## 1 Introduction  
The cast in situ piles (bored pile) are widely used in practical engineering projects such as buildings, roads, and bridges due to their simple construction technology and high bearing capacity [1, 2, 3, 4, 5]. In the bored pile installation process, the drilling machinery will agitate, cut, and squeeze the soil under the pile base, which will largely affect the base soil resistance. Moreover, the residues remained at the pile base will also hamper the mobilization of the pile base bearing capacity [6, 7]. During the bored pile installation process, the slurry with a certain density is remained in the pile hole to keep the pile hole stable. Hence, a sort mud layer will be formed around the pile shaft, which will reduce the frictional capacity of the pile-soil interface. In addition, the stress release and disturbance generated by the drilling process will also decrease the shaft capacity [8, 9]. It should also be noted that the shaft capacity of the pile is mobilized before the base capacity. The displacement required to fully mobilize the pile base resistance is about 10-15% of the pile diameter; whereas, the displacement required to fully mobilize the pile shaft resistance is only 0.5-2% of the pile diameter [10, 11]. The pile base capacity is normally not fully mobilized under the working load [12].  
The disadvantages of the bored piles mentioned above promote the development and application of post-grouting technology. As a pile foundation reinforcement technology, the post-grouting technology at the pile base was first applied to the construction of Maracaibo Bridge in Venezuela. The bearing capacity of the pile foundation was improved by grouting the bottom of the drilled and buried precast piles. Subsequently, this technology was gradually applied in the pile foundation engineering and has been constantly innovated and developed [13; 14; 15; 16; 17]. With the rapid development of urban construction, many super-long piles appeared to support the super high-rise building, high-speed railway, and highway, etc. [18; 19]. Since it is very difficult to take vertical super long side holes due to the inability to place drilling equipment on the site, the traditional side-hole transmission wave method will no longer be applicable to the detection of pile quality and grouting depth [20]. At present, the method to evaluate the bearing capacity of super-long piles usually adopts the static load test [21; 22], and the method to test the strength of the soil under the pile base usually adopts the dynamic penetration method [23; 24]. However, due to the high cost of the static load test and the large compression of the pile shaft, the traditional static load test cannot effectively compare the bearing capacity of super long piles before and after grouting. In addition, the conventional dynamic penetration method cannot test the strength of the soil before and after grouting under the same pile base.  
In view of the above problems, this study will adopt the transmission wave method in the pile to solve the problem of difficulty in taking vertical super long side holes and can effectively evaluate the pile quality and grouting depth. The dynamic penetration method in this study will bury two pre-embed pipelines at the same time, and the changes in the bearing capacity before and after grouting can be tested on one pile. The innovative method in this study can not only effectively solve the above problems, but also can be used for the design of base post-grouting of super long piles in similar geological conditions.