Abstract: It is often necessary to use pile foundation to deal with soft soil foundation for the newly built gate station near the seawall. If there is a concealed riprap layer in the construction area, it will affect the construction progress of pile foundation and increase the construction cost. Furthermore, the spatial distribution characteristics of riprap also play a decisive role in the selection of pile foundation construction technology. In view of the blindness problem in using the drilling method to investigate the irregular concealed riprap, we adopt the comprehensive investigation technology combining the ground penetrating radar with the high density resistivity method as well as the drilling verification in the anomaly area of geophysical prospecting to investigate the underground riprap in the pile foundation construction area of a gate station. According the survey results, we select the pile foundation construction scheme based on rotary drilling and use the construction technology combination of well point precipitation and excavate and backfill earth work for the block stone area with depth less than 6 m. For the riprap whose local depth is more than 8 m, we use the spiral drilling or core drilling to dig out, then the construction efficiency of the pile foundation is raised remarkably. Several conclusions are drawn as follows:firstly, ground penetrating radar and high density resistivity method were complementary to the detection of the shallow subsurface riprap. The former has high resolution and can reflect the electromagnetic anomalies of some sporadic stone, but it is difficult to distinguish the interface of riprap and the soil. Its abnormal characteristics are shown as messy wave form and strong amplitude as well as accompanying multiple reflection in the longitudinal direction. The latter is more suitable for the detection of relatively concentrated riprap layers. The resistivity of larger particle riprap is greater than 100 Ω·m and the resistivity of crushed stone near the groundwater level is less than 30 Ω·m. The research results can provide reference for the high precision detection of block stones within the depth of six meters in the soft soil construction area and the selection of corresponding pile foundation construction technology.