## 1 Introduction  
Widely distributed throughout the world, clay is a fine-grained natural soil material that has good plasticity when wet but hardens and becomes brittle and is no longer plastic when dried or fired [1]. Clay is one of the materials commonly used in civil engineering projects. Due to its properties related to the mineralogical composition and water content, the clay layer often requires the use of various methods to strengthen/improve it or even completely replace it.  
Expanded polystyrene foam granules (EPS granules) have the advantages of light weight, high compressibility, good stability, long-term durability, low moisture absorption and good thermal insulation, making them an ideal lightweight material for the improvement of other materials. Many scholars have conducted feasibility studies on EPS polypropylene as a road base filler. [PERSON] et al. [2] found that the EPS particle admixture and particle size had a relatively obvious effect on the unconfined compressive strength of lightweight soil; they found that a higher EPS dosage or larger particle size reduces the unconfined compressive strength of the soil. [PERSON] et al. [3] and [PERSON] et al. [4] studied the dynamic shear modulus, dynamic elastic modulus and damping ratio characteristics of EPS-modified lightweight soil; [PERSON] et al. [5] studied the degree of nonuniformity in the stress-strain distributions of EPS particles; and [PERSON] et al. [6] investigated the static earth pressure characteristics of lightweight soil mixed with EPS particles behind a retaining wall. The results of all the above studies indicated that EPS-modified lightweight soils have good engineering properties. Sisal fibre has good mechanical properties and is widely used in military and civilian industries. As a high-quality natural fibre, it has many excellent properties, such as corrosion resistance, high yield, high fibre volume and high crack resistance. It consists mainly of cellulose, hemicellulose and lignin along with other small amounts of pectin and waxes, and these elements give it durability, strength and stiffness. [PERSON] et al. [7] found that sisal fibres could significantly improve the cohesion of the soil, but the increase in the angle of internal friction was small. [PERSON] et al. [8] studied the early crack resistance of sisal fibre-modified concrete. [PERSON] et al. [9] investigated the effect of organic polyurethane polymers and sisal fibres on the mechanical properties of sandy soils. [PERSON] et al. [10] studied the unconfined compressive strength of soils improved with composites of fly ash and sisal fibre. [PERSON] et al. [11] studied the strength, stability and curing of sisal fibres combined with an ionic curing agent under static loading.  
In summary, scholars at home and abroad have studied the static loads of soils modified with EPS and sisal fibres and achieved a large number of scientific results, while less research has been conducted using dynamic loads. In addition, no one has yet studied the mechanics of EPS and sisal fibres incorporated into clay at the same time. The study of the dynamic properties of EPS-sisal-modified lightweight soils is particularly relevant because of the more complex situation of the forces affecting the soil under dynamic loading conditions. Based on this background, this study compared and analysed the effects of different sisal fibre admixtures on the dynamic properties of EPS-modified lightweight soils under the same surrounding pressure and frequency conditions and established a normalised model for the calculation of the decay of the dynamic elastic modulus. The microstructure and pore characteristics of the EPS-sisal-modified lightweight soils were analysed from a micro perspective through nuclear magnetic resonance and scanning electron microscopy. All the above research results will provide a reference basis for practical related projects.