## 1 Introduction  
Red bed formations are extensively employed in various engineering projects, such as highways, railways, water resources, and hydropower applications [1; 2]. Different from typical filling materials such as rocky soil and industrial slag, which have high friction coefficients, are easily compressed, and exhibit good permeability, red bed soft rock undergoes significant variations in strength due to differences in mineral composition and cementing materials. Particularly in atmospheric conditions or under the influence of wet-dry cycles, rock blocks can disintegrate into soil or even become mud. Therefore, its use as a filling material for highways can easily lead to roadbed settlement. Engineers and researchers have addressed specific challenges related to the unique physical properties of red layer formations, including efforts to improve the use of red layer soft rock as a filling material [2]. [PERSON] et al. conducted numerical simulations of the stability of red layer soft rock slopes in the Zhetang channel area of the Xinjiang reservoir using the Mohr-Coulomb criterion. The results validated that the safety factor of red layer soft rock slopes meet design requirements [3]. In previous research, the road performance of red bed soft rock as a filler has been extensively examined through field tests, involving the analysis of the chemical composition of typical red bed soft rock and its correlation with disintegration resistance and physical properties [4]. [PERSON] et al. proposed a transformative approach for red bed soft rock, involving a sequence of 'pre-disintegration-raking-compaction,' effectively eliminating the water sensitivity of red layer soft rock and creating a dense, impermeable embankment. This method ensures the water stability and strength of the roadbed, meeting the highway roadbed index requirements [5]. [PERSON] et al. [6] simulated four kinds of red bed embankments with optimal water content and compaction degrees of 87%, 90%, 93% and 97%,respectively, through centrifugal model tests, obtained settlement, compaction degree and acceleration, and analyzed the relationship between construction period and post-operation settlement. [PERSON] et al. [7] simulated the natural disintegration process of red bed soft rock in the atmosphere by using the circulation methods of sprinkling and blowing, and conducted particle screening and quality analysis of the disintegration products, drawing the particle mass change curve under different particle size conditions. On this basis, the control index of construction technology is put forward, which can be used to guide the construction of a highway embankment project to control the quality of roadbed. [PERSON] et al. [8] used geotechnical centrifugal model tests to analyze the settlement characteristics of red bed soft rock fillers under different compaction degrees, reasonably determined the compaction parameters of embankment filling, and verified the suitability of red bed soft rock fillers for embankment filling. Through a series of laboratory tests, researchers tested the expansion rate, triaxial shear strength and compression modulus of red bed soft rock under subgrade load, studied the change rule of CBR load ratio, compaction properties and unconfined strength of red layer soft rock, and verified the applicability of using red layer mudstone as the filling material of high-speed railway subgrade [9; 10]. Numerous studies on red bed soft rock subgrade materials and their parameters have been conducted both in China and internationally [11; 12; 13]. Due to limitations imposed by on-site conditions during construction operations, there is a current need to propose a method for the quantitative assessment of the physical and mechanical characteristics of soil-rock mixtures when using red bed soft rock as a filling material. Additionally, specific control methods for ensuring the safety and stability of high-fill embankments tailored to the unique features of mountainous roads need to be developed.