## 1 Introduction  
Soft soil is widely distributed close to lakes and the middle and lower reaches of rivers all over the world. The Pearl River Delta, located on the southeast coast of China, serves as one example of such an area. The physical and mechanical properties of soft soils are highly intricate, characterized by low strength, high moisture content, high compressibility, and high sensitivity to external disturbances [1]. Improper treatment of the soft soil base can result in a range of practical engineering issues, including uneven ground settlement and large post-work settlement over a longer period. These issues can impact the structural stability and long-term safety of various buildings constructed on such soil [2, 3]. Hence, it is imperative to improve soft soil conditions before utilizing it as a foundation for any building. Vacuum preloading, surcharge preloading, and combined preloading are commonly used practical methods to enhance soft soil conditions [4, 5, 6, 7, 8].  
Figure 1 illustrates the schematics of vacuum preloading and surcharge preloading methods. These methods aim to drain the pore water from the ground through a pre-loaded vertical and horizontal drainage system, and as the pore water dissipates, the pore volume of the soil then decreases, consolidation and settlement of the foundation occurs, and the effective stress and strength of the foundation soil gradually increases. The drainage consolidation models of the ground by preloading are presented [9, 10, 11, 12, 13].  
Some innovations are provided to improve the vacuum preloading method. For example, the expansion of dissolved gas in water, thermal preloading, and prefabricated boosters are used to accelerate water drainage [6, 7, 14, 15]. The optimal surcharge preloading rate minimizes the lateral displacement [16], and the sealing membrane with clay reduces the total cost by replacement [17].  
Some case studies have shown that vacuum preloading resulted in more consolidation degree and less consolidation settlement than surcharge preloading [18; 19; 20; 21]. The consolidation settlement is composed of immediate, primary, and secondary consolidation settlements [5; 22]. The immediate and primary consolidation settlements of the ground are mostly completed during the preloading stage. However, during the service stage, when the ground is used as a foundation for buildings, secondary consolidation settlements continue to occur due to soft soil creep under the additional stress. The uneven secondary consolidation settlement in the service stage will reduce the safety of the building on the ground and will even destroy the building. Therefore, it is necessary to investigate the impact of vacuum and surcharge preloading on the immediate, primary, and secondary consolidation settlement of ground to minimize the detrimental effects of uneven secondary consolidation settlement on the building [19; 23]. However, such effects have not been thoroughly studied.  
This study outlines an improvement project for soft soil ground in Zhuhai City and conducts a case comparative study to investigate the diversity of primary and secondary consolidation settlement resulting from ground improvement by vacuum preloading and surcharge preloading, including the effects of excess pore water pressure, horizontal displacement, and soil strength.