1. Introduction
With the development of urbanization, land use is becoming increasingly tense and, with the development of rivers and oceans, structures often need to be built on soft soil. The soft soil has a low strength and large deformation, so it needs to be improved. Since adding cement to soft soil can improve its strength, reduce deformation, reduce permeability and obtain a good cost-effectiveness, soil-cement is widely used as a subgrade reinforcement for retaining wall waterproofing in dam and port constructions, etc. [
1,
2,
3]. Cao et al. [
4] added ultra-fine cement (UFC) to peat soil, which significantly improved the unconfined compressive strength of soil-cement, but the increase in the compressive strength was not obvious when high amounts of the UFC was added. Wu et al. [
5] found that the elastic modulus had a linear relationship with the unconfined compressive strength. Wei et al. [
6] found that soil-cement reinforcement can effectively improve the pile soil resistance, and thus, improve the horizontal bearing performance of the pile foundation. Wang et al. [
7] studied the engineering characteristics of Xiamen marine silt cement-soil and found that, when the cement mixing ratio was less than 18%, the compressive strength and the cohesion of cement-soil increased approximately linearly, and when the cement mixing ratio was greater than 18%, the growth rate of the strength index decreased. Soil-cement structures are located below ground level and are exposed to the eroding effects of groundwater and seawater year-round. Therefore, in addition to the strength, research on the impermeability of cement-soil is also very important, which is an essential index to evaluate the engineering performance of cement-soil [
8,
9,
10].
With the growth of related structural engineering requirements, many scholars began to study the permeability of soil-cement. The research on soil-cement permeability mainly focuses on the soil quality, cement content, curing age and environmental conditions. The research on the influence of the soil quality on the permeability of cement-soil includes Toshimitsu et al. [
11], who examined the relationship between the soil-cement permeability and the soil structure compactness through microscopic testing, The results showed that the smaller the pore size and the better the compactness of the soil, the better the impermeability of cement-soil. Lu et al. [
12] investigated the permeability of soil-cement containing silty sand and silty clay. The results showed that, with the increase in the cement content and curing age, the permeability coefficient of the silty sand decreased faster than that of the silty clay. The research on the influence of the cement content on the permeability of cement-soil includes Hong and Gong [
13], who performed permeability tests to establish that the permeability coefficient of soil-cement decreased with the increase in the cement content and curing age. It was considered that the optimum cement mixing ratio for engineering application is 10%. Zhang et al. [
14] determined the relationship between the soil-cement permeability coefficient and the cement content using a regression analysis. Zhu et al. [
15] conducted an indoor permeability test on cement-soil with different cement contents and ages. The results showed that the permeability coefficient decreased with the increase in the cement content and age. Tao et al. [
16] studied the influence of the cement content on the micropore distribution and the permeability of cement-soil. It was found that the permeability coefficient of cement-soil decreased with the increase in the cement content, which decreased sharply in the range of low cement content (4%~12%) and slowly in the range of high cement content (15%~25%). The research on the influence of age on the permeability of cement-soil includes Zhang et al. [
17], who obtained the quantitative relationship between the permeability coefficient of cement-soil and the curing age using a regression analysis. Jiao et al. [
18] studied the influence of age on the permeability of cement-soil. The results showed that the permeability coefficient of cement-soil gradually decreased with the increase in age. After the age was more than 28 days, the reduction rate of the permeability coefficient with age decreased. A prediction formula for the long-term permeability coefficient of cement-soil was proposed. The research on the influence of environmental conditions on the permeability of cement-soil includes Yang et al. [
19], who considered the influence of a seawater environment on the permeability of cement-soil. The research showed that the permeability coefficient of cement-soil increased with the increase in age under the curing condition in seawater, which also showed that the cement-soil deteriorated in the seawater environment. Chen et al. [
20] conducted a permeability test on cement-soil under a sewage environment. The test results showed that the permeability coefficient of cement-soil decreased gradually before the age of 60 days. After the age of 60 days, due to the erosion of cement-soil, the permeability coefficient increased gradually with the increase in age. The research on the influence of additives on the permeability of cement-soil includes Pang et al. [
21], who studied the effect of adding fly ash and lime on the impermeability of cement-soil. The results showed that the effect of adding fly ash alone on the impermeability of cement-soil was small, but the effect was obvious after adding lime. In view of the soil properties studied, a reasonable ratio relationship was proposed. Chen et al. [
22] conducted a permeability test on cement composite soil. The results showed that the permeability coefficient of cement composite soil decreased gradually with the increase in the cement content, bentonite content and fly ash content. The cement content had the greatest influence on the permeability coefficient of cement-soil, followed by the fly ash and bentonite. Mo et al. [
23] studied the influence of the cement content, desulfurized gypsum content, fly ash content and water–cement ratio on the permeability of cement-soil. The results showed that the influence degree of the four factors on the permeability of the soil was the water–cement ratio > fly ash content > cement content > desulfurized gypsum content. Cui et al. [
24] studied the influence of the fly ash content on the permeability of cement-soil. The results showed that the permeability coefficient of fly ash cement-soil with a cement content of 60% was higher than that of pure cement-soil before curing for 14 days. However, it became lower after curing for 28 days. Hu et al. [
25] mixed different amounts of MgO into the fiber cement-soil and carried out a permeability test. The test results showed that there were some defects when the content of MgO was too high or too low, and the optimal content was 1%. Chen et al. [
26] studied the influence of ferronickel slag powder on the permeability of cement-soil. The results showed that the addition of ferronickel slag powder can improve the impermeability of cement-soil in a clean water environment and a seawater environment, especially in the latter. When the content of ferronickel slag powder was more than 20%, the influence of the increase in the ferronickel slag powder content on the impermeability of cemented-soil gradually decreased.
It can be seen from the above studies that some scholars improved the impermeability of soil-cement by enhancing the compactness of soil-cement, increasing the cement content and adding various additives. From both an economic and engineering perspective, incorporating various admixtures has proven more cost-effective than increasing the cement content and has become a more obvious and effective strategy to improve the performance of soil-cement. The most important point arising from the above investigations is that appropriate admixtures can be selected to meet variable engineering conditions to improve the performance of specific features of the soil-cement materials in order to satisfy the design requirements of different projects [
27,
28,
29]. Basalt fiber is widely used in engineering projects involving concrete due to its proven chemical stability, excellent temperature resilience, high-tensile properties, good dispersion capability and highly cost-effective performance. Despite these key attributes, its application in soil-cement to date has not been extensive.
At present, the research on basalt fiber applied to soil-cement mainly focuses on strength. The research on the compressive strength is as follows. Niu et al. [
30] found that it was not suitable to use long fibers for reinforcement. On the one hand, it is easy to form cluster phenomenon, and on the other hand, it is easy to hang on the mixer. Ma et al. [
31] found that the appropriate content of basalt fiber and sand can jointly promote the improvement of the soil-cement compressive strength. Chen [
32] found that the addition of basalt fiber can significantly improve the compressive strength of the cement-soil samples at different ages, but with the increase in the fiber content, its reinforcement effect gradually weakened. The research on tensile strength includes Zhang et al. [
33], who found that the order of influence of the fiber variables on the tensile properties was the length, content and diameter. A 9mm length of basalt fiber with the content of 1.5% are the optimal single mixing parameters of basalt fiber. Shen et al. [
34] found that a fiber length of 9 mm + 12 mm mixed in a 3:1 ratio had the best tensile and residual strength. Chen [
35] found that a too high fiber content could not produce an obvious strength increase, but the addition of basalt fiber enhanced the plasticity of cement-soil. The research on the shear strength includes Chen [
36], who found that the incorporation of basalt fiber significantly improved the cohesion of cement-soil, significantly improved the strength of cement-soil and increased its plastic deformation. Chen et al. [
37] found through a triaxial test that the stress–strain curve can be roughly divided into three stages: the linear elastic stage, plastic yield stage and peak softening stage. The research on the splitting strength includes Cao et al. [
38], who found that excessive basalt fibers had a negative effect. When the content of basalt fiber was 1.5%, its impact splitting strength and absorption energy reach the maximum value. The research on fatigue performance includes Chen et al. [
39], who found that basalt fiber can improve the fatigue properties of cement-soil mainly by limiting the formation and expansion of fatigue cracks, thus improving the fatigue life of cement-soil.
Through the above research, it was found that the diameter, length and content of basalt fiber affected the various strengths of cement-soil. When adding basalt fiber, it should not be too long to avoid agglomeration and twining. Although basalt fiber can enhance the strength and improve the plastic deformation ability, a too high basalt fiber content cannot significantly improve the strength and will increase the project cost.
With the development of marine engineering, it is urgent to solve the problem of the impermeability of cement-soil. However, there are few studies on the impermeability of basalt fiber cement-soil, and the impermeability of cement-soil in marine environment has not been given due attention. Therefore, it is necessary to study the impermeability of basalt fiber-enhanced soil-cement under the sea. In the study of the basalt fiber soil-cement strength, it was found that the strength increased continuously with the increase in the basalt fiber content, but an excessive content caused poor workability and the improvement effect was not obvious. Therefore, this paper mainly studies the influence of the basalt content on the permeability. Through the permeability and chloride ion permeability tests, the optimal basalt content was obtained. The influencing mechanism of the basalt content on the impermeability of soil-cement was microscopically scrutinized through an SEM examination of the microstructure of the compound.
The chloride ion permeability test in this test method is often used in permeability tests of high- and medium-strength concrete. However, few studies were identified that applied chloride ion permeability testing to establish the permeability of soil-cement. As soil-cement is generally located in environments rich in groundwater and seawater, it is essential to consider the eroding effects of chloride ions on this material. Therefore, this study carried out a chloride ion permeability test based on the permeability test and obtained the optimal content of basalt fiber cement-soil through a comprehensive analysis of the results of the two tests to improve the credibility. This study could, in the present authors’ view, serve as a valuable reference for soil-cement usage in projects with impermeability requirements.