Views: 0 Author: Mick Chan Publish Time: 2025-10-09 Origin: Site
The rotary drilling rig, as the "core tool" for pile foundation drilling, its selection and application directly determine the efficiency and quality of the project. However, in subway pile foundations and bridge engineering, due to differences in engineering functions, geological conditions, and load requirements, the selection logic, operation parameters, and working condition response strategies of the rotary drilling rig are significantly different. This article starts from the core demands of these two types of projects and dissects the key points of differentiated application of the rotary drilling rig.
The functions of the pile foundations for subway and bridge projects are different, which directly leads to essential differences in the core requirements for rotary drilling tools:
Subway pile foundation: "Limited space + Anti-disturbance + Environmental protection priority"
Subway pile foundations are mostly located in the urban core area, with narrow construction space (often adjacent to buildings and underground pipelines), and need to pass through complex strata such as soft soil, sand layers, and confined water layers; at the same time, vibration and noise need to be strictly controlled (to avoid affecting surrounding residents and existing buildings), the pile foundation mainly focuses on "uplift resistance, anti-horizontal load" (to support the subway tunnel structure and resist ground subsidence and lateral pressure), and the precision of hole formation is extremely high (the diameter deviation needs to be ≤ ±5mm, and the verticality deviation ≤ 0.5%).
Bridge pile foundation: "Deep hole heavy load + Hard rock breakthrough + Stability priority"
Bridge pile foundations are mostly located in the wild (across rivers, valleys, and land), with relatively open construction space, but need to bear the huge vertical load of the bridge superstructure (such as a single pile of a cross-river bridge can have a bearing capacity of several thousand tons); geological conditions often involve hard rock layers such as moderately weathered rock and slightly weathered rock, and some pile foundations are deeper than 50m (such as a cross-sea bridge), the core requirements are "deep hole hole formation efficiency" and "hole wall stability" (to avoid hole wall collapse and ensure vertical bearing capacity).
The geological differences between the two types of projects and the functions of the pile foundations directly determine the "type preference" of the rotary drilling tools. The specific differences are as shown in the table below:
Typical Scenario Examples
When the subway foundation passes through soft soil - sandy layers in the urban area, a double-bottom sand extraction bucket is preferred: Its bottom double-door design enables rapid soil extraction, and the drainage holes on the bucket wall allow the mud to evenly enter the hole, balancing the pressure on the hole wall and preventing the sand layer from collapsing. If there are densely located underground pipelines in the area, a short spiral drill bucket (diameter ≤ 1.2m) can reduce the disturbance to the pipelines by using "small-diameter hole formation + segmented expansion hole formation".
When the bridge foundation is constructed in the middle-aged weathered rock layer of a cross-river bridge, a cone-shaped tube drill must be used: When the cone-shaped drill bit at the bottom rotates with the drill rod, it breaks the hard rock through "compression + cutting", and the single drilling depth can reach 1.5-2m (30% more efficient than the ordinary rock-boring cone), and the tube structure can avoid the rock debris from scattering at the bottom of the hole, reducing the number of cleaning operations.
From "Precision Control" to "Efficiency Primacy"
Apart from the type of drill tools, the operational parameters of rotary drilling tools (rotation speed, down pressure, and lifting speed) also need to be dynamically adjusted according to engineering requirements. The difference lies in the balance between "precision orientation" and "efficiency orientation".
1. Rotation speed and down pressure: Adaptation to ground hardness and disturbance control
Subway pile foundation: Low rotation speed + Low down pressure, preventing ground disturbance
In soft soil and plastic soil layers, the rotation speed of the drill tool is controlled at 15-25 r/min (to avoid hole wall liquefaction and collapse due to high-speed rotation), and the down pressure is set at 50-80 kN (only "light pressure and slow drilling" is needed to penetrate the soft soil, reducing the compression on the surrounding soil); if passing through sand layers, the rotation speed needs to be further reduced to 10-15 r/min, and combined with mud circulation, the sand particles are "smoothly brought into" the drill bucket to avoid the sand layer being lifted off the hole wall.
Bridge pile foundation: High rotation speed + High down pressure, improving rock breaking efficiency
In moderately weathered rock layers, the rotation speed is increased to 30-45 r/min (to enhance the cutting force of the teeth on the rock through high-speed rotation), and the down pressure is set according to the hardness of the rock layer at 120-200 kN (hard rock requires "high-pressure drilling" to break through the rock structure); for deep holes exceeding 50m in depth, "segmental pressure increase" (low pressure at the hole opening and high pressure at the hard rock section at the bottom of the hole) is required to avoid pressure loss caused by long-distance transmission of the drill pipe.
2. Lifting speed: Balancing hole formation efficiency and hole wall stability
Subway pile foundation: Low-speed lifting, preventing hole wall collapse
After the drill bucket is filled with soil, the lifting speed is controlled at 0.5-0.8 m/s (about 2-3 m/min), and it needs to be maintained at a constant speed (to avoid sudden changes in speed causing disturbance of the mud in the hole, which may lead to sand layer piping phenomenon); if close to existing buildings, the lifting speed needs to be reduced to 0.3-0.5 m/s, and real-time monitoring through the body level gauge is required to prevent the body from tilting and pulling the hole wall.
Bridge pile foundation: Medium-high speed lifting, improving deep hole efficiency
Due to the spacious construction space and the presence of mud or steel pipe protectors on the hole wall, the lifting speed can be set at 1.0-1.5 m/s (about 4-6 m/min); for heavy-duty drill tools such as the embedded rock bucket, a "first slow then fast" method (low speed at the bottom to avoid rock debris falling, and high speed in the middle and upper sections for efficiency improvement) is required to balance efficiency and hole bottom cleanliness.
"Customized Adaptation of Drilling Tools"
Metro and bridge engineering often encounter special working conditions. Rotary drilling tools need to be "structurally modified" or "combined applications" to cope with these situations. The specific differences are as follows:
1. Metro pile foundation: Strategies for dealing with "space constraints + complex strata" for drilling tools
Narrow space (such as the transfer node of metro stations): Use "short sections of drill pipe + small-sized drill bucket" (the diameter of the drill bucket is ≤ 1.0m, and the single section length of the drill pipe is ≤ 3m), combined with a foldable drill bucket wall (deployed to take soil during operation, folded through narrow passages when lifting), to solve the problem of "drilling tools unable to vertically rise and descend";
Saturated water layer (such as metro in coastal cities): Use "tubular drill bucket with water-stop ring", the drill bucket wall is equipped with a rubber water-stop ring, during drilling it adheres to the hole wall, reducing the infiltration of saturated water into the hole; at the same time, the bottom of the drill bucket is added with "drainage valve" to avoid excessive water pressure in the hole when lifting causing mud to spurt.
2. Bridge pile foundation: Strategies for dealing with "deep holes of hard rock + harsh environment" for drilling tools
Ultra-deep holes (such as the depth of bridge piles for cross-river bridges 60m+): Use "graded drill bucket combination" (the upper part uses double-bottom sand捞 bucket for soft soil layers， and the lower part replaces with a toothed wheel tube drill)， and the drill bucket is equipped with "bottom monitoring sensors"， which transmit real-time data on rock layer hardness and wear of the drill bucket； to avoid blindly increasing pressure causing the drill pipe to break；
Underwater pile foundation (such as cross-sea bridges)： Use "anti-corrosion drill bucket"， the drill bucket wall and teeth are made of corrosion-resistant alloy materials， and the connection part is equipped with a sealing rubber ring to prevent seawater from entering the hydraulic system； at the same time， the bottom of the drill bucket is designed with "self-cleaning structure" to avoid sea sand adhering and affecting the efficiency of soil extraction.
The quality acceptance standards for rotary drilling equipment operations vary for the two types of projects, with the core differences lying in "hole wall protection" and "bearing foundation":
For subway pile foundations: The core is "hole wall integrity"
The requirements are that after the drilling operation, the hole wall should have no shrinkage or collapse (detected by a hole diameter gauge, with the hole diameter deviation ≤ ±5mm), and the thickness of the sediment at the bottom of the hole should be ≤ 3cm (due to the high uplift resistance requirement of subway pile foundations, excessive sediment thickness will reduce the bond strength between the pile body and the stratum); and the vibration speed during the drilling operation should be controlled at ≤ 0.15cm/s (to avoid affecting the surrounding building foundations).
For bridge pile foundations: The core is "hole depth accuracy + hard rock penetration depth"
The requirements are that the deviation of the drilling depth should be ≤ +10cm/-5cm (detected by a measuring rope + heavy hammer, to ensure the vertical bearing capacity of the pile foundation), the penetration depth of the hard rock section should comply with the design requirements (such as hard rock with medium weathering should penetrate ≥ 1.5 times the pile diameter); the wear of the drill bucket should be controlled at ≤ 5mm / 100 hours (to avoid the drill bit wear causing a smaller hole diameter and affecting the load transfer).
Conclusion: The essence of the differences is "engineering demand-oriented"
The differentiation of rotary drilling equipment in subway and bridge projects is not merely a "tool difference", but a "precise adaptation" to the engineering functions, geological conditions, and environmental restrictions - subway pile foundations require the drilling equipment to be "light, small, and stable", prioritizing precision and environmental friendliness; bridge engineering requires the drilling equipment to be "strong, tough, and efficient", focusing on overcoming challenges of hard rock and deep holes. Only by clearly identifying the core engineering requirements can the right drilling equipment be selected and the parameters be used accurately, ultimately achieving "dual excellence in quality and efficiency" for pile foundation drilling.
