## 3 Construction Processes  
Before the implementation of improvements, the length of the PVDs and the total pressures of surcharge preloading and vacuum preloading were fixed at 21.0 m and 80 kPa, respectively. According to the Code for Foundation Treatment, the PVDs were a cross-section of 100 mm \(\times\) 4 mm with an equivalent diameter of 66 mm, and they were designed to be installed in a triangular pattern at a spacing of 1.2 m in the two improving regions. Secondly, a 0.5 m thick sand cushion was laid on the two regions. In the vacuum preloading region, the vertical draining pipe, vacuum pump, horizontal draining pipe, sealing membrane (geo-members), and sealing ditch were successively installed by on-scene operation. The vacuum pump was then started, and the vacuum pressure under the membranes achieved \(-\)80 kPa on the 20 th day, and this was maintained for 63 days. The total period of vacuum preloading was 83 days, and in the surcharge preloading region, the water collection well, sealing membrane, blind ditch, and surcharge load were also successively applied by on-scene operation. The surcharge load material was selected to be clay soil with a unit weight of 17.8 kN/m\({}^{3}\), and the surcharge fill was constructed up to 4.5 m height with an equivalent pressure of 80.1 kPa in 30 days, which then lasted for 53 days. Figure 2: Schematic of the field test. During the total period of vacuum preloading and surcharge preloading, various parameters were monitored in the two regions targeted for improvement, including ground settlement, stratified settlement, horizontal displacement, and excess pore water pressure. The number and sequence of monitoring points and holes are tabulated in Table 2, and the layout of monitoring points and holes is plotted in Figure 2. In order to monitor the settlement amount and settlement process of each soil layer and to grasp the deformation of each soil layer under vacuum, ground vertical settlement and horizontal displacement were monitored using a LeicaNA730 balance level with an accuracy of 1.2 mm/km and a LeicaTZ08 total station with an accuracy of 1 s, respectively. In order to monitor the settlement amount and settlement process of each soil layer and to grasp the deformation of each soil layer under vacuum pre-pressure and mound pre-pressure, eight CY-7090 high-precision stratified settlement gauges with a measurement accuracy of 1 mm were fixed at intervals of 3.0 m in a 24 m stratified settlement tube, respectively, and the stratified settlement of the soil was obtained by monitoring the synchronous sinking amount of the magnetic settlement ring fixed on the settlement tube with the settlement of the foundation. It is worth mentioning that the accuracy of the measurement results is greatly influenced by the selection of the reference point, and it is necessary to ensure that the settlement tube enters the soil-bearing layer, otherwise, the compression and settlement of the soil at the bottom of the tube produces relatively large measurement errors. The pore water pressure was monitored using a KXR-3030 vibrating wire pore water pressure transducer, buried at the monitoring location in a one-hole multi-point pattern. The KXR-3030 vibrating wire pore water pressure transducer has a resolving power of \(\leq\)0.08% F.S (Full scale) and a combined error of \(\leq\)1.5% F.S. It mainly consists of a probe, a frequency reading instrument, and a cable, which translates the change in pore water pressure by measuring the frequency change of the internal steel string. Seven vibrating wire pore water pressure transducers were buried at 3.0 m intervals in each excess pore water pressure hole from a depth of 3 m to 21 m.  
The shear strength of the soil is related to the stability of the foundation, and the direct shear test is a common method to determine the shear strength of the soil, with the advantages of simple operation and high-test efficiency.