## 1 Introduction  
Soft soil subgrades with high compressibility and low strength are often encountered in engineering practice. Generally, the mechanical properties of these subgrades cannot meet the requirements of practical engineering conditions. It is necessary to make ground improvements to avoid serious damage to the roadbed, pavement, and even the upper road structures [1; 2; 3; 4]. As a typical example of the chemical improvement method, cement soil is widely applied to various roadbed treatment projects due to its advantages of good integrity, strong water stability, and low cost [5; 6; 7; 8; 9]. However, a large number of tests show that, due to the defects of low tensile strength, large brittleness, and poor deformability, cement soil is not suitable for every kind of project [10; 11; 12; 13]. In response to the above problems, many researchers have proposed a variety of improved methods based on different materials [14; 15; 16; 17; 18; 19; 20], and found that the use of fiber reinforcement technology as a method for improving the mechanical properties performed well in terms of soil improvement [21; 22].  
In recent years, studies [23; 24; 25] show that adding some fibers into the cemented soil can improve the tensile strength, toughness, bearing capacity of soil, and brittle failure to some extent. [PERSON] et al. [26]conducted unconfined tests on cement soil samples with different fiber mass contents; their results showed that the modification of a soil sample was optimum with a fiber mass content of 3%00-5%00. [PERSON] et al. (2017) carried out unconfined compressive tests on cemented soft soil samples with different fiber mass contents and found that the compressive strength first increases and then decreases as the fiber mass content increases in samples. When the fiber mass content was around 5%00, the compressive strength reached its maximum value. [PERSON] et al. (2018) conducted unconfined tests on cement loess samples with different polypropylene (PP) fiber mass contents and found that fibers have the best effect on the compressive strength and ductility in samples with 4%0 fiber mass content. [PERSON] et al. (2019) performed unconfined tests on PP fiber cement soft soils with different lengths and contents and pointed out that the compressive strength and tensile strength of cement soft soils increased with the increase in fiber mass content within a certain range, while the fiber length had little effect. By using the direct tensile test and split test results, [PERSON] and [PERSON] (2019) investigated the effect of randomness in orientation of fibers on the tensile strength of a sample. In their study, two types of fibers were considered, namely, PP and polyvinyl alcohol (PVA) fibers. A prediction model was proposed for the tensile strength of cement-admixed clays. The above literature review suggests that fiber reinforcement is a good soil improvement measure, but few have considered the effect of confining pressure. In reality, the ground improvement level is often buried below the ground level. Sometimes the buried depth is as high as 30 m or even deeper. In such circumstances, the confining pressure is likely to affect the mechanical behavior of the cemented soils.  
In this study, a series of triaxial unconsolidated undrained (UU) tests was carried out under different confining pressures. The brittleness index was introduced as the criterion for identifying brittle failure. The modification effects of different confining pressures and fiber mass contents on cement subgrade soil are discussed to provide a reference for the application of polypropylene fiber-cement-treated subgrade soil (PCS) in practical engineering problems.