Effect of Straw Characterization on the Mechanical Behavior of Compacted Straw-Reinforced Soils

Abstract

Straw reinforcement improves the mechanical properties of soil matrices by uniformly incorporating dispersed straw materials, demonstrating advantages in strength enhancement, toughness improvement, and deformation control. This study aims to compare the reinforcement effects of different types of straw on soil and clarify the optimal method for straw-based soil stabilization. For wheat straw-reinforced soil using different processing methods (straw segment, straw powder, and straw ash) and mass contents, the basic geotechnical properties of each mixture were first determined. Triaxial tests were then performed under varying confining pressures and compaction conditions to assess the strength and modulus characteristics of the different reinforced soil specimens, and the microstructural characteristics of fiber-reinforced soil were investigated using scanning electron microscopy (SEM) analysis. The experimental results indicated that the strength and ductility of soils increased significantly with the addition of straw. The optimal performance of straw-reinforced soils occurred at 0.3% content. The elastic modulus increased by 85%, 64%, and 57% under confining pressures of 50 kPa, 100 kPa, and 200 kPa, respectively. At 200 kPa, straw segments provided the highest modulus increase of 57%, while straw ash achieved the greatest strength improvement of 97%. Furthermore, considering both compaction effects and cost efficiency, a compaction degree of 95% is recommended for straw-reinforced soil in engineering applications. Based on scanning electron microscopy, it was observed that the distribution characteristics of different straw types within the soil exhibit distinct patterns. This study aims to provide data to support the efficient utilization of straw materials in engineering applications.

1. Introduction

The development of a healthy and well-organized straw crop industry is crucial for addressing China’s “three rural” issues, accelerating rural revitalization and promoting the construction of ecological civilization [1]. As a major agricultural country, China produces a wide variety of crop residues in large quantities, with a broad geographic distribution. However, the low-carbon treatment methods for straw waste remain underdeveloped, and conventional straw disposal practices include incineration, returning to the field, and conversion into feed or fuel. Straw incineration releases a large amount of pollutants such as NO, CO, and particulate matter, which degrade air quality and pose a threat to human health [2]. Returning straw to the field leads to an increase in the probability of pests and disease outbreaks, alters weed dynamics, and alterations in the soil microbiome [3]. Additionally, the production of fodder and fuel from straw is economically inefficient and suffers from low treatment efficiency [4]. Therefore, it is urgent to develop an efficient and sustainable method for the utilization of straw.

Straw belongs to biomass fibers and can serve as a reinforcing material for soils. Reinforced soil-related research has a long history, with the French engineer Vidal H [5] being the first to introduce the concept of modern soil reinforcement in the early 1960s. This concept involves incorporating strips into the soil. Utilizing interfacial strength and other factors, the tensile strength of the reinforcement is combined with the compressive strength of the soil, thereby improving the overall stability of the soil. Early studies primarily focused on reinforced materials such as geomembranes, geotextiles, geocells, and others [6]; these reinforced materials can traverse the shear zone of the soil body and utilize their inherent strength to resist the external load [7,8,9]. To further enhance the reinforcing effect, researchers have made improvements based on traditional reinforcing materials and developed a variety of innovative reinforcing technologies. Among these, fibers have increasingly gained attention as a potential soil reinforcement material [10].

The fiber reinforcement materials that are widely used in geotechnical engineering can be divided into synthetic fibers and natural fibers, in which polypropylene fibers and glass fibers and others, as synthetic materials blended into the soil body can significantly improve its unconfined compressive strength and tensile and shear strength and also increase the damage fracture toughness of the soil samples [11,12,13]. Natural fibers have also been favored by many scholars due to their advantages such as low cost and environmental friendliness, and their reinforcing effect has also been proven [14,15,16,17]. In fact, before the introduction of synthetic fibers, human beings used natural fiber materials such as straw to reinforce the soil thousands of years ago, and a long period of research and application has proven the applicability and safety of this type of reinforcement material.

In summary, straw-reinforced soil can be widely applied in geotechnical engineering, and it is also suitable for projects with high ecological requirements, such as slope stabilization and desertification control [18,19,20]. The application of reinforced soil technology in engineering significantly reduces construction costs and carbon emissions. For instance, the embankment along the park road near Joe Pool Lake in Texas, USA, employed fiber-reinforced soil technology, resulting in a 40% reduction in total project costs and a 60% decrease in carbon emissions [21], and this method helps to further solve the problem of resource utilization of crop straw. Current research on straw reinforcement techniques has yielded substantial findings, with the majority of studies having predominantly focused on reinforcement efficacy evaluations of diverse straw material compositions and parametric variations. However, empirical data remain insufficient concerning the influence of morphological configurations within identical straw types on soil reinforcement. Comparatively underexplored are investigations into enhanced soil compaction behaviors induced by intra-material morphological variations, with particularly limited analytical attention devoted to microscale reinforcement mechanisms. Therefore, this study investigated the impacts of three straw morphological forms (chopped segments, ground powder, and combustion-derived ash) based on existing treatment methods involving straw utilization as livestock feed through mechanical grinding, open-field burning, mass admixture, and specimen compaction on the mechanical properties of straw-reinforced soil using triaxial tests. Using SEM experiments, the reinforcement mechanism of different straw states was analyzed, aiming to provide data support for the efficient utilization of straw materials in engineering.

2. Test Content

2.1. Test Materials

The soil used in this study was collected from Batang County, Ganzi Prefecture, SiChuan Province, and the soil samples were yellowish brown, with fine particles and relatively uniform particle size, with a clayey composition. After air drying, the soil samples were sieved to remove impurities larger than 2 mm. The soil samples and grading curves are shown in Figure 1 ($C_u=d_{60}/d_{10},C_c=d_{30}^2/d_{60} \times d_{10}$, included among these $d_{10},d_{30},\mathrm{and}{d}_{60}$ comprise sieved weight as a percentage of 10%, 30%, and 60% particle size, respectively).

The reinforcing material was derived from wheat straw grown in the selected loess. After natural air drying, the wheat straw was cut into segments with an average length of 8–10 mm and similar diameters, followed by sieving to ensure uniformity (Figure 2a). The remaining straw segments were then ground in a grinder, and the resulting straw powder was sieved to obtain a homogeneous particle size distribution (Figure 2b). Subsequently, the straw powder was mixed with ethanol in a container and ignited. After complete combustion, the residual straw ash was sieved to remove unburned residues, as shown in Figure 2c.

The basic physical parameters of straw-reinforced soil in different states and mass content are presented in

Table 1. Basic physical parameters of straw-reinforced soil.

Status/Content	Plastic Limit (%)	Liquid Limit (%)	Maximum Dry Density (g·cm3)	Optimum Moisture Content (%)
0%	22.1 ± 0.5	45.84 ± 0.6	1.94 ± 0.05	19.7 ± 0.5
0.3% segment	21.26 ± 0.6	67.30 ± 0.7	1.92 ± 0.06	16.2 ± 0.7
0.6% segment	20.84 ± 0.8	74.72 ± 0.7	1.88 ± 0.08	15.7 ± 0.7
0.9% segment	19.60 ± 0.9	78.57 ± 0.9	1.85 ± 0.11	15.3 ± 0.8
0.3% powder	17.19 ± 0.6	46.23 ± 0.6	1.90 ± 0.06	15.6 ± 0.6
0.3% ash	16.25 ± 0.6	53.75 ± 0.7	1.91 ± 0.05	15.8 ± 0.5

. As the straw content increased, the integrity of the specimens decreased, resulting in reductions in their plastic limit, maximum dry density, and optimum water content. In contrast, the incorporation of the straw restricted water flow in the soil samples, resulting in an increase in the liquid limit. Compared to the straw segment-reinforced soil, straw powder and straw ash are more water absorbent, which caused saturation of the soil samples and further decreased the plastic limit, maximum dry density, and optimum water content.

2.2. Test Program

In natural or engineering environments, soil typically exists under a three-dimensional stress state. Compared to other testing methods (e.g., direct shear test, uniaxial compression test), the triaxial test enables precise simulation of this condition by applying controlled confining pressure and axial stress. Consequently, the triaxial test was selected to evaluate the reinforced soil specimens. The triaxial test was conducted according to the ASTM D2850 [22] Standard for Geotechnical Test Methods. The diameter of the specimen was 39.1 mm, and the height was 80 mm. Due to the poor drainage properties of loess, water usually cannot typically be discharged quickly during compression; therefore, this test was performed as an Unconsolidated Undrained (UU) test [23,24,25]. Studies by Chen et al. [26] and Ensani et al. [27] have demonstrated that when the moisture content of reinforced soil reaches its optimum level, the interaction between the reinforcement and soil is optimized, and the soil strength is maximized. However, with further increases in moisture content, the strength of fiber-reinforced soil decreases significantly. To ensure consistency in testing conditions, this study adopted the optimum moisture content for each type of reinforced soil. Based on the optimum moisture content of different reinforced soils, the required masses of reinforced soil and water for each group were proportioned. Straw fibers with varying morphologies were uniformly dispersed into the soil and thoroughly mixed. Subsequently, the prepared water was evenly sprayed onto the straw-expansive soil mixture with continuous agitation to gradually achieve the predetermined optimum moisture content. The formulated reinforced soil mixtures were then transferred into standardized containers, with each group being properly labeled and hermetically sealed with parafilm. All prepared soil specimens underwent 24 h static curing prior to sample preparation. According to the specification requirements, the test process is divided into stages, including specimen preparation, installation, shear, and others. During the specimen preparation stage, the soil sample and straw material were stirred evenly and compacted in three layers [28], with the surface scraped after each layer to enhance the adhesion between layers and improve the integrity of the soil sample.

The TSZ-2 fully automatic triaxial apparatus was utilized in this experiment. During the shear stage, the shear rate is maintained at a constant 0.8 mm/min to ensure the specimen’s strength is fully mobilized. According to Hao’s research [29], under low confining pressure (50 kPa, 100 kPa), the damage state of straw-reinforced soil presents the characteristics of “plastic bulging damage”. When the confining pressure is 200 kPa, the specimen’s damage clearly reaches the critical state. ASTM D2850 [22] specifies a confining pressure range of 50–400 kPa. For instance, a confining pressure of 50 kPa is typically adopted for shallow soil layers, while 200 kPa is recommended for deep soil applications, such as pile foundations. Therefore, the tests using pressures of 50 kPa, 100 kPa, and 200 kPa represent three common engineering confining pressure levels [30,31,32]. In addition, numerous studies have shown that the optimum quality content in fiber-reinforced soils is approximately 0.5% [11,33]. Based on this, straw quality content levels of 0%, 0.3%, 0.6%, and 0.9% were selected in this study to investigate the effect of straw content. For the compaction study, three common engineering compact levels, namely, 92%, 95%, and 98%, were selected [34,35]. In addition, at 20 °C temperature and 60% relative humidity, three replicate tests were conducted for each condition to minimize experimental errors. The experimental conditions are presented in

Table 2. Experimental scheme of straw-reinforced soil.

Status/Content	Confining Pressure/kPa	Compaction/%	Repetition
0%	50/100/200	95	3
0.3% Straw segment	50/100/200	95
0.6% Straw segment	50/100/200	95
0.9% Straw segment	50/100/200	95
0.3% Segment/ash/powder	200	95
0.3% Straw segment	200	92/95/98

.

3. Test Results and Analysis

In this paper, the stress–strain curve represents the strength of each specimen, and the data obtained from the triaxial test are processed to derive the stress–strain curve according to ASTM D2850 [22] and previous studies [36,37]. The deviator stress $q$ is calculated as follows:

$$q=({\sigma }_1-{\sigma }_3)=P/A$$

(1)

where ${\sigma }_1$ and ${\sigma }_3$ are the axial stress and confining stress, respectively; $P$ is the axial load. $A$ is the average cross-sectional area:

$$A=A_0/(1-\epsilon )$$

(2)

where $A_0$ is the initial cross-sectional area of the specimen. In addition, $\epsilon$ is the axial strain:

$$\epsilon =\ln L/L_0$$

(3)

where $L$ is the current length of the specimen, and $L_0$ is the initial length of the specimen. Moreover, the correction for deviations caused by the rubber membrane $\Delta ({\sigma }_1-{\sigma }_3{)}_m$ is calculated as follows:

$$\Delta ({\sigma }_1-{\sigma }_3{)}_m=4E_m{t}_m{\epsilon }_1/D$$

(4)

where $\Delta ({\sigma }_1-{\sigma }_3{)}_m$ is the membrane correction to be subtracted from the measured deviator stress, $D$ is the diameter of the specimen ($D=\sqrt{4A/\mathsf{\pi }}$), $E_m$ is the modulus of elasticity of the membrane (1100 kPa), and $t_m$ is the thickness of the membrane (3 mm).

3.1. Effects of Different Confining Pressures and Straw Segment Content on Soil Properties

Figure 3 shows the axial stress–strain curves of reinforced soil with different straw segment contents (0%, 0.3%, 0.6%, 0.9%) under the same testing conditions (each reinforced soil sample is prepared at optimum moisture content and 95% compaction). As shown in the figure, the stress–strain curve of the soil samples increased rapidly to the peak stress in the initial stage. Then, the stress began to plateau as strain increased. Finally, the soil samples failed, and the strength was reduced. For all confining pressures, the strength of soil was significantly increased by the addition of wheat straw, resulting in a delayed peak strength and increased ductility of the specimens. This behavior is consistent with observations in other enhanced soils (including other straw and traditional soil fabrics, etc.) [38,39,40,41]. Compared with data from Wang X et al. [40], at 50 kPa, 100 kPa and 200 kPa, the strength was increased by 30–60%, while the wheat straw-reinforced soil increased by about 80%. However, influenced by factors such as straw type and segment length, with the same parameters (0.6% content, 100 kPa), Wang Y et al. [42] reported no significant improvement in strength. Under different confining pressures, the peak strength of reinforced soils with the three contents showed little variation, indicating that the internal friction angle of the soil samples was low.

The initial modulus E (defined as the secant modulus at 1% strain) was used to evaluate the effect of wheat straw on stiffness. As shown in Figure 4, the initial modulus of soil samples was maximum at 0.3% straw section content. Under confining pressures of 50 kPa, 100 kPa, and 200 kPa, the mean elastic modulus values were 5.17 MPa, 5.40 MPa, and 5.76 MPa, respectively, corresponding to increases of 85%, 64%, and 57%, respectively, compared to the unreinforced soil baseline. After which, the values gradually decreased with increasing straw section content. This decline may be due to the fact that higher straw section content reduces the homogeneity of soil samples. Considering both strength and modulus, 0.3% straw mass content is the optimum content for this study.

Figure 5 shows the effect of straw segment content on cohesion (c) and internal friction angle (φ). Compared to the plain soil, the cohesion increases in the reinforced soil, while the internal friction angle remains relatively unchanged. The results suggest that the cohesion at the straw–loam interface is higher than that of the soil itself. The external load is transferred to the straw through this highly cohesive interface, with the tensile strength of the straw segment contributing to the increased shear strength of the soil.

Figure 6 illustrates the Failure morphology of the specimens at the end of the test. The shear band in the plain soil specimen is straight, while the shear band in the reinforced soil specimen is arc shaped. This observation suggests that the straw incorporated into the soil extends through the shear zone, although the overall strength increases. This also results in an uneven strength distribution within the shear zone and a reduction in the integrity of the specimen.

3.2. Influence of Different Straw States on the Properties of Straw-Reinforced Soil

Triaxial tests were conducted under a confinement pressure of 200 kPa with 0.3% straw content to assess the stress–strain behavior and mechanical properties of three straw morphological forms (segments, powder, and ash) compacted to a 95% compaction degree. The results are shown in Figure 7. The stress–strain curves of each specimen are similar in shape. The mean modulus values for straw segment, straw ash, and straw powder-reinforced soils were 5.76 MPa, 5.65 MPa, and 4.22 MPa, respectively, representing increases of 57%, 54%, and 15%, respectively, compared to unreinforced soil. Correspondingly, the mean strength values reached 273.38 kPa, 329.60 kPa, and 271.86 kPa, with improvements of 67%, 97%, and 66%, respectively, relative to the unreinforced baseline. The specimen with straw segment shows a significantly higher modulus compared to the specimen with straw powder, which is probably due to the substitution of some clay particles with the relatively softer straw ash and powder, while the soil particles bond with the straw ash and powder to form particles with larger pores, resulting in a decrease in the modulus [43].

The specimens mixed with straw ash exhibited the highest strength and had approximate initial modulus to the straw segment specimens. Previous studies suggest that straw ash contains a large amount of siliceous material. When straw ash is mixed with clay and water, the silica in straw ash reacts with various cations (such as Ca²⁺, K⁺) in the soil to form cementitious compounds [44]. This effectively promotes the adhesion between soil particles, thereby enhancing the strength of the soil samples. The chopped straw segments exhibit greater elastic recovery potential post-compaction, demonstrating enhanced resistance to cyclic loading in practical engineering applications. Ash-reinforced soil achieves higher densification, yet over-compaction may induce particle crushing, necessitating consideration of ash’s long-term stability. The intermediate particle size distribution of straw powder limits its ability to effectively fill pores or establish a reinforcing skeleton, resulting in non-uniform stress distribution and the lowest compaction efficiency among the three morphological forms.

3.3. Effects of Different Compaction Levels on the Properties of Straw-Reinforced Soil

In practical applications, both compaction and cost effectiveness must be considered for rammed soil; however, the effect of compaction on the mechanical properties of straw-reinforced soil has been less studied, the stress–strain curves and their characteristics for the 0.3% straw segment-reinforced soil specimens at 92%, 95%, and 98% of the three common compaction states in the project are show in Figure 8. From the initial modulus and peak strength of the specimen, it can be observed that higher compaction leads to increased strength and modulus. Higher compaction reduces the pore space within the soil, enhancing its integrity and providing a more stable force transfer path under external loads, thereby improving its resistance to these loads. When the compaction degree increased from 92% to 95%, the strength improved by 10%, and the sample modulus rose by 44%. However, when the compaction degree exceeded 98%, both strength and modulus exhibited only marginal increases of 3% and 9% compared to the 95% compaction level, indicating a significant decline in growth rates. Therefore, a 95% compaction level is considered optimal for straw-reinforced soil, considering both performance and economic factors.

3.4. Microanalysis of Reinforcement/Soil Interface Interactions in Straw-Reinforced Soils

Figure 9 show SEM images and pore size proportions magnified by 2000× under different reinforcement conditions. As shown in Figure 9a, plain soil consists of particles with varying sizes accompanied by substantial pore spaces, presenting a relatively loose soil structure. The addition of straw segments into the soil causes a significant amount of clay particles to adhere to the previously smooth surface of the straw segments. Moreover, filamentous fibers exist between the straw segments and the soil, linking the two (Figure 9b). When shear deformation or failure occurs under external loading, the embedded straw segments are subjected to tensile stress. The magnitude of this stress is governed by interfacial adhesion and frictional resistance between the clay particles and straw surfaces. These interfacial forces effectively restrict relative sliding between the straw segments and the soil. Concurrently, the randomly distributed straw fibers establish a three-dimensional interlocking network, mechanically restraining soil particle displacement and deformation, thereby enhancing the macroscopic strength of the composite soil.

In contrast, Figure 9c demonstrates that straw powder exhibits weaker interfacial coupling with clay particles, relying primarily on sparse fibrous connections. Under load, the clay–straw powder interface undergoes particle misalignment and rearrangement due to insufficient bonding. Additionally, the straw powder-reinforced soil exhibits a loose structure. Although its porosity is reduced, the effective interfacial contact area remains limited. These limitations hinder the development of interfacial forces, resulting in inferior elastic modulus and strength compared to straw segment- and ash-reinforced soils.

The reinforcement mechanism of straw ash is depicted in Figure 9d. Straw ash significantly enhances soil compactness and modifies its microstructure through ion exchange, carbonation reactions, and ash–soil particle bonding. This process substantially enhances the interlocking interactions at the interfacial zones and elevates the interfacial frictional coefficient. When straw ash-reinforced soil is subjected to tensile loading, the encapsulating effect of surrounding high-strength straw ash agglomerates significantly amplifies the cohesive and frictional forces at the interfaces, particularly the interlocking friction. These enhanced interfacial forces effectively increase the resistance to fiber–soil matrix slippage and improve the fiber’s capacity to sustain tensile stresses. As shown in Figure 9e, compared to plain soil, reinforced soil exhibits a reduction in pore areas. Due to the relatively large dimensions of straw segments, their filling effect in soil matrix remains limited, while straw powder and straw ash demonstrate significant improvements.

4. Conclusions

A triaxial compression test was conducted on wheat straw-reinforced soil, and the effects of confining pressure, straw content, and sample condition on the mechanical behavior of the soil were assessed. The analysis based on the principle of single variable testing led to the following main conclusions:

(1) The increase in straw incorporation resulted in a decrease in the plastic limit, maximum dry density, and optimum moisture content, while increasing in the liquid limit of the specimens. Compared to the straw segment, straw powder and straw ash caused a more significant decrease in the plastic limit, maximum dry density, and optimum moisture content of the soil samples but a smaller increase in the liquid limit.

(2) Adding straw significantly increased soil strength, as seen in higher cohesion levels. Even a small amount of straw had a noticeable reinforcement effect. When the straw segment content was 0.3%, the specimen exhibited the highest initial modulus. Under confining pressures of 50 kPa, 100 kPa, and 200 kPa, the mean elastic modulus values were 5.17 MPa, 5.40 MPa, and 5.76 MPa, respectively, corresponding to increases of 85%, 64%, and 57%, respectively, compared to the unreinforced soil baseline. However, the initial modulus of the specimen decreased gradually with the content, which may be due to the increase in straw content and the decrease in the integrity of the soil samples. Considering both strength and modulus, 0.3% straw mass content is the optimum straw content.

(3) Comparing the different straw forms, at 200 kPa confining pressure, the mean modulus values for straw segment, straw ash, and straw powder-reinforced soils were 5.76 MPa, 5.65 MPa, and 4.22 MPa, respectively, representing increases of 57%, 54%, and 15%, respectively, compared to unreinforced soil. Correspondingly, the mean strength values reached 273.38 kPa, 329.60 kPa, and 271.86 kPa, with improvements of 67%, 97%, and 66%, respectively, relative to the unreinforced baseline. The increase in strength due to the incorporation of straw segments and straw powder is attributed to the stronger bond at the straw–loam interface compared to the soil itself, with the tensile strength of the straw contributing as external loads are transferred through the cohesive interface. The reinforcement effect of straw ash is primarily due to the formation of cementitious compounds with soil particles.

(4) The strength and modulus of the specimen increased with compaction. When the compaction degree increased from 92% to 95%, the strength improved by 10%, and the sample modulus rose by 44%. However, when the compaction degree exceeded 98%, both strength and modulus exhibited only marginal increases of 3% and 9%, respectively, compared to the 95% compaction level, indicating a significant decline in growth rates. Therefore, considering economic factors, 95% compaction is the optimal level for straw-reinforced soil.

(5) The reinforcement efficiency of straw-modified soils exhibits material-dependent micromechanical mechanisms. Straw segments enhance soil strength through interfacial adhesion and fiber–soil interlocking, restricting particle displacement. Straw powder demonstrates limited reinforcement due to weak interfacial coupling and structural porosity. Straw ash achieves optimal performance via physicochemical bonding (ion exchange/carbonation), forming cemented agglomerates that amplify cohesive friction and tensile resistance.

This study reveals the effect of straw morphology on soil strength. At a fiber content of 0.3%, both strength and modulus exhibit significant improvements. In soft soil foundations, the partial replacement of sand and gravel materials with straw-reinforced soil not only reduces construction costs but also decreases reliance on natural sand and gravel resources. The incorporation of straw fibers enhances toughness and improves fatigue resistance in subgrade materials. Additionally, utilizing straw in construction mitigates environmental pollution caused by open-air burning. In future work, we will study straw improved soil with high straw content and high strength based on the findings in this study.