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Journal of Arid Land
Journal of Arid Land  2023, Vol. 15 Issue (9): 1129-1142    DOI: 10.1007/s40333-023-0026-3
Research article     
Effects of loading rate on root pullout performance of two plants in the eastern Loess Plateau, China
ZHANG Chaobo1,*(), LI Rong1, JIANG Jing1, YANG Qihong2
1College of Water Resources Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
2Key Laboratory of the Regulation and Flood Control of Middle and Lower Reaches of the Changjiang River under Ministry of Water Resources, Changjiang River Scientific Research Institute, Wuhan 430010, China
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Abstract  

Root pullout performance of plants is an important mechanical basis for soil reinforcement by plant roots in the semi-arid areas. Studies have shown that it is affected by plant factors (species, ages, root geometry, etc.) and soil factors (soil types, soil moisture, soil bulk densities, etc.). However, the effects of loading rates on root pullout performance are not well studied. To explore the mechanical interactions under different loading rates, we conducted pullout tests on Medicago sativa L. and Hippophae rhamnoides L. roots under five loading rates, i.e., 5, 50, 100, 150, and 200 mm/min. In addition, tensile tests were conducted on the roots in diameters of 0.5-2.0 mm to compare the relationship between root tensile properties and root pullout properties. Results showed that two root failure modes, slippage and breakage, were observed during root pullout tests. All M. sativa roots were pulled out, while 72.2% of H. rhamnoides roots were broken. The maximum fracture diameter and fracture root length of H. rhamnoides were 1.22 mm and 7.44 cm under 100 mm/min loading rate, respectively. Root displacement values were 4.63% (±0.43%) and 8.91% (±0.52%) of the total root length for M. sativa and H. rhamnoides, respectively. The values of maximum pullout force were 14.6 (±0.7) and 17.7 (±1.8) N under 100 mm/min for M. sativa and H. rhamnoides, respectively. Values of the maximum pullout strength for M. sativa and H. rhamnoides were 38.38 (±5.48) MPa under 150 mm/min and 12.47 (±1.43) MPa under 100 mm/min, respectively. Root-soil friction coefficient under 100 mm/min was significantly larger than those under other loading rates for both the two species. Values of the maximum root pullout energy for M. sativa and H. rhamnoides were 87.83 (±21.55) mm·N under 100 mm/min and 173.53 (±38.53) mm·N under 200 mm/min, respectively. Root pullout force was significantly related to root diameter (P<0.01). Peak root pullout force was significantly affected by loading rates when the effect of root diameter was included (P<0.01), and vice versa. Except for the failure mode and peak pullout force, other pullout parameters, including root pullout strength, root displacement, root-soil friction coefficient, and root pullout energy were not significantly affected by loading rates (P>0.05). Root pullout strength was greater than root tensile strength for the two species. The results suggested that there was no need to deliberately control loading rate in root pullout tests in the semi-arid soil, and root pullout force and pullout strength could be better parameters for root reinforcement model compared with root tensile strength as root pullout force and pullout strength could more realistically reflect the working state of roots in the semi-arid soil.

Key words： plant roots      soil reinforcement      loading rate      root pullout properties      root-soil interaction      loess area     
Received: 19 April 2023      Published: 30 September 2023
Corresponding Authors: * ZHANG Chaobo (E-mail: zhangchaobo@tyut.edu.cn)
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ZHANG Chaobo
LI Rong
JIANG Jing
YANG Qihong
Cite this article:

ZHANG Chaobo, LI Rong, JIANG Jing, YANG Qihong. Effects of loading rate on root pullout performance of two plants in the eastern Loess Plateau, China. Journal of Arid Land, 2023, 15(9): 1129-1142.

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http://jal.xjegi.com/10.1007/s40333-023-0026-3     OR     http://jal.xjegi.com/Y2023/V15/I9/1129

Fig. 1 Pullout tests were conducted on roots embedded in remolded samples. (a), remolded samples; (b), root pullout test.
Species Loading rate (mm/min) Number of roots successfully tested Average root diameter (mm)
Medicago sativa L. 5 8 0.72±0.06a
50 7 0.74±0.04a
100 9 0.72±0.05a
150 8 0.71±0.06a
200 7 0.72±0.02a
Hippophae
rhamnoides L.		 5 6 1.35±0.13a
50 6 1.37±0.13a
100 12 1.37±0.09a
150 6 1.41±0.15a
200 6 1.40±0.16a
Table 1 Root diameters of different species used in the pullout tests
Fig. 2 Fracture diameter (a) and fracture length (b) of Hippophae rhamnoides roots under different loading rates. Different lowercase letters indicate significant differences among different loading rates at P<0.05 level. Bars are standard errors.
Fig. 3 Root displacement of Medicago sativa and Hippophae rhamnoides under different loading rates. Different lowercase letters indicate significant differences among different loading rates at P<0.05 level. Different uppercase letters indicate significant differences between two species at P<0.05 level. Bars are standard errors.
Fig. 4 Root peak pullout forces (a) and strengths (b) of Medicago sativa and Hippophae rhamnoides under different loading rates. Different uppercase letters indicate significant differences between two species at P<0.05 level; Different lowercase letters indicate significant differences between loading rates at P<0.05 level. Bars are standard errors.
Fig. 5 Correlation of root pullout force (a) and root pullout strength (b) of Hippophae rhamnoides roots in breakage and slippage failure with root diameter
Fig. 6 Root-soil friction coefficients (a) and root pullout energy (b) of Medicago sativa and Hippophae rhamnoides under different loading rates. Different lowercase letters indicate significant differences between loading rates at P<0.05 level. Different uppercase letters indicate significant differences between species at P<0.05 level. Bars are standard errors. Boxes in figure 6b indicate the IQR (interquartile range, 75th to 25th of the data). The median value is shown as a line within the box. Black square is shown as mean. Whiskers extend to the most extreme value within 1.5×IQR.
Fig. 7 Tensile force and strength (a) and elastic modulus (b) of roots in different diameter classes. Different lowercase letters indicate significant differences among different diameter classes at P<0.05 level. Different uppercase letters indicate significant differences between species at P<0.05 level. Bars are standard errors.
Fig. 8 Correlation of root pullout force and root tensile force with root diameter for Medicago sativa (a) and Hippophae rhamnoides (b), and correlation of root pullout strength and root tensile strength with root diameter for Medicago sativa (c) and Hippophae rhamnoides (d)
Parameter Root displacement Peak pullout force Pullout strength Root-soil friction coefficient Pullout energy
Medicago sativa
Loading rate -0.042 0.315 0.206 0.379* 0.011
Root diameter -0.305 0.627** -0.809** -0.435** -0.181
Hippophae rhamnoides
Loading rate -0.157 0.162 0.078 0.153 0.005
Root diameter -0.233 0.718** -0.544** 0.152 0.282
Fracture root diameter -0.422* 0.603** -0.265 0.212 -0.092
Fracture root length 0.212 0.017 0.186 0.164 0.172
Table 2 Correlation coefficients of root diameter and loading rate with pullout performance of Medicago sativa and Hippophae rhamnoides roots
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