Driven Pile Construction
The foundation piles used for driven piles are mainly precast reinforced concrete piles and prestressed reinforced concrete piles. The commonly used cross-section forms are solid square piles and hollow tubular piles. Tubular piles (including ordinary and prestressed types) are generally manufactured in factories by the centrifugal molding method. The main construction methods for driven piles include: hammer-driven piling, vibratory piling, water-jetting piling, and static pressing piling. Here, the construction method of hammer-driven piling is introduced.
1. Overview
Hammer-driven piling is generally suitable for medium-dense sandy soil and cohesive soil. Since hammer-driven piling relies on the impact energy of the pile hammer to drive the pile into the soil, the pile diameter should generally not be too large (not greater than 0.6 m), and the penetration depth should be around 40 m; otherwise, higher requirements are placed on the piling equipment. Piling equipment is the key to the quality and success of pile foundation construction and should be selected according to soil conditions, project quantity, pile type, specifications, dimensions, construction period, on-site water and power supply, and other conditions.
2. Key Construction Points
(1) Before piling, the main equipment components such as the pile frame, pile hammer, and power machinery should be inspected; before starting the hammer, it should be checked again whether the pile hammer, pile cap, and the central axis of the follower and the pile are aligned; when hammer-driven piling begins, the kinetic energy of various pile hammers should be strictly controlled: when using a drop hammer and a single-acting air hammer, the lifting height of the hammer should not exceed 0.50 m; when using a double-acting air hammer, the air valve can be opened less to reduce air pressure and intake volume, thereby reducing the number of hammer blows per minute; when using a diesel hammer, the fuel supply can be controlled to reduce the hammering energy; when the pile tip has been driven to the design elevation but the penetration rate still does not meet the requirements, driving should continue until the required penetration rate is reached. During piling, if any of the following occurs: a sudden drastic change in penetration rate; sudden inclination or displacement of the pile body; the pile does not penetrate and the pile hammer rebounds severely; the pile top is broken or the pile body cracks or deforms; or serious ground heave occurs around the pile, hammering should be stopped immediately, the cause should be identified, and construction may continue only after measures have been taken.
(2) During piling, attention should be paid to the following: there should be a 5–10 mm clearance between the pile cap and the pile periphery, so that the pile can rotate slightly freely within the pile cap during hammering, avoiding torsional stress exceeding the allowable limit in the pile body; the guide rod of the pile driver should be fixed to stabilize the pile body during driving; the guide rod should be set to ensure free up-and-down movement of the pile hammer; the top surface of the precast pile should be provided with a pile cushion suitable for the size of the pile cap, with its thickness determined by the cushion material, pile length, and the resistance at the pile tip; the cushion should be replaced promptly after the pile edge is broken; the selected pile cap should distribute the hammer impact force evenly over the pile top surface.
3. Hammer-Stop Control Criteria for Hammer-Driven Piling
(1) When the design pile tip elevation is in hard plastic cohesive soil, gravelly soil, sandy soil above medium density, or weathered rock, based on changes in penetration and with reference to geological data, confirm that the pile tip has penetrated into that soil layer and the penetration rate has reached the control penetration rate.
(2) When the penetration rate has reached the control penetration rate but the pile tip elevation has not reached the design elevation, driving should continue for about 0.10 m (or 30–50 blows); if there is no abnormal change, hammering may be stopped; if the pile tip elevation is much higher than the design elevation, it should be reported to the relevant department for study and determination.
(3) When the design pile tip elevation is in general cohesive soil or other soft soil layers, elevation should be used as the control and penetration rate as a check. When the pile tip has reached the design elevation but the penetration rate is still relatively large, hammering should continue to bring it close to the control penetration rate.
(4) In the same pile foundation, the final penetration rates of all piles should be roughly similar, and the penetration depths should not differ too much, so as to avoid uneven settlement of the foundation. If soil conditions vary too greatly, causing excessive differences in penetration rate or penetration depth among piles, it should be reported to the relevant department for study and separate hammer-stop criteria should be formulated. For specially designed piles, when the design pile tip elevations differ (such as abutment piles of arch bridges), they should be handled according to the design requirements.
From the start of piling, the pile position and the verticality of vertical piles or the inclination of batter piles should be strictly controlled. During piling, methods such as pushing or pulling the pile head or pile body must not be used to correct deviation, so as to prevent cracking of the pile body and increase of additional bending moment in the pile body.