Application of active-source surface waves in urban underground space detection: a case study of Rongcheng County, Hebei, China

Abstract

Active-source surface wave exploration is advantageous because it has high imaging accuracy, is not affected by high-speed layers and has a low cost. It has unique advantages in the investigation of shallow surface structures. For the development and utilization of urban underground space, there are two important parameters in the shallow surface, namely, shear wave velocity (vs.) and site predominant period, which determine the elevation and aseismic grade of the building design. The traditional method mainly obtains the above two parameters through the testing and measurement of drilling samples. However, this method is extremely expensive and time consuming. Therefore, this paper uses the multichannel surface wave acquisition method to extract the fundamental dispersion curve of the single shot data using the phase shift method and obtains the vs. characteristics in the uppermost 40 m by inversion. According to the vs. profile, the following two conclusions were obtained. First, the study area can be roughly divided into 5 layers, among which the layers 0-8 m, 14-20 m, and 20-30 m are low-velocity layers, corresponding to miscellaneous fill, a water-bearing sand layer and a sand layer. Therefore, the vs. is relatively low, and the layers at 8-14 m and 30-40 m are high-velocity layers that are mainly composed of clay, with a relatively better compactness and relatively high vs. values. In addition, the low-speed anomaly suddenly appears in the high-speed area at 20-40 m. Combining this with geological data suggests that it is an ancient river channel. Second, according to the vs. value, the V_se(equivalent shear wave velocity) was calculated. The construction site soil is classified as Category III, with good engineering geological conditions. In addition, according to vs., we calculate the site predominant period to be 0.56-0.77 s. Therefore, in the overall structural design of foundation engineering, the natural vibration period of the structure should be strictly controlled to avoid the predominant period of the site.