Typically, soil samples must be crushed into particles for laboratory research. Thus, an efficient mechanism to ensure a uniform particle size is essential. We previously developed a rod mill device that performs well, but video analysis indicated that the shear forces applied by the rod were more effective than the compressive forces applied by the mill. The mechanism for this phenomenon is unclear. This study focused on clarifying the relationship between compressive load and abrasion when crushing dried and hardened soil particles. Soil pellets of the same size were prepared, and model experiments were performed, where vertical compression and abrasion were applied to the pellets until they fractured. The results showed that soil pellets were fractured easily when an abrasive load was continuously applied in the circumferential direction. Additionally, the load required to fracture the soil pellets was much lower than the required vertical compressive load. The rod mill device was previously thought to fracture soil aggregates by gradually stripping soil particles away from the surface. However, our experimental results clarified that the fracture process started at the center and subsequently induced the entire pellet’s sudden failure.
This is the study to understand the crushing mechanism quantitatively for the development of rod mill device for soil. When a soil sample is examined at a research institute, the soil must first be crushed into fine-grained particles, with a median diameter of less than 2 mm, to satisfy industrial standards. This crushed material consists of a large amount of aggregated soil, with great variability in size and hardness. Thus, a mechanism for grinding the soil efficiently is needed. Several methods have been devised, including human-powered methods, using a pestle, mortar, and sieves, as well as mechanical methods, where a rod or ball is rotated against the soil being crushed. Crushing is intended to reduce the size of a solid and has a broad meaning. It covers breaking ores and other solids as well as the fine crushing of grains and seeds. Soil is an aggregate, consisting of a combination of soil particles, and the crushing and sizing of soil (i.e., sieving and classification) for chemical analysis differs from breaking or fine crushing soil that must be separated into particles less than, or equal to, a desired size. In other words, if the force holding the soil particles together is removed, they fall apart.
We have proposed a rod mill device, in which a rod is turned inside a container holding the soil to be crushed. This device can crush, dry, and harden soil aggregates in a short time. The necessary factors for improving device performance further were obtained by comparing the soil aggregates’ yield stresses and the forces acting on them within the container. We visualized the action of the rod and soil aggregates during the crushing process using a high-speed camera installed in the container [
Shear forces are generated when compressive forces are applied to soil particles with varying sizes and morphologies, and slightly elastic behavior has been observed. Defossez and Richard used a finite element method to demonstrate numerically that stress is transmitted into lower layers by applying vertical loads to the soil surface [
Stresses frequently propagate through preferential paths, rather than homogenously, which isolates bulk volumes that are under less stress than the preferential path. This is because different assemblages coexist in soils because of small differences in particle size and shape [
As particulate matter, soil has complex reactions, and the relationship between individual particles’ properties and the material’s reaction is not fully understood. Normally, soil is air-dried before it is used for physical and chemical tests. However, dried and hardened soil aggregates do not have uniform sizes and shapes and are, thus, unsuitable for quantitative tests. This study focused on clarifying the relationship between the compressive load and abrasion when crushing dried and hardened soil particles. Soil particles from the same source were prepared into soil pellets of the same size, and a model experiment was performed where varying loads were applied to soil pellets with a resin plate to cause the pellets to fracture. The results indicated that the pellets were destroyed immediately when a crack formed at the circumference toward the center of the pellet.
The soil used for the soil pellet was collected from the stratum called Kanto Loam which is widely distributed around Tokyo. The Kanto Loam was formed by sedimentation of volcanic ash and volcanic debris which were weathered by wind and rain and has high cohesiveness with 25% to 40% contents of silt and clay. The soil pellets used in the experiments were fabricated to ensure uniform mass and morphology. The average soil aggregate volume, when crushed with a rod mill device, is 1 × 10−6 m3; thus cylindrical soil pellets with a diameter of 1 × 10−2 m and length of 1 × 10−2 m were formed for the experiments, as shown in
Step 1: Insert an acrylic cylinder with an inner diameter of 1 × 10−2 m and length of 2 × 10−2 m into a convex stainless steel stand with a diameter of 1 × 10−2 m and height of 5 × 10−3 m. Place the cylinder and stand on an electronic scale and zero the balance.
Step 2: Place 1 × 10−3 kg of soil in the acrylic cylinder.
Step 3: Gradually pour 0.4 × 10−3 kg of pure water into the soil and allow it to penetrate.
Step 4: Insert another stainless steel stand into the acrylic cylinder from above and compress by hand.
Step 5: Remove the acrylic cylinder from the stands and place the cylinder in an air dryer for approximately 15 min.
Step 6: Remove the soil pellet from the acrylic cylinder by inserting a convex stainless steel stand with a diameter of 1 × 10−2 m and length of 2 × 10−2 m.
Step 7: Once the soil pellet is removed from the acrylic cylinder, dry the soil pellet to 1 × 10−3 kg before using it in the test.
A soil particle density test (JIS A 1202) and soil moisture content test (JIS A1203) were conducted on the volcanic ash-derived soil used to manufacture the soil pellets. (JIS is an abbreviation for the Japan Industrial Standard.)
m = m s + m w + m a (1)
and the water content w is defined as
w = m w m s × 100 . (2)
| Soil Test Items | Test Results |
|---|---|
| Soil particle density (ρs) g/cm3 | 2.432 |
| Natural water content (w)% | 17.70 |
| Void ratio (e)% | 1.720 |
The soil pellet mass m = 1.02 × 10−3 kg, and the air mass in Equations (1) and (2) can be assumed to be 0. Thus, ms = 0.862 × 10−3 and mw = 0.153 × 10−3 kg. These results can be used to determine soil pellet volume. The soil pellets were 1 × 10−2 m in diameter and 1 × 10−2 m in length, yielding a soil pellet volume of 0.785 × 10−6 m3. The relationship between the soil pellet volume (v) and the volumes of the solid (vs), water (vw), and air (va) is given by
v = v s + v w + v a . (3)
The soil particle density ρ ѕ is 2.432, as shown in
ρ ѕ = m s V s (4)
Thus, vs = 0. 354 × 10−6 m3, vm = 0.153 × 10−6 m3.
v a = v − v s + v w (5)
Based on the soil pellet volume v = 0. 785 × 10−6 m3 and Equation (5), va = 0.251 × 10−6 m3.
The void ratio e is given by
e = v a + v w v s × 100 (6)
Thus, the soil pellets had an e value of 1.141.
To reproduce the breaking of soil pellets due to abrasion, an experiment was performed with the reciprocating rotary load test device shown in
The soil pellet was subjected to abrasion by a holding plate at five load levels with the reciprocating rotary load test device. The holding plate’s movement distance was set to 120 mm, and the reciprocating cycle was set to 0.17 Hz (i.e., 10 reciprocations per minute). As the holding plate reciprocated in the horizontal direction, the soil pellet rotated counterclockwise and clockwise on the rail plate. Because of the rolling principle, the movement distance of the soil pellet was 60 mm, which was half that of the holding plate. The soil pellet revolved approximately twice per reciprocation. The coefficient of static friction between the soil pellet and the rail plate was 0.44. As in the reciprocating rotary load test, the rolling friction coefficient of the soil pellet’s rotating surface (peripheral surface) was 0.05 for both the rail and holding plates, and the shear force was negligible. The load was changed in five stages and applied to ten soil pellets at each stage.
| Specifications | |
|---|---|
| Travel distance | 120 mm |
| Reciprocating speed | 10 times/min |
| Power supply | AC 100 − 240 V, 50/60 Hz |
| Dimensions | W380 mm × D590 mm × 310 mm |
| Equipment weight | Approx. 22 kg |
The number of revolutions of each soil pellets was measured until breakage occurred.
We conducted unconfined compressive tests to study the yield stress at which the soil pellet fractured to determine its homogeneity and strength. The yield stress was measured by a corresponding displacement to investigate the amount of force needed to fracture the soil pellet using the Instron 5566. The measured values of the maximum yield stress are shown in the plot in
The ε values were calculated by the following formula.
ε = Δ H H 0 (7)
Here, ∆H is the change of vertical displacement of upper table and H0 is the initial diameter of the soil pellet.
In contrast, the size and mass of air-dried soil aggregates vary naturally when
fractured in ordinary situations. Soil aggregates are groups of soil particles that bind more strongly with each other than with adjacent particles. The yield point and strain of the air-dried soil aggregates were measured, using an unconfined compressive test device. The average and the standard deviation obtained from these measurements are shown in
Regarding this elastic behavior, Mütze derived a stress behavior model in which particle bed stress behavior was such that the particles moved into interstitial spaces, and individual particles formed elasticity at low pressure levels, whereas, at higher pressure levels, the stress behavior model led to fracture [
| Number of samples n = 10 | Air-dried soil aggregate mass Kg | Maximum yield stress N | Compressive strain ε |
|---|---|---|---|
| Average | 0.86 × 10−3 | 52 | 0.17 |
| Standard deviation | 0.23 × 10−3 | 36 | 0.037 |
The soil pellet bulged slightly in the horizontal direction until the vertical load displacement reached the maximum yield stress. Until reaching this point, the soil pellet acted as an elastic body. In both the aggregated soil and the soil pellets used in the normal test, when the vertical load displacement reached the maximum yield stress, large cracks split the soil lumps. However, it did not break into small lumps (particles) at once, which would occur in a crushing container.
The reciprocating rotary test results are shown in
Furthermore, it is understood that the number of rotations needed for breakage to occur decreases as the load increases. In other words, as the load increases, less time is needed for the pellet to break (i.e., fewer rotations). The correlation coefficient between the load and the average number of rotations required to break the soil pellet was 0.9353, meaning it was closely correlated to the inverse proportion. This demonstrated the relationship between the mass of the rod that crushed the soil mass in the container and the rotational speed of the container in the rod mill. In the previous research, the aggregated soil mass
used for the normal crushing test had a distorted shape, so it was a test in which the cantilever contacted only part of the fixed state. However, the soil in the container of the crushing device is rubbed against the crushing rod and the container while the soil is also rotating intricately due to the rotation of the container and the complicated movement of the crushing rod. This experiment using soil pellets is believed to be able to simulate the behavior occurring in these real crushing containers.
In the reciprocating rotary load test at 8 N, the soil pellets broke from the center, as they did in the compression test. We also observed the breaking process at 4 N in detail.
a dynamic load to the contact point in the circumferential direction. Applying a static load and a dynamic load in the circumferential direction reduces the bond strength between soil particles, a crack is generated between the soil particles inside the soil pellet, and the soil pellet systematically ruptures.
To examine the breakage of dried and hardened soils, we performed model experiments on soil pellets of equal size, soil quality, and particle size to clarify the relationship between compressive loads and friction.
Our conclusion is listed as below:
1) Soil pellets were used in the experiments to disclose how collapse dried and hardened soils.
2) Soil pellets were all made of equal size, soil quality, and particle size to clarify the relationship between compressive loads and friction.
3) Soil pellets were fractured easily when loads were applied continuously in the circumferential direction.
4) The loads necessary for fracture on a soil pellet was smaller than when using vertical compressive loads.
5) In the experiment by using a rod mill device, soil particles were gradually stripped away from the aggregate surface.
6) The fracture process of soil pellets started at the center of the aggregate and subsequently induced the sudden failure of the entire aggregate.
7) It is concluded from the experiments that the rotation of the soil pellets caused disintegration (breakage and collapse) of themselves.
We wish to thank the basic physical data of soil pellet by the General Building Research Corporation of Japan (GBRC) and large number of compression test of soil pellet by the Saitama Industrial Technology Center (SAITEC). Funding for the development of rod mill from the Saitama Prefecture is gratefully acknowledged.
The authors declare no conflicts of interest regarding the publication of this paper.
Oishi, M., Kubota, Y. and Mochizuki, O. (2020) Crushing Mechanism for Soil Particles. World Journal of Mechanics, 10, 69-82. https://doi.org/10.4236/wjm.2020.106006