- Email: [email protected]
SW—Soil and Water
J. agric. Engng Res. (2001) 78 (3), 325}332 doi:10.1006/jaer.2000.0611, available online at http://www.idealibrary.com on SW*Soil and Water
Direct Measurement of Soil Deformation using the Bead-grid Method K. Ohtomo; C. C. Andy Tan Department of Agro-environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Obihiro-shi 080-855, Japan; e-mail of corresponding author: [email protected] School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology, 2 George Street, GPO Box 2434 Brisbane Q4001, Australia; e-mail: [email protected] (Received 10 July 1999; accepted in revised form 18 July 2000; published online 23 January 2001)
Soil compaction is the alteration of physical properties of the soil and has signi"cant e!ect on crop production. With the introduction of larger and heavier agriculture machinery it is now a problem throughout the world. Signi"cant knowledge is still required to study and develop a model to predict soil compaction. Numerous theoretical models are now available but these are far from being able to resolve practical problems. A combination of experimental study and theoretical calculations may provide a solution and lead to the development of an e!ective predictive model. In this study, experiments involving bead-grid are used to measure soil deformation under an axial compressive load. Measurement of horizontal and lateral deformations was performed using precision potentiometers and the data transferred to a computer for the plotting of deformation contours and soil deformation analysis. The results show that the bead-grid method provides an e!ective and accurate means for measuring soil deformation. 2001 Silsoe Research Institute
1. Introduction Soil compaction is a major problem a!ecting agricultural soil and can lead to a reduction in crop production. It changes the compactness of the soil and alters the physical and biological properties of the soil. The impact on environmental conservation can lead to soil erosion and prevent surface water in"ltration. With the introduction of larger and heavier agriculture machines, the damage to the soil is enormous. Taylor (1985) reported that as a result of mechanization in agriculture, the problem of soil compaction is universal and is especially severe in developing nations. Research on soil compaction can be traced back as far as the 1950s. Work on soil compaction covers the following areas: determination of pressure distribution in the soil due to wheel load, tyre in#ation pressure and soil condition; the e!ect of compressive pressure on the porosity of di!erent types of soil and moisture contents; determination of the extent of compactness and pressure distribution in the soil under the tyres; and soil deformation and smearing caused by slip in relation to the transmitted force from the tyres. These research areas have 0021-8634/01/030325#08 $35.00/0
been investigated since the early 1960s. Soehne (1958) published a fundamental piece of work on pressure distribution in the soil under tyres. From this work, although based on the work developed by Froehlich (1934), the principles of static and kneading soil compaction were detailed and it was shown that the porosity decreases with the logarithm of the applied pressure. Froehlich showed that compaction and plastic #ow increase with water content and the measurement of porosity under tyres follows the same basic relation with the pressure in the contact area as soil compaction during static or kneading compaction tests. Since the 1970s, there has been a tremendous rise in work on soil compaction. Work on soil compaction depending on initial soil strength (Harris, 1971; Raghavan & Mckyes, 1977; Taylor et al., 1982), caused by total axle load and depth of soil compaction by Taylor and Gill (1984) and the shear stresses on the soil due to relative motion of tyre and soil by Raghavan and McKyes (1977) and by Raghavan et al. (1978). Other research work on soil compaction includes the management concept applied to optimization of traction and mobility in the "elds by Taylor (1989) and the relationship
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between soil structure and strength and the e!ect on crop response by Hakansson et al. (1988). The e!ects of soil compaction on emergence and initial growth of cotton and sugar beet plants were investigated by Gemtos and Lellis (1997). Among other things, the investigation concluded that compaction delay emergence and reduced root growth in both crops. Jorajuria and Draghi (1997) conducted an experimental study to determine the e!ect of soil compaction due to passage of heavy and light tractors on the "eld. Although the results were not conclusive, they reported that on the basis of tra$c intensity, the lighter tractor, with a large number of passes, can do as much or even greater damage than the heavier tractor with fewer passes. Although much research has been carried out in soil compaction, there are still unresolved problems. This is re#ected in the paper published by Schafer et al. (1992) on the future research needs in soil compaction. They listed seven major objectives for future research work which can be broadly classi"ed as development of models which can predict the e!ect of soil compaction; development of standards and speci"cations; and management of soil compaction. It is envisaged that these objectives will now be the thrust of future research. Development of reliable predictive models for soil compaction based on only theoretical calculations has limitations and cannot accurately predict the outcome as there are too many unknowns due to nature that are beyond human control. Wood and Wells (1985) performed an experimental test to characterize soil deformation by the measurement of grid point displacement and converted the measurements to volumetric strain of di!erent soil densities. The grid consisted of "ve parallel lines of marble dust and the corners of each square were de"ned by the position of the grid points. The results on soil bulk density determination were close to those obtained from gamma-ray density gauge readings and con"rmed that the work could supplement density gauge readings. Visualization and characterization of soil deformation of wheel on wet soil in a tank using string-grid can be seen from Ohtomo (1987). The technique provides a simple means of characterizing soil deformation in experimental tests and can also be seen in Basnet et al. (1966) where a cylinder was used to contain the soil. Although the technique is useful in identifying soil deformation boundary points, it does not provide horizontal displacement of soil compaction. In this paper, the bead-grid method was applied to measure the vertical and horizontal soil deformations due to compressive loads. The method allows the deformation in the various layers of the soil under load to be reconstructed in a computer and enables the measurement of horizontal and vertical displacements along the various sections of the cylinder to be stored in the computer for analysis of soil deformation. The measurement
method and data acquisition system have great potential for use in experimental soil compaction tests.
2. Materials and methods The apparatus used in this study consist of a hydraulic press with a loading capacity of about 110 kg cm\ to compact the soil, a steel cylinder capable of splitting into halves, a needle assembly to guide the bead-grid and beads of di!erent colours.
2.1. Steel cylinder The steel cylinder used to contain the soil specimen in the experimental test is shown in Fig. 1. The cylinder has a diameter of 400 mm and a depth of 400 mm and consists of two parts, which can be dismantled into two halves. The split cylinders are assembled together with a long pin and four hinges. Two base plates are welded along the side edges of the split cylinder to provide additional thickness for the location of stopper. The centre of the stopper is located in line with the 5 mm split cylinder hole. Each base plate has 14 in-line stopper and cylinder holes and the centres are arranged so that the centres are in line with the respective holes on the other half of the cylinder. The holes are drilled at intervals of 20 mm and are essential to have the holes perfectly in line as the grid lines have to run parallel to the bottom of the cylinder.
2.2. Needle guide The needle guide assemble used for the placement of bead-grid consists of four parts and can be seen in Fig. 2.
Fig. 1. Schematic diagram of the cylinder: (a) side view of assembled cylinder; (b) sectional view of cylinder; all dimensions in mm
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Fig. 2. Needle guide assembly: (a) xrst-stage needle guide; (b) second-stage needle guide; (c) xnal-stage needle guide for long needle and (d) stopper for starting position of bead-grid; all dimensions in mm
These components are required to guide the drilling operation of a small long hole for the bead-grid through the compacted soil. The "rst- and the second-stage needle guide assemblies have linear guides inside, which are used to guide a pin of 6 mm diameter through the hole. The end of the pin has a 2 mm diameter needle and a length of either 50 or 180 mm, depending on the needle assembly. The "nal stage needle guide assembly has a 2 mm diameter hole drilled through its centre and is used to guide a 550 mm long needle through the compacted soil. The stopper is used to hold the needle assemblies in position and for locating the starting point of the bead-grid.
2.3. Bead-grid Each bead-grid consists of a long nylon string and has 66 blue and 195 yellow beads. A set of three yellow beads is arranged after each blue one, which is used as a spacer.
The mean diameters of the blue and yellow beads are 2)09 and 2)01 mm, respectively. The respective standard deviations (SDs) are 0)02 and 0)01 mm. The average distance between the blue ones is 6)09 mm. The diameter of the beads is slightly larger than the needle diameter to hold the bead-grid in position by means of friction between the bead and the soil. The same bead-grid is used throughout the test to maintain consistency throughout the experiments.
2.4. Building grids The placement of grids was arranged as follows. Firstly, a long straight hole was drilled through the compacted soils using three needle guide assemblies shown in Fig. 2. Then the beads were inserted into the holes until the front blue bead reaches the surface of the stopper. This was then followed by a set of three yellow beads. The
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process was repeated with a blue and three yellow beads until all the beads were installed. The stopper was installed to hold the needle assemblies and was removed when the bead-grid was in position. The "rst grid was located at a depth of 10 mm beneath the soil surface. The accuracy of the bead-grid is determined from the overall measurement of the grid line before and after installation in the compacted soil and worked out to be about 0)027 mm from the original size.
2.5. Soil The soil used in the experiment is classi"ed as sandy clay. Clods of soil were removed with a sieve after the drying process. The clod size is set at less than 2 mm. Before conducting the experiment, the soil was kept in a container and saturated with water. The soil was then left to dry naturally in-door for a period of about 2 weeks depending on the temperature to obtain a condition close to an optimum condition. The optimum soil condition is achieved when the soil maintains a certain density capable of holding the soil together during testing. The soil density for this condition is determined at 1)38 t m\.
3. Experiments and deformation measurement The split steel cylinder shown in Fig. 1 was used to contain the soil specimen compressed to the required bulk density. The amount of soil required in the experiment was determined from the actual soil density and the cylinder was "lled to a "nal soil height of 330 mm. The required soil was "lled in three stages. In each stage the soil was compacted to a depth of 110 mm by means of a circular plate driven by a hydraulic press. After three compactions, the total height of the soil accumulates a depth of 330 mm. The needle guide assemblies were then applied to guide the placement of the horizontal bead-grid across the cylinder. Fourteen rows of bead-grid were used in the experiment with each grid parallel to the base of the cylinder. The stages of experimental test are shown in Fig. 3. Tests were performed using a circular plate of 160 mm diameter and with depths of penetration from 30 to 50 mm in steps of 5 mm. The pressure on the plate was gradually applied to compress the soil to the desired depth of penetration. At the end of each compaction, the cylinder was placed in a horizontal position with the upper half of the split cylinder removed to allow for visualization and deformation measurement of the bead-grid. A layer of the com-
Fig. 3. Flow diagram of experimental procedure: (a) soil compaction process; (b) cylinder placed in horizontal direction for visualization of bead-grids; (c) position measuring system
pacted soil was shaved o! to allow for visualization of the grids and bead centres for measurement of soil deformation. The soil specimen was then transported to the position measuring system. The top view of the position measuring system is shown in Fig. 4. The system consists of a computer with an analogue to digital converter card installed for data acquisition and analysis and a position measurement device capable of translating in the X and > directions. The location of the compacted soil is indicated by dotted lines in Fig. 4. The motions along the X- and >-axis were controlled by a set of precision linear screw guides and were manually operated. The linear guide has an accuracy of 0)037 mm and is determined from the di!erence between the scale value and the measured data. A magnifying eyepiece was installed to visually locate the bead centre. The coordinates of the bead centre was recorded by the potentiometers and transmitted to the computer. The process was then repeated for all the bead centres.
4. Results and discussion The properties of the soil used in the experimental studies are shown in Table 1. The soil had water gravimetric values ranged from 34)1 to 38)1%. The average soil density for the three specimens is 1)38 t m\. Water content in soil plays a very important part in the deformation process. With excessive water content, small pockets of air may be trapped and results in air bubbles. This will lead to soil disintegration from the main body. In this work, the water content is controlled to eliminate this problem, although the soil can be considered wet. On the other hand, if the soil is too dry the soil particle maintain a loose pack con"guration and has no bondage property to keep the soil in the appropriate shape. The pressure distribution over the entire surface of the circular plate is not uniform. However, the manner in which the pressure is distributed within the soil and the
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Fig. 4. The top view of bead centre position measuring device (PMD)
extent of deformation caused depends on the characteristics of the soil and the depth of penetration. The magnitude and the distribution of the pressure exerted on the soil surface are essential for the prediction of stress distribution in the soil. Figures 5 and 6 show the photographs and contour plots of the horizontal and vertical deformations, respectively, of the soil deformation under compressive loads for 30, 40 and 50 mm depths of penetration. It can be seen that the contour plots using the data stored in the computer closely resemble those in the photographs. In Figs 5(a) and 6(a), the soil was compressed to a depth of 30 mm and represents a situation of light compressive load, while as those shown in Figs 5(b) and 6(b) represent a situation of medium loading. A compressive load to drive the circular plate to a depth of penetration of 50 mm represents a situation of heavy load and the results of soil deformation can be seen in Figs 5(c) and 6(c). The horizontal and vertical deformation contours in Fig. 6 resemble pressure distributions in the soil body and show varying shapes along the cross-section underTable 1 Soil properties Depth of penetration, mm 30 40 50
Water content, %
Soil density, t m!3
34)1 38)1 36)5
1)38 1)38 1)38
neath the load. The pressure distributions, arbitrarily plotted on the deformation contours, show that as the depth of penetration increases, the deformation pressure penetrates deeper. For the range of depths of penetration the respective depths of deformation ranged from 170, 210 and 250 mm. These are visible from Figs 6(a), (b) and (c), respectively. The pressure distribution under the plate has a conical shape of varying gradient, which is di!erent from those predicted in theory and shown by Zelinin (1950). The theory predicts that maximum pressure occurs directly under the centre of the load and decreases towards the spreading edge. The theory assumed an in"nite width surface and pressure cone is a straight line drawn from the edge on the compressed surface to a point along the centreline of the surface area. The split cylinder has a "nite circumference and restricts the horizontal motion of the soil hence the deformation contours assumed a parabola shape underneath the load region. Another observation from the horizontal and vertical deformation contours is the maximum slope of the deformed plan around the edge of the plate shown in Fig. 6. Table 2 shows the angles of slope measured from the horizontal axis along the depths of deformation from 30 to 210 mm from the surface of the soil for the range of compressive loads. The angle of slope increases as the depth of penetration increases and the gradient of slope decreases. The vertical deformation along the centreline under the compressive load is shown in Fig. 7. Vertical deformations along the other cross-sections were also recorded. Although not plotted, the results show similar trends with the gradient of the slope varying depending
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Fig. 5. Photographs of soil deformation with a 160 mm circular plate and depths of penetration (a) 30, (b) 40 and (c) 50 mm
on the initial depth of soil penetration. From the "gure, it can be seen that the gradient of the deformation plot of the 30 mm depth of penetration is relatively gentle compared with that of the 50 mm depth of penetration. However, it is interesting to note that regardless of the depth of penetration, they converge to a maximum depth of about 290 mm for a plate size of 160 mm diameter. The results represent the limit of pressure transmission in the soil, which is dependent on the soil characteristics.
Fig. 6. Measured horizontal and vertical deformations in soil using bead-grid due to 160 mm plate: (a) penetrated depth 30 mm; (b) penetrated depth 40 mm; and (c) penetrated depth 50 mm
The horizontal deformation for the three loading conditions measured along the centreline of the applied load can be seen in Fig. 8. It can be seen that there is very little movement in the horizontal direction. However, there is a slight movement around the centre of the load region, about 0)5}0)8 mm. There is hardly any movement at the edge of the cylinder.
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Table 2 Slope of deformed plane (deg) around the edge of the loading plate Depth from soil surface, mm
Depth of penetration, mm 30
40
50
30 50 70 90 110 130 150 170 190 210
47)0 26)0 16)0 10)0 7)0 5)0 2)0 1)5 1)0 0)5
67)5 35)0 24)0 15)0 11)5 9)0 5)0 3)5 2)5 1)5
86)0 69)0 45)0 30)0 20)0 15)0 11)0 8)5 6)5 4)5
The plots of soil compaction density along the centreline of the compressed soil for the range of depths of penetration are shown in Fig. 9. The "gure shows a fairly consistent compact soil density just underneath the surface of the plate for all loading conditions. The initial loading condition does not alter the characteristics of the soil and has a value of about 1)65 t m\. This value is determined from the deformation of the four adjacent beads of the bead-grid which form an approximate square using the same method proposed by Wood and Wells (1985). The results show that as the soil deformation gets deeper the density decreases, with the exception of the fourth layer of the bead-grid, which shows an increase in soil density. The soil density eventually settled down to the original value of about 1)38 t m\ after a depth of deformation of more than 270 mm from the soil surface for all depths of penetration.
Fig. 7. Vertical deformation along the centreline for three penetration depths: , 30 mm; , 40 mm, , 50 mm
Fig. 8. Horizontal deformation along the centreline for three penetration depths: , 30 mm; , 40 mm, , 50 mm
5. Conclusion The experiment simulates a compressive load on soil compaction test using a circular plate. The placement of bead-grid across the cylinder was achieved by using a three-stage bead-grid alignment assembly. The system allows a small hole, 2 mm, be drilled through the compacted soil. The design of a split cylinder allows one-half of the compacted soil to be removed to allow for visual observation and measurement of the vertical and horizontal deformation contours. The introduction of a horizontal and vertical displacements measurement system incorporating a computer with an analogue to digital converter software to acquire and reproduce the displacement data enables e$cient analysis and plotting of soil deformation. The deformation contours resemble pressure distribution in the soil and a cone of parabola surface can be
Fig. 9. Soil density along the centreline for xve penetration depths: , 50 mm; , 45 mm, , 40 mm; , 35 mm; , 30 mm
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reconstructed to represent the pressure distribution in the soil. It was observed that the depth of maximum deformation is independent of initial compressive load. Furthermore, the slope of the deformation plan provides a valuable tool for development of a soil compaction model. The vertical deformation along the centreline due to a compressive load is dependent on the initial depth of penetration of the loading plate. The gradient of the deformation process against the depth of deformation increases with initial depth of penetration of the loading plate. The results show a steep increase in deformation initially and gradually levels o! as the soil depth increases. There is very little horizontal movement except around the centre of the soil depth. The soil density stays fairly constant for all loading conditions after a depth of deformation of more than 270 mm from the soil surface is achieved.
References Basnet B B; Ohtomo K; Tullberg J N; Gupta M L (1966). Visualisation and characterisation of soil deformation in the laboratory using string-grid technique. Proceedings of the Conference on Engineering in Agriculture and Food Processing, Paper No. SEAg96/099 Froehlich O K (1934). Drukverteilung in Braugrunde [Formulae of Boussinesq]. J Springer, Berlin Gemtos T A; Lellis Th (1997). E!ects of soil compaction, water and organic matter contents on emergence and initial plant growth of cotton and sugar beet. Journal of Agricultural Engineering Research, 66, 121}131
Hakansson I; Voorhes W B; Riley H (1988). Vehicle and wheel factor in#uencing soil compact and crop response in di!erent tra$c regime. Soil and Tillage Research, 18, 239}282 Harris W L (1971). The soil compaction process. In: Compaction of Agriculture Soil. ASAE Monograph, pp 9}44 Jorajuria D; Draghi L (1997). The distribution of soil compaction with depth and the response of a perennial forage crop. Journal of Agriculture Engineering Research, 66, 261}265 Ohtomo K (1987). Deformation of soil compaction under wheels*the maximum depth on wet condition. Research Bulletin, Obihiro University, 15(3), 67}76 Raghavan G S V; McKyes E (1977). Laboratory study to determine the e!ect of slip-generated shear on soil compaction. Canadian Agriculture Engineering, 19, 40}42 Raghavan G S V; McKyes E; Beaulieu B (1978). Clay soil compaction due to wheel slip. Transactions of the ASEA, 21(4), 646}649 Schafer R L; Johnson C E; Koolen A J; Gupta S C; Horn R (1992). Future research needs in soil compaction. Transactions of the ASAE, 35(6), 1761}1770 Soehne W (1958). Fundamentals of pressure distribution and soil compaction under traction tires. Agriculture Engineering, 39, 279}281, 290 Taylor J H (1989). Controlled tra$c*a soil compaction management concepts. SAE Transaction, 95(3), 1090}1099 Taylor J H (1985). Soil compaction: &where's the beet?'. In: Soil Compaction (Randall C R, ed). Ohio State University, Columbus, OH Taylor J H; Gill W R (1984). Soil compaction*state of the art report. Journal of Terramechanics, 21(2), 195}213 Taylor J H; Trouse A C; Burt E C; Bailey A C (1982). Multipass behaviour of a pneumatic tire in titled soils. Transactions of the ASAE, 25(5), 1229}1231 Wood R K; Wells L G (1985). Characterizing soil deformation by direct measurement within the pro"le. Transactions of the ASAE, 28(6), 1300}1303 Zelinin A N (1950). Basic Physics of the Theory of Soil Cutting. USSR Academy of Sciences
Direct Measurement of Soil Deformation using the Bead-grid Method K. Ohtomo; C. C. Andy Tan Department of Agro-environmental Science, Obihiro University of Agriculture and Veterinary Medicine, Obihiro-shi 080-855, Japan; e-mail of corresponding author: [email protected] School of Mechanical, Manufacturing and Medical Engineering, Queensland University of Technology, 2 George Street, GPO Box 2434 Brisbane Q4001, Australia; e-mail: [email protected] (Received 10 July 1999; accepted in revised form 18 July 2000; published online 23 January 2001)
Soil compaction is the alteration of physical properties of the soil and has signi"cant e!ect on crop production. With the introduction of larger and heavier agriculture machinery it is now a problem throughout the world. Signi"cant knowledge is still required to study and develop a model to predict soil compaction. Numerous theoretical models are now available but these are far from being able to resolve practical problems. A combination of experimental study and theoretical calculations may provide a solution and lead to the development of an e!ective predictive model. In this study, experiments involving bead-grid are used to measure soil deformation under an axial compressive load. Measurement of horizontal and lateral deformations was performed using precision potentiometers and the data transferred to a computer for the plotting of deformation contours and soil deformation analysis. The results show that the bead-grid method provides an e!ective and accurate means for measuring soil deformation. 2001 Silsoe Research Institute
1. Introduction Soil compaction is a major problem a!ecting agricultural soil and can lead to a reduction in crop production. It changes the compactness of the soil and alters the physical and biological properties of the soil. The impact on environmental conservation can lead to soil erosion and prevent surface water in"ltration. With the introduction of larger and heavier agriculture machines, the damage to the soil is enormous. Taylor (1985) reported that as a result of mechanization in agriculture, the problem of soil compaction is universal and is especially severe in developing nations. Research on soil compaction can be traced back as far as the 1950s. Work on soil compaction covers the following areas: determination of pressure distribution in the soil due to wheel load, tyre in#ation pressure and soil condition; the e!ect of compressive pressure on the porosity of di!erent types of soil and moisture contents; determination of the extent of compactness and pressure distribution in the soil under the tyres; and soil deformation and smearing caused by slip in relation to the transmitted force from the tyres. These research areas have 0021-8634/01/030325#08 $35.00/0
been investigated since the early 1960s. Soehne (1958) published a fundamental piece of work on pressure distribution in the soil under tyres. From this work, although based on the work developed by Froehlich (1934), the principles of static and kneading soil compaction were detailed and it was shown that the porosity decreases with the logarithm of the applied pressure. Froehlich showed that compaction and plastic #ow increase with water content and the measurement of porosity under tyres follows the same basic relation with the pressure in the contact area as soil compaction during static or kneading compaction tests. Since the 1970s, there has been a tremendous rise in work on soil compaction. Work on soil compaction depending on initial soil strength (Harris, 1971; Raghavan & Mckyes, 1977; Taylor et al., 1982), caused by total axle load and depth of soil compaction by Taylor and Gill (1984) and the shear stresses on the soil due to relative motion of tyre and soil by Raghavan and McKyes (1977) and by Raghavan et al. (1978). Other research work on soil compaction includes the management concept applied to optimization of traction and mobility in the "elds by Taylor (1989) and the relationship
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2001 Silsoe Research Institute
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between soil structure and strength and the e!ect on crop response by Hakansson et al. (1988). The e!ects of soil compaction on emergence and initial growth of cotton and sugar beet plants were investigated by Gemtos and Lellis (1997). Among other things, the investigation concluded that compaction delay emergence and reduced root growth in both crops. Jorajuria and Draghi (1997) conducted an experimental study to determine the e!ect of soil compaction due to passage of heavy and light tractors on the "eld. Although the results were not conclusive, they reported that on the basis of tra$c intensity, the lighter tractor, with a large number of passes, can do as much or even greater damage than the heavier tractor with fewer passes. Although much research has been carried out in soil compaction, there are still unresolved problems. This is re#ected in the paper published by Schafer et al. (1992) on the future research needs in soil compaction. They listed seven major objectives for future research work which can be broadly classi"ed as development of models which can predict the e!ect of soil compaction; development of standards and speci"cations; and management of soil compaction. It is envisaged that these objectives will now be the thrust of future research. Development of reliable predictive models for soil compaction based on only theoretical calculations has limitations and cannot accurately predict the outcome as there are too many unknowns due to nature that are beyond human control. Wood and Wells (1985) performed an experimental test to characterize soil deformation by the measurement of grid point displacement and converted the measurements to volumetric strain of di!erent soil densities. The grid consisted of "ve parallel lines of marble dust and the corners of each square were de"ned by the position of the grid points. The results on soil bulk density determination were close to those obtained from gamma-ray density gauge readings and con"rmed that the work could supplement density gauge readings. Visualization and characterization of soil deformation of wheel on wet soil in a tank using string-grid can be seen from Ohtomo (1987). The technique provides a simple means of characterizing soil deformation in experimental tests and can also be seen in Basnet et al. (1966) where a cylinder was used to contain the soil. Although the technique is useful in identifying soil deformation boundary points, it does not provide horizontal displacement of soil compaction. In this paper, the bead-grid method was applied to measure the vertical and horizontal soil deformations due to compressive loads. The method allows the deformation in the various layers of the soil under load to be reconstructed in a computer and enables the measurement of horizontal and vertical displacements along the various sections of the cylinder to be stored in the computer for analysis of soil deformation. The measurement
method and data acquisition system have great potential for use in experimental soil compaction tests.
2. Materials and methods The apparatus used in this study consist of a hydraulic press with a loading capacity of about 110 kg cm\ to compact the soil, a steel cylinder capable of splitting into halves, a needle assembly to guide the bead-grid and beads of di!erent colours.
2.1. Steel cylinder The steel cylinder used to contain the soil specimen in the experimental test is shown in Fig. 1. The cylinder has a diameter of 400 mm and a depth of 400 mm and consists of two parts, which can be dismantled into two halves. The split cylinders are assembled together with a long pin and four hinges. Two base plates are welded along the side edges of the split cylinder to provide additional thickness for the location of stopper. The centre of the stopper is located in line with the 5 mm split cylinder hole. Each base plate has 14 in-line stopper and cylinder holes and the centres are arranged so that the centres are in line with the respective holes on the other half of the cylinder. The holes are drilled at intervals of 20 mm and are essential to have the holes perfectly in line as the grid lines have to run parallel to the bottom of the cylinder.
2.2. Needle guide The needle guide assemble used for the placement of bead-grid consists of four parts and can be seen in Fig. 2.
Fig. 1. Schematic diagram of the cylinder: (a) side view of assembled cylinder; (b) sectional view of cylinder; all dimensions in mm
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Fig. 2. Needle guide assembly: (a) xrst-stage needle guide; (b) second-stage needle guide; (c) xnal-stage needle guide for long needle and (d) stopper for starting position of bead-grid; all dimensions in mm
These components are required to guide the drilling operation of a small long hole for the bead-grid through the compacted soil. The "rst- and the second-stage needle guide assemblies have linear guides inside, which are used to guide a pin of 6 mm diameter through the hole. The end of the pin has a 2 mm diameter needle and a length of either 50 or 180 mm, depending on the needle assembly. The "nal stage needle guide assembly has a 2 mm diameter hole drilled through its centre and is used to guide a 550 mm long needle through the compacted soil. The stopper is used to hold the needle assemblies in position and for locating the starting point of the bead-grid.
2.3. Bead-grid Each bead-grid consists of a long nylon string and has 66 blue and 195 yellow beads. A set of three yellow beads is arranged after each blue one, which is used as a spacer.
The mean diameters of the blue and yellow beads are 2)09 and 2)01 mm, respectively. The respective standard deviations (SDs) are 0)02 and 0)01 mm. The average distance between the blue ones is 6)09 mm. The diameter of the beads is slightly larger than the needle diameter to hold the bead-grid in position by means of friction between the bead and the soil. The same bead-grid is used throughout the test to maintain consistency throughout the experiments.
2.4. Building grids The placement of grids was arranged as follows. Firstly, a long straight hole was drilled through the compacted soils using three needle guide assemblies shown in Fig. 2. Then the beads were inserted into the holes until the front blue bead reaches the surface of the stopper. This was then followed by a set of three yellow beads. The
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process was repeated with a blue and three yellow beads until all the beads were installed. The stopper was installed to hold the needle assemblies and was removed when the bead-grid was in position. The "rst grid was located at a depth of 10 mm beneath the soil surface. The accuracy of the bead-grid is determined from the overall measurement of the grid line before and after installation in the compacted soil and worked out to be about 0)027 mm from the original size.
2.5. Soil The soil used in the experiment is classi"ed as sandy clay. Clods of soil were removed with a sieve after the drying process. The clod size is set at less than 2 mm. Before conducting the experiment, the soil was kept in a container and saturated with water. The soil was then left to dry naturally in-door for a period of about 2 weeks depending on the temperature to obtain a condition close to an optimum condition. The optimum soil condition is achieved when the soil maintains a certain density capable of holding the soil together during testing. The soil density for this condition is determined at 1)38 t m\.
3. Experiments and deformation measurement The split steel cylinder shown in Fig. 1 was used to contain the soil specimen compressed to the required bulk density. The amount of soil required in the experiment was determined from the actual soil density and the cylinder was "lled to a "nal soil height of 330 mm. The required soil was "lled in three stages. In each stage the soil was compacted to a depth of 110 mm by means of a circular plate driven by a hydraulic press. After three compactions, the total height of the soil accumulates a depth of 330 mm. The needle guide assemblies were then applied to guide the placement of the horizontal bead-grid across the cylinder. Fourteen rows of bead-grid were used in the experiment with each grid parallel to the base of the cylinder. The stages of experimental test are shown in Fig. 3. Tests were performed using a circular plate of 160 mm diameter and with depths of penetration from 30 to 50 mm in steps of 5 mm. The pressure on the plate was gradually applied to compress the soil to the desired depth of penetration. At the end of each compaction, the cylinder was placed in a horizontal position with the upper half of the split cylinder removed to allow for visualization and deformation measurement of the bead-grid. A layer of the com-
Fig. 3. Flow diagram of experimental procedure: (a) soil compaction process; (b) cylinder placed in horizontal direction for visualization of bead-grids; (c) position measuring system
pacted soil was shaved o! to allow for visualization of the grids and bead centres for measurement of soil deformation. The soil specimen was then transported to the position measuring system. The top view of the position measuring system is shown in Fig. 4. The system consists of a computer with an analogue to digital converter card installed for data acquisition and analysis and a position measurement device capable of translating in the X and > directions. The location of the compacted soil is indicated by dotted lines in Fig. 4. The motions along the X- and >-axis were controlled by a set of precision linear screw guides and were manually operated. The linear guide has an accuracy of 0)037 mm and is determined from the di!erence between the scale value and the measured data. A magnifying eyepiece was installed to visually locate the bead centre. The coordinates of the bead centre was recorded by the potentiometers and transmitted to the computer. The process was then repeated for all the bead centres.
4. Results and discussion The properties of the soil used in the experimental studies are shown in Table 1. The soil had water gravimetric values ranged from 34)1 to 38)1%. The average soil density for the three specimens is 1)38 t m\. Water content in soil plays a very important part in the deformation process. With excessive water content, small pockets of air may be trapped and results in air bubbles. This will lead to soil disintegration from the main body. In this work, the water content is controlled to eliminate this problem, although the soil can be considered wet. On the other hand, if the soil is too dry the soil particle maintain a loose pack con"guration and has no bondage property to keep the soil in the appropriate shape. The pressure distribution over the entire surface of the circular plate is not uniform. However, the manner in which the pressure is distributed within the soil and the
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Fig. 4. The top view of bead centre position measuring device (PMD)
extent of deformation caused depends on the characteristics of the soil and the depth of penetration. The magnitude and the distribution of the pressure exerted on the soil surface are essential for the prediction of stress distribution in the soil. Figures 5 and 6 show the photographs and contour plots of the horizontal and vertical deformations, respectively, of the soil deformation under compressive loads for 30, 40 and 50 mm depths of penetration. It can be seen that the contour plots using the data stored in the computer closely resemble those in the photographs. In Figs 5(a) and 6(a), the soil was compressed to a depth of 30 mm and represents a situation of light compressive load, while as those shown in Figs 5(b) and 6(b) represent a situation of medium loading. A compressive load to drive the circular plate to a depth of penetration of 50 mm represents a situation of heavy load and the results of soil deformation can be seen in Figs 5(c) and 6(c). The horizontal and vertical deformation contours in Fig. 6 resemble pressure distributions in the soil body and show varying shapes along the cross-section underTable 1 Soil properties Depth of penetration, mm 30 40 50
Water content, %
Soil density, t m!3
34)1 38)1 36)5
1)38 1)38 1)38
neath the load. The pressure distributions, arbitrarily plotted on the deformation contours, show that as the depth of penetration increases, the deformation pressure penetrates deeper. For the range of depths of penetration the respective depths of deformation ranged from 170, 210 and 250 mm. These are visible from Figs 6(a), (b) and (c), respectively. The pressure distribution under the plate has a conical shape of varying gradient, which is di!erent from those predicted in theory and shown by Zelinin (1950). The theory predicts that maximum pressure occurs directly under the centre of the load and decreases towards the spreading edge. The theory assumed an in"nite width surface and pressure cone is a straight line drawn from the edge on the compressed surface to a point along the centreline of the surface area. The split cylinder has a "nite circumference and restricts the horizontal motion of the soil hence the deformation contours assumed a parabola shape underneath the load region. Another observation from the horizontal and vertical deformation contours is the maximum slope of the deformed plan around the edge of the plate shown in Fig. 6. Table 2 shows the angles of slope measured from the horizontal axis along the depths of deformation from 30 to 210 mm from the surface of the soil for the range of compressive loads. The angle of slope increases as the depth of penetration increases and the gradient of slope decreases. The vertical deformation along the centreline under the compressive load is shown in Fig. 7. Vertical deformations along the other cross-sections were also recorded. Although not plotted, the results show similar trends with the gradient of the slope varying depending
330
K . O H TO M O ; C . C. A N D Y T AN
Fig. 5. Photographs of soil deformation with a 160 mm circular plate and depths of penetration (a) 30, (b) 40 and (c) 50 mm
on the initial depth of soil penetration. From the "gure, it can be seen that the gradient of the deformation plot of the 30 mm depth of penetration is relatively gentle compared with that of the 50 mm depth of penetration. However, it is interesting to note that regardless of the depth of penetration, they converge to a maximum depth of about 290 mm for a plate size of 160 mm diameter. The results represent the limit of pressure transmission in the soil, which is dependent on the soil characteristics.
Fig. 6. Measured horizontal and vertical deformations in soil using bead-grid due to 160 mm plate: (a) penetrated depth 30 mm; (b) penetrated depth 40 mm; and (c) penetrated depth 50 mm
The horizontal deformation for the three loading conditions measured along the centreline of the applied load can be seen in Fig. 8. It can be seen that there is very little movement in the horizontal direction. However, there is a slight movement around the centre of the load region, about 0)5}0)8 mm. There is hardly any movement at the edge of the cylinder.
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331
Table 2 Slope of deformed plane (deg) around the edge of the loading plate Depth from soil surface, mm
Depth of penetration, mm 30
40
50
30 50 70 90 110 130 150 170 190 210
47)0 26)0 16)0 10)0 7)0 5)0 2)0 1)5 1)0 0)5
67)5 35)0 24)0 15)0 11)5 9)0 5)0 3)5 2)5 1)5
86)0 69)0 45)0 30)0 20)0 15)0 11)0 8)5 6)5 4)5
The plots of soil compaction density along the centreline of the compressed soil for the range of depths of penetration are shown in Fig. 9. The "gure shows a fairly consistent compact soil density just underneath the surface of the plate for all loading conditions. The initial loading condition does not alter the characteristics of the soil and has a value of about 1)65 t m\. This value is determined from the deformation of the four adjacent beads of the bead-grid which form an approximate square using the same method proposed by Wood and Wells (1985). The results show that as the soil deformation gets deeper the density decreases, with the exception of the fourth layer of the bead-grid, which shows an increase in soil density. The soil density eventually settled down to the original value of about 1)38 t m\ after a depth of deformation of more than 270 mm from the soil surface for all depths of penetration.
Fig. 7. Vertical deformation along the centreline for three penetration depths: , 30 mm; , 40 mm, , 50 mm
Fig. 8. Horizontal deformation along the centreline for three penetration depths: , 30 mm; , 40 mm, , 50 mm
5. Conclusion The experiment simulates a compressive load on soil compaction test using a circular plate. The placement of bead-grid across the cylinder was achieved by using a three-stage bead-grid alignment assembly. The system allows a small hole, 2 mm, be drilled through the compacted soil. The design of a split cylinder allows one-half of the compacted soil to be removed to allow for visual observation and measurement of the vertical and horizontal deformation contours. The introduction of a horizontal and vertical displacements measurement system incorporating a computer with an analogue to digital converter software to acquire and reproduce the displacement data enables e$cient analysis and plotting of soil deformation. The deformation contours resemble pressure distribution in the soil and a cone of parabola surface can be
Fig. 9. Soil density along the centreline for xve penetration depths: , 50 mm; , 45 mm, , 40 mm; , 35 mm; , 30 mm
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K . O H TO M O ; C . C. A N D Y T AN
reconstructed to represent the pressure distribution in the soil. It was observed that the depth of maximum deformation is independent of initial compressive load. Furthermore, the slope of the deformation plan provides a valuable tool for development of a soil compaction model. The vertical deformation along the centreline due to a compressive load is dependent on the initial depth of penetration of the loading plate. The gradient of the deformation process against the depth of deformation increases with initial depth of penetration of the loading plate. The results show a steep increase in deformation initially and gradually levels o! as the soil depth increases. There is very little horizontal movement except around the centre of the soil depth. The soil density stays fairly constant for all loading conditions after a depth of deformation of more than 270 mm from the soil surface is achieved.
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