How to select a drilling rig for pile foundation operation

Comparison of drilling rig solutions: bored piles are not affected by the groundwater level. They can penetrate various soils with complex textures or large changes in hardness and softness (such as various types of sand, clay, crushed gravel, pebble layers, weathered rocks and multi-layered rock layers) to reinforce the foundation. They have many advantages such as large bearing capacity, simple construction equipment, convenience and speed, and good safety. There are many processes for bored pile construction. In the entire construction process, the key link is drilling. The speed of drilling and the quality of the hole are dominant in the entire pile foundation construction process. Drilling machinery is the dominant machinery for bored pile foundation construction. Therefore, the comparison of drilling machinery is particularly important. Which drilling rigs can drill holes in this stratum, and what is the efficiency and consumption of drilling? Only by comprehensively analyzing the performance indicators of various drilling rigs can we find the most suitable construction drilling rig for this project.

Impact drill

The impact drill is a relatively traditional drilling machine. It relies on the impact hammer to impact and smash, the slag barrel to remove the slag, and the up and down reciprocating impact to split and smash the soil and rocks. Some of them are squeezed into the hole wall, and the ordinary mud is used to protect the wall. It is suitable for all common fill layers, clay layers, dense sand layers, round gravel layers and angular gravel composite interlayers, but the penetration is slow in large boulder, pebble layers and slightly weathered strata, and the impact hammer is easily damaged. Impact drills have the characteristics of wide stratum adaptability, fast construction speed, small site environment requirements, and low cost. But at the same time, the impact drill has high labor intensity, the mud circulation requires a large area of mud return pool, the generated mud is not easy to transport, and the construction vibration and noise are large.

Rotary drill

The rotary drill is a hole-forming machine with the longest application time, the widest range, and the largest market share in my country. Except for the difficulty of drilling in pebble and gravel layers, this type of drill has good applicability in various other common strata. Rotary drilling rigs are divided into positive circulation and reverse circulation according to different slag removal methods. Reverse circulation is further divided into three types: pump suction, air lift, and bottom hole pumping (jet). Positive circulation is often used in plain soil layers, clay layers, and sandy soil layers, and reverse circulation is often used in pebble layers, sand and pebble interlayers, rock layers, and bottom hole cleaning. Depending on the stratum, the drill bit can be in different forms, especially when drilling into hard rock layers, a roller cutter bit or a roller drill bit is required. The drilling diameter of the rotary drilling rig can reach 2 to 5 meters and the depth can reach 100 meters. Positive and reverse circulation rotary drilling rigs have the characteristics of a wide range of applications, good wall protection effect, high hole quality, no vibration and noise during construction, easy operation of the machine, and low cost. However, at the same time, its hole-forming efficiency in complex strata is low, the water consumption and mud discharge at the construction site are large, the hole expansion rate is difficult to control, and it is difficult to construct on pebbles, boulders, and bedrock. Especially in hard strata, the progress is slow, and the construction cost rises sharply.

Impact reverse circulation drilling rig

Impact reverse circulation drilling rig is a new type of large-diameter drilling rig that combines traditional impact drilling rigs with reverse circulation slag removal technology. That is, the impact drill bit is used to impact the rock at a high frequency to break the rock, and then the reverse circulation slag removal method is used to discharge the broken rock chips out of the hole in a timely manner. This drilling method not only retains the advantages of traditional impact drilling rigs such as low construction cost, clean hole cleaning, less sediment, and wide adaptability to strata, but also overcomes its shortcomings of inability to continuously discharge slag and repeated crushing. It is widely used in construction of various complex strata. Impact reverse circulation drilling rigs and rotary drilling rigs each have their own advantages. Generally, when constructing in clay, sub-clay, silty soil layers, and silty sand layers, rotary drilling rigs are 1.1 to 1.3 times faster than impact reverse circulation drilling rigs, and the cost consumption is lower. However, in the construction of gravel and bedrock, impact reverse circulation drilling is obviously 2 to 3 times faster than rotary drilling, especially in some hilly and mountainous areas. The superiority of impact reverse circulation in the construction of complex strata, i.e., gravel and gravel-and-stone layers, is more obvious. In terms of cost consumption, in the construction of gravel, boulders, and bedrock, impact drilling is efficient, while rotary drilling rigs have slow drilling speeds and long hole-forming cycles, and the cost is more than 3 times higher than that of impact drilling. In the case of large boulders, large rocks, and granite with high hardness, it is difficult for rotary drilling rigs to drill, and only impact reverse circulation drilling rigs can be used to complete the work. However, at the same time, the vibration of impact reverse circulation drilling has a great impact on the surrounding environment, especially when impacting the hard foundation rock surface at the bottom, the impact vibration produces a lot of noise to the surrounding area. In addition, the drilling rig has high power, high power consumption, complex structure, large volume, and high relocation and transportation costs.

Rotary Drilling Rig

Rotary drilling rig is a new type of pile foundation hole construction method that has developed fastest in recent years. It rotates the drill rod and the drill bucket, uses the weight of the drill bucket and hydraulic pressure as the drilling pressure, fills the drill bucket with soil debris and lifts the drill bucket out of the ground. The drill bucket rotates, digs, lifts, unloads soil and places mud to protect the wall, and the hole is formed through repeated cycles. The rotary drilling rig is suitable for all kinds of clay, silt, dense sand, silty soil, artificial backfill soil and strata containing some pebbles and gravel. With the help of the weight of the drill tool and the pressure of the drill rig, the rake teeth cut into the soil layer. Under the action of the rotary torque, the drill bucket rotates at the same time to cooperate with different drill tools, which is suitable for dry (spiral) and wet (rotary bucket) hole-forming operations. At present, the maximum drilling diameter of the rotary drilling rig is 3m, the maximum drilling depth is 120m (mainly concentrated within 40m), and the maximum drilling torque is 620kNm. The construction method of rotary drilling has the characteristics of reliable construction quality, fast drilling speed, high drilling efficiency, strong adaptability, etc. It greatly shortens the construction period, has less waste slurry, low noise, low pollution and protects the environment. It overcomes the shortcomings of mechanical drilling, such as a lot of sediment at the bottom of the hole, low friction resistance on the pile side, and poor mud management, and greatly improves the construction quality. However, its disadvantage is that it is easy to collapse the hole when drilling in loose sand layers with a large thickness, and the speed is slow when drilling in pebble layers with a large pebble content. It is not suitable for rock construction in hard rock layers. The one-time investment cost of the rotary drilling rig is relatively large, but the economic and technical indicators such as the cost of drilling rig are lower than those of other methods. It should be considered comprehensively in combination with the scale of the project, the amount of work and the construction progress. At present, this process has considerable economic benefits in my country.