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E-mail: [email protected] Whatsapp: +8613647327093 Tel: +86-731-8403-0163
Impact Hammer Piling is one of the most traditional and widely used deep foundation construction methods. It utilizes the impact energy generated by the free fall or forced descent of a heavy hammer to drive piles (typically precast piles) into the ground to the designed depth.
Preventing pile fracture during impact hammer piling is a crucial core aspect of pile foundation engineering construction. It is not only related to the project's quality, safety, and progress but also directly affects the overall project cost and long-term reliability. This article will elaborate on precautions to prevent pile fracture from three aspects: pile quality and preparation, construction equipment and process control, and geological conditions and investigation.
This is the foundation for preventing fracture. If the pile itself has defects, even the best construction techniques can hardly avoid problems.
1. Pile Concrete Strength and Quality:
1) Adequate Strength: Ensure the concrete strength of the precast pile meets the design grade (usually requiring 100% of the design strength before driving).
2) Manufacturing Quality: Prevent defects such as honeycombs, surface pitting, and cracks. Key parts like the pile head and tip should be reinforced to avoid local crushing.
3) Reinforcement Configuration: Ensure the dimensions, specifications, quantity, and spacing of the reinforcement cage comply with the design requirements. Especially, the densification of spiral stirrups at critical sections like the pile head and tip can effectively confine the concrete and improve shear and tensile capacity.
4) Curing: Sufficient curing is key to ensuring concrete strength and durability, preventing strength reduction due to shrinkage cracks.
2. Pile Manufacturing and Transportation:
1) Pile Straightness: The pile must be straight, without initial bending. Otherwise, additional bending moments will be generated during hammering, making it highly prone to fracture.
2) Pile Head and Tip Quality:
Pile Head: This is the part that directly bears the hammer impact. It must be flat and sturdy. The internal reinforcement mesh at the pile head (pile head mesh) is crucial, effectively preventing the pile head from being shattered. If the pile head is damaged, it should be repaired or cut flat before driving.
Pile Tip: Select the appropriate pile tip form (e.g., open, closed, conical) based on geological conditions, ensuring its strength can penetrate the soil layers smoothly and guide the pile.
3) Careful Handling and Storage: During lifting, transportation, and on-site storage, reasonable lifting points (usually at designated points) must be used to avoid impact and dropping. The storage area should be level with correct support point locations. For multi-layer stacking, padding timber should be aligned vertically to prevent cracks in the pile body.
This is the key controllable aspect during the construction process.
1. Principle of "Heavy Hammer, Low Drop": This is the most important principle for preventing pile head crushing and pile body fracture.
Heavy Hammer: Select a pile hammer with sufficient weight, whose impact force is adequate to sink the pile but not excessive.
Low Drop: Use a relatively low drop height for hammering. A low drop produces an impact force with a longer duration and a more gradual change in magnitude, facilitating stress wave propagation downward and avoiding excessive compressive and tensile stresses. Conversely, a "light hammer, high drop" produces sharp stress peaks, which can easily damage the pile head or cause tensile cracking in the pile body.
2. Hammer Type Selection: Different hammer types like diesel hammers and hydraulic hammers have their own characteristics. Selection should be based on pile type, geology, and environmental requirements.
3. Pile Cap and Cushion Selection:
Function: The pile cap and cushion (typically made of elastic materials like wood planks, gunny sacks, cardboard, phenolic resin) are crucial "buffers".
Requirements: The pile cap must be aligned correctly and match the pile head. The cushion's thickness and hardness should be appropriate – it should both cushion and uniformly distribute the force, without being too soft and causing excessive energy loss. Check constantly during construction and replace cushions that are crushed or excessively compressed promptly.
4. Pile Verticality Control:
Alignment: The pile rig leader must be parallel to the pile body, ensuring the hammer, cap, and pile are on the same vertical line.
Verticality: Use two theodolites in two perpendicular directions to strictly correct the pile's verticality during initial placement and within the first 1-2 meters of driving. If inclination is detected, forcibly correcting it by dragging with the rig is strictly prohibited; the pile should be extracted and re-positioned.
5. Piling Sequence:
In soft soil foundations,an improper piling sequence may cause previously driven piles to be lifted by subsequently driven piles due to soil displacement, generating tensile stresses and leading to fracture. The sequence should follow "deep before shallow, long before short, large before small, center before periphery" or use a jumping sequence to reduce soil displacement effects.
1. Detailed Geotechnical Investigation:
A detailed geological report is essential before construction, understanding soil layer distribution, hard/soft interlayers (especially hard clay layers, gravel layers), boulders, bedrock surface undulations, etc.
2. Prevention of "Runaway Pile":
When the pile tip suddenly moves from a hard layer into a soft layer (e.g., penetrating a hard crust into a soft mud layer) or encounters a cavity, the pile may sink rapidly and suddenly, like a "free fall." At this moment, the pile body bears huge tensile stresses under its self-weight and residual hammering kinetic energy, leading to fracture.
Measures: In areas where such strata are expected, provide early warning, use even lower drop heights, and closely monitor changes in pile penetration. Once signs of runaway pile are detected, stop hammering immediately. Wait for the pile to stabilize before resuming with very low energy.
3. Handling "Hard Layers" and "Boulders":
When encountering hard interlayers or boulders, the immense resistance can cause severe bouncing of the pile, making it prone to fracture.
Measures: Do not force high-energy hammering. Slow the pace, use low-energy impacts, or consider pre-drilling, using steel shoe methods, etc., to assist penetration.
4. Final Set Criteria and Penetration Rate Control:
Penetration Rate: Refers to the sinking distance of the pile per series of final blows (typically 10 blows). It is an important indicator for judging whether the pile has reached the design bearing capacity.
Control: A reasonable final penetration rate needs to be set. If the penetration rate is already very small (e.g., less than 2mm/blow) but the pile hasn't reached the predetermined elevation, it may indicate the tip has encountered an obstacle or entered an extremely hard layer. In this case, stop hammering, analyze the cause, rather than blindly pursuing the elevation and continuing to drive, which could cause fatigue failure of the pile.
Continuous Monitoring: Throughout the driving process, continuously monitor the hammering sound, observe whether the pile exhibits abnormal bouncing or tilting, and monitor changes in penetration rate.
Problem Handling: Once cracking, severe inclination, or abnormal penetration rate of the pile is detected, stop hammering immediately. Analyze the cause jointly with the design, supervision, and other relevant units to determine a solution (such as adding supplemental piles, pressure grouting for reinforcement, etc.).
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