Pile foundation is a core foundation form of buildings, composed of multiple columnar members. Its key function is to penetrate weak strata and transfer the full load of the building structure to the underlying stiffer and denser soil layers or bedrock (i.e., the bearing stratum). According to load transfer mechanisms, pile foundations fall into two major categories: friction piles and end-bearing piles. Friction piles do not rest directly on the bearing stratum, but transfer loads mainly through friction between the pile shaft and surrounding soil. End-bearing piles are seated directly on hard strata and bear loads primarily via the bearing capacity of the bearing stratum. A schematic diagram of pile foundation classification is shown below:

Based on construction methods, foundation piles are divided into precast piles and cast-in-place piles. Bored piles fall into the category of cast-in-place end-bearing piles. Besides bored piles, cast-in-place pile foundations also include bulb-expanded piles, vibro-compacted sand piles, jet grouting piles, and deep mixing piles, which are widely used in engineering scenarios with various geological conditions.

Ⅰ Advantages and Disadvantages of Bored Piles

Compared with precast piles, bored piles have become increasingly popular in modern engineering due to their strong adaptability and economic efficiency. With current technological innovations, their core advantages and disadvantages are as follows:
    1. Core Advantages

（1）Controllable cost and low steel consumption

Precast piles require factory production, long-distance transportation, and on-site hoisting. They are subjected to complex forces such as bending, impact, and vibration during transit and hoisting, leading to high requirements for steel strength and large steel consumption. In contrast, the reinforcement cages of bored piles can be fabricated and installed on-site without long-distance transportation, significantly reducing steel loss and costs associated with transportation and hoisting, presenting remarkable economic benefits.

（2）Excellent large-diameter hole-forming capability

With the upgrading of equipment and technology, breakthroughs have been made in the maximum drilling diameter of bored piles. At present, single rock-socketed bored piles with a diameter of 7 m have been achieved in domestic offshore projects. Internationally, mainstream equipment can drill diameters of 4.7–6 m, and special-purpose equipment can reach up to 10.4 m. Limited by manufacturing, transportation, and hoisting conditions, precast piles still struggle to exceed 1 m in diameter and cannot meet the needs of heavy-load engineering.

（3）Hole depth adaptable to ultra-deep engineering requirements

Current hole-forming depths of bored piles far exceed traditional levels, reaching 60–70 m for conventional projects. Through process optimization, depths can exceed 120 m in special projects. For instance, an extra-long bored pile with a depth of over 120 m and a diameter of 2.8 m was successfully constructed in a super high-rise building project in Southwest China. Restricted by hoisting capacity, precast piles cannot approach such depths and are unsuitable for projects with deep bearing strata.

（4）Flexible optimization of pile structure

Bored piles can be under-reamed according to engineering needs to effectively enhance uplift resistance. They can also be directly socketed into bedrock through drilling, integrating the pile with the bearing stratum and greatly improving single pile bearing capacity, whose design value can exceed 30,000 tons. Precast piles cannot be socketed into bedrock, limiting the improvement of bearing capacity.

（5）Low impact on surrounding environment

Bored pile construction generates no significant vibration or stratum extrusion, avoiding damage to surrounding soil structures. Impact drilling can even compact the soil to a certain extent. Precast pile driving often causes ground heave and may damage adjacent building foundations. In addition, bored pile construction produces low noise, meeting modern green construction requirements, whereas traditional piling equipment generates excessive noise and has been banned in urban areas in some countries.

（6）Wide geological applicability

It is adaptable to cohesive soil, silt, sand, gravel, pebbly strata, and weathered rock. Especially for complex conditions such as thick soft strata in marine plains and karst geology, construction difficulties can be effectively resolved through process optimization (e.g., combined short and long casing schemes), making it far more adaptable than precast piles.

   2. Existing Disadvantages

Despite remarkable advances in bored pile technology, several issues remain to be improved:

    First, cleaning sediment at the pile base is still challenging. Although optimized slag discharge processes can reduce sediment, complete removal requires high-precision equipment. Excessive sediment may lead to settlement after pile forming.

     Second, the mud cake on the borehole wall has not been fully resolved, which reduces shaft friction and impairs bearing capacity.

  Third, borehole diameter control is difficult. Over-excavation causes concrete waste, while under-excavation hinders reinforcement cage installation, especially in loose and collapsible strata.

    Fourth, conventional mud-retaining wall technology causes construction site pollution. Although mud-free and green wall-protection technologies have been applied, they are not yet widely adopted.

    Fifth, quality control and inspection demand high technical expertise, and traditional testing methods have limited accuracy. New technologies such as electrical monitoring are being gradually promoted to enable real-time and precise monitoring of concrete placement height and pile quality, but further standardization is required.

   Overall, the advantages of bored piles far outweigh their disadvantages, and with technological innovation, their application prospects are increasingly promising.

Ⅱ Basic Requirements and Characteristics of Bored Pile Drilling

   1. Borehole diameter matched to engineering requirements

The conventional pile diameter of domestic bored piles ranges from 400 mm to 1500 mm, and can reach 2.8–7 m in large-scale projects. International practice has developed diameters of 2000–5000 mm and even larger. Large-diameter drilling requires supporting large-scale drilling equipment, special tools, and advanced borehole stabilization measures (e.g., new polymer mud wall protection, full casing follow-up) to prevent borehole collapse and ensure hole-forming quality.
   2. Borehole depth customized to needs, breaking traditional limits

The borehole depth is generally within 30 m for conventional projects. In special projects such as super high-rise buildings and offshore wind farms, it can reach 70–120 m, with the deepest approaching 110 m. The specific depth depends on pile purpose, burial depth of the bearing stratum, and engineering load requirements, breaking the traditional limitation of shallow drilling.

   3. Complex drilled strata with high targeted requirements

Drilling mostly occurs in loose surface overburdens, including soil, sand, gravel, and pebbly strata. Such strata have high water content and poor borehole stability, prone to collapse, quicksand, and diameter reduction. Quicksand may cause adjacent ground subsidence, expansive strata may lead to borehole shrinkage, and hole collapse frequently occurs in thick soft marine plain strata, requiring targeted adaptive processes.

  4. Strict well-forming quality requirements and upgraded inspection technologies

Key technical indicators are further refined: base sediment thickness shall not exceed 100 mm, and is controlled to 20–40 mm in high-precision projects; borehole deviation shall not exceed 0.1%–1% of the pile length. Advanced processes such as the Bentonite pile limit verticality deviation to 0.226%–0.5% of pile length; pile position deviation shall not exceed 50–100 mm, and can be controlled to 20–30 mm in high-precision projects. Borehole over- and under-excavation must strictly comply with design specifications. Therefore, continuous improvement of drilling technology, optimization of tool performance, and adoption of new inspection methods such as electrical monitoring and core drilling sampling are necessary to ensure well-forming quality.

  5. Limited construction sites and raised efficiency requirements

   In modern engineering, pile layouts are dense, mostly located in narrow urban core areas, deep-water zones, and other confined sites with tight construction schedules. Concentrated equipment and personnel tend to cause cross-interference. Current process optimizations (e.g., coordinated operation of dual concrete mixing-transporting-pumping vessels, pile head breaking-free technology) effectively improve construction efficiency, shorten the construction period, reduce site occupation and interference, and achieve efficient construction.

Ⅲ Pile Shaft Drilling Technologies

   Given the complexity of geological conditions and diversity of engineering requirements, pile shaft drilling technologies must be selected and optimized accordingly. The core processes and application scenarios are as follows:

   Full-face reverse-circulation drilling remains the mainstream method due to its high slag discharge efficiency and good hole-forming quality, suitable for most strata. For extra-large pebbly strata, impact drilling can be adopted, though attention must be paid to its limited borehole diameter; specialized rock-breaking tools can be used if necessary.

   For large-diameter shafts encountering large pebbles and boulders, button cutters or full-cast cutter drilling should be used to improve rock-breaking efficiency and ensure borehole dimensions. For loose, collapsible strata or quicksand layers, a combined process of positive-circulation drilling and reverse-circulation slag removal effectively prevents borehole collapse and over-excavation.

   In addition, with technological development, various new drilling processes have been gradually promoted:

● The Bentonite pile (hydraulic full-casing bored pile) adopts full-casing drilling without mud protection, featuring no noise or vibration and high hole-forming quality, suitable for construction in sensitive areas and complex strata.

●In offshore and deep-water engineering, technologies such as pressurized vertical shaft boring machines and dual-vessel coordinated operation overcome construction challenges of deep water, large diameters, and extra-long piles, greatly improving construction efficiency and quality.

●The breaking-free pile head drilled pressure grouting process precisely controls concrete dosage, reduces waste, and avoids mud pollution, complying with green construction requirements and suitable for medium-shallow bored piles.