## 1 Introduction  
Engineering waste soil usually refers to the soil generated in the construction and demolition of all kinds of buildings, structures, roads, pipelines and so on. With the acceleration of urbanization in China, an astonishing amount of waste soil is excavated and disposed of in spoil grounds or landfills each year [1]. For example, the output of waste soil from Shaoxing in the Zhejiang province reached 20 million tons in 2020, accounting for about 80% of construction waste, and it is growing at an annual rate of over 10% [2]. Massive waste soil has brought a series of problems, such as high disposal costs, occupation of urban land, damage to the ecological environment and potential safety hazards [3]. On the other hand, the crushed rock used as subgrade materials is costly and in shortage due to the prohibition of large-scale mining [4]. Therefore, there is an urgent need to explore sustainable methods of resource utilization for engineering waste soils. To address these challenges, an effective and economical strategy entails utilizing substantial quantities of waste soil as subgrade filling materials after soil stabilization [5], providing huge social, economic and environmental benefits. Various types of waste soils are generated in urban construction, each with distinct properties. In the coastal areas of the Zhejiang North Plain, for instance, the waste soil is characterized by fine particles, high moisture content, significant porosity and low strength, rendering it unsuitable for direct use in roadbed filling. Therefore, the key to achieving the goal is to improve the mechanical properties and to bear the capacity of the waste soils so that they meet the requirements of the subgrade filling. An efficient approach involves the use of additives as stabilizers, which are proven to enhance the subgrade performance and water resistance of waste soils [6]. Line, often combined with cement, is frequently employed for soil stabilization due to its cost-effectiveness, accessibility and environmental friendliness [7]. After undergoing lime treatment, soils that contain a large amount of clay minerals exhibit significant improvements in their physical and mechanical properties, including compaction and consolidation properties, compressive and shear strength, swelling and shrinking deformation, durability and micro-pore structure, etc. [8; 9; 10; 11; 12; 13; 14], and the bearing capacity is one of the key mechanical properties for subgrade filler [15; 16; 17]. The hydration reaction of lime consumes large amounts of water in the soil and creates a high pH environment, and then the flocculation reaction and the pozzolanic reaction enhance the soil mechanical indexes, which form three main gels (calcium silicate hydrates, calcium aluminate hydrates and calcium sulfoaluminate hydrates) bonding the particles within the clay aggregates [18]. The strength of lime-treated soil undergoes a gradual enhancement over several weeks due to the time-dependent pozzolanic reaction [19]. In terms of the improvement effect of mechanical indexes, the optimal amount of lime varies with soil type, mineralogy, composition and particle size distribution. According to [PERSON] [20], the optimum addition of lime for the compressive strength of montmorillonite was approximately 4% by weight, whereas for kaolinite and quartz, it ranged between 4% and 6%. The work of [PERSON] [10] indicated a significant reduction in both the swell percentage and swell pressure of expansive soils when the lime addition exceeded 6%. [PERSON] [21] reported that a minimum of 6% lime is required as an additive to improve the plasticity properties of soft clay. Soil properties typically vary significantly from one region to another, and the mixing ratio plays a crucial role in influencing the enhanced soil mechanics, improvement cost and duration of improvement. Therefore, a comprehensive assessment of the effectiveness and enhancement of the engineering properties of treated soils is indispensable for the design and construction of subgrades.