[H-TT17] New Developments in Shallow Geophysics

[HTT17-04] Differences in P-wave velocities due to velocity logging in overlapping sections of horizontal boring surveys in sedimentary rock tunnel ground

*Kenji OKAZAKI¹, Toshiyuki KURAHASHI¹ (1.Civil Engineering Research Institute for cold region, Public Works Research Institute )

Keywords:tunnel, velocity logging , P-wave velocity

P-wave velocity is one of the evaluation indexes in the ground classification of tunnels. At the time of design, it is measured from the ground surface of the tunnel, and at the time of construction, it is measured at the core, inside the borehole, and rock fragments by the horizontal boring survey from the tunnel face. In addition, the P-wave velocity at the time of construction is used for reviewing the ground classification as the value closest to the excavation site. In one horizontal boring survey, about 100 m is excavated, but the next horizontal boring survey may be carried out before the tunnel excavation reaches the deepest part excavated in the horizontal boring survey. Therefore, in the first horizontal boring survey and the next horizontal boring survey, there may be an overlapping section of about 30 m. The P-wave velocity by the velocity logging of the first horizontal boring survey is the P-wave velocity about 100 m ahead of the tunnel face and in the state where the ground before tunnel excavation is not loosened. On the other hand, the P-wave velocity by the velocity logging of the next horizontal boring survey is the P-wave velocity in the state where loosening occurs due to tunnel excavation at the tunnel face. It is important information for reviewing the ground classification and deciding the selection of auxiliary construction method by knowing the change of P wave velocity in advance.

Therefore, in this survey, we collected P-wave velocities by velocity logging in the overlapping section (8 boreholes total length 197 m) at the time of the horizontal boring survey (104 boreholes in total) for 5 tunnels (total length 11,104 m) with sedimentary rock tunnel ground. Then, the difference between the P-wave velocity by the velocity logging of the first horizontal boring survey and the velocity logging of the next horizontal boring survey was investigated.

As a result of the survey, the overlapping section was 25 m on average. The P-wave velocity by velocity logging in the first horizontal boring survey was 2.5-3.2 Km/s (average 2.7 Km/s). On the other hand, the P-wave velocity by velocity logging in the next horizontal boring survey was 1.0 to 2.8 Km/s (average 2.2 Km/s). From this results, it was clarified that the P wave velocity in the overlapping section decreased by about 17% due to the excavation of the tunnel.