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Thin-section study of soil materials near perforations in corrugated subsurface drains
Geoderma, 43 (1989) 337-347 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
337
Thin-Section Study of Soil Materials near Perforations in Corrugated Subsurface Drains
JACQUES GALLICHAND,ROBERT LAGACEand MICHEL CAILLIER Ddpartement des Sols, Universit~ Laval, Ste-Foy, Que. GIK 7P4 (Canada) (ReceivedNovember 9, 1987;accepted after revision September 1, 1988)
ABSTRACT
Gallichand, J., Lagac~, R. and Caillier, M., 1989. Thin-section study of soil materials near perforations in corrugated subsurfacedrains. Geoderma, 43: 337-347. Laboratory experiments with drainage simulators were performed using a sample of weakly structured sandy loam and four different types of corrugated subsurface drains. Followingthe experiments, thin sections were made to investigate the soil arrangement near drain perforations. Results show that soil aggregates can form bridges that will stop the flow of sediment into the drain. Bridges formed of individual soil particles were observedat drain openings. For the weakly structured sandy loam studied, the soil material near the drain is not homogeneous.Preferential channels leading to drain openings were observednear 16 of 38 perforations.
INTRODUCTION T h e purpose of the investigation was to study the a r r a n g e m e n t s of soil particles an d aggregates a r o u n d perforations of subsurface drains in order to improve the u n d e r s t a n d i n g of t he sedimentation process. T h e soil sample was a weakly structured sandy loam not expected to form coherent masses. Trials were conducted in the laboratory to per m i t full control of experimental conditions. A r r an g emen ts of soil particles and aggregates near drain openings were studied by means of t hi n sections. Several investigators have used t hi n sections to study changes in soil materials adjacent to or within drains. Pat erson and Mitchell (1978) pr e pa r ed t h i n sections to examine t he stratification of sedim e n ts in tile drains. Sole-Benet (1979) pr e pa red t hi n sections of sediments in a study of the t r a n s p o r t and deposition within drains. Pet erson (1979) used t h i n sections to study materials clogging t he joints of tile drains.
0016-7061/89/$03.50
© 1989 Elsevier Science Publishers B.V.
338 MATERIALS AND METHOD
Laboratory experiments were performed with six drainage simulators to study the effect of four different types of drains on the level of sedimentation. The drainage simulators consisted of five sections: a central section simulating a trench and, on both sides, an intermediate section and a water supply section. The intermediate section was filled with a sandy material and its purpose was to simulate flow conditions in the field. Fig. 1 presents a schematic diagram of a drainage simulator. The soil sample used in the simulators was taken from drain depth ( 1.5 m) in a field where heavy drain sedimentation has been observed. The soil sample, classified as sandy loam according to the Canadian Soil Classification System (Commission Canadienne de Pddologie, 1978), has the following particle-size distribution: 13% of particles smaller than 0.002 mm, 42% between 0.002 m m and 0.05 mm, and 35% between 0.05 m m and 0.1 mm. The four corrugated drain types ( 100 m m diameter) had perforation dimensions of 0.7×5.0 m m (small), 1.1X6.7 mm (medium), 2.0X27.1 m m (large1 ) and 2.1 X 8.1 mm (large-2). The perforations were located on the bottoms of the drain corrugations. To simulate a drain installed by a trenching machine, the soil material below the drain was packed to the approximate natural soil density. Once the drain was installed, soil material with a friable consistency was shovelled into the trench to a depth of about 500 mm above the top o f the drain. Water was sprinkled on a diffusing material until water appeared in the drain. The water WATERSUPPLY SECTION-~
[~,~--700
.... ~-~490--~700--
~ ]
,.T.--.o,.1. /A
J
z 1240
.W
!A ° ALL OIMENSIONS INm m
Fig. 1. Schematic diagram of a drainage simulator.
339 TABLE
I
General information on the experiments and on the thin sections
Experiment Simulator Drain type
Duration of experiment (days )
Sedimentation Thin Number of Position level (mm) section perforations
14
1
Small
142
15
3
Medium
Low (1-5) Low (l-S)
16
5
Larger-2 240
Medium (5-15)
17
6
Medium
10
Very low (traces )
19
7
Large-1
8
High (32-73)
13
8
Small
10
142
Very low (traces )
4 5 1 2 3 7 8 9 10 11 12 13 14 6 15 16
8 3 2 1 2 3 2 2 1 2 3 2 1 2 2 2
THIN S E C T I O N ~--- SAMPLING BOX
J
.~ ~
r
TOP OF DRAIN
I/
CORRUGATED DRAIN
THIN S E C T I O N BOTTOM OF DRAIN
Fig. 2. Schematic representation of a corrugated drain and a sample box.
Top Bottom Top Top Bottom Top Top Bottom Top Top Bottom Top Bottom Bottom Top Bottom
340
level in the water supply tank was then slowly raised. The final water level was such as to produce ponded water conditions in the trench and was kept constant throughout the duration of the experiment by means of a float valve. Table I presents the level of sedimentation obtained for each experiment. After completion of the experiments in the beginning of 1983, the soil materials in the simulators were allowed to dry undisturbed until the beginning of 1985 when samples were taken for examination. This long drying time did not produce any features noticeable in the thin sections. In each simulator, galvanized steel boxes, measuring 250 X 250 mm and open at either end, were used to sample a 150 mm length of drain and the soil material surrounding it. The sizes of the boxes used left 70 mm of soil material outside the drain to be studied. Once sampled, the soil blocks were oven-dried at 70°C for 3 weeks, then impregnated with a blend of polylite, styren and acetone. This method was described by Caillier et al. (1987). After the impregnation, the resin was allowed to harden for a period of 3 months. The blocks were then cut parallel to the drain length. From the cut blocks, thin sections 0.030 mm thick were prepared with the soil material in a section perpendicular to the perforation length. As presented in Table I, 16 thin sections were made for a total of 38 perforatiorls. Fig. 2 shows the positions of the drain, the sampling box, the corrugations and the perforations. RESULTS AND DISCUSSION
Observations of the thin sections The soil material in the vicinity of each of the 38 perforations was studied in order to detect patterns of soil particles and/or aggregates that could develop in drain corrugations. An effort was made to determine the possible effect of sedimentation leveland position of a perforation (top and bottom of drain) on the arrangement of soil material near drain perforations. Out of the thin sections adjacent to the 38 perforations, 24 clearly showed visible aggregates nearby. The general trend was for soil aggregates to occur mainly near drains with low to high sedimentation levels. For drains with very low sedimentation levels, the soil material near the perforation was more like a compact mass. Measurements of perforation widths and aggregate dimensions were made in cases where perforations were surrounded by observable aggregates. These measurements showed that for high sedimentation levels the aggregates were always smaller than the perforation width, whereas for very low sedimentation levels the aggregates were larger than the perforation width. For low to medium sedimentation levels, the aggregates were either larger than the perforation width or smaller with formation of bridges. For 26 of the 38 perforations bridge-type formations were present at drain
341
openings. The bridges observed could be classified into two categories: aggregate-bridges and particle-bridges. Aggregate-bridges were formed by stable soil aggregates larger than or of the same order of magnitude as the drain perforations. These aggregate-bridges were clearly mechanical with the aggregates behaving as stable structural units that could resist the hydraulic force of seeping water. Fig. 3 shows a typical aggregate-bridge formed by four aggregates. Aggregate-bridges were observed at 11 perforations. In four instances, one of which is shown in Fig. 4, the inflow of sediment seems to have been blocked by a single soil aggregate just slightly larger than the drain perforation. Because thin sectLons can show only two dimensions, the perforation may not be completely blocked even though that seems so in the photomicrograph. Water could still flow between aggregates and also between aggregates and the edges of the perforation outside of the field shown in the thin section.
Fig. 3. Bridge formed by several soilaggregates (top of drain; × 14).
342
Fig. 4. Single aggregate blocking a perforation (top of drain; )< 18).
Particle-bridges were observed at 15 perforations. Cohesion between individual soil particles rather than mechanical bridging is believed to provide the resistance to hydraulic forces. Zavlasky and Kassif (1965) showed that mechanical bridging is highly improbable when the hole assumes a magnitude several times that of soil particles. The bridge-like shape is due to an even distribution of the hydraulic gradient over the surface. Fig. 5 shows a particlebridge. Aggregate-bridges were identified mainly for drains with low to high sedimentation levels, whereas particle-bridges were observed for very low to low sedimentation levels. The macroporosity was measured within each corrugation of the drain pipe that contained a perforation. Measurements were made with a planimeter on enlarged projections of the thin sections. In this paper macroporosity refers to the fraction of the drain corrugation not occupied by aggregates or masses of soil material. On the average, the macroporosity at the bottom of the drain {0.35 ) was higher than at the top (0.28). This result is consistent with findings of Lagac~ (1983) in which the hydraulic conductivity in a corrugation was found to be greater at the bottom than at the top of the drain. A higher macroporosity at the bottom of the drain decreases losses in the hydraulic head close to the drain and results in a greater contribution of the drain bottom to the total flow rate. The cases with high macroporosity in the corrugations represent two dis-
343
Fig. 5. Bridge formed of soilparticles(top of drain; × 16).
Fig. 6. Empty corrugation at the bottom of a drain (bottom of drain; × 16).
344 tinct soil arrangements. The first arrangement, illustrated in Fig. 3, shows that the macroporosity is a "structural porosity" having large voids between the aggregates in the drain corrugation. This case can be associated with the development of a natural filter near the perforation. The natural filter also contributes to a decreased loss in hydraulic head around the drain by increasing the permeability of the soil material in the drain corrugations. In the second arrangement, illustrated in Fig. 6 and occurring at the bottoms of the drains, macroporosity is high because the soil mass does not penetrate into the drain corrugations. The drain was initially laid down on a horizontal surface of soil material and in contrast to the top of the drain soil particles or aggregates are not forced into corrugations by gravity. Moreover, the hydraulic gradient at the point where the water leaves the soil material was not large enough to
Fig. 7. Preferential channel near a perforation (top of drain; × 12).
345 overcome the cohesive forces between soil particles or aggregates. This type of soil arrangement also tends to decrease the loss of hydraulic head by increasing the opening area which in turn reduces entrance resistance. Distinctive preferential channels toward the drain opening were present at 16 of 38 perforations. The preferential channels were observed for all drains regardless of the sedimentation level. This implies that for the weakly structured sandy loam used the soil mass around the drain does not have uniform hydraulic conductivity. A great part of the water and sediment may be carried into the drain in preferential channels. Fig. 7 presents a typical example of a preferential channel. Implications on sedimentation of drains From the thin sections examined, some observations can be made about the process of drain sedimentation and its effect on the arrangement of soil particles and/or aggregates at drain openings for the case of weakly structured soil materials. According to established theory (Luthin et al., 1968), the soil material will become unstable and invade the drain if the hydraulic gradient in that material at the drain opening is greater than the critical hydraulic gradient for that soil material. This might be true for a cohesionless homogeneous mass but in the case of a weakly structured soil material, the soil mass near a perforation cannot be considered homogeneous. The soil material in drain corrugations is often composed of aggregates surrounded by large pores. In such cases, the tractive force exerted by the flowing water at the surfaces of the aggregates may play a determining role in the detaching process and the transport of soil particles or aggregates through drain perforations. This aspect is further exemplified by the presence of preferential channels near openings where the flowing water can steadily detach soil particles from the channel walls and thus cause considerable sedimentation in the drain. Aggregates may also be carried in preferential channels. If not enough aggregates arrive at the perforation within the same time interval, bridges will not form and the inflow of sediment will continue unless an aggregate larger than the perforation blocks the opening. The thin sections examined indicate that soil aggregates can form bridges that will resist the hydraulic force of seeping water. When aggregates are smaller than drain openings both aggregates and soil particles will enter a drain unless and until a bridge is formed. This is illustrated in Fig. 8 which shows a chimney made through the sediment inside the drain by the inflowing water. The chimney has been filled with soil aggregates and particles of smaller diameters than the drain opening. Stable soil aggregates can therefore be viewed as basic units controlling sedimentation into drains.
346
Fig. 8. Chimney and soil aggregates inside the drain (bottom of drain; X 9). SUMMARYAND CONCLUSIONS Six drainage simulators were used to study the effect of various corrugated subsurface drains (100 mm in diameter) on sedimentation with use of weakly structured sandy loam. Thin sections were made to study the arrangement of soil particles and aggregates in the vicinities of drain perforations. The following conclusions can be drawn from observations of the thin sections. (1) Stable soil aggregates are clearly identifiable near the perforations and appear to be a determining factor in controlling sedimentation in subsurface drains installed in weakly structured soil material.
347 (2) Stable aggregates smaller than the drain openings can form bridges that will stop the inflow of sediment. (3) Out of the 38 thin sections studied, 16 showed preferential channels. Thus, the soil material around the drain cannot be considered homogeneous and the theory of a critical hydraulic gradient is not applicable to weakly structured soil materials.
REFERENCES Caillier,M., Richard, G., Bourbeau, G. and Blackburn, M., 1987. Technique de fabrication des lames minces de sol.Note Technique, D~partement des Sols,Universitd Laval. Commission Canadienne de P~dologie, 1978. Le Syst~me Canadien de Classificationdes Sols. Publication 1646, Agriculture Canada, 170 pp. Lagac~, R., 1983. Predicting Drain SiltingPotential.Ph.D. Thesis, North Carolina State University,Raleigh, N.C. Luthin, J.N., Taylor, G.S. and Prieto, C., 1968. Exit gradients into subsurface drains. Hilgardia, 39(15): 419-428. Paterson, E. and Mitchell,B.D., 1978. Erosion deposits in tile-drains.Agric.Water Manage., 1 (4): 311-317. Peterson, F.F., 1979. Particlesize,structureand mineralogy of clogged drain openings. In: Factors Influencing Water and ParticlesMovement into Drains. U.S. Department of Agriculture,ARRW-8/June 1979, pp. 1-18. Sole-Benet, M.A., 1979. Contribution ~ l'~tudedu colmatage mineral des drains. M~moire no. 13, Centre Technique du G~nie Rural des Eaux et des For~ts, 251 pp. Zavlasky, D. and Kassif, G., 1965. Theoretical formulation of piping mechanism in cohesive soils. Gdotechnique, 15 (3): 305-316.
337
Thin-Section Study of Soil Materials near Perforations in Corrugated Subsurface Drains
JACQUES GALLICHAND,ROBERT LAGACEand MICHEL CAILLIER Ddpartement des Sols, Universit~ Laval, Ste-Foy, Que. GIK 7P4 (Canada) (ReceivedNovember 9, 1987;accepted after revision September 1, 1988)
ABSTRACT
Gallichand, J., Lagac~, R. and Caillier, M., 1989. Thin-section study of soil materials near perforations in corrugated subsurfacedrains. Geoderma, 43: 337-347. Laboratory experiments with drainage simulators were performed using a sample of weakly structured sandy loam and four different types of corrugated subsurface drains. Followingthe experiments, thin sections were made to investigate the soil arrangement near drain perforations. Results show that soil aggregates can form bridges that will stop the flow of sediment into the drain. Bridges formed of individual soil particles were observedat drain openings. For the weakly structured sandy loam studied, the soil material near the drain is not homogeneous.Preferential channels leading to drain openings were observednear 16 of 38 perforations.
INTRODUCTION T h e purpose of the investigation was to study the a r r a n g e m e n t s of soil particles an d aggregates a r o u n d perforations of subsurface drains in order to improve the u n d e r s t a n d i n g of t he sedimentation process. T h e soil sample was a weakly structured sandy loam not expected to form coherent masses. Trials were conducted in the laboratory to per m i t full control of experimental conditions. A r r an g emen ts of soil particles and aggregates near drain openings were studied by means of t hi n sections. Several investigators have used t hi n sections to study changes in soil materials adjacent to or within drains. Pat erson and Mitchell (1978) pr e pa r ed t h i n sections to examine t he stratification of sedim e n ts in tile drains. Sole-Benet (1979) pr e pa red t hi n sections of sediments in a study of the t r a n s p o r t and deposition within drains. Pet erson (1979) used t h i n sections to study materials clogging t he joints of tile drains.
0016-7061/89/$03.50
© 1989 Elsevier Science Publishers B.V.
338 MATERIALS AND METHOD
Laboratory experiments were performed with six drainage simulators to study the effect of four different types of drains on the level of sedimentation. The drainage simulators consisted of five sections: a central section simulating a trench and, on both sides, an intermediate section and a water supply section. The intermediate section was filled with a sandy material and its purpose was to simulate flow conditions in the field. Fig. 1 presents a schematic diagram of a drainage simulator. The soil sample used in the simulators was taken from drain depth ( 1.5 m) in a field where heavy drain sedimentation has been observed. The soil sample, classified as sandy loam according to the Canadian Soil Classification System (Commission Canadienne de Pddologie, 1978), has the following particle-size distribution: 13% of particles smaller than 0.002 mm, 42% between 0.002 m m and 0.05 mm, and 35% between 0.05 m m and 0.1 mm. The four corrugated drain types ( 100 m m diameter) had perforation dimensions of 0.7×5.0 m m (small), 1.1X6.7 mm (medium), 2.0X27.1 m m (large1 ) and 2.1 X 8.1 mm (large-2). The perforations were located on the bottoms of the drain corrugations. To simulate a drain installed by a trenching machine, the soil material below the drain was packed to the approximate natural soil density. Once the drain was installed, soil material with a friable consistency was shovelled into the trench to a depth of about 500 mm above the top o f the drain. Water was sprinkled on a diffusing material until water appeared in the drain. The water WATERSUPPLY SECTION-~
[~,~--700
.... ~-~490--~700--
~ ]
,.T.--.o,.1. /A
J
z 1240
.W
!A ° ALL OIMENSIONS INm m
Fig. 1. Schematic diagram of a drainage simulator.
339 TABLE
I
General information on the experiments and on the thin sections
Experiment Simulator Drain type
Duration of experiment (days )
Sedimentation Thin Number of Position level (mm) section perforations
14
1
Small
142
15
3
Medium
Low (1-5) Low (l-S)
16
5
Larger-2 240
Medium (5-15)
17
6
Medium
10
Very low (traces )
19
7
Large-1
8
High (32-73)
13
8
Small
10
142
Very low (traces )
4 5 1 2 3 7 8 9 10 11 12 13 14 6 15 16
8 3 2 1 2 3 2 2 1 2 3 2 1 2 2 2
THIN S E C T I O N ~--- SAMPLING BOX
J
.~ ~
r
TOP OF DRAIN
I/
CORRUGATED DRAIN
THIN S E C T I O N BOTTOM OF DRAIN
Fig. 2. Schematic representation of a corrugated drain and a sample box.
Top Bottom Top Top Bottom Top Top Bottom Top Top Bottom Top Bottom Bottom Top Bottom
340
level in the water supply tank was then slowly raised. The final water level was such as to produce ponded water conditions in the trench and was kept constant throughout the duration of the experiment by means of a float valve. Table I presents the level of sedimentation obtained for each experiment. After completion of the experiments in the beginning of 1983, the soil materials in the simulators were allowed to dry undisturbed until the beginning of 1985 when samples were taken for examination. This long drying time did not produce any features noticeable in the thin sections. In each simulator, galvanized steel boxes, measuring 250 X 250 mm and open at either end, were used to sample a 150 mm length of drain and the soil material surrounding it. The sizes of the boxes used left 70 mm of soil material outside the drain to be studied. Once sampled, the soil blocks were oven-dried at 70°C for 3 weeks, then impregnated with a blend of polylite, styren and acetone. This method was described by Caillier et al. (1987). After the impregnation, the resin was allowed to harden for a period of 3 months. The blocks were then cut parallel to the drain length. From the cut blocks, thin sections 0.030 mm thick were prepared with the soil material in a section perpendicular to the perforation length. As presented in Table I, 16 thin sections were made for a total of 38 perforatiorls. Fig. 2 shows the positions of the drain, the sampling box, the corrugations and the perforations. RESULTS AND DISCUSSION
Observations of the thin sections The soil material in the vicinity of each of the 38 perforations was studied in order to detect patterns of soil particles and/or aggregates that could develop in drain corrugations. An effort was made to determine the possible effect of sedimentation leveland position of a perforation (top and bottom of drain) on the arrangement of soil material near drain perforations. Out of the thin sections adjacent to the 38 perforations, 24 clearly showed visible aggregates nearby. The general trend was for soil aggregates to occur mainly near drains with low to high sedimentation levels. For drains with very low sedimentation levels, the soil material near the perforation was more like a compact mass. Measurements of perforation widths and aggregate dimensions were made in cases where perforations were surrounded by observable aggregates. These measurements showed that for high sedimentation levels the aggregates were always smaller than the perforation width, whereas for very low sedimentation levels the aggregates were larger than the perforation width. For low to medium sedimentation levels, the aggregates were either larger than the perforation width or smaller with formation of bridges. For 26 of the 38 perforations bridge-type formations were present at drain
341
openings. The bridges observed could be classified into two categories: aggregate-bridges and particle-bridges. Aggregate-bridges were formed by stable soil aggregates larger than or of the same order of magnitude as the drain perforations. These aggregate-bridges were clearly mechanical with the aggregates behaving as stable structural units that could resist the hydraulic force of seeping water. Fig. 3 shows a typical aggregate-bridge formed by four aggregates. Aggregate-bridges were observed at 11 perforations. In four instances, one of which is shown in Fig. 4, the inflow of sediment seems to have been blocked by a single soil aggregate just slightly larger than the drain perforation. Because thin sectLons can show only two dimensions, the perforation may not be completely blocked even though that seems so in the photomicrograph. Water could still flow between aggregates and also between aggregates and the edges of the perforation outside of the field shown in the thin section.
Fig. 3. Bridge formed by several soilaggregates (top of drain; × 14).
342
Fig. 4. Single aggregate blocking a perforation (top of drain; )< 18).
Particle-bridges were observed at 15 perforations. Cohesion between individual soil particles rather than mechanical bridging is believed to provide the resistance to hydraulic forces. Zavlasky and Kassif (1965) showed that mechanical bridging is highly improbable when the hole assumes a magnitude several times that of soil particles. The bridge-like shape is due to an even distribution of the hydraulic gradient over the surface. Fig. 5 shows a particlebridge. Aggregate-bridges were identified mainly for drains with low to high sedimentation levels, whereas particle-bridges were observed for very low to low sedimentation levels. The macroporosity was measured within each corrugation of the drain pipe that contained a perforation. Measurements were made with a planimeter on enlarged projections of the thin sections. In this paper macroporosity refers to the fraction of the drain corrugation not occupied by aggregates or masses of soil material. On the average, the macroporosity at the bottom of the drain {0.35 ) was higher than at the top (0.28). This result is consistent with findings of Lagac~ (1983) in which the hydraulic conductivity in a corrugation was found to be greater at the bottom than at the top of the drain. A higher macroporosity at the bottom of the drain decreases losses in the hydraulic head close to the drain and results in a greater contribution of the drain bottom to the total flow rate. The cases with high macroporosity in the corrugations represent two dis-
343
Fig. 5. Bridge formed of soilparticles(top of drain; × 16).
Fig. 6. Empty corrugation at the bottom of a drain (bottom of drain; × 16).
344 tinct soil arrangements. The first arrangement, illustrated in Fig. 3, shows that the macroporosity is a "structural porosity" having large voids between the aggregates in the drain corrugation. This case can be associated with the development of a natural filter near the perforation. The natural filter also contributes to a decreased loss in hydraulic head around the drain by increasing the permeability of the soil material in the drain corrugations. In the second arrangement, illustrated in Fig. 6 and occurring at the bottoms of the drains, macroporosity is high because the soil mass does not penetrate into the drain corrugations. The drain was initially laid down on a horizontal surface of soil material and in contrast to the top of the drain soil particles or aggregates are not forced into corrugations by gravity. Moreover, the hydraulic gradient at the point where the water leaves the soil material was not large enough to
Fig. 7. Preferential channel near a perforation (top of drain; × 12).
345 overcome the cohesive forces between soil particles or aggregates. This type of soil arrangement also tends to decrease the loss of hydraulic head by increasing the opening area which in turn reduces entrance resistance. Distinctive preferential channels toward the drain opening were present at 16 of 38 perforations. The preferential channels were observed for all drains regardless of the sedimentation level. This implies that for the weakly structured sandy loam used the soil mass around the drain does not have uniform hydraulic conductivity. A great part of the water and sediment may be carried into the drain in preferential channels. Fig. 7 presents a typical example of a preferential channel. Implications on sedimentation of drains From the thin sections examined, some observations can be made about the process of drain sedimentation and its effect on the arrangement of soil particles and/or aggregates at drain openings for the case of weakly structured soil materials. According to established theory (Luthin et al., 1968), the soil material will become unstable and invade the drain if the hydraulic gradient in that material at the drain opening is greater than the critical hydraulic gradient for that soil material. This might be true for a cohesionless homogeneous mass but in the case of a weakly structured soil material, the soil mass near a perforation cannot be considered homogeneous. The soil material in drain corrugations is often composed of aggregates surrounded by large pores. In such cases, the tractive force exerted by the flowing water at the surfaces of the aggregates may play a determining role in the detaching process and the transport of soil particles or aggregates through drain perforations. This aspect is further exemplified by the presence of preferential channels near openings where the flowing water can steadily detach soil particles from the channel walls and thus cause considerable sedimentation in the drain. Aggregates may also be carried in preferential channels. If not enough aggregates arrive at the perforation within the same time interval, bridges will not form and the inflow of sediment will continue unless an aggregate larger than the perforation blocks the opening. The thin sections examined indicate that soil aggregates can form bridges that will resist the hydraulic force of seeping water. When aggregates are smaller than drain openings both aggregates and soil particles will enter a drain unless and until a bridge is formed. This is illustrated in Fig. 8 which shows a chimney made through the sediment inside the drain by the inflowing water. The chimney has been filled with soil aggregates and particles of smaller diameters than the drain opening. Stable soil aggregates can therefore be viewed as basic units controlling sedimentation into drains.
346
Fig. 8. Chimney and soil aggregates inside the drain (bottom of drain; X 9). SUMMARYAND CONCLUSIONS Six drainage simulators were used to study the effect of various corrugated subsurface drains (100 mm in diameter) on sedimentation with use of weakly structured sandy loam. Thin sections were made to study the arrangement of soil particles and aggregates in the vicinities of drain perforations. The following conclusions can be drawn from observations of the thin sections. (1) Stable soil aggregates are clearly identifiable near the perforations and appear to be a determining factor in controlling sedimentation in subsurface drains installed in weakly structured soil material.
347 (2) Stable aggregates smaller than the drain openings can form bridges that will stop the inflow of sediment. (3) Out of the 38 thin sections studied, 16 showed preferential channels. Thus, the soil material around the drain cannot be considered homogeneous and the theory of a critical hydraulic gradient is not applicable to weakly structured soil materials.
REFERENCES Caillier,M., Richard, G., Bourbeau, G. and Blackburn, M., 1987. Technique de fabrication des lames minces de sol.Note Technique, D~partement des Sols,Universitd Laval. Commission Canadienne de P~dologie, 1978. Le Syst~me Canadien de Classificationdes Sols. Publication 1646, Agriculture Canada, 170 pp. Lagac~, R., 1983. Predicting Drain SiltingPotential.Ph.D. Thesis, North Carolina State University,Raleigh, N.C. Luthin, J.N., Taylor, G.S. and Prieto, C., 1968. Exit gradients into subsurface drains. Hilgardia, 39(15): 419-428. Paterson, E. and Mitchell,B.D., 1978. Erosion deposits in tile-drains.Agric.Water Manage., 1 (4): 311-317. Peterson, F.F., 1979. Particlesize,structureand mineralogy of clogged drain openings. In: Factors Influencing Water and ParticlesMovement into Drains. U.S. Department of Agriculture,ARRW-8/June 1979, pp. 1-18. Sole-Benet, M.A., 1979. Contribution ~ l'~tudedu colmatage mineral des drains. M~moire no. 13, Centre Technique du G~nie Rural des Eaux et des For~ts, 251 pp. Zavlasky, D. and Kassif, G., 1965. Theoretical formulation of piping mechanism in cohesive soils. Gdotechnique, 15 (3): 305-316.