Anchor piles are suitable for foundation reinforcement and inclination correction of multi-story industrial and civil buildings on foundation soils such as silt, cohesive soil, artificial fill, mucky soil, and loess. They have strong adaptability and good economy for medium and small construction projects, old city reconstruction projects, mountain geological conditions in village and town construction, and treatment of quality accidents in pile foundations of new projects. They are particularly suitable for situations such as cracking or tilting of superstructures caused by uneven settlement of foundations, building additions or factory expansion, construction of multi-story buildings in dense building groups or near precision instrument workshops, and treatment of pile foundation engineering accidents, but should be used with caution in deep silt layers.

 

　　I. Project Overview

 

　　The project is a six-story frame structure with a building area of 7652m2, a column grid of 8.0×8.0m, and seismic fortification intensity of 7 degrees. The project adopts single-column single-pile, with large and concentrated loads, complex stratum structure, thick silt layer, and deep bedrock. The design adopts bored cast-in-place pile foundation. One month after the completion of bored cast-in-place piles, static load tests were randomly conducted on one pile of different diameters. The results showed that pile #1 failed when loaded to 30% of the ultimate load value; piles #2 and #3 failed when loaded to 40% of the ultimate load value. Therefore, it was determined that the engineering piles of the project did not meet the design requirements and needed reinforcement. In order to quickly and reliably handle the pile foundation accident without delaying the construction period due to the treatment of pile foundation quality accidents, it was decided to use anchor static pressure piles for reinforcement to compensate for the insufficient bearing capacity of bored cast-in-place piles.

 

 

　　Since the foundation pit has been excavated, it is impossible to add piles, and soil squeezing is serious, so adding piles is also impractical. In the initial discussion, pressure grouting was considered to reinforce the foundation soil, and each pile cap was connected into a whole to form a pile-raft foundation, so that the pile raft and foundation soil work together to bear the superstructure loads. Because the theory of pile-soil interaction is not yet mature, and most of the broken piles are located on the outside, it was not adopted in the end. Instead, it was unanimously agreed to use post-anchor static pressure piles for foundation reinforcement, which does not affect the construction period and is safe and feasible.

 

　　Small-section precast reinforced concrete column-type members are pressed into the soil layer using anchor rods around them and a load-bearing frame as foundation piles, connected into a whole through independent caps or beam-type caps to form a pile foundation, and the underpinning technology is used to support the superstructure loads, completing foundation reinforcement and inclination correction, thereby achieving the purpose of improving foundation bearing capacity and controlling settlement. It fully utilizes the respective characteristics of anchor rods and small concrete piles, which can effectively provide the supporting reaction force required for reinforcement and inclination correction, and reduce the adverse additional settlement caused during the implementation process. However, due to the short pile segments, the number of joints in the entire pile is large, which increases the initial deflection of the pile and affects the integrity of the pile and its ability to resist longitudinal buckling. Therefore, a single pile should not enter the bearing layer too deeply, nor should it be pressed into incompressible hard rock and gravel layers. It must be used with caution in deep silt layers.

 

　　(I) Process flow:

 

　　(II) Construction

 

　　1. Pile fabrication. Anchor static pressure piles have the characteristics of small size and uniform specifications, suitable for factory production or on-site production. According to the requirements of pile body and foundation pile bearing capacity, pile section sizes of 200mm×200mm, 250mm×250mm, and 300mm×300mm are selected. Pile segments are generally 2-2.5m, and the entire anchor pile length consists of one first segment and multiple intermediate segments. The reinforcement ratio of the pile body is not less than 0.6%, the diameter of the main reinforcement should not be less than φ10, the concrete strength grade is C30, and the coarse aggregate particle size is ≤40 crushed stone or crushed river pebbles. The formwork can be removed and lifting can be carried out only after the concrete strength of the pile reaches more than 70% of the design strength.

 

　　The cross-sectional size of the anchor pile is 250×250, and the single length L=2.5m. The pile is required to enter the residual sandy cohesive soil layer. The single pile bearing capacity is calculated as 436kN. In principle, the number of added piles is such that the single pile bearing capacity of the bored cast-in-place pile is taken as 30% of the original design value, and the remainder is borne by the anchor piles. Through calculation, it is determined as four-pile, five-pile, and six-pile caps (as shown in Figure 2). When designing the cap, the requirement of a certain spacing between the original pile and the static pressure pile and between the static pressure piles must be considered. Eccentricity of reaction force due to pile addition is not allowed, and factors such as the reduction of single pile bearing capacity due to group pile effect should be noted.

 

　　Reserve pile openings according to the number of added piles, with an upper opening of 300×300 and a lower opening of 400×400, and reserve anchor bars. After construction reaches the third floor, pile pressing begins.

 

　　2. Stand and press piles. (1) For frame structure buildings, pile pressing can generally begin only after the concrete of the fourth floor slab is completed; for brick-concrete structures, generally after the concrete of the third floor slab is completed (specifically determined by the designer based on soil geological conditions and pressing force); (2) The pile pressing frame can be designed according to the pressing force; (3) The pile pressing frame should be kept vertical and should be tightly and firmly anchored to the anchor rods. During construction, it must be checked and adjusted at any time; (4) The pile segment must be kept vertical when positioned, so that the jack and the pile segment axis are on the same vertical line. A 30mm-40mm thick wooden board or multiple layers of burlap bags should be placed on the pile top, and a steel cap should be put on before pile pressing; (5) Pile pressing construction must not be interrupted midway, and should be pressed to the design elevation and required pressure value in one go, generally following this double control requirement; (6) The pressure gauge should be calibrated and qualified by a qualified unit, and a recent test qualification certificate should be provided before pile pressing; (7) The construction unit should promptly provide construction records such as pile length and pressing force.

 

　　3. Anchor welding pile connection. When the pile bears horizontal force or pull-out force, welded joints should be used; when the pile only bears vertical pressure, sulfur mortar joints can be used. (1) The anchor bars for pile connection should be cleaned and straightened in advance. The length of anchor bars, depth of anchor bar holes, and platform position should be checked to meet design requirements before pile connection; (2) After cleaning, the anchor bar holes should be dry, free of impurities and contamination. It is strictly forbidden to cut the anchor bar length due to insufficient hole depth; (3) After the upper pile segment is positioned, insert the anchor bars into the anchor bar holes, check for correct alignment and uniform gaps, lift the pile 10cm, install the sulfur mortar clamp, and then pour sulfur mortar; (4) When connecting piles, the anchor bar holes should be filled with sulfur mortar first, and spread to a thickness of 10-20mm. The pouring time should not exceed 2 minutes, and the upper pile segment should be immediately lowered vertically; (5) When preparing sulfur mortar, the temperature should be strictly controlled within the range of 140℃-145℃, and the pouring temperature should not be lower than 140℃. Burnt sulfur mortar must not be used; (6) It is strictly forbidden to mix unmelted sulfur mortar particles, sand, gravel, fragments, or wood chips into the pile top surface; (7) After pouring sulfur mortar, the pause time for pile connection should be determined by tests according to the temperature during pile pressing, and should be more than 7 minutes before continuing pile pressing; (8) The mix proportion and main physical and mechanical performance indicators of sulfur mortar must meet the requirements of Appendix II of the "Technical Specification for Anchor Static Pressure Piles" (YBJ227-91).

 

　　4. Pile sealing and load bearing. (1) Before pile sealing, sampling inspection should be conducted to meet design requirements, and pile sealing can only be carried out after acceptance; (2) Before pile sealing, debris in the pile pressing hole must be cleaned up, accumulated water removed, and laitance on the hole wall and pile surface cleaned; (3) The sealing material must be C30 concrete mixed with micro-expansion early-strength admixture and compacted; (4) Anchor rods should be welded with cross reinforcement to strengthen the anchoring capacity of the seal, so that the pile and cap form a whole. The welding length for double-sided welding is ≥5d (d is the diameter of the cross reinforcement); (5) Pile sealing work should be carried out symmetrically and evenly.

 

 

　　After the completion of anchor static pressure piles, three static load test piles conducted by the testing center of City A all met the design requirements. The construction unit invited relevant personnel from design, construction, and quality supervision stations to organize appraisal and acceptance. It was unanimously agreed that the reinforcement had reached the expected requirements. After pile acceptance, the precast pile heads were chiseled open, concrete fragments and other debris in the holes were removed, the walls of the reserved pile holes were cleaned, and the positions of pile head reinforcement were adjusted. Then, dry hard concrete was poured in one go and vibrated to seal. The cumulative settlement measured at five settlement observation points of the building was within the range of 4-7mm, all meeting the requirements of national standards. The six-story frame structure project in the new industrial zone of City A adopted anchor static pressure piles for reinforcement and achieved good results, receiving unanimous praise.