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Terramechanics research in the Soil Section of the Scottish Institute of Agricultural Engineering
Journal of Terramechanics, Vol. 20. No. 1, pp. 61-67, 1983. Printed in Great Britain.
0022-4898/83 $03.00+0.00 Pergamon Press Ltd. © 1983 ISTVS
TERRAMECHANICS RESEARCH IN THE SOIL SECTION OF THE SCOTTISH INSTJTUTE OF AGRICULTURAL ENGINEERING D. J. CAMPBELL*
Summary--This is a documentary article describing the Institute's work concerning terramechanics. A brief history'of the Soil Section outlines its purpose in relating the characteristics of agricultural machinery, the physical properties of soils and crop growth behaviour. The facilities of the Soil Section are described and its success in developing a highresolution gamma-ray probe to measure soil bulk density and a recording penetrometer is described. Soil compaction measurements have been carried out for alternatives to the conventional tractor wheel and use of a drop-cone to measure soil plastic limit has been developed. Current efforts are focused on performance measurements of multiple-wheel configurations, four-wheel-drive tractors, consideration of soil compaction under modern crawler tracks and research on the effects on crop yield of soil loosening below normal ploughing depth. Development of prediction models for soil compaction is a major interest. HISTORY AND OBJECTIVES THE Soil Section at S I A E was established in 1963 within the A g r i c u l t u r a l D e p a r t m e n t o f the Institute. It was led by Dr. B. D. Soane f r o m its inception until his a p p o i n t m e n t as H e a d o f the A g r i c u l t u r a l D e p a r t m e n t , the a u t h o r being a p p o i n t e d H e a d o f the Soil Section in 1980. The general objective o f the w o r k o f the Section is to measure a n d i n t e r p r e t the effects o f agricultural vehicles a n d i m p l e m e n t s on the physical p r o p e r t i e s o f soils a n d the subsequent effect o f these p r o p e r t i e s on crop growth, quality a n d yield. The i n f o r m a t i o n g a t h e r e d is used b o t h to i m p r o v e the selection a n d m a n a g e m e n t o f existing agricultural machines a n d to develop new machines a n d m o d i f i c a t i o n s to existing machines for b o t h research a n d c o m m e r c i a l purposes. The p r o g r a m m e o f w o r k falls into f o u r separate categories with p a r t i c u l a r emphasis on Scottish soil a n d climatic conditions where a p p r o p r i a t e . The categories are: (1) (2) (3) (4)
The d e v e l o p m e n t o f m e t h o d s for m e a s u r i n g soil physical p r o p e r t i e s in the field a n d in the l a b o r a t o r y . A s t u d y o f the effects o f wheel a n d soil p r o p e r t i e s on soil c o m p a c t i o n including an a t t e m p t to m o d e l the c o m p a c t i o n process. A study o f the effect o f traffic intensity a n d control, including zero traffic, on soil physical p r o p e r t i e s a n d on crop g r o w t h a n d yield. A study o f soil suitability for r e d u c e d a n d zero cultivation systems for cereals.
*Head, Soil Section, Scottish Institute of Agricultural Engineering, Bush Estate, Penicuik, Midlothian EH26 0PH, Scotland. 61
62
D . J . CAMPBELL RESEARCH FACILITIES
From an initial staff of three in 1963, the Section has expanded to its present complement of eleven research and support staff and one post-graduate student. All staff are housed in a building which was purpose-built in 1975 and which includes, in addition to office accommodation, laboratories which allow the routine measurement of a wide range of soil physical properties and a laboratory equipped to handle radioactive gases and solids. A separate building houses a soil tank (bin) which is 15 m long and which is used in the study of soil compaction by full-scale machines. Computing facilities include both a microcomputer and access to a mainframe computer. Other facilities in the Institute include a library, a photographic darkroom, a drawing office and extensive mechanical and electronic engineering workshops. Field experiments are undertaken on both commercial farms and on 35 ha of Institute land. SOME ACHIEVEMENTS Some of the work of the Section, for example that concerned with reduced cultivations for cereals, is perhaps of only peripheral interest to those concerned with terramechanics but a selection of achievements which are of direct interest are described below. Much of the early effort of the Section was devoted to the development of techniques for measuring soil physical properties since the absence of suitable techniques was a major obstacle to progress. A high resolution gamma-ray probe to measure soil bulk density Interest in gamma-ray equipment arose because earlier core sampling techniques gave only a mean bulk density for the depth of the core, and thin layers of high density, which are important in relation to root growth and water infiltration, went undetected. Several versions of the twin probe gamma-ray apparatus have been produced over the years culminating in the current high resolution version [1] which employs a 5 mCil36Cs source, a NaI (T1) detector and a source-detector separation of 220 mm. Only unattenuated gamma-rays are counted and the resultant spatial resolution allows layers of soil as thin as 10 mm to be tested and also allows accurate measurements within 20 mm of the soil surface (Fig. l). In the wide range of compaction and tillage experiments in which the equipment has been employed at SIAE, the most important effects were always found within 100 mm of the soil surface. This is the region in which earlier versions of the equipment, which employed a Geiger-Mfiller detector, were in error with bulk density values close to the soil surface being grossly over-estimated. A high resolution system, the Shire, based on the prototype developed at SIAE is now available commercially from RTD Pantatron Systems Ltd., Fern Street, Motherwell, Strathclyde, ML1 2A J, Scotland. A recording penetrometer to measure soil cone resistance The resistance met by a 30 ° cone of 12.8 mm base diameter as it is forced into the soil is an empirical measurement of soil strength which is widely used despite the dependence of cone resistance on many factors other than the inherent strength characteristics of the soil. Nonetheless, because such measurements can be very easily
TERRAMECHANICS RESEARCH IN THE SOIL SECTION
63
FIG. 1. High resolutiongamma-rayprobe for measuringsoildensitytogetherwith equipment for making access holes and for soil water content sampling. made, they are valuable for measuring changes in soil strength in replicated compaction and tillage experiments where it is known that soil type and certain inherent soil properties will remain largely constant. Hitherto, many penetrometers have been accurate but expensive, bulky and often tractor mounted. Alternatively they have been simple but of doubtful accuracy and have required two operators if the large quantities of data necessitated by soil variability are to be recorded. The Bush recording penetrometer (Fig. 2) developed at SIAE combines the performance and accuracy associated with sophisticated heavy instruments with the lightness and convenience associated with the simpler hand-held versions. Maximum penetration is 500 mm and during each penetration, cone resistance at each of up to 15 depths may be either read on a digital display or the data may be fed to an optional solid-state data collector. Use of the data collector [2] allows at least 800 penetrations per day to be made by a single operator. At the end of the day the data are transferred directly from the collector to the computer in the laboratory. Such a system keeps sources of operator error to a minimum. The Bush penetrometer is available from Findlay Irvine Ltd., Bog Road, Penicuik, Midlothian, Scotland. Alternatives to the conventional tractor wheel
As larger and heavier tractors are increasingly employed in agricultural systems, the risk of soil compaction increases. Consequently, commercially available alternatives
64
D. ,1. CAMPBELL
FIG. 2, "'Bush" recording soil penetrometer in use with electronic data collecl¢~T
to the t r a c t o r wheel which may offer a d v a n t a g e s in terms of c o m p a c t i o n have been e x a m i n e d at S I A E . F o r such alternatives to be effective in m i n i m i s i n g c o m p a c t i o n , the c o n t a c t pressure between wheel a n d soil must be reduced in c o m p a r i s o n with the s t a n d a r d tyre, a l t h o u g h it is i m p o r t a n t to r e m e m b e r that this statement is valid only for a given wheel l o a d a n d that load p e r se is also i m p o r t a n t . I n c r e a s i n g the width o f tyres can reduce the contact pressure and hence reduce the intensity o f c o m p a c t i o n . Such increases can be fairly small as in wide-section traction tyres o r very large as in low g r o u n d pressure tyres, Such tyres can be expensive and, in terms o f soil response, a c h e a p e r alternative m a y be dual c o n v e n t i o n a l tyres or a c o n v e n t i o n a l tyre fitted with a cage wheel. A l t h o u g h these alternatives p r o d u c e a slightly smaller m a x i m u m increase in, for e x a m p l e , soil bulk density a n d a shallower rut, the v o l u m e o f soil c o m p a c t e d a n d the rut cross-sectional area is greater t h a n for a single tyre. Cage wheels i n t e n d e d as a substitute for, rather than an a d d i t i o n to, the c o n v e n t i o n a l t r a c t o r wheel were recently d e v e l o p e d at the N I A E , Silsoe, E n g l a n d and tested for their c o m p a c t i o n characteristics at S I A E . These open, fiat-lugged, steel wheels with lugs at 30 ° to the wheel radius (Fig. 3) were f o u n d to c o m p a c t the soil to an extent which was always very m a r k e d l y less than for a c o n v e n t i o n a l tyre. M o r e o v e r , the steel wheel always p r o d u c e d a shallower rut which h a d in effect been cultivated by the action o f the lugs [3]. C o m m e r c i a l l y p r o d u c e d versions of these e x p e r i m e n t a l
TERRAMECHANICS RESEARCH IN THE SOIL SECTION
65
FIG. 3. Experimentalcage wheel undergoing compaction tests in an indoor soil tank (bin) at SIAE.
wheels which are known as Yield Wheels, are available from Cabcraft Ltd., Four Ashes Industrial Estate, Four Ashes, Wolverhampton, Staffordshire, England. Use o f a drop-cone to measure soil plastic limit The plastic limit of a soil has in the past been related to its ease of management, the number and strength of clods it contains, its particle size distribution, its water release characteristics and its angle of internal friction. Despite this fundamental importance of the plastic limit in all branches of soil science, routine plastic limit determination has remained unchanged from the method developed by Casagrande in 1932 which is empirical, subjective and suffers from poor reproducibility and repeatability. In an attempt to overcome these deficiencies, a new method involving a drop-cone penetrometer was developed at SIAE [4]. Briefly the method consists of allowing a 30 ° cone of mass 80 g to sink freely into the soil at a selection of water contents, the water content corresponding to the minimum of the water content-cone penetration curve being defined as the cone penetrometer plastic limit. This water content is always numerically less than, but correlates closely with, the Casagrande plastic limit, the
66
D.J. CAMPBELL
difference between the two being typically between 3 and 12% w / w and generally increasing with clay content. The new method is not subjective and reproducibility is at least as good as for the Casagrande method even when inexperienced operators carry out the test. Unlike the Casagrande method, there is a cone penetrometer plastic limit for all soils which is more consistent with field experience of soil behaviour. Also, and possibly more importantly, the cone penetrometer plastic limit is more related to the water content at which the soil changes from the brittle to the plastic state and hence to soil behaviour in the field. This was demonstrated by showing that the cone penetrometer plastic limit corresponded to turning points in the variation with water content of soil cohesion, soil-metal friction, soil susceptibility to compaction and implement draught [5].
CURRENT PROBLEMS Preliminary work at SIAE has illustrated the increased importance of a suitable tractor front wheel when rear wheels are fitted which produce a minimum of compaction. In such circumstances, a front tyre which has a poor performance in terms of compaction produced can detract from the improved performance produced by the rear wheel. This illustrates the need to consider the tractor as a whole rather than to treat each wheel in isolation when considering compaction. This point is also important when four-wheel drive is considered. There is now mounting evidence that a four-wheel drive tractor offers advantages not only in terms of increased traction but also of reduced compaction in comparison with an otherwise identical two-wheel drive tractor. The effect, which is thought to be associated with the better weight distribution and enhanced contact area under the front tyres of the four-wheel drive tractor, will shortly be investigated in detail. The crawler tractor is often suggested as a means of avoiding compaction, but results obtained at SIAE suggest that the advantages are similar to those offered by dual conventional tyres. It is possible that the performance of the crawler would be enhanced if less vibration were transmitted to the soil by the tracks. Civil engineers recognise vibration as an accepted means of compacting soil. Further work is required to investigate the compaction produced by crawlers fitted with modern tracks. As the weight of tractors and other agricultural vehicles has increased over the years, so has concern that soil below the depth of cultivation may be compacted and that any such compaction may be difficult or expensive to alleviate by cultivation. A project starting shortly at SIAE will examine the extent to which soil loosening below the normal depth of ploughing influences crop growth and yield in Scottish soil and climatic conditions. Much work has been completed on the compaction produced by alternative wheel systems. However, many remain untested and in order to have information quickly available on both the untested wheels and on new wheels as they become available, an attempt is being made to model the process of soil compaction. Preliminary work showed that, for a limited range of initial soil conditions, satisfactory agreement could be obtained between measured and predicted bulk densities and that the Critical State Theory of soil mechanics could be applied to problems of compaction in unsaturated soils. However, much work remains to be done if compaction by contrasting wheel
TERRAMECHANICS RESEARCH IN THE SOIL SECTION systems on a wide range of soil conditions
67
is t o b e p r e d i c t e d f r o m s o i l a n d w h e e l
properties.
REFERENCES [1] J. K. HENSHALL and D. J. CAMPBELL,The calibration of a high resolution gamma-ray transmission system for measuring soil bulk density and an assessment of its field performance. J. SoilSci. 34 (1983) (in press). [2] M. F. O'SULLIVAN, J. W. DICKSON,J. K. HENSHALLand G. ANDERSON,Electronic collection and storage of data from a hand-held recording soil penetrometer. J. agric. Engng Res. 28 (1983) (in press). [3] J. W. DICKSON~D. J. CAMPBELLand J. K. HENSHALL,An assessment of seedbed compaction by open, flat-lugged, steel, tractor wheels. J. agric. Engng Res. 28, 45-60 (1983). [4] D. J. CAMPBELL,Plastic limit determination using a drop-cone penetrometer. J. Soil Sci. 27, 295-300 (1976). 15] D. J. CAMPBELL,J. V. STAFFORDand P. S. BLACKWELL,The plastic limit, as determined by the drop-cone test, in relation to the mechanical behaviour of soil. J. Soil Sci. 31, 11-24 (1980).
0022-4898/83 $03.00+0.00 Pergamon Press Ltd. © 1983 ISTVS
TERRAMECHANICS RESEARCH IN THE SOIL SECTION OF THE SCOTTISH INSTJTUTE OF AGRICULTURAL ENGINEERING D. J. CAMPBELL*
Summary--This is a documentary article describing the Institute's work concerning terramechanics. A brief history'of the Soil Section outlines its purpose in relating the characteristics of agricultural machinery, the physical properties of soils and crop growth behaviour. The facilities of the Soil Section are described and its success in developing a highresolution gamma-ray probe to measure soil bulk density and a recording penetrometer is described. Soil compaction measurements have been carried out for alternatives to the conventional tractor wheel and use of a drop-cone to measure soil plastic limit has been developed. Current efforts are focused on performance measurements of multiple-wheel configurations, four-wheel-drive tractors, consideration of soil compaction under modern crawler tracks and research on the effects on crop yield of soil loosening below normal ploughing depth. Development of prediction models for soil compaction is a major interest. HISTORY AND OBJECTIVES THE Soil Section at S I A E was established in 1963 within the A g r i c u l t u r a l D e p a r t m e n t o f the Institute. It was led by Dr. B. D. Soane f r o m its inception until his a p p o i n t m e n t as H e a d o f the A g r i c u l t u r a l D e p a r t m e n t , the a u t h o r being a p p o i n t e d H e a d o f the Soil Section in 1980. The general objective o f the w o r k o f the Section is to measure a n d i n t e r p r e t the effects o f agricultural vehicles a n d i m p l e m e n t s on the physical p r o p e r t i e s o f soils a n d the subsequent effect o f these p r o p e r t i e s on crop growth, quality a n d yield. The i n f o r m a t i o n g a t h e r e d is used b o t h to i m p r o v e the selection a n d m a n a g e m e n t o f existing agricultural machines a n d to develop new machines a n d m o d i f i c a t i o n s to existing machines for b o t h research a n d c o m m e r c i a l purposes. The p r o g r a m m e o f w o r k falls into f o u r separate categories with p a r t i c u l a r emphasis on Scottish soil a n d climatic conditions where a p p r o p r i a t e . The categories are: (1) (2) (3) (4)
The d e v e l o p m e n t o f m e t h o d s for m e a s u r i n g soil physical p r o p e r t i e s in the field a n d in the l a b o r a t o r y . A s t u d y o f the effects o f wheel a n d soil p r o p e r t i e s on soil c o m p a c t i o n including an a t t e m p t to m o d e l the c o m p a c t i o n process. A study o f the effect o f traffic intensity a n d control, including zero traffic, on soil physical p r o p e r t i e s a n d on crop g r o w t h a n d yield. A study o f soil suitability for r e d u c e d a n d zero cultivation systems for cereals.
*Head, Soil Section, Scottish Institute of Agricultural Engineering, Bush Estate, Penicuik, Midlothian EH26 0PH, Scotland. 61
62
D . J . CAMPBELL RESEARCH FACILITIES
From an initial staff of three in 1963, the Section has expanded to its present complement of eleven research and support staff and one post-graduate student. All staff are housed in a building which was purpose-built in 1975 and which includes, in addition to office accommodation, laboratories which allow the routine measurement of a wide range of soil physical properties and a laboratory equipped to handle radioactive gases and solids. A separate building houses a soil tank (bin) which is 15 m long and which is used in the study of soil compaction by full-scale machines. Computing facilities include both a microcomputer and access to a mainframe computer. Other facilities in the Institute include a library, a photographic darkroom, a drawing office and extensive mechanical and electronic engineering workshops. Field experiments are undertaken on both commercial farms and on 35 ha of Institute land. SOME ACHIEVEMENTS Some of the work of the Section, for example that concerned with reduced cultivations for cereals, is perhaps of only peripheral interest to those concerned with terramechanics but a selection of achievements which are of direct interest are described below. Much of the early effort of the Section was devoted to the development of techniques for measuring soil physical properties since the absence of suitable techniques was a major obstacle to progress. A high resolution gamma-ray probe to measure soil bulk density Interest in gamma-ray equipment arose because earlier core sampling techniques gave only a mean bulk density for the depth of the core, and thin layers of high density, which are important in relation to root growth and water infiltration, went undetected. Several versions of the twin probe gamma-ray apparatus have been produced over the years culminating in the current high resolution version [1] which employs a 5 mCil36Cs source, a NaI (T1) detector and a source-detector separation of 220 mm. Only unattenuated gamma-rays are counted and the resultant spatial resolution allows layers of soil as thin as 10 mm to be tested and also allows accurate measurements within 20 mm of the soil surface (Fig. l). In the wide range of compaction and tillage experiments in which the equipment has been employed at SIAE, the most important effects were always found within 100 mm of the soil surface. This is the region in which earlier versions of the equipment, which employed a Geiger-Mfiller detector, were in error with bulk density values close to the soil surface being grossly over-estimated. A high resolution system, the Shire, based on the prototype developed at SIAE is now available commercially from RTD Pantatron Systems Ltd., Fern Street, Motherwell, Strathclyde, ML1 2A J, Scotland. A recording penetrometer to measure soil cone resistance The resistance met by a 30 ° cone of 12.8 mm base diameter as it is forced into the soil is an empirical measurement of soil strength which is widely used despite the dependence of cone resistance on many factors other than the inherent strength characteristics of the soil. Nonetheless, because such measurements can be very easily
TERRAMECHANICS RESEARCH IN THE SOIL SECTION
63
FIG. 1. High resolutiongamma-rayprobe for measuringsoildensitytogetherwith equipment for making access holes and for soil water content sampling. made, they are valuable for measuring changes in soil strength in replicated compaction and tillage experiments where it is known that soil type and certain inherent soil properties will remain largely constant. Hitherto, many penetrometers have been accurate but expensive, bulky and often tractor mounted. Alternatively they have been simple but of doubtful accuracy and have required two operators if the large quantities of data necessitated by soil variability are to be recorded. The Bush recording penetrometer (Fig. 2) developed at SIAE combines the performance and accuracy associated with sophisticated heavy instruments with the lightness and convenience associated with the simpler hand-held versions. Maximum penetration is 500 mm and during each penetration, cone resistance at each of up to 15 depths may be either read on a digital display or the data may be fed to an optional solid-state data collector. Use of the data collector [2] allows at least 800 penetrations per day to be made by a single operator. At the end of the day the data are transferred directly from the collector to the computer in the laboratory. Such a system keeps sources of operator error to a minimum. The Bush penetrometer is available from Findlay Irvine Ltd., Bog Road, Penicuik, Midlothian, Scotland. Alternatives to the conventional tractor wheel
As larger and heavier tractors are increasingly employed in agricultural systems, the risk of soil compaction increases. Consequently, commercially available alternatives
64
D. ,1. CAMPBELL
FIG. 2, "'Bush" recording soil penetrometer in use with electronic data collecl¢~T
to the t r a c t o r wheel which may offer a d v a n t a g e s in terms of c o m p a c t i o n have been e x a m i n e d at S I A E . F o r such alternatives to be effective in m i n i m i s i n g c o m p a c t i o n , the c o n t a c t pressure between wheel a n d soil must be reduced in c o m p a r i s o n with the s t a n d a r d tyre, a l t h o u g h it is i m p o r t a n t to r e m e m b e r that this statement is valid only for a given wheel l o a d a n d that load p e r se is also i m p o r t a n t . I n c r e a s i n g the width o f tyres can reduce the contact pressure and hence reduce the intensity o f c o m p a c t i o n . Such increases can be fairly small as in wide-section traction tyres o r very large as in low g r o u n d pressure tyres, Such tyres can be expensive and, in terms o f soil response, a c h e a p e r alternative m a y be dual c o n v e n t i o n a l tyres or a c o n v e n t i o n a l tyre fitted with a cage wheel. A l t h o u g h these alternatives p r o d u c e a slightly smaller m a x i m u m increase in, for e x a m p l e , soil bulk density a n d a shallower rut, the v o l u m e o f soil c o m p a c t e d a n d the rut cross-sectional area is greater t h a n for a single tyre. Cage wheels i n t e n d e d as a substitute for, rather than an a d d i t i o n to, the c o n v e n t i o n a l t r a c t o r wheel were recently d e v e l o p e d at the N I A E , Silsoe, E n g l a n d and tested for their c o m p a c t i o n characteristics at S I A E . These open, fiat-lugged, steel wheels with lugs at 30 ° to the wheel radius (Fig. 3) were f o u n d to c o m p a c t the soil to an extent which was always very m a r k e d l y less than for a c o n v e n t i o n a l tyre. M o r e o v e r , the steel wheel always p r o d u c e d a shallower rut which h a d in effect been cultivated by the action o f the lugs [3]. C o m m e r c i a l l y p r o d u c e d versions of these e x p e r i m e n t a l
TERRAMECHANICS RESEARCH IN THE SOIL SECTION
65
FIG. 3. Experimentalcage wheel undergoing compaction tests in an indoor soil tank (bin) at SIAE.
wheels which are known as Yield Wheels, are available from Cabcraft Ltd., Four Ashes Industrial Estate, Four Ashes, Wolverhampton, Staffordshire, England. Use o f a drop-cone to measure soil plastic limit The plastic limit of a soil has in the past been related to its ease of management, the number and strength of clods it contains, its particle size distribution, its water release characteristics and its angle of internal friction. Despite this fundamental importance of the plastic limit in all branches of soil science, routine plastic limit determination has remained unchanged from the method developed by Casagrande in 1932 which is empirical, subjective and suffers from poor reproducibility and repeatability. In an attempt to overcome these deficiencies, a new method involving a drop-cone penetrometer was developed at SIAE [4]. Briefly the method consists of allowing a 30 ° cone of mass 80 g to sink freely into the soil at a selection of water contents, the water content corresponding to the minimum of the water content-cone penetration curve being defined as the cone penetrometer plastic limit. This water content is always numerically less than, but correlates closely with, the Casagrande plastic limit, the
66
D.J. CAMPBELL
difference between the two being typically between 3 and 12% w / w and generally increasing with clay content. The new method is not subjective and reproducibility is at least as good as for the Casagrande method even when inexperienced operators carry out the test. Unlike the Casagrande method, there is a cone penetrometer plastic limit for all soils which is more consistent with field experience of soil behaviour. Also, and possibly more importantly, the cone penetrometer plastic limit is more related to the water content at which the soil changes from the brittle to the plastic state and hence to soil behaviour in the field. This was demonstrated by showing that the cone penetrometer plastic limit corresponded to turning points in the variation with water content of soil cohesion, soil-metal friction, soil susceptibility to compaction and implement draught [5].
CURRENT PROBLEMS Preliminary work at SIAE has illustrated the increased importance of a suitable tractor front wheel when rear wheels are fitted which produce a minimum of compaction. In such circumstances, a front tyre which has a poor performance in terms of compaction produced can detract from the improved performance produced by the rear wheel. This illustrates the need to consider the tractor as a whole rather than to treat each wheel in isolation when considering compaction. This point is also important when four-wheel drive is considered. There is now mounting evidence that a four-wheel drive tractor offers advantages not only in terms of increased traction but also of reduced compaction in comparison with an otherwise identical two-wheel drive tractor. The effect, which is thought to be associated with the better weight distribution and enhanced contact area under the front tyres of the four-wheel drive tractor, will shortly be investigated in detail. The crawler tractor is often suggested as a means of avoiding compaction, but results obtained at SIAE suggest that the advantages are similar to those offered by dual conventional tyres. It is possible that the performance of the crawler would be enhanced if less vibration were transmitted to the soil by the tracks. Civil engineers recognise vibration as an accepted means of compacting soil. Further work is required to investigate the compaction produced by crawlers fitted with modern tracks. As the weight of tractors and other agricultural vehicles has increased over the years, so has concern that soil below the depth of cultivation may be compacted and that any such compaction may be difficult or expensive to alleviate by cultivation. A project starting shortly at SIAE will examine the extent to which soil loosening below the normal depth of ploughing influences crop growth and yield in Scottish soil and climatic conditions. Much work has been completed on the compaction produced by alternative wheel systems. However, many remain untested and in order to have information quickly available on both the untested wheels and on new wheels as they become available, an attempt is being made to model the process of soil compaction. Preliminary work showed that, for a limited range of initial soil conditions, satisfactory agreement could be obtained between measured and predicted bulk densities and that the Critical State Theory of soil mechanics could be applied to problems of compaction in unsaturated soils. However, much work remains to be done if compaction by contrasting wheel
TERRAMECHANICS RESEARCH IN THE SOIL SECTION systems on a wide range of soil conditions
67
is t o b e p r e d i c t e d f r o m s o i l a n d w h e e l
properties.
REFERENCES [1] J. K. HENSHALL and D. J. CAMPBELL,The calibration of a high resolution gamma-ray transmission system for measuring soil bulk density and an assessment of its field performance. J. SoilSci. 34 (1983) (in press). [2] M. F. O'SULLIVAN, J. W. DICKSON,J. K. HENSHALLand G. ANDERSON,Electronic collection and storage of data from a hand-held recording soil penetrometer. J. agric. Engng Res. 28 (1983) (in press). [3] J. W. DICKSON~D. J. CAMPBELLand J. K. HENSHALL,An assessment of seedbed compaction by open, flat-lugged, steel, tractor wheels. J. agric. Engng Res. 28, 45-60 (1983). [4] D. J. CAMPBELL,Plastic limit determination using a drop-cone penetrometer. J. Soil Sci. 27, 295-300 (1976). 15] D. J. CAMPBELL,J. V. STAFFORDand P. S. BLACKWELL,The plastic limit, as determined by the drop-cone test, in relation to the mechanical behaviour of soil. J. Soil Sci. 31, 11-24 (1980).