Pile-Sinking Mechanism of Jacked Piles

　　During pile-sinking construction, when the pile tip “penetrates” into the soil, the initial stress state of the undisturbed soil is disturbed, causing compression deformation of the soil below the pile tip. The soil generates corresponding resistance to the pile tip. As the pile penetration pressure increases, when the stress on the soil at the pile tip exceeds its shear strength, the soil undergoes rapid deformation and reaches ultimate failure. The soil then experiences plastic flow (cohesive soil) or densification, lateral displacement, and downward drag (sandy soil). At the ground surface, cohesive soil may heave upward, while sandy soil may be dragged downward. At depth, due to the pressure of the overlying soil layers, the soil mainly squeezes horizontally around the pile, completely destroying the soil structure adjacent to the pile. The large radial pressure also causes significant disturbance to the soil near the pile. At this time, the pile shaft is inevitably subjected to the resistance of pile-side friction and pile-tip resistance caused by the strong normal resistance of the soil. When the static pressure at the pile top exceeds these resistances during pile sinking, the pile will continue to “penetrate” and sink. Otherwise, it will stop sinking.

　　During pile pressing, the foundation soil is strongly disturbed, and the actual shear strength of the soil around the pile differs greatly from the static shear strength of the foundation soil. As the pile sinks, relative shear displacement occurs between the pile and the surrounding soil. Due to the shear strength of the soil and the adhesion between pile and soil, the soil generates friction on the pile surface. When the soil around the pile is relatively hard, the shear plane occurs at the pile-soil interface; when the soil around the pile is relatively soft, the shear plane generally occurs in the soil adjacent to the pile surface. In cohesive soil, as the pile sinks, the shear strength of the soil around the pile gradually decreases until it drops to the remolded strength. In sandy soil, except for loose sand, the shear strength changes little. The pile-side friction exerted by each soil layer on the pile is not a constant value but a variable value that significantly decreases as the pile continues to sink. The friction in the lower part of the pile plays a significant role in the pile-sinking resistance, accounting for 50–80% of the pile-sinking resistance. It is proportional to the strength of the soil around the pile and inversely proportional to the penetration depth of the pile.

　　In cohesive soil, the compressive strength of the soil at the pile tip significantly decreases under the effects of disturbance, remolding, and excess pore water pressure. In sandy soil, dense sand experiences a reduction in compressive strength due to relaxation effects, while loose sand experiences an increase in compressive strength due to densification effects. In layered foundations, the pile tip resistance in hard soil is also affected by the clay layer at the interface. When the overlying layer is soft soil, within the critical depth, the pile tip resistance increases with the penetration depth into the hard soil. When the underlying layer is soft soil, within the critical thickness, the pile tip resistance decreases with increased penetration into the hard soil.

　　Generally, the pile friction resistance is divided into three zones from top to bottom: the upper cavity zone, the middle slip zone, and the lower compression zone. During construction, the duration of interruption due to pile splicing or other factors has no significant effect on the continued sinking of the pile tip resistance, but it greatly affects the increase in pile-side friction. The increase in pile-side friction is proportional to the interruption time and related to the characteristics of the foundation soil layers. Therefore, in static pile sinking, the structure and position of pile splicing should be reasonably designed to avoid stopping the pile tip in hard soil layers for pile splicing.

　　Final Pressure and Ultimate Bearing Capacity

　　After the completion of jacked pile construction, the pore water pressure in the soil begins to dissipate, the soil consolidates and its strength gradually recovers, the upper pile cavity zone is filled, the middle pile slip zone disappears, and the pressure in the lower pile compression zone decreases. Only then does the pile begin to acquire ultimate bearing capacity in the engineering sense. From extensive engineering practice, the final ultimate bearing capacity of longer jacked piles in cohesive soil is greater than the final pressure during pile pressing. In some soft soil areas with high soil consolidation coefficients, the final vertical ultimate bearing capacity of a single jacked pile can be one to two times higher than the final pressure value. However, for short piles in cohesive soil, although the friction resistance increases after a period of strength recovery, the proportion of side friction in the ultimate bearing capacity of the pile does not differ much due to the short pile length, and the final ultimate bearing capacity may not reach the final pressure of the pile. Therefore, the final pressure and ultimate bearing capacity of a pile are two different concepts. Some designers and constructors new to jacked piles often confuse the two. Their numerical values are not necessarily equal, mainly depending on the pile length, the properties of the soil around the pile, and the soil at the pile tip, but they are also related. Shantou City has summarized local experience and proposed its own approach: for projects with high design bearing capacity, the final pressure value should preferably reach 1.5–1.7 times the design value, and repeated pressing should be considered depending on soil conditions and pile layout; for medium-length piles of 14–21 m, the final pressure should be controlled at more than 1.7–2 times the design value, and repeated pressing should be performed 3 times; for short piles less than 14 m, the final pressure should be controlled at more than 2–2.5 times the design value, and repeated pressing should be performed 3–5 times.

　　The compilation group of the Guangdong Province "Technical Specification for Jacked Pile Foundations" conducted statistical analysis of a large amount of pile foundation data and proposed an empirical formula for the relationship between the ultimate bearing capacity of a pile and the final pressure:

　　When L≤14m, Quk=αRsm = (0.6–0.85)Rsm

　　When 14m＜L＜21m, Quk=αRsm = (0.75–1.05)Rsm

　　When L≥21m, Quk=αRsm = (1.0–1.20)Rsm

　　Where: Quk—standard value of vertical ultimate bearing capacity of a single jacked pile

　　Rsm—final pressure value of the jacked pile

　　For longer piles and better soil conditions, the upper limit of the soil recovery coefficient α can be used; otherwise, the lower limit is used. This empirical formula has two applications: First, given the final pressure, pile length, and soil conditions, the ultimate bearing capacity of the jacked pile can be roughly estimated. For example, if the final pressure is 3600 kN, the pile length is 7 m, and the soil recovery coefficient is taken as 0.6, the estimated ultimate bearing capacity of the pile is Quk = 0.6 × 3600 = 2160 kN. Second, given the vertical ultimate bearing capacity of a single pile, pile length, and soil conditions, the final pressure value during pile pressing can be selected. For example, if the pile length is 7 m, the soil recovery coefficient is taken as 0.7, and the required ultimate bearing capacity of the pile is 2400 kN, then the final pressure value should be Rsm ≥ 2400/0.7 = 3430 kN, so a 3600 kN pile pressing machine can be selected for pressing.

　　Analysis and Treatment of Common Quality Accidents

　　Summarizing some cases of consultation and treatment of quality accidents at jacked pile sites by the Civil Engineering and Architecture Society in recent years, such as the Huajianyuan project in Xiaogang, Baiyun District, the residential building project of Sun Yat-sen University, the residential building project of the Agricultural Reclamation Company, the police dormitory in Xiaogang Middle, the Century Plaza commercial and residential building project, etc., some common problems can be identified.

　　1 Pile Shaft Uplift

　　Since jacked piles are displacement piles, when there are many piles in the site and the pile spacing is relatively close, the later pressed piles often squeeze and uplift the already pressed piles, especially for short piles, easily forming so-called "hanging-foot piles." When such piles are subjected to static load tests, the initial settlement is relatively large and the curve is steep, but when the pile tip reaches the bearing layer, the bearing capacity increases significantly and the settlement curve tends to flatten. This is a typical curve of pile shaft uplift. In addition to excessive static load settlement, pile shaft uplift may cause the joint to be pulled apart and the pile tip to become detached, while greatly increasing the horizontal squeezing force on surrounding piles, leading to pile inclination and deviation. In terms of treatment, the pile pressing sequence should be reasonably arranged before construction. For a single building, it is generally required to press the piles in the center of the site first, then the surrounding piles; press the piles with deeper bearing layers first, then the shallower ones. After pile shaft uplift occurs, repeated pressing is generally used to restore the pile foundation to normal service, but caution should be exercised for foundations subjected to horizontal loads.

　　2 Problems with Pre-drilling and Pile Pressing

　　To prevent excessive soil squeezing effects between piles, or when the soil is too hard and the pile shaft is short, the pre-drilling and pile pressing technique is often used in construction. That is, first drill a hole with a diameter slightly smaller than the pipe pile, with a depth of (2/3–1)L of the pile length, and then press the pipe pile along the pre-drilled hole. The pre-drilling should be followed by immediate pressing, and the interval should not be too long; otherwise, water accumulation in the hole may soften the soil at the pile tip, and after the water dissipates, a certain gap may remain at the bottom of the hole; secondly, the accumulated water may seep out along the outer wall of the pile, weakening the pile-side friction.
For pre-drilling and pile pressing in harder soil, there may also be a phenomenon where the pile tip cannot reach the bottom of the pre-drilled hole. After construction, water accumulation at the bottom of the hole softens the soil, causing the bearing capacity to fail to meet design requirements.

　　3 Improper Sealing of Pile End

　　When there is a gap at the pile tip, the pressure difference of groundwater head can cause water outside the pile to enter the inner cavity of the pipe pile through the gap. If the soil near the pile tip is muddy soil, it is easily softened by water, directly affecting the bearing capacity of the pile. For the welding quality of the pile shoe, it is required that there be no gap or misalignment between the pile shoe and the end plate, ensuring full weld seams without pores. Welding should be performed symmetrically, welding time should be properly controlled, and after welding is completed, it should be naturally cooled for about 10 minutes before driving, because high-temperature weld seams become brittle when encountering water and are prone to cracking. A relatively effective remedial technical measure in engineering is the "core-filling concrete" method, which involves immediately pouring about 1.2 m high C20 fine stone concrete to seal the bottom after the pipe pile is pressed. This prevents water leakage at the pile end, balances the water pressure near the pile end, allows the soil at the pile end to bear three-phase pressure, and maintains stable bearing capacity.

　　4 Cracking at Pile Top (Bottom)

　　As pile pressing machines become larger, with the heaviest reaching 6800 kN, for harder soil, the pipe pile may still not reach the design elevation. Under repeated pressing, strong transverse stress is generated in the pile shaft. If the pile is still reinforced with conventional stirrups, the concrete at the pile top may crack due to insufficient tensile strength, producing vertical cracks, which brings great difficulty to treatment. Another situation is when the pipe pile suddenly enters a hard bearing layer from a soft soil layer without a transition layer, the pile machine oil pressure rises rapidly, and the instantaneous impact force on the pile shaft can also easily cause cracking at the pile top. If the hard bearing layer is uneven, the pile shoe cannot penetrate the soil, causing the pile head to break and crush, and the pile machine oil pressure drops again. When pressing again, the pressure is unstable, and the measured pile length by suspension line is found to be shorter than the embedded part. In terms of treatment, the pile tip form should be improved in advance (conical pile tips are easy to slip), and afterwards pressure grouting should be used to bond the broken concrete at the pile bottom, with appropriate reduction of the design bearing capacity.

　　5 Geological Fault Zones

　　Many areas in Guangzhou urban districts are located in geological fault fracture zones, such as the Luhu fault zone and the Shougouling fault zone. When jacked piles are used in these sites, due to the influence of tectonic faults, the stratum structure is altered, and the fracture zones, as groundwater channels, often soften the bearing layer. Although the final pressure and pile length requirements are met during pile pressing, the pile may fail in static load tests. The range of failed pile lengths can occur from 8 m to 30 m, completely inconsistent with the empirical formula statistically derived from the specification. In the Shougouling fault zone, there was a case where pressing a pile to 80 m still could not stop, indicating that due to the broken soil and water lubrication, the shear strength of the soil is basically lost, and the pressure no longer increases with pile length. This requires special attention. For cases with soft and hard interlayers, especially when the hard interlayer is not thick, during construction the pile tip reaches the hard interlayer. Due to the reverse effect of excess pore pressure, the final pressure of the pile meets the design requirements. However, after construction, as the pore pressure dissipates, the shear strength of the soil has not yet recovered. During static load testing, the soil at the pile tip bears greater pressure, which is transmitted to the soft underlying layer, causing increased compression of that layer, and consequently increased settlement at the pile top, failing to meet displacement requirements.

　　6 Foundation Pit Excavation

　　As jacked piles are gradually used in high-rise buildings, foundation pit excavation is inevitable. Whether it is necessary to first support and excavate before pile sinking should be considered based on the excavation depth. Excavating while driving piles is not advisable. If piles are driven first and then excavated, symmetrical and uniform excavation should be considered. For example, excavating in the middle and piling soil around the edges will cause a huge difference in soil height between the surroundings and the center. At the same time, excess pore water pressure and vibration will cause the pipe piles to tilt or break. Therefore, a reasonable foundation pit excavation plan is essential.

　　Conclusion

　　The pile-sinking mechanism of jacked piles is very complex and is related to soil properties, soil layer arrangement, hard soil layer thickness, number of piles, pile spacing, construction sequence, progress, etc., and requires further research. Problems encountered during jacked pile construction are also varied. The most commonly used treatment method is to increase the final pressure and perform repeated pressing. Often, after static load tests reveal that a pile is unqualified, additional static load tests or large-strain tests are required to determine the distribution of unqualified piles in a larger area. Sometimes the foundation pit has already been excavated, the pile heads have been cut off and their positions are difficult to determine, the pile pressing machine has been withdrawn from the site, and repeated pressing or supplementary piling is difficult. In such cases, other measures must be taken to treat unqualified piles, such as grouting reinforcement, reducing the pile bearing capacity standard, or enlarging the pile cap. It is believed that with the continuous enrichment of engineering practice, more material will be provided for the formulation of jacked pile specifications.