1 Project Overview
The second phase of Gezhouba City Garden is located on Qiaokou Road, Qiaokou District. The above-ground part consists of five high-rise buildings (Buildings 7#, 8#, 9#, 10#, and 11#) and a five-story commercial street, with two basement levels. According to current exploration, the old foundation piles of the former Seafood City under Building 11 need to be extracted: pile cross-section 400*400 mm, pile length 15 m (two-section piles), approximately 216 piles.
2 Project Characteristics
Based on the on-site pile location plan and geological documents, and considering the requirements of subsequent construction, the pile foundation removal must meet the following requirements:
2.1 The piles must be completely removed in one successful attempt, without broken piles or leaving part of the pile underground; if a pile breaks, measures must be taken to remove the broken pile from the ground.
2.2 During extraction, the stability of the soil around the pile should be maintained as much as possible, and the impact on nearby soil should be minimized;
2.3 After extraction, backfill and reinforcement should be carried out promptly, ensuring the treatment effect is as close as possible to the original pile-soil condition.
3 Engineering Geological Conditions
The soil layers at the site, from top to bottom, are: (1) miscellaneous fill, (2-1) clay, (2-2) silty clay with silt, (3-1) silty sand with silt, (3-2) fine silty sand, (3-3) fine sand, (3-3a) silty clay, (3-4) medium-fine sand, (3-5) gravelly sand, (4-1) strongly weathered silty mudstone, (4-2) moderately weathered silty mudstone.
4 Analysis of Pile Extraction Principle
Pile extraction is the process of pulling deeply buried piles (including precast square piles, bored cast-in-place piles, etc.) out of the soil. During extraction, the self-weight of the pile and the friction between the pile and the soil must be overcome.
From a mechanical perspective, the extraction force required must be greater than the sum of the pile's self-weight and the friction between the pile and the soil.
There is friction between the pile and the soil. For precast square piles, the contact surface with the soil is relatively smooth, so the friction is relatively small. However, for bored cast-in-place piles, the friction resistance is greater. Combined with the pile's self-weight, it is very difficult to pull the pile directly out of the soil using mechanical equipment, so technical measures are needed for extraction.
Regardless of the technical measures adopted, from a mechanical perspective, the goal is to reduce the required extraction force, i.e., to reduce the sum of the pile's self-weight and the friction between the pile and the soil. From the current technical perspective, the measures include:
(1) Isolating the pile from the soil to reduce friction resistance;
(2) Isolating the pile from the soil by dividing the pile into several sections, thereby reducing both friction resistance and the self-weight of each extracted pile section.
5 Pile Extraction Construction
Based on the geological conditions, pile foundation type, and mature construction experience from similar pile extraction projects, the following scheme is proposed: full casing with dual high-pressure air jet and water jet, vibratory casing sinking and hole cleaning method.
5.1 Main Construction Equipment for Pile Extraction
Based on actual conditions, steel casings with a diameter of Ф1000 are used. The main equipment includes:
(1) 150T crawler crane, used for lifting and placing steel casings, vibratory casing sinking equipment, and integrally extracting the steel casing and pile;

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(2) Ф1000 steel casing, used to isolate the pile from the soil;
(3) Vibratory casing sinking equipment, used to vibrate the steel casing into the soil;
(4) Auxiliary sinking equipment, used for water and air jetting during casing sinking to wash and cut the soil around the pile;
5.2 Pile Extraction Construction Process
The construction process flow is: determine pile head → vibrate steel pipe → high-pressure water and air jetting → lift and extract pile → backfill pile hole → vibrate and extract steel pipe → backfill and compact pile hole → cement mixing pile for pile hole reinforcement.
5.2.1 Determine pile head position: Based on the original pile design positions, set out the original axis. Given the actual site conditions, slope excavation is not suitable. Considering the long time elapsed and pile position errors, first use seismic geophysical methods to preliminarily determine the general location of the piles, then use a small auger to detect the specific position of a pile and determine its exact position on the drawings. Renumber according to old drawings, establish a new pile position coordinate system, set out all pile positions using polar coordinates, verify with a small auger, and mark them. Use an excavator to dig a 1-meter-deep trench. In the trench, use conventional manual hole digging pile construction technology to manually excavate a 1000 mm diameter, 2 m deep hole, with support as excavation proceeds. At 2 m below the surface, use a steel probe to find the exact position of the square pile and determine the pile to be extracted.
5.2.2 Considering the deep embedment of the pile top, it is necessary to adopt bamboo tube support measures and equip a water pump for dewatering. Given the small spacing between piles, the skip-extraction method is adopted. After extraction, the holes are promptly backfilled and compacted to prevent collapse and hole connection between adjacent piles.
5.2.3 Vibratory sinking of steel pipe
Based on pile diameter, cross-sectional area, and depth, a fabricated casing with several water and air channels and sufficient stiffness is vibrated into the ground using a 150T crawler crane with a 135 kW zero-current start vibratory hammer to 50-100 cm below the pile toe elevation. During sinking, verticality is controlled. After sinking in place, the vibratory hammer fixed at the top of the steel casing is removed.
5.2.4 High-pressure water and air jetting
During sinking, the air compressor and high-pressure water pump are started to break and replace the soil around the pile inside the casing and to wash the pile until it is independently separated.
5.2.5 Lifting and extracting the pile
A wire rope (pre-set at a certain depth inside the casing) is fixed to the middle-lower part of the lower pile section. A 150T crawler crane is used to tighten and lift the pile, and transport it to a nearby storage site. 5.2.6 Vibratory extraction of steel casing
Start the vibratory hammer fixed on the steel casing, and use the 150T crawler crane to lift while vibrating to extract the steel casing.

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5.2.7 After extraction of old piles, the pile holes must be backfilled and reinforced. Reinforcement uses 500 cement mixing piles with 5% cement content (18 kg per meter), and the pile length is the same as the old square piles. Reinforcement details for each pile cap are shown in the following figures: 5.2.8 Broken pile reporting procedure
If a pile breaks during extraction for various reasons, immediately treat the broken pile and monitor the entire process until the broken pile is successfully treated.
6 Technical Points for Pile Extraction Construction
Due to the large power of the vibratory hammer and the high lifting height, a 150T crawler crane is required, and roadbed boxes must be laid according to ground strength.
6.1 The steel pipe pile is extracted from about 3 m above the pile toe to prevent uneven stress on the pile body and pile breakage, ensuring the entire pile can be extracted.
6.2 Before extraction, strictly follow procedures to remove soil inside the casing using high-pressure water and air jetting to minimize the friction on the pile exterior, facilitating pile extraction;
6.3 During pressurized lifting, strictly control the lifting speed to ensure the integrity of the pile.
6.4 If a pile breaks, repeatedly lower the casing to encase the broken pile body, then fix the lifting point and extract the remaining pile body to ensure thorough extraction. Since this project uses the vibratory casing sinking method for pile extraction, even in extremely complex broken or deviated pile conditions, the casing can be used to remove relatively short broken piles.
In response to the above possible situations, the following treatment plan is specifically formulated:
During lifting preparation, the estimated uplift force on the pile is about 600 KN, and the wire rope is 38# or above with a safety factor of more than 3 times. If the pile body has already broken before lifting, the clamp must be fixed very close to the pile tip, and another wire rope fixed at the pile top. When extracting the lower half, simultaneously use the auxiliary hoist to lift the upper half, and lift the broken pile out of the hole in sections.
6.5 Treatment of old piles and backfilling of pile cap pits: The extracted old piles should be transported away promptly. The excavated pits from pile extraction must be densely backfilled and reinforced with compaction grouting or jet grouting.
7 Project Conclusion
This pile extraction technology was very successfully applied in this project, creating favorable conditions for subsequent construction. The needs of engineering practice are the driving force for technological development. With the demand for large-scale urban renovation and demolition, pile foundation extraction equipment and technology are continuously updated, and pile extraction construction will become faster.