Quality control of cup-shaped foundations

A cup-shaped foundation is a type of independent foundation. When the upper structure of a building is supported by a frame structure, or by single-story rack or gantry structures, square or rectangular independent foundations are commonly used; such foundations are referred to as independent foundations or columnar foundations. The independent foundation is the basic form of foundation under columns; when columns are made from precast components, the foundation is designed in a cup-shaped format, and the column is inserted into and secured within this cup shape, which is why it is called a cup-shaped foundation. (1) Construction procedure: Site marking and setting out → Manual excavation → Inspection of the soil quality in the foundation pit → Pouring of the concrete cushion → Installing rebar for the foundation pad → Setting up formwork → Inspection of the rebar → Pouring of concrete → Setting up formwork for the bottom of the ground beam → Installing rebar for the ground beam → Setting up side formwork for the ground beam → Concrete pouring → Manual curing → Construction of the local foundation masonry → Inspection → Layered backfilling of soil. (II) Construction of the cap foundation: 1. For the construction of the cup-shaped foundation base, please refer to the sections on formwork and concrete works below. 2. Cast-in-place cap foundation: After digging test trenches, a C10 concrete cushion is poured using brick formwork. Then, the axis lines are marked on the surface of the cushion using the positioning stakes; the forms for the column base caps and beams as well as the boundaries of the columns are accurately positioned. Lines are drawn at the intervals specified in the design to divide the area into sections, and the spacing between the reinforcement holes in the cap bottom plate is strictly controlled, with the reinforcement being tied together in accordance with construction standards. When binding the reinforcement for KJ columns and frame columns, a stirrup must be used at the bottom to secure them; once in place, the ends of the column reinforcements should be firmly welded to the reinforcement on the base slab. The column members are arranged in rows horizontally and columns vertically according to the project’s axis, forming a network-like structure that is securely fixed in place using a full-scale steel pipe scaffold, thereby preventing the column reinforcements from shifting due to vibration. The cap foundation uses 18 mm thick nine-ply plywood, 50×100 mm wooden beams, and stirrups at intervals of @500 mm; it is securely supported. The concrete used has a grade of C30 and is mixed on-site, then delivered to the work area using two-wheel dump trucks to ensure proper strength and workability of the concrete. An insert vibrator is used for vibration, with each layer having a thickness of 300 mm. Within the cap, vibration is carried out first in the corners and then in the center, ensuring thorough compaction; the construction is completed in one go in accordance with relevant standards. The construction joint for cast-in-place columns is left on the surface of the cap. After the final setting of the cap foundation, it should be kept moistened with water for no less than 14 days. (III) Earthback filling: Earthback filling is carried out after the structure has passed the acceptance inspection and the concrete strength has reached the specified requirements. 1. Based on the characteristics of the project, the type of filler, the designed compaction coefficient, and the construction conditions, the moisture content range for the filler is reasonably set at 8%, while the thickness of the soil layer applied is 300 mm. As for the number of compaction passes, a frog rammer is generally used to compact each layer at least three times; when compacting in layers, it is required that one full compaction be followed by half a compaction. 2. Remove standing water and organic impurities from the foundation pit. 3. The moisture content of the fill soil must be checked before backfilling ； If it is too high, measures such as loosening the soil, drying it evenly, mixing in dry soil, or replacing the soil can be taken; if it is too low, pre-wetting the soil with water can be employed. The so-called “water ramming” method of using water to settle the soil is strictly prohibited. 4. The upper and lower water pipes must be installed firmly. When backfilling the area around the pipes and the trench, in order to prevent displacement of the pipe’s centerline, soil should be manually filled around the pipes and compacted; this work should be carried out on both sides simultaneously. Only after a layer of soil 0.5 meters or more has been piled up above the top of the pipes can a rammer be used for compaction, as long as this does not damage the pipes. At the splice joints, as well as around the anti-corrosion insulation layer or cables, fine-grained material should be used for backfilling. 5. After the soil backfilling is completed, the ring cutter method is used to take samples in layers in order to determine that the density of the backfilled soil is not less than 90%.