Cap construction.
1) Construction process.
The cushion cap construction process is shown in the figure.
2) Construction method.
In the section with good geological conditions and low groundwater level, the foundation pit of the bearing platform adopts slope excavation. The excavation of the bearing cap on the slope of the existing road subgrade depends on the size of the slope earth pressure to select steel sheet pile protection or bored pile row pile protection, and take necessary measures.
1) Foundation pit excavation: the foundation pit is excavated by excavator, manual construction, when the excavation to 200mm away from the base, stop mechanical excavation, change to manual bottom cleaning, to ensure that the base is not disturbed. And when there is water seepage in the foundation pit, the surrounding drainage ditch and collection pit should be done outside the foundation in the foundation pit, and the water should be pumped in time to prevent the foundation pit from being soaked in water.
2) chisel out the over-filled pile head concrete that extends into the cushion cap: the over-filled pile head concrete that extends into the cushion cap adopts the circumferential cutting method.
First of all, the elevation of the excavated foundation pit is measured, and the design pile top elevation and circumferential tangent cutting line are marked on the pile body (identified with red paint).
Pile head circumferential cutting: use a cutting machine to cut a 4cm incision along the red paint mark of the pile body to prevent the pile head from destroying the pile edge when chiseling, and strictly control the cutting depth during cutting to avoid cutting the steel bar.
Strip out the steel bar: use manual or air picks to chisel out the hole pile reinforcement above the pile top, but it is strictly forbidden to bend the main reinforcement of the pile head locally to cause damage and cannot be corrected, and chisel off part of the concrete protective layer of the pile above the cutting line, strip out the main reinforcement of the reinforcement, so that the reinforcement is separated from the pile and slightly bent to the outside.
Chisel out the fracture ring: Punch the hole symmetrically at the ring tangent line at the bottom of the pile above the cutting line, and drive it into as symmetrically and horizontally as possible to ensure that the fracture surface remains in the same horizontal plane.
Pile head lifting: After the pile head is broken after the steel drill is driven along the cutting line, the pile head is pulled out with a crane, and the lifting is as light as possible to avoid damage to the steel bar.
Manual refurbishment: After the pile head is pulled out, there will be some uneven places on the fracture surface, which need to be manually chiseled, and the excess part will be chiseled off and polished according to the design elevation.
3) Rebar binding: The steel bar production is carried out in the steel bar processing plant, and then the steel bar made and formed is transported to the site for binding. There are two methods of steel bar binding: manual and electric welding. Special attention should be paid to the welding of pile reinforcement and cushion cap reinforcement. Because the cushion cap is a one-time pouring, it is necessary to bind the pier body reinforcement or pier body stubble reinforcement according to the design drawing.
4) Formwork installation: The cushion cap formwork adopts a composite steel formwork, which is fixed with tie bolts and auxiliary supports to ensure sufficient strength, stiffness and flatness. After the template is assembled in blocks at the assembly site, it is transported to the site by car, lifted and installed by truck crane, the formwork is tightly jointed, the surface is smooth, the support is firm, and the surface is coated with release agent. After the formwork is installed in place, measure its center position and verticality. The used formwork should be repaired to meet the flatness requirements before it can be reused.
5) pouring concrete: the cushion cap concrete is carried out according to the large-volume concrete construction technology, and the concrete is provided by the concrete centralized mixing station of the bid section, and the concrete mixer truck arrives at the pouring site, and the concrete is pumped into the mold, poured in layers, and vibrated in layers. The plug-in vibrator is used to vibrate, and the vibrator can be slowly withdrawn when the concrete is no longer sinking, the surface begins to be slurry, the bubbles no longer appear, and the surface is shiny. The two-stage reinforced concrete cushion cap adopts a step-by-step vertical mold pouring and health-keeping, and the health maintenance time of each stage is not less than 7 days, and the top-level concrete will cover all the foundation after sprinkling health.
3) Foundation pit backfilling.
When the concrete of the cushion cap is poured and reaches the condition of demoulding, the formwork is removed in time and the backfill is carried out, the foundation pit backfill must be carried out symmetrically, the filler meets the requirements of the design and specification, the vibration rammer and small roller are adopted, the backfill height is lower than the top surface of the cushion cap 10cm is appropriate, and the entire foundation pit is completed after the pier body construction is completed, so as to ensure that the thickness of the foundation covering soil is not less than 30cm.