In view of the high cost and poor accuracy in monitoring the internal layered settlement of foundation soil mass, this study has introduced an ultrasonic ranging technique into the layered settlement monitoring of civil engineering foundations. It has also developed a special ultrasonic monitor capable of generating different frequencies (e.g., 200kHz, 400kHz). It has also selected two oscilloscopes that can match the frequencies, the pipe fitting materials, reflecting materials, and propagation mediums that are related to the burying of the ultrasonic monitoring system into a soil mass. After that, it has calibrated the propagation velocity of ultrasonic waves under different water temperatures, tested the ranging technique, and obtained an effective range of at least 45 m. Then, it has carried out actual measurement comparison and error analysis, which show that the accuracy of the ultrasonic monitoring system is in the order of millimeters. Finally, an indoor ultrasonic-monitored settlement simulation test has been carried out. The ultrasonic-monitored burying system has been buried into the soil mass, and the ultrasonic monitoring system has been used to measure the layered settlement of the soil mass after rolling compaction and compression. The comparison with the actual measurements shows that the measurements by the ultrasonic monitoring system are highly accurate. The ultrasonic monitoring system can therefore be used to monitor the settlement of foundation soil mass.
