【Excerpts】Seven steps analysis pile foundation construction, 3 minutes quickly master the practical points

Introductory remarks

Pile foundation is widely used because of its high bearing capacity, good stability, small settlement and difference deformation, stable and fast settlement, strong seismic performance and the ability to adapt to various complex geological conditions. The following has arranged the pile foundation construction points, take a look together.

01  Pile production

       The precast pile formwork should be made of steel formwork, which has sufficient stiffness and should be flat and accurate in size. The methods of making square pile include parallel method, interval method and overlapping method. Due to the limitation of the site, the overcasting method is used. The number of overlapping layers is determined according to the allowable load and construction conditions on the ground, but should not exceed 4 layers. Isolation layer (such as linoleum, kraft paper, plastic paper, paper tendon ash, etc.) should be made between piles. The pouring of the upper pile or adjacent pile shall be carried out only after the concrete of the lower pile or adjacent pile reaches 30% of the design strength. Because the construction of overlapping method needs to be lifted after the concrete of the upper pile, the whole pile can also be made into a step shape.

       The coarse aggregate of precast pile concrete shall be gravel or open pebble, and the particle size shall be 5~40mm. Concrete strength grade is commonly used C30~C40, should be used with mechanical mixing, mechanical vibration, from pile top to pile tip continuous pouring tamping, one time. After production, water shall be not less than 7d.

 02  Lifting, transportation, and stacking of piles

Pile lifting

       Reinforced concrete precast pile should be lifted when the concrete reaches 70% of the design strength, and 100% of the design strength can be transported and piling. In advance, measures must be taken and qualified. When lifting, the lifting point should be reasonably selected to prevent the bending damage during the lifting process. If there is no ring and the design is not specified, the number and position of the binding points are determined according to the length of the pile, which should conform to the principle of the minimum lifting bending moment, and can be tied according to the position.

Pile transportation

       The concrete strength of the pile body during transportation shall reach 100% of the design strength. It can be transported by a flat plate truck, and the transportation should be kept stable. The pile stacking site should be smooth and solid, with no uniform subsidence. The pad position shall be the same as the lifting point and remain on the same plane. Each layer of cushion wood should be aligned up and down.

Pile stacking

       The piling of piles should be carried out in turn according to the requirements of piling, and the number of stacking layers should not exceed 4 layers. The pile body should be transported to the pile driving position, which should be arranged within the working radius of the lifting hook attached to the pile driving frame, and the lifting direction should be taken into account to avoid turning.

03  Pile foundation positioning

Technical requirements for pile foundation construction measurement

       (1) Technical requirements for building axis measurement. Building pile foundation positioning measurement, generally according to the architectural design or design units provided by the measurement control point or baseline related data and the new building, first set building positioning rectangular control network, building positioning measurement, and then according to the building positioning rectangular control network, set the building pile axis, finally according to the pile axis to measure the cap pile.

       (2) Technical requirements for elevation measurement. The leveling point is set before the elevation introduction, its position shall not be affected by the construction, it should be easy to use and save, the number is generally not less than 2~3. The elevation measurement can be carried out according to the fourth leveling measurement method and requirements, the round-trip is poor, the attachment or loop closure difference should not be greater than ± 20L, L is the length of the horizontal route, in km. The pile site elevation measurement is generally measured by the ordinary level scatter method, and the elevation measurement error should not be greater than ± 1cm.

Measurement quality of pile foundation construction shall be controlled

       （1）Building positioning rectangular network needs to be buried diameter of 8cm, 35cm long large pile, pile position should be easy to operate, and easy to save, and in the wooden pile nail as the central sign, the pile should be reinforced with cement protection, in the construction should pay attention to protection, should be checked before use. For large or more complex projects and long construction period, cement piles with 10cm 10cm on top, 12cm 12cm on bottom and 80cm long should be buried as the long-term control point.

       （2）The inspection work must be strengthened, including all the calculation data of the pile position measurement line diagram. The second person must be 100% inspection, confirmed to the site test. After the building positioning measurement results meet the requirements, the building pile axis can be measured for the positioning measurement of the building. The building pile axis measurement is carried out after the completion of the building positioning rectangular network measurement. It is based on the building positioning rectangular network, and adopts the internal division method. Polar coordinate method is generally used to measure the center point of the circle of complex buildings. The pilot pile of the pile axis should be driven into the small wooden pile, and the small iron nail should be nailed on the top of the pile as the central point of the pile axis. For easy preservation and use, the pile top is required to be flush with the ground, and sprinkle white ash around the lead pile. After the completion of the pile axis measurement, the length between the pile axis and the length of the pile axis should be detected in time. The difference between the actual quantity distance and the design length is required, and the single row pile position should not exceed ± 1cm, and the cluster pile position shall not exceed ± 2cm. The measurement of the cap pile position can only be conducted after the pile position axis inspection meets the design requirements.

Quality control of cap and pile position measurement

       The measurement and setting of the building cap pile is based on the foundation of the pile axis. The pile foundation design is divided into group piles and single row piles according to the needs of the building. In the process of design, the bearing pile and the axis can be selected according to the relationship between the position and the pile given in the design. For the measurement of the pile position of complex buildings, the data provided by the design can not be directly used directly, but can only be measured after conversion. After the measurement and setting of the cap pile position, small wooden piles should be driven as the pile position mark, and white ash is sprinkled to facilitate the construction of the pile foundation. After the measurement and setting of the cap piles, it should be detected in time. The difference between the real distance between the cap piles should not be greater than ± 2cm, and the difference between the real distance between the adjacent cap piles should not be greater than ± 3cm. Only after the pile point is tested and meets the design requirements can it be transferred to the pile foundation construction unit for pile foundation construction.

04  Site preparation

       （1）Obstructive processing. Before pile sinking, requirements should be made to the city management, water supply, power supply, gas, telecommunications, housing management and other relevant units, to seriously deal with obstacles at high altitude, above ground and underground. And a comprehensive inspection of the site around the buildings, underground pipelines, if there are dangerous buildings or dangerous structures, must be strengthened or take vibration isolation measures or demolition.

       （2）Site levelling. The piling site must be smooth and solid, and roads should be laid if necessary, compacted by the roller, and drainage ditches should be dug around the site. The approach route of pile making materials and the route of pile forming to the pile driving site shall not be disturbed.

       （3）Copy flat line. The leveling point shall be set in the pile driving site, and its position shall not be affected by the pile driving, and the number shall be no less than two to flatten the site and check the soil depth of the pile. Each pile position of the pile foundation is controlled according to the axis of the building.

       （4）Before construction, the technical person in charge of the construction site and the construction staff should comprehensively check the construction preparation hole by hole, and carry out technical safety disclosure and safety education step by step, so that safety and technical management can be implemented in thought, organization and measures.

       （5）Special personnel shall be responsible for recording the vertical center line, axis, pile diameter, pile length and bedrock soil, including the concealed acceptance records of steel cage and pile concrete, and compiled and distributed to relevant units and sent to the technical department for archiving after completion.

       （6）The height of the wall formwork depends on the soil quality, generally can be used 50~100cm.

05  Pile foundation detection points

       （1）During low strain detection, for the rock-embedded pile, the base time domain reflection signal is a single reflection wave and in the same direction as the hammer signal; the measured signal is complex and irregular, which cannot be accurately evaluated; the pile is verified by static load method or core drilling method.

       （2）During high strain detection, the pile defect; or the pile defect affects the horizontal bearing capacity; when the single penetration degree is large, the pile base is strong and the reflection peak is wide, the side resistance wave and the end resistance wave are weak, that is, when the waveform shows the vertical bearing properties and the geological conditions in the survey report, the static load method can be used for further verification.

       （3）The bottom of rock-embedded pile reflects strongly in the same direction, and there is no obvious end resistance reflection after time 2L / C, which can be verified by core drilling method.

       （4）The shallow defects of the pile body can be verified by excavation.

       （5）The precast pile with cracks in the pile body or joint can be verified by the high stress change method.

       （6）When the quality problem of pile concrete is found by single hole core detection, it is appropriate to add drilling verification in the same foundation pile.

       （7）For piles or class piles whose integrity category cannot be specified in the low stress transformer test, appropriate methods such as static load method, core drilling method, high stress transformer method and excavation can be verified and tested according to the actual situation.

       （8）When the sampling inspection results of single pile bearing capacity or core drilling method do not meet the design requirements, the reasons shall be analyzed and the sampling inspection shall be expanded after confirmation.

       （9）When the sum of piles is more than 20% of the number of piles found by the method of low detection, the original detection method (acoustic transmission method can be changed by drilling method), and continue to expand the sampling inspection in the unchecked piles.

06  Measures for handling common pile foundation problems

Adhesion method

       This method can be used when the soil depth of the precast pile is insufficient, or when the pile is lifted due to the uplift of the soil.

Pile filling method

       Pile filling method The pile repair method is based on the pile repair scheme issued by the design unit in conjunction with the opinions of the design, supervision and the owner. However, this method has large investment and long construction period, so it is difficult to be recognized by all parties.

Supplement and send combination method

       When the pile is driven into the foundation by the way of joint connection, if the quality of the pile is not up to standard, it is possible to remove the joint in the process of sinking. At this time, the pile foundation can be combined for the treatment of pile foundation. For suspicious piles, it should be repeated to make the pile sink, which can tighten the disjointed pile connection again and make the joint has vertical bearing capacity; the piles can be repaired, and some complete piles can be properly appropriately, so that the foundation can not only meet the bearing capacity requirements, but also improve the seismic load of the building foundation.

The correction method

       If the pile body is found that the pile body is inclined and the pile length is not long, and intact and not broken, or the pile body is inclined due to the excavation of the foundation pit but the pile is still complete, the pile body can be partially dug, and then the jack can be used to correct the pile body.

The cap expansion

       （1）If there is a large deviation in the pile position, the original cap design size cannot meet the normative requirements of the structure, so the pile deviation can be corrected by appropriately expanding the area of the cap.

       （2）Pile and soil work together. If a single pile cannot meet the designed bearing capacity requirements, it is necessary to expand the cap, and the upper load of the natural foundation and the pile should be taken into account.

       （3）Pile foundation quality problems. In the construction, there is easy to appear uneven pile foundation quality. In order to prevent the cloud settlement of the cap layout caused by the pile foundation quality problems in the later period and to improve the seismic capacity of the building, the integral pile foundation cap can be used, so as to improve the integrity of the foundation.

Composite foundation method

       （1）It is to use the principle of the joint action of pile and soil, make appropriate treatment to the foundation, improve the bearing capacity of the foundation, and share the load of the pile foundation more effectively. The common methods are as follows.

       （2）Under the land base. Before the construction of pile foundation cap, dig a certain depth of soil, replace the sand filling layer, and then the cap is constructed on the artificial foundation and pile foundation.

       （3）Add soil-soil piles between the piles. When the bearing capacity of the pile can not meet the design requirements, the method of dry spraying cement into the soil between the pile can be used to form a composite foundation.

Modify the pile type or pile driving parameter method

       （1）Change the pile type. Such as prefabricated square pile to prestressed pipe pile.

       （2）Change the soil depth of the pile. For example, when the precast pile encounters thick dense silt or silt layer in the penetration process, it is difficult to sink or even break the pile accident. At this time, shortening the pile length, increasing the number of piles, and taking the dense silt layer (expanded soil layer) can be used as the bearing layer.

       （3）Change the pile position. If the pile encountered hard, small underground obstacles, make the pile tilt, or even fracture, the pile position can be used to change the pile again.

       （4）Voltage pile equipment. When the pile sinking depth can not meet the design requirements, the large tonnage pile frame and heavy hammer and low strike method can be adopted.

07  Safety precautions

Technical measures to prevent collapse safety

       （1）Make good hole protection planning to prevent surface water from entering the hole.

       （2）Before the excavation process, the wall protection scheme should be designed according to different geological conditions. During the excavation process, the geological conditions must be carefully reviewed, and the hole wall protection work should be strictly done well according to different geological conditions.

       （3）For the hole wall requiring concrete protection, the excavation shall be excavated strictly in accordance with the specified footage, and the formwork can be removed after the concrete reaches the design strength.

Safety measures for ventilation in the hole

       （1）Manual hole digging pile should be ventilated in the hole. When the hole depth is greater than 5m, the ventilation pipe should be used to send air into the hole. Operators should work for about 2h, and should rest outside the hole.

       （2）For the special geological area, we should dig the hole in the process of harmful gas detection.

Protection measures in the hole

       （1）For the lifting wire rope, there should be no less than 10 times the safety reserve, and regularly check its wear.

       （2）The bucket load slag can not be too full, to prevent the broken slag scattered.

       （3）The lower hole operator should wear a safety helmet, for the special hole position, should also tie the life rope.

Anti-electric shock measures

       （1）The installation and removal of all power supply and circuits on the construction site must be operated by the certified electrician.

       （2）Electrical equipment must be strictly grounded or zero protection and installed leakage protector, each pile hole electricity must be divided into a brake, it is strictly prohibited to use a brake.

       （3）The cable on the hole must be more than 2m above, and the ground must be buried in the soil. The cable and wire in the hole must adopt anti-wear, moisture-proof and anti-break protection measures.

       （4）The lighting in the hole should be safe mine lamp or safe voltage below 12V. The operator in the hole should wear working gloves and wear insulating rubber shoes.

——Source: Zhulong Forum

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