## 1 Introduction  
Soil nailed retaining structures are widely used in slope supporting, tunnel retaining, and as bracing structure for foundation pits to limit deformation and improve safety. The advantages of soil nailing include its simple structure, flexible operation, and high cost-effectiveness [1, 2, 3]. Soil nails have a high ratio of circumference to cross-section area, and therefore rely essentially on friction resistance for the load transfer. The soil nail force is transferred to the surrounding soils through soil-grout interface friction, which arises from the relative displacement between the soil and the grout. The shear strain required to mobilize the nail force is very small; therefore, soil nails usually start to function soon after the excavation when deformation occurs. Soil nailing is one of the in situ soil reinforcement techniques which are widely utilized by civil engineers in underground construction, slope stability, housing construction on inclined or vertical cuts, etc. A vast amount of research has been conducted on the retaining structure. As for in situ or laboratory tests, some studies were focused on investigating the pullout behavior of pressure-grounded soil nails [4; 5; 6; 7]. Numerical models were established to examine the contribution of soil nails towards the slope stability [8; 9; 10; 11]. However, the soil conditions in which soil nailing can be applied are limited and the deformation is difficult to control. Therefore, for projects in urban areas, a soil nail-prestressed anchor composite retaining structure is preferable. This is because the additional prestressed anchors can assist, in addition to the soil nails, to limit the displacement of the slope surface [12; 13; 14; 15]. A typical layout of the soil nail-prestressed anchor composite retaining structure is shown in Figure 1.  
In such a composite retaining structure, the combined contribution of soil nails and prestressed anchors is expected to provide higher resistance against rainfall or overloaded-induced failure [16]. In the existing literature, the interaction mechanism between soil nails and prestressed anchors is investigated mostly based on numerical studies [17]. Very limited model and in situ test results [18; 19; 20] and analytical theories and methods were reported despite the widespread use of such retaining structures. This is due to (i) model and in situ tests being expensive, complicated, and time-consuming to conduct. For example, many delicate instruments are needed, such as stress and strain gauges which are prone to damage during testing and installation; (ii) in situ tests may disturb the scheduled construction process and the in situ soil layers. In addition, previous studies were mainly focused on the behavior of such retaining structures installed in dense materials [21]; the corresponding behaviors in loose fill materials need to be further investigated. In this paper, a full-scale in situ test on a composite soil nailed retaining structure enhanced with prestressed anchors installed in loose materials was carried out to investigate the mechanical behavior of soil nails and prestressed anchors during excavation. Great precaution was taken during the in situ tests to avoid possible damages to the instruments. For the installed instruments, obvious signboards were set next to them, and the staff on site were reminded to avoid rolling over, covering of, and collision with the monitoring instruments. In addition, daily inspection was strengthened and replacements were done immediately in case of damage or abnormality. Therefore, the survival ratio of gauges was ensured to be as high as 99%. A total of 90 strain gauges were installed, and only one was damaged. The effects of the prestressing force and unbonded length of the prestressed anchors on the nail forces were investigated.