Abstract: In areas with thick overburden soils, due to the large depth of drilling, complex composition and particles of the bedrock, single hole wave velocity test method often cannot accurately determine the integrity of the bedrock. In situ testing methods such as cross hole wave velocity test method can be used to qualitatively or quantitatively evaluate and analyze the integrity of the bedrock, to provide accurate geological basis for preliminary design. This article presents the detailed geotechnical investigation of the Jiangxinzhou middle ventilation shaft in the second phase of Nanjing Metro Line No. 4. The process of bedrock exploration in the thick overburden area of the Yangtze River floodplain is introduced by using the cross hole wave velocity test method, which is excited by the middle borehole and received by both side of the boreholes. Boreholes are reasonably arranged at both ends of a deep excavation for complex bedrock strata. The longitudinal wave test is carried out using electric spark source to excite longitudinal waves. Based on comprehensive analysis of laboratory test results and geological boreholes data, the bedrock integrity coefficient was evaluated and obtained.  The integrity of the bedrock strata information is further refined. This information provides a reliable geological basis for optimizing the insertion depth of the diaphragm wall and controlling the total construction cost of the project.

Key words: Thick Overburden, Cross Hole Wave Velocity Test Method, Deep Excavation, Diaphragm Wall