Keywords: hand-dug cast-in-place pile, construction, supervision, safety, quality, acceptance
Using hand-dug cast-in-place pile as foundation has the advantages of simple mechanical equipment, convenient construction operation, small occupation of construction site, little influence on surrounding buildings, reliable construction quality, comprehensive construction, shortened construction period and low cost. Therefore, it is widely used. Taking the city where the author lives as an example, most of the brick-concrete structures, bottom frames and frame buildings that have been built and under construction use manual digging piles, which is mainly caused by geological conditions and other factors in this area. Manual digging cast-in-place pile is suitable for clay, loam and clay layers with good soil quality and low groundwater level. It can be used as pile foundation in high-rise buildings, public buildings and hydraulic buildings, and has the functions of supporting, anti-sliding and retaining soil. Not suitable for soft soil, quicksand, soil layer with high groundwater level and large water inflow.
At present, the author briefly describes the key points of construction and quality control of hand-dug cast-in-place piles, and the key points of construction and quality acceptance put forward here should be the key to ensure the quality and safety of the whole project.
1, data collection:
Before the formal construction, the following engineering data are necessary:
1. 1 engineering geology of construction site and necessary water level geological data;
1.2 Summary of the blue prints.the design of pile foundation construction drawing;
1.3 data of underground pipelines (pipes and cables) in the construction site and adjacent areas;
1.4 technical performance data of main construction machinery and its supporting equipment;
1.5 pile foundation construction organization design or construction scheme;
1.6 quality inspection report of building materials (cement, sand, stone, steel bar) for pile foundation reinforced concrete;
2, quality management measures before construction:
2. 1 The pile location, number, construction sequence, water and electricity lines and temporary facilities shall be marked on the construction plan;
2.2 make construction operation plan and labor organization plan;
2.3 make the supply plan of mechanical equipment, tools and materials;
2.4 Formulate technical measures for seasonal (winter and rainy season) construction;
3. Safety measures:
The following safety measures should be taken for manually dug cast-in-place piles:
3. The1hole is equipped with an emergency ladder for personnel to go up and down the well; Construction personnel must wear safety helmets when entering the hole; The electric hoists and hanging cages used shall be safe and reliable, and equipped with automatic locking safety devices.
3.2 Before starting work every day, it is necessary to detect the toxic and harmful gases underground and take adequate safety measures. When the excavation depth of the pile hole exceeds 10m, special air supply equipment shall be provided to deliver clean air to the underground.
3.3 The excavated earthwork shall be transported away from the orifice in time, and shall not be piled up around the orifice 1m, and the passage of motor vehicles shall not affect the safety of the shaft wall.
3.4 All power supplies and circuits on the construction site must be installed and dismantled by certified electricians; Electrical appliances must be strictly grounded and connected to zero, and leakage protectors should be used. Never use more than one brake. Safety miner's lamp or safety lamp below 12V shall be used for lighting.
4. Construction and quality acceptance of bored piles:
4. 1 In order to check whether the geological data, testing equipment, technology and construction technical requirements are appropriate, the pile should be "drilled" before construction.
4.2 Before opening the hole, according to the survey datum and baseline of the construction unit, the center of the hole pile shall be released by cross method after review by the supervisor. The pile location should be accurately positioned and set out, and positioning gantry piles should be set outside the pile location. And appoint someone to be responsible.
4.3 When the clear distance between piles is less than 2 times the pile diameter and less than 2.5m, interval excavation shall be adopted.
4.4 The deviation between the center line of the first paragraph of retaining wall and the design axis shall not be greater than 20mm；; The top surface of the well circle should be higher than the site 150~200mm, and the wall thickness should be higher than the lower shaft wall 100~ 150mm.
4.5 The construction of reinforced concrete retaining wall with well circle shall ensure that the thickness of retaining wall, the strength of reinforcement and concrete meet the design requirements; The lap length of upper and lower retaining walls should not be less than 50mm；; Each retaining wall is completed on the same day; Retaining wall formwork shall be removed after 24h; When honeycomb and water leakage are found in the retaining wall, it should be reinforced in time to prevent accidents.
4.6 When digging to the design elevation, there should be no water at the bottom of the hole. After the final hole, the silt on the retaining wall, the residue at the bottom of the hole and water should be cleaned up, and then the concealed works should be accepted. After acceptance, the back cover and concrete of the cast-in-place pile body shall be closed immediately.
4.7 The allowable deviation of hole formation shall meet the following requirements: pile diameter 50mm, verticality 0.5%, and pile position 50 mm.. The enlarged part at the bottom should be dug into a frustum shape according to the design to ensure the size.
5, reinforcing cage production and quality acceptance:
5. 1 rebar shall be accepted when it comes into the site, with quality certificate, mechanical performance test and welding test, and can be used only after it is qualified.
5.2 covered electrode should have a quality certificate, and the model should be compatible with the performance of steel bars.
5.3 The reinforcing cage shall be made in strict accordance with the design, and the position of the main reinforcement shall be controlled by the reinforcement positioning bracket. The allowable deviation of main reinforcement spacing is10mm; Stirrup or spiral reinforcement spacing allowable deviation plus or minus 20 mm; The allowable deviation of reinforcing cage diameter is 65438±00mm；; The allowable deviation of reinforcing cage length is 50 mm 。
5.4 The neck hoop should be located outside the main reinforcement, so as to strengthen the hoop effect on the reinforcing cage without increasing the construction difficulty. Generally, the main reinforcement has no hook.
5.5 When handling and hoisting the reinforcing cage, deformation should be prevented; Before placing, the situation in the hole must be checked again to ensure that there is no landslide and sediment in the hole; When placing, aim at the hole, be steady, slow and straight, and avoid colliding with the hole wall. It is forbidden to pier and twist the cage.
5.6 Pay attention to the elevation of reinforcing cage. After the reinforcing cage reaches the design position, it should be fixed with technical reinforcement (hanging reinforcement and anti-floating reinforcement) to prevent the reinforcing cage from sinking or floating due to buoyancy on the concrete.
5.7 The thickness of reinforced protective layer is 70mm without retaining wall and 35mm with concrete retaining wall. The protective layer is made of cement mortar blocks. When there is no concrete retaining wall, it is forbidden to replace it with clay brick or short steel bar head (the water absorption of brick and the corrosion of short steel bar head will cause a chain reaction of steel cage corrosion). Every 1.5-2m is divided into blocks, with 3 blocks in each group, and the circumferential distance is 120, and each group is arranged in a plum blossom shape. The allowable deviation of protective layer is 65438 00 mm. 。
5.8 When the hole depth is different from the design depth, the length of reinforcing cage should be changed accordingly, but the length of reinforcing cage of friction pile can be unchanged.
6 concrete pouring construction:
6. 1 Concrete shall be poured as soon as possible after the quality of pore-forming is qualified. Before pouring concrete, the hole should be cleaned, and the hole wall and bottom must be cleaned without scum and looseness. The thickness of sediment at the bottom of the hole is 50 mm for end bearing piles, 100 mm for friction end bearing piles and end bearing friction piles, and 300 mm for friction piles.
6.2 When there is groundwater and the amount of seepage is not large, the accumulated water in the hole should be eliminated first, and then the concrete should be poured by string pipe method. The height from the end of the string pipe to the bottom of the hole should not be greater than 2m, and the concrete should be vibrated by an internal vibrator. If the amount of water seepage is too large, it is not convenient to drain the accumulated water, then the conduit method should be used to pour concrete underwater.
6.3 The coarse aggregate of concrete can be gravel or pebbles, and its maximum particle size should not be greater than 50mm, and not greater than 1/3 of the clear distance of main reinforcement.
6.4 Insist on feeding according to the mixture ratio, and the slump of concrete should not be too large, preferably 5-8cm, vibrating every 50cm in time, and concrete pouring should be carried out continuously. When the slump loss is greater than 5cm/h, the ratio should be adjusted.
6.5 For the quality control of concrete mixture, the mixing time of each plate of concrete shall not be less than 90 seconds. When mixing, a slump test must be done to adjust the fluidity and have good cohesiveness. Slump loss should be observed during pouring to guide the adjustment of concrete mixture ratio. The mixed concrete should be used immediately, and it is forbidden to pour it into the pile hole to prevent segregation.
6.6 Pay attention to the elevation of the concrete at the pile head, which should be properly beyond the design elevation, so as to ensure that the pile head enters the cap 50~ 100mm after the laitance layer is chiseled.
6.7 Specimen must be reserved for pile concrete. For piles with a diameter greater than 1m, each pile shall be made into 1 group of test pieces, and each 1 100m3 concrete and each pouring machine-team shall be no less than1group, with 3 pieces in each group. The production of test pieces must be objective and true, and it is forbidden to "start with a small stove".
6.8 Pay attention to slow setting when the temperature is higher than 30℃, and pay attention to frost resistance when the temperature is lower than 0℃.
7, pile quality inspection:
7. 1 Strength and quality inspection of concrete block and dynamic inspection of pile body. Dynamic detection of pile body includes large strain and small strain, which can measure pile length, diameter reduction, diameter expansion, broken pile and estimate concrete strength. Pile quality inspection and dynamic inspection must be qualified.
7.2 It is suggested that conditional sampling should be carried out according to 1~2%, and vertical static load test should be carried out according to slow maintenance load method, which must meet the design requirements.
8, other key points to ensure quality:
8. 1 The concrete pouring process must be carried out on the sidelines, and the whole process should be controlled and strictly controlled.
8.2 Follow-up inspection in time and evaluate the quality results in time. Bibliography:
1, Building Construction Technology, Lu Xun et al., Tongji University Press, 1999.
2. Technical Specification for Building Pile Foundation (JGJ94-94)
3. Code for Design of Concrete Structures (GBJ 10-89), revised in parts 93 and 96.
4. Code for Construction and Acceptance of Concrete Structures (GB50204-92)
5. Code for Design of Building Foundation (GBJ7-89)
6. Practical Handbook of Civil Structural Engineering, Tang, Heilongjiang Science and Technology Press, 200 1.