Evaluation and improvement of soil driller performance for well cellar type tobacco transplantation using DEM

Abstract

The well-cellar style transplanting (WCST) method has been shown to offer benefits in terms of improving the nutrient absorption capacity of tobacco plants, but conventional well-cellar devices tend to compact the soil, restricting the growth of tobacco roots. To address this issue, the discrete element method was applied in this study to investigate the impact of different soil drillers on the torque requirements and the profile characteristics of well-cellar holes. The performance of an auger with three rod diameters (20, 40, 60 mm), three values for the spiral lead (30, 60, 90 mm) and three rotation speeds (200, 400, 600 r/min) was evaluated, and the results showed that a lower mean torque was required as the auger rod diameter decreased and the rotation speed increased. The soil disturbance was 73.10% higher for an auger with a rod 20 mm in diameter than for a rod 60 mm in diameter. The weight variation in the auger trials was primarily affected by the rotation speed. Compared with a 200 r/min cylinder-cone bit, an optimized auger with a rod diameter of 40 mm, spiral lead of 90 mm, and rotation speed of 200 r/min had a lower peak torque and caused less soil disturbance, with looser soil around the well-cellar hole wall. The optimized design for the auger enhanced the formation of the well cellar, thereby promoting improved root growth and nutrient uptake in tobacco plants.

Keywords:
soil driller; well-cellar-type transplantation; discrete element method; auger; torque

Introduction

The well-cellar-style transplanting (WCST) method is a highly effective agricultural practice that was designed to enhance the establishment of a crop under variable climatic conditions. In this approach, tobacco seedlings are cultivated in precisely formed cylindrical well cellars (with diameters of 8–10 cm and a depth of 18 ± 2 cm) along ridges, and this method is used across 719,000 ha of tobacco cultivation regions (Lin et al., 2015). The transplantation of seedlings into well-cellar cavities induces rapid physiological recovery, facilitated by a buffered microclimate akin to greenhouse conditions; this promotes early root initiation and enhanced efficiency of macronutrient uptake, which are helpful for the growth, development and yield of tobacco crops. Since the size of the well cellar is strictly determined (Li & Yan., 2012), the quality of formation of the hole has attracted attention from tobacco farmers.

Well-cellar transplanting is currently achieved using three primary types of hole-forming device (Fan et al., 2020; Xu et al., 2023; Yu et al., 2018): a human-powered knapsack soil driller (Figure 1(a)), an intermittent well-cellar soil driller (Figure 1(b)), and a continuous well-cellar soil driller (Figure 1(c)). In all three cases, cylinder-cone drill bits are rotated while positioned vertically. During the hole-forming process, the soil particles move downward and are squeezed out of the hole area; this means that there is compacted soil around the well-cellar hole, which hinders tobacco seedlings from taking root, and it is therefore important to develop other types of drilling mechanism. The development of existing well-cellar soil drillers depends entirely on experimental methods, and the mechanical aspects of the well-cellar hole forming processes of various soil drillers are highly complex and difficult to model accurately. There are two main reasons for this: (i) there are discrepancies between models based on the discrete element method (DEM) and real ridges (Vucajnk et al., 2012), particularly in southwestern China, which is characterized by unique karst landforms; and (ii) traditional methods such as soil bin experiments and field tests cannot accurately capture the soil disturbance and forces acting on the ridge soil under unknown and unpredictable working conditions (Liu et al., 2023; Zhang et al., 2021).

Figure 1
Well-cellar hole-forming devices used in southwestern China: (a) manually powered knapsack soil driller; (b) intermittently movable well-cellar soil driller; (c) continuous well-cellar soil driller.

Numerical methods have been shown to offer significant advantages in investigating the complex geometry of soil-engaging tools, particularly in terms of time cost (Zhao et al., 2020a). Several studies have been conducted using DEM to investigate the interaction between the soil and the hole-forming device with high fidelity (Shmulevich, 2010). Park et al. (2021) predicted the excavation performance of an earth pressure balance shield tunnel boring machine under different operating conditions, although these authors did not take into account soil conditioning or the additional support pressure in the chamber. Wang et al. (2022) investigated the auger mechanism associated with soil lifting and throwing when building a fish-scale pit on a slope. Ding et al. (2022) studied the use of a spiral end-effector for digging holes in a seedling substrate for plug trays. Yang et al. (2022) examined the impacts of different shapes for the hanging cup on the soil action mechanism, cavitation performance, and planting quality. Their findings indicated that the longitudinal dimensions of the hole with a conical hanging cup were smaller than for a multilateral hanging cup. Wang et al. (2023) reported that a bionic hole-forming device caused less soil disturbance and reduced the sowing resistance compared to a traditional device using a planting mode involving opening holes in film for sowing. However, these studies did not cover well-cellar hole forming mechanism for WCST, nor did they take the field ridge soil into consideration.

In view of the above, the objectives of this study were as follows: (i) to validate the rationality of the ridge model for WCST through combined field and simulation experiments with a commonly used cylinder-cone type drill bit; and (ii) to determine the optimal structure and working speed for a self-designed auger based on the modelling results.

Material And Methods

Overview of the Machine

A widely used cylinder-cone-type drill bit (Shi et al., 2023; Wang et al., 2014; Xu et al., 2023) was first used to calibrate the parameters of the soil used in this research, through a comparison of results from field and simulation experiments. It consisted of a cylinder 80 mm in diameter and 180 mm in length, as shown in Figure 2(a), and the head was fitted with a conical tip with a bottom radius of 40 mm, an apex angle of 90°, and a friction angle 45° to achieve a lower tip resistance for cohesion soils, as theorized by Baligh (1975) and Browning (2005). The drill bit was rotated while moving vertically downward, in either a clockwise or counterclockwise direction.

Figure 2
Drill bits used with the WCST method in southwestern China: (a) a typical cylinder-cone-type drill bit; (b) the self-designed auger; (c) results of a strength analysis.

Based on the differences in the position of the single-thread spiral device during the hole-forming process, the construction process of a well cellar can be divided into three phases. The first is the soil penetration phase, in which the conical drill bit cuts into the soil, driven by the input power. Due to the soil compaction effect, an initial hole shape is formed, which acts as a basis for the subsequent soil cutting and discharge processes. The second phase involves soil cutting: as the single-thread spiral hole-forming device continues to drill downward, the soil on the inner wall of the well cellar is cut and stripped by the spiral blades. The cut soil particles are accumulated between the blades along the spiral grooves, under the combined action of the forced thrust from the spiral surface and the constraint of the inner wall of the hole. The final phase is soil discharge: after the device has drilled to a depth that meets the agronomic requirements of the well cellar, it is lifted upward. The accumulated soil particles are conveyed upward along the grooves, under the spiral thrust from the upper surface of the spiral blades, and are finally forced out of the well cellar and scattered on the ground around the mouth of the hole.

Several self-designed augers with different structures and rotation speeds were constructed and imported individually into the soil bin model at different simulation needs. The performance of each auger was examined using a calibrated soil model. The auger consisted of a rod, a spiral blade and a conical tip, with the spiral blade wrapped around the rod in a cylindrical helix curve (Figure 2(b)). The diameter and length of the auger were set to 80 mm and 180 mm, respectively, according to the agronomic requirements of the WCST method in southwestern China. According to the size requirements for the well cellar and a strength analysis (Figure 2(c)), three rod diameters of 20, 40 and 60 mm were considered, with three spiral leads of 30, 60 and 90 mm under rotation speeds of 200~600 r/min based on the strength requirements. The self-designed auger was moved up and down at a speed of 360 mm/s. Three-dimensional models of the self-designed augers were created using MotionView (Version 2023, Altair Engineering Inc.). As shown in Table 1, these parameters gave a total of nine augers with different spiral blades, which were examined using an orthogonal experimental design method.

Table 1
Orthogonal experimental design for determining the optimal structure and working speed of the self-designed auger

Trial no.	Rod diameter (mm)	Spiral lead (mm)	Rotation speed (r/min)
1	20	30	200
2	20	60	400
3	20	90	600
4	40	30	400
5	40	60	600
6	40	90	200
7	60	30	600
8	60	60	200
9	60	90	400

Development of the DEM model

Field soil condition

Soil samples were taken from the ridge in a WCST planting system in Linquan Village, Bijie City, Guizhou Province (27.04°N，105.91°E) in the transplanting season at the end of April. Five soil cores with a diameter of 50 mm and a depth of 30 cm (Blake, 1965) were taken from the ridge and cut into 10 cm sections (0–10 cm depth) to measure the bulk density, moisture content and soil porosity. According to the Standards (2015), the average bulk density and moisture content on the ridge were obtained as 1.49±0.20 g/cm³ (wet-based) and 16.5%±0.15% (wet-based), respectively, through the oven-drying method. The porosity was determined as 25.8% through the saturation method.

Development of the soil model

To simulate the interaction between the soil particles and the drill bits, a soil bin model was developed using DEM. The dimensions of the model were set to 400 mm in length, 400 mm in width, and 300 mm in height, thus ensuring that the drill bits were 120 mm away from the bottom of the soil bin (in its lowest position) and 160 mm away from sidewalls, to avoid possible edge effects. The soil model contained 88,090 soil particles, which were constructed as spherical elements with a radius of 4 mm, a value that was chosen based on the size of the domain and the available computing power. To enable a precise analysis of the movement of the soil at different ridge depths, the soil was divided into six layers: the surface layer (0–5 cm), the shallow layer (5–10 cm), the upper middle layer (10–15 cm), the lower middle layer (15–20 cm), the deep layer (20–25 cm) and the subsoil layer (25–30 cm).

Prior to well-cellar drilling operations in southwestern China, fractured ridge particles will naturally reconsolidate following a sedimentation period. To represent this particular soil rheology, we developed a soil model based on the Edinburgh elasto-plastic adhesion model (EEPA), which is commonly used to simulate cohesive and adhesive clay soils (Aikins et al., 2023; Ma et al., 2019; Yan et al., 2022). As shown in Table 2, the values for the Poisson's ratio and shear modulus of the soil particle parameters were obtained as 0.4 and 1.2×10⁶ Pa, respectively, from triaxial test measurements. The coefficients of static and rolling friction between the ridge soil particles were 0.67 and 0.03, respectively, according to the slope experiments, with values of 0.55 and 0.35 between the soil and the agricultural tool, respectively. The other model parameters established by EDEM (Version 2022, Altair Engineering Inc.) were selected from the related literature (Liu et al., 2023; Mohajeri et al., 2021; Yang et al., 2024; Zhao et al., 2020b). For the soil, the coefficient of restitution was 0.47, the contact plasticity ratio was 0.7, the slope exponent was 1.5, the tensile exponent was two, and the tangential stiff multiplier was 0.29. The drill bit had a density of 7,850 kg/m³ and a shear modulus of 7.98×10¹⁰ Pa, with a coefficient of restitution between the soil and steel of 0.45. The surface energy f₀ and constant pull-off force Δγ for the soil, which have been shown to vary greatly in different research studies and are difficult to measure accurately through physical experiments, were calibrated in this research.

Table 2
Parameters of the DEM model.

Property	EEPA	Source
Poisson’s ratio for clay soil	0.4	Measured
Shear modulus for clay ridge soil	1.2×106 pa	Triaxial test Measurement
Coefficient of restitution: soil–soil	0.47	Yang et al. (2024)
Coefficient of static friction: soil–soil	0.67	Slope experiment
Coefficient of rolling friction: soil–soil	0.03	Slope experiment
Poisson’s ratio for steel	0.3	Zhao et al. (2020)
Density of steel	7850 kg/m3	Zhao et al. (2020)
Shear modulus for steel	7.98×1010 pa	Liu et al. (2023)
Coefficient of restitution: soil–steel	0.45	Zhao et al. (2020)
Coefficient of static friction: soil–steel	0.55	Slope experiment
Coefficient of rolling friction: soil–steel	0.35	Slope experiment
Constant pull-off force (f0)	-0.01~0 N	To be calibrated
Surface energy (Δγ)	5~50 J/m2	To be calibrated
Contact plasticity ratio	0.7	Zhao et al. (2020)
Slope exponent	1.5	Zhao et al. (2020), Mohajeri et al. (2021)
Tensile exponent	2	Zhao et al. (2020), Mohajeri et al. (2021)
Tangential stiff multiplier	0.29	Zhao et al. (2020)

EXPERIMENTAL METHOD

A method based on simulation and field experiments was adopted (Figure 3). Tests were carried out at the experimental site of the China National Tobacco Corporation Guizhou Provincial Company in Linquan village.

Figure 3
Field experiments: (a) model of the test equipment; (b) test equipment with a cylinder-cone-type drill bit mounted; (c) photograph of a well-cellar hole on the ridge after drilling.

The cylinder-cone-type drill bit was mounted on the testing platform in the field experiment, and its torque and the uprightness of the hole during hole-forming events were measured. The drill bit rotated at 200 r/min and moved up and down at 360 mm/s. The contact model between the soil particles and the drill bits was based on

a Hertz–Mindlin (no-slip) condition, and it was assumed that the other particle properties were not associated with the factors examined here. In this paper, a calibration experiment involving field trials and simulation was performed to determine the optimal soil model by evaluating the relative error in the torque data and hole morphology data from the cylinder-cone-type drill bit at different drilling depths, with three replications of each test. In each replication, the drill bit was operated in an area 20 m long with a hole spacing of 50 cm and a ridge spacing of 120 cm. The same calibrated particle properties were used for each of the drill bits in the simulation.

Data collection and processing

The torque of the drill bit was monitored throughout the well-cellar type hole forming process using DEM. Figure 4 displays a typical torque requirement curve for the drill bit moving up and down during the hole-forming process in the field experiment. Formation of the

Figure 4
Typical torque requirement curve during the formation of a single well-cellar hole.

well-cellar hole started when the drill bit initially touched the top surface of the ridge, and ended as the drill bit left this surface. The torque was computed at depths of zero to 180 mm over 1 s for a complete drill bit rotation, and the mean torque was calculated from 0.1 to 0.5 s during a single downward soil-drilling movement of the auger.

Measurement of the well-cellar hole profile

To evaluate the soil disturbance due to the drill bits, the displacement of 12 soil particles under the ridge soil was monitored in three dimensions, with the particles used as tracers. The arrangement of these tracers is shown in Figure 5(a), in an area measuring 40 mm horizontally and 150 mm vertically. Tracers were

Figure 5
Soil disturbance and movement during the formation of a hole: (a) cross-sectional view of the drill bits and monitored soil particles; (b) mass variation of the hole drilling area, obtained based on the grid bin group method.

placed every 20 mm in the horizontal direction, from the middle to both sides, and every 50 mm in the vertical direction, from 25 mm to 175 mm. To observe and analyze the interaction between the soil and the drill bit, the soil bin model was bisected in the lateral direction, so that the position of the drill bit in the soil bin could be monitored from a side view in real time.

The displacement of the soil particle tracers during the formation of the hole was monitored to calculate the average soil disturbance. The disturbance distance L_d was calculated using [eq. (1)]:

$L_d=\sqrt{x^2+y^2+z^2}$ (1)

Where:

x is the forward displacement of the soil particle tracers;

y is the lateral displacement, and

z is the vertical displacement.

As depicted in Figure 5(b), the change in weight in the hole-forming area before and after drilling was measured using a weighing device in the field experiment, and using a total mass sensor with a grid bin group in the simulation. The grid bin group had dimensions of 100 mm horizontally along the ridge surface, 100 mm vertically along the ridge surface, and 150 mm perpendicularly to the ridge surface. The axis of the hole was positioned at the center of the grid bin group to allow for precise and comprehensive monitoring of the weight changes in the specific area surrounding the well-cellar hole. This setup enabled an evaluation of the uniformity of the hole, both in practical field conditions and in the simulations.

During the process of forming a well-cellar hole, the device can lead to soil compaction within the hole, especially at the bottom. To evaluate the state of compaction at the bottom of the hole, penetration tests were conducted in both the field experiments (using a 6110FS-SC900 spectrum soil compaction meter) and in the simulation (Figure 6). A probe was used to penetrate the soil ridge, and the maximum compressive force was determined by multiplying the maximum soil penetration resistance by the working area. In the simulation, this force was directly obtained at the post-processing stage. The force exerted from 18 cm to 25 cm of the bottom of the hole was measured at a constant speed of 40 mm/s at three points at the bottom. This method provided valuable data on the soil compaction levels at the bottom of the well-cellar hole, thus allowing for an assessment of the impact of the hole-forming devices on soil compaction in that specific area.

Figure 6
Simulation of a penetration test for detection of soil compaction.

Results and Discussion

Torque requirements for the self-designed augers

As illustrated in Figure 7, all of the self-designed augers had a similar torque profile, regardless of their operational and structural parameters: the torque surged sharply upon initial contact with the soil, fluctuated during downward penetration until the target depth was reached, and then gradually decreased during retraction as the soil–auger interaction diminished. The variation trend was similar to that reported in previous studies by Park et al. (2021) and Shi (2014). The maximum peak torque in the orthogonal trials of the augers was 11.73 N•m, 45.44% lower than for the cylinder-cone-type drill bit, which had a peak torque of 21.50 N•m. The auger required less downward pressure than the drill bit during drilling, due to a more efficient method of evacuating the material shavings as the soil was continuously compressed during the working process (Eliud, 2011). However, an inconsistency was found, as shown in Figure 8, in that the peak torque of the augers in orthogonal trials 1, 3, 8, and 9 occurred near the lowest drilling position, whereas the peak torque for the auger in orthogonal trial 7 occurred in the initial drilling process, and the peak torque in the other trials was observed during the downward drilling process. The variation in the torque may be related to the structural and working parameters of the auger, and may be because the peak torque of the soil screw conveyor process was much smaller than that of the auger head as it interacted with the soil, especially when the spiral blade is occupied with small diameter. This effect is consistent with the results reported by Li et al. (2019).

Figure 7
Variation in the torque during the process of forming the well-cellar hole in the orthogonal trials.

Figure 8
Positions of the self-designed auger with peak torque in the orthogonal trials.

As shown in Figure 9, the simulation results showed that 21.13% less mean torque was required as the auger rotating speed was increased. This finding is consistent with research by Shi et al. (2014) and Tian et al. (2015), except that the trend noted above was not significant, whereas the rod diameter of the auger had a significant effect (p = 0.05) on the mean torque. The mean torque for a rod diameter of 60 mm and blade width of 20 mm was increased by more than twice, with a p-value of 0.05, compared to a rod diameter of 20 mm and a blade width of 60 mm during a single downward soil drilling process. This result seems inconsistent with previous work by Park et al. (2021) and Eliud (2011), who reported that the required torque is higher when the size of the blade is greater, since the resistance increases when the friction area between the auger and the soil is increased. One possible reason for this disparity may be that the rod diameter was kept constant in previous studies and only the blade diameter was increased, with a varying contact area and size of the hole, whereas in this study, the overall size of the auger remained unchanged to ensure a suitable hole size for real agriculture. To enable a more specific analysis, the contact area as the auger penetrated the soil was calculated as shown in [eq. (2)].

Figure 9
Effect of three factors on the mean torque: (a) rod diameter; (b) spiral lead; (c) rotation speed.

$S=S_0+S_1+S_2=\frac{\pi D^2}{4\cos \delta /2}+\pi D\left(vt\right)+2(D_a-D)\sqrt{(\pi D{)}^2+(vt{)}^2}$ (2)

Where:

S is the contact area as the auger penetrates the soil (mm²),

S₀ is the contact area between the conical tip and the soil (mm²),

S₁ is the contact area between the auger rod and the soil (mm²),

S₂ is the contact area between the spiral blade and the soil (mm²),

D_a is the diameter of the auger (80 mm),

L is the length of the auger (180 mm),

δ is the apex angle (90°), and

v is the downward speed of the auger (360 mm/s).

As can be seen from Figure 10, the contact area as the auger penetrates the soil increases as the rod diameter is increased from 20 to 60 mm. Hence, the effect of the rod diameter on the mean torque can be seen to be consistent with previous work by Shi et al. (2014) and Eliud (2011), in that less torque is needed when the friction area is decreased.

Figure 10
Contact area as the auger penetrates the soil: (a) diagram showing the contact area; (b) graph of the variation in contact area with diameter and time.

Well-cellar hole profiles for the self-designed augers

The average soil movement paths in the forward, lateral and vertical directions were calculated for the 12 monitored particles (Figure 11). The average values for the forward displacement of the different augers ranged from 6.13 to 13.83 mm (Figure 11(a)). As shown in Figure 11(b), for the lateral displacement, the soil tracers moved first in a positive direction and then in a negative

Figure 11
Average soil particle paths in the different trials: (a) forward displacement; (b) lateral displacement; (c) vertical displacement.

one, following a sinusoidal curve, except for trial 5. From Figure 11(c), it can be seen that the soil particle moved much further in the vertical direction than in the forward and lateral directions. The vertical displacements were in the range 41.00–97.54 mm for the different augers. The total soil disturbance was significantly affected by the rod diameter, with a value 73.10% higher for an auger with a rod 20 mm in diameter than for a 60 mm rod diameter.

As shown in Table 3, the order in which the three factors affected the soil disturbance in the forward direction was rotation speed > spiral lead > rod diameter, whereas in the lateral direction, the order was rotation speed > rod diameter > spiral lead. For the vertical direction, the order was rod diameter > spiral lead > rotation speed. Soil disturbance was mainly manifested in the vertical direction, and the total soil disturbance was significantly influenced by the rod diameter in the orthogonal trials of the augers, which may be caused more by the movement of penetrating speed. This result was comparable with the findings reported by Wang et al. (2022) for soil disturbances induced by augers. These authors reported the disturbed area including the volume occupied by the spiral blades and the out-of-pit caused by the compression of the cutting end of the spiral blade.

Table 3
Effects of three variables on displacement in orthogonal trials (p = 0.05).

Disturbance distance (mm)	Rod diameter		Spiral lead		Rotation speed
	F-value	P-value	F-value	P-value	F-value	P-value
Forward direction	0.445	0.661	0.880	0.462	4.292	0.070
Lateral direction	1.595	0.278	0.265	0.776	3.061	0.121
Vertical direction	12.894	0.007	0.484	0.639	0.040	0.961
Total	11.415	0.009	0.486	0.637	0.082	0.923

The formation of well cellars was mainly concentrated at soil depths of 0–15 cm. The standard deviation of the soil weights at depths of 0–5, 5–10 and 10–15 cm of the well-cellar hole area was calculated using [eq. (3)] to estimate the uniformity of the holes.

$\sigma =\sqrt{[{(W_1-\overline{W})}^2+{(W_2-\overline{W})}^2+{(W_3-\overline{W})}^2]/(n-1)}$ (3)

Where:

Tmi, Tsi (i=1,2,3,4) are the measured and simulated values of the torque, respectively, at 0.1, 0.3, 0.5, and 0.7 s;

Wdj and Wsj (j=1,2,3,4) are the measured and simulated values of the soil layer weights at depths of 0–50, 50–100, 100–150 and 150–200 mm.

As shown in Table 4, the standard deviation for the 0–15 cm soil layer reached a maximum of 107.25 in trial 8 and a minimum of 24.31 in trial 2. The order in which the factors affected the weight variation in the auger trials was rotation speed > rod diameter > spiral lead, and none of these effects were significant. This result differed from the findings of Shi et al. (2023), who reported that the order of the effects on the well diameter using an auger in northern China were auger diameter, spiral lead, and rotational speed; in their study, the diameter of the auger was varied in the range 90–110 mm to meet the requirements for agriculture in southern China.

Table 4
Soil layer weights at a depth of 0–20 cm for different augers (*Sd means standard deviation of the shallow soil layer).

Soil layer (cm)	Soil weight (g)
	Original	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5	Trial 6	Trial 7	Trial 8	Trial 9
0–5	635.07	159.97	172.95	171.59	186.63	198.25	182.52	236.53	258.41	263.19
5–10	637.13	244.05	213.97	248.15	207.13	231.06	219.44	265.93	229.69	272.76
10–15	639.18	338.39	241.32	254.30	319.25	259.77	419.74	427.26	485.36	432.73
15–20	642.59	689.84	594.06	573.55	705.49	646.70	706.17	732.15	723.94	724.63
Sd*	/	60.61	24.31	35.39	54.39	20.96	97.23	78.24	107.25	73.22

As can be seen from Table 5, the influence of each of these factors on the soil compaction at the bottom of the well-cellar hole is not significant, according to the simulated penetration tests. The compressive force ranged from 9.04 to 14.66 N for all the augers, with an average value of 11.32 N. The downward movement of the drill bit itself will still have a certain impact on the bottom soil. The outward discharge of soil during the formation of the hole appears to alleviate some of the compression at the bottom of the well-cellar. The soil compaction increases by 24.28% and 28.84% at the bottom of the well-cellar hole as the auger rotating speed is increased from 200 to 400 and 600 r/min, respectively.

Table 5
Compressive force at the bottom of the well-cellar hole (mean values within a column followed by the same letter are not significantly different; P=0.05).

Rod diameter (mm)	Compressive force (N)	Spiral lead (mm)	Compressive force (N)	Rotating speed (r/min)	Compressive force (N)
20	11.05a	30	10.43 a	200	9.43 a
40	10.80 a	60	11.47 a	400	11.72 a
60	11.45 a	90	11.39 a	600	12.15a

Parameter optimization for the auger

To obtain the optimal parameters of the auger, the optimization module of Design Expert software was used to conduct a multi-objective optimization analysis in which the optimization objectives were minimization of the average torque 𝑇̅, disturbance distance L_d(D, H, n), weight deviation W_d(D, H, n), and compressive force F_c(D, H, n), with weights of 0.30, 0.15, 0.25, 0.30, respectively. The objective function and constraint conditions were as follows:

$\{\begin{array}{c}min\overline{T}(D,H,n)\\ min{L}_d(D,H,n)\\ min{W}_d(D,H,n)\\ min{F}_c(D,H,n)\\ \text{s.t.}\{\begin{array}{c}20\le D\le 60\\ 30\le H\le 90\\ 200\le n\le 600\end{array}\end{array}$ (4)

Two sets of optimal results were obtained through these calculations: auger I had a rod diameter of 20 mm, a spiral lead of 60 mm and a rotation speed of 200 r/min, while auger II had a rod diameter of 40 mm, a spiral lead of 90 mm, and rotation speed of 200 r/min.

Performance comparison of the optimized auger and CC drill

The working performance of the optimized augers and CC drill was compared using EDEM analysis. As shown in Table 6, auger II was found to have the lowest peak torque and soil disturbance, the highest weight standard deviation, and the lowest compressive force.

Table 6
Performance comparison of the optimized auger and CC drill.

Drill type	Peak torque (N•m)	Total distance Ld (mm)	Weight standard deviation (%)	Compressive force (N)
Auger I	11.35	90.94	79.66	9.83
Auger II	6.49	36.28	112.43	7.89
CC drill with rotational speed 200 r/min	19.94	52.19	54.08	19.87

At the same rotational speed of 200 r/min, the CC drill needed 75.68% and three times more torque than augers I and II, respectively. The peak torque was affected by the soil–drill bit interactions as the soil was cut and moved during the process of drilling the hole. The whole process is shown in Figure 12. From Figures 12(a) and (b), it can be seen that soil cutting started when the auger interacted with the soil surface. The spiral blade cut the lifted soil, which was then thrown from the spiral blades directly by the centrifugal force, and reached the ground surface during the downward drilling process between zero and 0.5 s. In the traditional CC drilling process (Figures 10(c), (d)), the soil was cut and compressed as the CC drill feeding, and the soil around the area of the hole visibly sank between zero and 0.5 s. There was no soil in the hole thrown out of the soil surface, and the deformation of the soil layer did not retract when the cylinder-cone-type drill bit returned to the soil surface. The torque required to cut the soil was mainly due to the penetration resistance and the friction force. Soil cutting is more difficult with a CC drill, since the soil is continuously compressed and the penetration resistance is increased; this is consistent with the findings of Jafar & Singh (2009) and resulted in a compressive force for the CC drill that was nearly twice as high as for the augers.

Figure 12
Soil particle velocities during the hole forming process: (a), (b) side and top views of the simulated drilling process for auger II; (c), (d) side and top views of the simulated CC drilling process.

The soil movement was mainly in a vertical direction in each of the drilling processes. As shown in Figure 13, the path of soil movement in the vertical direction for the augers showed a sinusoidal trend, with movement first downward until 0.5 s and then upward drilling of the 0–15 cm soil layer. In the conical tip area for the 15–20 cm soil layer, the spiral blade did not reach the bottom, so the soil mainly moved downward. In the 0–5 cm soil layer, the values for the soil particle disturbance for augers I and II reached 18.51 and 17.04 mm, respectively, rising to 41.18 and 29.53 mm in the 5–10 cm layer, 19.65 and 27.08 mm for the 10–15 cm layer, and 5.35 and 13.30 mm for the 15–20 cm layer. These results were comparable with the findings reported by Wang et al. (2022) for the path of soil movement induced by an earth auger.

Figure 13
Average soil movement within the hole area in the vertical direction: (a) in the 0–50 mm layer; (b) in the 50–100 mm layer; (c) in the 100–150 mm layer; (d) in the 150–200 mm layer.

In contrast to the movement of soil particles under the effects of an auger, the soil particles moved downwards in each soil layer under the effects of the CC drill. The downward distance for the soil particles with the CC drill reached values of 47.48, 64.66, 39.27, and 11.36 mm for the 0–5 cm, 5–10 cm, 10–15 cm, and 15–20 cm soil layers, respectively. These results were similar to those of previous studies conducted by Park et al. (2021) and Eliud (2011), which showed that the movement of soil is not uniform but is affected by the pit wall, and that the pit wall does not exert longitudinal compression on the surrounding soil but mainly affects the soil beneath the drill.

To compare the performance of the three types of drill, a range method was used based on the following formulae:

$r_{\mathrm{ij}}=\frac{Y_{\mathrm{maxi}}-Y_{\mathrm{ij}}}{Y_{\mathrm{maxi}}-Y_{mini}},i=1,2,3;j=1,2,3,4$ (5)

$R_r=\left[\begin{array}{cccc}{r}_{11}& r_{12}& r_{13}& r_{14}\\ r_{21}& r_{22}& r_{23}& r_{24}\\ r_{31}& r_{32}& r_{33}& r_{34}\end{array}\right]$ (6)

$W=P\cdot R_r$ (7)

Where:

r_ij is the membership value obtained from the indicator value corresponding to the i^th decision variable solution of the indicator Y_j;

Y_ij is the indicator value corresponding to the i^th decision variable solution of the indicator Y_j;

Y_maxi is the maximum value of indicator Y_j;

Y_mini is the minimum value of indicator Y_j;

P is the vector of weights for the average torque, disturbance distance, weight deviation and compressive force, with values [0.30, 0.15, 0.25, 0.30]. The overall score W is then [0.495, 0.750, 0.480]. Auger II (with a rod diameter of 40 mm, spiral lead of 90 mm, and rotation speed of 200 r/min) has a higher score than auger I and the CC drill, meaning that it has better overall operational performance.

Conclusions

In this study, DEM was adopted to simulate the interaction between soil particles and various augers with differing geometries and rotation speeds. The results show that DEM was a helpful tool in terms of understanding the soil–drill bit interactions and the effects of the rod diameter, spiral lead and rotation speed on the torque requirements and the profile of the well-cellar hole. The augers required more torque as the rotation speed was increased, and the rod diameter also had a significant effect on the torque. The soil disturbance was mainly in the vertical direction, and was significantly influenced by the rod diameter. The weight variation of the well-cellar hole was not significantly influenced by the rotating speed and the structural parameters.

The operational performance of the two augers with optimized parameters and a traditional CC drill were evaluated. The auger cut the soil and lifted it, and the soil was then thrown from the spiral blades directly by the centrifugal force, reaching the ground surface during the downward drilling process. In the downward drilling process with a traditional CC drill, the soil was cut and compressed, and the soil visibly sank. The overall performance of an auger with a rod diameter of 20 mm, a spiral lead of 60 mm, and a rotation speed of 200 r/min was comparable to that of a CC drill with a rotation speed of 200 r/min. The auger with a rod diameter of 40 mm, a spiral lead of 90 mm, and a rotation speed of 200 r/min yielded the lowest peak torque, soil disturbance and compressive force.

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• Data Availability Statement:
  The data used to support the findings of this study are available from the corresponding author upon request.
• Funding:
  This work received financial support from the Natural Science Foundation of Shandong Province (ZR2024ME137), the Science and Technology Program of Guizhou Province (QKHZDZX[2024]004), the Science and Technology Project of Guizhou Company of China National Tobacco Corporation( 2024XM20), the Key Project of Shandong Provincial Company of China National Tobacco Corporation (KN334), the Shandong Tobacco Industry technology system (SDARS-25-04), the Technology Project of China National Tobacco Corporation (110202301006), and the Agricultural Science and Technology Innovation Program of Shandong Academy of Agricultural Sciences (CXGC2025C16).

Edited by

• Area Editor:
  João Paulo Arantes Rodrigues da Cunha

Data availability

The data used to support the findings of this study are available from the corresponding author upon request.

Publication Dates

• Publication in this collection
  15 June 2026
• Date of issue
  2026

History

• Received
  17 Oct 2025
• Accepted
  8 Feb 2026