Finite Element Structural Analysis and Optimization of Sustainable Oil-Absorbing Concrete Slope Retaining Wall

Abstract

Addressing the issue of oil pollutants and their impact on environmental sustainability, this study prepared sustainable oil-absorbent concrete through particle size adjustment and chemical modification methods. The effects of alkaline activators and seashell powder on the strength and oil absorption performance of the sustainable oil-absorbent concrete were investigated. Based on this, retaining wall blocks with different structural forms were designed for use as oil-absorbing functional concrete materials. Retaining walls with different structural forms and arrangements were calculated by ABAQUS, and their stress and displacement were compared to select the best structural form and arrangement. The research findings indicate that NaOH adversely affected the oil absorption capacity of sustainable oil-absorbent concrete, resulting in a decrease in oil absorption from 207.70 kg/m³ to 104.56 kg/m³; however, it enhanced the compressive strength of the concrete, increasing the 28-day compressive strength by 5.02%. The incorporation of seashell powder exerted a detrimental effect on both the compressive strength and oil absorption performance of the sustainable oil-absorbent concrete. The finite element analysis results show that L-shaped retaining wall bricks with vegetation cavity had better anti-deformation ability, and under the inverted arrangement, the maximum deformation of the retaining wall was 1.148 mm, which was the smallest of all working conditions. This study provides an effective reference for the design of sustainable oil-absorbing concrete retaining walls with oil adsorption capacity.

1. Introduction

As energy utilization has continued to increase, oil spills have occurred frequently. Marine oil spills often pollute coastal and estuary water bodies with large amounts of oily wastewater, which then spreads to freshwater bodies [1,2,3]. Therefore, solving the problem of oil pollution in water bodies and protecting the water environment are of great significance to sustainable development. Mineral admixtures and modified materials can improve the oil adsorption capacity of concrete. However, studies have shown that the strength of this type of concrete is far less than 10 MPa, and its scope of application is limited [4,5].

Zhou et al. developed a fluorinated nano-silica modified self-curing epoxy resin hydrophobic coating, which was utilized to modify the surface of basalt fiber-reinforced resin composite (BFRP), resulting in a 40% reduction in the water absorption rate without compromising its strength [6]. However, the internal addition of hydrophobic modified materials has a greater impact on concrete [7]. Feng et al. added a low-cost aqueous stearic acid emulsion (SAE) to cement mortar. The results showed that the addition of SAE reduced the water absorption of cement mortar by 86.06%, but its compressive strength decreased compared with the control group, with a decrease of about 20% [8]. Yu et al. modified the overall hydrophobicity of concrete by adding a silane emulsion and studied its capillary water absorption performance and compressive strength. The results showed that when 0.5% silane emulsion was added, the compressive strength of concrete at 28 days decreased by about 20%. When the amount added was further increased to 2%, the compressive strength of concrete at 28 days decreased by 49.6% compared with the control group [9].

In order to improve the low strength of geopolymer hydrophobic concrete, Maraghechi et al. used a mixed solution of NaOH and water glass to stimulate the activity of geopolymer mortar prepared from recycled glass powder and fly ash. They ultimately discovered that NaOH with a concentration of 4 mol/L could stimulate the reactivity of recycled glass powder, achieving a compressive strength of 21 MPa after 28 days of curing [10]. Lee et al. used a mixture of NaOH and water glass to stimulate the strength of fly ash slag geopolymer, and the 28-day strength increased from 15.35 MPa to 22.60 MPa [11]. Ruengsillapanun et al. investigated the mechanical properties of fly ash concrete activated with NaOH and observed that the maximum strength was achieved at a NaOH concentration of 6 mol/L, with a 28-day strength reaching 17.1 MPa [12]. This shows that alkaline activators such as NaOH can be used to improve the strength of concrete with a large amount of mineral admixtures [13]. Oderji et al. studied the effects of slag content and different alkaline activators on the mechanical properties of geopolymers and found that the activation effects of different types of activators were Na₂SiO₃+NaOH>Na₂SiO₃>NaOH [14].

Chitosan, as the second largest biomass in nature, is widely found in the seashells of shellfish, shrimps, crabs and other crustaceans and has a wide range of uses [15,16,17]. In coastal areas in the late 18th century, seashells were used in concrete and to make a specific type of building material called “Tabby” [18,19]. Kuo et al. showed that when seashell waste was crushed and sieved to make seashell powder with a particle size of 0.075–2.36 mm and added to concrete, its strength first increased and then decreased with the amount of seashell added. The optimal amount was 5%, and its 28-day strength could be increased by 37.29% [20]. As an inert material, seashell powder has a weak reaction with the other cementitious materials in concrete, improving its strength due to the physical filling effect [21,22]. Studies have found that chitin can be extracted from seashells through acid treatment and converted into chitosan through deacetylation [23,24,25,26]. Li et al. added N-carboxymethyl chitosan to a fly ash-based polymer and increased its 28-day unconfined compressive strength by 3.5% [27]. Zhao et al. added 0.6% chitosan to dicalcium silicate (C₂S) and found that chitosan could induce C₂S to form a dense carbonate matrix to improve the strength of concrete [28]. This indicates that acid–base-modified seashell powder is expected to improve the strength of highly adsorbable concrete.

Traditional engineering retaining wall technology mostly analyzes slope stability problems from a mechanical perspective [29]. Existing retaining walls are mainly divided into three types: concrete retaining walls, mortar-made stone retaining walls and prefabricated ecological slope protection block retaining walls [30]. Prefabricated ecological slope protection block retaining walls are prefabricated in factories. By planting vegetation on an ecological retaining wall (Figure 1), not only can the environment around the building be improved, but a building microclimate can be created [31]. Wu et al. used ecological chain retaining walls to treat black and smelly water bodies. The combination of retaining wall blocks and vegetation can not only improve the environment but also help stabilize river banks [32]. Cai studied the applications of ecological frame retaining walls in embankment projects and found that ecological frame retaining walls have flood control benefits, ecological benefits, environmental benefits and social benefits [33]. Feng studied the mix ratio of concrete for grass retaining walls and obtained a concrete mix ratio that could meet the needs of plant growth on the ecological slopes of small- and medium-sized rivers [34].

Lin et al. designed an H-type gravity mutual-assisted steel slag concrete retaining wall brick and used ANSYS to perform numerical simulation analysis on its mechanical properties and stability. The results showed that the new retaining wall block had a concave–convex structure, which enhanced the bite force and overall stability between the retaining wall bricks [35]. Nie et al. designed an ecological herringbone frame retaining wall structure and used ANSYS to analyze its mechanical response. The results showed that the slope foot was the most unfavorable stress position, and the structural form that combined a cement mortar base and precast concrete blocks had the lowest deformation resistance [36]. Dang et al. used ANSYS to conduct a numerical study on the new type of assembled concrete hollow blocks in the seawall structure and found that under the action of waves and the soil behind the wall, the retaining wall structure established by the new retaining wall blocks enhanced the stability of the seawall structure and reduced settlement [37]. Zhang et al. used the finite element method to study the evolution law of the shear zone of ecological retaining walls. The results showed that a soft soil foundation has a greater impact on the damage to ecological retaining walls [38].

Absorbent concrete is mostly used for non-load-bearing structural components, and research on its mechanical properties as a load-bearing material has not been reported. In this paper, oil absorption was used as the functional goal of concrete design, and a compressive strength of more than 15 MPa was used as its structural goal. The blocks and arrangement of the vegetation retaining wall were designed by multi-size coupling, the deformation and mechanical properties of the vegetation retaining wall were analyzed by combining in situ tests and ABAQUS finite element simulation, and an optimized design scheme was obtained. This study can provide certain theoretical guidance for the design and development of vegetation retaining wall bricks with an oil adsorption function.

2. Experiments

In previous studies, our team found that when the cementitious material ratio was 70% Grade II fly ash, 10% S95 slag, and 20% P·II cement, the silane content was 3% and the water–cement ratio was 0.5, the concrete showed high oil adsorption performance [39,40]. Therefore, the above mix ratio was used as the basis for the design of high-adsorption concrete.

2.1. Raw Materials

P II 42.5 Portland cement was obtained from Qinhuangdao Asano Co., Ltd., (Qinhuangdao, China) S95 granulated blast furnace slag and secondary fly ash were produced by Qinhuangdao Suizhong Thermal Power Plant (Qinhuangdao, China), and their chemical compositions are shown in

Table 1. Main chemical components of cementitious materials.

Materials	CaO/%	SiO2/%	Al2O3/%	SO3/%	Fe2O3/%	K2O/%	Density/kg·m−3	Specific Surface Area/m2·kg−1
Cement	63.4	20.2	4.95	3.33	3.32	0.94	2994	474
Class II fly ash	4.48	42.7	44.8	0.61	3.31	1.14	2144	420.4
S95 Slag	32.3	32.9	17.4	2.81	0.69	0.61	2854	461.1

. Artificial sand was used as the fine aggregate, and its fineness modulus was 3.48, classified as zone II coarse sand. The coarse aggregate was composed of granite gravel with a particle size of 5 mm to 20 mm and a good gradation distribution, which met the test requirements. The parameters of the coarse and fine aggregates are shown in

Table 2. Aggregate parameters.

Aggregate Type	Apparent Density/kg·m−3	Bulk Density/kg·m−3	Water Absorption/%	Crushing Indicators
Granite crushed stone	2833	1510	2.50	11.80
Machine-made sand	2882	1710	—	—

. Both the coarse and fine aggregates were sourced from Qinhuangdao Municipal Building Materials Group (Qinhuangdao, China). Seashells were obtained from scallop seashell waste from the Bohai Bay scallop farm, and water was obtained from the Qinhuangdao Water Plant (Qinhuangdao, China). For this study, 96% pure sodium hydroxide solid was selected, which was produced by Tianjin Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). Arowana brand soybean oil was purchased from Qinhuangdao. Silane was purchased from Shanxi Jingchen Building Materials Co., Ltd. (Jincheng, China).

2.2. Sample Preparation

First, the silane, NaOH and other materials were dissolved in water. The cementitious material, modified seashell powder and fine aggregate were added to the mixer and stirred for 30 s, then the required water was added to the mixer and stirred for 50 s to obtain cement mortar and finally the coarse aggregate was added to the mixer and stirred for 60 s. The mixture was poured into a mold with dimensions of 100 mm × 100 mm × 100 mm and 100 mm × 100 mm × 400 mm, placed on a vibration table for 35–40 s to vibrate and form, covered with a polyvinyl chloride (PVC) film, placed in a room at 20 °C for 24 h and then demolded. It was then placed in a standard curing room at (20 ± 2) °C and a with relative humidity of not less than 95% for curing to the required age. The matching example is as follows (

Table 3. Concrete mix ratio.

Number	Cementitious Materials/kg·m−3			NaOH /kg·m−3	Seashells/kg·m−3	Silane /kg·m−3	Machine-Made Sand/kg·m−3	Gravel /kg·m−3	Water/kg·m−3
	Cement	Fly Ash	Slag
D0.5	494	—	—	—	—	—	594	1102	247
CA1	100	346	48	—	—	—	594	1102	247
YCA1	100	346	48	—	—	14.82	594	1102	247
ACA1	100	346	48	4.94	—	14.82	594	1102	247
BCA1	100	346	48	—	49.4	14.82	594	1102	247

):

The flow chart of this study is shown in Figure 2. A porous concrete system was constructed using mineral admixtures, and the concrete was modified using seashell powder, NaOH and silane. The effects of seashell powder, NaOH and silane on the adsorption and strength of concrete were explored. The structural form and arrangement of the retaining wall bricks were then designed, concrete retaining wall bricks were made according to the designed dimensions using the optimal concrete mix ratio and a model verification test was carried out on the artificial sand slope. Finally, a full-scale model was established and a simulation analysis was carried out to select the best structural form and arrangement.

2.3. Compressive Strength Test

The compressive strength of the test block was tested according to the provisions of the Standard Test Method for Mechanical Properties of Ordinary Concrete [41]. Three samples were randomly selected from the same mix ratio of the sustainable oil absorption concrete made during the test. The size was 100 mm × 100 mm × 100 mm, and the loading rate was 0.3–0.5 MPa/s. Each sample was continuously and uniformly loaded until it broke and the concrete strength was recorded.

Figure 3 shows the compressive strength of concrete with different mix ratios. The 28-day strengths of the CA1 sample with mineral admixtures and the YCA1 sample with silane were 60.18% and 72.84% lower than that of the control group D0.5 sample, respectively. This may be because the mineral admixtures replaced a large amount of cement, resulting in a decrease in the content of hydration products such as C-S-H gel, and the silane was wrapped on the surface of the cementitious material particles, which hindered the hydration of fly ash, further resulting in a decrease in the amount of hydration products generated and a decrease in strength [42].

The 28-day strength of the ACA1 sample with 1% NaOH added increased by 5.02% compared with the YCA1 sample, reaching 12.55 MPa, and the 90-day strength reached 14.37 MPa. This may be because the addition of NaOH promoted the hydration reaction of fly ash and generated more hydration products. Under the action of the alkali activator, the fly ash and mineral powder underwent the following chemical reactions:

$$\mathrm{S}\mathrm{i}(\mathrm{O}\mathrm{H}{)}_4+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}⟶{\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{i}\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(1)

$$2\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}{)}_3+2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}⟶2{\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}\mathrm{O}}_2+4{\mathrm{H}}_2\mathrm{O}$$

(2)

Under the action of the alkali activator, the mineral admixtures dissolved to form sodium silicate and sodium aluminate, and then the two continued to react to form hydration products such as C-S-H. When seashell powder was added to the sample, the 28-day strength of the BCA1 sample decreased by 13.72% compared with the YCA1 sample. The results show that seashell powder is not conducive to the strength development of sustainable oil absorption. This may be because the main component of seashell powder is CaCO₃, which has difficulty reacting with other cementitious material components and only plays a physical filling role in sustainable oil absorption [22]. In addition, since seashells are porous materials, their own strength is low. This loosens the internal structure of the concrete, decreases the bonding effect between the cementitious material and the aggregate, and ultimately leads to a decrease in strength.

2.4. Oil Adsorption Test

In the oil adsorption experiment, soybean oil was used to prepare a water–oil mixture to simulate the oil adsorption of concrete in a water–oil environment. The test process was as follows: the specimen was placed in a forced air-drying oven at 60 °C until the daily mass loss of the specimen reached 0.2%. The specimen was placed in a clean, independent container. Subsequently, 300 g of water and 40 g of oil were weighed and placed in a magnetic mixer to create a water–oil mixture. The specimen was placed in the container to ensure that the liquid level of the final water–oil mixture was 20 mm from the bottom of the specimen. Every 48 h, a separatory funnel was used to separate the remaining oil and water in the container and weigh their respective masses.

Three specimens were grouped together, and the final results were averaged to calculate the oil adsorption amount K as shown in Equation (3). The test is stopped until the mass change was less than 0.2% for two consecutive tests. Finally, the oil adsorption curve was obtained.

$$K={\displaystyle \frac{m_1-m_0}{V_c}}$$

(3)

where K is the oil absorption per unit volume of concrete; m₀ is the mass of residual oil in the container; m₁ is the initial mass of oil; and V_c is the volume of the specimen.

Figure 4a shows the oil and water absorption of concrete with different mix ratios after 20 days of adsorption. As a hydrophilic material, the water absorption per unit volume of the D0.5 sample reached 56.79 kg/m³, and the oil absorption per unit volume reached 66.61 kg/m³. Mineral admixtures such as fly ash increase the porosity of concrete. The water absorption per unit volume of the CA1 sample increased by 65.47% compared with the D0.5 sample, and the oil absorption per unit volume increased by 95.13% compared with the D0.5 sample, reaching 93.97 kg/m³ and 129.98 kg/m³, respectively.

After adding silane, the hydrophobic material combines with concrete, making the internal pores of the concrete hydrophobic. The water absorption per unit volume of the YCA1 sample decreased by 88.50% compared with the D0.5 sample to 14.95 kg/m³, and the oil absorption per unit volume increased by 265.73% to 207.70 kg/m³. This greatly improved the hydrophobic and oil absorption performance of concrete [43]. After adding NaOH, the oil absorption decreased significantly compared with YCA1, with a decrease of 49.66%. This is because silane may hydrolyze and become ineffective in a NaOH environment, which reduces its oil absorption performance. The oil absorption per unit volume of the YCA1 sample decreased by only 6.53% after adding seashell powder, because the seashell powder hardly reacted with the components in the concrete and only played a physical filling role. On the other hand, seashell powder is a porous material, which allows oil absorption performance to be maintained at a high level.

Figure 4b shows the curve of the oil absorption of concrete with different mix ratios changing with time. This is because oil has a higher viscosity than water [44,45]. During the adsorption process, the water in the environment first invades the internal pores of the concrete, causing the concrete to enter a saturated state prematurely. After the addition of silane, the adsorption saturation time is significantly delayed, and the concrete reaches a saturated state after about 10 days of adsorption. This may be because the hydrophobic material prevents the water in the environment from entering the concrete, providing more adsorption space for the oil, and the oil has a high viscosity and enters the concrete pores slowly, resulting in a delay in the adsorption saturation time.

By modifying the concrete, its oil absorption per unit volume reached up to 207.7 kg/m³, 265.73% higher than that of ordinary concrete. Oil-absorbing concrete can be used to absorb oil pollution on coastlines and rivers. Its efficient oil absorption performance will help to purify the polluted environment more quickly and provide strong support for ecological protection and water quality restoration.

3. Simulation Analysis of Sustainable Oil-Absorption Retaining Wall Bricks

Taking rectangular and foot-stepped L-shaped retaining wall bricks as the basic structural forms, four different structural forms of retaining wall bricks (Figure 5) and three arrangement methods (Figure 6) were designed, totaling six working conditions. Through ABAQUS, the deformation and stress distribution of retaining wall bricks with different structural forms and different arrangements under the action of deadweight and vehicle load behind the wall were compared, and the optimal retaining wall brick structure and arrangement were determined.

This study designed two types of retaining wall bricks, rectangular and foot-stepped L-shaped, by controlling the retaining height. The integrity of the foot-stepped L-shaped retaining wall bricks was found to be between the protection offered by rigid concrete slab slopes and flexible stone masonry slopes. While having good integrity, it also fully improved the disadvantage of the poor adaptability of integral slope protection to soil deformation. A three-view drawing of the designed high-adsorption concrete retaining wall brick is shown in Figure 7.

The working condition table is as follows (

Table 4. Working condition classifications.

Condition No.	Arrangement	Type	Condition No	Arrangement	Type
Working condition 1	Ⅰ	A1-1	Working condition 4	II	B1-2
Working condition 2	Ⅰ	A1-2	Working condition 5	III	B1-3
Working condition 3	Ⅱ	B1-1	Working condition 6	III	B1-4

):

3.1. Retaining Wall Brick Test and Model Verification

In order to verify the rationality of the finite element model, tests were carried out on an artificial slope made of sand, and a 1:1 model specimen was made according to the size of the retaining wall bricks designed for working condition 4. The retaining wall bricks were prepared according to the alkali-activated high-adsorption concrete mix ratio given in this paper (see the BCA1 mix ratio in Table 3 for details), and were laid on the artificial slope. The load was applied step by step on the soil behind the wall with autoclaved fly ash bricks to obtain the vertical strain of the concrete retaining wall bricks under the load.

The loading steps in the test are as follows:

(1)

Construct according to the size of the retaining wall bricks in working condition 4 and ensure that the retaining wall bricks are placed on the slope.

(2)

Paste the strain gauge on the surface of the retaining wall bricks and test the strain value of the corresponding point under the action of load.

(3)

Place the retaining wall bricks on the simulated slope in the arrangement shown in Figure 8, and place several heavy objects at the foot of the slope to constrain the horizontal displacement of the retaining wall bricks, as shown in Figure 8.

(4)

Measure the mass of the loaded bricks and take the average value of the three bricks as 2.19 kg/brick. Connect the pasted strain gauge to the strain gauge. The loading area was 575 mm × 720 mm. Fifteen loads were loaded step by step in each layer. The load effect was converted to 0.00159 MPa/layer.

(5)

Record the strain values corresponding to each point when the number of load layers is 2, 3, 4 and 5; that is, when the load is 0.00318 MPa, 0.00477 MPa, 0.00636 MPa and 0.00795 MPa.

A finite element model was established to verify the test results. Due to the limited size and small load, and the fact that the retaining wall bricks are the last line of defense to preventing slope instability, the constitutive model used for the retaining wall bricks in the model verification stage was the Drucker–Prager model, which is used to simulate the plastic behavior of concrete. The constitutive parameters of the concrete were all taken from the high-absorption concrete material used in this study. The specific mix ratio is shown in Table 3 for the BCA1 mix ratio. The Mohr–Coulomb model was adopted for the soil. The parameters of the retaining wall brick material and the soil body are shown in the following

Table 5. Finite element simulation specific parameters.

Material	Density/t·mm−3	Elastic Modulus/MPa	Poisson’s Ratio	Friction Angle/°	Expansion Angle/°	Cohesive Yield Stress/kPa
Concrete	2.5 × 10−9	27,065	0.18	60.09	—	1.74 × 103
Soil behind the wall	1.765 × 10−9	6.11	0.3	38.6	8.6	11.2

:

In the verification model, the boundary conditions of the model were as follows: the side parallel to the retaining wall bricks limited the horizontal displacement of the Z axis, the side perpendicular to the retaining wall bricks limited the horizontal displacement of the X axis and a fixed constraint was set in the soil at the bottom of the model to limit its displacement and rotation. A load surface was set in the action area, its material parameters were consistent with the soil and its contact with the soil was tie-binding.

The normal contact between the soil and the retaining wall bricks was set to “hard” contact, the tangential contact was set to the “penalty” friction formula and the friction coefficient was set to 0.35; the contact settings between the retaining wall bricks were similar to the contact between the soil and the retaining wall bricks. The normal contact was set to “hard” contact, the tangential contact was set to the “penalty” friction formula, and the friction coefficient was set to 0.4. The gravity action value was 10 N/kg, and the load action area was a uniform load.

3.2. Grid Division

The finite element analysis model of the retaining wall bricks and soil was established using ABAQUS (as shown in Figure 9). The model was meshed in ABAQUS, where the retaining wall bricks were simulated using eight-node hexahedral elements (C3D8Rs). The retaining wall bricks were divided into 6 layers of units along the height direction, 8 layers of units along the length direction and 4 layers of units along the width direction. The soil behind the retaining wall bricks was simulated using the same unit type and was divided into 13 layers of units along the Y axis.

3.3. Contact Settings

In the process of establishing a ABAQUS finite element analysis model, the contact settings should be restored to the actual situation as much as possible, and the ABAQUS model should be simplified. There is contact between the retaining wall bricks and the soil. The contact surface setting is shown in Figure 10. The contact position between the retaining wall bricks is represented by “brick-brick” in the figure, and the contact position between the retaining wall bricks and the soil is represented by “brick-soil” in the figure.

In ABAQUS, the slip formula between the retaining wall bricks was set as finite slip, the contact normal behavior was defined as “hard” contact, the tangential behavior was defined as the “penalty” friction formula and the friction coefficient was set to 0.4 according to the experimental test parameters. The setting between the retaining wall bricks and the soil was the same as the setting between the retaining wall bricks. The sliding formula was finite slip, the contact normal behavior was defined as “hard” contact, the tangential behavior was defined as the “penalty” friction formula and the friction coefficient was set to 0.35 according to the test results.

3.4. Loading and Boundary Conditions

According to the actual load conditions, the weight of the retaining wall bricks and the soil was considered, and the gravity acceleration was set to 10 N/kg in the opposite direction of the Y axis. The vehicle load was considered on the upper rear part of the retaining wall bricks. According to the (JTGD302015) “Highway Roadbed Design Code”, the vehicle load is taken as 20 kN/m when the wall height is less than 2 m; when the wall height is greater than 10 m, it is taken as 10 kN/m. The model wall height is 1 m, so the vehicle load is 20 kN/m.

In ABAQUS, an overall model of the retaining wall bricks and soil was established according to the above conditions, the load was applied for finite element analysis and the vertical strain value that was the same as the test point, namely E22, was extracted. The test data were compared with the simulation data, as shown in

Table 6. Test strain and calculated strain.

Load/MPa	0.00318		0.00477		0.00636
Strain Gauge Location	Test Strain	Finite Element Analysis Strain	Test Strain	Finite Element Analysis Strain	Test Strain	Finite Element Analysis Strain
1	3.0 × 10−6	2.7 × 10−6	4.1 × 10−6	3.8 × 10−6	3.7 × 10−6	4.3 × 10−6
2	1.2 × 10−6	1.5 × 10−6	1.4 × 10−6	1.1 × 10−6	3.0 × 10−6	3.2 × 10−6
3	3.9 × 10−7	4.4 × 10−7	1.3 × 10−7	1.1 × 10−7	2.4 × 10−7	1.9 × 10−7
4	3.7 × 10−7	4.4 × 10−7	1.4 × 10−7	1.1 × 10−7	2.3 × 10−7	1.9 × 10−7
5	3.3 × 10−6	2.5 × 10−6	3.1 × 10−6	3.8 × 10−6	3.6 × 10−6	4.3 × 10−6
6	1.4 × 10−6	1.2 × 10−6	1.3 × 10−6	1.1 × 10−6	2.7 × 10−6	3.2 × 10−6
7	−3.7 × 10−6	−4.3 × 10−6	−5.9 × 10−6	−6.8 × 10−6	−6.9 × 10−6	−8.5 × 10−6

.

Figure 11a shows the vertical strain contour of the retaining wall bricks. The results show that there is tensile strain on the outside of the retaining wall, and the vertical strain shows a decreasing trend from the top of the wall to the corner. This is because under the action of the load, the soil behind the wall deforms downward, causing the retaining wall to tend to tilt toward the soil. Therefore, the retaining wall at the top of the wall deforms first, generating vertical tensile stress on the surface, thereby generating tensile strain. With the action of the load, a tension–compression combination zone appears in the middle of the retaining wall bricks at the top of the wall, resulting in an increase in the vertical strain; that is, the strain gauge test area at point 7# is in the tension–compression combination zone, where the strain value is the largest, and as the load increases, the tensile strain area also increases. The retaining wall bricks at the foot of the wall are constrained by the retaining wall bricks at the top of the wall and the soil, resulting in a decrease in the strain value of the lower monitoring point.

The vertical strain of retaining wall bricks was tested by designing an experiment, and the field test was simulated by combining ABAQUS 2022 finite element analysis software. The vertical strain values at different test points and under different loads were calculated. The final test results were compared with the experimental results. Under the same load, as the test points changed, the change trends of each test point were basically the same. The error δ between the test results and the simulation results was calculated according to Equation (4):

$$\delta ={\displaystyle \frac{E_m-E_c}{E_c}}$$

(4)

where δ is the relative error between the strain value of the simulation result and the strain value of the test result; E_m is the vertical strain value of the simulation result; and E_c is the vertical strain value of the test result.

The experimental errors are shown in

Table 7. Table of errors between test results and simulation results.

Strain Gauge Location	Error/%
	0.00318 MPa	0.00477 MPa	0.00636 MPa
1	8.9	7.0	17.1
2	23.5	20.9	5.4
3	12.9	18.5	19.1
4	20.0	21.3	17.5
5	23.8	21.8	21.4
6	18.5	18.4	18.4
7	16.6.	13.3	22.8

. The errors between the experimental and simulation results are both around 20%, which is within the acceptable range, proving the rationality of the modeling method. Some test points show large errors, which may be caused by the temperature and other objective conditions in the experimental environment.

3.5. Analysis of Finite Element Results of Full-Scale Model

Using ABAQUS to apply loads and calculate, the stress cloud diagram of the retaining wall bricks was obtained. Figure 12 shows the stress cloud of the oil-absorption concrete slope retaining block wall as a whole under various working conditions. The figure shows that the stress of the blocks at the bottom of the oil-absorption concrete slope retaining block wall is relatively large, and the maximum compressive stress is 0.161 MPa. According to the actual test results, the compressive strength of high-absorption concrete is 15 MPa, which is greater than 0.161 MPa, and the design of the retaining wall bricks meets the requirements. Under normal use, on the same horizontal line, the stress of the retaining wall bricks on the side close to the soil is relatively large, and the stress of the retaining wall bricks on the side away from the soil is relatively small. The outer side of each retaining wall of the new retaining wall bricks has an exposed part relative to the previous level, and there is a vegetation cavity inside, which can be used to plant flowers and plants to improve the urban environment. Figure 12b shows that in working condition 2, a stress concentration occurs where the cavity wall is connected, causing the stress at this location to be relatively large. By comparison, it was found that the stress of the retaining wall bricks on the upper side of the retaining wall was relatively small. The lower the retaining wall stress is, the greater the stress is, and the greater the force on the retaining wall bricks is.

Figure 12 shows the deformation cloud diagram of the oil-absorption concrete slope retaining block wall under different working conditions. The results show that the largest deformation area of the oil-absorption concrete slope retaining block wall is the side of the top retaining wall brick close to the soil. As the load is transferred downward, the deformation of the soil decreases. At the same time, due to the constraint between the retaining bricks, the deformation between the retaining bricks at the toe of the oil-absorption concrete slope retaining block wall decreases. The figure shows that the minimum deformation of the oil-absorption concrete slope retaining block wall occurs in working condition 6, which is 1.148 mm.

Figure 13 shows the stress and deformation cloud diagram of the retaining wall bricks at the location where the oil-absorption concrete slope retaining block wall displacement is the largest under different working conditions. The figure shows that the retaining wall bricks are subjected to a trend of pulling up and pressing down, and by comparison, it can be seen that the compressive stress of the retaining wall bricks is one order of magnitude greater than the tensile stress, which means that the retaining wall bricks are mainly subjected to pressure under the load conditions studied.

The stress cloud map results show that due to the presence of vegetation cavities in the vegetation retaining wall bricks, a stress concentration occurs at the junction of the cavity wall of the retaining wall bricks, and by comparing the deformation cloud maps of retaining wall bricks under different working conditions, it is found that the displacement on the side close to the soil is larger, and the displacement on the side away from the soil is smaller. Combining the stress and deformation cloud maps, this may be caused by the oil-absorption concrete slope retaining block wall shifting toward the soil side under the uniformly distributed load behind the wall.

The maximum values of stress and displacement of the retaining wall bricks under various working conditions are shown in

Table 8. Maximum stress and displacement of retaining wall bricks under different working conditions.

Working Conditions	1	2	3	4	5	6
Smax/MPa	−8.2 × 10−3	−2.1 × 10−2	−2.0 × 10−2	−6.3 × 10−2	−3.4 × 10−2	−1.0 × 10−1
Smax/MPa	6.1 × 10−3	1.7 × 10−2	5.9 × 10−3	2.5 × 10−2	9.6 × 10−3	3.3 × 10−2
Umax/mm	1.480	1.425	1.538	1.312	1.218	1.148

Note: In the table, positive values of S_max indicate tensile stress, and negative values indicate compressive stress.

. The data from working conditions 3 to 6 show that the displacement of the retaining wall bricks in working conditions 3 and 5 is greater than that in working conditions 4 and 6, but the maximum stress of the retaining wall bricks in working conditions 3 and 5 is less than that in working conditions 4 and 6. This shows that under the same load, the vegetation foot-stepped L-shaped retaining wall bricks have better anti-deformation ability than the foot-stepped L-shaped retaining wall bricks. In working conditions 3 to 6, the maximum displacement of the retaining wall bricks in working conditions 3 and 5 is greater than that in working conditions 4 and 6, and among the calculated maximum displacements of the retaining wall bricks, the displacement of the retaining wall bricks in working condition 6 is the smallest, with a value of 1.148 mm. This shows that the inverted arrangement is better than the upright arrangement.

Of the working conditions, working condition 1 refers to ordinary rectangular retaining wall bricks, working condition 3 refers to upright foot-stepped L-shaped retaining wall bricks, and working condition 5 refers inverted foot-stepped L-shaped retaining wall bricks. The total displacement from large to small is in the order of working condition 3 > working condition 1 > working condition 5. The total displacement of the oil-absorption concrete slope retaining block wall in working condition 3 is 33.97% higher than that in working condition 5. This is because under the action of load, the oil-absorption concrete slope retaining block wall as a whole tilts toward the side of the soil body. The upright foot-stepped L-shaped oil-absorption concrete slope retaining block wall has a stronger force on the soil body due to the upward movement of the center of gravity, causing a greater deformation of the soil body, resulting in a greater displacement of the oil-absorption concrete slope retaining block wall in working condition 3 than in working condition 1. In working condition 5, the inverted arrangement enhances the restraint effect and integrity between the retaining wall bricks, which are optimal among the three conditions, thereby reducing the total displacement of the oil-absorption concrete slope retaining block wall.

Working condition 2 refers to vegetated rectangular retaining wall bricks, working condition 4 refers to upright vegetated foot-stepped L-shaped retaining wall bricks and working condition 6 refers to inverted vegetated foot-stepped L-shaped retaining wall bricks. The total displacement from large to small is in the order of working condition 2 > working condition 4 > working condition 6. The total displacement of the oil-absorption concrete slope retaining block wall in condition 2 is 24.13% higher than that in condition 6. This is because, with the same amount of concrete, the L-shaped oil-absorption concrete slope retaining block wall with vegetation has a lighter mass and better integrity than the rectangular oil-absorption concrete slope retaining block wall with vegetation. After the inverted arrangement, the results of condition 6 show that the stress of the inverted L-shaped oil-absorption concrete slope retaining block wall with vegetation is the largest among the six conditions, which indicates that the force between the retaining wall bricks in condition 6 is enhanced, providing the oil-absorption concrete slope retaining block wall with a stronger anti-deformation ability.

Table 9. Maximum stress and displacement of oil-absorption concrete slope retaining block wall under different working conditions.

Working Conditions	1	2	3	4	5	6
Smax/MPa	−1.7 × 10−2	−7.0 × 10−2	−5.4 × 10−1	−1.4 × 10−1	−6.1 × 10−2	−1.6 × 10−1
Smax/MPa	1.1 × 10−2	3.0 × 10−2	5.3 × 10−1	1.1 × 10−1	2.8 × 10−2	6.8 × 10−2
Umax/mm	1.480	1.425	1.538	1.312	1.218	1.148

Note: In the table, positive values of S_max indicate tensile stress, and negative values indicate compressive stress.

shows the maximum stress and displacement of the oil-absorption concrete slope retaining block wall under different working conditions. The results show that the compressive stress of the oil-absorption concrete slope retaining block wall is greater than the tensile stress, indicating that the retaining wall bricks are mainly subjected to pressure. Affected by the foundation and the lower structure, the displacement of the oil-absorption concrete slope retaining block wall shows an increasing trend from the bottom to the top of the wall. Under the load, the maximum displacement of the oil-absorption concrete slope retaining block wall occurs in working condition 3 of the upright foot-stepped L-shaped retaining wall bricks, with a value of 1.538 mm, and the minimum displacement occurs in working condition 6, which is 1.148 mm. In order to ensure the stability of the retaining wall, the displacement limit of the retaining wall bricks is 5–10% of the thickness of the retaining wall bricks. The thickness of the retaining wall bricks is designed to be 50 mm; that is, the displacement limit of the retaining wall bricks is 2.5–5 mm, which meets the stability requirements of the soil behind the wall.

4. Conclusions

This paper achieved sustainable oil absorption by adjusting the mix ratio of concrete, and studied the effects of alkali activators and external seashells on its strength and oil absorption performance. Different structural forms and arrangements were designed, and finite element analysis was performed using ABAQUS. The best structural form and arrangement were selected. The following conclusions can be drawn:

By adjusting the concrete mix ratio, sustainable oil absorption was successfully achieved, and its oil absorption performance could reach 207.70 kg/m³. NaOH has an adverse effect on the oil absorption performance of concrete, causing the oil absorption performance to drop from 207.70 kg/m³ to 104.56 kg/m³, but NaOH has an effect on improving the strength of oil absorption, and the 28-day strength can be increased by 5.02%.

Hydrophobic silane is beneficial for improving the oil absorption performance of oil-adsorbing concrete, which can be increased by 265.73%. Seashell powder has an adverse effect on the oil absorption performance and strength of oil-absorbent concrete, causing the 28-day strength and oil absorption performance to decrease by 13.72% and 6.53%, respectively.

Foot-stepped L-shaped concrete bricks, retaining wall bricks with four different structural forms and three arrangements were analyzed through ABAQUS finite element analysis. The loads were self-weight and vehicle loads, and the force and deformation were calculated. The results show that under the same load, the vegetation foot-stepped L-shaped retaining wall bricks have better anti-deformation ability than the foot-stepped L-shaped retaining wall bricks and other structural forms.

The inverted vegetation foot-stepped L-shaped oil-absorption concrete slope retaining block wall produced the smallest total displacement of 1.148 mm, which was 22.43% lower than the total displacement of the ordinary rectangular retaining wall. Therefore, the vegetation foot-stepped L-shaped retaining wall bricks were used as the preferred structural form, and the inverted arrangement method was adopted to achieve the best performance for an oil-adsorbing L-shaped retaining wall.