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SW—Soil and Water
J. agric. Engng Res. (2001) 79 (1), 1}13 doi:10.1006/jaer.2000.0692, available online at http://www.idealibrary.com on SW*Soil and Water
REVIEW PAPER
A Review of Soil}tine Models for a Range of Soil Conditions H. P. W. Jayasuriya; V. M. Salokhe Agricultural, Aquatic Systems and Engineering Program, Asian Institute of Technology, Bangkok, Thailand; e-mail of corresponding author: [email protected] (Received 13 April 2000; accepted in revised form 2 February 2001; published online 23 April 2001)
This review paper summarizes various research "ndings on soil}tine interactions under di!erent test and soil conditions and rearranges them into de"ned categories in order to evaluate the research trends. E!orts have been made to correlate results observed by di!erent researchers, some of which were without proper raw data given or other supporting information. It is observed that not all the work has been conducted in a systematic manner. Some speci"c tasks and areas have been covered leaving gaps even at preliminary levels. The study reveals that the best solution might be to have a "nite number of arrays of tillage models based on speci"c conditions of soils, tools and failure modes. Heading toward a unique or a universal model would be an impossible task due to fuzzy conditions of the working environment. It is suggested that computer-aided methods be used to create a supporting database of model parameters in order to utilize them for any speci"c condition or design purposes. 2001 Silsoe Research Institute There is a vast development in the area of soil}structure interaction in civil engineering in which the stability of buildings and structures has been studied mostly under static or seismic conditions. Most of the analyses in civil engineering problems, however, are based on initial failure point (or yield point) and micro level failure and deformations. Their major concern is to avoid soil failure; thus their designs or theories are based on the same concept of failure prevention. In the "eld of agricultural engineering, soil failure is a requirement and the concern is not only aimed at the yield point, but also the subsequent failure process after the initial failure. Unlike civil engineers, agricultural engineers are working on minimizing forces required for soil failure and looking for corresponding optimum conditions. Many research studies have been conducted to analyse tillage operations, most aimed at optimizing the draught power requirement in order to minimize the power wasted.
1. Introduction Land preparation with machines provides greater reliability and enables the farmer to achieve his targets at the right times regardless of many natural disturbances. Optimizing tillage is one of the major objectives in mechanized farming to achieve economically viable crop production systems. When considering the di!erent conditions, mechanisms and variables involved in the soil}tool interaction system, this becomes a di$cult task. More and more complex situations need to be handled during the process as the behaviour of soil varies even with a slight change in moisture content, compaction level, type of soil minerals or texture as well as the implement parameters and its manner of motion. Many researchers have worked on major soil types such as sandy, clayey and loamy soils (Tables 1 and 5) but the overall outcome still seems unsatisfactory to utilize these "ndings in a systematic way. Design of "eld machinery and implements is based not only on the machine or implement performance but also includes knowledge of the behaviour of soils and soil}implement interactions. Table 1 summarizes research "ndings of soil}tool interaction studies conducted under di!erent test conditions in order to observe soil failure and deformation patterns and their outcomes. 0021-8634/01/050001#13 $35.00/0
2. Di4erent approaches Four major methods could be identi"ed in recent approaches to solve problems in the area of soil}tool interactions and failure mechanisms. Firstly, using empirical or semi-empirical formulae based on geometry, and static 1
2001 Silsoe Research Institute
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Table 1 Summary of research 5ndings on soil failure and deformation in di4erent soils and under di4erent test conditions Reference
Tool condition
Soiltype/condition
Failure type
Kawamura (1952)
Inclined tool
Dry, frictionalcohesive
Progressive shear; avg. shear angle 313; curved shear surface
Mean shear angle increased with tool depth and reduced with increasing rake angle; shear angle as predicted
Selig and Nelson (1964)
Four wide blades Inclined (forward) Vertical Vertical
Dry sand
Failure angle greater than theoretical (45! /2)
-doDry, silty clay
Passive shear. Failure patterns as per passive pressure theory Passive shear
Inclined (forward)
Dry, plastic } clay
Tensile failure
Siemens et al. (1965)
Inclined and vertical tines
Dry, arti"cial soil
Progressive shear
Formation of soil blocks at uniform intervals
Hettiaratchi and Reece (1967)
Vertical tines 25 mm width, Aspect ratio 1}6 Inclined (forward)
Dry sand
Progressive shear
Stationary soil wedge for all aspect ratios
Dry sand
Progressive shear
Failure angle always less than (45! /2) -do-
Hettiaratchi and Reece (1974)
Varying rake angles Dry sand of 0}1803
Elijah and Weber (1971)
Inclined (forward) Flat blades: model; full size
Arti"cial soils: wet Four distinct failure modes: The inter-relationship was sand fric.-cohesive shear plane; bending; found due to changes in clay Field soils: silty clay tensile; #ow content, moisture and loam agric. silt texture loam (di!. moist.)
Godwin and Spoor (1977)
Inclined narrow tines
Dry sandy loam
Sta!ord (1981)
Narrow tine 40 cm width, 150 cm, depth Rake 453, 903 Rake 453 Rake 903 Rake 453
Clay
Harison (1982)
Inclined (forward)
Godwin et al. (1984)
Tines 25 mm wide, 453 rake angle
Moisture Moisture Moisture Moisture
Di!erent zones in wedge: Rankine; kinematic; complex-kinematic; transition
Remarks
Rajaram and Oida (1989)
3-D failure with crescent type for narrow tools Failure from shear to #ow as speed, moisture increase
18% 28% 28% 38%
Rigid-brittle failure -doFlow-type failure -do-
Below plastic range Above plastic range
Silty loam; moist. 18}20%
Progressive shear with formation of soil block
Inter-relationship between soil wedge and ridge
Compacted sandy loam soil
Soil failure function of Failure boundaries function of interaction between tines relative position of tines
Salokhe and Flat cage wheel lug Bangkok clay; Gee-Clough (1987) with angles 0, 15, moisture 49%; 30, 453 CI 140 kPa Rajaram (1987)
Progressive shear with stationary soil wedge
Variation of rupture geometry with rake angle and interface
Vertical tines; width 3, 12, 15 cm
Bangkok clay: moisture 5)2% 8)3% 18)6% 42)0%
Narrow/wide tines: vertical; inclined
Dry loose sand
Elliptical wedge formed contrary to theoretical failure Collapse-type failure Fracturing-type failure Chip-forming type Flow-type failure
Limitations of passive pressure theory for clayey soils Cyclic behaviour of soil forces with tine movement up to 28% moisture. Limitations of passive pressure theory
Collapse-type for wide; Collapse-type failure in frictional-type for narrow periodical manner for wide. Frictional-#ow-type for narrow tines
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Table 1*Continued Reference
Tool condition
Soiltype/condition
Failure type
Remarks
Sharma (1990)
Flat rigid tines; rake angle 15}1403; aspect ratio 4
Dry sand
Progressive shear; average shear angle 263
No distinct zones of soil failure as described by theory
Wang (1991)
Wide tines; rake angle 25}1253
Bangkok clay; moisture 44}52%
Plastic deformation
Formation of elliptical soil wedge for rake angle greater than 903
Salokhe and Pathak (1992)
Flat vertical tines
Dry sand
Progressive shear; shear angle (453! /2)
Failure patterns did not match with passiive pressure theory
Makanga (1997)
Flat tines with rake angles 50, 90, 1303; aspect ratio 0)5}3)3
Silty loam; moisture 5)2%, 21% and 33)5%
Progressive shear type: a!ected by tool parameters; failure in regularcycles
E!ect due to depth and width individually. No distinct zones as in passive pressure theory. Shape of rupture pro"le same for all rake angles
Jayasuriya and Salokhe (1998)
Flat narrow tines with rake angles 50, 90, 1303
Dry lateritic soil and two gradations; passing sieve no. 10 and 4
Progressive shear type; force}displacement signals in cyclic nature; true shape of 3-D failure geometry
Parabolic failure curves for frontal and in the furrow direction. True 3-D geometry not spherical as assumed in many previous models; lateral failure boundaries
Note: 3-D three dimensional: , soil}soil internal friction angle: CI, cone penetration resistance.
or dynamic conditions, has been described by many researchers in which the models have been developed for speci"c conditions (Tables 2}4). There is not a wellde"ned method to choose a model for a selected soil or other speci"ed conditions. The second method is the dimensional analysis approach (Table 2) in which the most critical aspect is to identify the critical model parameters and their form in the Pi-term. The third method is the use of "nite element method (FEM) (Table 2), by which prediction could be done without a preliminary assumption of soil failure pattern and it can calculate stress, displacement, velocity or acceleration of soil. This includes the approximation of de"ned constitutive relationship, with a selected model (e.g., linear-elastic, anisotropic-elastic, non-linear-elastic, elastic-plastic, strain-softening, cam-clay, etc.) characterizing important properties of some common soils and with soil}tool interface properties. The method has limitations for dynamic situations with successive failure common in tillage operations. The fourth method, the arti"cial neural network (ANN) modelling technique as sighted by Kushwaha and Zhang (1998), possesses the possibility and scope for modelling application by combining with the "nite element method (FEM) to simulate tine draught in dynamic situations, leading to computerized tool design.
The method seemed to be best for handling fuzzy variables but may have limitations for generalized models. Table 2 alone provides summarized details of research work reviewed by di!erent approaches and objectives.
3. Constraints and limiting factors Development of models for evaluating soil reaction forces, deformation and failure behaviour under quasistatic or dynamic conditions was given attention in recent studies. However, there is still no well-de"ned, generalized theoretical model to predict the behaviour of soil}tool interactions. Most of the studies have been carried out for selected or in situ soil conditions. Those models or results were not #exible enough to adjust or generalize to any other conditions. These limiting factors hinder the development of implement design, though they have a greater importance in improving e$ciency in land preparation process. Most of the research studies were based on theoretical failure mechanisms with empirical formulas by shear, or based on passive earth pressure theory, which are extensively used in civil engineering applications. However, the experiments done in agricultural soils have shown that soil failure can be shear, fracturing, chip forming or #ow and cannot be generalized for all soil types and
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Table 2 Summary of soil+tool interaction studies and prediction models developed with di4erent approaches in chronological order Researcher Kawamura (1952) Soehne (1956) Payne (1956) Row and Barns (1961) Selig and Nelson (1964) O'Callaghan et al. (1964) Siemens et al. (1965) O'Callaghan et al. (1965) Hettiaratchi et al. (1966) Hettiaratchi and Reece (1967) Gill and Vandenberg (1967) Larson et al. (1968) Sirohi and Reaves (1969) Elijah and Weber (1971) Hettiaratchi and Reece (1974) Hettiaratchi and Reece (1975) Wismer et al. (1976) Godwin and Spoor (1977) McKyes and Ali (1977) Yong and Hanna (1977) Gee-Clough et al. (1978) Wadhwa (1980) Kuckzewski (1981) Sta!ord (1981) Harison (1982) Perumpral et al. (1983) Sta!ord (1984) Swick and Perumpral (1985) Godwin et al. (1985) Liu Yan and Homzhi-Min (1985) Salokhe and Gee-Clough (1987) Salokhe and Gee-Clough (1988) Rajaram (1987) Rajaram and Oida (1989) Thakur and Godwin (1989) Suministrado et al. (1990) Sharma et al. (1990, 1992) Chi and Kushwaha (1990) Wang (1991) Salokhe and Pathak (1992) Niyamapa et al. (1992) Hatibu and Hettiaratchi (1993) Zhang and Kushwaha (1995) Fielke (1996) Godwin and Wheeler (1996) Makanga et al. (1996) O'Dogharty et al. (1996) Wheeler and Godwin (1996) Desbiolles et al. (1997a) Hettiaratchi (1997) Jayasuriya and Salokhe (1998) Kushwaha and Zhang (1998) Desbiolles et al. (1999) Jayasuriya (1999) Desbiolles and Godwin (2000) Saunders et al. (2000)
Condition
Approach
Purpose
Inclined tines, e!ect of rake, shear Inclined tines, failure by shear Vertical narrow tines, all loamy soils Inclined tines, 2-D, adhesion term Wide blades, dif. angles, sand, clay Vertical plate tine, deep, shallow Nearly vertical tines, dry soil, passive Inclined plate tines, deep, shallow Wide blades, c- soil, 2-D, passive pre. Wide blades, c- soil, 3-D, plastic equilb. General application, optimisation Mouldboard plough, agric. soils Cultivator sweep design, 2-D failure Inclined tines, arti"cial/"eld soils Wide blades, c- soil, rupture geom. Wide blades, c- soil, boundary wedge Plane/bulldozer blade, disc, sand, clay Inclined narrow tines, c- soil, 2-D Narrow blades, rupture distance Plane soil cutting, 2-D general cases Mouldboard ploughs, di!erent soils Distorted model, general application Model plough bodies, soil parameters E!ect- tine geometry, speed, moisture Inclined blades, wedge formation Narrow blades, di!. depth width rake Di!. rake, speed e!ect, two failure modes Narrow blades, dynamic e!ect Concave discs, cutting, scrubbing Narrow blades, 3-D Single lug,2-D,BKK clay,wedge Multiple lug, 2-D, BKK clay, spacing Vertical tines, clay, di!. moist. four pattern Wide, narrow tines, sand, two patterns Force prediction models for rotary tools (review) Mouldboard plough, with assumptions Di!erent #at tines, sand, 2-D models General applications, 3-D Di!erent #at tines, BKK clay, 2-D Flat vertical tines, sand, shear failure Triaxial tests, silty and sandy loam Triaxial test, sand, loam, clay, four factors Blades/tines, cutting resistance, 3-D Sweeps, tool edge e!ect, 2-D Land anchors Di!erent #at tines, loam soil, 2-D Tilted spherical plough disc Single and multiple tines at high speed Standard tine, sandy loam and clay soils Concave discs, c- soil, 3-D Inclined tines, dry compressible soil, 3-D General applications, dominant factors Cone index used for draught prediction Flat tines, di!erent rakes, dry c- soil Use of soil strength factors Use of Mohr}Coulomb criteria and inertia
EX TH, EX TH, EX TH, EX EX TH TH, EX TH TH TH FEM, TH SIM, EX SIM, EX EX TH TH SIM TH, EX TH TH, FEM TH, SIM, EX TH, SIM TH, SIM EX EX TH, EX TH, EX TH, EX TH TH, FEM EX EX EX EX
FS PM PM PM FS PM PM, FS PM PM, FS PM, FS PM PM PM, FS FS PM, FS PM, FS PM PM, FS PM, FS PM, FS PM PM PM FS FS PM PM, FS PM PM PM, FS FS FS FS FS
* TH TH, EX TH, FEM EX, FEM EX EX EX TH, EX EX TH, EX EX TH TH,EX TH, EX TH, SIM EX TH, FEM, ANN TH,EX EX TH,EX EX
* PM PM, FS PM, FS FS FS FS FS PM PM, FS PM FS PM PM PM PM PM, FS PM, FS PM PM, FS PM PM, FS
Note: FS, failure study; PM, prediction models; TH, theoretical; EX, experimental; SIM, similitude; FEM, "nite element method; ANN, arti"cial neural network; c, cohesion; , soil}soil internal friction angle; c- ,frictional-cohesive soil, 2-D, two dimensional; 3-D, three dimensional; BKK clay, Bangkok clay.
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Table 3 Tillage force prediction models developed without considering dynamic e4ect Researcher
Tool/test condition
Remarks
Payne (1956)
Vertical narrow tools in sandy loam, silty loam, clayey loam
Used passive pressure theory, aspect ratio '2 considered as wide
Row and Barns (1961)
Inclined blades/tines
2-D failure, adhesion term included
O'Callaghan et al. (1964, 1965)
Vertical plate (1965), inclined (1965) deep and shallow
Horizontal and vertical rupture. Transition at depth/width ratio"0)6
Hettiaratchi et al. (1966)
Wide plates Rake angles vary (453! /2) to (1803! )
(1966) 2-D failure, (1967) 3-D failure, passive pressure (1974) 2-D failure, rapid calculation method taking plastic equilibrium
Godwin and Spoor (1977)
Narrow inclined tines Frictional-cohesive soil
Models for shallow, deep cases, 3-D and side crescent, wedge considered
Perumpral et al. (1983)
Narrow blades, extended for depths, widths, rake angles
No side wedge component, friction angle found function of failure angle
Sharma et al. (1990, 1992)
Flat tines, in sand Rake angles, aspect ratios varied
Two models for rake angles up to (903! ) and between (903! ) and 903
Zhang and Kushwaha (1995)
Narrow blades, tines Two ranges of rake angles
3-D failure, two models rake up to (903! ) and above that range
Desbiolles et al. (1997a)
Standard tine in sandy-loam and clay soils
Draught related to soil strength and tool geometry
Makanga (1997)
Narrow blades, loam soil
E!ect of aspect ratio, rupture pro"le
Hettiaratchi (1997)
Concave discs, for c- soils
3-D, rapid calculation method
Desbiolles et al. (1999)
Di!erent tillage implements
Use of cone penetrometer data
Jayasuriya (1999)
Flat tines, dry c- soils
E!ect of compaction, con"ne condition
Hettiaratchi and Reece (1967, 1974)
Note: c, cohesion; , soil}soil internal friction angle; c- , frictional-cohesive soil, 2-D, two dimensional; 3-D, three dimensional.
conditions (Salokhe, 1986; Rajaram & Gee-Clough, 1988; Sharma, 1990). It was revealed that the inadequacy of present theoretical considerations in generalizing the situation, since the e!ect of moisture content, stress history, cyclic failure mechanism instead of instantaneous failure could not be taken into account by the existing failure theories. Rajaram and Erbach (1997) also emphasized the importance of better understanding of true failure mechanisms and use of in situ soil properties in developing better theoretical models for tillage applications. According to the characteristics of soil or classi"cations de"ned in various standards, it is di$cult to "nd clear boundaries between some important properties of typical soils. There are still no clear methods to separate soils into homogeneous, isotropic or anisotropic categories by its measurable properties. It has also been found that some clay minerals behave in a di!erent manner
depending on their composition and moisture content. Some soils possess special properties that have not been considered in any model developed so far. Thixotropy, sensitivity, colloidal activity, dilation and sensitivity to stress history are some of the factors found to be di$cult for use in analytical approaches in developing theoretical models (Raymond et al., 1984; Terzaghi, 1959; Terzaghi & Peck, 1948). In order to understand general principles of soil failure mechanisms and corresponding theoretical considerations, it would be worthwhile to review research "ndings in the relevant areas of soil engineering as well as the area of soil failure principles under tillage operations. Better understanding of the mechanical behaviour of soils under di!erent situations, and soil failure and deformation mechanisms, would be a pre-requisite. The models developed so far are not in precise form with clearly justi"ed model parameters and so have limitations in
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Table 4 Tillage force prediction models developed considering dynamic e4ects Researcher
Tool/test condition
Remarks
Soehne (1956)
Inclined tool
Soil failure due to repeated shear. Acceleration component included
Siemens et al. (1965)
Nearly vertical tool Dry arti"cial soil
Used passive earth pressure theory. Acceleration component included
Sirohi and Reaves (1969)
Cultivator sweep similitude
Assumed soil failure in 2-D mode
Wismer et al. (1976)
Plane blade and disc systems Similitude method for clay and sand soils
Separate models for clay and sand considering frictional and cohesive properties of soil
Gee-Clough et al. (1978)
Similitude method for ploughs working on di!erent soils
Mouldboard ploughs were tested, good correlation obtained
Sta!ord (1981, 1984)
Inclined & vertical tool, clay Moisture 18}49% Speed 5 mm s\}5 m s\
Governing parameters; tool speed, moisture, tool geometry. Inertia component included
Suministrado et al. (1990)
Mouldboard plough
Mathematical model assuming a constant relative speed, ploughs bottom as stationary and soil slice in motion
Wheeler and Godwin (1996)
Single and multiple tines
Force prediction models at high speeds
Kushwaha and Zhang (1998)
Simulation by arti"cial neural network (ANN) method
Advantages of ANN method. Ability to model system input parameters having fuzzy and uncertain properties
Saunders et al. (2000)
Mouldboard plough
Models use Mohr}Coulomb criteria and inertia e!ects
Note: 2-D, two-dimensional; ANN, arti"cial neural network.
application. This is mainly due to the di$culties in quanti"cation of the mechanical behaviour of the soil mass in those applications attempted individually. Most of the theoretical models have been based on assumptions taken into account during the model development process. Due to the fuzzy behaviour under actual "eld conditions, a majority of the studies did not consider the dynamic e!ects and were done in laboratory soil bins. Many have used quasi-static conditions and simulations for dynamic condition. Tables 3 and 4 show the research work on development of tillage models with and without considering dynamic e!ects in the situation. Identi"cation of governing parameters and their exact form was found to be the most critical factor in this process where recent research work and developed models are concerned. The inter-relationship of some major parameters, and changes in application environments or test conditions restricted their use as common parameters. This has resulted in di$culties in modelling and generalization of models for application. Based on various situations, it can be expected that arrays of models can predict soil forces and failure patterns for di!erent conditions. This emphasizes the importance of better understanding of true soil properties and the exact
behaviour of agricultural soils before the modelling process. It can also be pointed out that consideration must be given to application requirements in the tillage operation, rather than modifying theories applied in civil engineering applications as mentioned by recent researchers (Rajaram & Erbach, 1997; Kushwaha & Zhang, 1998). It is also important to conduct future research covering major soil types and major areas, as described in standard soil classi"cation systems, in order to achieve the best outcome and completeness. Soils with special properties or characteristics should be given second priority based on the results and usefulness in application. Therefore, these factors justify the importance of rearranging all "ndings in a logical manner under well-de"ned categories, and the necessity of a database of di!erent "ndings and properties.
4. Rearranging research 5ndings on soil failure mechanisms and reaction models All research "ndings on soil deformation and failure mechanisms were rearranged in a tabulated matrix form, for di!erent soil types and conditions, in order to
Table 5 Summary of soil failure mechanisms categorized under di4erent soil types and conditions Condition
In frictional (sandy) soils
Wet=in the range of liquid limit
No research studies found
In frictional-cohesive soils Elijah and Weber (1971) Inclined (forward) #at blades (Silty clay loam, silty loam). Flow-type failure observed
In cohesive (clayey) soils Rajaram (1987) Vertical tines with varying width of 3, 12, 15 cm in Bangkok Clay soil at 42% moisture (PL, 25%; LL, 54%) Flow-type soil failure with hyperbolic behaviour of soil forces and limitations of passive pressure theory was observed Wang (1991) Wide tine with varying rake angles 25+1253 in Bangkok clay. Soil moisture 52% (LL, 54%) Plastic deformation (#ow-type soil failure) Formations of elliptical soil wedge in front of the tine for rake angles greater than 903 were also observed
Wet=Below or Closer to Plastic Limit
No research studies found
No other research studies found
Elijah and Weber (1971) Inclined (forward) #at blades in "eld soils (silty clay loam, silty loam) Four distinct failure patterns and interrelationship due to changes in clay content, moisture content and speed Bending-type failure observed
Staword (1981) Narrow tine with size 40/150 mm, two rake angles 45, 903 in clay Failure patterns changed from shear to #ow as speed, moisture increased Flow-type failure observed at 28 and 38% moisture, above the plastic range
Makanga (1997) Flat tines with rakes 50, 90, 1303, aspect ratios varying between 0)5 and 3)3, soil moisture 33)5%, (PL, 23%; LL, 51%) silty loam soil Plastic-type failure patterns; in repetitive and periodic manner and found a!ected by depth and width No distinct zones found as described in passive pressure theory. Shapes of the rupture pro"les the same for all rake angles
Rajaram (1987) Vertical tines, width of 3, 12, 15 cm in Bangkok clay at soil moisture 28.6% (PL, 25%; LL, 54%) Chip forming-type-soil failure with hyperbolic behaviour of soil forces and limitations of passive pressure theory
Elijah and Weber (1971) Inclined (forward) #at blades in "eld soils (silty clay loam, silty loam). Four distinct failure patterns an interrelationship due to changes in clay content, moisture content and speed Tensile-type failure observed
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Wet=above plastic limit
Wang (1991) Wide tine with varying rake angles 25+1253 in Bangkok clay at soil moisture 44% (PL, 25%; LL, 54%) Plastic deformation (#ow-type soil failure). Formations of elliptical soil wedge in front of the tine for rake angles greater than 903 Higher tool forces were obtained compared to the condition with soil moisture content of 52% Staword (1981) Narrow tine 40/150 mm two rake angles 45, 903 tested on clay Failure patterns changed from shear to #ow as speed, moisture increased Rigid-brittle-type failure patterns at 18% and 28% moisture
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Table 5*Continued Condition
In frictional (sandy) soils Elijah and Weber (1971) Inclined (forward) 6at blades in "eld soils (Silty clay loam, silty loam). Shear-plane-typefailure observed.
In frictional-cohesive soils Makanga (1997) Flat tines with rake angles 50, 90, 1303 and aspect ratios varying between 0)5}3)3, silty loam, soil moisture 21% (PL, 23%) Tensile-type failure patterns in repetitive and periodic manner. Failure patterns found a!ected by depth and width
In cohesive (clayey) soils Rajaram (1987) Vertical tines of width 3, 12, 15 cm in Bangkok clay at soil moisture 18.3% (PL, 25%) Fracturing-type-soil failure with cyclic behaviour of soil forces. Limitations of passive pressure theory
Harison (1982) Inclined (forward) tines on silty loam soil, moisture at 18}20% Progressive shear failure with formation of soil blocks. Interrelationship of soil wedge and ridge Kawamura (1952) Inclined (forward) 6at tools Rajaram et al. (1987) Vertical tines width of 3, 12, Selig and Nelson (1964) Four in dry, c- soil 15 cm in Bangkok clay at soil moisture 5)2% wide blades, inclined (forward) and vertical. Passive shear failure Progressive-shear-type failure with curved shear (PL, 25%). and failure patterns according to surfaces and the average shear angle of 313 Collapsing-type-soil failure with cyclic behaviour passive pressure theory. Failure Mean shear angle increased with tool of soil forces. Limitations of passive pressure angle found greater than depth, reduced with increasing rake angle. theory for clayey soils theoretical value of (453! ). Shear angle as passive pressure theory No other research studies found Hettiaratchi and Reece (1967) Vertical and inclined (forward) tines with 25 mm wide, aspect ratios 1}6 were used on dry sand Progressive-shear-type-failure with stationary soil wedge for all aspect ratios was observed
Selig and Nelson (1964) Four wide blades, inclined (forward) and vertical, in Dry slity clay and dry plastic clay soils Passive shear failure and failure patterns as passive pressure theory for vertical tines, tensile failure for inclined tines. Failure angle less than theoretical value of (453! ).
Hettiaratchi and Reece (1974) Tines with varying rake angles 0}1803 on dry sand. Wedge zones, Rankine, Kinematic, Complex-Kinematic, Transition identi"ed. Variation of rupture geometry with rake Sharma (1990) Flat rigid tines with rake angle 15+1403 and aspect ratio four on dry sand Progressive-shear-type failure with average shear angle of 263. No distinct zones of soil as described by theory. Variation of rupture distance with rake angle
Siemens et al. (1965) Inclined and vertical 6at tools in dry-arti"cial soils Progressive-shear-type failure patterns. Formation of soil blocks at uniform intervals for all tool rake angles tested Elijah and Weber (1971) Inclined (forward) 6at blades in "eld soils (Silty clay loam, silty loam) Shear-plane-type failure
No other research studies found
H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
Dry Soil
Salokhe and Pathak (1992) Flat vertical tines on dry sand Progressive-shear-type failure The failure patterns not match with passive pressure theory. Shear angle found (45! /2)
Godwin and Spoor (1977) Inclined narrow tines on dry sandy loam soil Progressive-shear-type failure with stationary soil wedge. three-dimensional crescent type failure mechanism for the upper part and twodimensional failure for the lower part.
Rajaram and Oida (1989) Narrow Makanga (1997) Flat tines with rake angles 50, 90, and Wide tines on dry loose 1303, aspect ratio varying 0)5}3)3 on dry sand. Collapse-type failure for (moisture 5)2%) silty loam soil. wide tines in periodic manner. Progressive-shear-type-failure in regular Frictional-6ow-type for narrow tines cycles. Failure patterns found a!ected by depth and width and there was no direct correlation with aspect ratio No distinct zones were found as described in passive pressure theory. Shapes of the rupture pro"les were the same for all rake angles Note. PL, plastic limit; LL, liquid limit; c, cohesion; , soil}soil internal friction angle; c- , frictional cohesive soil S O IL - TI N E M O D E LS
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highlight gaps and to identify the possible mechanisms. It was found that there was a necessity to carry out tests to obtain further boundaries of failure mechanisms possible in cohesive and frictional-cohesive soils with moisture content between plastic and liquid limit. More precisely, the sticky limit of the soil could be considered as the point at which the failure mechanism could have a transition. Some investigations on scouring of implements dealt with sticky limit of soils. Among the studies on soil failure mechanisms, the identi"cation of four distinct failure patterns or mechanisms in all major soil types with the change of soil moisture content could be considered as the turning point of the study area. This was initially reported by Elijah and Weber (1971) and further studied by Sta!ord (1981, 1984) and later con"rmed by Rajaram and GeeClough (1988). Based on their observations four failure mechanisms could be identi"ed as follows: (1) &progressive-shear' or &shear-plane-type' failure in all the soil types under dry condition with cyclic variation in force}time curve; (2) &fracturing-' or &tensile-type' failure in frictional-cohesive and cohesive soils when the moisture content increases closer to the plastic limit, with cyclic variation in the force}time curves; (3) &chip-forming-type' or &plastic-type' failure in frictional-cohesive and cohesive soils when moisture content increased above the plastic limit with cyclic (low amplitude) hyperbolic shape of force}time curves; and (4) &-ow-type' failure in frictional-cohesive and cohesive soils when moisture content was closer to the liquid limit with a hyperbolic shape of force}time relationship. The progressive-shear- or shear-plane-type failure mechanisms were observed by many researchers who worked with dry frictional and frictional-cohesive soils. The force}time curve was of a cyclic nature. Sharma (1990), Makanga (1997) and Jayasuriya (1999), who worked with sandy, loamy and lateritic type soils, respectively, con"rmed these results. The speci"cations of the force}time curve, amplitude, frequency and wavelength were analysed in many studies. However, the relationships of these parameters between di!erent soil types and conditions have not been studied completely so far. Uniform frequencies and amplitudes were observed in frictional soils and became irregular when changed to frictional-cohesive and cohesive soils under dry condition. The e!ect of the parameters, such as soil compaction and soil gradation (Jayasuriya, 1999) on failure mechanisms, force}displacement signal speci"cations and soil reactions, showed the existence of de"nite relationships among parameters and failure modes.
Fig. 1. Analysis of force}displacement signal specixcations for comparison
Figure 1 shows the predictable variation of force} displacement behaviour for di!erent soils and with the wave speci"cations. The shape of the force}time curve changed from cyclic to hyperbolic when moisture content increased from a dry to wet-saturated condition. Rajaram and GeeClough (1988), Wang (1991) and Makanga (1997), each con"rmed the plastic- and #ow-type soil failures for different tines tested by changing moisture content closer to liquid limits, while working in Bangkok clay and silty loam soil respectively. In general, the numerical value of the moisture content does not show any direct relationship with the change of soil failure patterns in di!erent soils. However, the &consistency limits' or &Atterberg limits' (plastic limit, liquid limit, etc.) show clear boundaries of di!erent failure mechanisms in frictional-cohesive and cohesive soils. The exact boundaries seemed to depend on the soil particle size distribution, mineral types and the composition.
5. New categorization It can also be stated that most research work has been conducted either in dry soils or under wet-saturated conditions. The experiments conducted so far have not clearly identi"ed the critical boundaries of di!erent soil failure mechanisms or any other criteria, as in dry soils, for wet soil condition. But some zones, where di!erent soil failure mechanisms take place, can be identi"ed corresponding to the four failure mechanisms described previously. They are: (1) dry zone, in which soil tends to collapse or pulverize easily by shear; (2) wet, below (closer to) plastic limit, where fracturing- or tensile-type failure occurs; (3) wet, above plastic limit, where chip-formingor plastic-type failure occurs; and (4) wet, in the range of liquid limit, where #ow-type or plastic deformation occurs.
S O IL - TI N E M O D E LS
The soil reaction forces and the soil failure mechanisms seem to possess a relationship with increasing moisture content in frictional-cohesive and cohesive soils. Unlike the failure mechanism, the soil reaction force could be dependent upon many other factors involving soil parameters, tine parameters and experimental conditions. The e!ects of some parameters were investigated in many studies but the form or role of some have yet to be investigated. Table 5 shows a matrix of soil deformation and failure mechanisms arranged by major soil types: frictional, frictional-cohesive and cohesive for varying moisture contents. Based on the observations, when rearranged in matrix form, the following points can be identi"ed for discussion. The rearranged literature shows the approaches used in di!erent studies. It is revealed that the research studies have been conducted in the major soil types of sandy (frictional), loamy (frictional-cohesive) and clayey (cohesive), in order to study the overall picture of failure mechanisms and behaviour. However, the soil reaction prediction models could not be found in a precisely speci"ed manner, thus limiting applicability in di!erent conditions. It is clear that this problem could be solved by developing arrays of models under speci"cally de"ned categories involving soil types and conditions. Use of model parameters involving soil properties and characteristics in prediction models is found to be most critical. The form of the parameter representing the situation should be the best to provide optimum results. The process of summarizing and rearranging the literature in a logical way revealed many uncertainties in the area of failure mechanisms and tillage models. Lack of availability of raw data and details of experimental conditions were the most crucial ones. Most post-research recommendations by researchers were to eliminate approximations used during the modelling process and to look at the feasibility of application in actual "eld conditions. However, Rajaram and Erbach (1997) pointed out the necessity of identifying di!erent approaches for solving the problem of tillage modelling. Their major point was whether the classical soil theories used in soil engineering studies could be readily applicable to agricultural soils and applications where soil failure in an e$cient manner is the prime objective. Kushwaha and Zhang (1998) opted for di!erent theoretical approaches with updated technology. They proposed the arti"cial neural network (ANN) method, combined with "nite element method (FEM), leading to computer-aided tillage tool design as a promising solution. This seemed to be the best where time and speed are considered as in a dynamic situation. All work in the design stage will then be with computer simulations and no material waste will take place except for computer
11
time. Optimization could be possible in the design stage itself, up to a certain extent. Their recommendation is to use the dynamic e!ects in prediction models, tool wear and the non-uniformity conditions as an input to static or quasi-static prediction models without much di$culty.
6. Proposed research directions and scope (1) Changes in soil failure mechanisms when moisture content is closer to the sticky limit of the soil need to be studied. There could be another failure mechanism at the sticky limit of the soil at which the soil tends to stick over the surface of the tool causing changes in soil #ow patterns. (2) The draught force and the soil failure mechanisms seem to possess a relationship with increasing moisture content in frictional-cohesive and cohesive soils. (3) The feasibility of applying theories in the area of laminar #uid #ow mechanics to the behaviour of wet clay under tool motion could be studied. The e!ect on laminar #ow with stationary obstacles with di!erent shapes may be simulated the other way around for moving tools in wet clay. The same principle may be extended to well-graded frictional dry soils which also show laminar-(#ow-) type failure. Frictionalcohesive and partially saturated soils could be modelled under turbulent category. (4) The role and best form of some parameters in prediction models needs further evaluation and clari"cation in order to arrange them in the best "tting manner. (5) Studies on void removal mechanisms in soils under dry and unsaturated conditions and wet conditions due to tool movement and their e!ect on soil failure mechanisms and reaction forces need to be conducted. (6) Studies under dynamic conditions somehow need to be conducted in order to solve soil}tool interaction modelling problems in actual "eld conditions. Arrays of models could be expected under di!erent soil categories and need to tabulate systematically for easy use. (7) The arti"cial neural network modelling technique proposed by Kushwaha and Zhang (1998) seems a better solution for soil}tool interaction modelling because it can handle fuzzy and non-uniform input variables for the dynamic range of the problem. This should automatically produce a database of tillage parameters and arrays of prediction models for di!erent soil and test conditions. Arti"cial neural network models need training with series of data collected from each category to de"ne hidden parameters inside the model.
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H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
7. Conclusions Review of research "ndings on soil}tool interactions revealed many important factors and gaps that could be used to direct future research towards new tasks and with new methods. Obtaining arrays of tillage models for speci"c conditions instead of generalized models, formation of a database of tillage parameters and use of arti"cial neural networks and "nite element modelling techniques and other new approaches is considered as promising recommendations for rapid improvement in the research area. It is also revealed that the slow development in the area for the past few decades was due to individual approaches by various researchers on speci"c targets and arbitrarily selected test conditions. An e!ort to include all important research "ndings relevant to the area is made. However, any further relevant "ndings that are not presented or discussed here could be combined for the advancement of the research area.
References Chi L; Kushwaha R L (1990). A non-linear 3-dimensional "nite element analysis of soil failure with tillage tools. Journal of Terramechanics, 27(4), 243}266 Desbiolles J M A; Godwin R J (2000). In situ soil strength characterisation for tillage draft prediction. Fourth International Confernce on Soil Dynamics, Adelaide, Australia, 26}30 March Desbiolles J M A; Godwin R J; Kilgour J; Blackmore, B S (1997). A novel approach to the prediction of tillage tool draught using a standard tine. Journal of Agricultural Engineering Research, 66(4), 295}309 Desbiolles J M A; Godwin R J; Kilgour J; Blackmore, B S (1999). Prediction of tillage implement draught using cone penetrometer data. Journal of Agricultural Engineering Research, 73(1), 65}76 Elijah D L; Weber J A (1971). Soil failure and pressure patterns for #at cutting blades. Transactions of ASAE, 14(4), 781}785 Fielke J M (1996). Interaction of cutting edge of tillage implements with soil. Journal of Agricultural Engineering Research, 63(1), 61}72 Gee-Clough D; McAllister M; Pearson G; Evernden D W (1978). The empirical prediction of tractorimplement "eld performance. Journal of Terramechanics, 15(2), 81}94 Gill W R; Vandenberg G E (1967). Soil dynamics in tillage and traction. Agricultural Handbook No. 316, United States Department of Agriculture Godwin R J; Seig D A T; Allot M (1985). The development and evaluation of force prediction model for agricultural discs. Proceedings of International Conference on Soil Dynamics, Auburn, Alabama, 31 August } 4 September, Vol. II, 250}265 Godwin R J; Spoor G (1977). Soil failure with narrow tines. Journal of Agricultural Engineering Research, 22(4), 213}228
Godwin R J; Spoor G; Soomro M S (1984). The e!ects of tine arrangement on soil forces and disturbance. Journal of Agricultural Engineering Research, 30(1), 47}53 Godwin R J; Wheeler P N (1996). An investigation into the soil mechanics and land anchors. Journal Agricultural Engineering Research, 63(1), 53}59 Harison H P (1982). Soil reaction from laboratory studies with an inclined blade. Transactions of the ASAE, 25(1), 7}12 Hatibu N; Hettiaratchi D R P (1993). The transition from ductile #ow to brittle failure in unsaturated soils. Journal of Agricultural Engineering Research, 54(4), 319}328 Hettiaratchi D R P (1997). Predicting the draught requirements of concave agricultural discs. Journal of Terramechanics, 34(3), 209}224 Hettiaratchi D R P; Reece A R (1967). Symmetrical threedimensional soil failure. Journal of Terramechanics, 4(3), 45}67 Hettiaratchi D R P; Reece A R (1974). The calculation of passive soil resistance. Geotechnique, 24(3), 289}310 Hettiaratchi D R P; Reece A R (1975). The boundary wedge in two-dimensional soil failure. Geotechnique, 25(2), 197}220 Hettiaratchi D R P; Witney B D; Reece A R (1966). The calculation of passive pressure in two-dimensional soil failure. Journal of Agricultural Engineering Research, 11(2), 89}107 Jayasuriya H P W (1999). Modelling soil}tine interactions in lateritic soils. AIT D. Eng. Dissertation, DISS No. AE-99-1, Asian Institute of Technology, Bangkok, Thailand (unpublished) Jayasuriya H P W; Salokhe V M (1998). Measurement of 3dimensional soil failure and deformation by superposition method. Proceedings of 5th Asia-Paci"c Regional Conference of International Society for Terrain Vehicle Systems (ISTVS). Seoul, Korea, 20}22 October, pp. 123}136 Kawamura N (1952). Study of the plough shape (3). Study on soil cutting and pulverisation (2). Society of Agriculcultural Machinery Journal, 14(3), 65}71 Kuckzewski J (1981). Soil Parameters for predicting draft of a model plough bodies. Journal of Agricultural Engineering Research, 26(3), 193}201 Kushwaha R L; Zhang Z X (1998). Evaluation of factors and current approaches related to computerized design of tillage tools: A review. Journal of Terramechanics, 35(2), 69}86 Larson L W; Lovely W G; Bockhop C W (1968). Predicting draft forces using model mouldboard plough in agricultural soils. Transactions of the ASAE, 11(5), 665}668 Liu Yan; Hou Zhi-Min (1985). Three-dimensional non-linear "nite element analysis of soil cutting by narrow blades. Proceedings of International Conference on Soil Dynamics. Auburn, AL, Vol. 2, pp. 338}347 Makanga J T; Salokhe V M; Gee-Clough D (1996). E!ect of tine rake angle and aspect ratio on soil failure patterns in dry loam soil. Journal of Terramechanics, 33(5), 233}252 Makanga J T (1997). Soil-tine interaction in loam soil. AIT D. Eng. Dissertation, DISS. No. AE-97-1, Asian Institute of Technology, Bangkok, Thailand (unpublished) McKyes E; Ali O S (1977). The cutting of soil by narrow blades. Journal of Terramechanics, 14(2), 43}58 Niyamapa T; Namikawa K; Salokhe V M (1992). Soil failure under undrained quasi-static and high speed triaxial compression test. Journal of Terramechanics, 29(2), 195}205 O:Callaghan J R; Farrely K M (1964). Cleavage of soil by tined implements. Journal of Agricultural Engineering Research, 9(3), 259}270
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O'Callaghan J R; McCullen P J (1965). Cleavage of soil by inclined and wedge shaped tines. Journal of Agricultural Engineering Research, 10(3), 248}254 O:Dogherty M J; Godwin R J; Hann M J; Al-Ghazal A A. (1996). A geometrical analysis of inclined and tilted spherical plough discs. Journal Agricultural Engineering Research, 63(3), 205}217 Payne P C J (1956). The relationship between mechanical properties of soil and the performance of simple cultivation implements. Journal of Agricultural Engineering Research, 1(1), 23}50 Perumpral J V; Grisso R D; Desai C S A (1983). Soil tool model based on limit equilibrium analysis. Transactions of the ASAE, 26(4), 991}995 Rajaram G (1987). Force}time behaviour of tine implements. M.Eng. Thesis No AE-87-14, Asian Institute of Technology, Bangkok, Thailand (unpublished) Rajaram G; Erbach D C (1997). Soil failure by shear versus modi"cation by tillage: a review. Journal of Terramechanics, 33(6), 265}272 Rajaram G; Gee-Clough D (1988). Force}distance behaviour of tine implements. Journal of Agricultural Engineering Research, 41(2), 81}98 Rajaram G; Oida A (1989). The nature of soil failure as applied to agricultural engineering. Journal of Japanese Society of Agricultural Machinery, 51(6), 3}11 Row R J; Barns K K (1961). In#uence of speed on elements of draft of a simple tillage tool. Transactions of the ASAE, 4(1), 55}57 Salokhe V M (1986). Cage wheel blocking in wet clay soils, AIT D. Eng. Dissertation No. AE-86-1, Asian Institute of Technology, Bangkok, Thailand (unpublished) Salokhe V M; Gee-Clough D (1987). Formation of boundary wedge on a single lug in wet clay soil. Journal of Agricultural Engineering Research, 38(2), 113}125 Salokhe V M; Gee-Clough D (1988). Wet clay soil behaviour under multiple cage wheel lugs. Journal of Agricultural Engineering Research, 39(2), 57}70 Salokhe V M; Pathak B K (1992). E!ect of aspect ratio on soil failure pattern generated by vertical #at tines at low strain rates in dry sand. Journal of Agricultural Engineering Research, 53(3), 169}180 Saunders C; Godwin R J; O:Dogherty M J (2000). Prediction of soil forces acting on mouldboard ploughs. Fourth International Confernce on Soil Dynamics, Adelaide, Australia, 26}30 March Selig E T; Nelson R D (1964). Observations of soil cutting with blades. Journal of Terramechanics, 1(3), 777}782 Sharma V K (1990). Soil}tool interactions for tools of simple shape in dry sand. AIT D.Eng. Dissertation, DISS. No. AE-90-1, Asian Institute of Technology, Bangkok, Thailand (unpublished) Sharma V K; Singh G; Gee-Clough D (1990). Soil}tool interactions in sand. Proceedings of the Tenth International Conference of the ISTVS, Kobe, Japan, 20}24 August
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Sharma V K; Singh G; Gee-Clough D (1992). Soil failure caused by #at tines in sand. Part*1. Rake angles 50}90. Agricultural Engineering Journal, 1(2), 71}91 Siemens J C; Weber J A; Thornburn T H (1965). Mechanics of soil as in#uenced by model tillage tools. Transactions of the ASAE, 8(1), 1}7 Sirohi B S; Reaves C A (1969). Similitude technique applied to performance studies of cultivator sweeps. Transactions of the ASAE, 12(6), 786}789 Soehne W (1956). Some basic considerations of soil mechanics as applied to the agricultural engineering. Grundlagen der Landtechnik, 7(11), NIAE Translation No 531 Sta4ord J V (1981). An application of critical state soil mechanics: The performance of rigid tines. Journal of Agricultural Engineering Research, 26(5), 387}401 Sta4ord J V (1984). Force prediction models for brittle and #ow of soil by draft tillage tools. Journal of Agricultural Engineering Research, 29(1), 51}60 Suministrado D C; Koike M; Konaka T; Yuzawa S; Kuroishi I (1990). Prediction of soil reaction forces on mouldboard plough surface. Journal of Terramechanics, 27(4), 307}320 Swick W C; Perumpral J V (1985). A model for predicting dynamic soil tool interaction. Proceedings of International Conference on Soil Dynamics, Auburn, Alabama, 15}17 June, Vol. II, pp. 384}399 Terzaghi K (1959). Theoretical Soil Mechanics. Chapman and Hall Ltd, J. Wiley and Sons Inc., New York Terzaghi K; Peck R B (1948). Soil Mechanics in Engineering Practice. Chapman and Hall Ltd, J. Wiley and Sons Inc., New York. Thakur T C; Godwin R J (1989). The present state of force prediction models for rotary powered tillage tools. Journal of Terramechanics, 26(2), 121}138 Wadhawa D S (1980). Force prediction equation using distorted model as an analogue device. Journal of Terramechanics, 17(3), 119}130 Wang JinJun (1991). Stress-strain relationship in wet clay soil and some applications. AIT D. Eng. Dissertation No. AE-911, Asian Institute of Technology, Bangkok, Thailand (unpublished) Wheeler P N; Godwin R J (1996). Soil dynamics of single and multiple tines at speeds up to 20 km/h. Journal of Agricultural Engineering Research, 63(3), 243}250 Wismer R D; Fertag D R; Schafer R L (1976). Application of similitude to soil-machine systems. Journal of Terramechanics, 13(3), 153}182 Yong R N; Ezzat A Fattah; Nicolas Skiadas (1984). Vehicle Traction Mechanics. Elsevier Publishers, Amsterdam Yong R N; Hanna A W (1977). Finite element analysis of plane soil cutting. Journal of Terramechanics, 14(3), 103}125 Zhang J I; Kushwaha R L (1985). A modi"ed model to predict soil cutting resistance. Soil and Tillage Research, 34(3), 157}168
REVIEW PAPER
A Review of Soil}tine Models for a Range of Soil Conditions H. P. W. Jayasuriya; V. M. Salokhe Agricultural, Aquatic Systems and Engineering Program, Asian Institute of Technology, Bangkok, Thailand; e-mail of corresponding author: [email protected] (Received 13 April 2000; accepted in revised form 2 February 2001; published online 23 April 2001)
This review paper summarizes various research "ndings on soil}tine interactions under di!erent test and soil conditions and rearranges them into de"ned categories in order to evaluate the research trends. E!orts have been made to correlate results observed by di!erent researchers, some of which were without proper raw data given or other supporting information. It is observed that not all the work has been conducted in a systematic manner. Some speci"c tasks and areas have been covered leaving gaps even at preliminary levels. The study reveals that the best solution might be to have a "nite number of arrays of tillage models based on speci"c conditions of soils, tools and failure modes. Heading toward a unique or a universal model would be an impossible task due to fuzzy conditions of the working environment. It is suggested that computer-aided methods be used to create a supporting database of model parameters in order to utilize them for any speci"c condition or design purposes. 2001 Silsoe Research Institute There is a vast development in the area of soil}structure interaction in civil engineering in which the stability of buildings and structures has been studied mostly under static or seismic conditions. Most of the analyses in civil engineering problems, however, are based on initial failure point (or yield point) and micro level failure and deformations. Their major concern is to avoid soil failure; thus their designs or theories are based on the same concept of failure prevention. In the "eld of agricultural engineering, soil failure is a requirement and the concern is not only aimed at the yield point, but also the subsequent failure process after the initial failure. Unlike civil engineers, agricultural engineers are working on minimizing forces required for soil failure and looking for corresponding optimum conditions. Many research studies have been conducted to analyse tillage operations, most aimed at optimizing the draught power requirement in order to minimize the power wasted.
1. Introduction Land preparation with machines provides greater reliability and enables the farmer to achieve his targets at the right times regardless of many natural disturbances. Optimizing tillage is one of the major objectives in mechanized farming to achieve economically viable crop production systems. When considering the di!erent conditions, mechanisms and variables involved in the soil}tool interaction system, this becomes a di$cult task. More and more complex situations need to be handled during the process as the behaviour of soil varies even with a slight change in moisture content, compaction level, type of soil minerals or texture as well as the implement parameters and its manner of motion. Many researchers have worked on major soil types such as sandy, clayey and loamy soils (Tables 1 and 5) but the overall outcome still seems unsatisfactory to utilize these "ndings in a systematic way. Design of "eld machinery and implements is based not only on the machine or implement performance but also includes knowledge of the behaviour of soils and soil}implement interactions. Table 1 summarizes research "ndings of soil}tool interaction studies conducted under di!erent test conditions in order to observe soil failure and deformation patterns and their outcomes. 0021-8634/01/050001#13 $35.00/0
2. Di4erent approaches Four major methods could be identi"ed in recent approaches to solve problems in the area of soil}tool interactions and failure mechanisms. Firstly, using empirical or semi-empirical formulae based on geometry, and static 1
2001 Silsoe Research Institute
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H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
Table 1 Summary of research 5ndings on soil failure and deformation in di4erent soils and under di4erent test conditions Reference
Tool condition
Soiltype/condition
Failure type
Kawamura (1952)
Inclined tool
Dry, frictionalcohesive
Progressive shear; avg. shear angle 313; curved shear surface
Mean shear angle increased with tool depth and reduced with increasing rake angle; shear angle as predicted
Selig and Nelson (1964)
Four wide blades Inclined (forward) Vertical Vertical
Dry sand
Failure angle greater than theoretical (45! /2)
-doDry, silty clay
Passive shear. Failure patterns as per passive pressure theory Passive shear
Inclined (forward)
Dry, plastic } clay
Tensile failure
Siemens et al. (1965)
Inclined and vertical tines
Dry, arti"cial soil
Progressive shear
Formation of soil blocks at uniform intervals
Hettiaratchi and Reece (1967)
Vertical tines 25 mm width, Aspect ratio 1}6 Inclined (forward)
Dry sand
Progressive shear
Stationary soil wedge for all aspect ratios
Dry sand
Progressive shear
Failure angle always less than (45! /2) -do-
Hettiaratchi and Reece (1974)
Varying rake angles Dry sand of 0}1803
Elijah and Weber (1971)
Inclined (forward) Flat blades: model; full size
Arti"cial soils: wet Four distinct failure modes: The inter-relationship was sand fric.-cohesive shear plane; bending; found due to changes in clay Field soils: silty clay tensile; #ow content, moisture and loam agric. silt texture loam (di!. moist.)
Godwin and Spoor (1977)
Inclined narrow tines
Dry sandy loam
Sta!ord (1981)
Narrow tine 40 cm width, 150 cm, depth Rake 453, 903 Rake 453 Rake 903 Rake 453
Clay
Harison (1982)
Inclined (forward)
Godwin et al. (1984)
Tines 25 mm wide, 453 rake angle
Moisture Moisture Moisture Moisture
Di!erent zones in wedge: Rankine; kinematic; complex-kinematic; transition
Remarks
Rajaram and Oida (1989)
3-D failure with crescent type for narrow tools Failure from shear to #ow as speed, moisture increase
18% 28% 28% 38%
Rigid-brittle failure -doFlow-type failure -do-
Below plastic range Above plastic range
Silty loam; moist. 18}20%
Progressive shear with formation of soil block
Inter-relationship between soil wedge and ridge
Compacted sandy loam soil
Soil failure function of Failure boundaries function of interaction between tines relative position of tines
Salokhe and Flat cage wheel lug Bangkok clay; Gee-Clough (1987) with angles 0, 15, moisture 49%; 30, 453 CI 140 kPa Rajaram (1987)
Progressive shear with stationary soil wedge
Variation of rupture geometry with rake angle and interface
Vertical tines; width 3, 12, 15 cm
Bangkok clay: moisture 5)2% 8)3% 18)6% 42)0%
Narrow/wide tines: vertical; inclined
Dry loose sand
Elliptical wedge formed contrary to theoretical failure Collapse-type failure Fracturing-type failure Chip-forming type Flow-type failure
Limitations of passive pressure theory for clayey soils Cyclic behaviour of soil forces with tine movement up to 28% moisture. Limitations of passive pressure theory
Collapse-type for wide; Collapse-type failure in frictional-type for narrow periodical manner for wide. Frictional-#ow-type for narrow tines
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Table 1*Continued Reference
Tool condition
Soiltype/condition
Failure type
Remarks
Sharma (1990)
Flat rigid tines; rake angle 15}1403; aspect ratio 4
Dry sand
Progressive shear; average shear angle 263
No distinct zones of soil failure as described by theory
Wang (1991)
Wide tines; rake angle 25}1253
Bangkok clay; moisture 44}52%
Plastic deformation
Formation of elliptical soil wedge for rake angle greater than 903
Salokhe and Pathak (1992)
Flat vertical tines
Dry sand
Progressive shear; shear angle (453! /2)
Failure patterns did not match with passiive pressure theory
Makanga (1997)
Flat tines with rake angles 50, 90, 1303; aspect ratio 0)5}3)3
Silty loam; moisture 5)2%, 21% and 33)5%
Progressive shear type: a!ected by tool parameters; failure in regularcycles
E!ect due to depth and width individually. No distinct zones as in passive pressure theory. Shape of rupture pro"le same for all rake angles
Jayasuriya and Salokhe (1998)
Flat narrow tines with rake angles 50, 90, 1303
Dry lateritic soil and two gradations; passing sieve no. 10 and 4
Progressive shear type; force}displacement signals in cyclic nature; true shape of 3-D failure geometry
Parabolic failure curves for frontal and in the furrow direction. True 3-D geometry not spherical as assumed in many previous models; lateral failure boundaries
Note: 3-D three dimensional: , soil}soil internal friction angle: CI, cone penetration resistance.
or dynamic conditions, has been described by many researchers in which the models have been developed for speci"c conditions (Tables 2}4). There is not a wellde"ned method to choose a model for a selected soil or other speci"ed conditions. The second method is the dimensional analysis approach (Table 2) in which the most critical aspect is to identify the critical model parameters and their form in the Pi-term. The third method is the use of "nite element method (FEM) (Table 2), by which prediction could be done without a preliminary assumption of soil failure pattern and it can calculate stress, displacement, velocity or acceleration of soil. This includes the approximation of de"ned constitutive relationship, with a selected model (e.g., linear-elastic, anisotropic-elastic, non-linear-elastic, elastic-plastic, strain-softening, cam-clay, etc.) characterizing important properties of some common soils and with soil}tool interface properties. The method has limitations for dynamic situations with successive failure common in tillage operations. The fourth method, the arti"cial neural network (ANN) modelling technique as sighted by Kushwaha and Zhang (1998), possesses the possibility and scope for modelling application by combining with the "nite element method (FEM) to simulate tine draught in dynamic situations, leading to computerized tool design.
The method seemed to be best for handling fuzzy variables but may have limitations for generalized models. Table 2 alone provides summarized details of research work reviewed by di!erent approaches and objectives.
3. Constraints and limiting factors Development of models for evaluating soil reaction forces, deformation and failure behaviour under quasistatic or dynamic conditions was given attention in recent studies. However, there is still no well-de"ned, generalized theoretical model to predict the behaviour of soil}tool interactions. Most of the studies have been carried out for selected or in situ soil conditions. Those models or results were not #exible enough to adjust or generalize to any other conditions. These limiting factors hinder the development of implement design, though they have a greater importance in improving e$ciency in land preparation process. Most of the research studies were based on theoretical failure mechanisms with empirical formulas by shear, or based on passive earth pressure theory, which are extensively used in civil engineering applications. However, the experiments done in agricultural soils have shown that soil failure can be shear, fracturing, chip forming or #ow and cannot be generalized for all soil types and
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H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
Table 2 Summary of soil+tool interaction studies and prediction models developed with di4erent approaches in chronological order Researcher Kawamura (1952) Soehne (1956) Payne (1956) Row and Barns (1961) Selig and Nelson (1964) O'Callaghan et al. (1964) Siemens et al. (1965) O'Callaghan et al. (1965) Hettiaratchi et al. (1966) Hettiaratchi and Reece (1967) Gill and Vandenberg (1967) Larson et al. (1968) Sirohi and Reaves (1969) Elijah and Weber (1971) Hettiaratchi and Reece (1974) Hettiaratchi and Reece (1975) Wismer et al. (1976) Godwin and Spoor (1977) McKyes and Ali (1977) Yong and Hanna (1977) Gee-Clough et al. (1978) Wadhwa (1980) Kuckzewski (1981) Sta!ord (1981) Harison (1982) Perumpral et al. (1983) Sta!ord (1984) Swick and Perumpral (1985) Godwin et al. (1985) Liu Yan and Homzhi-Min (1985) Salokhe and Gee-Clough (1987) Salokhe and Gee-Clough (1988) Rajaram (1987) Rajaram and Oida (1989) Thakur and Godwin (1989) Suministrado et al. (1990) Sharma et al. (1990, 1992) Chi and Kushwaha (1990) Wang (1991) Salokhe and Pathak (1992) Niyamapa et al. (1992) Hatibu and Hettiaratchi (1993) Zhang and Kushwaha (1995) Fielke (1996) Godwin and Wheeler (1996) Makanga et al. (1996) O'Dogharty et al. (1996) Wheeler and Godwin (1996) Desbiolles et al. (1997a) Hettiaratchi (1997) Jayasuriya and Salokhe (1998) Kushwaha and Zhang (1998) Desbiolles et al. (1999) Jayasuriya (1999) Desbiolles and Godwin (2000) Saunders et al. (2000)
Condition
Approach
Purpose
Inclined tines, e!ect of rake, shear Inclined tines, failure by shear Vertical narrow tines, all loamy soils Inclined tines, 2-D, adhesion term Wide blades, dif. angles, sand, clay Vertical plate tine, deep, shallow Nearly vertical tines, dry soil, passive Inclined plate tines, deep, shallow Wide blades, c- soil, 2-D, passive pre. Wide blades, c- soil, 3-D, plastic equilb. General application, optimisation Mouldboard plough, agric. soils Cultivator sweep design, 2-D failure Inclined tines, arti"cial/"eld soils Wide blades, c- soil, rupture geom. Wide blades, c- soil, boundary wedge Plane/bulldozer blade, disc, sand, clay Inclined narrow tines, c- soil, 2-D Narrow blades, rupture distance Plane soil cutting, 2-D general cases Mouldboard ploughs, di!erent soils Distorted model, general application Model plough bodies, soil parameters E!ect- tine geometry, speed, moisture Inclined blades, wedge formation Narrow blades, di!. depth width rake Di!. rake, speed e!ect, two failure modes Narrow blades, dynamic e!ect Concave discs, cutting, scrubbing Narrow blades, 3-D Single lug,2-D,BKK clay,wedge Multiple lug, 2-D, BKK clay, spacing Vertical tines, clay, di!. moist. four pattern Wide, narrow tines, sand, two patterns Force prediction models for rotary tools (review) Mouldboard plough, with assumptions Di!erent #at tines, sand, 2-D models General applications, 3-D Di!erent #at tines, BKK clay, 2-D Flat vertical tines, sand, shear failure Triaxial tests, silty and sandy loam Triaxial test, sand, loam, clay, four factors Blades/tines, cutting resistance, 3-D Sweeps, tool edge e!ect, 2-D Land anchors Di!erent #at tines, loam soil, 2-D Tilted spherical plough disc Single and multiple tines at high speed Standard tine, sandy loam and clay soils Concave discs, c- soil, 3-D Inclined tines, dry compressible soil, 3-D General applications, dominant factors Cone index used for draught prediction Flat tines, di!erent rakes, dry c- soil Use of soil strength factors Use of Mohr}Coulomb criteria and inertia
EX TH, EX TH, EX TH, EX EX TH TH, EX TH TH TH FEM, TH SIM, EX SIM, EX EX TH TH SIM TH, EX TH TH, FEM TH, SIM, EX TH, SIM TH, SIM EX EX TH, EX TH, EX TH, EX TH TH, FEM EX EX EX EX
FS PM PM PM FS PM PM, FS PM PM, FS PM, FS PM PM PM, FS FS PM, FS PM, FS PM PM, FS PM, FS PM, FS PM PM PM FS FS PM PM, FS PM PM PM, FS FS FS FS FS
* TH TH, EX TH, FEM EX, FEM EX EX EX TH, EX EX TH, EX EX TH TH,EX TH, EX TH, SIM EX TH, FEM, ANN TH,EX EX TH,EX EX
* PM PM, FS PM, FS FS FS FS FS PM PM, FS PM FS PM PM PM PM PM, FS PM, FS PM PM, FS PM PM, FS
Note: FS, failure study; PM, prediction models; TH, theoretical; EX, experimental; SIM, similitude; FEM, "nite element method; ANN, arti"cial neural network; c, cohesion; , soil}soil internal friction angle; c- ,frictional-cohesive soil, 2-D, two dimensional; 3-D, three dimensional; BKK clay, Bangkok clay.
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Table 3 Tillage force prediction models developed without considering dynamic e4ect Researcher
Tool/test condition
Remarks
Payne (1956)
Vertical narrow tools in sandy loam, silty loam, clayey loam
Used passive pressure theory, aspect ratio '2 considered as wide
Row and Barns (1961)
Inclined blades/tines
2-D failure, adhesion term included
O'Callaghan et al. (1964, 1965)
Vertical plate (1965), inclined (1965) deep and shallow
Horizontal and vertical rupture. Transition at depth/width ratio"0)6
Hettiaratchi et al. (1966)
Wide plates Rake angles vary (453! /2) to (1803! )
(1966) 2-D failure, (1967) 3-D failure, passive pressure (1974) 2-D failure, rapid calculation method taking plastic equilibrium
Godwin and Spoor (1977)
Narrow inclined tines Frictional-cohesive soil
Models for shallow, deep cases, 3-D and side crescent, wedge considered
Perumpral et al. (1983)
Narrow blades, extended for depths, widths, rake angles
No side wedge component, friction angle found function of failure angle
Sharma et al. (1990, 1992)
Flat tines, in sand Rake angles, aspect ratios varied
Two models for rake angles up to (903! ) and between (903! ) and 903
Zhang and Kushwaha (1995)
Narrow blades, tines Two ranges of rake angles
3-D failure, two models rake up to (903! ) and above that range
Desbiolles et al. (1997a)
Standard tine in sandy-loam and clay soils
Draught related to soil strength and tool geometry
Makanga (1997)
Narrow blades, loam soil
E!ect of aspect ratio, rupture pro"le
Hettiaratchi (1997)
Concave discs, for c- soils
3-D, rapid calculation method
Desbiolles et al. (1999)
Di!erent tillage implements
Use of cone penetrometer data
Jayasuriya (1999)
Flat tines, dry c- soils
E!ect of compaction, con"ne condition
Hettiaratchi and Reece (1967, 1974)
Note: c, cohesion; , soil}soil internal friction angle; c- , frictional-cohesive soil, 2-D, two dimensional; 3-D, three dimensional.
conditions (Salokhe, 1986; Rajaram & Gee-Clough, 1988; Sharma, 1990). It was revealed that the inadequacy of present theoretical considerations in generalizing the situation, since the e!ect of moisture content, stress history, cyclic failure mechanism instead of instantaneous failure could not be taken into account by the existing failure theories. Rajaram and Erbach (1997) also emphasized the importance of better understanding of true failure mechanisms and use of in situ soil properties in developing better theoretical models for tillage applications. According to the characteristics of soil or classi"cations de"ned in various standards, it is di$cult to "nd clear boundaries between some important properties of typical soils. There are still no clear methods to separate soils into homogeneous, isotropic or anisotropic categories by its measurable properties. It has also been found that some clay minerals behave in a di!erent manner
depending on their composition and moisture content. Some soils possess special properties that have not been considered in any model developed so far. Thixotropy, sensitivity, colloidal activity, dilation and sensitivity to stress history are some of the factors found to be di$cult for use in analytical approaches in developing theoretical models (Raymond et al., 1984; Terzaghi, 1959; Terzaghi & Peck, 1948). In order to understand general principles of soil failure mechanisms and corresponding theoretical considerations, it would be worthwhile to review research "ndings in the relevant areas of soil engineering as well as the area of soil failure principles under tillage operations. Better understanding of the mechanical behaviour of soils under di!erent situations, and soil failure and deformation mechanisms, would be a pre-requisite. The models developed so far are not in precise form with clearly justi"ed model parameters and so have limitations in
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Table 4 Tillage force prediction models developed considering dynamic e4ects Researcher
Tool/test condition
Remarks
Soehne (1956)
Inclined tool
Soil failure due to repeated shear. Acceleration component included
Siemens et al. (1965)
Nearly vertical tool Dry arti"cial soil
Used passive earth pressure theory. Acceleration component included
Sirohi and Reaves (1969)
Cultivator sweep similitude
Assumed soil failure in 2-D mode
Wismer et al. (1976)
Plane blade and disc systems Similitude method for clay and sand soils
Separate models for clay and sand considering frictional and cohesive properties of soil
Gee-Clough et al. (1978)
Similitude method for ploughs working on di!erent soils
Mouldboard ploughs were tested, good correlation obtained
Sta!ord (1981, 1984)
Inclined & vertical tool, clay Moisture 18}49% Speed 5 mm s\}5 m s\
Governing parameters; tool speed, moisture, tool geometry. Inertia component included
Suministrado et al. (1990)
Mouldboard plough
Mathematical model assuming a constant relative speed, ploughs bottom as stationary and soil slice in motion
Wheeler and Godwin (1996)
Single and multiple tines
Force prediction models at high speeds
Kushwaha and Zhang (1998)
Simulation by arti"cial neural network (ANN) method
Advantages of ANN method. Ability to model system input parameters having fuzzy and uncertain properties
Saunders et al. (2000)
Mouldboard plough
Models use Mohr}Coulomb criteria and inertia e!ects
Note: 2-D, two-dimensional; ANN, arti"cial neural network.
application. This is mainly due to the di$culties in quanti"cation of the mechanical behaviour of the soil mass in those applications attempted individually. Most of the theoretical models have been based on assumptions taken into account during the model development process. Due to the fuzzy behaviour under actual "eld conditions, a majority of the studies did not consider the dynamic e!ects and were done in laboratory soil bins. Many have used quasi-static conditions and simulations for dynamic condition. Tables 3 and 4 show the research work on development of tillage models with and without considering dynamic e!ects in the situation. Identi"cation of governing parameters and their exact form was found to be the most critical factor in this process where recent research work and developed models are concerned. The inter-relationship of some major parameters, and changes in application environments or test conditions restricted their use as common parameters. This has resulted in di$culties in modelling and generalization of models for application. Based on various situations, it can be expected that arrays of models can predict soil forces and failure patterns for di!erent conditions. This emphasizes the importance of better understanding of true soil properties and the exact
behaviour of agricultural soils before the modelling process. It can also be pointed out that consideration must be given to application requirements in the tillage operation, rather than modifying theories applied in civil engineering applications as mentioned by recent researchers (Rajaram & Erbach, 1997; Kushwaha & Zhang, 1998). It is also important to conduct future research covering major soil types and major areas, as described in standard soil classi"cation systems, in order to achieve the best outcome and completeness. Soils with special properties or characteristics should be given second priority based on the results and usefulness in application. Therefore, these factors justify the importance of rearranging all "ndings in a logical manner under well-de"ned categories, and the necessity of a database of di!erent "ndings and properties.
4. Rearranging research 5ndings on soil failure mechanisms and reaction models All research "ndings on soil deformation and failure mechanisms were rearranged in a tabulated matrix form, for di!erent soil types and conditions, in order to
Table 5 Summary of soil failure mechanisms categorized under di4erent soil types and conditions Condition
In frictional (sandy) soils
Wet=in the range of liquid limit
No research studies found
In frictional-cohesive soils Elijah and Weber (1971) Inclined (forward) #at blades (Silty clay loam, silty loam). Flow-type failure observed
In cohesive (clayey) soils Rajaram (1987) Vertical tines with varying width of 3, 12, 15 cm in Bangkok Clay soil at 42% moisture (PL, 25%; LL, 54%) Flow-type soil failure with hyperbolic behaviour of soil forces and limitations of passive pressure theory was observed Wang (1991) Wide tine with varying rake angles 25+1253 in Bangkok clay. Soil moisture 52% (LL, 54%) Plastic deformation (#ow-type soil failure) Formations of elliptical soil wedge in front of the tine for rake angles greater than 903 were also observed
Wet=Below or Closer to Plastic Limit
No research studies found
No other research studies found
Elijah and Weber (1971) Inclined (forward) #at blades in "eld soils (silty clay loam, silty loam) Four distinct failure patterns and interrelationship due to changes in clay content, moisture content and speed Bending-type failure observed
Staword (1981) Narrow tine with size 40/150 mm, two rake angles 45, 903 in clay Failure patterns changed from shear to #ow as speed, moisture increased Flow-type failure observed at 28 and 38% moisture, above the plastic range
Makanga (1997) Flat tines with rakes 50, 90, 1303, aspect ratios varying between 0)5 and 3)3, soil moisture 33)5%, (PL, 23%; LL, 51%) silty loam soil Plastic-type failure patterns; in repetitive and periodic manner and found a!ected by depth and width No distinct zones found as described in passive pressure theory. Shapes of the rupture pro"les the same for all rake angles
Rajaram (1987) Vertical tines, width of 3, 12, 15 cm in Bangkok clay at soil moisture 28.6% (PL, 25%; LL, 54%) Chip forming-type-soil failure with hyperbolic behaviour of soil forces and limitations of passive pressure theory
Elijah and Weber (1971) Inclined (forward) #at blades in "eld soils (silty clay loam, silty loam). Four distinct failure patterns an interrelationship due to changes in clay content, moisture content and speed Tensile-type failure observed
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Wet=above plastic limit
Wang (1991) Wide tine with varying rake angles 25+1253 in Bangkok clay at soil moisture 44% (PL, 25%; LL, 54%) Plastic deformation (#ow-type soil failure). Formations of elliptical soil wedge in front of the tine for rake angles greater than 903 Higher tool forces were obtained compared to the condition with soil moisture content of 52% Staword (1981) Narrow tine 40/150 mm two rake angles 45, 903 tested on clay Failure patterns changed from shear to #ow as speed, moisture increased Rigid-brittle-type failure patterns at 18% and 28% moisture
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Table 5*Continued Condition
In frictional (sandy) soils Elijah and Weber (1971) Inclined (forward) 6at blades in "eld soils (Silty clay loam, silty loam). Shear-plane-typefailure observed.
In frictional-cohesive soils Makanga (1997) Flat tines with rake angles 50, 90, 1303 and aspect ratios varying between 0)5}3)3, silty loam, soil moisture 21% (PL, 23%) Tensile-type failure patterns in repetitive and periodic manner. Failure patterns found a!ected by depth and width
In cohesive (clayey) soils Rajaram (1987) Vertical tines of width 3, 12, 15 cm in Bangkok clay at soil moisture 18.3% (PL, 25%) Fracturing-type-soil failure with cyclic behaviour of soil forces. Limitations of passive pressure theory
Harison (1982) Inclined (forward) tines on silty loam soil, moisture at 18}20% Progressive shear failure with formation of soil blocks. Interrelationship of soil wedge and ridge Kawamura (1952) Inclined (forward) 6at tools Rajaram et al. (1987) Vertical tines width of 3, 12, Selig and Nelson (1964) Four in dry, c- soil 15 cm in Bangkok clay at soil moisture 5)2% wide blades, inclined (forward) and vertical. Passive shear failure Progressive-shear-type failure with curved shear (PL, 25%). and failure patterns according to surfaces and the average shear angle of 313 Collapsing-type-soil failure with cyclic behaviour passive pressure theory. Failure Mean shear angle increased with tool of soil forces. Limitations of passive pressure angle found greater than depth, reduced with increasing rake angle. theory for clayey soils theoretical value of (453! ). Shear angle as passive pressure theory No other research studies found Hettiaratchi and Reece (1967) Vertical and inclined (forward) tines with 25 mm wide, aspect ratios 1}6 were used on dry sand Progressive-shear-type-failure with stationary soil wedge for all aspect ratios was observed
Selig and Nelson (1964) Four wide blades, inclined (forward) and vertical, in Dry slity clay and dry plastic clay soils Passive shear failure and failure patterns as passive pressure theory for vertical tines, tensile failure for inclined tines. Failure angle less than theoretical value of (453! ).
Hettiaratchi and Reece (1974) Tines with varying rake angles 0}1803 on dry sand. Wedge zones, Rankine, Kinematic, Complex-Kinematic, Transition identi"ed. Variation of rupture geometry with rake Sharma (1990) Flat rigid tines with rake angle 15+1403 and aspect ratio four on dry sand Progressive-shear-type failure with average shear angle of 263. No distinct zones of soil as described by theory. Variation of rupture distance with rake angle
Siemens et al. (1965) Inclined and vertical 6at tools in dry-arti"cial soils Progressive-shear-type failure patterns. Formation of soil blocks at uniform intervals for all tool rake angles tested Elijah and Weber (1971) Inclined (forward) 6at blades in "eld soils (Silty clay loam, silty loam) Shear-plane-type failure
No other research studies found
H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
Dry Soil
Salokhe and Pathak (1992) Flat vertical tines on dry sand Progressive-shear-type failure The failure patterns not match with passive pressure theory. Shear angle found (45! /2)
Godwin and Spoor (1977) Inclined narrow tines on dry sandy loam soil Progressive-shear-type failure with stationary soil wedge. three-dimensional crescent type failure mechanism for the upper part and twodimensional failure for the lower part.
Rajaram and Oida (1989) Narrow Makanga (1997) Flat tines with rake angles 50, 90, and Wide tines on dry loose 1303, aspect ratio varying 0)5}3)3 on dry sand. Collapse-type failure for (moisture 5)2%) silty loam soil. wide tines in periodic manner. Progressive-shear-type-failure in regular Frictional-6ow-type for narrow tines cycles. Failure patterns found a!ected by depth and width and there was no direct correlation with aspect ratio No distinct zones were found as described in passive pressure theory. Shapes of the rupture pro"les were the same for all rake angles Note. PL, plastic limit; LL, liquid limit; c, cohesion; , soil}soil internal friction angle; c- , frictional cohesive soil S O IL - TI N E M O D E LS
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H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
highlight gaps and to identify the possible mechanisms. It was found that there was a necessity to carry out tests to obtain further boundaries of failure mechanisms possible in cohesive and frictional-cohesive soils with moisture content between plastic and liquid limit. More precisely, the sticky limit of the soil could be considered as the point at which the failure mechanism could have a transition. Some investigations on scouring of implements dealt with sticky limit of soils. Among the studies on soil failure mechanisms, the identi"cation of four distinct failure patterns or mechanisms in all major soil types with the change of soil moisture content could be considered as the turning point of the study area. This was initially reported by Elijah and Weber (1971) and further studied by Sta!ord (1981, 1984) and later con"rmed by Rajaram and GeeClough (1988). Based on their observations four failure mechanisms could be identi"ed as follows: (1) &progressive-shear' or &shear-plane-type' failure in all the soil types under dry condition with cyclic variation in force}time curve; (2) &fracturing-' or &tensile-type' failure in frictional-cohesive and cohesive soils when the moisture content increases closer to the plastic limit, with cyclic variation in the force}time curves; (3) &chip-forming-type' or &plastic-type' failure in frictional-cohesive and cohesive soils when moisture content increased above the plastic limit with cyclic (low amplitude) hyperbolic shape of force}time curves; and (4) &-ow-type' failure in frictional-cohesive and cohesive soils when moisture content was closer to the liquid limit with a hyperbolic shape of force}time relationship. The progressive-shear- or shear-plane-type failure mechanisms were observed by many researchers who worked with dry frictional and frictional-cohesive soils. The force}time curve was of a cyclic nature. Sharma (1990), Makanga (1997) and Jayasuriya (1999), who worked with sandy, loamy and lateritic type soils, respectively, con"rmed these results. The speci"cations of the force}time curve, amplitude, frequency and wavelength were analysed in many studies. However, the relationships of these parameters between di!erent soil types and conditions have not been studied completely so far. Uniform frequencies and amplitudes were observed in frictional soils and became irregular when changed to frictional-cohesive and cohesive soils under dry condition. The e!ect of the parameters, such as soil compaction and soil gradation (Jayasuriya, 1999) on failure mechanisms, force}displacement signal speci"cations and soil reactions, showed the existence of de"nite relationships among parameters and failure modes.
Fig. 1. Analysis of force}displacement signal specixcations for comparison
Figure 1 shows the predictable variation of force} displacement behaviour for di!erent soils and with the wave speci"cations. The shape of the force}time curve changed from cyclic to hyperbolic when moisture content increased from a dry to wet-saturated condition. Rajaram and GeeClough (1988), Wang (1991) and Makanga (1997), each con"rmed the plastic- and #ow-type soil failures for different tines tested by changing moisture content closer to liquid limits, while working in Bangkok clay and silty loam soil respectively. In general, the numerical value of the moisture content does not show any direct relationship with the change of soil failure patterns in di!erent soils. However, the &consistency limits' or &Atterberg limits' (plastic limit, liquid limit, etc.) show clear boundaries of di!erent failure mechanisms in frictional-cohesive and cohesive soils. The exact boundaries seemed to depend on the soil particle size distribution, mineral types and the composition.
5. New categorization It can also be stated that most research work has been conducted either in dry soils or under wet-saturated conditions. The experiments conducted so far have not clearly identi"ed the critical boundaries of di!erent soil failure mechanisms or any other criteria, as in dry soils, for wet soil condition. But some zones, where di!erent soil failure mechanisms take place, can be identi"ed corresponding to the four failure mechanisms described previously. They are: (1) dry zone, in which soil tends to collapse or pulverize easily by shear; (2) wet, below (closer to) plastic limit, where fracturing- or tensile-type failure occurs; (3) wet, above plastic limit, where chip-formingor plastic-type failure occurs; and (4) wet, in the range of liquid limit, where #ow-type or plastic deformation occurs.
S O IL - TI N E M O D E LS
The soil reaction forces and the soil failure mechanisms seem to possess a relationship with increasing moisture content in frictional-cohesive and cohesive soils. Unlike the failure mechanism, the soil reaction force could be dependent upon many other factors involving soil parameters, tine parameters and experimental conditions. The e!ects of some parameters were investigated in many studies but the form or role of some have yet to be investigated. Table 5 shows a matrix of soil deformation and failure mechanisms arranged by major soil types: frictional, frictional-cohesive and cohesive for varying moisture contents. Based on the observations, when rearranged in matrix form, the following points can be identi"ed for discussion. The rearranged literature shows the approaches used in di!erent studies. It is revealed that the research studies have been conducted in the major soil types of sandy (frictional), loamy (frictional-cohesive) and clayey (cohesive), in order to study the overall picture of failure mechanisms and behaviour. However, the soil reaction prediction models could not be found in a precisely speci"ed manner, thus limiting applicability in di!erent conditions. It is clear that this problem could be solved by developing arrays of models under speci"cally de"ned categories involving soil types and conditions. Use of model parameters involving soil properties and characteristics in prediction models is found to be most critical. The form of the parameter representing the situation should be the best to provide optimum results. The process of summarizing and rearranging the literature in a logical way revealed many uncertainties in the area of failure mechanisms and tillage models. Lack of availability of raw data and details of experimental conditions were the most crucial ones. Most post-research recommendations by researchers were to eliminate approximations used during the modelling process and to look at the feasibility of application in actual "eld conditions. However, Rajaram and Erbach (1997) pointed out the necessity of identifying di!erent approaches for solving the problem of tillage modelling. Their major point was whether the classical soil theories used in soil engineering studies could be readily applicable to agricultural soils and applications where soil failure in an e$cient manner is the prime objective. Kushwaha and Zhang (1998) opted for di!erent theoretical approaches with updated technology. They proposed the arti"cial neural network (ANN) method, combined with "nite element method (FEM), leading to computer-aided tillage tool design as a promising solution. This seemed to be the best where time and speed are considered as in a dynamic situation. All work in the design stage will then be with computer simulations and no material waste will take place except for computer
11
time. Optimization could be possible in the design stage itself, up to a certain extent. Their recommendation is to use the dynamic e!ects in prediction models, tool wear and the non-uniformity conditions as an input to static or quasi-static prediction models without much di$culty.
6. Proposed research directions and scope (1) Changes in soil failure mechanisms when moisture content is closer to the sticky limit of the soil need to be studied. There could be another failure mechanism at the sticky limit of the soil at which the soil tends to stick over the surface of the tool causing changes in soil #ow patterns. (2) The draught force and the soil failure mechanisms seem to possess a relationship with increasing moisture content in frictional-cohesive and cohesive soils. (3) The feasibility of applying theories in the area of laminar #uid #ow mechanics to the behaviour of wet clay under tool motion could be studied. The e!ect on laminar #ow with stationary obstacles with di!erent shapes may be simulated the other way around for moving tools in wet clay. The same principle may be extended to well-graded frictional dry soils which also show laminar-(#ow-) type failure. Frictionalcohesive and partially saturated soils could be modelled under turbulent category. (4) The role and best form of some parameters in prediction models needs further evaluation and clari"cation in order to arrange them in the best "tting manner. (5) Studies on void removal mechanisms in soils under dry and unsaturated conditions and wet conditions due to tool movement and their e!ect on soil failure mechanisms and reaction forces need to be conducted. (6) Studies under dynamic conditions somehow need to be conducted in order to solve soil}tool interaction modelling problems in actual "eld conditions. Arrays of models could be expected under di!erent soil categories and need to tabulate systematically for easy use. (7) The arti"cial neural network modelling technique proposed by Kushwaha and Zhang (1998) seems a better solution for soil}tool interaction modelling because it can handle fuzzy and non-uniform input variables for the dynamic range of the problem. This should automatically produce a database of tillage parameters and arrays of prediction models for di!erent soil and test conditions. Arti"cial neural network models need training with series of data collected from each category to de"ne hidden parameters inside the model.
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H . P. W . JA Y A SU RI YA ; V. M. S AL O K H E
7. Conclusions Review of research "ndings on soil}tool interactions revealed many important factors and gaps that could be used to direct future research towards new tasks and with new methods. Obtaining arrays of tillage models for speci"c conditions instead of generalized models, formation of a database of tillage parameters and use of arti"cial neural networks and "nite element modelling techniques and other new approaches is considered as promising recommendations for rapid improvement in the research area. It is also revealed that the slow development in the area for the past few decades was due to individual approaches by various researchers on speci"c targets and arbitrarily selected test conditions. An e!ort to include all important research "ndings relevant to the area is made. However, any further relevant "ndings that are not presented or discussed here could be combined for the advancement of the research area.
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