## 1 Introduction
The experimental data show that compared with the traditional equal diameter bolt, the ultimate bearing capacity of the enlarged head bolt can be increased by 20% to 30%, and the bearing capacity can be increased by 60% under some optimized design conditions [15]. By optimizing the geometric shape of the end of the anchor rod the redistribution of the stress field is realized, which provides a new solution to effectively solve the key technical problems such as an insufficient bearing capacity and the difficult deformation control of traditional anchor rods. Taiwanese scholars [PERSON] et al. [16] pointed out in their research that an expansion bolt with a cylindrical structure at the bottom is one of the most widely used types of enlarged-head bolts in engineering practice. [PERSON] et al. [17] proposed an innovative ship-anchored grouting open soil anchor structure, which significantly improved the mechanical properties and engineering applicability of the anchorage system by introducing an anchor claw design. [PERSON] [18] innovatively proposed an umbrella-shaped anti-pulling anchor structure for the technical problem that the grouting method is limited in its application in a non-soft soil layer due to grouting pressure. [PERSON] et al. [19] innovatively realized the self-expansion function of the anchorage body by introducing the umbrella-shaped reaction plate structure into the bolt system. [PERSON] et al. [20; 21] developed a new type of anti-pulling device by combining an umbrella anchor head and a tension anchor cable, and systematically evaluated its mechanical properties through a series of pull-out tests. In order to further verify the practical application effect of the technology, [PERSON] [22] and [PERSON] [23] et al. carried out field tests on the engineering site of the Z2 straight tower of the double-circuit transmission line from Beilun Power Plant to Cangyan in Ningbo, Zhejiang Province. The test results showed that the umbrella anchor shows significant advantages in bearing capacity and has great engineering application potential. Through the combination of theoretical calculation and experimental research, [PERSON] et al. [24] carried out a systematic optimization design study on the enlarged-head bolt. The results show that there are significant differences in the ultimate pull-out force of the enlarged-head bolt with different geometric shapes: the truncated cone-shaped (umbrella-shaped) tension bolt shows the best bearing performance and its ultimate pull-out force is the largest. The ladder-shaped bolt is the second, the semi-elliptical bolt is the third, and the cylindrical bolt has the smallest pull-out force. Based on the problems of insufficient bearing capacity, and difficult deformation control and limited engineering land in the existing research, combined with the inspiration of previous research results, this paper innovatively combines the umbrella-shaped enlarged-head bolt technology and the grouting bolt anchor cable theory [25; 26; 27; 28; 29] with the prestressed diffusion effect of a bolt tail support, and proposes a new umbrella-shaped enlarged-head hollow grouting bolt structure with an expansion pipe. The structure adopts an umbrella-shaped expansion head design at the head of the anchor rod, and an expansion sleeve structure is introduced at the tail. Through double optimization, the contact area between the anchor rod and the rock and soil mass is significantly increased, thereby expanding the support influence range of a single-anchor rod. This design not only optimizes the stress distribution and load transfer mechanism, but also effectively reduces the number of bolts used and reduces the engineering cost. In order to comprehensively evaluate the performance of the structure, this research method comprehensively uses theoretical analysis, mechanical performance evaluation, experimental research, and numerical simulation, and it is verified by comparing it with experimental data from ordinary bolts.