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Discussion on Construction of Horizontal Directional Drill in Pebble

As we all know, horizontal directional drilling rigs are most difficult to construct in the pebble layer and crushed stone layer. The main reason is that it is difficult to guide and does not form holes.

 

Firstly, when it comes to selecting the appropriate machinery for guiding the pebble layer, it is crucial to consider the torque and thrust requirements. Consequently, opting for a small machine is not recommended due to the small and short drill pipe typically equipped with such machines. These drill pipes are prone to bending and fractures at the joints, especially during the handling of pebbles or crushed stones. For pebble layers with particle sizes less than 50 mm, it is advisable to use drill rods of at least 73 drill rod size, and a machine weighing around 32 tons is recommended. For particle sizes ranging from 50-80 mm, drill rods of at least 89 drill rod size should be used, and a machine weighing around 45-60 tons is recommended. For particle sizes of 80-150 mm, drill rods of at least 140 drill rod size are recommended, and a machine weighing around 200-350 tons is suitable. If the formation has a diameter greater than 150 mm, it is not recommended to use horizontal directional drilling directly; alternative auxiliary processes should be considered.

Secondly, it is not advisable to use a guide plate as a guide bit in pebble strata due to its wide area and large angle, which can easily alter the orientation of the pebbles. Instead, an eagle claw-guided drill bit or a screw drill with three teeth is recommended as a guide tool. A longer drill collar is preferable as it enhances the effectiveness of the guide and minimizes changes in direction.

Thirdly, although the functionality of guidance instruments is generally similar, there are differences in terms of earthquake resistance, power consumption, and signal strength. Since vibration within the pebble layer is a common occurrence, stability should be prioritized when choosing a guidance instrument. Some recommended options include DCI Lunar Eclipse, Falcon Series, Geomagnetism, Sharewell Geomagnetism, Goldland Commander Series, and Guideline 5.

Next, when selecting a reamer for pebble strata, the use of a flow path reamer is not recommended as it can easily lead to sticking. Instead, a circular-cut reamer or an extruding reamer with an alloy cutter head on the outer ring, known as a rock reamer, could be used for large rig constructions. However, the effectiveness of using a rock reamer for small drill rigs may not be as good due to lower torque, which can result in sticking.

In terms of pipe pulling, for projects involving a single-hole tube, it is best to use a separate puller for the pull tube head. Additionally, a self-made protective tube cover should be used behind the reamer to prevent pebble displacement. For perforated pipe projects, the puller reamer is an ideal choice, with self-contained splitters and a self-made bushing extending to the head of the engineered pipe.

Regarding mud ratio, while mud doesn’t provide significant hole protection in pebble layers, it helps retain mud in the channel and reduces friction. Therefore, for pebble mud, it is advisable to maintain a mud ratio of at least 80 seconds for optimal results.

When encountering formations with excessive pebble and gravel content and high water content, it is recommended to employ high-grade quick-setting concrete for water and solidification before utilizing rock drilling tools for construction. This approach becomes necessary to prevent curvature changes and drill pipe fractures caused by the lifting of drill pipes during reaming.

Furthermore, for pebble layers that can be addressed through excavation and replacement methods, this approach should be prioritized, especially when the pebble layer is not deeper than the entry and exit points. In such cases, the pebbles can be removed and replaced first, followed by direct installation of the engineering pipe into the subsequent layer, ensuring maximum safety and reliability.

During the reaming process, it is essential to equip the back of the hole reamer with a drill pipe. This is necessary as once the pebble layer is guided and reamed, it becomes impossible to push the hole or revert to the original state. Neglecting this precautionary measure could lead to accidents resulting in losses, as observed in previous construction cases.

Lastly, the construction risks associated with pebble layers are significant. Therefore, it is crucial to include provisions in the bidding process that allow for price adjustments specifically for pebble layer encounters. Failure to address this aspect may result in reputation gains for the project but potential financial losses.

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