Influencing Factors and Prediction Model for the Antierosion Performance of Cement-Improved Loess Compacted Using Different Compaction Methods

Abstract

To analyze the antierosion performance of cement-improved loess (CIL), several influencing factors have been investigated based on two different compaction methods, which include the quasi-static compaction method (QSCM) and the vertical vibration compaction method (VVCM). Then, a prediction model for the cumulative erosion mass loss (CEML) has been established. The effects of erosion on the strength deterioration of CIL were also studied. The results show that, compared with QSCM, specimens compacted using the VVCM have better antierosion performance. As the cement content and the compaction coefficient are increased by 1%, the antierosion performance is increased by 16% and 6.2%, respectively. The eroding time has a significant effect on the antierosion performance of CIL, and the CEML increases linearly with an increase in the eroding time. The compressive strength of CIL decreases significantly due to erosion, and based on the average deterioration degree of the specimens, the design criteria for strength of CIL are proposed, which can provide reference for the design of CIL.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

In recent decades, loess has become an invaluable material in the construction of intercity railways. However, in its application, the unique porous structure of loess makes it vulnerable to water erosion. When the subgrade is exposed to adverse climate and hydrological conditions for a long time, a hydrodynamic load formed by the accumulated water under the traffic load is repeatedly applied to the subgrade filler. When the cohesion of the subgrade filler is less than the shear stress from the hydrodynamic load, the fine particles on the surface of the base are removed, which results in a deterioration in the performance of the subgrade in service and thus affects the safety of train operations.

To find methods of improving the erosion resistance of the subgrade, studies have researched how the strength of the subgrade filler could be improved and how eroding tests could be optimized according to the actual situation of the subgrade and have proposed antierosion measures. In particular, extensive attention has been paid to efforts that seek to improve the strength of subgrade materials, and some chemical binders, such as cement [1–4], lime [5], nano-MgO [6], coal ash [7], fiber [8], marine-dredged soil [9], sustainable regenerated binding materials [10], and manganese slag [11], have been applied to the subgrade to investigate their effect on the water stability and the erosion resistance of the soil subgrade. Researchers at home and abroad have found that using the aforementioned materials (especially cement) not only greatly increases the strength of the subgrade, but also additionally improves the water damage resistance. Furthermore, Narloch and Woyciechowski found that specimens that have a cement content of not less than 6% show almost no symptoms of water erosion [4].

Other scholars have conducted tests using different erosion test methods [12–14]. In France, a rotary brush made of steel wire, which originated from the French roads department, was applied to test the antierosion performance of a specimen. Specifically, the rotary wire brush was used to continuously brush the surface of the specimen, and the erosion mass of each specimen was used as the index for evaluating the antierosion performance. But the results of these studies were not in accordance with a real-life situation given that water was not involved in the test process [15]. In Australia, a vibration platform was applied to test the erosion resistance, and a specimen, which had been placed inside a steel cylinder, was vibrated using the vibration platform. The water erosion and the pumping caused by the hydrodynamic load were also considered during the test [16]. In China, an experimental method for studying the erosion resistance of semirigid base materials using the material testing system (MTS) was proposed, and this method has been compiled into the Test Methods of Materials Stabilized with Inorganic Binders for Highway Engineering (JTG E51-2009) (JTG E51-2009) [17]. Recently, some new approaches have been taken in evaluating the water damage, and the antierosion performance of inorganic-binder-stabilized soil has also been put forward [13, 14]. In addition, some scholars have also sought to establish the correlation function between the materials stabilized using an inorganic binder and the erosion parameters, and an erosion prediction model has been proposed, based on the analysis of the eroding test [18].

The aforementioned research has focused mostly on the erosion resistance of cement-stabilized materials compacted using the standardized test method: hammer quasi-static compaction method (QSCM). However, studies have indicated that the mechanical properties of the specimens compacted using QSCM are not consistent with that of field core samples, due to the renovation of compaction equipment, and their correlation is less than 70% [19–21]. Therefore, the standardized test method cannot accurately predict the mechanical properties of cement-stabilized materials. In China, vertical vibration compaction method (VVCM) is another compaction method proposed based on the principle of vibration compaction equipment and the actual compaction conditions, and the aforementioned correlation is as high as 90%, thus attracting increasing attention in recent years [22]. VVCM has been widely applied in the mechanical properties of cement improved loess (CIL), while the application of VVCM in the erosion resistance for CIL has rarely been studied.

To address this gap in the research, it is necessary to investigate the erosion resistance of CIL compacted using the VVCM and QSCM. Moreover, cement improved loess (CIL) is a complex that is composed of many materials, and its antierosion properties are closely correlated with its composition characteristics, including cement content, compaction coefficient, and eroding time. Therefore, this study, which is based on the Xi’an-Hancheng intercity railway project, has conducted a systematic and comprehensive study on the factors that influence the revealing of the erosion mechanism of CIL, and a prediction model for the cumulative erosion mass loss (CEML), based on the influencing factors, has been established. In addition, the effects of erosion on the strength deterioration of CIL have also been studied, because they can significantly affect the design criteria for strength of CIL.

2. Materials and Test Methods

2.1. Materials

The physical properties of the site in which the loess specimens used in this study were obtained are shown in Table 1. The cement used was Portland cement P·O 42.5, produced by Shaanxi city. The specific standard is in accordance with Common Portland Cement (GB175-2007) [23] and the technical indicators of this cement are listed in Table 2.

Table 1

Physical properties of loess.

Collapsibility grade of loess	Density (g/cm3)	Liquid limit (%)	Plastic limit (%)	Plasticity index	Passing ratio (by mass) as a function of sieve size (%)
					0.25–0.075	0.075–0.05	0.05–0.01	0.01–0.005	≤0.005
IV	2.74	26.4	15.7	10.7	2.47	7.22	53.43	13.83	23.05

Table 2

Technical indicators of cement.

Test index	Surface area ratio (Blaine method) (m2/kg)	Soundness (mm)	Setting time (min)		Compressive strength (MPa)		Flexural strength (MPa)
			Initial time	Final time	3 d	28 d	3 d	28 d
Results	329	2.05	219	426	22.3	48.2	5.8	8.06
Specific requirement	≥300	≤5.0	≥45	≤600	≥17.0	≥42.5	≥3.5	≥6.5

2.2. Specimen Preparation Methods

In this study, the antierosion properties of CIL were investigated using specimens that had been fabricated using two compaction methods: the vertical vibration compaction method (VVCM) and the hammer quasi-static compaction method (QSCM). The cement content (mass fraction) was 2%, 3%, 4%, and 6% and the compaction coefficient was 0.92, 0.95, and 0.97, respectively.

2.2.1. VVCM

VVCM involves two procedures. First, the maximum dry density and optimum water content were determined, and then the specimen was fabricated with different compaction coefficients. In this study, the cylindrical specimens (Φ100 mm × h 100 mm) were compacted using vertical vibration testing equipment (VVTE) [19, 20]. The VVTE crucial in VVCM is displayed in Figure 1, and the working parameters of VVTE based on previous research are presented in Table 3 [21, 22, 24]. The vibration compaction time of CIL specimens in the aforementioned compaction coefficients was determined by controlling the height of specimens.

[figures omitted; refer to PDF]

Table 3

Working parameters of vertical vibration testing equipment (VVTE).

Frequency (Hz)	Nominal amplitude (mm)	Mass (kg)
		Upper system	Lower system	Working weight
35	1.2	120	180	300

2.2.2. QSCM

To determine the maximum dry density and optimum water content of CIL, specimens (Φ100 mm × h 100 mm) were fabricated per the heavy compaction test (HCT-Z1) in the Code for Soil Test of Railway Engineering (TB10102-2010) [25]; the technical parameters of HCT-Z1 are listed in Table 4. In a similar way, QSCM was applied to prepare testing specimens (Φ100 mm × h 100 mm).

Table 4

The standard technical parameters of the heavy compaction test (HCT-Z1).

Compaction device specifications						Testing conditions
Hammer			Compaction mold			Compaction work (kJ/m3)	Number of layers	Striking times	Maximum particle size (mm)
Mass (kg)	Bottom diameter (mm)	Drop distance (mm)	Inner diameter (mm)	Height (mm)	Volume (mm)
4.5	51	457	102	116	947.4	2659	5	25	5

2.2.3. Specimen Preparation Procedure

After having been compacted using VVCM and QSCM, the specimens were demolded and placed into a standard curing room with a temperature of (20 ± 2)°C and a humidity of 95%. According to the strength-growth equation of the CIL [26], the compressive strength at 28 days is 80% of the ultimate strength, and so 28 days has been taken as the curing age of the specimen in the erosion resistance test.

2.3. Erosion Resistance Test

In the erosion resistance test (ERT), a newly developed scouring test device was used. The schematic diagram of the ERT is detailed in Figure 2, and the erosion testing equipment is illustrated in Figure 3. The erosion test comprises three parts: the system for generating erosion pressure, the control system, and a container for the testing specimen. The scouring effect of the hydrodynamic load generated by pumping on the railway filler during the process of train operations can be better simulated in the scouring container by installing a telescopic rod connected with a piston under the stress loading device. The control console is equipped with a temperature control knob and a frequency control knob, which can be adjusted to simulate the antierosion performance of the railway filler under different temperatures and working conditions. The outer wall of the scouring container is also provided with a water circulation pipeline and a water flow regulating valve. The speed of water circulation can be altered by controlling the water flow regulating valve to adjust the hydrodynamic pressure on the specimen during the scouring process and to judge the antierosion performance of railway filler under different hydrodynamic loads.

[figure omitted; refer to PDF]

Given that ERT was not included in the Code for Soil Test of Railway Engineering (TB 10102-2010) [25], it was conducted following the method proposed in the Test Methods of Materials Stabilized with Inorganic Binders for Highway Engineering (JTG E51-2009) [27]. Before the test, the cured specimen was immersed in water for 24 hours before placing into the scouring container. Afterward, the scouring container was firmly placed on the testing machine, and water was injected into the container until the height of the water was 5 mm higher than the top surface of the specimen. During the test, the hydrodynamic load was 60 kPa and the scouring frequency was 100 times/min. The scoured mud was poured into the container at 5 min intervals and was allowed to precipitate for 12 hours before being dried and weighed. The test was terminated after 30 minutes of scouring. Three replicate specimens were used in the test, and the erosion resistance of CIL was evaluated using the indexes of cumulative erosion mass loss (CEML), erosion mass loss ratio (EMLR), and average erosion rate.

The EMLR was calculated according to$$\begin{array}{}\text{(1)}& P=\frac{{\displaystyle {\sum }_{i=1}^6{m}_{f_i}}}{m_0},\end{array}$$where $P$ is erosion mass loss ratio of the specimen, (%), $m_{f_i}$ is the erosion mass of the specimen at the ith time, and the CEML is the sum of $m_{f_i}$, (g), i is the time (with each 5 min interval recorded as one time), and m₀ is the mass of the specimen.

The average erosion rate was calculated using $$\begin{array}{}\text{(2)}& V_t=\frac{{\displaystyle {\sum }_{i=1}^6{m}_{f_i}}}{30},\end{array}$$where V_t is the erosion rate of specimen in 30 min intervals.

2.4. Unconfined Compressive Strength

Following the Code for Soil Test of Railway Engineering (TB10102-2010) [25], specimens that had been cured for 27 days were immersed in water for 24 hours, and then an unconfined compressive strength (UCS) test was conducted using a WAW-100 universal material testing machine. It should be noted that the code recommends that the loading rate should be controlled at 1 mm/min and that six replicate specimens should be tested. A photograph of the UCS test is displayed in Figure 4.

[figure omitted; refer to PDF]

These findings can be ascribed to the strength formation mechanism of CIL. The strength of CIL is mainly a result of the cohesion between cement and soil particles and the friction between soil particles. The cohesion of loess is closely related to various physical and chemical forces between particles, including Coulomb force (electrostatic force), van der Waals force, and cementation. The internal friction angle of loess is attributed to the interlocking force produced by the contact area and the intercalation between particles. Given that the loess used was the same throughout this study, the strength is mainly correlated with the cemented materials. When the cement content is relatively high, the cement can be evenly distributed and well wrapped around the soil particles. This will enhance the bonding force and produce a more compact structure with a higher strength within the specimen, and thus, it would be difficult for the soil particles to be washed away by the water flow. Accordingly, the CEML is reduced, and the antierosion property is improved. However, when the cement content is low, it is often difficult to evenly combine the cement slurry with soil particles, and as a result, the bonding force between the soil particles is poor and the compressive strength is reduced; thus, the soil particles are easily washed away under the hydrodynamic load, and the antierosion properties are relatively weak.

The erosion deterioration factor proposed to investigate the effects of erosion on the compressive strength was calculated using equation (5), and the results are listed in Table 9. Moreover, a comparison of the compressive strength of the specimen before and after ERT is illustrated in Figure 11:$$\begin{array}{}\text{(5)}& DR=\frac{q_E}{q_u}\times 100\%,\end{array}$$where DR is the erosion deterioration factor of specimen (%), q_E is the compressive strength of the specimen after ERT (MPa), and q_u is the compressive strength of the specimen (MPa).

Table 9

The results of the compressive strength of the specimen before and after ERT.

Compaction method	Compaction coefficient	Cement content (%)	qu (MPa)	qE (MPa)	DR (%)
VVCM	0.92	2	1.13	0.77	68.1
		3	1.47	1.11	75.5
		4	1.62	1.27	78.4
		6	2.29	1.79	78.2
	0.95	2	1.53	1.16	75.8
		3	1.82	1.41	77.5
		4	2.22	1.82	82.0
		6	2.68	2.26	89.2
	0.97	2	2.00	1.56	78.0
		3	2.46	1.98	80.5
		4	2.74	2.38	86.9
		6	3.39	2.96	84.4
QSCM	0.92	2	1.03	0.68	66.0
		3	1.30	0.88	67.7
		4	1.49	1.06	71.1
		6	2.03	1.49	73.4
	0.95	2	1.34	0.91	67.9
		3	1.60	1.11	69.4
		4	1.90	1.50	78.9
		6	2.44	1.94	79.5
	0.97	2	1.65	1.12	67.9
		3	2.03	1.62	79.8
		4	2.26	1.83	81.0
		6	2.80	2.30	82.1

[figures omitted; refer to PDF]

As is evident from Figure 11, the compressive strength of CIL is significantly decreased after ERT, in which the erosion deterioration factors for specimens compacted using VVCM and QSCM are 79.5% and 73.7%, respectively. This indicates that the hydrodynamic load not only has an effect on the specimen but also causes damage, to a certain extent, to the internal structure of the specimen. This produces microdamage and microcracks inside the specimen, which causes a reduction in the compressive strength. This can be ascribed to the fact that the damage of CIL is caused by the process of microstructural damage. Under cyclic dynamic loading, fine particles with low cohesion are eroded first, which results in defects on the surface of the specimen. With an increase in the eroding time, the defects on the surface of the specimen deepen further, and the water content in the specimen also increases sharply. Due to the collapsibility of loess, the deterioration of the compressive strength is aggravated by the invasion and erosion of water loading. These findings are consistent with research that shows that the damage of the materials under cyclic erosion of hydrodynamic pressure is a cumulative process that can result in the strength degradation of the material properties [13, 28–31].

3.4. Design Criteria for Strength of CIL Based on the Antierosion Performance

According to the Code for Design of Railway Earth Structure (TB10001-2016) [32], the UCS of CIL is used as the technical indicator in the design of subgrade materials. However, studies have indicated that the design value of UCS is calculated based on the train load, while the long-term stability under the influence of erosion is not considered [33]. As is detailed from Table 9 and Figure 11, the compressive strength of CIL is significantly affected by the erosion test. Therefore, the erosion deterioration characteristics should be considered. Based on the design theory of strength control, the design criteria of CIL are calculated according to equations (6) and (7):$$\begin{array}{}\text{(6)}& q_u\ge {\sigma }_{\max },\text{(7)}& q_u={\eta }_e\cdot q_u,\end{array}$$where [q_u] is the strength considering the influence of erosion on CIL (MPa); q_u is the strength of CIL before erosion deterioration (MPa); η_e is the erosion deterioration factor of CIL; σ_max is the maximum stress of subgrade (MPa).

The design criteria for UCS of CIL considering the antierosion performance is obtained by substituting equation (7) into equation (6):$$\begin{array}{}\text{(8)}& q_u\ge \frac{{\sigma }_{\max }}{{\eta }_e}.\end{array}$$

In this study, the erosion deterioration factors for specimens compacted using VVCM and QSCM are 79.5% and 73.7%, respectively. According to the Code for Design of Railway Earth Structure (TB10001-2016) [31], the UCS of CIL is listed in Table 10. By substituting the aforementioned stress and erosion deterioration factor into equation (8), the revised design standard for UCS of CIL can be calculated, and the results are listed in Table 10.

Table 10

The design criteria for UCS of CIL based on the antierosion performance.

Railway classification	Design speed (km/h)	Unconfined compressive strength (MPa)
		The bottom layer of the subgrade bed		The embankment below subgrade bed
		Specification value	Revised value	Specification value	Revised value
Ballast track	120, 160	≥0.35	≥0.48	≥0.20	≥0.28
	200			≥0.25	≥0.34
Ballastless track	250, 300	≥0.35	≥0.48	≥0.25	≥0.34
	350

4. Conclusions

The influencing factors on the antierosion performance of CIL, which include the compaction method, the cement content, the compaction coefficient, and the eroding time, have been investigated systematically and prediction models of CEML have been established based on a newly developed scouring test device. The following conclusions can be drawn from this research:

(1) Compared with QSCM, the antierosion performance of CIL compacted using VVCM can be significantly improved, and the erosion mass loss ratio can be reduced by at least 10%.

(2) There was found to be a strong correlation between the antierosion performance of CIL and influencing factors such as the cement content and the compaction coefficient. As the cement content and compaction coefficient were increased by 1%, the erosion resistance increased by 16% and 6.2%, respectively.

(3) The eroding time has a significant effect on the CEML of CIL, with the CEML increasing linearly with an increase in the eroding time.

(4) The prediction model of CEML based on the influencing factors has been established, and the correlation coefficient is higher than 90%.

(5) The compressive strength of CIL decreases significantly due to the dynamic erosion loading. The average deterioration factor of CIL compacted by VVCM is 79.5%, and that of the specimen compacted using QSCM is 73.7%. Based on the erosion deterioration factor, the revised design criteria for strength of CIL is proposed.

The results of this investigation are of great significance in the application of loess and the improvement of subgrade durability in the construction of intercity railways. Further field erosion tests and an investigation into the micromechanisms of cement improved loess after ERT will be conducted.

Acknowledgments

This work was supported by the Science and Technology Project from Shaanxi Provincial Department of Transportation (Grant nos. 18-02K and 19-27K) and the Scientific Research of Central Colleges of China for Chang’an University (Grant no. 300102218212).

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