Pile foundation construction is the first link in the construction project. It is related to the stability and safety of the building. This article will discuss the basic knowledge of pile foundation, the construction method of prestressed pipe piles, the selection and use of pile driving equipment, the precautions for pile construction, and the key points of quality control of prestressed pipe piles.
Basic Knowledge of Pile Foundation
Definition and composition of pile foundation
Pile foundation is the most widely used form of deep foundation, mainly composed of piles sunk into the soil and caps or cap beams connected to the top of the piles. Its function is to transfer the load of the upper building to the deep soil layer with strong bearing capacity, or to compact the soft soil layer to improve the bearing capacity and density of the foundation soil.
Classification of Piles
Pile can be divided into end-bearing piles and friction piles according to the force conditions.
End-bearing piles pass through soft soil layers to reach hard soil layers or rock layers, and the upper structure load is mainly borne by the resistance of the rock layer; friction piles are completely set in soft soil layers, and the bearing capacity is improved by compacting the soil.
According to the construction method, they can be divided into precast piles and cast-in-place piles. Precast piles include driven piles, water-driven piles, vibration-driven piles and static pressure piles, while cast-in-place piles include bored cast-in-place piles and excavated cast-in-place piles.
(From: ASCE LBRARY)
Construction Method of Prestressed Pipe Piles
Scope of application of pipe piles
Prestressed high-strength concrete pipe piles (PHC) .
Prestressed concrete pipe piles (PC) are suitable for the foundation of industrial and civil buildings, and can also be used for the foundation design of railways, highways and bridges, ports, water conservancy, municipal engineering and other projects.
Prestressed concrete thin-walled pipe piles (PTC) are suitable for the foundation of industrial and civil buildings, but areas with seismic fortification intensity greater than 7 degrees need to be verified separately.
Main Construction Methods
The construction methods of pipe pile foundation include hammering, static pressure, hole driving (pressing), drilling and piling, and central excavation. The hammering method uses a diesel hammer or hydraulic hammer, and the static pressure method uses equipment such as a clamping hydraulic pile driver.
Selection and Use of Pile Driving Equipment
Setting of pile cap and cushion
The pile cap should have sufficient strength, rigidity and resistance to hitting, and should be made into a cylindrical shape. The depth of the cylinder used to cover the pile head should be 350 to 400mm, and the inner diameter should be 20 to 30mm larger than the outer diameter of the pipe pile. When piling, an elastic pad should be set between the pile cap and the pile head. The thickness of the pad should be uniform and the thickness after hammering and compaction should not be less than 120mm.
Pile driver and pad setting
The pile driver should be made into a cylindrical shape and should have sufficient strength, rigidity and resistance to hitting. The length of the pile driver should meet the requirements of pile driving concentration, and the upper and lower end surfaces should be flat and perpendicular to the central axis of the pile driver. The lower end face of the pile driver should be opened to connect the inner cavity of the pile with the outside world.
Precautions for Pile Driving
Use of pile hammer
The pile hammer must not be started before the pile hammer is completely placed on the pile head and all the required preparations and inspections are completed. The pile hammer must not be started when it is off the pile frame guide rail or parked on the pad to prevent accidents. To prevent eccentric pile driving, the axis line of the pile and the center line of the pile should be as close as possible to ensure that the pile head is evenly stressed.
Understanding of the soil quality at the construction site
It is necessary to understand the soil quality at the construction site to avoid the phenomenon of chasing hammers. After self-sinking pile delivery, do not rush to engage in gear construction. The upper piston should be raised to a certain height with the landing gear hook, so that it can fall freely and hit it in neutral with its own weight, and the operation is repeated. Then observe the penetration of the pile into the soil. When the penetration is less than 1.7m, the gear can be engaged.
Key points of Quality Control of Prestressed Piles
Quality control before construction
The finished piles should be inspected for appearance quality according to the selected standard drawing or design drawing. Inspection of welding rods, pile sinking and other materials and equipment for butt-jointing piles.
Quality Control During Construction
Depth of driving (static pressure), control standard of stopping hammer (pile sinking resistance) and verticality of pile body (frame). Quality of pile connection, interval time of pile connection and integrity of pile top. Number of hammer blows per meter of penetration (pile sinking resistance), number of hammer blows in the last 1m (pile sinking dynamic resistance), penetration of the last three arrays and pile tip elevation, etc.
Quality Control After Construction
Inspect the quality of pile body and welding quality after pile sinking through pile hole lighting. Pile position deviation, pile top elevation, penetration, low strain dynamic measurement, static load test.
Summary
The quality of pile foundation construction is directly related to the stability of the building. The preparatory work for the pile foundation construction needs to be done well. For example: choose the right pile type and piling equipment, etc.
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References:
Bauer Group Technical Manual: Advanced Piling Equipment Specifications.
