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Improvement of Planosol Solum: Part 7, Mechanical Properties of Soils
J. agric. Engng Res. (1998) 70, 177—183 Article Number: ag980264
Improvement of Planosol Solum: Part 7, Mechanical Properties of Soils H. Jia1; F. Liu1; H. Zhang1; C. Zhang1; K. Araya2; M. Kudoh2; H. Kawabe2 1 Hejiang Agricultural Resesarch Institute, Jiamusi, Heilongjiang, People’s Republic of China; 2 Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan (Received 10 March 1997; accepted in revised form 2 January 1998)
The planosol solum in China is a binary mixture of soil particles where silt forms the frame structure and clay fills the pore spaces. It is extremely hard and impermeable and has particular mechanical properties. This paper deals with the mechanical properties of the three horizons (Ap, Aw and B) of the planosol solum as an aid to understanding the draught requirement of a three-stage subsoil mixing plough for improvement of the planosol solum. Pseudogley soil which is a typical heavy clay soil in Japan was also tested for comparison. Tensile strength, compressive strength, shearing strength, soil-metal friction as static properties and soil brittleness as a dynamic property were determined. The results show that the cohesive strengths of all soils, except the B horizon, had maximums at particular soil water contents. These values were nearly the same as the plastic limits. The B horizon did not have a maximum value in the range of soil water content studied. The cohesion of the pseudogley soil was the smallest and that of the Aw horizon was the largest. The Ap and Aw horizons had the same trend in brittleness; the impact energy required to fracture both soils increased with decreasing soil water content. The required impact energy of the B horizon showed a quite different trend from that of the other soils and was a maximum at 30% d.b. soil water content. When the B horizon was dry, the required impact energy decreased and it became brittle. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b. In this range, the tensile strength of the Aw horizon was the largest, followed by the B horizon, Ap horizon and pseudogley soil. Comparing the cohesion at soil water contents in excess of 20% d.b., the tensile strength of the pseudogley soil was about one-seventh of its cohesion but the tensile strengths of all planosols was about onehalf of their cohesive values. ( 1998 Silsoe Research Institute 0021-8634/98/060177#07 $30.00/0
Notation E f P R ¹ c c@ d g l m p r 1 r 2 w a p b
p
p p c p t q q m / h
impact energy, N m normal (perpendicular) load, N radius of gyration ("0)333 m) torque measured, N m cohesion, Pa adhesion, Pa diameter of soil specimen, m gravitational acceleration ("9)8 m/s) length of soil specimen, m mass of pendulum ("1)99 kg) outer diameter ("0)05 m) inner diameter ("0)03 m) soil water content, % d.b. angle from which pendulum is released, deg angle before pendulum comes to rest, after breaking specimen, deg normal (perpendicular) stress, Pa compressive stress, Pa tensile stress, Pa shearing stress, Pa frictional resistance, Pa angle of soil-internal friction, deg angle of soil-metal friction, deg
1. Introduction Planosol solum is widely distributed in the Shanjiang plain of Heilongjiang province of the People’s Republic of China, near the border with Russia, and has a low crop yield. Figure 1 shows the planosol solum of a cultivated field at farm number 853 in the Hosei district. The first
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particles. As a result, the bulk density of the soil increases. The Aw soil does not have a tridisperse (ternary) but a bidisperse (binary) mixture in which silt mainly forms the frame structure and clay fills the pore spaces. To disturb the bidisperse mixture characteristics of the Aw soil, the Aw and B horizons should be mixed one to one and the clay percentage should be increased. If organic matter is mixed in as well, a lower soil hardness is achieved. This paper deals with the determination of the mechanical properties of planosol solum to estimate the draught force of a special plough, the three-stage subsoil mixing plough5 for improvement of the planosol solum. Pseudogley soil which is a typical heavy clay soil in Japan was also tested for comparison.
2. Experimental details
Fig. 1. Typical plansol solum at farm number 853, Hosei District, China (Each graduation on the scale represents 100 mm)
horizon (Ap) is a humic soil which is suitable for plant growth and has a thickness of about 200 mm. The second horizon (Aw) is a lessivage soil which is dense and impermeable and has a thickness of about 200 mm. The third horizon (B) below about 400 mm depth is diluvial heavy clay.1,2 With the impermeable Aw horizon, plants suffer both from drought and excess water content. The soil hardness of the Aw horizon is more than 5)0 MPa (30° cone angle, 16 mm base diameter), and the roots of plants cannot penetrate the Aw horizon. Zhao et al.3 conducted field cultivation tests with plots of soy beans, corn and beets for three years with soils from the Aw and B horizons mixed in various proportions. The most suitable proportion of B/Aw, where a good yield was obtained, was over the range 1/1 to 0)5/1. Araya4 investigated the structure of planosol solum and found that the particle size distribution of the Aw horizon was such that the soil was very susceptible to soil compaction. Soil compaction is a physical phenomenon, established in powder technology, where small particles fill the pore spaces of frame structures produced by larger
Tensile strength, compressive strength, shearing strength, soil—metal friction as static properties and soil brittleness as a dynamic property were determined. The tensile strength of soils are generally small and a conventional tensile test for steels cannot be used because the specimen has to be gripped by a chuck. A radial compression test6 was conducted whereby a cylindrical soil specimen with the dimensions of diameter d" 29 mm and length l"15 mm was radially compressed as shown in Fig. 2. Kawamoto6 reported that with a normal load, P, distributed over the length of the specimen (lineloading), a tensile stress, p , is produced transverse to the t central line of the specimen of magnitude: 2P p" t ndl
(1)
The compression test was conducted using the same testing machine shown in Fig. 2, but a cylindrical soil specimen with the dimension of diameter d"29 mm and length l"48 mm was arranged vertically and loaded along its axis. The compressive stress, p , is given by c P p" (2) c (d/2)2 n In both tensile and compression tests, the deflection of the specimen in the direction of the load and the amount of load were sensed and recorded on a x—y recorder. The shearing and soil—metal friction tests were conducted by a ring (annular) shear tester.7 A ring with vanes was used for the shearing tests and a flat ring for the soil—metal friction tests. From the measured load and torque, the normal and shearing stresses were calculated. The normal stress is given by P p" n(r2!r2 ) 2 1
(3)
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Fig. 2. Radial compression test
where p is the normal stress, r is the outer diameter of 1 the ring, r is the inner diameter of the ring and P is the 2 normal load. With the torque measured by a torque wrench, the shearing stress or the frictional resistance is given by 3¹ q" (4) 2n(r3!r3 ) 2 1 where q is the shearing stress or frictional resistance and ¹ is the measured torque. Graphs were drawn of q versus f (p) for each soil and each soil water content. The cohesion (c) or adhesion (c@) was determined as the value of q when p"0 and the angle of internal friction (/) or the angle of soil—metal friction (d) was determined from the slope of the plot of q versus f (p). The soil brittleness test was conducted by a Charpy steel shock tester8 which was modified for soil as shown in Fig. 3. The Charpy impact tester was normally used for testing the brittleness of notched steel specimens. The table used for the steel specimens was replaced by one for soil specimens. The soil specimen of diameter 29 mm and length 48 mm which was same as that for the compression test, was placed on the table, and the energy required for breaking by one blow of a pendulum was measured. The energy required for breaking soil is given by E "m gR(cos b !cos a ) (5) f p p p where E is the required impact energy, g is the gravif tational acceleration, m is the mass of the pendulum, R is p the radius of gyration, a is the angle from which the p pendulum is released and b is the angle before it comes p to rest after breaking the specimen. Both angles are measured with respect to the vertical, the first angle clockwise and the second angle anticlockwise. Three to five runs were made for each condition. The specimens for the shearing strength and soil—metal fric-
Fig. 3. Charpy shock tester
tion tests were all made of the disturbed soils which were taken from the different horizons of the planosol solum and pseudogley soil. The soil water content were adjusted to the required value and the soil was compacted in a steel mould. The pseudogley soil specimen was compacted to a hardness of about 0)5 MPa on Yamanaka’s hardness tester scale7 (25° cone angle, 17 mm base diameter) which was the same as soil of the soil bin test. For the planosol, since 1990, the soil hardness of each horizon of the field has been measured by the Yamanaka hardness tester at different soil water contents and, hence, the values of the Yamanaka hardness tester scale of each horizon of the field at a certain soil water content are known. The planosol specimen at a certain soil water content in the laboratory was compacted to give the same as the known value of Yamanaka hardness tester scale of the field soil, against the same water content. The specimens for the tensile, compression and brittleness tests were also made of the same disturbed soils. The water contents of the disturbed soils were first adjusted around the liquid limit and then the specimens were formed by a plastic mould and dried in air. Hence, all
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Table 1 Mechanical composition of soils in this study Planosol solum Soil Depth, mm Coarse sand, % (2—0)2 mm) Fine sand, % (0)2—0)02) Silt, % (0)02—0)002) Clay, % ((0)002) Bulk density, Mg/m3 Void ratio Plastic limit, % d.b. Liquid limit, % d.b. Organic matter, mg/kg Soil texture
Pseudogley
Ap
Aw
B
0—600 20)0 25)0 30)0 25)0 1)20 0)57 24)0 35)0 13)1 SiC*
0—200 5)8 16)4 49)8 28)0 1)06 1)36 27)0 39)0 49)1 SiC
200—400 6)2 15)4 48)8 29)6 1)53 0)64 18)0 32)0 11)7 SiC
400—600 1)3 9)3 28)7 60)7 1)46 0)78 26)0 60)0 12)9 HCs
*SiC is silty clay. sHC is heavy clay.
specimens were different from the original state in the field. Soils in this study were all heavy clay soils, pseudogley soil in Japan, and Ap, Aw and B horizons of planosol solum in China as shown in Table 1. The pseudogley soil is a typical heavy clay soil in Japan and its texture is silty clay. The particle size distributions of the Ap and Aw horizons of the planosol are nearly the same and both textures are silty clay, but the Ap horizon contains much more organic matter than the Aw horizon. The texture of the B horizon is different from the Ap and Aw horizons and is heavy clay.
3. Results and discussion 3.1. Cohesion The value of q when p"0 on the lines of q"f (p), namely, the cohesion, c, is shown in Fig. 4. The cohesion of all soils, except the B horizon, showed a maximum at particular soil water contents. These soil water contents were nearly same as the plastic limits in Table 1 except for the B horizon. The B horizon did not have a maximum value in the range of soil water contents studied. The B horizon was a heavy clay and, hence, in the range less than the plastic limit, the soil strength exceeded the capacity of the ring shear tester and so results could not be obtained. The cohesion of the pseudogley soil in Japan was the smallest and that of the Aw horizon was the largest. The compositions of the Ap and Aw horizons were very similar except for the organic matter content but their cohesions were not same. This is because the Ap
Fig. 4. Cohesion (c) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
horizon has an aggregate structure which is affected by the presence of organic matter and, hence, the actual particle size distribution of the Ap horizon may deviate from that of the bidisperse mixture.4
3.2. Angle of internal friction From the lines of q versus f (p), the slopes of the lines (/), the angle of internal friction, were determined and are shown in Fig. 5. The angles of internal friction of all soils increased as soil water content decreased and hence the shearing strength depends more on the normal stress at lower soil water contents. Soil water content had rather less effect on the / of the Ap horizon.
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Fig. 5. Angle of internal friction (/) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
3.3. Adhesion The value of q when p"0 on the lines of q versus m m f (p), namely, the adhesion (c@), is shown in Fig. 6. The adhesion of the pseudogley soil was about 3 kPa, regardless of soil water content. The adhesion of the Ap horizon was maximum, at about 6 kPa at 28% d.b. soil water content. The commonly occurring soil water content of the Ap horizon in the fields was from 26% (dry year) to 39% d.b. (wet year) and hence, the maximum adhesion with a soil water content of 28% d.b. can occur in the fields. The adhesion of the Aw horizon varied but showed a minimum at 24% d.b. soil water content. The commonly occurring soil water content of the Aw horizon in the fields was from 20% (dry year) to 25% d.b. (wet year) and hence, the maximum value, 10 kPa at 20% d.b., can occur in the fields. The B horizon showed the same trend as the Aw horizon, and the c@ value was a minimum at 30% d.b. soil water content and increased at both lesser and greater soil water contents. The commonly occurring soil water content of the B horizon was from 25% (dry year) to 32% d.b. (wet year) and hence the resistance by adhesion between soil and plough body will be increased in a dry year but not in a wet year. Comparing the adhesions in Fig. 6 with the cohesions in Fig. 4, the adhesions are less than one-fifth of the cohesions and, hence, no soil will adhere and accumulate on the plough bodies.9
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Fig. 6. Adhesion (c@) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
are shown in Fig. 7. The values of h of all soils increasesd with decreasing soil water content and hence friction depends very much on the normal stress at lower soil water content as does the shearing strength. At a soil water content of more than 30% d.b., the values of d decreased and the friction did not depend on the nromal stress.
3.5. Brittleness The impact energy (E ) to cause fracture is shown in f Fig. 8. The value of E of the pseudogley soil was about f 2 N m regardless of soil water content and, hence, did not become more brittle when it became drier. The Ap and Aw horizons had the same trend in E value. The f
3.4. Angle of soil-metal friction From the lines of q versus f (p), the slopes of the lines m (h), the angle of soil-metal friction, were determined and
Fig. 7. Angle of soil-metal friction (h) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
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Fig. 9. A vertical crack produced by radial compression Fig. 8. Fracture energy (E ) as a function of soil water content f (w): K Pseudogley; ) Ap; m Aw and | B
brittleness of both soils decreased at lower soil water content. The E value of the B horizon showed a quite f different trend from that of the other soils with a maximum at 30% d.b. soil water content. When the soil was dry, the E value decreased, the soil becoming more f brittle, and it was easily broken by impact. The E values of all soils decreased at higher soil water f content. That was due to the low soil strength when the soils were wet.
3.6. ¹ensile strength A vertical crack was produced in the specimen by radial compression as shown in Fig. 9. The maximum load, P, was determined and the tensile strength, p , was t calculated by Eqn (1). The results are shown in Fig. 10. The tensile strength of the pseudogley soil was essentially unaffected by soil water content. The tensile strengths of the planosol increased steeply at lower soil water content. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b.. In this range, the tensile strength of the Aw horizon is the largest, followed by the B horizon, Ap horizon and pseudogley soil. Comparing the cohesion c in Fig. 4 at soil water contents of more than 20% d.b., the value of p of the t pseudogley soil is about one-seventh of the value of c but the p values of all planosols are about one-half of c. t
indicating a shear failure, as shown in Fig. 11. The maximum load, P, was used to calculate the compressive strength, p , as in Eqn (2). The results are shown in c Fig. 12. The compressive strengths of all soils increased at lower soil water content; that of the B horizon was the largest.
4. Conclusions 1. The cohesions of all soils except the B horizon became a maximum at particular soil water contents. These soil water contents were nearly the same as the plastic limits. The B horizon did not have a maximum value in the range of soil water contents studied. The cohesion of the pseudogley soil in Japan was the smallest and that of the planosol Aw horizon was the largest.
3.7. Compressive strength In the compression tests, the specimen was always broken with a slanting crack at about 45° to the axis
Fig. 10. Tensile strength (p ) as a function of soil water content t (w): K Pseudogley; ) Ap; m Aw and | B
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Fig. 11. Compressive failure by shearing stress
2. The Ap and Aw horizons showed the same trend in brittleness. The impact energy required to fracture both soils increased at the lower soil water contents and hence the brittleness decreased. The required impact energy of the B horizon showed a quite different trend from that of the other soils with a maximum at 30% d.b. soil water content. When the B horizon was dry, the required impact energy decreased and it became brittle. 3. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b. In this range, the tensile strength of the Aw horizon was the largest, followed by the B horizon, Ap horizon and pseudogley soil. At soil water contents in excess of 20% d.b., the tensile strength of the pseudogley soil was about one-seventh of the cohesion but the tensile strength of all planosols was about one-half of the cohesion.
Fig. 12. Compressive strength (p ) as a function of soil water c content (w): K Pseudogley; ) Ap; m Aw and | B
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References 9 1
Akazawa T Soil and pasture in Shanjiang plain (I). Journal of Hokkaido-Heilongjiang Science Cooperative Institute, 1986, 17, 13—25
Akazawa T Soil and pasture in Shanjiang plain (II). Journal of Hokkaido-Heilongjiang Science Cooperative Institute, 1987, 26, 11—30 Zhao D; Liu F; Jia H Transforming constitution of planosol solum. Journal of Chinese Scientia Agricultura Sinica 1989, 22(5), 47—55 Araya K Influence of particle size distribution in soil compaction of planosol (Bai Jiang Tu) solum. Journal of Environmental Science Laboratory, Senshu University, 1991, 2, 181—192 Araya K; Kudoh M; Zhao D; Liu F; Jia H Improvement of planosol solum: Part 6, Field experiments with a threestage subsoil mixing plough. Journal of Agricultural Engineering Research, 1996, 65, 151—158 Kawamoto T Applied Elasticity, p. 98. Tokyo: Kyoritsu Press Ltd., 1968 The Japanese Society of Soil Mechanics and Foundation Engineering Soil Tests, p. 215. Tokyo: Sanmi Press Ltd., 1992 Japanese Society of Mechanical Engineers Handbook of Mechanical Engineering, Pp. 6—55. Tokyo: Sanmi Press, 1977 Ai H Studies on soil sticking prevention of plow for volcanic ash soil. Bulletin of Faculty of Agriculture, Tokyo University of Agriculture and Technology, 1973, 17, 1—51
Improvement of Planosol Solum: Part 7, Mechanical Properties of Soils H. Jia1; F. Liu1; H. Zhang1; C. Zhang1; K. Araya2; M. Kudoh2; H. Kawabe2 1 Hejiang Agricultural Resesarch Institute, Jiamusi, Heilongjiang, People’s Republic of China; 2 Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan (Received 10 March 1997; accepted in revised form 2 January 1998)
The planosol solum in China is a binary mixture of soil particles where silt forms the frame structure and clay fills the pore spaces. It is extremely hard and impermeable and has particular mechanical properties. This paper deals with the mechanical properties of the three horizons (Ap, Aw and B) of the planosol solum as an aid to understanding the draught requirement of a three-stage subsoil mixing plough for improvement of the planosol solum. Pseudogley soil which is a typical heavy clay soil in Japan was also tested for comparison. Tensile strength, compressive strength, shearing strength, soil-metal friction as static properties and soil brittleness as a dynamic property were determined. The results show that the cohesive strengths of all soils, except the B horizon, had maximums at particular soil water contents. These values were nearly the same as the plastic limits. The B horizon did not have a maximum value in the range of soil water content studied. The cohesion of the pseudogley soil was the smallest and that of the Aw horizon was the largest. The Ap and Aw horizons had the same trend in brittleness; the impact energy required to fracture both soils increased with decreasing soil water content. The required impact energy of the B horizon showed a quite different trend from that of the other soils and was a maximum at 30% d.b. soil water content. When the B horizon was dry, the required impact energy decreased and it became brittle. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b. In this range, the tensile strength of the Aw horizon was the largest, followed by the B horizon, Ap horizon and pseudogley soil. Comparing the cohesion at soil water contents in excess of 20% d.b., the tensile strength of the pseudogley soil was about one-seventh of its cohesion but the tensile strengths of all planosols was about onehalf of their cohesive values. ( 1998 Silsoe Research Institute 0021-8634/98/060177#07 $30.00/0
Notation E f P R ¹ c c@ d g l m p r 1 r 2 w a p b
p
p p c p t q q m / h
impact energy, N m normal (perpendicular) load, N radius of gyration ("0)333 m) torque measured, N m cohesion, Pa adhesion, Pa diameter of soil specimen, m gravitational acceleration ("9)8 m/s) length of soil specimen, m mass of pendulum ("1)99 kg) outer diameter ("0)05 m) inner diameter ("0)03 m) soil water content, % d.b. angle from which pendulum is released, deg angle before pendulum comes to rest, after breaking specimen, deg normal (perpendicular) stress, Pa compressive stress, Pa tensile stress, Pa shearing stress, Pa frictional resistance, Pa angle of soil-internal friction, deg angle of soil-metal friction, deg
1. Introduction Planosol solum is widely distributed in the Shanjiang plain of Heilongjiang province of the People’s Republic of China, near the border with Russia, and has a low crop yield. Figure 1 shows the planosol solum of a cultivated field at farm number 853 in the Hosei district. The first
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particles. As a result, the bulk density of the soil increases. The Aw soil does not have a tridisperse (ternary) but a bidisperse (binary) mixture in which silt mainly forms the frame structure and clay fills the pore spaces. To disturb the bidisperse mixture characteristics of the Aw soil, the Aw and B horizons should be mixed one to one and the clay percentage should be increased. If organic matter is mixed in as well, a lower soil hardness is achieved. This paper deals with the determination of the mechanical properties of planosol solum to estimate the draught force of a special plough, the three-stage subsoil mixing plough5 for improvement of the planosol solum. Pseudogley soil which is a typical heavy clay soil in Japan was also tested for comparison.
2. Experimental details
Fig. 1. Typical plansol solum at farm number 853, Hosei District, China (Each graduation on the scale represents 100 mm)
horizon (Ap) is a humic soil which is suitable for plant growth and has a thickness of about 200 mm. The second horizon (Aw) is a lessivage soil which is dense and impermeable and has a thickness of about 200 mm. The third horizon (B) below about 400 mm depth is diluvial heavy clay.1,2 With the impermeable Aw horizon, plants suffer both from drought and excess water content. The soil hardness of the Aw horizon is more than 5)0 MPa (30° cone angle, 16 mm base diameter), and the roots of plants cannot penetrate the Aw horizon. Zhao et al.3 conducted field cultivation tests with plots of soy beans, corn and beets for three years with soils from the Aw and B horizons mixed in various proportions. The most suitable proportion of B/Aw, where a good yield was obtained, was over the range 1/1 to 0)5/1. Araya4 investigated the structure of planosol solum and found that the particle size distribution of the Aw horizon was such that the soil was very susceptible to soil compaction. Soil compaction is a physical phenomenon, established in powder technology, where small particles fill the pore spaces of frame structures produced by larger
Tensile strength, compressive strength, shearing strength, soil—metal friction as static properties and soil brittleness as a dynamic property were determined. The tensile strength of soils are generally small and a conventional tensile test for steels cannot be used because the specimen has to be gripped by a chuck. A radial compression test6 was conducted whereby a cylindrical soil specimen with the dimensions of diameter d" 29 mm and length l"15 mm was radially compressed as shown in Fig. 2. Kawamoto6 reported that with a normal load, P, distributed over the length of the specimen (lineloading), a tensile stress, p , is produced transverse to the t central line of the specimen of magnitude: 2P p" t ndl
(1)
The compression test was conducted using the same testing machine shown in Fig. 2, but a cylindrical soil specimen with the dimension of diameter d"29 mm and length l"48 mm was arranged vertically and loaded along its axis. The compressive stress, p , is given by c P p" (2) c (d/2)2 n In both tensile and compression tests, the deflection of the specimen in the direction of the load and the amount of load were sensed and recorded on a x—y recorder. The shearing and soil—metal friction tests were conducted by a ring (annular) shear tester.7 A ring with vanes was used for the shearing tests and a flat ring for the soil—metal friction tests. From the measured load and torque, the normal and shearing stresses were calculated. The normal stress is given by P p" n(r2!r2 ) 2 1
(3)
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Fig. 2. Radial compression test
where p is the normal stress, r is the outer diameter of 1 the ring, r is the inner diameter of the ring and P is the 2 normal load. With the torque measured by a torque wrench, the shearing stress or the frictional resistance is given by 3¹ q" (4) 2n(r3!r3 ) 2 1 where q is the shearing stress or frictional resistance and ¹ is the measured torque. Graphs were drawn of q versus f (p) for each soil and each soil water content. The cohesion (c) or adhesion (c@) was determined as the value of q when p"0 and the angle of internal friction (/) or the angle of soil—metal friction (d) was determined from the slope of the plot of q versus f (p). The soil brittleness test was conducted by a Charpy steel shock tester8 which was modified for soil as shown in Fig. 3. The Charpy impact tester was normally used for testing the brittleness of notched steel specimens. The table used for the steel specimens was replaced by one for soil specimens. The soil specimen of diameter 29 mm and length 48 mm which was same as that for the compression test, was placed on the table, and the energy required for breaking by one blow of a pendulum was measured. The energy required for breaking soil is given by E "m gR(cos b !cos a ) (5) f p p p where E is the required impact energy, g is the gravif tational acceleration, m is the mass of the pendulum, R is p the radius of gyration, a is the angle from which the p pendulum is released and b is the angle before it comes p to rest after breaking the specimen. Both angles are measured with respect to the vertical, the first angle clockwise and the second angle anticlockwise. Three to five runs were made for each condition. The specimens for the shearing strength and soil—metal fric-
Fig. 3. Charpy shock tester
tion tests were all made of the disturbed soils which were taken from the different horizons of the planosol solum and pseudogley soil. The soil water content were adjusted to the required value and the soil was compacted in a steel mould. The pseudogley soil specimen was compacted to a hardness of about 0)5 MPa on Yamanaka’s hardness tester scale7 (25° cone angle, 17 mm base diameter) which was the same as soil of the soil bin test. For the planosol, since 1990, the soil hardness of each horizon of the field has been measured by the Yamanaka hardness tester at different soil water contents and, hence, the values of the Yamanaka hardness tester scale of each horizon of the field at a certain soil water content are known. The planosol specimen at a certain soil water content in the laboratory was compacted to give the same as the known value of Yamanaka hardness tester scale of the field soil, against the same water content. The specimens for the tensile, compression and brittleness tests were also made of the same disturbed soils. The water contents of the disturbed soils were first adjusted around the liquid limit and then the specimens were formed by a plastic mould and dried in air. Hence, all
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Table 1 Mechanical composition of soils in this study Planosol solum Soil Depth, mm Coarse sand, % (2—0)2 mm) Fine sand, % (0)2—0)02) Silt, % (0)02—0)002) Clay, % ((0)002) Bulk density, Mg/m3 Void ratio Plastic limit, % d.b. Liquid limit, % d.b. Organic matter, mg/kg Soil texture
Pseudogley
Ap
Aw
B
0—600 20)0 25)0 30)0 25)0 1)20 0)57 24)0 35)0 13)1 SiC*
0—200 5)8 16)4 49)8 28)0 1)06 1)36 27)0 39)0 49)1 SiC
200—400 6)2 15)4 48)8 29)6 1)53 0)64 18)0 32)0 11)7 SiC
400—600 1)3 9)3 28)7 60)7 1)46 0)78 26)0 60)0 12)9 HCs
*SiC is silty clay. sHC is heavy clay.
specimens were different from the original state in the field. Soils in this study were all heavy clay soils, pseudogley soil in Japan, and Ap, Aw and B horizons of planosol solum in China as shown in Table 1. The pseudogley soil is a typical heavy clay soil in Japan and its texture is silty clay. The particle size distributions of the Ap and Aw horizons of the planosol are nearly the same and both textures are silty clay, but the Ap horizon contains much more organic matter than the Aw horizon. The texture of the B horizon is different from the Ap and Aw horizons and is heavy clay.
3. Results and discussion 3.1. Cohesion The value of q when p"0 on the lines of q"f (p), namely, the cohesion, c, is shown in Fig. 4. The cohesion of all soils, except the B horizon, showed a maximum at particular soil water contents. These soil water contents were nearly same as the plastic limits in Table 1 except for the B horizon. The B horizon did not have a maximum value in the range of soil water contents studied. The B horizon was a heavy clay and, hence, in the range less than the plastic limit, the soil strength exceeded the capacity of the ring shear tester and so results could not be obtained. The cohesion of the pseudogley soil in Japan was the smallest and that of the Aw horizon was the largest. The compositions of the Ap and Aw horizons were very similar except for the organic matter content but their cohesions were not same. This is because the Ap
Fig. 4. Cohesion (c) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
horizon has an aggregate structure which is affected by the presence of organic matter and, hence, the actual particle size distribution of the Ap horizon may deviate from that of the bidisperse mixture.4
3.2. Angle of internal friction From the lines of q versus f (p), the slopes of the lines (/), the angle of internal friction, were determined and are shown in Fig. 5. The angles of internal friction of all soils increased as soil water content decreased and hence the shearing strength depends more on the normal stress at lower soil water contents. Soil water content had rather less effect on the / of the Ap horizon.
I M PR O VEM EN T O F P LA NO SO L S O LU M : PA R T 7
Fig. 5. Angle of internal friction (/) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
3.3. Adhesion The value of q when p"0 on the lines of q versus m m f (p), namely, the adhesion (c@), is shown in Fig. 6. The adhesion of the pseudogley soil was about 3 kPa, regardless of soil water content. The adhesion of the Ap horizon was maximum, at about 6 kPa at 28% d.b. soil water content. The commonly occurring soil water content of the Ap horizon in the fields was from 26% (dry year) to 39% d.b. (wet year) and hence, the maximum adhesion with a soil water content of 28% d.b. can occur in the fields. The adhesion of the Aw horizon varied but showed a minimum at 24% d.b. soil water content. The commonly occurring soil water content of the Aw horizon in the fields was from 20% (dry year) to 25% d.b. (wet year) and hence, the maximum value, 10 kPa at 20% d.b., can occur in the fields. The B horizon showed the same trend as the Aw horizon, and the c@ value was a minimum at 30% d.b. soil water content and increased at both lesser and greater soil water contents. The commonly occurring soil water content of the B horizon was from 25% (dry year) to 32% d.b. (wet year) and hence the resistance by adhesion between soil and plough body will be increased in a dry year but not in a wet year. Comparing the adhesions in Fig. 6 with the cohesions in Fig. 4, the adhesions are less than one-fifth of the cohesions and, hence, no soil will adhere and accumulate on the plough bodies.9
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Fig. 6. Adhesion (c@) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
are shown in Fig. 7. The values of h of all soils increasesd with decreasing soil water content and hence friction depends very much on the normal stress at lower soil water content as does the shearing strength. At a soil water content of more than 30% d.b., the values of d decreased and the friction did not depend on the nromal stress.
3.5. Brittleness The impact energy (E ) to cause fracture is shown in f Fig. 8. The value of E of the pseudogley soil was about f 2 N m regardless of soil water content and, hence, did not become more brittle when it became drier. The Ap and Aw horizons had the same trend in E value. The f
3.4. Angle of soil-metal friction From the lines of q versus f (p), the slopes of the lines m (h), the angle of soil-metal friction, were determined and
Fig. 7. Angle of soil-metal friction (h) as a function of soil water content (w): K Pseudogley; ) Ap; m Aw and | B
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H . J IA E¹ A¸ .
Fig. 9. A vertical crack produced by radial compression Fig. 8. Fracture energy (E ) as a function of soil water content f (w): K Pseudogley; ) Ap; m Aw and | B
brittleness of both soils decreased at lower soil water content. The E value of the B horizon showed a quite f different trend from that of the other soils with a maximum at 30% d.b. soil water content. When the soil was dry, the E value decreased, the soil becoming more f brittle, and it was easily broken by impact. The E values of all soils decreased at higher soil water f content. That was due to the low soil strength when the soils were wet.
3.6. ¹ensile strength A vertical crack was produced in the specimen by radial compression as shown in Fig. 9. The maximum load, P, was determined and the tensile strength, p , was t calculated by Eqn (1). The results are shown in Fig. 10. The tensile strength of the pseudogley soil was essentially unaffected by soil water content. The tensile strengths of the planosol increased steeply at lower soil water content. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b.. In this range, the tensile strength of the Aw horizon is the largest, followed by the B horizon, Ap horizon and pseudogley soil. Comparing the cohesion c in Fig. 4 at soil water contents of more than 20% d.b., the value of p of the t pseudogley soil is about one-seventh of the value of c but the p values of all planosols are about one-half of c. t
indicating a shear failure, as shown in Fig. 11. The maximum load, P, was used to calculate the compressive strength, p , as in Eqn (2). The results are shown in c Fig. 12. The compressive strengths of all soils increased at lower soil water content; that of the B horizon was the largest.
4. Conclusions 1. The cohesions of all soils except the B horizon became a maximum at particular soil water contents. These soil water contents were nearly the same as the plastic limits. The B horizon did not have a maximum value in the range of soil water contents studied. The cohesion of the pseudogley soil in Japan was the smallest and that of the planosol Aw horizon was the largest.
3.7. Compressive strength In the compression tests, the specimen was always broken with a slanting crack at about 45° to the axis
Fig. 10. Tensile strength (p ) as a function of soil water content t (w): K Pseudogley; ) Ap; m Aw and | B
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Fig. 11. Compressive failure by shearing stress
2. The Ap and Aw horizons showed the same trend in brittleness. The impact energy required to fracture both soils increased at the lower soil water contents and hence the brittleness decreased. The required impact energy of the B horizon showed a quite different trend from that of the other soils with a maximum at 30% d.b. soil water content. When the B horizon was dry, the required impact energy decreased and it became brittle. 3. The commonly occurring soil water content in the actual planosol fields was more than 20% d.b. In this range, the tensile strength of the Aw horizon was the largest, followed by the B horizon, Ap horizon and pseudogley soil. At soil water contents in excess of 20% d.b., the tensile strength of the pseudogley soil was about one-seventh of the cohesion but the tensile strength of all planosols was about one-half of the cohesion.
Fig. 12. Compressive strength (p ) as a function of soil water c content (w): K Pseudogley; ) Ap; m Aw and | B
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References 9 1
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Akazawa T Soil and pasture in Shanjiang plain (II). Journal of Hokkaido-Heilongjiang Science Cooperative Institute, 1987, 26, 11—30 Zhao D; Liu F; Jia H Transforming constitution of planosol solum. Journal of Chinese Scientia Agricultura Sinica 1989, 22(5), 47—55 Araya K Influence of particle size distribution in soil compaction of planosol (Bai Jiang Tu) solum. Journal of Environmental Science Laboratory, Senshu University, 1991, 2, 181—192 Araya K; Kudoh M; Zhao D; Liu F; Jia H Improvement of planosol solum: Part 6, Field experiments with a threestage subsoil mixing plough. Journal of Agricultural Engineering Research, 1996, 65, 151—158 Kawamoto T Applied Elasticity, p. 98. Tokyo: Kyoritsu Press Ltd., 1968 The Japanese Society of Soil Mechanics and Foundation Engineering Soil Tests, p. 215. Tokyo: Sanmi Press Ltd., 1992 Japanese Society of Mechanical Engineers Handbook of Mechanical Engineering, Pp. 6—55. Tokyo: Sanmi Press, 1977 Ai H Studies on soil sticking prevention of plow for volcanic ash soil. Bulletin of Faculty of Agriculture, Tokyo University of Agriculture and Technology, 1973, 17, 1—51