The Novel Application of a Geosynthetic as Vegetation Substrate for Ecological Restoration on Steep Concrete and Rock Slopes

Abstract

Civil, transportation, and hydraulic projects often result in concrete or rocky slope surfaces that have difficultly sustaining vegetation due to the lack of suitable substrate. A geosynthetic-based vegetation substrate was proposed to replace traditional soil-based vegetation substrates for vegetation restoration on steep concrete or rock surfaces. The geosynthetic vegetation substrate (GVS) provides the following four key functions for vegetation restoration: 1. Germination environment for seeds. 2. Room for root development and vegetation fixation. 3. Allowing water and nutrients to be transported and stored within the substrate. 4. Sufficient strength to support vegetation on steep or vertical surfaces. An 8-month field study revealed the following: vegetation leaf length peaked at over 400 mm by the 100th day, with annual fresh biomass reaching 2.99 kg/m² (94% from stems/leaves). The geosynthetics maintained 91.6% to 99.5% of initial tensile strength and 82.9% to 98.2% creep resistance. These findings establish GVS as a viable solution for ecological restoration on engineered slopes.

1. Introduction

Ecological damage refers to single or multiple events that disrupt the symbiotic relationship between organisms and their habitats [1]. The severity of ecological damage resulting from these events can range from minor to catastrophic. While minor ecological damage may be self-repaired by ecosystems, severe ecological damage exceeds the ecosystem’s capacity for self-recovery, potentially leading to complete ecological degradation [2,3]. Internationally, significant attention has been devoted to ecosystem restoration [4,5]. In 2019, the United Nations declared 2021 to 2030 as the “UN Decade on Ecosystem Restoration”, with the aim of intensifying efforts to prevent, halt, and restore damage to global ecosystems [6]. By designating ecosystem restoration as a global priority, the United Nations highlights its critical role in sustainable development [7].

With the expansion of infrastructure, ecosystems such as forests, grasslands, and rivers have experienced varying degrees of ecological degradation [1]. Infrastructure construction has resulted in steep land surfaces composed of concrete or rock. During construction, the land surface ecosystem is often damaged beyond its capacity for self-repair [8]. After construction, these slopes are characterized by hardness, low water retention, poor surface adhesion, and a lack of suitable substrates for vegetation establishment [9]. Since vegetation is a critical component of ecosystems [10,11,12,13], its absence reduces biodiversity and exerts significant adverse impacts on sustainable development [14]. In other words, the harsh conditions on concrete and rock slopes hinder vegetation restoration [15], thereby complicating environmental rehabilitation and the pursuit of sustainable infrastructure development.

Recently, a series of slope vegetation restoration techniques were developed. The most commonly employed techniques include guest soil spraying [16,17,18], vegetation concrete spraying [19], and the use of planting bags [20]. Guest soil spraying was effectively implemented on a rock slope with a 57-degree inclination along the Chengwu Expressway [21]. Vegetation concrete spraying proved effective in achieving robust vegetation growth and greening effects, as observed by Kim and Park [22] on riverbank slopes, with Chang et al. [23] successfully applying it along the Hechi–Baise Expressway and Mohammed et al. [24] confirming its long-term protective capabilities on highway embankments. The planting bag technique stabilized slopes by preventing soil runoff and facilitating root growth, thereby meeting ecological restoration requirements on slopes of varying steepness [20,25].

The aforementioned techniques have demonstrated varying degrees of success In restoring vegetation on concrete and rock slopes. However, these techniques face significant challenges when applied to steep rock and concrete slopes. First, they often suffer from poor stability, especially on steep slopes. In guest soil spraying applications, base materials may accumulate at the slope toe due to erosion [16], thereby requiring additional measures to control runoff and channeling. Although vegetation concrete offers a stronger substrate than guest soil spraying, it still faces stability issues on steep slopes. The incorporation of seeds and organic materials weakens the bond between cement particles, diminishing the original concrete strength and leading to cracking [26]. In the planting bag technique, water absorption increases the bags’ weights, leading to slippage and overall instability [27]. Second, irrigation presents additional challenges. On guest soil-sprayed surfaces, irrigation must be carefully controlled to prevent erosion. Vegetation concrete requires large volumes of water due to its low field capacity, increasing the risk of slope waterlogging under excessive irrigation [28]. The planting bag technique imposes strict limits on water volume within the bags, as excess water rapidly increases their weight, thereby undermining slope stability [27]. Third, all these techniques face challenges related to rapid nutrient loss and difficulties in nutrient replenishment on steep slopes. These methods require periodic fertilization. In guest soil spraying, binder additives reduce soil microbial activity, thereby impairing organic matter decomposition efficiency. Zhang et al. emphasized the need for experimental research on substrates to enhance fertility and microbial activity [29], thereby improving their capacity for self-driven organic matter decomposition. Combined with rain-induced erosion and frequent irrigation, these factors lead to rapid nutrient depletion that is difficult to reverse. Meanwhile, due to the incorporation of cement, vegetation concrete exhibits elevated alkalinity, which not only hampers vegetation growth but also constrains microbial activity and organic matter decomposition [30]. The substantial water requirements of the vegetation concrete technique also lead to significant nutrient leaching [28]. By contrast, fertilizer must be periodically added to the bagged soil to meet vegetation growth requirements [27]. Because the soil is enclosed within bags, the planting bag technique poses challenges for nutrient replenishment through fertilization.

Table 1. Ecological slope protection technique.

Technique	Scope of Application	Disadvantages
Guest soil spraying	Suitable for gentle slopes of rock and concrete.	1. The poor stability of the substrate 2. Rapid nutrient leaching 3. Substrate accumulation at the slope’s base
Vegetation concrete	Suitable for rock and concrete slopes with a maximum inclination of 85 degrees.	1. The risk of detachment 2. Diminishes the initial strength 3. Increases the alkalinity of the substrate 4. The lower part is submerged
Planting bag	Suitable for concave areas on rock and concrete slopes with a maximum inclination of 65 degrees.	1. Poor stability, results in slippage 2. Regular addition of fertilizer

presents the appropriate applications and associated drawbacks of the three aforementioned vegetation restoration techniques. As shown in Table 1, reduced stability, irrigation difficulties, and nutrient depletion represent the primary challenges for current techniques on steep rock and concrete slopes. As a result, soil-based ecological restoration methods are unsuitable for steep concrete and rock slopes. To mitigate environmental damage and enhance sustainability on such slopes, a novel ecological restoration technique that does not depend on soil as a substrate is required.

The “Geosynthetic-Vegetation-Substrate (GVS)” was proposed, which innovatively integrates materials, structural design, and ecological functions to address ecological restoration on steep concrete and rock slopes. It lies in a multi-layered composite structure combining high-strength geotextiles (tensile strength up to 300 kN/m) with biodegradable cover layers (e.g., coconut fiber, pulp paper), which mimic soil functions (seed germination, root anchorage, water/nutrient storage) while resolving stability and nutrient supply challenges inherent to traditional substrates on steep slopes. The biodegradable layer, embedded with seeds via organic adhesives, creates a humid microenvironment to promote germination and gradually decomposes to release nutrients. Leveraging the geotextile’s high hydraulic conductivity (permittivity 1–2 s⁻¹), a recirculating irrigation system enables vertical water penetration and residual water recovery, reducing irrigation demand to 0.8–1.0 L per cycle. Vegetation roots penetrate the geotextile to form a self-sustaining network regulated by matrix suction (−30 kPa to −100 kPa), aligning with plant water potential requirements. By overcoming limitations of traditional soil-based methods in stability, resource efficiency, and long-term ecological maintenance, GVS establishes a scalable engineering solution for sustainable slope restoration.

2. Materials and Methods

2.1. Conceptual Mechanism of Geosynthetic Vegetation Substrate (GVS)

A suitable vegetative substrate for steep concrete and rock slopes must support vegetation both botanically and mechanically. According to Plant Physiology [31], a vegetation substrate should have the following characteristics: 1. Allow free water infiltration. 2. Possess adequate water and nutrient storage. 3. Maintain a suitable pH range. 4. Exhibit appropriate porosity to store organic matter. Moreover, the substrate must be sufficiently porous to facilitate root penetration and secure plant anchorage on steep slopes. Geotextiles, as porous fabric materials, share soil-like properties conducive to vegetation growth, including stable pH, unrestricted water infiltration, and porous spaces for organic matter storage and root anchorage. Although most geotextiles are hydrophobic and cannot retain substantial water, incorporating wicking fabrics can significantly enhance the matrix suction of the material. Additionally, geotextiles are substantially lighter than soil, especially at high saturation levels, thereby reducing stress on slope stability. Furthermore, the high transmissivity of geotextiles allows water and nutrients to move freely within their plane, delivering these resources directly to plant roots. This property can replace conventional spray or sprinkler irrigation systems, thereby reducing overall water consumption. By collecting and reusing excess water, nutrient leaching losses can be minimized.

Building on the similarities between soil and geotextiles, this study proposes a new slope ecological restoration technique employing a geosynthetic vegetation substrate (GVS) coupled with a recirculating irrigation system.

The geosynthetic vegetation substrate (GVS) consists of a geotextile base layer bonded to a biodegradable cover layer. Grass seeds are sandwiched between the base and cover layers, as demonstrated in Figure 1a. Due to the high tensile strength of the geotextile, the GVS sheet can be suspended from the slope crest. The recirculating irrigation system includes a drip irrigation setup at the top of the GVS and a runoff collection system at its base. When irrigation commences, water infiltrates the base layer from above and moves downward. As the biodegradable cover layer becomes moistened, it creates a humid microenvironment conducive to seed germination. During germination, herbaceous plant roots grow toward water sources, penetrating the base layer and thereby facilitating robust vegetation anchorage. The root system’s water uptake from the GVS is driven by differences in suction potential. Most grass roots can absorb water from substrates with water potentials ranging from −30 kPa to −100 kPa. Meanwhile, the suction characteristics of geotextiles are analogous to those of soil and can be described as a function of water content [32,33]. Therefore, by adjusting irrigation frequency, the suction within the geotextile can be regulated, enabling the GVS to continuously supply water to vegetation (Figure 1b). Vegetation roots continuously draw water from the GVS, which is subsequently transported through the plant and transpired via leaf stomata. This water transport supports photosynthesis and promotes vegetative growth. Over time, the biodegradable layer and senescent leaves accumulate on the GVS, supplying nutrients to younger plants.

Excess irrigation water is collected at the base of the GVS and subsequently reused for irrigation. The collected water is reused for irrigation. This collection process serves the following functions: 1. Reducing the amount of water required for irrigation. 2. Preserved the water-soluble nutrients leached by water movement. 3. Prevent water-soluble nutrient and pesticide pollution. 4. Collecting precipitation during rainfall.

2.2. GVS Composition

To evaluate the feasibility of GVS as an ecological restoration technique for steep rock and concrete slopes, a series of small-scale outdoor vegetation growth tests were conducted in Sichuan Province, China. Two types of geotextiles and three types of biodegradable materials were employed to construct GVS samples (Figure 2). Considering the local climatic conditions, two herbaceous species—tall fescue (Festuca arundinacea) and Bermuda grass (Cynodon dactylon)—were selected as restoration vegetation. The growth test was conducted under minimal management conditions, providing only water.

As a base layer, one woven geotextile and one non-woven geotextile were selected. The properties of the two geotextiles are summarized in

Table 2. Properties of two geotextiles.

Property	Woven	Non-Woven
Apparent opening size O95 (mm)		0.19
Nominal mass (g/m2)	730	250
Tensile strength (kN/m)	300	19
Permittivity (sec−1)	1	2

Note: Above data were obtained from TenCate Geosynthetics Asia website (https://www.solmaxchina.com/Content/2462149.html, accessed on 30 January 2025).

.

The biodegradable cover layer provides initial seed attachment and a suitable germination environment. Therefore, it must 1. protect seeds against erosion, 2. be capable of maintaining moisture, and 3. allow leaves to penetrate. Based on these requirements, three materials—coconut fiber, tissue paper, and pulp paper—were selected as cover layers.

Table 3. Composition of test samples.

Samples	Base Layer		Biodegradable Cover Layer			Vegetation
	Woven	Non-Woven	Coconut Fiber	Tissue Paper	Pulp Paper	Tall Fescue	Bermuda Grass
G1 *	√	√	√				√
G2	√		√				√
G3 *	√	√	√				√
G4	√		√			√
G5	√		√			√
G6	√				√	√
G7	√			√		√
G8	√				√		√

*: Consisted of layer of woven geotextile over layer of non-woven geotextile.

summarizes the composition of the tested specimens.

The vegetation species selected were tall fescue (Festuca arundinacea) and Bermuda grass (Cynodon dactylon), commonly used for ecological restoration in Sichuan. The selection of tall Fescue (Festuca arundinacea) and Bermuda grass (Cynodon dactylon) for ecological restoration of highly steep rock slopes in Sichuan is grounded in their complementary ecological functions and synergistic restoration mechanisms. As a cool-season species, tall Fescue anchors rock fissures with its vertical root system (1–2 m depth) and adapts to Sichuan’s moderate climate (annual mean 12–18 °C) via C3 photosynthesis, while its cold tolerance (−15 °C) addresses winter vegetation coverage gaps. Bermuda grass, a warm-season species, utilizes C4 photosynthesis to thrive under high temperatures (>35 °C) and intense light, with stolons expanding at 1–2 cm/day to form dense turf that reduces surface runoff by 60–80% and mitigates erosion.

To prepare the coconut fiber samples, grass seeds were first mixed with an organic adhesive, then evenly applied to the base layer. The coconut fibers were subsequently unfolded, trimmed to match the base dimensions, and securely stitched onto the base layer. Preparation of the tissue paper samples followed a similar procedure. The grass seeds were combined with the same organic adhesives and uniformly applied to the base layer. A single layer of tissue paper was then placed over the seeded surface. As the adhesive dried, the tissue paper adhered to the geotextile.

To produce the pulp sample, grass seeds, water, organic adhesives, and commercially ready paper pulp are mixed, creating a slurry. Then, the slurry is poured into a mesh frame with a pore size of 0.2 mm, allowing the mixture to adhere evenly to the mesh. The frame is then air-dried for 48 h. Once dried, the pulp paper containing seeds was taken and then stitched to the base layer.

2.3. Test Setup

The small-scale outdoor vegetation growth experiment took place in Jinkouhe District, Leshan, Sichuan Province, China (103°08′ E, 29°25′ N). The region’s annual average temperature ranges from 9 to 23 °C, with peak temperatures reaching approximately 32 °C from June to August and lows around 6 °C from December to February. Annual precipitation averages 1505 mm, with most rainfall occurring in August (Figure 3).

Two stainless steel boards 1.5 m long and 1 m wide were utilized to simulate slope surfaces at an angle of 73 degrees. The two steel boards were placed facing northwest. Four test specimens were placed over the boards parallelly, as demonstrated in Figure 4. Each sample measures 1 m in length and 35 cm in width. The specimens were irrigated by a dripping system installed at the top edge of the board. The irrigation system was driven by a pump placed within a 2 m³ water tank that serves as the reservoir of the system. The pump was controlled by a timer so that the specimens were watered every two hours for two minutes. Each sample received 0.8–1.0 L of water in each irrigation cycle. A water collecting tray was placed at the bottom of the stainless steel boards so that the runoff water from the specimens was collected. When the water level in the collecting tray reaches a pre-set level, a pump in the tray turns on automatically and drives the runoff water back to the water tank. The water tank was refilled with tap water as necessary.

2.4. Test Methods

In this study, vegetation germination and sustained growth served as key indicators of GVS feasibility. Vegetation growth was evaluated based on stem and leaf length, root thickness, and leaf/root biomass. The experiment began in October 2021 and concluded in May 2022. Length measurements were non-destructive and performed periodically. Other tests, which would disturb the specimens, were conducted only at the experiment’s conclusion. At the end of the study, root layer thickness was measured, and leaves, stems, and roots were trimmed from the substrate and weighed.

The base layer degradation was also a concern for GVS as the material was not placed in soil and various factors such as UV degradation and biological clogging can affect its performance. During the test, although the biodegradable cover layer, in the initial stage, and vegetation, in the later stage, can provide some protection to the geotextile against sunlight, the integrity of the geotextile base layer would undoubtably be affected by UV degradation. The tensile strength and elongation of the base layer were tested per ISO 10319 [34] to evaluate the material degradation due to UV exposure. The roots of the vegetation and microorganisms residing in the pores of the geotextile can potentially alter the pore structure and thus impact the permeability and transmissivity. This, in turn, may reduce the water uptake efficiency of the vegetation. Therefore, a Scios 2 scanning electron microscope was employed. Sample preparation included the following: cutting 42 geotextile segments (1 cm × 1 cm) post-experiment; vacuum drying for 24 h followed by gold sputtering (10 nm thickness); and observation under 15 kV acceleration voltage. The transmissivity tests on the geotextile were conducted with a falling head method per ASTM D1987-22 [35] after the growth test.

3. Results and Discussion

3.1. Sustained Vegetation Establishment

Over the course of an eight-month outdoor vegetation growth experiment, the proposed geosynthetic vegetation substrate (GVS) demonstrated a consistent capacity to facilitate seed germination and foster sustained vegetation development. Figure 5 shows that, by the 162nd day of the experimental period, a visibly established stand of herbaceous plants had successfully emerged on the GVS-treated slope. These visual confirmations provide compelling evidence that the GVS not only supports the initial establishment of vegetation from seed but also maintains conditions conducive to ongoing plant growth. Such findings suggest that the GVS can effectively promote root anchorage, nutrient availability, and moisture retention, thereby enabling herbaceous species to thrive. Ultimately, these results underscore the potential of the GVS as a viable ecological restoration strategy, particularly for challenging environments where conventional soil-based techniques may be less effective.

3.2. Leaf Length

Figure 6 presents the average leaf lengths for the different experimental groups (G1–G8). Overall, as anticipated, tall Fescue (G4, G5) with a coconut fiber cover layer exhibited a higher growth rate and longer leaf lengths compared to the other specimens. Tall Fescue growth rate accelerated during warmer seasons; however, both tall Fescue and Bermuda grass showed reduced growth under low winter temperatures. It is worth noting that during the period between the 48th and 125th days of the experiment, the study area in the Jinkou River region experienced winter season conditions (December, January, and February), characterized by a maximum temperature of 10 °C and a minimum temperature of 0 °C. Despite the low temperatures, the tall Fescue group displayed continued growth, indicating the vegetation’s strong adaptability to the local climate. The longest leaf length of the tall Fescue was observed at around the 100th day into the experiment, reaching over 400 mm. In the following spring (starting from the 144th day of the experiment), a decrease in leaf length was observed on G4 and G5. This reduction likely resulted from older, taller leaves being replaced by new growth. The tissue paper and pulp paper samples seemed to have a negative impact on the growth of tall Fescue, as G6 (pulp paper) and G7 (tissue paper) presented similar leaf lengths as the Bermuda grass samples, which were significantly lower than that of G4 and G5. The Bermuda grass samples, as well as G6 and G7, maintained relatively stable leaf length through the first 150 days of the test at around 100 mm. After day 150, the leaf lengths increased in all samples except G4 and G5, reflecting the onset of spring growth. Additionally, head sprouting was observed in tall Fescues on samples G4 and G5 near the end of the test. This indicates that the tall Fescue can potentially propagate on GVS.

Leaf length measurements indicate that the geosynthetic vegetation substrate (GVS) is capable of supporting not only the germination but also the sustained growth of vegetation on steep slopes, including those inclined at 75°. Among the tested biodegradable cover layers, the choice of material significantly influenced the growth performance of tall Fescue. Both tissue and pulp paper layers limited tall Fescue growth, whereas the coconut fiber layer exerted a less pronounced effect. The robustness and vertical growth habit of tall Fescue, evidenced by its relatively long leaves, further underscores its suitability as an ecological restoration species in the Jinkouhe region. These findings highlight the importance of tailoring substrate materials and plant species selection to local conditions, ensuring more resilient and sustainable ecological restoration outcomes on challenging terrain.

3.3. Development of Root System

Observations indicate that during the germination process, vegetation roots can penetrate the composite substrate. As the root system further develops, it forms an almost textile-like mat between the base layer and the steel board, with most of the root fibers growing downward. This network of roots firmly anchors the vegetation to the substrate, supporting the vegetation developed on a 75° slope. The supporting steel plate apparently restricted downward root growth, causing the roots to develop along the interface between the base layer and the steel plate. This root behavior, forming a layer beneath the base layer rather than within geotextile voids, deviated from initial expectations.

Figure 7 shows the final root layer thickness at the end of the experiment. According to Figure 7, root layer thickness shows no clear correlation with the plant species. The thickest root layer was recorded on sample G6 by tall Fescue at over 2.1 mm. Root layer thicknesses of sample G1 (Bermuda grass) and G4 (tall Fescue) were similar at around 1.5 mm. Overall, the average root thickness of the tall Fescue groups (i.e., 1.11 mm) was greater than that of the Bermuda grass (0.83 mm). However, the root thickness of tall Fescue showed greater deviation. The effect of the cover layer on root layer thickness remains unclear. The tissue paper cover layer (G7) exhibited the thinnest root layer. In contrast, both G6 and G8 employed pulp paper covers but yielded significantly different root thicknesses. The effect of adding a non-woven geotextile beneath the woven layer was also inconclusive. Although both G1 and G3 included a non-woven geotextile, they displayed markedly different root thicknesses.

3.4. Biomass of Leaves and Roots

Due to the unavailability of the necessary experimental equipment at the trial site for assessing the dry biomass of vegetation, the authors solely measured the fresh biomass of the samples following the eight-month experiment, as depicted in Figure 8. The average annual fresh biomass of the tall Fescue groups (G4, G5, G6, G7) amounted to 2.99 kg/m², comprising 2.82 kg/m² of stem and leaf and 0.17 kg/m² of root. In contrast, the average annual fresh biomass of the Bermuda grass groups (G1, G2, G3, G8) was 2.20 kg/m², with 2.12 kg/m² of stem and leaf and 0.08 kg/m² of root. Furthermore, as shown in Figure 8c, fresh biomass seems to increase with root layer thickness. This suggests that thicker root systems correspond to better plant development and higher biomass accumulation.

3.5. Deterioration of Mechanical Properties

After the eight-month outdoor experiment, only minor changes in the geotextile’s mechanical properties were observed. Tensile strength tests were conducted in accordance with the Wide-Width Test standard for geotextiles (ISO 10319). The reductions in tensile strength and maximum load elongation are presented in Figure 9a. The results indicate that the strength retention rate ranged from 91.6% to 99.5%, and the creep performance retention rate ranged from 82.9% to 98.2%.

It was hypothesized that vegetation shading could mitigate UV-induced geotextile deterioration. As depicted in Figure 9b, coconut fiber (G1, G2, G3, G4, G5) benefits vegetation development (average annual fresh biomass accumulation is 2.86 kg/m²), correspondingly, the tensile strength reduction was only 5.0%. In contrast, the sample with tissue paper (G7) demonstrated the most significant strength reduction at 8.4%, corresponding to lowest average annual fresh biomass accumulation at 0.77 kg/m². These results support the hypothesis that well-established vegetation can protect the geotextile base layer from UV-related strength losses.

3.6. Deterioration of Transmissivity

After the eight-month field experiment, the transmissivity of the geotextile base layer was tested using the constant-head method (ASTM D1987-22), and the resulting reductions are shown in Figure 10. The transmissivity reduction in tested geotextile specimens ranged between 50% to 70%.

Transmissivity deterioration in geotextile materials typically arises from the obstruction of pore spaces between fibers. In this study, two potential factors were considered: (1) root intrusion and (2) biological clogging caused by microorganisms. To determine the primary cause of transmissivity deterioration, the test specimens were subdivided into 42 smaller segments for SEM observation. Figure 11a shows the unused geotextile fibers. As shown in Figure 11a, the unused geotextile fibers are smooth, with no foreign material attached. Figure 11b shows the only observed vegetation roots among the 42 segments. These roots appear as tubular fibers with relatively smooth surfaces and diameters much smaller than the geotextile fibers, continuously adhering to a geotextile strand. The pore spaces were not occupied by roots. Instead, they contained clusters or films of unidentified clogging substances, as illustrated in Figure 11c. Although the chemical composition of these clogging materials was not determined, their environmental context strongly suggests a biological origin. The 50–70% transmissivity reduction (Figure 10) correlates strongly with the continuous biofilm coverage observed (Figure 11c), which increases flow path tortuosity and reduces effective porosity. While root-induced mechanical clogging was negligible in this study, we acknowledge that long-term (5+ years) root lignification may alter clogging dynamics. Microscopic observations thus indicate that biological clogging, rather than root intrusion, predominantly drives the observed transmissivity reduction. Future work will incorporate quantification and microbial community profiling to precisely characterize bioclogging progression.

Furthermore, the rarity of root presence along the geotextile fibers supports the notion that vegetation roots preferentially develop beneath the geotextile base layer rather than permeating its internal fiber structure.

4. Conclusions

This study proposed a geosynthetic vegetation substrate (GVS) for the ecological restoration of steep concrete and rock slopes. To evaluate the feasibility of the proposed GVS, an eight-month vegetation growth experiment was conducted. The study monitored plant growth and examined the mechanical and hydraulic properties of the geotextile base layer. The following conclusions can be drawn from this study:

• The eight-month outdoor vegetation growth experiment has validated the significant ecological restoration potential of the proposed geosynthetic vegetation substrate (GVS) on 73° steep engineered slopes. This study demonstrates that the GVS system effectively sustains vegetation health through tall Fescue’s continuous low-temperature growth (0–10 °C), superior annual fresh biomass production (2.99 kg/m²) with strong root thickness, and complete reproductive cycle evidenced by heading, collectively validating its engineering viability for steep slope restoration.
• The biodegradable layer, embedded with seeds via organic adhesives, creates a humid microenvironment to promote germination and gradually decomposes to release nutrients, so that significantly influences vegetation growth. Based on the aforementioned mechanism, the selection criteria for ecological restoration materials should emphasize biodegradable cover materials with three-dimensional porous architecture, superior hydroponic retention efficacy, and mechanical stability to satisfy the engineering requirements of vegetation rehabilitation.
• During the initial growth phase, plant roots penetrate the base layer. Over time, they form an interwoven root network beneath the GVS, rather than occupying the pore spaces between fibers. This root fixation can support the vegetation’s weight on slopes of 73°.
• Over the eight-month test period, the geotextile base layer’s tensile strength decreased by an average of 5.5%. Furthermore, vegetative shading mitigated strength loss, as samples with lower biomass accumulation exhibited more pronounced reductions.
• The transmissivity of the geotextile base layer decreased by 50–70% after the eight-month test. Based on the microscopic observation, the major cause of transmissivity deterioration was biological clogging, as very little root was observed between the geotextile fibers.

5. Limitation

Future studies should expand the experimental scale and diversity (e.g., varying slopes and climatic conditions) to validate GVS’s adaptability in complex environments. Long-term monitoring is necessary to assess material degradation, vegetation succession, and carbon sequestration dynamics. Further research should focus on optimizing biodegradable cover materials or developing UV-resistant geotextile composites. While the current study focused on standard geotextiles, we recognize the potential of advanced fiber composites like rubberized asphalt with tire fabric fibers. We will expand the discussion to include the following: (1) a comparative analysis of natural vs. synthetic fiber reinforcement strategies; and (2) field validation plans using hybrid geotextiles embedded with recycled tire textiles [36,37]. These enhancements will strengthen the technical foundation for GVS optimization.