Study on the anti-scouring performance of a new type of cast-in-situ vegetation concrete for slope protection

ABSTRACT

Ecological river slope protection techniques have received considerable attention for sustainable development. This study focuses on a newly developed material called cast-in-situ vegetation concrete (CVC), which incorporates vegetation and punched holes. A series of anti-scouring tests were conducted to optimize its construction parameters. Wave scouring tests on bare slopes and rainfall scouring tests on vegetated slopes were carried out to evaluate the effects of flow velocity, rainfall intensity, slope gradient, punched hole parameters, and construction thickness on underlying soil erosion. Additionally, high-flow scouring tests were conducted to examine the overall survival status of vegetation after scouring under different construction thicknesses and perforation parameters. The results indicate that flow velocity, rainfall intensity, and slope gradient are positively correlated with soil erosion. Among these, Perforation Group A (4.5 cm in depth, 2.5 cm in diameter, 2.8 cm in spacing) demonstrated superior performance to Perforation Group B (4.0 cm in depth, 1.0 cm in diameter, 3.5 cm in spacing), exhibiting lower erosion volume and better vegetation-reinforced slope protection effects. In terms of promoting stable vegetation survival, construction thicknesses of 6 cm or 10 cm were found to be preferable to 15 cm. Therefore, the parameters of Perforation Group A with a thickness of 6 cm or 10 cm are recommended. These findings offer practical guidance for the application of Cast-in-place Vegetation Concrete (CVC) technology in ecological slope protection.

Keywords:
Cast-in-situ vegetation concrete; Side slope protection; Anti-scouring performance test; Construction process parameters; Punched hole parameters

1.INTRODUCTION

With rapid economic development and urbanization, human activities and construction projects have significantly impacted rivers and related ecosystems. To meet flood control requirements, rigid materials such as concrete and rubble masonry, which possess anti-scouring, anti-freezing, and anti-collision capabilities, have been widely used for river slope protection.

However, these conventional slope protection materials suffer from disadvantages such as poor ecological stability, complicated manufacturing and installation processes, and high cost [¹]. A more critical drawback is their low eco-friendliness. The extensive use of impermeable materials directly isolates the underlying soil from the external environment, affecting biodiversity and disrupting the ecological balance [²,³,⁴,⁵]. Moreover, the widespread use of cement and concrete can lead to soil hardening and salinization, which are highly detrimental to the growth of flora, fauna, and microorganisms [⁶, ⁷].

In recent years, ecological sustainability has become a focus of contemporary research across various disciplines. Consequently, the development of concrete materials that incorporate ecological and environmental benefits has gained significant attention. Porous eco-concrete, also known as vegetation concrete, is a green building material based on porous concrete with planting functionality [⁸,⁹,¹⁰]. It has demonstrated excellent performance in slope protection engineering, attracting worldwide attention and spurring extensive research.

For instance, LEE et al. [¹¹] selected a cement containing α-hemihydrate calcium sulfate to design a low-alkali binder that effectively reduces the pore alkalinity of vegetation concrete [¹¹,¹²,¹³]. GANAPATHY et al. [¹⁴] investigated new environmentally friendly ultra-low alkalinity cementitious materials to improve vegetation growth [¹⁵]. RAJA and KUMAR [¹⁶] found that incorporating cupola slag in cement or aggregates enhanced properties such as strength, water permeability, resistance to aggressive environments, and chloride ion penetration resistance [¹⁶, ¹⁷]. CHEN et al. [¹⁸] and AHMAD et al. [¹⁹] examined the effect of aggregate type on the durability of green concrete and concluded that increasing aggregate texture and angularity helps improve compressive strength. VIEIRA et al. [²⁰] and LYU et al. [²¹] conducted a full life-cycle assessment of vegetation concrete, integrating environmental factors into their evaluation [²⁰,²¹,²²]. CHENG et al. [²³] reported that composite vegetated concrete significantly improved vegetation coverage and slope compressive strength, effectively promoting slope vegetation and ecological sustainability [²³, ²⁴]. KONG et al. [²⁵] investigated the effect of municipal solid waste (MSW) on the vegetative properties of modified vegetation concrete and explored the impacts of cement, fertilizer, and straw on plant growth and the anti-scouring ability of concrete with different mix ratios. WANG et al. [²⁶] demonstrated that vegetation concrete exhibits good scouring resistance under conditions such as snowmelt, heavy rainfall, and high-flow velocities.

Although vegetation concrete provides a modern approach to slope protection, studies have shown that plant growth within it does not achieve desired outcomes. For example, plant height and coarseness in vegetation concrete were found to be only half of those in ordinary soil.

Therefore, developing vegetation concrete that supports optimal plant growth remains an important research direction. Cast-in-situ vegetation concrete, with its inherent high porosity and additional punched holes from the construction process, provides more space for vegetation and microorganisms, thereby benefiting biodiversity and environmental protection. As a type of porous eco-concrete prepared through an improved construction process, it is particularly suitable for ecological slope protection. ZHANG et al. [²⁷] investigated the structural compatibility of vegetation concrete with plants and analyzed the relationship between the sand permeability coefficient and pore structure characteristics [²⁸]. WU et al. [²⁹] developed high-green, low-carbon plant-compatible eco-concrete (LCPEC) with self-releasing fertilizer properties, using multi-solid-waste-based low-alkalinity cementitious materials (LACM) and self-combusted gangue aggregates (SCGAs). XU et al. [³⁰] analyzed the role of plants in the erosion process and found that root systems improve soil structure through entanglement and consolidation, thereby increasing slope shear resistance. SHAMSABADI et al. [³¹], through laboratory-simulated rainfall experiments, found that runoff generation on red soil slopes is influenced by both slope gradient and rainfall intensity. Specifically, runoff increased with steeper slopes and higher rainfall intensity.

In summary, the aforementioned studies have contributed significantly to understanding the properties, low alkalinity, and pore structures of vegetation concrete. However, few studies have addressed cast-in-situ vegetation concrete with punched holes. Therefore, this paper focuses on slope scouring resistance tests to explore the effects of construction process parameters (punched hole parameters and construction thickness), slope gradients, and soil type on the performance of cast-in-situ vegetation concrete. This research aims to provide guidelines for engineering applications.

2. MATERIALS AND METHODS

2.1. Materials

2.1.1. Cement

The ordinary Portland cement of P.O 42.5 was used in this test, and its main property indices are shown in Table 1.

Table 1
Cement property indices.

PROPERTY	SETTINGTIME, MIN		STABILITY	STRENGTH, MPA
	INITIAL	FINAL		FLEXURAL (3D)	COMPRESSION (3D)	FLEXURAL (28D)	COMPRESSION (28D)
Results	280	315	Qualified	5.3	26.4	7.8	46.6
Requirement	>45	≤600	Qualified	≥3.5	≥17.0	≥6.5	≥42.5

2.1.2. Aggregate

Basalt aggregates with the particle size of 5~20mm were selected. The relative apparent density reached 2.770, the bulk density was 1.620g/cm³, and the silt content was less than 1%.

2.1.3. Special additive

The special additive from Shanghai Jiuding Vegetation Concrete Co. ltd was used to fabricate the concrete. It is a type of light yellow liquid, and the dosage was 12kg/m³. The main components of the additive include water-retaining agent, organic fertilizer, rooting agent, and alkali-reducing agent, which can improve the water locking and fertilizer holding capacity, provide water and nutrient components required for normal growth of vegetation, and reduce the alkalinity in the pores to make the pH value range suitable for plant growth in the cast-in-situ vegetation concrete, thus improving the survival rate of vegetation.

2.1.4. Mixing water

Ordinary tap water was used in this research.

2.2. Mix ratio design

According to the《Technical Specifications for Pervious Concrete Pavement》 (CJJ/T 135-2009), the absolute volume method was used to carry out the Mix ratio design for cast-in-situ vegetation concrete. proportioning design, The raw material usage per unit volume was shown in Table 2.

Table 2
Cast-in-situ vegetation concrete mixing ratio.

AGGREGATE (kg/m3)	CEMENT (kg/m3)	WATER (kg/m3)	SPECIAL Additive (kg/m3)
1587	288	87	12

2.3. Preparation process

2.3.1. Mixing process

The cast-in-place vegetated concrete was prepared using an HJS-60 twin-shaft forced mixer, typically employing a single-stage feeding method. During the experiment, all aggregates, cement, water, and specialized admixtures were added to the mixer at once in sequence and mixed for 5–8 minutes. The mixing process was monitored to ensure the aggregates were fully coated with the cement paste. The discharge criteria required that the vegetated concrete showed no slurry runoff, had a smooth surface, and exhibited a metallic luster. After mixing, the material was discharged for specimen molding.

2.3.2. Molding method

Based on preliminary research and trial results, a combined “rodding + tamping” method was adopted for specimen molding. The freshly mixed vegetated concrete was placed into the mold in three layers: the first layer was 3 cm thick, followed by a second layer of 3 cm. Each layer was rodded 10–15 times with an φ8 mm steel bar and tamped 10–15 times with appropriate force. Finally, suitable aggregates were used to fill the surface voids, and the surface was gently smoothed without scraping to preserve edge integrity. After molding, the specimens were covered with film to prevent moisture loss and left to stand for 36 hours.

2.3.3. Curing process

Various curing methods, such as standard curing, natural curing, and water tank curing, were considered. Due to the designed high porosity and rapid moisture loss of the vegetated concrete, and the lack of ideal curing conditions on construction sites, water tank curing was selected per the experimental requirements. The demolded specimens were placed in a water tank at room temperature, arranged neatly with adequate spacing. The water level was maintained above the top surface of the specimens, and the water was regularly changed. Curing was continued for 28 days.

2.4. Performance testing

After 28 days of curing, the compressive strength of the cast-in-place vegetated concrete was tested in accordance with the “Standard for Test Methods of Concrete Physical and Mechanical Properties” (GB/T 50081-2019).

The connected porosity, defined as the ratio of interconnected pores to the apparent volume of the specimen, was indirectly calculated by determining the mass difference between the water-saturated specimen and the oven-dried specimen, divided by the mass of an equivalent volume of water. This parameter was used to evaluate whether the internal porosity of the specimen could meet the requirements for vegetation survival and root development. To ensure the accuracy of the porosity test results, three sets of measurements were taken, and the maximum value was recorded.

For the pH test, the vegetated concrete specimens were immersed in a container filled with water until fully submerged. The container was sealed with plastic wrap to prevent water evaporation and minimize external influences. After 24 hours of immersion, the pH value of the solution was measured using a digital pH meter. The water was replaced repeatedly until the pH stabilized, at which point the final reading was recorded.

The results are listed in Table 3. The compressive strength is 7.5 MPa, the connected porosity is 27.5%, and the pH value is 9.8.

Table 3
Technical index of cast-in-situ vegetation concrete.

TEST METRICS	TEST RESULTS (AVERAGE)	TEST METHODS
Compressive strength	7.5 MPa	GB/T50081-2019
Porosity	27.5%	GB/T 50082-2009
PH value	9.8	GB/T 50107-2010

2.5. Test methods

2.5.1. Sample preparations

(1) Molding

Molds with the slope gradient of 1:1.0, 1:1.5, and 1:2.0 were designed, as shown in Figure 1. The design dimensions of the molds were shown in Table 4.

Figure 1
Molds with different slope gradient.

Table 4
Design dimensions of the molds.

SLOPE	SLOPE HEIGHT (cm)	SLOPE WIDTH (cm)	VERTICAL SLOPE HEIGHT (cm)
1:1.0	80	60	20~30
1:1.5	60	60	20~30
1:2.0	40	60	20~30

(2) Soil paving

Clay and sand soil were selected as the under laying soil. The plastic limit index of them was 19.6% and 6.5% respectively. During the paving process, the soil was paved in the molds from bottom to upside, and then compacted by a press plate. The soil was paved by two layers one after another, and each layer needs to be compacted by 80 times to ensure the compactness.

(3) Casting and punching

After mixing, the cast-in-situ vegetation concrete was poured on the soil surface in the mold and casted evenly from bottom to the top. The construction thickness of 6cm, 10cm and 15cm were selected. Meanwhile, two sets of holes were punched by customized tools: Set A had the hole depth of 4.5cm, hole diameter of 2.5cm, and hole distance of 2.8cm. Set B had the hole depth of 4.0cm, hole diameter of 1.0cm, and hole distance of 3.5cm, as shown in Table 5 and Figure 2. To be clear, the hole depth was controlled less than 75% of the ­minimum construction thickness to avoid the negative impact on the overall slope protection structure. Besides, hole diameter and distance determine the hole amount in the unit area, which influence the growth density, vegetation coverage and root development in the unit area. Therefore, two sets of parameters A and B were designed for comparative study, so as to provide guidelines for engineering applications.

Table 5
Punched hole parameters of Set A and Set B.

NUMBER	HOLE DEPTH (cm)	HOLE DIAMETER (cm)	HOLE DISTANCE (cm)
A	4.5	2.5	2.8
B	4	1.0	3.5

Figure 2
Illustration of punched holes.

(4) Curing

After construction, the cast-in-situ vegetation concrete was covered with straw mulch and cured by spraying water to the required curing time.

2.5.2. Simulated wave scouring test

Simulated wave scouring tests were conducted on the bare slope samples without vegetation, so as to explore the anti-erosion capability of cast-in-situ vegetation concrete, and to determine the appropriate construction thickness and punched hole parameters under different slope gradient and soil type.

(1) Experimental design

The influencing factors including slope gradient, soil types, punched hole parameters and construction thicknesses were all considered for the simulated wave scouring test. The parameters were summarized in Table 6 Overall, 36 sets of combination tests were conducted.

Table 6
Parameters and Levels for wave scouring test.

LEVEL	TEST PARAMETERS
	SLOPE GRADIENT	SOIL TYPE	CONSTRUCTION THICKNESS (cm)	PUNCHED HOLE PARAMETERS
1	1:1.0	Clay, Sand	6,10,15	A, B
2	1:1.5
3	1:2.0

Design of Wave Scour Velocity Parameters. The determination of experimental flow velocities was primarily based on long-term hydrological records of representative rivers in the Yangzhou region (e.g., the Ancient Canal and Hangou). During normal stages, flow velocities in these rivers typically range from 0.3 to 0.7m s⁻¹, whereas velocities of 0.8–1.5m s⁻¹ can be reached during flood seasons or intense rainfall events. To systematically evaluate the scour resistance of vegetated concrete under varying hydraulic conditions, representative velocities of 0.5, 0.9 and 1.4m s⁻¹ were selected. The lowest velocity represents low-flow regimes, the intermediate value simulates moderate discharge increases caused by conventional rainfall, and the highest velocity corresponds to short-duration heavy rainfall or local confluence-induced intensified scouring. This design covers the velocity spectrum commonly encountered in slope-engineering practice and introduces a gradient of scour intensities. Each velocity was maintained for a continuous 12h scouring period to monitor the long-term hydraulic performance evolution of the material.

(2) Experimental procedures

The specific procedures of the Simulated wave scouring test were as follows:

Step 1: the concrete sample was lift by a forklift and put into a designed cistern with the dimension of 3m × 1m × 1m. The water level was controlled as half of the height of the sample.

Step 2: Wave was made by a water pump (shown in Figure 3) to simulate the scouring process. Three flow rate of 0.5m/s, 0.9m/s and 1.4m/s were selected, which was monitored by a Flow Rate Meter (shown in Figure 4). and the scouring process lasted for 12h for each flow rate, as shown in Figure 5.

Figure 3
Wave making pump.

Figure 4
Flow velocity monitoring.

Figure 5
Scoring process with the flow velocity of 1.4m/s.

Step 3: The sample was moved out of the cistern after 12h scouring process.

Step 4: Taking the four corners of the concrete in the mold before scouring test as reference area. After scouring test, the settlement of the four corners were measured by vernier caliper to calculate the average settlement. Afterwards, the soil erosion amount can be expressed by equation 1.

(1) $$Q=H_{\alpha }{S}_{\alpha }\gamma \cos \theta$$

In the form: Q— —Soil erosion amount, g;

H_α— —Average settlement, mm;

S_α— —Slope erosion area, cm³;

γ— —Soil density, g/cm³;

θ— —Slope gradient.

2.5.3. Vegetation planting

Tall fescue was chosen for this test due to its advantages of strong adaptability, vigorous root system, superior acid and alkali soil resistance, and fast growth rate. The vegetation planting procedures were as follows:

Step 1: Vegetation planting. Soil with a thickness of 3~5cm was laid on the surface of the cast-in-situ vegetation concrete. Then water-retaining agent (20~50g/m²) and tall fescue seeds (30~38g/m²) were spread. Afterwards, a layer of soil with a thickness of 1~2cm was laid to cover the seeds. The soil had to be watered and moistened completely.

Step 2: Maintenance. After vegetation process, the seeds have to be watered in the morning and evening for 2~3 weeks. Straw mulch was used to keep the moisture and prevent sudden temperature drop.

Step 3: Fertilization. Fertilization is beneficial to promote the growth of tall fescue. Generally, when the rate of emergence is more than 90%, fertilizer (50–100g/m²) can be applied to supplement the nutrients for plants.

Step 4: Observation. the emergence and growth of tall fescue were observed and recorded, as shown in Figure 6.

Figure 6
Growing process of tall fescue.

It is found that tender buds emerged on the slope about 10 days after planting. During the next week, grass germinated one after another and the bud tips turned red. In 2–3 weeks, all grass on the slope have sprouted. In one month, tall fescue grew well to form green seedlings with a certain height. After watering and fertilizing for about 3 months, tall fescue grew so well and turned out to be turf state, which can be used for simulated rainfall scouring test and high-speed flow scouring test.

2.5.4. Simulated rainfall scouring test

Simulated rainfall scouring test tests were carried out on the slope samples with vegetation, so as to explore the runoff erosion resistance and slope protection performance of cast-in-situ vegetation concrete enduring rainfalls.

(1) Experimental design

The influencing factors including slope gradient, punched hole parameters and construction thicknesses were all considered for the simulated rainfall scouring test. The testing parameters were summarized in Table 7.

Table 7
Parameters and levels for rainfall scouring test.

	SLOPE GRADIENT	CONSTRUCTION THICKNESS (cm)	SURFACE HOLE PARAMETERS	RAINFALL INTENSITY (L/min)
1	1:1.0	6, 10	A, B	3.2, 4.8, 6.4
2	1:1.5
3	1:2.0

(2) Experimental procedures

The specific procedures of the simulated rainfall scouring test were as follows:

Step 1: Device development. A self-made rainfall simulation device is developed, which is composed of a support device and a rainfall system, as shown in Figure 7. The support device is constructed of PVC pipe, with the bottom contour of 1m × 1m, and the height can be adjusted between 2m and 3m through multiple connecting ports to meet the test requirements of different slope heights. The rainfall system is composed of water pump, water pipe, regulating valve, pressure gauge and sprinkler. The universal atomizing nozzle produced by Zhejiang changtuo company is adopted, which can produce uniform raindrop distribution under low pressure. The sprinkler is installed on the top of the support, and the vertical distance from its water outlet to the slope is fixed at 2.5m to ensure that the end speed of raindrops is close to the natural rainfall. Rainfall intensity is controlled by regulating valve and precision water pressure gauge (range 0–0.6MPa, accuracy ± 0.5%FS). In order to evaluate the quality of rainfall spatial distribution, the rainfall uniformity in the 1m × 1m core test area was calibrated by the “rain cone array method” before the formal scouring test. By measuring the amount of water collected by 9 uniformly arranged rain cones within 30 minutes of stable rainfall, the rainfall uniformity is calculated to reach 86.5% according to the formula of Christensen uniformity coefficient, which meets the requirements of scouring test for rainfall spatial consistency. The effective rainfall coverage area of the device is about 0–2m², which can be expanded by adjusting the support layout.

Figure 7
Schematic diagram of simulated rainfall.

Step 2: Vegetation check. After 3 months, when the tall fescue turned out to be turf state, it’s suitable for rainfall scouring test.

Step 3: Scouring start. Open the spray-head and regulate the valve to control the rainfall intensity according to the designed value. According to the rainfall situation in Jiangsu, Zhejiang and Shanghai, the representative rainfall intensity was selected to carry out the experiment. Set the rainfall intensity as 3.2L/min, 4.8L/min and 6.4L/min for 60min to carry out the rainfall test.

Step 4: Runoff velocity measurement. When the runoff on the slope was stable, the runoff velocity V0 was measured by staining agent (KMnO4 solution) method.

Step 5: Runoff erosion calculation. The rainwater mixture produced by the runoff erosion with soil was diverted to a water collecting box at the bottom side of the slope with a plastic film. Then the rainwater mixture was set for 12h, and the sediment was dried and weighed. Which was marked as runoff erosion amount W.

Step 6: Hydrodynamics parameters calculation. The Froude number Fr and runoff shear force were calculated according to the test results. Froude number refers to the ratio of the inertia force and gravity of the water flow, which is used to determine the flow pattern of the water flow. It can be expressed by equation 2.

(2) $$Fr=\frac{V_0}{\sqrt{gh}}$$

In the form: V₀— —The runoff velocity, m/s, measured from step 4;

g— —Water gravity acceleration, m/s², g = 9.8 m/s²;

h— —The height of the slope sample, h.

Runoff shear refers to the force of water flow along a slope, which can be calculated by equation 3.

(3) $$\tau =\gamma RJ$$

In the form: τ— —Runoff shear force, N/m²;

γ— —Volume weight of water, kg/m²;

R— —The hydraulic radius can be replaced by the average water depth on the slope, m;

J— —The hydraulic energy slope, expressed by the slope to surface ratio.

2.5.5. High-speed flow scouring test

High-speed flow scouring test was introduced to investigate the combination performance of plants and vegetation concrete structures, and to evaluate whether concrete structure and plant roots can resist water erosion.

(1) Experimental design

The influencing factors including punched hole parameters and construction thicknesses were considered for the high-speed flow scouring test. The parameters and levels were summarized in Table 8.

Table 8
Parameters and levels for high-speed flow scouring test.

CONSTRUCTION THICKNESS (cm)	PUNCHED HOLE PARAMETERS	FLOW RATE (L/min)
6,10	A,B	8

(2) Experimental procedures

Step 1: Vegetation growth Assessment. The leaf height, stem diameter and root length of the tall fescue on the surface of the vegetation concrete were measured by a tape rule (shown in Figure 8).

Figure 8
Illustration of vegetation growth measurement.

Step 2: Sample cut. Typical areas with punched holes or without holes were selected and cut to be pieces with the area of around 10cm × 10cm. Then, the tall fescue coverage rate per unit area was detected by grid method.

Step 3: Scouring start. A high-pressure washer produced by Black Cat Group (Hong Kong) Cleaning Machinery Co., Ltd. is used, fitted with a cylindrical nozzle 1.2 mm in diameter that generates a concentrated, columnar water jet. At the rated working pressure the unit delivers a steady flow of 8 L min⁻¹, corresponding to an outlet-pressure range of 6–9 MPa. The axis of the lance is held at a 90° impingement angle to the specimen surface, and the perpendicular distance from the nozzle exit to the surface is fixed at 30 cm. The cut sample block is rinsed omnidirectionally for one hour.

Step 4: Vegetation situation checks. After the scouring test, the status of roots and leaves of tall fescue was observed, and vegetation coverage rate was calculated by grid method (as shown in Figure 9). It will be considered to be qualified when the rate exceeds 80%.

Figure 9
Plant height 24cm.

3. RESULTS AND DISCUSSIONS

3.1. Results of Simulated wave scouring test

3.1.1. Impact of punched hole parameters on wave erosion

The influence of punched hole parameters on soil erosion was illustrated in Figure 10. As shown in Figure 10 barely no difference can be observed in soil erosion amount no matter which set of punched hole parameters were used for the vegetation concrete, when other factors are the same.

Figure 10
Results of soil erosion amount when different punched hole parameters used

This is because that cast-in-situ vegetation concrete possesses a continuous porous structure with very rough and permeable surface. During the wave scouring test, the water can permeate into the porous structure directly. Therefore, additional hole structure has little impact on the wave climbing or refluxing on the slope. However, the main function of the punched holes is to provide more space for plant growth [³²], which will be investigated during the rainfall scouring test. The experimental study by SIRICO et al. [³²] demonstrated that well-designed pore structures (such as appropriate diameter and spacing) can significantly promote root system development and enhance vegetation survival rates by improving soil contact and water availability. Anyway, punched hole parameters of Set A will be used for the following analysis of wave scouring test.

3.1.2. Impact of scouring rate on wave erosion

The effect of scouring rate on the soil erosion amount was shown in Figure 11. The missing data in the bar graph is due to the absence of wave erosion. It can be found from Figure 11 that there is a positive relationship between scouring rate and soil erosion amount in both clay and sandy soils. Taking underlying clay soil as an example, when the scouring rate was 0.5m/s, few soil erosion amount could be observed. When the scouring rate increased to 1.4 m/s, soil erosion amount intensified up to 359.33g. Meanwhile, obvious soil erosion could be found since the water near the cast-in-situ vegetation concrete became turbid.

Figure 11
Results of soil erosion amount when different scouring rate used

This is because that when the wave speed is higher, the wave can climb higher along the slope. Thus, more area of underlying soil would be infiltrated by the wave water.

3.1.3. Impact of slope gradient on wave erosion

The effect of slope gradient on the soil erosion amount was illustrated in Figure 12. As shown in Figure 12, when the soil type is the same, the overall soil erosion amount first increases and then decreases with the increasing of the slope gradient. The soil erosion amount reached the highest value when the slope was 1:1.5.

Figure 12
results of soil erosion amount when different slope gradient used

To elucidate the underlying mechanism of how slope gradient affects scouring magnitude, this study analyzes the interplay between wave run-up theory and flow shear stress distribution. Wave-induced scouring is governed by two competing factors: the effective area covered by wave run-up and the flow shear stress acting on the slope surface.According to the classical formula proposed by HUNT JUNIOR [³³], R/H₀ = ξ (where H₀ is the wave height and ξ is the Iribarren number, related to slope gradient and wave steepness), the wave run-up height increases with steeper slopes, thereby expanding the effective scouring area and tending to enhance erosion.

However, the direct driving force of erosion originates from the flow shear stress (τ). On gentle slopes (e.g., 1:2.0), flow velocity is low, resulting in limited shear stress and minimal erosion. When the slope increases to 1:1.5, the downslope component of gravity becomes more pronounced, altering the velocity profile and significantly increasing bed shear stress, which leads to intense scouring. As the slope further steepens to 1:1.0, the flow regime shifts from sheet flow to impact-dominated behavior, reducing the flow residence time on the slope. Consequently, flow energy is converted more into impact pressure rather than sustained shear stress, causing a decline in effective, continuously acting bed shear stress.

Therefore, the influence of slope gradient on scouring magnitude results from the combined effect of these two factors: from gentle slopes to 1:1.5, both the effective area and shear stress increase, leading to a peak in erosion; beyond this point, although the effective area remains relatively large, the dominant reduction in shear stress causes the erosion magnitude to decrease.

3.1.4. Impact of soil type on wave erosion

The effect of soil type on the soil erosion amount were shown in Figure 13. It can be seen from Figure 13 that when the slope gradient was the same, the soil erosion amount of samples using sand underlying soil was 94.32 to 226.38 g more than that of using clay one. Taking the condition of slope gradient of 1:1.0, scouring rate of 0.9m/s and thickness of 6 cm for example, the soil erosion amount of samples using sand was 3.2 times that of using clay. This is because that the particle size of sand is relatively larger and has superior permeability. Meanwhile, the bonding ability between the particles is poor, and it will soften and collapse when it encounters water. On the contrary, the clay itself presents inferior permeability with very fine particles. The binding ability between particles is strong, which can contribute to enhancing its anti-erosion ability with water. However, when the moisture content tends to be saturated, the clay will become fluid slurry causing erosion lose. Overall, soil type has a pronounced impact on the soil erosion. Slope treatment should be carried out if necessary to ensure the safety of slope protection.

Figure 13
Results of soil erosion amount when different soil type used.

3.1.5. Impact of construction thickness on wave erosion

The influence of construction thickness on the soil erosion amount was illustrated in Figure 14. It was found from Figure 14 that the thickness of cast-in-situ vegetation concrete was able to impact the of soil erosion amount significantly, presenting the tendency of the soil erosion decreasing as the thickness increasing.

Figure 14
Results of soil erosion amount when different thickness used.

When the scouring rate was as slow as 0.5m/s, no clay soil erosion could be observed. However, sand soil erosion decreased with an increasing thickness. Specifically, compared with samples of 6cm thickness, the sand soil erosion amount reduced by 37% and 54% for sample with 10 cm and 15 cm thickness, respectively. When the scouring speed increased up to 1.4m/s, clay soil erosion amounts of samples with 6cm and 10cm thickness were mostly within 150g. The leaching amount was very small with a maximum value of 71.9g when the thickness reached 15cm. As for the sand soil, the erosion amount was around 280.84g~359.33g for the samples with the thickness of 6cm. However, the erosion amount reduced by 28% and 67% on average when the thickness increased to 10cm and 15cm, respectively. Overall, the thickness of 15cm can guarantee a safe erosion amount of less than 255.31g for sand soil. This is due to the increased bonding points of internal pore structure caused by the bigger thickness, which prevents the soil from leaching by erosion [³⁴]. All data above are averages at three slope gradients.

In summary, based on the effect of scouring speed, slope gradient, soil type and other parameters, suitable construction thickness of cast-in-situ vegetation concrete was proposed for slope protection. The recommended thicknesses presented in Table 9 are derived from the wave scour test results, which represent the most severe hydraulic loading condition encountered in this study (a flow velocity of 1.4 m/s sustained for 12 hours). The selected thickness for each soil type and slope gradient is the minimum value that demonstrated successful resistance to scour, ensuring both structural integrity and the promotion of vegetation establishment under these demanding conditions.

Table 9
Recommended thickness of cast-in-situ vegetation concrete for slope protection(flow velocity of 1.4 m/s for a continuous 12-hour duration).

SLOPE	SOIL TYPE	RECOMMENDED THICKNESS (cm)
1:2.0	Clay	6
1:1.5	Clay	10
1:1.0	Clay	10
1:2.0	Sand	10
1:1.5	Sand	15
1:1.0	Sand	15

3.2. Simulated rainfall scouring test

The samples with underlying clay soil were used for simulated rainfall scouring test. According to the recommended Thickness from wave scouring test, the construction thicknesses of 6 cm and 10 cm were selected for rainfall scouring test.

3.2.1. Impact of punched hole parameters on runoff erosion

The influence of punched hole parameters on the of runoff erosion amount was shown in Figure 15. It can be found from Figure 15 that both types of the punched holes can slow down the runoff flow. It’s noticed that the leaves of tall fescue can reduce the splashing of rain on the slope, and the dense and strong stems can intercept the runoff water. Thus, the tall fescue can play a role in weakening runoff erosion, presenting obvious capability to consolidate soil and protect slope. However, the runoff erosion amount of the sample with Set A hole parameters was less than that of sample with Set B.

Figure 15
Results of runoff erosion amount when different hole parameters used

The relationship of runoff shear force and runoff erosion amount was shown in Figure 16. it can be seen from Figure 16 that bigger runoff shear force can result in higher erosion amount. No matter which slope gradient was used, the samples with Set B hole parameters suffered higher runoff shear force and presented more runoff erosion loss. This is because tall fescue grew better in Set A holes due to the bigger hole diameter, and had superior capability to intercept the runoff. Therefore, all the following analysis of this section were performed using the results from samples with Set A hole parameters.

Figure 16
Runoff erosion amount versus runoff shear force when different hole parameters used

3.2.2. Impact of rainfall intensity on runoff erosion

The effect of rainfall intensity on runoff erosion was illustrated in Figure 17. it can be seen from Figure 17 that runoff erosion amount increased with increasing rainfall intensity. For instance, when the slope gradient was 1:1.5 and thickness was 6cm, the runoff erosion amount increased from 141.8g to 221.8g when the rainfall intensity increased from 3.2L/min to 6.4L/min. Besides, it increased by 56% for the concrete with the thickness of 10cm.

Figure 17
Results of runoff erosion amount when different rainfall Intensity used

The hydrodynamics parameters of samples with the thickness of 10cm were calculated. The Froude numbers Fr undering different testing conditions were summarized in Table 10. Froude number Fr is an indicator to estimate the flow state of a fluid. The flow can be recognized as Laminar flow, Transitional flow and turbulence flow when Fr is less than 1.0, equals 1.0, and more than 1.0, respectively. It can be seen from Table 10 that the flow state of runoff depended on the runoff intensity and slope gradient. The Fr number increased with an increasing in rainfall intensity and slope gradient. When the rainfall intensity is more than 4.8L/min and the slope gradient reach 1:1.0, the runoff on the vegetation concrete slope was prone to be in the turbulence state.

Table 10
Froude number Fr of 10cm paving thickness specimen.

SLOPE	SURFACE HOLE PARAMETERS	RAINFALL INTENSITY (L/min)
		3.2	4.8	6.4
1:2.0	A	0.63	0.71	0.82
	B	0.65	0.73	0.88
1:1.5	A	0.79	0.90	0.95
	B	0.83	0.94	0.99
1:1.0	A	0.98	1.10	1.18
	B	1.05	1.14	1.22

The relationship between runoff shear force and soil erosion amount was illustrated in Figure 18. It can be seen from Figure 18 that the runoff shear force is positively correlated with rainfall intensity, and the bigger rainfall intensity, could lead to higher runoff erosion amount. For instance, when the samples with the slope gradient of 1:1.0 and thickness of 10cm, the runoff erosion amount increased by 25% and 49% when the rainfall intensity increased from 3.2L/min to 4.8L/min and 6.4L/min. This can be explained by the fact that when the rainfall intensity was as low as 3.2L/min, it takes a long time for runoff to be produced. The well grew tall fescue can intercept the runoff effectively to reduce the erosion. When the rainfall intensity increased to 6.4L/min, the runoff formed fast and presented strong runoff shear force, which will destroy the adhesion between soil particles and make the soil particles dispersed, leading to higher erosion amount.

Figure 18
Runoff erosion amount versus runoff shear force when different rainfall intensity used

The transition in flow regime fundamentally influences the runoff scouring capacity. In subcritical flow (Fr < 1), characterized by laminar or low-energy conditions, the sediment transport capacity is limited. However, as rainfall intensity and slope increase, the transition to supercritical turbulent flow (Fr > 1) significantly intensifies erosion. Turbulence enhances near-bed velocity fluctuations and vertical momentum exchange, resulting in instantaneous shear stresses that substantially exceed the time-averaged values. This vigorous mixing action more effectively dislodges soil particles and disrupts inter-particle cohesion. Consequently, the sharp increase in soil erosion observed under high rainfall intensities and steep slopes (e.g., 1:1.0) is not merely a result of increased flow volume but is fundamentally driven by the shift to a more energetic and chaotic turbulent state, which dramatically enhances the flow’s sediment-entraining capacity.

3.2.3. Impact of slope gradient on runoff erosion

The effect of slope gradient on runoff erosion was shown in Figure 19. It shows that the runoff erosion amount was positively correlated with slope gradient under each thickness and rainfall intensity. For instance, when the samples endured the rainfall intensity of 6.4L/min, the runoff erosion amount increased from 189.6g to 259.3g when the slope gradient increased from 1:2.0 to 1:1.0 for the concrete thickness of 6cm. Besides, runoff erosion amount of sample with the thickness of 10cm was at the same level as that of the sample with the thickness of 6cm. It indicated that no obvious difference in soil erosion can be found for the vegetated vegetation concrete with different thickness.

Figure 19
Results of runoff erosion amount when different slope gradient used.

The relationship between slope gradient and runoff shear force was shown in Figure 20. It can be seen from Figure 20. that when the rainfall intensity was the same, the runoff shear force increased with the increasing slope gradient. For instance, when the construction thickness was 10cm, the punched hole parameter was Set A, and the rainfall intensity was 6.4L/min, the runoff shear force of the samples with the slope of 1:1.5 and 1:1.0 increases by 19% and 41%, respectively, compared to that of the samples with slope of 1:2.0.

Figure 20
Runoff erosion amount versus runoff shear force when different slope gradient used

To sum up, it is found that slope gradient and rainfall intensity are the main reasons for runoff erosion, and punched hole parameters has slight effect and construction thickness has no obvious effect on it. With the increase in slope gradient, runoff is significantly affected by gravity, which accelerates soil erosion. With the increase in rainfall intensity, runoff shear force is further enhanced, and the flow on the slope becomes turbulent, resulting in fast runoff erosion. According to the rainfall scouring test results, the appropriate construction thickness and punched hole parameters of cast-in-situ vegetation concrete with vegetation are proposed, as shown in Table 11.

Table 11
Recommended construction thickness and punched hole parameters of cast-in-situ vegetation concrete with vegetation.

SLOPE	CONSTRUCTION THICKNESS (cm)	PUNCHED HOLE PARAMETERS
1:2.0	6	A
1:1.5	10	A
1:1.0	10	A

3.3. High-speed flow scouring test

3.3.1. Vegetation growth assessment before scouring

The leaf height, stem diameter and root length of the tall fescue and its coverage rate per unit area were summarized in Table 12. In addition, No.3 and No.6, which were ordinary vegetation concretes without punched holes, were used for comparison.

Table 12
Vegetation growth.

NUMBER	THICKNESS (cm)	PUNCHED HOLE PARAMETERS	GROWTH HEIGHT (cm)	ROOT DIAMETER, (cm)	ROOT LENGTH, (cm)	VEGETATION COVERAGE RATE (%)
1	6	A	21.3~25.6	0.13~0.28	6.5	94.1
2	6	B	20.4~24.8	0.10~0.25	2.8	93.2
3	6	/	16.8~21.7	0.08~0.24	2.3	88.7
4	10	A	20.6~24.8	0.12~0.29	4.8	95.7
5	10	B	19.6~23.9	0.10~0.24	1.9	94.5
6	10	/	17.1~22.5	0.08~0.24	1.1	86.6

It can be seen from Table 12 that samples with punched holes, no matter with Set A or Set B, presented higher vegetation coverage rate up to over 93%. Comparatively speaking, coverage rate of the ordinary vegetation concretes without punched holes just reached 88.7% and 86.6% for sample with thickness of 6cm and 10cm, respectively. This is because the punched holes can provide more space and restore water and nutrients for tall fescue to grow. In this way, the tall fescue can grow more quickly through the pores within the cast-in-situ vegetation concrete and reach the underlying soil to develop roots. Besides, coverage rate of samples with parameters Set A was slightly higher than that of samples with parameters Set B, since the hole diameter was bigger in Set A.

In addition, construction thickness affected the root growth to some extent. It becomes more difficult for plant roots to pass through the cast-in-situ vegetation concrete and to reach the underlying soil, as the thickness increasing. The root length of tall fescue on the samples with the thickness of 6cm was 1.5 times than that of the samples with the thickness of 10cm. The growth time of this planting was as short as 3 months. Given adequate growth time, tall fescue can complete the intended growth target on the cast-in-situ vegetation concrete samples with the thickness of 10cm.

3.3.2. Vegetation situation checks after scouring

After the high-speed flow scouring test, the status of roots and leaves of tall fescue, the vegetation coverage rate of each sample were checked and examined. the results were shown in Table 13. The results were shown in Table 13.

Table 13
Vegetation situation after high-speed flow scouring.

NUMBER	THICKNESS (CM)	PUNCHED HOLE PARAMETERS	SITUATION AFTER SCOURING
			TALL FESCUE	UNDERLYING SOIL	CAST-IN-SITU VEGETATION CONCRETE
1	6	A	Intact and qualified	None	Intact
2	6	B	Intact and qualified	None	Intact
3	6	/	Intact and qualified	None	Intact
4	10	A	Intact and qualified	None	Intact
5	10	B	Intact and qualified	None	Intact
6	10	/	Intact and qualified	None	Intact

As can be seen from Table 13, all the samples of the cast-in-situ vegetation concrete have not been damaged with intact structure. The surface soils have been washed away. The tall fescue was all intact and undamaged, resting on the surface of the cast-in-situ vegetation concrete, no matter Set A or Set B parameters were used.

From the perspective of the thickness, as the high-speed flow scouring lasting, the tall fescue just bowed down and the root system was gradually exposed, without falling off, washing away and other phenomena, no matter the thickness of 6cm or 10cm was used. This is due to the fact that most of the tall fescue has grew within the pore structures of the vegetation concrete.

Taking into the results of rainfall scouring test and high-speed scouring test, the thickness of 6cm or 10cm, is suitable for plant growth and can shorten the construction period. They were recommended as the thickness of the cast-in-situ vegetation concrete with underlying clay soil.

4. CONCLUSIONS

This paper developed a novel cast-in-situ vegetation concrete with punched holes. Simulated wave scouring test, rainfall scouring test and high-speed flow scouring test were conducted to investigate the scouring resistance of this vegetation concrete when being used as slope protection materials. Under the above-mentioned testing conditions, the following conclusions could be drawn.

• (1)

  The results of simulated wave scouring test for the bare slope without vegetation showed that flow rate was positively correlated with wave erosion amount. The influence of slope gradient on wave erosion presented the tendency of increasing firstly and then decreasing, and the wave erosion amount reached the maximum value when the slope gradient was 1:1.5. The soil type had a significant effect on the wave erosion amount. When sand soil was used, the erosion amount was 94.32 to 226.38 g more than that of clay soil. However, the wave erosion amount can be effectively reduced by increasing the thickness of the concrete.

• (2)

  Tall fescue was selected for the vegetation planting of cast-in-place vegetation concrete. The tall fescue grew well and presented a vegetation coverage rate of over 90% after 90 days of growth.

• (3)

  The results of simulated rainfall scouring test for the slope with vegetation showed that runoff erosion amount was positively correlated with slope and rainfall intensity. When the rainfall intensity was more 6.4L/min and the slope gradient was 1:1.0, the runoff on the vegetation concrete slope turned into the turbulence state. This led to higher runoff shear force, and resulted in increased runoff erosion amount, which increased by 41% compared to that of samples with the slope of 1:2.0.

• (4)

  Although construction thickness had no significant effect on runoff erosion amount, the punched hole parameters did. Runoff erosion amount of samples with hole parameters Set A(Hole depth of 4.5cm, Hole Diameter of 2.5cm, and Hole distance of 2.8cm) was less than that of samples with hole parameters Set B(Hole depth of 4cm, Hole Diameter of 1cm, and Hole distance of 3.5cm). Therefore, hole parameters of Set A were recommended for engineering application.

• (5)

  The results of high-speed flow scouring test showed that after certain time’s scouring, no damages could be observed in both tall fescue and cast-in-situ vegetation concrete under various conditions, and the combination of both was intact. Besides, the growth time of tall fescue was as short as 3 months. The root length of it in the samples with concrete thickness of 6cm was 1.5times than that of samples with thickness of 10cm, indicating that thinner thickness can contribute to the rapid growth of root into the underlying soil. Therefore, thinner thickness is recommended if it meets the requirements for slope protection.

• (6)

  By synthesizing the results from wave scour, rainfall scour, and high-flow jet tests, and balancing the structural safety requirements against scouring with the ecological need to promote rapid root penetration, this study proposes appropriate construction thicknesses for Cast-in-place Vegetation Concrete (CVC). For clay subsoils, the recommended thickness is 6 cm for a 1:2.0 slope, and 10 cm for both 1:1.5 and 1:1.0 slopes. For sandy subsoils, the recommended thickness is 10 cm for 1:2.0 and 1:1.5 slopes, and 15 cm for a 1:1.0 slope. This recommendation system aims to achieve an optimal balance between protective performance and ecological function. The selection of relatively thinner configurations (e.g., 6 cm) not only meets the anti-scouring requirements but also significantly facilitates the extension and anchorage of plant roots into the underlying soil.

5. ACKNOWLEDGMENTS

This study is financially supported by the National Natural Science Foundation of China (Grant No. 52178439) and the Yangzhou Government-Yangzhou University Cooperative Platform Project for Science and Technology Innovation (No. YZ2020262).

6. DATA AVAILABILITY

Data will be made available on request.

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Publication Dates

• Publication in this collection
  02 Feb 2026
• Date of issue
  2026

History

• Received
  04 Sept 2025
• Accepted
  02 Dec 2025