Raft foundation, abbreviated as raft, is a common application form in foundation engineering. Especially in modern times, with the massive construction of high-rise buildings, the requirements for foundation bearing capacity are increasingly high. Raft foundation, with its characteristics of large load-bearing capacity, strong resistance to settlement, and good overall stability, has received attention and development, becoming a highlight of deep foundation engineering.
　　Raft foundation, as the name implies, is a foundation that itself looks like a whole raft slab. It is composed of a bottom slab, beams, etc., commonly known as "full foundation". Generally, raft foundations have relatively shallow embedment depth, and sometimes no embedment is required. After the concrete pouring of the bottom slab is completed, only watering for curing for no less than two weeks is needed. The entire construction is relatively simple, and the raft foundation does not damage the site soil, so it is more green and natural. The underground space created by the raft foundation has no excessive internal wall partitions, is spacious, and can be fully utilized, such as for parking lots, supermarkets, etc. This is another reason why raft foundations are favored.
　　The reason why raft foundations have good integrity and strong bearing capacity, and can effectively prevent uneven settlement of local structures, is that the entire large bottom slab and beam frame are constructed of steel bars and poured with concrete as an integrated structure. To ensure the perfect realization of the engineering design, it is necessary to strictly follow the operation technology and strictly control the construction quality during construction. 　　I. Preparation before construction
　　1. Material preparation. Mainly cement, sand and gravel, water reducer, early strength agent, steel bars, and cushion blocks needed for construction.
　　2. Mechanical equipment and tools. Equipment mainly includes: mixer, belt conveyor, immersion vibrator, plate vibrator, dump truck, motorized dump truck, mixer truck, and concrete pump truck (for pumped concrete), etc. Tools include: large and small flat shovels, tremie, chute, rubber hose, concrete discharge chute, hopper, hand-pushed rubber wheelbarrow, trowel, etc.
　　3. Working conditions. The construction organization design or construction plan has been prepared, including earthwork excavation, foundation treatment, deep foundation pit dewatering and support, formwork and concrete pouring procedures and methods, and protection of adjacent buildings, etc.
　　The soil conditions and elevation of the foundation base, and the dimensions of the foundation axis have been identified and inspected, and the concealed inspection procedures have been completed.
　　The formwork has been inspected, meets the design requirements, and the pre-inspection procedures have been completed.
　　Mark or snap the concrete pouring height marks on the trench side, wall surface, or formwork, and drive leveling stakes at intervals of about 3m.
　　The steel bars, bolts, embedded parts, heating and sanitation, electrical and other pipelines embedded in the foundation have been installed, signed by all disciplines, accepted by the quality inspection department, and the concealed inspection procedures have been completed.
　　The concrete mix proportion has been determined by the laboratory and adjusted and reviewed according to on-site materials; the background scale has been checked; and the pre-pouring briefing has been conducted, and test molds are ready.
　　The temporary water and power supply lines for construction have been set up, and the construction machinery and equipment have been installed in place and tested for normal operation.
　　The concrete pouring procedure, method, and quality requirements have been technically briefed in detail at all levels.
　　4. Determine the construction method to be adopted. There are two methods:
　　(1) First tie the steel bars of the bottom slab and beams and the upper column dowel bars on the cushion layer, pour the bottom slab concrete first, and after reaching a strength of more than 25%, support the beam side formwork on the bottom slab, and pour the beam concrete.
　　(2) Adopt the method of supporting the bottom slab and beam steel bars and formwork at the same time, support the beam side formwork with concrete buttresses or steel legs, and fix them firmly, and pour the concrete continuously in one go. However, both methods should ensure the correct position of the beams and column dowel bars, and the concrete should be poured continuously in one go. 　　II. Construction process
　　1. Pit excavation: if there is groundwater, artificial dewatering should be used to lower the groundwater level to 50 cm below the pit bottom, so that earthwork excavation and foundation structure construction can be carried out in dry conditions.
　　2. During pit earthwork excavation, care should be taken to maintain the original structure of the pit bottom. If mechanical excavation is used, the soil layer 20-40 cm above the pit bottom should be removed manually to avoid over-excavation or damage to the foundation soil. If there is local soft soil or over-excavation, replacement and compaction should be carried out. Pit excavation should be continuous. If the next process cannot be carried out immediately after the pit is excavated, a layer of 150-200 mm above the base should be left unexcavated, and excavated to the design pit bottom elevation when the next process is constructed, to avoid disturbance of the foundation soil.
　　3. When the length of the raft foundation is very long (over 40 m), consideration should be given to leaving a continuous post-pouring strip at an appropriate position in the middle to avoid temperature shrinkage cracks and facilitate segmented flow construction; for super-thick raft foundations, measures should be taken to reduce the heat of hydration of cement and the pouring temperature into the mold, to avoid excessive temperature shrinkage stress causing cracks in the foundation bottom slab.
　　4. Before concrete pouring, remove silt and debris from the foundation or cushion layer, and there should be no standing water in the pit; wooden forms should be watered and wetted, and board seams and holes should be sealed.
　　5. When the pouring height exceeds 2 m, use tremie or chute (pipe) to prevent segregation. Concrete should be poured in layers continuously, with each layer thickness of 250-300 mm.
　　6. During concrete pouring, always observe whether the formwork, steel bars, embedded iron parts, reserved holes, and pipes are displaced. If deformation or displacement is found, stop pouring, and after handling before the initial setting of concrete, continue pouring.
　　7. After the concrete is poured and vibrated densely, smooth with a wooden trowel or trowel with an iron trowel.
　　8. After the foundation pouring is completed, the surface should be covered and watered for curing for no less than 7 days. If necessary, insulation curing measures should be taken, and the foundation should be prevented from being soaked.
　　9. Set settlement observation points on the foundation bottom slab, observe and analyze regularly, and keep records.
　　III. Quality standards
　　1. The cement, water, aggregate, admixtures, etc. used in concrete must comply with construction specifications and relevant regulations.
　　2. The concrete mix proportion, raw material measurement, mixing, curing, and construction joint treatment must comply with the provisions of the construction specifications.
　　3. The test blocks for evaluating concrete strength must be sampled, made, cured, and tested according to regulations, and their strength must meet the design requirements and evaluation standards.
　　4. The specification, shape, size, quantity, anchorage length, and joint setting of steel bars in the foundation must comply with the design requirements and construction specifications.
　　5. Concrete should be vibrated densely, with honeycomb area not greater than 400 cm2 and hole area not greater than 100 cm2.
　　6. Check in detail against the allowable deviation and inspection methods of the foundation.
　　IV. Protection after completion
　　1. Formwork removal should be carried out only when the concrete strength can ensure that its surface and edges and corners will not be damaged.
　　2. Only when the poured concrete strength reaches more than 1.2 MPa can pedestrians walk on it or the next process be constructed.
　　3. During construction, properly protect the heating and sanitation, electrical, concealed pipes, and door openings, etc., and do not collide.
　　4. Reserved holes, embedded bolts, and iron parts in the foundation should be set according to design requirements, and concrete shall not be chiseled afterwards.
　　5. If the foundation embedment depth exceeds that of adjacent buildings (structures), proper protection measures should be taken. 　　V. Safety measures during construction
　　1. During foundation construction, first check the soil quality and slope gradient of the foundation pit and trench sides. If cracks, sliding, etc. are found, reinforce in time. Stacked materials should be kept at least 1 m away from the pit edge. Ladders or ramps should be provided for deep foundation pits, and it is forbidden to step on formwork or supports.
　　2. For raft foundation pouring, set up sturdy scaffolding platforms and runways, and lay scaffold boards tightly to prevent stones from falling; when using handcarts, motorized dump trucks, hoppers, etc. for pouring, there should be dedicated personnel for unified command, scheduling, and feeding to ensure no collision accidents; when using tremie for feeding, prevent blockage to avoid unhooking accidents; for pumped concrete pouring, take measures to prevent blockage and pipe burst.
　　3. Operators of vibrators must wear rubber shoes, connect electricity safely and reliably, and set up dedicated protective grounding wires to avoid the danger of live wires leaking electricity. In case of failure, cut off the power immediately for repair; if the wire is worn, replace it in time.
　　4. Construction personnel should wear safety helmets and soft-soled shoes; tools should be put in tool bags; when transporting concrete and passing objects to the pit, do not throw.
　　5. In rain, snow, and freezing weather, anti-slip measures should be taken on the scaffolding, and only after cleaning ice, frost, and snow before construction can one go on the scaffolding; work should be stopped in winds above level 5.
　　6. On-site mechanical equipment and electric tools should be equipped with leakage protectors, each machine should be set separately, not shared, to ensure electrical safety; for night construction, sufficient lighting should be installed.
　　VI. Precautions during construction
　　1. Concrete should be poured in layers and vibrated densely in layers to prevent honeycomb and pitted surfaces and incomplete compaction; at the root of the hanging formwork (formwork, slab), after the bottom slab under the beam is poured, stop for 0.5-1.0 h, and after settlement, pour the upper beam to avoid the phenomenon of rotten neck at the root.
　　2. During concrete pouring and vibrating, prevent cushion blocks from displacement, steel bars from being close to the formwork, or insufficient vibration causing exposed steel bars.
　　3. To strictly maintain the correct elevation of the concrete surface, pay attention to avoid movement of leveling stakes, or excessive or insufficient paving thickness; carefully level during operation, and support the formwork firmly.
　　4. For pouring thicker raft slabs, take measures to prevent temperature shrinkage cracks, and strengthen curing to prevent cracks.