## 2 Materials and Methods  
### Instruments  
Cone tip resistance and sleeve friction are two important measurement parameters in a CPT test [13; 16]; however, the friction measuring device is complex and consists of several sophisticated components [40]. In order to simplify the measuring device and quickly obtain the CPT data, cone side pressure rather than sleeve friction was measured in our study. To obtain soil cone tip resistance and cone side pressure at different depths, a portable measuring device was developed (Figure 1). The device was mainly composed of a tripod, a ball screw slide table, a stepper motor, and a conical rod. The stepper motor was installed onto the tripod to ensure measuring stability. Its output shaft was connected with the screw of the ball screw sliding table through a shaft coupling to ensure good force transmission capability. During the operation of the device, the sliding table was driven up and down by the stepping motor, thus allowing the conical rod connected with the sliding block to penetrate the soil at a constant speed. To better penetrate the soil and measure the side pressure of the cone, the American Society of Agricultural Engineers (ASABE, St. Joseph, MI, USA) standard cones were modified by using two semi-cones to form a new cone. A film sensor was placed at the contact surface between the two semi-cones to measure the cone side pressure as the new cone penetrates the soil. The other end of the conical rod was linked to a load cell to detect the vertical penetration resistance of the cone, i.e., cone tip resistance. An automatic measuring and collection system was designed for real-time acquisition of sensor data and to control the movement of the conical rod (Figure 2). An STM32 single chip with type of STM32F103C8T6 provided by STMicroelectronics (Carrollton, Dallas, TX, USA) was used as the core processor for this system. A constant frequency control signal was given to the stepper motor driver by the output comparing mode of the timer inside the processor, which caused the stepper motor to rotate uniformly so that the cone-rod penetrated the soil at a consistent speed. Two Hall sensors with type of NJK-5002C provided by AOTORO (Yueqing, Zhejiang, China) were used as limit switches to ensure that the cone could reach the required penetration depth, and an SD card was used to store the output values of the sensors. The cone penetration depth (S) is calculated as follows [41]:  
\[S=\frac{M\cdot T}{C} \tag{1}\]  
where \(S\) is the cone depth into the soil; \(M\) is the pitch of the thread screw rod; \(T\) is the number of rotary steps in the stepper motor; \(C\) is the number of steps needed by the motor to rotate one circle. The load cell was selected based on the conical rod shape and the expected maximum resistance from the tested soil. A round stick was chosen and used to manufacture the cone, the apex angle of which accorded with the ASABE Standard (30-deg). After soil resistance tests in the research area and an investigation into the available commercial products, the DYZ-101 cylindrical pressure measuring sensor with a diameter of 25.4 mm from Da Yang Sensing System Engineering Co., Ltd. (Bengbu, China) was selected. This sensor has a measuring range of 0-2940 N and features high precision, large dynamic response, small volume, and strong anti-interference capability. Additionally, the film sensor was a Flexiforce A301 film pressure sensor purchased from Tekscan Inc. (Boston, MA, USA). This sensor is small in size, has a large measuring range (0-436 N), and can be easily integrated and installed into the sections of the semi-cones. The sensor data were processed in parallel through a series of data processing units. The DYZ-101 sensor was powered by a 3 V power supply and can output 20 mV voltage signals under rated load (10 kN). The output signals from the sensor were linearly magnified by a magnifier and collected by the first interface of a built-in analog-to-digital converter (ADC) in the STM32 single-chip.