In mountain tunnel construction, the Percussion Wireline Sampler (PS-WL Method) is frequently utilized for horizontal boring in geotechnical surveys. With this method, there are occasions when work safety decreases and core samples cannot be obtained due to a large quantity of high-pressure groundwater encountered during drilling operations, which makes it hard to achieve the predetermined length of boring. Considering these factors, it was necessary to consider and develop a new method to obtain core samples which would be capable of avoiding the above-mentioned problems when there was a large quantity of high-pressure groundwater encountered during the operations. To meet this requirement, we have developed the Percussion Single Reverse Drilling Method (PS-SR Method) and conducted a confirmation test and field test. This horizontal core boring method combines the benefits of drilling by the Rotary Percussion Drilling Machine and drilling system of the reverse circulation-type horizontal core boring method, which was put to practical use during the construction of the SEIKAN Tunnel. We focused on the point that efficient core transportation and high-speed drilling are possible by using drainage flow of groundwater. We also concluded that high speed drilling and efficient core boring can be performed even when encountering a large quantity of high-pressure groundwater that cannot be tackled by the PS-WL Method. Through our confirmation test, the practicability of the PS-SR Method was confirmed. Thus, we concluded that the PS-SR Method was able to streamline core boring and save labor. Through the field test, it was confirmed that the PS-SR Method had sufficient applicability as a horizontal boring method of about 100 m to 150 m to be carried out from the tunnel face in actual ground.

1. Introduction

Recently, an advancing horizontal boring method for geotechnical survey which starts from near the tunnel face of tunnel, the Percussion Wireline Sampler (PS-WL Method), has been used frequently, and is able to drill and obtain core sample at high speed. However, when the PS-WL method encounters a large quantity of high-pressure groundwater during drilling operations, the wireline core barrel housing the core is squeezed under pressure of groundwater, and violently ejected on the conductor pipe side, this creates safety problems for workers. Another problem with this method is that setting the wireline core barrel requires a long time, and in cases where the wireline core barrel cannot be set successfully, it is impossible to obtain core samples. In these cases, a predetermined length of drilling cannot be completed within a scheduled time span which affects the progress of tunneling. Considering these problems, we felt the necessity of figuring out a boring method which is able to drill continuously and obtain core samples even when encountering a large quantity of high-pressure groundwater. To cope with these problems, we developed a new type of horizontal drilling, equipped with both the functions of high-speed drilling and efficient transportation of core sample even when encountering a large quantity of high-pressure groundwater (Funahashi et al., 2018).

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