depth. Iron ore bodies exist in the interface between the Ordovician metaphase calcareousness rock and Yanshan stage diorite, with a depth of about 134 m to 679 m underground. Most of them are under groundwater table. Pumping water is absolutely necessary before exploiting the ore. When pumping groundwater, dynamical condition of water is altered, and many problems such as osmotic corrasion, osmotic pressure effect, settlement of concretion are triggered, and all of these play important roles in the development of ground subsidence.
2. Ground subsidence triggered by osmotic corrasion 2.1. Conditions of ground subsidence triggered by osmotic corrasion There are two kinds of conditions in which the
12
ground subsidence can be triggered by osmotic corrasion. The physical conditions are the existing of the seepage routes of osmotic corrasion and the overlying soil layer’s property of instability and easy to collapse [l]. The seepage routes are composed of mine-out area, laneways and the formed cracks consisted in the rock layer above the ore body. The dynamical condition is the soil osmotic corrasion triggered by groundwater. The former is the interior reason and the latter is the exterior one. Both of them are necessary to the development of ground collapse. Through the seepage routes, the running groundwater aggravates the osmotic corrasion on the fillings of rock dissolves and the overlying soil layer. And this accelerated the development of ground subsidence especially when large amount of groundwater is pumped out for underground mining.
2.2. Mechanical mechanism of osmotic corrasion Soil is a medium with run-through holes. Exiguous soil grains will be washed away with the scouring of osmotic water because of the dynamic hydraulic pressure of osmotic water when groundwater is pumped out. Dynamic hydraulic pressure is the scouring force of osmotic water on per volume unit of solid grain. The direction of the pressure is the same with the direction of osmotic water, and the computing formula is as follows,
where Gd is the dynamic hydraulic pressure; yw is the specific gravity of water; Z is the water table gradient; Ah is the water head difference; L is the length of osmotic route. There are two kinds of mechanical mechanisms of ground collapse which is triggered by pumping out groundwater. (1) If there are caves in the unconsolidated overlying soil layer or the underlying bedrock that both containing water, dynamic balance is kept between the seepage force of groundwater and the rock geotechnical body in the native state. When pumping out groundwater for underground mining, the groundwater table of karst descends below the unconsolidated formation, and large water table gradient is developed in the two water-bearing strata. With the effect of hydrodynamic pressure, groundwater and grains of overlying soil layer flow into the cavities in underlying bedrock, which trigger ground collapse in the karst area. (2) If the overlying soil layer is waterresisting claypan and there are caves in the underlying bedrock that contains water, the ground water of karst is artesian groundwater in native state, and the pene-
J. Univ. Sci. Technol. Beijing, VoL13,No.1, Feb 2006
trability of groundwater and geotechnical body is in dynamical balance. When pumping out groundwater for underground mining, the artesian groundwater loses its pressure with the decline of the water table, and a space with no pressure is developed between the two layers. All of these make the original effective stress in the overlying unconsolidated claypan increase, which in turn lead to the compression deformation of the claypan. Because of the compression deformation, the overlying claypan sinks towards the caves, which aroused ground collapse in this area [2-41.
2.3. Process of osmotic corrasion As Fig. 1 shows, the process of ground collapse that is triggered by osmotic corrasion is approximately divided into four phases. (1) Karst artesian groundwater seeping into the underlying bedrock, large amount of phreatic water flowing towards the mine-out area or artificial drainage laneway, and the water in the shallow bearing water strata with great permeability also seeping into the underlying bedrock, all of which lead to the groundwater table decline and the hydrodynamic pressure increase. With the alteration of the dynamic condition, the buoyancy on the overlying soil strata declines, and the original balance of groundwater is destroyed, which in turn lead to the increasing of the hydraulic gradient and the water velocity, and the corrasion effect of water is enhanced. So groundwater makes the incompacted fillings and shroud in the interstice alleyway of bedrock endure the effect of suffusion, erosion and undercutting in side direction. (2) With the suffusion and erosion of water, the incompacted fillings in the interstice alleyway of bedrock are washed away. The bottom soil of quaternary system collapses and is washed away, which lead to the formation of arch avalanche and the concealed soil cavities. (3) On one hand, the soil cavities are under the erosion and scouring of karst ground water after their formation, On the other hand, the ground water in overlying stratum as well as water seeping from the surface permeates down into the caves and accumulates, and replenishes the ground water. Then, vertical osmotic erosion is engendered on the overlying stratum. Under the hydrodynamic pressure of osmotic water and the overlying soil layer’s weight, the soil body collapses and is moved away. In turn the concealed soil cavities are enlarged, and ground settlement takes place. When the overlying soil layer’s weight is gradually approach the ultimate compressing or shearing strength of cavities, ground settlement is
Z.H. Ouyang et al., Seepage effects of groundwater and its make-up water on triggering ground subsidence
aggravated. Finally, under the stretching stress, ground cracks appear. (4)If the weight pressure of the overlying soil layer exceeds the ultimate compressing or shearing strength of the soil cavities, ground collapse and cracks will
13
take place. The reason is that not only the shearing stress in vertical direction but also stretching stress in horizontal direction is triggered during the collapse process.
Fig. 1. Formation process of subsidence triggered by osmotic corrasion: (a) groundwater table declined; (b) soil cavities appeared; (c) soil cavities enlarged; (d) ground collapse appeared. l-laneway; M i n e - o u t area; Mimestone; 4-cranny; --roundwater table; 6-quaternary system soil; 7 - t h e earth's surface; &oil cavity; %racks on the ground; lo-collapse pit.
3. Ground subsidence triggered by osmotic pressure effect
ration degree of soil. The other is that when the rainfall accumulates on the ground surface, weight of the water becomes dead load on soil.
Atmospheric precipitation is the main make-up water of groundwater. The mechanical mechanism of ground collapse that is triggered by atmospheric precipitation is osmotic pressure effect, which includes still water pressurizing effect, vertical osmotic effect, intenerate effect after sopping up water and negative pressure sealing effect. Among them vertical osmotic effect is hydrodynamic effect. Still water pressurizing effect and intenerate effect for sop up water are soil mechanical effect, and negative pressure sealing effect is pneumatics effect.
3.2. Sop softening effect
3.1. Still water pressurizing effect Still water pressurizing effect has two meanings. One is the increasing of the natural unit weight of the soil during or after the rainfall because the rain water leaks down and replenishes the groundwater, which leads to the increase of the moisture content and satu-
The meaning of sop softening effect is that the shearing stress of soil decreases with the increase of the moisture content of soil. Rainfall makes the soil become soft, and changes the mechanical property of soil. Soil above the mine-out area is unsaturated soil with low moisture content. From Fredlund shearing stress formula of unsaturated soil [5],
zf= c ' + ( 0 - u , )
tan 4' + (u, -u,) tan&,
(2)
where o is the positive pressure; u, is the pore air pressure; u, is the pore water pressure; qY is the effective angle of internal friction; 4 is the increment rate of strength. It can be seen that the shearing stress of unsaturated soil is decided by the following three factors: C' is the cohesion, (o-ua)tan4' is the friction strength of
J. Univ. Sci. Technol. Beijing, Vo1.13, No.1, Feb 2006
14
outside force, and (u, - u w ) tan& is the friction strength of matric suction force. Matric suction force can be described by Kelvin Capillary Model, (3)
where T, is the surface tension of capillary water film, N/m; R, is the curvature radius of capillary water film. Along with the increase of the moisture content of soil, the matric suction force decreases continuously. Charles [6] have obtained the characteristic curve (see Fig. 2) through the dehydration and sop contrast indoor test of soil with improved triaxial equipment. From Fig. 2, it can be seen that matric suction force decreases along with the increase of moisture content during both sop process and dehydration process. It can also be seen that matric suction force in sop process is obviously smaller than that in dehydration process with the same moisture content, which is to say, soil is softened rapidly in sop process. 44
Generally speaking, the drier the soil or the less the moisture content or saturation degree, the greater the permeability is and the less the perviousness is. In 1965 [7], Barden [8] had obtained the relationship between seepage coefficient and saturation (see Fig. 3). From Fig. 3, it can be seen that along with the increase of saturation degree of soil its gas permeability declines rapidly. For that reason, rainfall’s seepage makes the soil’s saturation degree and closeness increase, and negative pressure difference of vacuum comes into being easily [9-lo].
1
Saturation I % I
1 10 100 Suction force of soil I kPa
1000
Fig. 2. Relationship between water content and matric suction force of soil [6].
3.3. Negative pressure sealing effect Negative pressure sealing effect is the effect that low pressure easily comes into being in the soil caves when the groundwater table changes because the saturation of soil increases and the ratio between air content and water content decreases which leads to the worsening of the soil’s permeability. As to the unsaturated soil above the mine-out area, the ratio between the water content and the air content has great influence on the soil’s mechanical property especially the soil osmosis which includes gas permeability (k,) and perviousness (k,) because the fillings in the caves are the mixture of water and gas. According to Barden’s viewpoint, both k, and k, are the functions of porosity (n),configuration factor (S,) and saturation (A).
Fig. 3. Relationship between seepage coefficient and saturation [S].
4. Ground subsidence triggered by consolidation settlement The reason for the formation of pumping consolidation is that the soft soil layer bears a pressure which is equal to the gravity of the declined groundwater because of the decline of groundwater table and the pressure makes the soft soil consolidate, which has something to do with the distribution and permeability of soil layer. The pumping consolidation has something to do with the consolidation degree of soil. As to underconsolidated stratum, whether pumping or not, ground settlement will take place. As to normal consolidation stratum, the stress of stratum is in a balance state if there is no pumping, and no ground settlement will take place. However, when pumping is carried out, the original balance of stratum stress is destroyed because of the decline of ground water, which makes the effective stress increase and obvious deformation ap-
Z.H. Ouyang et al., Seepage effects of groundwater and its make-up water on triggering ground subsidence
pears. All of these will lead to the ground settlement, and the more the ground water table declines, the more the settlement takes place. As to overconsolidation stratum, if the additional load triggered by the decline of ground water is not exceeding the prophase consolidation pressure, there is no obvious deformation and no ground settlement. The pumping consolidation has some relation with the saturation degree of soil, too. As to saturated soil, consolidation is mainly triggered by the soil volume’s change after the interstitial water flows away from the soil body. As to the unsaturated soil, because the gas in soil body has great compressibility, consolidation is mainly considered as a generalized problem. That is to say, the coupling problem of the interstitial steam water, air, heat motion and deformation of soil body is mainly considered. Consolidation of soil includes primary consolidation and secondary consolidation. Settlement and deformation of the primary consolidation can be calculated according to the effective stress principle. And the settlement of the secondary consolidation can be calculated by the following formula:
where Cdi is the secondary consolidation coefficient of No. i soil layer; hi is the thickness of No. i soil layer; e,, is the original porosity ratio of No. i soil layer; t is the time of consolidation settlement lasts; tl is the time when the consolidation degree is 100%; n is the layer number of soil.
5. Conclusions (1) The internal course of ground collapse is the character of soil layer. When pumping groundwater, soil is infiltrated and eroded by water and the condition of ground collapse is created. Through the underground mine-out area and the formed cracks consisted in the rock layer above the ore body, soil grain is washed away rapidly. Additionally, the badly radial flow of the earth’s surface makes most atmospheric precipitation filtered into ground in a short time. All of these aroused ground collapse rapidly.
(2) Atmospheric precipitation makes the hydrostatic
15
load of the soil above cave increased. Because of sop softening and hydrodynamic corrasion, the suction and shearing stress of soil are reduced, and together with the vacuum suction pressure, ground collapse may be aroused. (3) Concretion settlement which is aroused by pumping groundwater is another important cause of ground settlement. And it has relation to the distribution, permeability, consolidation and saturation degree of soil layer.
(4)Decline of the groundwater table is only the exterior cause of ground collapse. And the internal causes are the mechanical property change of rock and soil and the coupling effect between water and soil.
References [l] P.B. Attewell and I.W. Farmer, Principles of Engineering Geology, Chapman and Hall, London, 1976. [2] K.J. Larson, H. Basagaolu, and M.A. Maritlo, Prediction of optimal safe ground water yield and land subsidence in the Los Banos-Kettleman City area, California, using a calibrated numerical simulation model, J. Hydrol., 242(2001), p.79. [3] P.R. Sheorey, J.P. Loui, K.B. Singh, et al., Ground subsidence observations and a modified influence function method for complete subsidence prediction, Znt. J Rock Mech. Min. Sci., 37(2000), p.801. [4] E. Rojas, J. Arzate, and M. Arroyo, A method to predict the group fissuring and faulting caused by regional groundwater decline, Eng. Geol., 65(2002), p.245. [5] Frudlund Delwyn G. and Rahardjo Harianto, Soil Mechanics for Unsaturated Soils, China Architecture & Building Press, Beijing, 1997. 161 W.W.Ng Charles and Y.W. Pang, Influence of state on soil-water characteristics and slop stability, J. Geotech. Geoenviron. Eng., 126(2000),No.2, p. 157. [7] S.K. Chaudhari and R.B. Somawanshi, Unsaturated flow of different quality irrigation waters through clay, clay loam and silt loam soils and its dependence on soil and solution parameters, Agric. Water Manage., 64(2004), p.69. [8] W.X. Huang, Engineering Character of Soil, Water Conservancy and Electric Power Press, Beijing, 1983. [9] Dvoralai Wulfsohn, Bankole A. Adams, and Delwyn G. Fredlund, Triaxial testing of unsaturated Agricultural soils, J. Agric. Eng. Res., 69(1998), No.4, p.317. [lo] P. Delage, M.D. Howat, and Y.J. Cui, The relationship between suction and swelling properties in a heavily compacted unsaturated clay, Eng. Geol., 50(1998), p.3 1.
Journal of University of Science and Technology Beijing Volume 13, Number 1, February 2006, Page 11
Seepage effects of groundwater and its make-up water on triggering ground subsidence Zhenhua Ouyang, Meijeng Cai, Changhong Li, and Mowen Xie Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing 100083, China (Received 2004-12-03)
Abstract: The functioning mechanism of groundwater and its make-up water in the process of ground subsidence was studied from such three aspects as osmotic corrasion, osmotic pressure effect and concretion effect. As to osmotic corrasion, its forming conditions, mechanical mechanism and process were analyzed. As to osmotic pressure effect, it was mainly studied from hydrostatic pressurizing effect, sop softening effect and negative pressure sealing effect. Through concretion and saturation of soil, the factors of concretion settlement were analyzed. The results showed that both groundwater and its make-up water are important triggering factors to ground subsidence.
Key words: groundwater; make-up water; ground subsidence; seepage effect
[This work was financially supported by the National Nature Science Foundation of China (N0.50490271). and the National Key Technologies R & D Program of China (N0.2004BA615A-18).]
1. Introduction Beiming River Iron Mine is 8 km away from Wu'an city, Hebei Province of China, and the geographical coordinates of this mine is 114"7'30" in east longitude and 36'45'0" in north latitude. The iron ore is embedding in the riverbed of Beiming River. At the end of February 2003, two ground collapse-pits and many collapse-fissures appeared on the earth surface around the measure-well, which made the well discarded as useless. The bigger one of the two ground collapse-pits is about 20 m long from east to west, and about 15 m wide from south to north. And the biggest collapse-fissure is about 400 mm in width. Now, the ground collapse becomes more and more severe, which menaces the mine production security and destroys the ground surface's environment. The reasons that arouse the ground settlement and collapse of Beiming Rive Iron Mine are not only the underground mining but also the co-function of many factors, in which the groundwater and its make-up water play an important role. There are mainly five kinds of replenishment approaches of groundwater: (1) atmospheric precipitation; (2) the drainage of mill plants on the ground; (3) water leakage of Beiming river; (4) water leakage of agricultural irrigation; (5) Correspondingauthor: Zhenhua Ouyang, E-mail:[email protected]
replenishment of side direction. In Beiming River Iron Mine, both the rock layer above the ore body and the wall rock are Ordovician metaphase calcareous rock which contains a lot of water. The rock body can be classified as dissolved rock cave cracks ore body with complicated conditions on hydrologic geology. Above the ore body is quaternary system loess and gravel abroad, which is about 150 m in depth. Iron ore bodies exist in the interface between the Ordovician metaphase calcareousness rock and Yanshan stage diorite, with a depth of about 134 m to 679 m underground. Most of them are under groundwater table. Pumping water is absolutely necessary before exploiting the ore. When pumping groundwater, dynamical condition of water is altered, and many problems such as osmotic corrasion, osmotic pressure effect, settlement of concretion are triggered, and all of these play important roles in the development of ground subsidence.
2. Ground subsidence triggered by osmotic corrasion 2.1. Conditions of ground subsidence triggered by osmotic corrasion There are two kinds of conditions in which the
12
ground subsidence can be triggered by osmotic corrasion. The physical conditions are the existing of the seepage routes of osmotic corrasion and the overlying soil layer’s property of instability and easy to collapse [l]. The seepage routes are composed of mine-out area, laneways and the formed cracks consisted in the rock layer above the ore body. The dynamical condition is the soil osmotic corrasion triggered by groundwater. The former is the interior reason and the latter is the exterior one. Both of them are necessary to the development of ground collapse. Through the seepage routes, the running groundwater aggravates the osmotic corrasion on the fillings of rock dissolves and the overlying soil layer. And this accelerated the development of ground subsidence especially when large amount of groundwater is pumped out for underground mining.
2.2. Mechanical mechanism of osmotic corrasion Soil is a medium with run-through holes. Exiguous soil grains will be washed away with the scouring of osmotic water because of the dynamic hydraulic pressure of osmotic water when groundwater is pumped out. Dynamic hydraulic pressure is the scouring force of osmotic water on per volume unit of solid grain. The direction of the pressure is the same with the direction of osmotic water, and the computing formula is as follows,
where Gd is the dynamic hydraulic pressure; yw is the specific gravity of water; Z is the water table gradient; Ah is the water head difference; L is the length of osmotic route. There are two kinds of mechanical mechanisms of ground collapse which is triggered by pumping out groundwater. (1) If there are caves in the unconsolidated overlying soil layer or the underlying bedrock that both containing water, dynamic balance is kept between the seepage force of groundwater and the rock geotechnical body in the native state. When pumping out groundwater for underground mining, the groundwater table of karst descends below the unconsolidated formation, and large water table gradient is developed in the two water-bearing strata. With the effect of hydrodynamic pressure, groundwater and grains of overlying soil layer flow into the cavities in underlying bedrock, which trigger ground collapse in the karst area. (2) If the overlying soil layer is waterresisting claypan and there are caves in the underlying bedrock that contains water, the ground water of karst is artesian groundwater in native state, and the pene-
J. Univ. Sci. Technol. Beijing, VoL13,No.1, Feb 2006
trability of groundwater and geotechnical body is in dynamical balance. When pumping out groundwater for underground mining, the artesian groundwater loses its pressure with the decline of the water table, and a space with no pressure is developed between the two layers. All of these make the original effective stress in the overlying unconsolidated claypan increase, which in turn lead to the compression deformation of the claypan. Because of the compression deformation, the overlying claypan sinks towards the caves, which aroused ground collapse in this area [2-41.
2.3. Process of osmotic corrasion As Fig. 1 shows, the process of ground collapse that is triggered by osmotic corrasion is approximately divided into four phases. (1) Karst artesian groundwater seeping into the underlying bedrock, large amount of phreatic water flowing towards the mine-out area or artificial drainage laneway, and the water in the shallow bearing water strata with great permeability also seeping into the underlying bedrock, all of which lead to the groundwater table decline and the hydrodynamic pressure increase. With the alteration of the dynamic condition, the buoyancy on the overlying soil strata declines, and the original balance of groundwater is destroyed, which in turn lead to the increasing of the hydraulic gradient and the water velocity, and the corrasion effect of water is enhanced. So groundwater makes the incompacted fillings and shroud in the interstice alleyway of bedrock endure the effect of suffusion, erosion and undercutting in side direction. (2) With the suffusion and erosion of water, the incompacted fillings in the interstice alleyway of bedrock are washed away. The bottom soil of quaternary system collapses and is washed away, which lead to the formation of arch avalanche and the concealed soil cavities. (3) On one hand, the soil cavities are under the erosion and scouring of karst ground water after their formation, On the other hand, the ground water in overlying stratum as well as water seeping from the surface permeates down into the caves and accumulates, and replenishes the ground water. Then, vertical osmotic erosion is engendered on the overlying stratum. Under the hydrodynamic pressure of osmotic water and the overlying soil layer’s weight, the soil body collapses and is moved away. In turn the concealed soil cavities are enlarged, and ground settlement takes place. When the overlying soil layer’s weight is gradually approach the ultimate compressing or shearing strength of cavities, ground settlement is
Z.H. Ouyang et al., Seepage effects of groundwater and its make-up water on triggering ground subsidence
aggravated. Finally, under the stretching stress, ground cracks appear. (4)If the weight pressure of the overlying soil layer exceeds the ultimate compressing or shearing strength of the soil cavities, ground collapse and cracks will
13
take place. The reason is that not only the shearing stress in vertical direction but also stretching stress in horizontal direction is triggered during the collapse process.
Fig. 1. Formation process of subsidence triggered by osmotic corrasion: (a) groundwater table declined; (b) soil cavities appeared; (c) soil cavities enlarged; (d) ground collapse appeared. l-laneway; M i n e - o u t area; Mimestone; 4-cranny; --roundwater table; 6-quaternary system soil; 7 - t h e earth's surface; &oil cavity; %racks on the ground; lo-collapse pit.
3. Ground subsidence triggered by osmotic pressure effect
ration degree of soil. The other is that when the rainfall accumulates on the ground surface, weight of the water becomes dead load on soil.
Atmospheric precipitation is the main make-up water of groundwater. The mechanical mechanism of ground collapse that is triggered by atmospheric precipitation is osmotic pressure effect, which includes still water pressurizing effect, vertical osmotic effect, intenerate effect after sopping up water and negative pressure sealing effect. Among them vertical osmotic effect is hydrodynamic effect. Still water pressurizing effect and intenerate effect for sop up water are soil mechanical effect, and negative pressure sealing effect is pneumatics effect.
3.2. Sop softening effect
3.1. Still water pressurizing effect Still water pressurizing effect has two meanings. One is the increasing of the natural unit weight of the soil during or after the rainfall because the rain water leaks down and replenishes the groundwater, which leads to the increase of the moisture content and satu-
The meaning of sop softening effect is that the shearing stress of soil decreases with the increase of the moisture content of soil. Rainfall makes the soil become soft, and changes the mechanical property of soil. Soil above the mine-out area is unsaturated soil with low moisture content. From Fredlund shearing stress formula of unsaturated soil [5],
zf= c ' + ( 0 - u , )
tan 4' + (u, -u,) tan&,
(2)
where o is the positive pressure; u, is the pore air pressure; u, is the pore water pressure; qY is the effective angle of internal friction; 4 is the increment rate of strength. It can be seen that the shearing stress of unsaturated soil is decided by the following three factors: C' is the cohesion, (o-ua)tan4' is the friction strength of
J. Univ. Sci. Technol. Beijing, Vo1.13, No.1, Feb 2006
14
outside force, and (u, - u w ) tan& is the friction strength of matric suction force. Matric suction force can be described by Kelvin Capillary Model, (3)
where T, is the surface tension of capillary water film, N/m; R, is the curvature radius of capillary water film. Along with the increase of the moisture content of soil, the matric suction force decreases continuously. Charles [6] have obtained the characteristic curve (see Fig. 2) through the dehydration and sop contrast indoor test of soil with improved triaxial equipment. From Fig. 2, it can be seen that matric suction force decreases along with the increase of moisture content during both sop process and dehydration process. It can also be seen that matric suction force in sop process is obviously smaller than that in dehydration process with the same moisture content, which is to say, soil is softened rapidly in sop process. 44
Generally speaking, the drier the soil or the less the moisture content or saturation degree, the greater the permeability is and the less the perviousness is. In 1965 [7], Barden [8] had obtained the relationship between seepage coefficient and saturation (see Fig. 3). From Fig. 3, it can be seen that along with the increase of saturation degree of soil its gas permeability declines rapidly. For that reason, rainfall’s seepage makes the soil’s saturation degree and closeness increase, and negative pressure difference of vacuum comes into being easily [9-lo].
1
Saturation I % I
1 10 100 Suction force of soil I kPa
1000
Fig. 2. Relationship between water content and matric suction force of soil [6].
3.3. Negative pressure sealing effect Negative pressure sealing effect is the effect that low pressure easily comes into being in the soil caves when the groundwater table changes because the saturation of soil increases and the ratio between air content and water content decreases which leads to the worsening of the soil’s permeability. As to the unsaturated soil above the mine-out area, the ratio between the water content and the air content has great influence on the soil’s mechanical property especially the soil osmosis which includes gas permeability (k,) and perviousness (k,) because the fillings in the caves are the mixture of water and gas. According to Barden’s viewpoint, both k, and k, are the functions of porosity (n),configuration factor (S,) and saturation (A).
Fig. 3. Relationship between seepage coefficient and saturation [S].
4. Ground subsidence triggered by consolidation settlement The reason for the formation of pumping consolidation is that the soft soil layer bears a pressure which is equal to the gravity of the declined groundwater because of the decline of groundwater table and the pressure makes the soft soil consolidate, which has something to do with the distribution and permeability of soil layer. The pumping consolidation has something to do with the consolidation degree of soil. As to underconsolidated stratum, whether pumping or not, ground settlement will take place. As to normal consolidation stratum, the stress of stratum is in a balance state if there is no pumping, and no ground settlement will take place. However, when pumping is carried out, the original balance of stratum stress is destroyed because of the decline of ground water, which makes the effective stress increase and obvious deformation ap-
Z.H. Ouyang et al., Seepage effects of groundwater and its make-up water on triggering ground subsidence
pears. All of these will lead to the ground settlement, and the more the ground water table declines, the more the settlement takes place. As to overconsolidation stratum, if the additional load triggered by the decline of ground water is not exceeding the prophase consolidation pressure, there is no obvious deformation and no ground settlement. The pumping consolidation has some relation with the saturation degree of soil, too. As to saturated soil, consolidation is mainly triggered by the soil volume’s change after the interstitial water flows away from the soil body. As to the unsaturated soil, because the gas in soil body has great compressibility, consolidation is mainly considered as a generalized problem. That is to say, the coupling problem of the interstitial steam water, air, heat motion and deformation of soil body is mainly considered. Consolidation of soil includes primary consolidation and secondary consolidation. Settlement and deformation of the primary consolidation can be calculated according to the effective stress principle. And the settlement of the secondary consolidation can be calculated by the following formula:
where Cdi is the secondary consolidation coefficient of No. i soil layer; hi is the thickness of No. i soil layer; e,, is the original porosity ratio of No. i soil layer; t is the time of consolidation settlement lasts; tl is the time when the consolidation degree is 100%; n is the layer number of soil.
5. Conclusions (1) The internal course of ground collapse is the character of soil layer. When pumping groundwater, soil is infiltrated and eroded by water and the condition of ground collapse is created. Through the underground mine-out area and the formed cracks consisted in the rock layer above the ore body, soil grain is washed away rapidly. Additionally, the badly radial flow of the earth’s surface makes most atmospheric precipitation filtered into ground in a short time. All of these aroused ground collapse rapidly.
(2) Atmospheric precipitation makes the hydrostatic
15
load of the soil above cave increased. Because of sop softening and hydrodynamic corrasion, the suction and shearing stress of soil are reduced, and together with the vacuum suction pressure, ground collapse may be aroused. (3) Concretion settlement which is aroused by pumping groundwater is another important cause of ground settlement. And it has relation to the distribution, permeability, consolidation and saturation degree of soil layer.
(4)Decline of the groundwater table is only the exterior cause of ground collapse. And the internal causes are the mechanical property change of rock and soil and the coupling effect between water and soil.
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