## 1 Introduction  
With the rapid development of urbanization, lots of high-rise buildings and buildings in underground spaces are constructed due to restrictions on land use in cities. Correspondingly, the foundation pits of these buildings are also dug increasingly deeper. Meanwhile, the construction of foundation pits faces many problems because of the complexity and sensitivity of use functions of the buildings, narrow construction space, and complex construction environment [1, 2]. Foundation trenches are the spaces between the foundation and retaining walls of foundation pits. Because foundation pits are dug deeper and deeper, the foundation trenches are increasingly narrower. For some deep foundation pits, the width of foundation trenches is even less than one meter. The backfill materials and backfilling methods of foundation trenches are of important significance for the overall stability of the buildings, the normal use of pipelines, and the safety of foundations [3; 4]. The backfilling method commonly used in engineering is to backfill and compact 2:8 lime soil in layers [5; 6]. In engineering practice, the method is found to have some drawbacks: Firstly, the compaction in layers needs compacting equipment, while large compacting machinery is not applicable to narrow foundation trenches, so artificial compaction can only be adopted. This causes low construction efficiency and delays the construction schedule. Secondly, compacted lime soil has a low load-bearing capacity, so the method is not suitable for special buildings that have requirements for load-bearing capacity. Thirdly, lime soil is not waterproof [7], so a large volume of retained water exists in foundation trenches, which applies huge additional pressure on the basement walls. Therefore, special waterproofing work needs to be conducted for underground structures of buildings, which substantially increases the overall cost of buildings. Cemented soil is a hard material of special engineering properties formed by mixing, vibrating, and curing the mixture composed of soil as the aggregate, Portland cement as the cementing material, and water as the reaction medium, and it is also termed as cement-stabilized soil [8; 9; 10]. Cemented soil is widely used in fields including roadbeds, canal lining and slope protection in water conservancy projects, and foundation reinforcement of buildings [11; 12]. Its economic, social, and environmental benefits have been long confirmed. However, engineering practice also finds that the strength and impermeability of cemented soil are closely related to the cement content, and cemented soil has high strength at room temperature, while its strength is greatly deteriorated under conditions of freeze-thaw cycles [13; 14; 15]. How to improve the strength, impermeability, and frost resistance of cemented soil, on the premise of ensuring the economic efficiency of the use amount of cemented soil, is the key to further promoting the application of cemented soil in cold regions rich in water [16; 17]. An effective measure to improve the strength of cemented soil is to dope additives to improve the compactness of structures [18; 19; 20]. Therefore, selecting an appropriate additive has become an important topic to improve the strength and endurance of cemented soil and also a research hot spot of researchers in China and abroad [21; 22; 23; 24; 25; 26; 27]. In such contexts, the concept of composite-cemented soil is emergent. Aiming at shortcomings of ordinary-cemented soil, the fly ash (FA)-water glass (WG)-composite-cemented soil material was developed based on a background project. The unconfined compressive strengths (UCS) of ordinary-cemented soil and cemented soil only doped with FA and WG (FA- and WG-cemented soil) were tested. The orthogonal test schemes for the UCS of the composite-cemented soil were established, and the contents of cement, FA, and WG were selected as variables to study the UCS of the composite-cemented soil after different curing periods.