DEM Simulation and Experimental Investigation of Draft-Reducing Performance of Up-Cutting Subsoiling Method Inspired by Animal Digging

Abstract

Overcoming high draft forces has long been a primary challenge in conventional subsoiling. To better utilize this agronomically advantageous technique, it is necessary to substantially reduce the draft. Inspired by the digging behaviors of fossorial animals, a low-draft up-cutting subsoiling method was proposed in this study. Discrete element method (DEM) simulations were employed to study the draft-reducing performance of up-cutting tools compared with regular tools. The results showed that the up-cutting motion reduced the draft by 63.07%, 63.84%, and 58.92%, respectively, at rake angles of 45°, 60°, and 75%, and by 79.73%, 63.84%, and 45.22%, respectively, at advancement velocities of 0.5 m·s⁻¹, 1 m·s⁻¹, and 1.5 m·s⁻¹. An increase in up-cutting velocity reduces the draft. Soil disturbance, particle velocity distribution, and soil deformation and movement patterns change in ways that contribute to this reduction. The draft-reducing performance of a chain subsoiler developed based on the principle of soil-breaking by animal digging was verified using field tests, exhibiting a draft-reduction amplitude approaching or greater than 30%. This study shows the great application potential of the up-cutting method in reducing subsoiling drafts and provides a theoretical basis for future research.

1. Introduction

For a subsoiler, the most significant aspect of the soil–tool interaction pertains to the soil-cutting procedure executed by a horizontally advancing tool (Figure 1). The high draft demand associated with this process has been a major constraint on the scale of its operation [1,2]. There is an urgent need to substantially lower the subsoiling resistance. This necessity is driven not merely by the motive to cut costs but also by the electrification and automation trends in agricultural machinery, along with the indispensable requirements for efficiency and safety [3].

Existing draft-reducing strategies mainly emphasize minimizing the friction and adhesion of the tool. Methods such as soil–tool interface lubrication [4,5] and using non-stick materials [6,7,8], biomimetic non-smooth surfaces [9,10,11], and vibration [12,13,14] have been extensively studied. These methods rest upon the presumption that soil that is in contact with the tool surface will always move along the tool surface (Figure 1c) [15,16]. However, because of the intrinsic constraints of the interface kinematics, when the tool surface is “synchronized” with the boundary wedge (a soil buildup hardly moves relative to the tool, Figure 1d) created by the soil in the failed state, the actual draft-reducing performance is weakened [17,18,19,20]. This is essentially because these methods fail to alter the patterns of soil failure and movement. To achieve a further reduction in the draft force, more radical strategies must be adopted to avoid soil clogging.

For many fossorial species, the intense demands of survival force them to be avid diggers. The intense digging behavior and sophisticated digging techniques of these “ecosystem engineers” have a huge impact on the ecology and environment [21,22]. These digging techniques mainly involve the control of digging tools to operate on the soil, as well as the soil reactions that result from such operations. Clearly, animals’ digging movements are not accomplished through parallel movements of their claws (or other body parts), but rather by a combination of complex, compound movements [23,24,25,26]. Learning from their digging techniques can provide insights into the design of low-draft subsoilers. Integrating the useful aspects of these techniques into the design of subsoilers is likely to be the optimal solution to the challenge of overcoming high draft forces in subsoiling.

A special device configuration with surface components (or elements) moving upwards during horizontal motion appears to offer greater potential to reduce drafts, possibly sharing similarities with animal digging. This motion combination has been verified to be effective in reducing drafts through powered discs, where the draft force decreased by more than 30% compared with free-rolling discs [27,28,29]. The draft-reducing performance of this combination has also been demonstrated in many other types of tools [30,31,32,33]. Subsoilers are characterized as narrow-shaped implements working at a depth substantially greater than their width. To enable the aforementioned tool motion within such subsoiling depths, an up-cutting continuous belt, as defined by Mellor [30] and other researchers, can be utilized. When the rake angle of the up-cutting continuous belt is less than 90°, there exists a high probability that it mimics the soil failure mechanism induced by animal digging. Previous research by the authors has tested a combination consisting of a subsoiler and an up-cutting motion [34]. Since the patterns of soil failure and movement change, the draft reduction is more than 50%.

However, the existing theoretical frameworks on continuous up-cutting devices (e.g., chain trenchers) fail to apply to the analysis of the subsoiling scenarios. This is due to the fact that the kinematic and dynamic analyses related to the continuous belt device pay insufficient attention to the soil failure ahead of the tool, let alone facilitate comparisons with the draft-reducing performance of conventional subsoilers [30,31,32,33]. Due to the lack of relevant research, explanations of the influencing factors and mechanisms of the draft force in up-cutting tools are incomplete. Therefore, this concept needs to be tested and further validated using more intuitive and accurate methods for draft-reducing ability.

Verifying the concept from a theoretical perspective using the discrete element method (DEM) and conducting tests with prototypes are appropriate choices for validating this concept. The DEM can effectively replicate the mechanical properties of granular materials [35,36] and avoid many drawbacks seen in experimental methods [37,38,39]. Its applicability and reliability have been widely verified [10,13,35,36,40,41,42,43,44,45], which serves as a guarantee for the successful theoretical analysis of up-cutting subsoiling. Designing and manufacturing prototype up-cutting tools and testing their draft-reducing performance and tillage quality under actual working conditions can make up for the deficiency of relevant machinery lacking practical applications in up-cutting subsoiling [30,32,33]. This provides valuable references and the most persuasive evidence for the feasibility of applying the above-mentioned concept to subsoiling.

This study aimed to explore an innovative method by which the draft force during subsoiling operations could be substantially reduced. To this end, this paper first elaborates on the principle of soil-breaking in animal digging and the concept of the up-cutting method. Subsequently, through discrete element method (DEM) simulations, a comparison was made between the soil-cutting models of up-cutting tools and conventional tools. Factors, such as the rake angle, advancement velocity, and up-cutting velocity, were considered, and their effects on the tool force, disturbance range, particle velocity distribution, soil deformation, and movement patterns were analyzed to reveal the draft-reduction performance and underlying mechanisms. Finally, field tests were conducted on a chain subsoiler designed and manufactured based on this principle to verify whether the up-cutting tools could significantly reduce the subsoiling draft and meet the quality requirements of subsoiling. This study provides enlightening insights regarding the reduction of the draft force in subsoiling. It also provides a theoretical basis for subsequent research, as well as scientific guidance for the application of a continuous up-cutting motion in subsoilers and other earth-moving devices.

2. Materials and Methods

2.1. The Principle of Soil-Breaking Inspired by Animal Digging

The general behavior related to animal digging is complex and random. However, a common feature of the digging behavior of many underground (or fossorial) species is the use of alternating patterns of soil-breaking and soil transport (Figure 2a) [22,23,25,46,47]. Their general digging behavior shows an alternating sequence of soil-breaking (scratch digging and chisel tooth digging) and the transport of soil (forelimb raking, hindlimb kicking, turn-around pushing, head-lift pushing, or shoveling). Scratch digging with the forelimbs is a typical soil-breaking motion.

Observation of the forelimb digging motions shows that each forelimb digging cycle can be summarized into four phases: penetrating, cutting, retracting, and resetting (Figure 2b) [48]. The forelimb digging cycle begins in the penetration phase (Figure 2b(I)), where the claws move forward to penetrate the soil, similar to the action of a curved hook piercing an object. Subsequently, it enters the longest cutting phase (Figure 2b(II)), where the claw tip begins to move along an almost straight trajectory below the ventral side of the sagittal plane, cutting the soil from the inside out. In the subsequent retracting stage, the entire forelimb begins to retract backwards and upwards from the soil (Figure 2b(III)). In the final resetting phase, the claws have detached from the soil, moved forward and upward, and returned to the vicinity of the initial penetration position, thus ending a single forelimb digging cycle (Figure 2b(IV)). The most important feature among these motions is the cutting direction from the inside out during the cutting phase. At the same time that the animal performs this motion, the affected soil separates from its original position, transforming from well-structured hard soil to loosened and fragmented soil, creating the necessary conditions for subsequent soil transportation. Then, animals will transport the soil in a timely manner during the digging process in order to reduce soil accumulation, lower the difficulty of subsequent digging, and ensure the smooth progress of digging work.

The main mechanical process of digging can be seen as a complex combination of penetration and cutting [49]. The interaction between simple flat tools when cutting soil in parallel can serve as the basis for understanding this process [15]. When a simple flat tool cuts the soil in parallel, the soil is mainly subjected to the force caused by the horizontal forward movement of the tool (Figure 3a). As a result, continuous compressive stress causes the deformation of the soil as well as forward displacement, which then accumulates in front of the tool. When the stress periodically exceeds the limit, repeated shear failure occurs (Figure 3a(s1)) [50]. Further movement of the tool will trigger a new failure process in front of the tool (Figure 3a(s2)).

However, when the tool moves outward from the inside of the substrate like a claw, the soil is subjected to tool forces mainly in an outward direction (Figure 3b). The direction of principal stresses is deflected toward the digging surface, producing tensile failure at the tool tip. Moreover, lower compressive stresses in the substrate are produced as the soil moves freely toward the free surface (digging surface) [16,34]. Tensile failure exploits the inherent weakness of the soil when subjected to tension [50], while shear in a low-stress state has been shown by critical-state soil mechanics to be the only condition that loosens the soil [19,51]. These two mechanical soil effects allow the soil to easily separate outward from the digging surface, avoiding non-essential periodic shear work and thereby reducing the force on the tool. Therefore, applying force from the inside of the soil to the outside may be an important factor that explains why animals have adopted these digging techniques in order to reduce labor.

The main process of subsoiling is similar to the forelimb digging motion of animals. The process of the horizontal movement of the subsoiler (Figure 3a) corresponds to the cutting phase in the digging cycle (Figure 2b(II)). The difference between the two is that animal claws cut the soil from the inside of the soil towards the outside of the free surface; the movement direction of subsoiling parallels the free surface of the undisturbed soil. Therefore, if the continuous cutting of the soil by the subsoiler can also make the soil movement move towards the outside of the free surface of the soil (Figure 3b), it can produce a similar effect as claw digging. This is difficult to achieve for a subsoiler that cuts soil through parallel motions, so it is necessary to change the structural design of existing subsoilers in order to apply the principle of animal digging to subsoiling.

The techniques of animal digging offer valuable ideas for studying the biomimetic design of subsoilers. Understanding the principles of these digging techniques and applying them to the design of subsoiling machinery potentially represents a more effective way of lowering the draft of subsoiling. This is also the main goal of the imitation of animal digging techniques. The key is to control how the tool moves in the soil and to make changes to the direction in which the tool applies force to the soil. This way, only a small force is needed to cause soil failure, thus greatly reducing the draft force in subsoiling.

The authors carried out DEM simulations of the tools, simultaneously executing both an up-cutting motion and a horizontal motion in the soil [34]. Preliminary results confirmed that such a tool–motion pattern induces the soil movement described above and effectively decreases the draft force encountered during soil cutting. In light of this, a more thorough and profound analysis regarding the concept was performed in the present study.

2.2. DEM Simulation of Up-Cutting Tools

The commercial software EDEM 2018 (DEM Solutions, Ltd., Edinburgh, UK) was employed to develop simplified models of up-cutting tools. Simulations were carried out on the soil failure process under different tools and operating conditions. The aim was to deeply analyze the mechanism by which the up-cutting motion reduces the draft force in subsoiling.

2.2.1. DEM Modeling

In the simulation, the tools were modeled as a series of simple, narrow tines. These tools could function directly as regular tools. When an up-cutting motion was added to their surfaces, they could also serve effectively as up-cutting tools (Figure 4a). The purpose of employing simple tool geometries in the analysis was to comprehensively understand the interaction between the soil and the tool moving in this manner. This understanding could then serve as the basis for a subsequent analysis of more complex conditions [6]. The tine geometries were defined using the following parameters: rake angle (α), width (w), and depth (d), where the width is 45 mm and the depth is 300 mm. Other settings relating to operation factors included the tool advancement velocity (v_t) and up-cutting velocity (u_t). The parameters were set according to the factors and levels in the subsequent experimental design.

Based on simulation experiences, a soil bin model was employed to hold the soil (Figure 4b). Its length, width, and height were specified as 1100 mm, 650 mm, and 600 mm, respectively. These dimensions were sufficient to accommodate the soil disturbance generated by the tools and were adequate to allow the tools to be in a stable cutting state [15,44].

The tine model was placed within the symmetry plane of the soil bin according to the designated cutting operation parameters. A positive x-axial translational constraint was implemented on each tool to uphold a constant advancement velocity. For each up-cutting tool, an extra conveyor belt constraint along the front surface of the tool was established. The constraint enables the tool to perform the up-cutting motion (Figure 4a). In an effort to prevent any possible confusion, the coordinate system of the DEM model is explained as follows: the x-axis, y-axis, and z-axis correspond to the longitudinal, transversal (or lateral), and vertical directions, respectively (Figure 4b).

The soil–tool interactions were specified by particle, material, and contact parameters in the EDEM software. These modeling parameters were set with reference to published data, which have been verified to be reasonable. Studies have indicated that even when a particle size larger than the actual size is employed, a reasonable simulation can still be carried out [37]. Therefore, the particle model was set to 3 mm spheres, which significantly improved the computational efficiency under limited computational power. The material properties were specified by the parameters of density (ρ), Poisson’s ratio (γ), and shear modulus (G). The Hertz–Mindlin non-slip contact model with Bonding was defined by parameters including the coefficient of restitution (e), friction coefficients (static ${\mu }_s$ and rolling ${\mu }_r$), stiffness per unit area (normal $\overline{K_n}$ and shear $\overline{K_s}$), critical strength values (normal $\overline{\sigma }$ and shear $\overline{\tau }$), and bond disk scale (R_m). The parameter combination defines a rule for the interaction between 65 Mn steel and typical hardened black loamy soil (clay 9.6%, silt 44.0%, and sand 46.4%) [44]. The specific values are presented in

Table 1. Model parameters and values specified in the EDEM software.

Category	Parameter	Value
Particle	Radius/mm	3
Material	Density of particle/(kg·m−3)	2620
	Poisson’s ratio of soil	0.33
	Shear modulus of soil/Pa	1.02 × 108
	Density of steel/(kg·m−3)	7850
	Poisson’s ratio of steel	0.3
	Shear modulus of steel/Pa	7.9 × 1010
Interaction	Coefficient of restitution, soil–soil	0.53
	Coefficient of static friction, soil–soil	0.63
	Coefficient of rolling friction, soil–soil	0.42
	Coefficient of restitution, soil–steel	0.27
	Coefficient of static friction, soil–steel	0.54
	Coefficient of rolling friction, soil–soil	0.28
	Bond Normal Stiffness/(N·m−3)	5 × 107
	Bond Shear Stiffness/(N·m−3)	5 × 107
	Bond Normal Strength/(Pa)	21,680
	Bond Shear Strength/(Pa)	21,680
	Bonded Disk Scale	0.5

.

The simulations ran at appropriate time steps, with a Rayleigh time step set to 5.13 × 10⁻⁵ s and a fixed time step set to 2.57 × 10⁻⁶ s, which was 5% of the former. The total simulation duration was 1 to 2 s (depending on the advancement velocity).

2.2.2. Experimental Design of the DEM Simulation

In the simulation, the simple and intuitive one-factor-at-a-time (OFAT) method was adopted. Each time, only the level of one factor changed, while all other factors remained constant. By comparing the response values at different levels, the independent effect of each factor was quickly verified (without considering the interactions between different factors on the response value for the time being). In addition, another purpose of using this experimental design was to explore the microscopic changes occurring in the soil medium when the response value (draft force) changed due to the variation of a single factor. Based on this, the mechanism by which the up-cutting motion reduces the draft force was analyzed.

As mentioned previously, the factors investigated in this study include the rake angle, the advancement velocity of the tool, and the up-cutting velocity. The settings of factors and levels are presented in

Table 2. Factors and levels in the simulation.

Level	Factor
	Rake Angle (α)/°	Advancement Velocity (vt)/m·s−1	Up-Cutting Velocity (ut)/m·s−1
Low	45	0.5	0.5
Medium	60	1	1
High	75	1.5	1.5

. Rake angles of 45°, 60°, and 75° were set based on the inclination of subsoiler shanks. The advancement velocities were set at 0.5 m·s⁻¹, 1 m·s⁻¹, and 1.5 m·s⁻¹, in accordance with actual field subsoiling speeds. The up-cutting velocities, based on the ratio to the 1 m·s⁻¹ basic advancement velocity, were set at 0.5, 1, and 1.5 (i.e., 0.5 m·s⁻¹, 1 m·s⁻¹, and 1.5 m·s⁻¹). When analyzing the variation of any factor, other factors were held constant: 60° rake angle, 1 m·s⁻¹ advancement velocity, and 1 m·s⁻¹ up-cutting velocity.

In addition, based on the above parameter combinations, the up-cutting velocity was set at 0 m·s⁻¹ for the purpose of simulating the process of soil cutting by regular tools. The purpose was to compare the performance differences between up-cutting tools and regular tools under different parameter combinations.

2.2.3. Measurements in the DEM Simulation

(1)

Tool Force and Soil Disturbance

The tool force and soil disturbance are the two main aspects of evaluating the tool’s performance. The magnitude of the tool force and the range of the soil disturbance increase as the tool advances, until the tool is in a stable cutting state [15]. Subsequently, they begin to exhibit periodic fluctuations within a relatively constant range, which is determined by the nature of the soil–tool interaction [15,44]. A complete failure boundary forms when the tool force is at its minimum [52], and the disturbance range at this moment is typical. In EDEM’s Analyst module, during the stable cutting state, the tool force was acquired over a period of time. Then, the average values of the draft force (F_x) and vertical force (F_z) were calculated. Meanwhile, the soil disturbance was acquired specifically at the moment of the minimum tool force. The disturbed particles are shown in a color sequence (based on a particle velocity of 0–1 m·s⁻¹) to differentiate them from undisturbed particles. Then, cross-sections parallel to the y-z plane were captured at 50 mm intervals within the disturbance range (Figure 4b). These sectional images were superimposed using the “stack” function in ImageJ software (V.1.51J, National Institutes of Health, Bethesda, MD, USA) to acquire the transversal disturbance characteristics (Figure 4c). The image of the clip in the symmetry plane was captured to acquire the longitudinal disturbance characteristics (Figure 4b,c). The disturbances were quantified by the color sequence, and the disturbance profiles were mapped.

(2)

Particle Velocity Distribution

The level of difficulty that the tool contends with during its advancement through the soil is reflected in the tool forces and soil disturbance. This aspect can be more precisely described through the velocity distribution of particles in the longitudinal section at the point of the minimum tool force. Consequently, to explain the mechanism of the tool–force change, the particle velocity distribution was analyzed.

Through the use of longitudinal disturbance images, analyzing the magnitude distribution of particle velocity became straightforward, since the particles were already color-coded according to velocity. By using color-coded particle motion velocity, the motion distribution characteristics of particles are represented by the motion trajectory of particles within 0.2 s. These trajectories are depicted by lines, with the color of each line representing the particle’s motion velocity (where red indicates that the particle velocity is close to or exceeds 1 m·s⁻¹, and dark blue indicates that the particle velocity is close to rest). At the same time, the length of the lines also represents the motion velocity of particles. The longer the lines, the greater the displacement of particles within 0.2 s.

(3)

Deformation and Movement Patterns

With the advancement of the tool, the soil is subject to changes in shape and position [15,53]. The patterns of such deformation and displacement alter in response to changes in the kinematic conditions of the interface. Thus, to further elucidate the mechanism of tool–force variation, the longitudinal deformation and movement of the soil were analyzed in detail.

At the start of soil cutting, a square-shaped slice section in the symmetry plane, measuring 100 mm × 100 mm, was marked red. During the cutting process, images of the slice were captured at time intervals of 0.1 s, and then superimposed utilizing ImageJ. The red square section served to demonstrate the soil’s deformation and displacement, with its changing positions and forms reflecting the movement of the tool.

2.3. Field Test

Based on the principle of up-cutting, a prototype of a chain subsoiler was designed and manufactured, and then field tests were carried out to verify if the draft performance of the chain subsoiler adopting the up-cutting principle could be improved and whether the tillage quality is fulfilled under field conditions.

2.3.1. Site and Equipment

The chain subsoiler consists of components such as the universal joint coupling, right-angle reducer, drive sprocket, driven sprocket, active and passive subsoiling sprockets, shank, tine, subsoiling chain, and cutting teeth (Figure 5). When the tractor travels, these components work together through a series of transmissions to drive the subsoiling chain to cut upwards, thereby loosening the soil (Figure 5a).

Although there were differences in the mechanical structures between the chain subsoiler and the standard conventional subsoiler, the external dimensions of the chain subsoiler were designed to be similar to those of the standard conventional subsoiler (Figure 5b). This design aimed to more effectively compare the differences in the resistance performance presented by the two due to their different operations.

In October 2024, the field tests were carried out in the agricultural experimental site of Jilin Agricultural University. This site location lies within the black soil region of northeast China. The soil is loam with a moisture content of 18%, a soil firmness of 300–500 kPa, and a bulk density of 1.32 g·cm⁻³, which is suitable for testing. A KUBOTA M704 tractor (KUBOTA Agricultural Machinery (Suzhou) Co., Ltd., Suzhou, China) was used to provide traction power. A field mechanical test system (Bona Science & Technology Co., Ltd., Harbin, China) (Figure 5d–f), designed for field experiments, was used to measure the draft force. This system dynamically measures the load attached to the equipment frame through sensors integrated on the three-point linkage and wirelessly transmits the measured data to a computer for storage. With a sensitivity of 0.045 kN, the upper limit of this system reaches 15 kN. Its maximum measurement range is 15 kN, and the sensitivity is 0.045 kN. In the control experiments, a standard subsoiler meeting the Chinese subsoiler standard JB/T 9788-2020, equipped with a medium-sized shank and a chisel tine, was adopted.

2.3.2. Experimental Design of the Field Test

A comparison was made between the draft performance of the conventional subsoiler and the chain subsoiler. The tillage depth was set to 0.25 m, 0.35 m, and 0.45 m; the advancement velocity was set to 3 km·h⁻¹, and the rate of the rotation of the power take-off was set to 540 rpm (the corresponding linear velocity of the subsoiling chain was approximately 1.6 m·s⁻¹, which is the up-cutting velocity). These parameters were set by referring to the operational parameters of the conventional subsoiler in field practice, considering the tractor’s power and safety.

2.3.3. Measurements in the Field Test

(1)

Tillage Force Measurement

When the field tests started, the subsoiler was pulled through the starting zone (10 m), the measuring zone (50 m), and the decelerating zone (10 m) steadily in sequence. Throughout the process, the mechanical test system constantly monitored the draft force data. The data collected in the measuring zone were taken as valid data due to their relatively stable features. However, given that the force in the stable cutting stage fluctuates periodically, the resulting draft force for a single working condition was determined by the average value of three repetitions of one measurement in the stable cutting stage.

(2)

Furrow Profile Mapping

Furrow profile mapping is an essential part of field tests and is necessary to verify the effect of soil disturbance. A furrow profile in the stable zone was selected and dug up at three random spots. The shape of the soil disturbance was determined using a furrow profiler. The effect of the soil disturbance was represented by analyzing the profiles.

3. Results

3.1. Results of the DEM Simulation

3.1.1. The Effects of the Rake Angle and Up-Cutting Motion

The tool force, as affected by the rake angle, was revealed by the simulation, as depicted in Figure 6. During the increase in the rake angle from 45° to 60° (reference level), and then to 75°, the draft force of the regular tools (Figure 6a) increased from 1344.50 N to 1655.21 N and 2084.81 N, while the up-cutting tools obtained values of 496.457 N, 598.59 N, and 856.469 N, which were 63.07%, 63.84%, and 58.92% lower than the values of the regular tools, respectively. These results clearly demonstrate that the draft force is significantly reduced under the influence of an up-cutting motion, and this reduction is even more drastic with the increase in the rake angle. In addition to the draft force, the vertical forces of the tools (Figure 6b) also showed distinct characteristics during this process. The vertical force of the regular tools changed from −480.93 N to −286.18 N and −56.79 N, while the up-cutting tools obtained values of −529.36 N, −472.26 N, and −436.65 N, respectively, increasing by 10.07%, 65.02%, and 668.89% in terms of absolute values compared with regular tools. Although the downward vertical force increased, the increased vertical force level remained within the conventional range, which was reasonable and acceptable. Compared with regular tools, the up-cutting tools showed relatively gentle changes in vertical force and were always at a higher level. These results indicate that the downward vertical force tends to increase due to the up-cutting motion, especially under larger rake angles. The above-mentioned force change was closely related to the corresponding soil disturbance, particle velocity distribution, and soil deformation motion patterns.

The soil disturbance, as affected by the rake angle, was also shown by the simulation (Figure 7a). For both regular tools and up-cutting tools, there are two main types of disturbance: longitudinal and transversal. The longitudinal disturbance, presented in the main figure, exhibits a shape similar to an approximate triangular area. This area is enclosed by the tool surface, the soil failure surface, and the uplifted soil surface. Meanwhile, the transversal disturbance, shown in the sub-figure, is a composite of uplifted soil aboveground and an inverted triangular area underground. During the increase in the rake angle from 45° to 60° (reference level), and then to 75°, the disturbance characteristics of regular tools and up-cutting tools showed similar trends. The shape of the longitudinal disturbance zone changed from a flat triangle to a narrow triangle, and the inclination of the failure surface increased. The height of the uplift, the distance of soil failure along the tool-moving direction, and the lateral disturbance area increased, but the longitudinal disturbance area decreased. These changes may be attributed to the fact that, as the rake angle increases, the soil changes from being mainly “lifted” to being more “pushed”, resulting in the corresponding disturbance characteristics mentioned above. Compared with regular tools, up-cutting tools increased the height of the uplift. However, they reduced the transversal and longitudinal disturbance areas, the failure distance, and the disturbance width. In addition, the failure strip produced by up-cutting tools (the dark blue soil particle strip in the figure) was straighter. This may be because up-cutting tools exert a stronger upward force on the soil during operation. This enhances the “lifting” effect and weakens the horizontal compression in the soil.

Figure 6 also shows the value of specific resistance, which is calculated by dividing the draft force by the area of underground disturbance, as presented in Equation (1) [54].

$$K_f={\displaystyle \frac{F_x}{A_f}}$$

(1)

where K_f represents the specific resistance (kN·m⁻²) and A_f represents the area of underground disturbance.

As shown in Figure 6, with the increase in the rake angle, the K_f value of the regular tools increases from 20.12 kN·m⁻² to 21.74 kN·m⁻² and 27.84 kN·m⁻². However, the K_f values of the up-cutting tools are 9.25 kN·m⁻², 10.73 kN·m⁻², and 15.07 kN·m⁻², which are much lower than those of the regular tools. The specific resistance of subsoiling reflects the draft requirement of a subsoiler to cause a given area of underground disturbance (transversal). A decrease in specific resistance leads to an increase in subsoiling efficiency [44]. The results suggest an improvement in subsoiling efficiency in the case of up-cutting.

The disturbance of the soil by the tool is closely linked to particle movement. The simulation results (Figure 7b) clearly showed that the rake angle of the tool had a significant impact on particle movement. When the tool rake angle gradually increased from 45° through the reference value of 60° to 75°, the changes in particle movement characteristics induced by both types of tools exhibited similar trends: Near the tool surface, the horizontal component of the particle velocity increased, and the particle velocity increased, which are both shown as long red lines in the figure. Near the failure surface, the particle velocity was relatively low for both types of tools, presented as short blue lines. In the middle transitional part of the disturbance zone, the particle velocity was at a medium level, indicated by green lines, and its movement direction was between that near the tool surface and that near the failure surface. These changes strongly demonstrate the transition of the soil’s stress state from being mainly “lifted” to being more “pushed”. Upon further comparisons between regular tools and up-cutting tools near the failure surface, although the particle movement direction of the up-cutting tool is deflected, the degree of deflection was small, and most particles move in an upward-sloping direction. The particle movement direction of regular tools was also deflected, but particles in the deeper part mainly moved horizontally forward, while those in the shallower part mainly moved upward at an angle. This difference indicates that the up-cutting tool strengthens the “lifting” effect and relatively weakens the “pushing” effect.

The deformation and movement of the square slices in the soil can be used to explain the changes in the forces exerted on the tools by the soil (see Figure 8). As the rake angle of the tools increased, under the action of both regular and up-cutting tools, the longitudinal compression of the slices intensified (became thinner longitudinally), and both the forward-pushing distance and upward-lifting height increased. However, compared with regular tools, the up-cutting tools made the slices thinner and caused forward rotation. In addition, the up-cutting tools lifted the slices to a greater height. This indicates that up-cutting tools can lift the soil upward, thus reducing the draft force. Notably, when the rake angle is small, up-cutting tools push the slice a shorter distance compared to regular tools. This indicates that the up-cutting motion can partially counteract the forward-pushing effect of the tools on the soil under such an angle.

In summary, the changes in tool forces are partly attributed to the variations in the soil disturbance area, the redistribution in particle velocity, and the pattern alteration in deformation and movement. The increase in the rake angle transforms the effect of the tools from “cutting” to “pushing” [35,45,54]. This implies changes in both the direction of the tool’s force on the soil and the direction of soil movement induced by these forces [1]. Consequently, the re-established soil equilibrium gives rise to an increase in tool force. However, the up-cutting motion modifies the disturbance area and particle velocity distribution, as well as the patterns of soil deformation and movement. It effectively mimics the soil behavior resulting from the digging actions of animals. These changes all play a part in decreasing the draft force.

3.1.2. The Effects of Advancement Velocity and Up-Cutting Motion

The tool force, as affected by the advancement velocity, was revealed by the simulation, as depicted in Figure 9. During the increase in the advancement velocity from 0.5 m·s⁻¹ to 1 m·s⁻¹ (reference level), and then to 1.5 m·s⁻¹, the draft force (Figure 9a) of the regular tools increased from 1311.04 N to 1655.21 N and 2135.86 N, while the up-cutting tools obtained values of 265.79 N, 598.59 N, and 856.47 N, which were 79.73%, 63.84%, and 45.22% lower than those of the regular tools, respectively. It can be observed that as the advancement velocity increased, the draft forces of both regular tools and up-cutting tools showed an upward trend. Although both are increasing, the draft force of the up-cutting tools was significantly lower than that of the regular tools. Moreover, as the advancement velocity rose, the percentage difference of the draft force between the up-cutting tools and the regular tools gradually decreased. This indicates that with the increase in advancing velocity, the advantage of the up-cutting tools in reducing the draft force weakens. Meanwhile, the vertical force (Figure 9b) of regular tools varied from −260.27 N to −286.18 N and −335.79 N. The vertical force of up-cutting tools changed from −384.06 N to −472.26 N and −552.7 N. The absolute values of the vertical force of up-cutting tools increased by 47.56%, 65.02%, and 64.60%, respectively, compared with those of regular tools. This indicates that up-cutting tools reduce the draft force while increasing the vertically downward force. The advancement velocity is a key factor affecting both the draft and vertical forces of the tools. The two types of tools show consistent trends in force changes, but there are significant differences in force magnitude and reduction ratios.

The effect of advancement velocity on soil disturbance (Figure 10a) can explain the changes in force. As the advancement velocity increased, the failure distance, transversal and longitudinal disturbance areas, and the width of both regular tools and up-cutting tools increased. This may be because the higher the advancement velocity, the greater the number of particles affected per unit time. Meanwhile, restricted by tool kinematics and surface friction, the upward movement of particles along the tool surface is limited. As a result, the effect of the tool can only be transmitted forward, which is one of the important causes of higher draft force. When the up-cutting tool advances forward, the soil in front is not only pushed but also lifted upward by the up-cutting motion, thus reducing the draft force. Compared with regular tools, up-cutting tools had a greater uplift height but a smaller failure distance and transversal and longitudinal disturbed areas. This may be because the upward particle movement counteracts the pushing effect of the tool in deeper soil layers.

As the advancement velocity increased, the K_f value of regular tools increased from 18.44 kN·m⁻² to 21.78 kN·m⁻² and then to 26.31 kN·m⁻² (Figure 9a). For the corresponding up-cutting tools, the K_f values were 6.46 kN·m⁻², 10.77 kN·m⁻², and 17.21 kN·m⁻², which are 64.97%, 50.55%, and 34.59% lower than those of regular tools. The up-cutting motion, as indicated by this, was capable of enhancing subsoiling efficiency by means of reducing the specific resistance at each advancement velocity.

The effect of particle velocity (Figure 10b) can also be used to explain the effects of advancement velocity and up-cutting motion. As the advancement velocity increased, changes occurred in the middle part of the disturbed area for both regular and up-cutting tools. For regular tools, the green area expanded. For up-cutting tools, this area was gradually replaced by particles that are affected by the tool surface, with a relatively high velocity and exhibiting a distinct forward trend. This demonstrates the transmission of the tool’s effect on the soil towards the front of the tool. As shown in Figure 10b, the particle velocity, as affected by the up-cutting tools, is significantly shifted upward compared with that of the regular tools, which proves the counteracting effect of the up-cutting motion on soil pushing.

Figure 11 exhibits how the velocity of advancement affects the deformation and movement of the slice. As the advancement velocity increased, the slice, acted on by both regular tools and up-cutting tools, experienced longitudinal compression deformation and distortion. It was also lifted upward and pushed forward. Compared with regular tools, up-cutting tools had a greater degree of forward rotation, a higher lift height, less compression deformation, and a shorter forward distance. This means that during the cutting process, up-cutting tools tend to make soil slices move obliquely upward as a whole, while regular tools more often cause soil slices to be broken under forced extrusion.

In summary, an increase in advancement velocity causes the effect of the tool on the soil to be transmitted more in the forward direction. The up-cutting motion, by mimicking the digging actions of animals, causes the soil to move upward, thus weakening the impact on the soil ahead and achieving the objective of reducing the draft force.

3.1.3. The Effects of Up-Cutting Velocity

The simulation showed the effect of the up-cutting velocity on the tool force (Figure 12). For the up-cutting tool only, when the up-cutting velocity rose from 0.5 m·s⁻¹ to 1 m·s⁻¹ and then to 1.5 m·s⁻¹, the draft force changed from 1212.07 N to 598.59 N and 361.89 N (Figure 12a). Therefore, the increase in the up-cutting velocity significantly inhibited the growth of the draft force. The higher the up-cutting velocity, the lower the draft force. This implies that a higher up-cutting velocity is more conducive to the tool’s horizontal advancement. At the same time, the vertical force changed from −471.35 N to −472.26 N and −463.89 N, with relatively small magnitude variations (Figure 12b). This may suggest that the advantages of a decreased draft force from higher up-cutting velocities far exceed the drawbacks of the vertical force from the up-cutting motion.

By observing the influence of the up-cutting velocity on soil disturbance (Figure 13a), it can be found that, compared with regular tools (0 m·s⁻¹), as the up-cutting velocity increased from 0.5 m·s⁻¹ to 1 m·s⁻¹ (the reference level) and 1.5 m·s⁻¹, the inclination of the failure surface of the up-cutting tools decreased (the inclination angle increased), gradually becoming closer to vertical. The transversal and longitudinal sections became narrower and longer. The total disturbed area and failure distance in the longitudinal and transversal sections decreased, the disturbance width remained nearly the same, and the uplift height increased. In addition, compared with regular tools, under the changes in the above-mentioned factors, the up-cutting motion changed the shape of the lateral disturbance area from U-shaped to V-shaped (Figure 7a, Figure 10a, and Figure 13a). This may be because the faster the up-cutting motion’s velocity, the greater the number of particles transported upward under the influence of the up-cutting motion. Regular tools merely guide the particles along the tool surfaces in a relatively limited way. Due to kinematic constraints and surface friction, the upward movement of particles is limited, and the cumulative effect transmitted to the front of the tool is an important reason for the generation of the draft force. In contrast, the increase in up-cutting velocity promotes the upward transportation of particles and reduces the cumulative effect of the soil in front of the tool due to the tool’s movement, thus reducing the draft force.

As the up-cutting velocity increased, the K_f value of the up-cutting tool dropped significantly. The specific resistance (K_f) of the up-cutting tools was lower than that of the regular tools (see Figure 12a, reference value). This indicates that increasing the up-cutting velocity can reduce the specific resistance. As a result, the subsoiling efficiency is substantially improved.

The simulation results show the influence of the up-cutting velocity on particle velocity (Figure 13b). As the up-cutting velocity increased, both regular and up-cutting tools exhibited similar changes in the middle part of the disturbed area. Gradually, this area was replaced by particles affected by the tool surface. These particles had a relatively high velocity and showed an obvious upward-moving trend. In the vicinity of the tool surface, the velocity of particles associated with the up-cutting tool was significantly influenced. The red area occupied the entire tool surface, the vertical upward component of particle movement significantly increased, and the velocity accelerated noticeably. Within 0.2 s, the particles were pushed to a higher position. This proves the contribution of the increase in up-cutting velocity to the upward transportation of particles.

Figure 14 shows the influence of the up-cutting velocity on the deformation and movement effects of the slices. Regarding the up-cutting velocity of the tool, under the action of the regular tool, the forward-pushing distance and upward-lifting height were at a certain level. For the up-cutting tool, as the up-cutting velocity increased, the longitudinal compression deformation of the square soil unit decreased, i.e., the longitudinal thickness, the degree of forward rotation, and the upward-lifting height increased, and the forward-pushing distance decreased. This indicates that increasing the up-cutting velocity promotes the oblique upward movement of the soil mass as a whole.

In summary, during the up-cutting motion that mimics the soil-disturbing behavior of animals’ digging, increasing the up-cutting velocity allows the tool to transport more soil upward, thus reducing the draft force.

3.2. Results of Field Tests

The results of the field tests present the draft-force-related performance of the conventional subsoiler and the chain subsoiler (Figure 15). The increase in the average draft force of both the conventional subsoiler and the chain subsoiler, as the results indicated, was associated with the increase in tillage depth. The draft force of the conventional subsoiler was 1.87 kN, 4.34 kN, and 8.73 kN when operated at a depth of 250 mm, 350 mm, and 450 mm, respectively. However, the chain subsoiler obtained draft forces of 1.33 kN, 2.52 kN, and 4.71 kN at these depths, respectively, which were 28.92%, 41.94%, and 46.03% lower than those of the conventional subsoilers. Moreover, with the increase in tillage depth, this reduction became more remarkable. This adequately shows that a chain subsoiler is highly efficacious in reducing the draft force in subsoiling.

Figure 15 also shows the specific resistance calculated from the draft force and the corresponding disturbance. It was indicated by the results that the specific resistance (K_f) values of the conventional subsoiler were 21.43%, 38.19%, and 50.50% higher than those of the chain subsoiler. This proves that the chain subsoiler effectively reduces specific resistance and improves subsoiling efficiency.

Figure 16 presents the typical soil disturbance profiles generated after subsoiling operations, corresponding to the conventional subsoiler (Figure 16a) and the chain subsoiler (Figure 16b), respectively. In the figure, key information, such as the horizontal plane of the original soil surface, the disturbed area, the depth, and the bottom horizontal plane of the furrow, is clearly marked. In terms of the overall shape, the general outlines of the disturbed areas of the two types of subsoilers are similar. Although there are certain differences regarding some specific details, such as the curvature of the edge of the disturbed area and the transition area between the bottom of the furrow and the soil on both sides, the overall differences are not significant. This is because the soil transported upward accumulates on both sides of the furrow, thus affecting the shape of the disturbed area. As for the relevant profile parameters, refer to

Table 3. Disturbance profile parameters of a conventional subsoiler and a chain subsoiler.

Type	Depth (mm)	Transversal Area (m2)	Disturbed Width (mm)	Uplift Height (mm)
Conventional Subsoiler	250	0.0727	490.99	86.62
	350	0.0780	513.33	93.32
	450	0.1110	502.47	94.88
Chain Subsoiler	250	0.0658	532.86	112.35
	350	0.0921	487.93	108.98
	450	0.1211	526.64	120.56

for details.

The above results suggested that there was no substantial difference in soil disturbance between the chain subsoiler and the conventional subsoiler. Moreover, no other obvious negative effects of the chain subsoiler were found. It suggests that the chain subsoiler, namely the proposed up-cutting subsoiling device, can significantly reduce the draft force and improve tillage efficiency while ensuring that the degree of soil disturbance meets the requirements, which makes it feasible and effective for practical applications.

4. Discussion

This study aimed to draw inspiration from the digging techniques of animals and explore a new type of low-draft subsoiling method. To this end, the principle of soil breaking, summarized from these digging techniques, was applied to the design of subsoilers. An innovative concept of up-cutting subsoiling was proposed. DEM simulations were carried out to analyze the draft-reducing performance and the mechanism of up-cutting. Finally, field tests were conducted using a chain subsoiler designed based on this principle to verify its draft-reducing performance.

The idea of applying digging techniques to machines with different operation modes may seem counterintuitive. There are significant differences in the forms of tool movement between parallel cutting in subsoiling operations and the digging actions of animals. However, from the perspective of the soil-breaking principle, the features of these two movement patterns are skillfully integrated using an up-cutting tool, thus reducing the draft force.

The DEM simulation results clearly show that when the up-cutting motion is introduced, the draft force is significantly reduced. Furthermore, as the up-cutting velocity increases, the draft force decreases. This indicates that the up-cutting motion effectively reduces the draft force. Field tests have also proven the draft-reducing feasibility of the chain subsoiler designed using this principle.

The fundamental reason why subsoiling with an up-cutting motion can reduce draft forces is that the tool exerts force from the interior of the soil and pushes the disturbed soil to the ground to reduce the draft force.

First of all, the range of soil disturbance varies as a result of up-cutting. The path along which the tool causes the least resistance is the longitudinal failure boundary [55]. The failure boundary occurs in the shape of a line or a logarithmic spiral. Up-cutting causes a change in the direction of external friction, resulting in a deflection of the principal stress axis [18]. As a result, the failure boundary rotates accordingly. This change in the volume of soil under load during the shearing cycle may directly contribute to the reduction of the draft. This is in line with the findings from powered discs from similar earth-moving operations [29].

Secondly, the particle velocity redistributes as a result of up-cutting. Under the up-cutting motion, soil does not continuously amass upon the tool surface to reshape the tool surface macroscopically, nor does there exist a boundary wedge synchronizing with the tool [17,19,20]. Therefore, the soil in front of the tool can be smoothly replaced, reducing the compressive force applied to the next piece of soil, thus reducing the draft force [29,45,56].

Finally, soil movement and deformation patterns are altered as a result of up-cutting, resulting in the whole-scale lifting of soil in front of the tool, thereby reducing the draft. By doing this, a primary shortcoming in the design concept of soil-loosening tools, which first compress the soil through the tool to generate shear and then loosen the soil, is evaded [19,50,57]. Conversely, some tensile stress might be generated in the unsupported soil ahead of the tool [50]. As is commonly known, compared with the shear strength brought about by compressive stress, the inherent tensile strength of soil is several orders of magnitude lower. Consequently, soil failure occurs more readily, and the draft force is reduced as a result of this situation [50].

It is worth noting that the up-cutting motion increases the downward vertical force. However, this force is close to that of a conventional tool with a rake angle of 45°. This vertical force is acceptable in that it falls within a reasonable range. It can be balanced by the soil reaction force without causing the negative impacts of excessive downward penetration and over-soil compaction [44].

Combined with the field test results, it can be considered that integrating an up-cutting motion with subsoiling has good prospects for draft-reducing applications. In agricultural soil tillage, to overcome the resistance between the soil and the tillage machinery, 30–50% of the total energy consumption is employed. Even a slight reduction in the draft force can bring huge benefits [1]. When the draft force is high, a high-horsepower tractor must be used for drafting to ensure the normal progress of subsoiling operations. Forcing operations will make components more prone to deformation, wear, and malfunction. As a result, the working efficiency of the subsoiler will be reduced, affecting the tillage quality [34]. After using an up-cutting motion to reduce the draft force, it is possible to use a light-duty tractor for traction, reducing the cost of the tractor [27]. This can avoid various tillage quality and efficiency problems caused by excessive resistance. In addition, the tractor can till at a faster speed, reducing slippage and improving tillage efficiency once again [29].

It should be noted that, due to limited space, this study is not exhaustive. Firstly, the soil conditions used in the simulation cannot replicate actual, real farmland soil in every dimension without deviation. There are also deviations between the obtained results and the actual ones. However, this difference is reasonable and acceptable. The purpose of this study is to explore the performance changes of a given tool under specific soil conditions. The set soil conditions can effectively present the corresponding change trends and are sufficient to meet the research needs. Under different soil conditions, the draft-reducing performance of the up-cutting tool should show a similar trend. This is due to the large-scale soil behavior caused by the soil–tool interaction, which can be described as the soil movement characterized by flow around the tool (Figure 1). Different soil failure modes may be similar in the “macro-scale” properties of soil movement [58,59]. Nevertheless, in future research, we plan to fully consider different soil conditions, especially varying suction and moisture levels, and deeply explore the draft-reducing performance of the up-cutting tool under these conditions, aiming to further reveal the internal laws of soil–tool interactions and provide more robust theoretical support for the development of related fields. The second issue is that the interaction between factors has not been analyzed, nor have the optimized parameters been determined, due to limited time and resources. However, this paper mainly focuses on the draft-reducing mechanism and analyzes the impact of a single factor on draft reduction to establish a basic understanding. Our subsequent research will focus on studying the interaction between factors and determining the optimal structural and operational parameters based on the existing results. A third concern is that more energy is consumed to meet the power requirement for driving the chain, and this is also highly correlated with the chain speed. Therefore, in subsequent research, it is necessary to study and analyze the rotary speed and power consumption of the chain subsoiler and the whole machine to further improve work efficiency and minimize power consumption.

However, these controversies do not impede the application of this principle. The significance of this article is that it has confirmed the significant reduction of the draft force using the up-cutting motion, the reasons for this reduction, and the practical feasibility of machines applying this principle. The excellent draft-reducing ability addresses the most urgent draft issues in subsoiling or other types of tillage, thus necessitating more attention. This technology has broad application prospects.

5. Conclusions

In this research, inspired by the soil-digging techniques of animals, an innovative up-cutting subsoiling method was proposed that can significantly reduce the draft force in subsoiling operations. The discrete element method (DEM) was adopted to conduct a detailed comparative simulation analysis of the draft performance of conventional tools and up-cutting tools under different rake angles, advancement velocities, and up-cutting velocities. Meanwhile, the internal mechanism by which up-cutting tools reduce the draft force under these conditions was deeply analyzed. Based on the theoretical analysis and simulation results, a prototype of the chain subsoiler was designed and manufactured according to this principle. Subsequently, field tests were carried out on the draft-reducing performance and soil disturbance effects of this subsoiler at different depths. The results indicate that the up-cutting tools based on the soil-breaking principle of animal digging techniques have fully exploited their advantages in reducing the draft force, showcasing excellent draft-reducing effects. This draft-reducing effect has been verified through the draft performance of the chain subsoiler in field tests. The key research results are outlined as follows:

(1) The soil-breaking effect induced by the up-cutting tools in the soil is highly similar in principle to that generated when animal claws dig up soil. This similarity lays an important prerequisite for the bionic application of animal digging techniques in the field of subsoiling tillage.

(2) Compared with regular tools, up-cutting tools significantly reduced the draft force under different factor levels. At rake angles of 45°, 60°, and 75°, the up-cutting tools decreased the draft force by 63.07%, 63.84%, and 58.92%, respectively. At advancement velocities of 0.5 m·s⁻¹, 1 m·s⁻¹, and 1.5 m·s⁻¹, the draft force decreased by 79.73%, 63.84%, and 45.22%, respectively. Although the up-cutting tools increased the vertical downward force, the increase still remained within a reasonable and acceptable range.

(3) Compared with regular tools, the variations in the soil disturbance area, the redistribution in particle velocity, and the pattern alteration in deformation and movement caused by up-cutting all contribute to the reduction in the draft force. The upward movement of the soil effectively prevents soil accumulation in front of the tool and alleviates soil compression deformation. These aspects, acting together, play a positive role in reducing the draft force during subsoiling.

(4) The chain subsoiler, designed and manufactured based on the up-cutting principle, demonstrates a good draft-reducing effect. The results of field tests confirm that, at different depths, the chain subsoiler with the up-cutting method has a draft-reduction amplitude close to or more than 30%. The soil disturbance it generates is similar to that of a standard subsoiler, fully meeting the subsoiling requirements for tillage quality.

This study demonstrates that up-cutting holds great potential in reducing subsoiling draft forces. Meanwhile, this research highlights the innovative direction and key design concepts for the design of future low-draft subsoilers based on the up-cutting principle, as well as other earth-moving machinery.