## 1 Introduction  
The coastal region of South China is located in the subtropics, suffering a warm and humid climate and flourishing vegetation, where the deposited soft soil is a thick layer and rich organic matter and has the \"three high (high water content, high organic matter content, and high compressibility), two low (low bearing capacity, low permeability) and one long (long deformation stability time)\" substantial regional characteristics. The engineering performances of this soft soil are extremely poor [1, 2, 3]. Therefore, the proposed pretreatment is required for engineering construction in the coastal soft soil area of South China. Various foundation treatment methods have been used for soft soils, among which cement mixing piles have been intensively studied [4, 5, 6] and widely used [7, 8, 9] because of their good reinforcement effect and simple construction. Engineering practice has found that the strength of cement mixing piles in South China's organic-rich coastal soft soil region is very low, although the cement content is high. The main reason is that the hydration products of cement have difficulty in forming a continuous cementitious structure skeleton in soft soil with a high organic matter content [10, 11, 12]. South China's coastal soft soil areas, especially the Pearl River Delta, have experienced three sedimentary cycles, forming geological inter-bedded layers of silty sand and soft clay [13]. The sand particles, which are used as a common aggregate, can be effectively mixed into the cement-solidified soils to improve the strength and deformation characteristics. [PERSON] et al. [14] prepared cement-solidified artificial soft soil with different sand particle content and investigated the effect of the sand particle content on the deformation characteristics of cement-solidified soft soil by ultrasonic and bending element tests. They found that sand particles can effectively increase the shear modulus of the cement-solidified soft soil. [PERSON] et al. [15] systematically summarized the effect of sand particle content on the unconfined compression strength (UCS) of cement-solidified soil and analyzed the microscopic control mechanism of the reinforcing effect of sand particles on cement-solidified soft soil in Lianyungang City by using the numerical simulation method. [PERSON] et al. [16] conducted a series of UCS tests on the cement-solidified Singa-porean marine soft clay with different sand particle content. The test results showed that the UCS of the cement-solidified samples decreases with increasing sand particle content, yielding findings opposite to those of [PERSON] et al. [15]. [PERSON] et al. [17] carried out a series of UCS tests and microscopic tests on solidified soft clay with different cement contents and sand particle contents. The test results showed that there is an optimal sand particle content for cement-solidified soil with different cement content; that is, the strength of the cemented soil increases first and then decreases with the increase of sand particle content. It is consistent with the research results of [PERSON] et al. [18] and [PERSON] et al. [19], but the optimal sand particle content is different. The above studies mainly investigate the influence of sand particle content on the strength of cemented soil. However, the influence of sand particle size on the mechanical properties of cemented soil has not been reported. In addition, the cement mixing piles in the coastal soft soil region of South China inevitably undergo a dry-wet cycle cycle of seawater and freshwater when suffering ebb and flow, dry and flood seasons and changing seasons, leading to strength deterioration and even structural destruction [20; 21]. The ability to resist the dry-wet cycle degradation directly determines the water stability and durability of cement mixing piles and then the success and failure of cement mixing pile reinforcement projects.