## 1 Introduction  
In recent years, with the rapid development of bridge engineering, pile foundation has attracted the attention of designers as an important part of highway bridge construction [1; 2; 3; 4]. On the one hand, some researchers established theoretical models to analyze the bearing characteristics of piles [5; 6; 7; 8]. On the other hand, some researchers studied the bearing characteristics of piles through field or indoor experiments [9; 10; 11; 12]. Moreover, some researchers studied the bearing characteristics of piles through simulation experiments [13; 14; 15]. In the process of pile foundation construction, various types of soil will be encountered. Loess is the dominant soil in Northwest China, accounting for 72.4% of the total loess area in China. In addition, China introduced the conception of the \"New Silk Road Economic Belt\" in 2013, and most of the soil in this area is loess, which brings great opportunity and challenge to the construction of pile foundation in loess area [16; 17; 18; 19; 20; 21]. When cast-in-place bored piles are used in loess area, the steel casing is usually used to stabilize the borehole wall or solve the slurry leakage problem of huge caves. Because of the difficulty in recycling, the steel casing is sometimes no longer recycled after pile forming, so as to form a concrete-filled steel tube composite structure with reinforced concrete materials, which is called steel tube-reinforced concrete piles in this study. When steel casings are recycled, they are called conventional reinforced concrete piles.  
There are some researches on pile foundation in loess area. [PERSON] et al. [22] tested three squeezed branch and plate piles in loess area and proposed the safety factor for the three piles. [PERSON] et al. [23] investigated the negative skin friction along piles caused by earthquake in loess area and developeda method to estimate the negative skin friction. [PERSON] et al. [24] found that down-hole dynamic compaction pile could improve 50 m collapsible loess when the pile spacing is reasonable by field tests. [PERSON] and [PERSON] [25] carried out water immersion tests to study the different pile types on negative skin friction of pile foundations in loess regions and found NSF of the bored concrete pile was smallest. [PERSON] et al. [26] used static load test and post-grouting technology; the settlement parameters of post-grouting piles in loess area were analyzed and optimized. However, the above research is on the bearing characteristics of concrete piles in loess area; there is little research on the effect of steel casing length on vertical bearing characteristics of steel tube-reinforced concrete piles in loess area. Compared with field tests, more and more centrifugal model tests have been applied recently in various engineering due to its virtue of quick construction and low engineering cost. Simultaneously, the results of centrifugal model tests are consistent with those of field tests, which had been confirmed by a large number of studies, suggesting that centrifugal model tests have a certain accuracy and reliability [27; 28; 29; 30; 31; 32].  
Based on this, the effect of steel casing on vertical bearing characteristics of steel tube-reinforced concrete piles in loess area was investigated by centrifugal model test, which includes the axial force, unit skin friction, tip resistance, shaft resistance, and ultimate bearing capacity. In addition, advantages of steel casing in pile foundation engineering were summarized.