**1. Introduction**

Soil treatment measures (e.g., cement, lime, fly ash) are commonly used to improve the strength and water retention capacity of soils [1]. Traditional materials such as Portland cement have long been used in geotechnical engineering to treat soil, but a large amount of CO2 is emitted during the production process of cement and lime [2]. One ton of Portland cement production has been shown to release approximately one ton of CO2, and one ton of lime output releases approximately 0.86 tons of CO2 [3]. Alternative materials, such as metakaolin and calcium hydroxide mixtures, alkaline aluminosilicate minerals, and fly ash– based inorganic polymer concrete, have therefore emerged to control and reduce carbon emissions [4–6]. Microbial soil treatment not only protects the ecological environment by controlling carbon emissions compared with traditional and inorganic methods but also significantly improves the strength and ductility of treated soils [7]. Microbial technology, namely biopolymers, has been applied to improve soil mass. For example, the unconfined compressive strength of 0.5% biopolymer was shown to be higher than that of 10% cement after treatment. The large-scale commercialization of biopolymers has good economic feasibility due to the high cost of cement in less-developed countries and can help improve the strength and durability of geotechnical engineering [8]. Driven by this huge catalytic potential and differing from traditional geotechnical engineering soil treatment technology, improved microbial soil treatment technology has been explored, including microbial and inanimate microbial improvement technology.

**Citation:** Zhang, J.; Meng, Z.; Jiang, T.; Wang, S.; Zhao, J.; Zhao, X. Experimental Study on the Shear Strength of Silt Treated by Xanthan Gum during the Wetting Process. *Appl. Sci.* **2022**, *12*, 6053. https:// doi.org/10.3390/app12126053

Academic Editor: Bing Bai

Received: 19 May 2022 Accepted: 13 June 2022 Published: 14 June 2022

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Since the onset of the 21st century, a series of studies have addressed different kinds of microbial technologies for soil treatment: synthetic hydrophilic polyacrylamide additives can improve the strength and stability of different soils [9]; casein and sodium caseinate biopolymers can improve the strength of sand [10]; the Panstrains of Enterobacteriaceae cultivated by cell-free fermentation liquid can stabilize soil [11]; and microbial-induced calcium precipitation technology (MICP) can strengthen and stabilize soil [12]. Recent studies showed that biopolymers could produce hydrogels and induce a pore-blocking effect, which can significantly reduce the permeability, whereas calcite precipitated by MICP does not have such a strong pore-blocking effect [13]. Inanimate microbial technology does not require a strict culture environment and offers greater advantages in enhancing the water stability of soil. Inanimate microbial technology can also produce hydrogel colloid and promote the transport of heavy metals in contaminated soil to improve the soil and protect the environment [14].

Numerous suitable biopolymers for soil improvement have emerged in recent years, and biopolymers have become popularized in engineering. For example, the commercial, large-scale production of composite fiber polymer has been applied [15], and the polymer lignin is widely used in manufacturing industries [16,17]. Polyacrylamide polymers have been widely used in the United States to irrigate land and control sand erosion and runoff protection, as well as to construct helicopter landing pads to reduce dust pollution [10]. Recent studies showed that gellan gum and agar gum could significantly improve soil durability [18], that xanthan gum can maintain water for vegetation growth in the soil to prevent desertification [19], and that both xanthan gum and gellan gum can improve the dynamic characteristics of sand [20]. Adding a small amount of biopolymer can greatly improve the soil strength, and guar gum is more advantageous for treating collapsible soil and clay using the wet mixing method, but xanthan gum is superior when treating silty fine-grained soil [21–23]. Xanthan gum can be used as a stabilizer for slope protection in geotechnical engineering. Xanthan gum also has a low application cost and very competitive price compared with other biopolymers [24].

In the late 20th century, Wallingford and Sanchez pointed out that xanthan gum is more capable of absorbing water than other polysaccharide polymers [25,26]. Xanthan gum can be used to effectively improve the water retention capacity of engineering soil, such as through hydraulic seepage barriers and underground pollution stabilization [14,24,27]. Zhou et al. [28] verified that xanthan gum significantly enhances the water retention of soil. Because xanthan gum can separate soil particles and fill soil pores, the pores of sand become larger [29], and its water retention performance is very strong [30], thus improving the water retention capacity of the soil.

Ayeldeen and Qureshi et al. [21,31] indicated that the addition of xanthan gum could improve the collapsibility resistance of collapsible soil, as well as the water disintegration resistance and strength of sandy soil. Cabalar et al. [32] showed that the compaction degree, viscosity, and strength of clays treated with xanthan gum were enhanced at low water content. Chang et al. [33] found that the unconfined compressive strength (UCS) of drying soil tended to stabilize upon increasing the xanthan gum content to a certain range. According to the UCS test results of Latifi [34], stability can be achieved by adding xanthan gum to bentonite and kaolinite at low water content. Soldo et al. [35] found that the strength of soil with 2% xanthan gum content is close to the maximum at low water content. Soldo and Sujatha et al. [36,37] indicated that water content is an important factor affecting soil strength and that the strength of silty sand and silt treated with xanthan gum is greatly improved at low water content. Engineering soil must usually be cured for a few days to reach a low water content state. Because the water content of engineering soil is low, the soil strength is generally high but easily affected by rainfall infiltration, which can reduce the soil strength and stability. Most soil treatment studies conducted strength tests in the range of high suction, but few tests have focused on soil strength over the entire range of water content of soil treated by xanthan gum. Recent studies showed a significant decrease in the wetting strength of sand after treatment with biopolymer

guar gum [38]. However, for sand treated with xanthan gum, the wetting strength was greater than the initial strength [39]. Based on the above research results, further in-depth tests were carried out to systematically study the influence of xanthan gum on the wetting strength characteristics of silt over the full range of water content.

The objective of this study was to investigate the effect and mechanism of the strength weakening characteristics of silt treated with xanthan gum (XG-silt) during the wetting process. A series of microscopic tests, water retention characteristics tests, and direct shear tests were carried out on XG-silt using scanning electron microscopy, mercury porosimeter, a WP4C dew point potential meter, and a Shear Trac-II test system. The experimental results qualitatively and quantitatively reveal the variation law and internal mechanism of the wetting strength characteristics of XG-silt, which provides a useful scientific basis for the design and construction of related geotechnical engineering projects.

## **2. Materials and Methods**
