In China, engineers have worked to create additional usable land for building construction by flattening the ridges of hills and filling in the adjacent valleys. China’s Loess Plateau comprises a type of soil (loess) with a large pore structure that can collapse and become unstable when exposed to groundwater. Conventional valley fill materials include remolded loess or remolded loess treated with cement, lime, gypsum, or other stabilizing additives. These stabilizers are often detrimental to the surrounding environment. Moreover, loess treated with conventional stabilizers exhibits excessive brittleness, which is not suitable for building foundations. Adequate stability of the building foundations in the filled valleys is required to ensure public safety. In this study, we tested 50 remolded loess samples treated with a lignin polymer compound to determine its potential as a valley fill material. Triaxial tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to study the mechanical characteristics of each sample, determine the effects of the lignin treatment on the loess, and identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered. Based on this study’s results, the optimum lignin content in the treated loess samples was 4%; lignin contents exceeding 4% decreased axial stress and increased dilation after saturation. The shear strength and strain-hardening phenomenon of the lignin-treated loess samples increased as the lignin content increased, while the excess pore water pressure decreased. Microscopically, the addition of lignin increased cohesion in the loess samples, while slightly contributing to the internal friction angle. The use of lignin as a stabilizing additive for valley fill material shows potential for controlling building foundation deformation by increasing soil strength and minimizing environmental impacts by maintaining the soil pH and limiting pollutant production.
Loess—a type of soil with a large pore structure—is found all over the world including in Asia, Europe, and North and South America [
Loess is eroded by water.
In China, engineers have worked to create additional usable land for building construction by flattening the ridges of hills and filling in the adjacent valleys. They are now applying these same land creation processes in the north and northwest areas of the country where loess concentrations are high. These areas are near the cities of Lanzhou and Yan’an in the Gansu and Shanxi Provinces, respectively. Figure
Land creation engineering.
Conventional valley fill materials include remolded loess or remolded loess treated with cement, lime, gypsum, or other stabilizing additives. These stabilizers are often detrimental to the surrounding environment [
Cuisinier et al. [
Results from these previous studies have demonstrated that the use of conventional stabilizing additives (cement, lime, and fly ash) is effective in improving the shear strength and compressibility of loess and has subsequently encouraged the widespread use of loess as a fill material in geotechnical engineering applications. However, the use of these conventional stabilizers has been found to have a detrimental effect on the environment although this topic has received much less scholarly focus. Rollings et al. [
An environmentally friendly alternative to the conventional stabilizing additives of cement, lime, and fly ash used to improve the strength and durability of loess is required. Lignin—a polymer compound produced in the paper industry—has shown some potential for stabilizing unstable soil [
Lignin offers several advantages over the conventional stabilizing additives of cement, lime, and fly ash. Lignin is less costly, more environmentally friendly, and readily available. Over 50 million tons of industrial lignin is produced worldwide annually [
In this study, we tested 50 remolded loess samples treated with lignin to determine its potential as a valley fill material. Triaxial tests, including unconfined compression (UC), isotropically consolidated undrained compression (ICUC), and consolidated drained compression (CDC) shearing tests, were used to study the mechanical characteristics of each sample and determine the effects of the lignin treatment on the loess. In addition, SEM and XRD were used to identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered.
To support testing in this study, loess was acquired in northwestern China near the city of Lanzhou in the Gansu Province at a depth of 10.5–12.5 m. Figure
Location of the research area and the sampling points.
Particle size distribution curve of loess soil.
The lignin used in this study was in the form of a nontoxic white powder that was soluble in water. Lignin, a polymer compound produced in the papermaking industry, contains hydrophilic groups including sulfonate, phenylic hydroxyl, and alcoholic hydroxyl and hydrophobic groups including the carbon chain [
In the laboratory, the loess was initially dried for 7–10 days to ensure a stable water content. A mortar was then used to crush the dry loess. The loess was subsequently sieved in preparation for testing.
Prepared samples included both treated and untreated loess. Treated loess samples were prepared using five lignin concentrations (0, 1, 2, 3, and 4%) based on the dry loess weight. Samples initially prepared with lignin concentrations higher than 4% became seriously dilated when saturated using the vacuum saturation method. Thus, the results for samples with 5% lignin were consistent with the results for samples with 4% lignin, eliminating the need to prepare and consider samples with lignin concentrations higher than 4%. The required amount of lignin was first mixed with the loess. Water was then added until the optimum water content was reached.
Specimens were prepared using static compaction with controlled static stress and axial displacement. After pouring the required amount of loess into the stainless steel barrel, the stainless steel head was lowered onto one side of the sample with a load, producing a half axial displacement. Figure
Mould of compacting samples. (1) Stainless steel barrel. (2) Stainless steel head. (3) Soil. (4) Heel block.
To investigate the shear strength of lignin-treated loess, UC, ICUC, and CDC tests were conducted on samples that were cured for 28 days. Preliminary UC test results for samples cured for 0, 4, 8, 12, 16, 20, 24, 28, 32, and 36 days revealed that the shear strength of the loess increased only marginally for cure times in excess of 28 days. Table
Summary of tests in the study.
Samples | Moisture content (%) | Density (g/cm3) | Plastic limit (%) | Liquid limit (%) | Void ratio | Specific gravity | Test type |
---|---|---|---|---|---|---|---|
MZS-0 | 9.11 | 1.73 | 26.08 | 16.31 | 0.71 | 2.71 | UC |
MZS-1 | 9.63 | 1.73 | 31.25 | 19.28 | 0.71 | 2.70 | UC |
MZS-2 | 9.65 | 1.73 | 33.12 | 18.09 | 0.70 | 2.69 | UC |
MZS-3 | 9.53 | 1.72 | 37.90 | 21.69 | 0.71 | 2.68 | UC |
MZS-4 | 9.51 | 1.72 | 43.17 | 21.70 | 0.69 | 2.66 | UC |
MZS-0 | 9.46 | 1.73 | 26.08 | 16.31 | 0.71 | 2.71 | ICUC + 80 kPa |
MZS-1 | 9.44 | 1.73 | 31.25 | 19.28 | 0.71 | 2.70 | ICUC + 80 kPa |
MZS-2 | 9.27 | 1.73 | 33.12 | 18.09 | 0.70 | 2.69 | ICUC + 80 kPa |
MZS-3 | 9.29 | 1.73 | 37.90 | 21.69 | 0.69 | 2.68 | ICUC + 80 kPa |
MZS-4 | 9.52 | 1.73 | 43.17 | 21.70 | 0.68 | 2.66 | ICUC + 80 kPa |
MZS-0 | 9.61 | 1.73 | 26.08 | 16.31 | 0.72 | 2.71 | ICUC + 140 kPa |
MZS-1 | 9.96 | 1.72 | 31.25 | 19.28 | 0.73 | 2.70 | ICUC + 140 kPa |
MZS-2 | 9.78 | 1.72 | 33.12 | 18.09 | 0.72 | 2.69 | ICUC + 140 kPa |
MZS-3 | 10.12 | 1.72 | 37.90 | 21.69 | 0.72 | 2.68 | ICUC + 140 kPa |
MZS-4 | 10.08 | 1.72 | 43.17 | 21.70 | 0.70 | 2.66 | ICUC + 140 kPa |
MZS-0 | 10.11 | 1.72 | 26.08 | 16.31 | 0.73 | 2.71 | ICUC + 200 kPa |
MZS-1 | 10.15 | 1.72 | 31.25 | 19.28 | 0.73 | 2.70 | ICUC + 200 kPa |
MZS-2 | 10.55 | 1.72 | 33.12 | 18.09 | 0.73 | 2.69 | ICUC + 200 kPa |
MZS-3 | 10.32 | 1.73 | 37.90 | 21.69 | 0.71 | 2.68 | ICUC + 200 kPa |
MZS-4 | 10.29 | 1.73 | 43.17 | 21.70 | 0.70 | 2.66 | ICUC + 200 kPa |
MZS-0 | 10.32 | 1.73 | 26.08 | 16.31 | 0.73 | 2.71 | ICUC + 300 kPa |
MZS-1 | 10.53 | 1.74 | 31.25 | 19.28 | 0.72 | 2.70 | ICUC + 300 kPa |
MZS-2 | 9.46 | 1.73 | 33.12 | 18.09 | 0.70 | 2.69 | ICUC + 300 kPa |
MZS-3 | 9.44 | 1.73 | 37.90 | 21.69 | 0.70 | 2.68 | ICUC + 300 kPa |
MZS-4 | 9.27 | 1.73 | 43.17 | 21.70 | 0.68 | 2.66 | ICUC + 300 kPa |
MZS-0 | 9.29 | 1.73 | 26.08 | 16.31 | 0.71 | 2.71 | CDC + 80 kPa |
MZS-1 | 9.52 | 1.72 | 31.25 | 19.28 | 0.72 | 2.70 | CDC + 80 kPa |
MZS-2 | 9.61 | 1.72 | 33.12 | 18.09 | 0.71 | 2.69 | CDC + 80 kPa |
MZS-3 | 9.96 | 1.72 | 37.90 | 21.69 | 0.71 | 2.68 | CDC + 80 kPa |
MZS-4 | 9.78 | 1.72 | 43.17 | 21.70 | 0.70 | 2.66 | CDC + 80 kPa |
MZS-0 | 10.12 | 1.72 | 26.08 | 16.31 | 0.74 | 2.71 | CDC + 140 kPa |
MZS-1 | 10.08 | 1.73 | 31.25 | 19.28 | 0.72 | 2.70 | CDC + 140 kPa |
MZS-2 | 10.11 | 1.73 | 33.12 | 18.09 | 0.71 | 2.69 | CDC + 140 kPa |
MZS-3 | 10.15 | 1.73 | 37.90 | 21.69 | 0.71 | 2.68 | CDC + 140 kPa |
MZS-4 | 10.55 | 1.74 | 43.17 | 21.70 | 0.69 | 2.66 | CDC + 140 kPa |
MZS-0 | 10.29 | 1.73 | 26.08 | 16.31 | 0.73 | 2.71 | CDC + 200 kPa |
MZS-1 | 10.32 | 1.73 | 31.25 | 19.28 | 0.72 | 2.70 | CDC + 200 kPa |
MZS-2 | 10.53 | 1.73 | 33.12 | 18.09 | 0.72 | 2.69 | CDC + 200 kPa |
MZS-3 | 9.46 | 1.73 | 37.90 | 21.69 | 0.70 | 2.68 | CDC + 200 kPa |
MZS-4 | 9.44 | 1.72 | 43.17 | 21.70 | 0.69 | 2.66 | CDC + 200 kPa |
MZS-0 | 9.27 | 1.74 | 26.08 | 16.31 | 0.70 | 2.71 | CDC + 300 kPa |
MZS-1 | 9.29 | 1.73 | 31.25 | 19.28 | 0.71 | 2.70 | CDC + 300 kPa |
MZS-2 | 9.52 | 1.73 | 33.12 | 18.09 | 0.70 | 2.69 | CDC + 300 kPa |
MZS-3 | 9.88 | 1.73 | 37.90 | 21.69 | 0.70 | 2.68 | CDC + 300 kPa |
MZS-4 | 1.02 | 1.73 | 43.17 | 21.70 | 0.55 | 2.66 | CDC + 300 kPa |
MZS-0 | 10.06 | 1.72 | 26.08 | 16.31 | 0.73 | 2.71 | SEM + XRD |
MZS-1 | 9.97 | 1.72 | 31.25 | 19.28 | 0.73 | 2.70 | SEM + XRD |
MZS-2 | 9.83 | 1.72 | 33.12 | 18.09 | 0.72 | 2.69 | SEM + XRD |
MZS-3 | 9.65 | 1.72 | 37.90 | 21.69 | 0.71 | 2.68 | SEM + XRD |
MZS-4 | 9.67 | 1.72 | 43.17 | 21.70 | 0.70 | 2.66 | SEM + XRD |
To best reflect the field conditions near the cities of Lanzhou and Yan’an in the Gansu and Shanxi Provinces, respectively, all triaxial tests were performed on saturated specimens. In the newly created land areas in Lanzhou and Yan’an, many of the filled valleys remain low-lying, with the groundwater level very close to the new ground surface. In these areas, full saturation is common. Rainfall and irrigation runoff will further infiltrate the filled valleys; the fill material encourages the horizontal movement of water throughout its mass, ensuring full saturation in low-lying areas.
Prior to ICUC and CDC testing, vacuum saturation methods were used to saturate the specimens for more than 24 hr. Next, back pressure saturation methods were used until each sample’s
To additionally reflect the field conditions near Lanzhou and Yan’an, effective confining pressures used in the triaxial tests were determined based on the in situ conditions of these areas. The valley depths near Lanzhou and Yan’an range from 6.0 to 36.0 m. Based on these depths, effective confining pressures were calculated as a function of the soil density and the coefficient of arth pressure at rest (
In addition to the triaxial tests, SEM and XRD were used to identify the microscopic mechanism affecting shear stress in the lignin-treated loess. Prior to SEM analysis, the loess samples were first freeze-dried and then positioned with an adhesive paste on a flat surface and sprayed with a conductive gold coating. The prepared samples were analyzed using a scanning electron microscope (KYKY-2800B) with amplifications of 100, 200, 500, and 600. Similarly, the freeze-dried loess samples were positioned on a glass wafer and subsequently analyzed in the X-ray chamber.
Figure
UCS results of treated loess.
Figure
Undrained shear (ICUC) behavior of treated loess with different contents of lignin at different effective confining pressures. (a) 80 kPa. (b) 140 kPa. (c) 200 kPa. (d) 300 kPa.
As shown in Figure
The peak deviator stress also rapidly increased as the effective confining pressure increased. In the untreated (0% lignin) loess samples, the residual stress was significant following the peak deviator stress. In the treated (1–4% lignin) loess samples, the deviator stresses increased continuously until the axial strain exceeded 15%. The deviator stress amplitude also increased as the effective confining pressure increased. As shown in Figure
As shown in Figure
Figure
Drained shear (CDC) behavior of treated loess with different contents of lignin at different effective confining pressures. (a) 80 kPa. (b) 140 kPa. (c) 200 kPa. (d) 300 kPa.
As shown in Figure
As shown in Figure
The results of both the ICUC and CDC tests indicated that the addition of lignin increased the shear strength of loess. The deviator stress was nearly constant when the axial strain was 15%. Stress-strain relationships from both tests also showed a strain-hardening behavior in the treated (1–4% lignin) loess samples. However, in the untreated (0% lignin) loess samples, the stress-strain relationships differed between the two tests. In the ICUC tests, the untreated samples showed a strain-softening behavior, while in the CDC tests, the untreated samples showed a strain-hardening behavior. This latter finding indicating a strain-hardening behavior is consistent with previously reported findings [
To analyze the mechanical effects of lignin, scanning electron microscopy (SEM) was used to compare the microstructures of the treated (1–4% lignin) and untreated (0% lignin) loess samples. Figure
SEM results of treated loess with different contents of lignin. (a) MZS-0. (b) MZS-1. (c) MZS-2. (d) MZS-3. (e) MZS-4.
In the untreated loess samples, the clay particles were adhered to larger particle surfaces. The grain outlines were distinct, and the adhesive material between particles was minimal. The embedded particles were compact but joined only by point contacts with obvious open voids. The pore structures in these samples were generally uniform.
In the treated loess samples, the adhesive content on the large particle surfaces increased as the lignin content increased. When the lignin content was 1-2%, the adhesive content on the silt (0.005–0.05 mm) and sand (>0.05 mm) surfaces increased, and the interparticle contact mode changed from point to surface contacts, which increased the material’s macroscopic mechanical properties. When the lignin content was 3-4%, the banded lignin fibers were apparent in a randomly distributed pattern around the loess particles. In these samples with higher lignin contents, the content of linear-shaped (strip) lignin fibers increased significantly, the adhesive content on the large particle surfaces increased, and loess particle aggregation increased. These collective findings suggest that the increased strength of lignin-treated loess is primarily attributable to the increase in strip lignin fibers and adhesion in the material’s microstructure.
To supplement the findings of the SEM analysis, X-ray diffraction (XRD) was used to compare the mineral compositions of the treated (1–4% lignin) and untreated (0% lignin) loess samples. The specific mineral composition of the loess included hornblende, gypsum, quartz, feldspar, calcite, pyroxene, and kaolin. Table
Mineral composition of loess and treated loess samples.
Samples | Hornblende (%) | Gypsum (%) | Quartz (%) | Feldspar (%) | Calcite (%) | Pyroxene (%) | Kaolin (%) |
---|---|---|---|---|---|---|---|
MZS-0 | 4.20 | 1.19 | 35.50 | 16.80 | 11.87 | 5.00 | 25.44 |
MZS-1 | 4.33 | 1.40 | 32.84 | 17.65 | 12.27 | 5.40 | 26.11 |
MZS-2 | 5.12 | 1.20 | 32.68 | 18.24 | 8.42 | 6.10 | 28.24 |
MZS-3 | 6.14 | 1.77 | 30.82 | 18.20 | 5.48 | 6.33 | 31.26 |
MZS-4 | 6.00 | 1.65 | 30.71 | 19.55 | 0.79 | 7.85 | 33.45 |
The mineral compositions of the treated and untreated loess samples were nearly consistent. In the treated loess samples, the clay mineral content was generally higher and increased as the lignin content increased. This finding suggests that an ion exchange occurs at certain water contents after the lignin is mixed with the loess, which results in the formation of new clay minerals and an increased clay mineral content.
In response to the need for an environmentally friendly alternative to conventional stabilizing additives (cement, lime, and fly ash) used to improve the strength and durability of valley fill material, we considered the effects of lignin on loess shear strength. In this study, we tested 40 remolded loess samples treated with lignin. Triaxial tests, SEM, and XRD were used to study the mechanical characteristics of each sample, determine the effects of the lignin treatment on the loess, and identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered.
Based on the results of the UC tests, compressive strengths generally increased as lignin content increased. The optimum lignin content in the treated loess samples was 4%. Lignin contents exceeding 4% decreased axial stress and increased dilation after saturation, which in turn affected the accurate test of mechanical properties.
Based on the results of the ICUC and CDC tests, shear strengths also increased as lignin content increased. Stress-strain relationships from both tests also showed a strain-hardening behavior in the treated (1–4% lignin) loess samples. However, in the untreated (0% lignin) loess samples, the stress-strain relationships differed between the two tests. In the ICUC tests, the untreated samples showed a strain-softening behavior, while in the CDC tests, the untreated samples showed a strain-hardening behavior. In the ICUC tests, the addition of lignin transformed the stress-strain relationship from strain softening to strain hardening, and the excess pore water pressure decreased as the strain increased. The decrease in excess pore water pressure is countered by the increase in shear strength. This collective behavior suggests good potential for lignin-treated loess in geotechnical engineering applications.
Results of the CDC tests also indicated the effects of different effective confining pressures on a material’s shear strength. A dilative shear response was observed for samples tested at low confining pressures (80 and 140 kPa), while a contractive shear response was observed for samples subjected to higher confining pressures (200 and 300 kPa). In each case, the responses were magnified in the treated loess samples.
Finally, results from the SEM and XRD analyses indicated that the addition of lignin altered the pore structure and material composition of the loess. These changes to the material’s microstructure increased cohesion in the loess samples, while slightly contributing to the internal friction angle. In samples with higher lignin contents (3-4%), the increased strength of lignin-treated loess was primarily attributable to the increase in both strip lignin fibers and adhesion in the material’s microstructure. Considering material composition, ion exchange following the addition of lignin resulted in the formation of new clay minerals and an increased clay mineral content.
Based on these collective results, the use of lignin as a stabilizing additive for valley fill material shows potential for controlling building foundation deformation by increasing soil strength. In addition, the use of lignin as an alternative stabilizing additive to cement, lime, and fly ash would minimize environmental impacts by maintaining the soil pH and limiting pollutant production.
The figures and tables reflecting data used to support the findings of this study are included within this paper.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to thank Mr. Wang Jun (China Earthquake Administration and Gansu Province) for his useful guidance. The authors would also like to acknowledge Mr. Liu Zhaozhao (Lanzhou University) and Mr. Fan Wenjun (Lanzhou University) for their assistance in sampling. This work was supported by the Opening Fund of the Key Laboratory of Loess Earthquake Engineering, China Earthquake Administration, and Gansu Province (Grant KLLEE-17-004); National Natural Science Foundation of China (Grant 51778590); and Fundamental Research Funds for the Central Universities (Grant lzujbky-2017-170).