I. Concept of Pile Cap

Pile cap: refers to the reinforced concrete platform set at the top of foundation piles to bear and distribute the loads transmitted from the pier body and connect the tops of each pile.

As a structure that transfers loads from top to bottom, the pile cap is often large in size, meaning a large amount of reinforced concrete, and usually involves construction measures for mass concrete.

II. Construction Process Flow of Pile Cap

III. Excavation of Pile Cap Foundation Pit

1. Excavation method

Generally, mechanical excavation (backhoe excavator) is used, supplemented by manual bottom cleaning and leveling. Usually, mechanical excavation is carried out to 20 cm above the base, and the remaining soil is removed manually to prevent disturbance of the foundation.

2. Excavation dimensions

Determined based on the pile cap dimensions, formwork and operation requirements, the need for drainage ditches and sumps, and slope protection requirements. Generally, the pit bottom is 0.5~1 m larger than the pile cap dimensions.
3. Excavation slope

The excavation slope should be based on the principle of ensuring slope stability, determined according to geological conditions, excavation depth, and specific site conditions, generally around 1:0.33-1:1.

4. Foundation pit with supported walls

5. Foundation pit drainage

1) Sump pit drainage method. Applicable in general cases except for severe quicksand.

2) Wellpoint drainage method. When the soil is poor with severe quicksand, groundwater level is high, excavation is deep, and the pit walls are difficult to stabilize, and ordinary drainage methods are difficult to solve, the wellpoint drainage method can be used.

3) Other drainage methods. For foundation pits with large soil permeability and deep excavation, sheet pile method or caisson method can be used.

6. Treatment of pile cap bottom

IV. Preparation before Pile Cap Construction

1. Construction of pile cap cushion layer

2. Construction of breaking pile heads

3. Pile testing: Invite a testing unit to conduct pile foundation inspection. The next process can only proceed after the testing unit issues a qualified test report.
4. Measurement and setting out: After pile testing is completed, surveyors shall set out the cross centerlines of the foundation based on the reference stakes, and set out the foundation formwork edge lines, as well as the pier column centerline, to ensure accurate positioning of the pile cap reinforcement.
5. Straightening pile foundation reinforcement

V. Pile Cap Reinforcement Construction

1. Raw materials

2. Reinforcement processing and installation
3. Reinforcement joints
4. On-site actual measurement items for reinforcement construction: 1) The number of joints in the same cross-section of load-bearing reinforcement, lap length, and quality of welded and mechanical joints shall meet the requirements of construction technical specifications.
2) During reinforcement installation, the number of reinforcement bars required by the design must be guaranteed. Load-bearing reinforcement must be straight and free from cracks and other damage on the surface.
3) Actual measurement: spacing of load-bearing reinforcement, spacing of stirrups and transverse horizontal reinforcement and spiral reinforcement, dimensions of reinforcement cage, thickness of protective layer, and position of embedded parts.

VI. Pile Cap Formwork Construction

1. Formwork type

2. Release agent: Use high-quality 30# machine oil (or salad oil). Before formwork installation, clean the formwork surface and evenly apply oil with cotton thread.
3. Formwork installation: After formwork installation, check its plane position, top elevation, node connections, and longitudinal and transverse stability. After self-inspection is qualified, invite supervision for inspection and acceptance. Concrete can be poured only after signing.

VII. Concrete Pouring Construction

1. Raw materials

2. Concrete pouring

3. Mass concrete 

VIII. Concrete Curing and Formwork Removal

1. Concrete curing

2. Formwork removal

After the concrete strength reaches above 2.5 MPa, with the supervisor's approval, the formwork can be removed. When removing formwork, first remove the bottom supports, then loosen the top tie bolts, fix the crane hook to the formwork lifting rings, remove the diagonal braces, evacuate all personnel from the trench, manually pry the formwork slowly with a wooden bar at the top of the pile cap to separate the formwork from the pile cap, under the unified command of a signalman, the crane slowly lifts and gradually moves the formwork away from the pile cap to prevent collision.
During formwork removal, the temperature difference between concrete and environment shall not exceed 15℃. When the temperature difference is 10-15℃, temporary covering measures shall be taken for the concrete surface after formwork removal.
For concrete cured by external heat sources, when the ambient temperature is below 0℃, the formwork can only be removed after the concrete has cooled to below 5℃.

IX. Earth Backfill

After formwork removal of the pile cap, clean the foundation trench promptly, remove all debris from the trench, and backfill the pile cap foundation trench only after passing supervision inspection.