## 1 Introduction  
With the depletion of shallow mineral resources, mining inevitably goes deeper, where the operation faces significantly more additional challenges [1, 2, 3, 4, 5]. At shallower depths, the ground stresses around the ore body are low, so only a minimal number of pillars need to be retained as necessary ground supports to ensure the safety of the underground operation [6, 7]. However, for deep mining, more in-depth geological investigations are required to help select the best mining method and design the most suitable pillar layout, to make the mining process safe and efficient [8, 9]. In deep mining, in situ stresses around the ore body are much higher and it is difficult to effectively control the pressure of surrounding rocks using only pillars. Therefore, in mining practice, a large number of mines try to fill the mined areas with tailings and other mining wastes to bear the stresses of surrounding rocks together with pillars [10]. Back-filling using tailings also brings additional environmental benefits by addressing the issues of accumulated tailings on the ground surface, hence helping promote the restoration of ecological environments after mining [11; 12]. At the early stage of the backfilling process, mine pillars are still the main stress-bearing structures and the backfill mainly plays the role of limiting the lateral displacement of mine pillars and improving their bearing capacity [13; 14; 15; 16]. With the development of backfill technology, the strength of backfilled material has been enhanced and the contact area between the backfill and the roof has been gradually increased. Therefore, the overall bearing capacity of the backfill has been increased significantly to improve its ability to limit the movement of the roof strata [17; 18; 19]. This potentially allows the mining of previously retained pillars to improve the recovery of mineral resources [20]. The original pillars are left temporarily to divide the mining area and to ensure that multiple slopes are mined simultaneously without interfering with each other. Subsequently, these temporary pillars are mined, with only a few thin walls (thin pillars) left permanently underground. These thin walls are mainly used to prevent the mixing of backfill materials and ones at each slope. At the same time, these permanent thin walls can also limit the lateral displacement of backfill materials and build a load-bearing system together with the backfill to maintain the stability of the goat [21]. There are a significant number of published studies on the load-bearing mechanism of the backfill and pillars in underground space [22; 23; 24]. [PERSON] et al. [25] investigated the joint bearing mechanism between the backfill and the surrounding rocks using indoor uniaxial compression tests, and the results indicated that pillars could enhance the uniaxial compressive strength and the elastic modulus of the backfill. [PERSON] et al. [26] used particle flow code numerical simulation software to investigate the load-bearing effects of support pillars consisting of backfill and coal walls under different conditions, and the results indicated that the width of the residual coal pillars had the greatest influence on the stability of the support pillars relative to factors such as mining depth, mining thickness, filling material, and filling ratio. However, most published research focuses on the interaction behavior of a single backfill material with the pillar, and there are few studies on the case of two backfill materials. In practice, more than one backfill material exists because of the economic benefits. In this study, based on elastic mechanics, the stress and deformation characteristics of permanently retained thin walls in panel barrier pillar slopes are solved when different backfill materials are used in adjacent slopes. The theoretical findings are verified by numerical simulations and collected monitoring data.