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Before pre-expanding, an optimized reaming plan should be formulated, and the maximum reaming diameter and number of reaming should be determined according to the equipment capacity, stratum conditions and site conditions. (The final reaming diameter should be 1.25～1.5 times of the diameter of the pipeline to be laid). When pre-reaming, choose a reasonable reamer type and diameter, and determine the next-level reaming method according to the previous reaming situation.

① Selection points of rock hole reamer:
Choose rock reamer according to the type of traversing stratum: The type of traversing stratum is an important factor in the correct selection of the size and type of rock reamer hole opener. When reaming soft rock formations, larger diameter reamers and fewer reamers are allowed for the number of holes, the size difference between two adjacent reamers can also be larger, the level difference is 100-200mm; the reaming diameter required when reaming hard rock formations should not change too much, the level difference is 50-150mm and multiple reaming is adopted. In order to achieve the best reaming efficiency and smooth discharge of drill cuttings.

② Key points of reaming operation control:
Different diameters and different types of rock reamers have different requirements on WOB, especially during the reaming of long-distance rock formations, reasonable tension and torque must be controlled to avoid excessive wear of the reamer. Most rock reaming accidents are caused by excessive pursuit of reaming speed and excessive drilling pressure. An important principle that should be followed in rock reaming operations is: the harder the rock to be broken, the greater the drilling pressure required and the lower the drilling speed required. Under the same drilling speed conditions, the larger the diameter of the reamer, the greater the linear velocity of the outer edge and the more severe the wear.

③ Key points of hole reaming mud control:
When reaming a rock layer, mud is also a very important factor. At this time, the main function of mud is to cool the reamer and carry drill cuttings. The mud is generally made up of water, bentonite and mud additives. The mud should have proper viscosity and fluidity, and the ability to carry drill cuttings should be strong, so that the drill cuttings will be suspended at the bottom of the hole. The flow of the mud can make the drill cuttings return smoothly ground. The reflow in the hole has a certain relationship with the unobstructed condition in the hole. If there is no reflow phenomenon, it may cause pipe laying failure, and the actual reason should be analyzed. If the hole is not smooth and the water in the mud also leaks, the remaining mud skin will accumulate and thicker, and the pipeline will be easily "hugged", causing great resistance to the pipe. In the case of normal pipe drawing, there will generally be a phenomenon of reflow. The actual mud displacement should be determined after calculation based on the amount of drill cuttings produced by each stage of reaming and the carrying capacity of the mud.

④ Control of key points of reaming:
The construction method of the reamer entering the rock layer: When the rock reamer enters the rock layer from the soft soil layer or enters the hard rock layer from the soft rock layer, the main point of operation is to first slow down the reaming speed, reduce the drilling pressure, and wait for the reamer body Increase the WOB after completely entering the rock layer or hard rock layer. If the drilling pressure applied during this process is too large, it is likely to cause jamming and damage the reamer. When pre-expanding the hole, slow down the speed of the hole in the place where the guide hole curve is not ideal and the boundary of the formation, and the curve should be adjusted appropriately.

⑤ In the process of hole cleaning, the mud displacement must be controlled to maximize the carrying out of drill cuttings. In addition, in order to clean up the drill cuttings in the hole, prevent mud hydration in the hole, drill cuttings settling, buy time for dragging the pipeline, and more importantly, check whether the tension and torque along the guide hole are normal.

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