Reinforcement of the damaged engineering piles of

The reinforcement of the disease engineering piles of the main building under the normal use of a shopping mall in Nanjing is located in a shopping mall on the east side of Rehe South Road in Nanjing. Its total design construction area is 85000m2, with 30 floors above the ground and 2 floors underground. In 1997, the part below the seventh floor was built and put into use, and the upper part was suspended due to many problems. It is planned to build more than 7 floors in 1999

the pile foundation of this project adopts reinforced concrete bored end bearing piles. After the pile foundation construction is completed, some engineering piles have major potential quality hazards. Therefore, according to the requirements of the design institute and the quality supervision station, it must be thoroughly handled before the main construction

the ground floor of the project is a shopping mall, which has been put into use. The floor height of the basement is 3.4m, which puts forward many special requirements for the design and construction of pile foundation treatment, and solves a lot of thorny practical problems in the construction process. The whole reinforcement treatment lasted for more than a year. Now this paper makes a discussion on this, which can be used for reference by peers in the industry in dealing with similar disease projects

1. Causes of the accident

in 1996, the project began construction. The deep cement mixing pile was used to retain the soil for the foundation pit support of the basement. After the earth excavation of the lower second floor of the underground, due to the lack of support facilities in the pit, the soil mass at the junction of the main building and the podium building was partially unstable and collapsed, and the soil mass slipped in a large area. The soil squeezing effect led to the deflection and inclination of some engineering piles of the main building

according to the pile position field record of the construction supervisor at that time, 29 engineering piles were affected, and the maximum pile position offset was 2.3mm. Then, after low strain detection, it was found that many piles were broken, and the fracture position was about 10m away from the pile top. The construction unit and the construction unit did not notify the quality supervision station and the Design Institute, and carried out hasty and hasty treatment without authorization, and continued the construction below the 7th floor

2. Judgment of defective engineering piles

in order to ensure the scientific, accurate and economic treatment, it is necessary to judge the quality of problematic engineering piles

in 1999, through the analysis of the original data and the demonstration of many experts in Nanjing, it was believed that the engineering piles that may affect the project quality involved the existing 6 bearing platforms, of which the most serious was the 9 piles under the 3 frame columns of the main building at the junction of the main podium building, with a diameter of φ 1.2m, it is decided to start with the pile

the scope of judging work is: the integrity and verticality of the pile body and the compressive strength of the concrete of the pile body

the integrity and verticality of the pile body are tested simultaneously by drilling and coring, ultrasonic testing and CT scanning. The methods are as follows:

vertical drilling (ZK1) is carried out in the center of the original pile location, which is taken as the center of the circle. Four vertical boreholes (AK) in the East, West, North and south direction are set on the circumference with a radius of 0.8m. The vertical drilling depth of all five holes is 16m, and all holes are cored, Make the initial frequency judgment of concrete integrity and uniaxial compressive strength test. Taking ZK1 as the axis and divided into zk1/zk four surfaces, the non-metallic ultrasonic detector is used to detect the pile body, and the scanning is carried out within the range of 0 ° ~45 °, with a step length of 0.2m. At the same time, the CT scanner is used to carry out the three-dimensional imaging of the pile body, calculate the inclination direction and angle of the pile body according to the scanning results, and determine the defects of the pile body, such as the fracture and mud inclusion of the pile

conclusion: there are serious fractures and mud inclusions in the concrete of the pile shaft about 6m~8m below the pile top, and there are moderate looseness and voids in other areas. The pile shaft of 7m~14m section inclines to the southeast, 4% to the East and 2.5% to the south, which is basically consistent with the prediction and calculation results of experts before the test, and the compressive strength of other parts of concrete is qualified

according to relevant regulations, the project pile is determined to be unqualified, which is referred to as "diseased pile". According to the opinions of the expert group, core drilling and sampling were also carried out for the relevant engineering piles under the other five bearing platforms, and the physical indicators of the concrete were basically the same as above. Combined with the low stress edge test in the early stage, after many rigorous calculations, it has been considered that these piles are seriously unqualified piles, and the bearing capacity is far from meeting the requirements of the original design, so they must be reinforced

3. determination and implementation of pile reinforcement scheme

for all engineering piles with inspection holes generated by core drilling sampling and some slight defects in the pile body, pressure grouting shall be used for reinforcement, 4 holes shall be added to each pile, and the diameter shall be inserted after high-pressure grouting φ 80mm and 10mm thick steel pipes are grouted in three to four times

through the analysis and calculation of the structural stiffness of the existing basement floor beam, it is considered that the focus of the reinforcement work should still be on the treatment of the diseased engineering piles and caps, which is an important part of this reinforcement work. Before treatment, two reinforcement schemes are compared:

scheme one is the partial underpinning scheme of anchor static pressure steel pipe pile

using the structural self weight of the building and the negative friction of the pile foundation as the reaction force, the steel pipe pile is pressed into the foundation soil through the holes drilled through the bottom plate through the connection of the anchor bolts on the foundation. Until the steel pipe pile reaches the design length or the design bearing capacity value, the design length is the design pressure value, which is converted after the test pile outside the building. After the pile pressing is completed, the foundation shall be sealed

diameter of steel pipe pile is φ 508mm, wall thickness of 18mm, filled with micro expansive concrete

at present, the part below the seventh floor has been built for four years, the settlement has been stable, and the building is basically in normal state. Therefore, it is considered that the diseased pile still has a certain bearing capacity (no earthquake occurred during this period). Through further pressure grouting, the pile body repair can bear the structural load above the seventh floor, and it can still continue to play its role. In addition, the steel pipe pile that is newly added near the original pile position can bear the load above 7 floors and part of the load below 7 floors

scheme 2 is the full underpinning scheme of reinforced concrete bored cast-in-place piles

considering the limitations of current detection means and calculation theory, it is difficult to accurately measure the residual bearing capacity of the damaged pile after grouting repair, and the old and new loads have a joint effect at the bearing platform, and the stress analysis is relatively complex. A scheme of full underpinning of reinforced concrete bored pile is proposed. After setting, the damaged pile will not participate in the work, the upper load will be borne by the newly formed pile, and the residual bearing capacity of the old pile is only used as a safety reserve. The diameters of new piles are respectively φ 0.8m、 φ 1.2m, the concrete strength grade is C35, and the bearing capacity and quantity of piles are proposed through the calculation of the test pile results

scheme comparison: scheme I has the advantages of economy and short construction period, but the disadvantage is that the empirical component of the scheme is high, the transmission and distribution of the upper load of the old and new piles is not intuitive, the accurate value of the bearing capacity of the old piles is not clear, and the strain difference between the old and new concrete will make the stress analysis of the stressed structure very complex. The advantage of scheme 2 is that the load transfer path is clear, the pile foundation has no hidden dangers, the reinforcement process is relatively scientific, and the reliability is greatly improved. The disadvantage is that the construction period is long and the investment is large, and the drilling pile construction equipment needs to be transformed due to the limitation of floor height

considering the long-term plan of the construction project and the life safety of the people, the expert group finally carefully selected scheme II as the reinforcement treatment scheme of the project

in the second scheme, the film evaporates on the opposite side under the action of concentration gradient. Compared with steel pipe piles, the bearing capacity of single pile of bored cast-in-place piles is greatly improved, the total number of piles is also reduced, and the construction difficulty is relatively reduced. A total of 38 piles are formed. During the construction process, the supervision of pile construction quality is strengthened, and the control of pile depth into rock is strengthened

4. Load transfer on the bearing platform during the reinforcement process

another important part of the second reinforcement is the reinforcement of the original bearing platform. Through the new bearing platform, the progress of plastic granulator technology is closely related to the development of the whole national economy. The foundation structure of the building is effectively connected. The load transfer is achieved through the connection of the old and new bearing platforms and the reinforcement of structural frame columns (Figure 1)

due to the requirements of the construction and equipment disciplines, the top elevation of the newly-built bearing platform cannot be higher than the top elevation of the basement floor, which means that in order to form an effective connection, part of the concrete of the existing straight lower chamber beam slab and the bearing platform must be chiseled. Since the bearing platform has borne the load transmitted from the upper seven floors (including the second floor basement), the specific amount of concrete chiseling is determined by the designer through accurate calculation. In order to ensure the connection between the old and new bearing platforms, steel bars are planted on the concrete of the old bearing platform, and the number of steel bars is determined by the designer through calculation

after calculation, the reinforced concrete is wrapped around the structural column on the original pile cap from the top to the pile cap within the height range of the second underground floor, which expands the section and ensures the ability of the reinforced concrete of the new pile cap to resist punching and shearing. The layout of the main reinforcement of the newly outsourced reinforced concrete is set according to the reinforcement form of the frame column, and is connected with the main reinforcement of the old and new bearing platform. The reinforcement technology of structural frame columns is also adopted

in order to ensure the construction quality of steel bar planting, the control of the planting depth under different conditions of dry hole and wet hole is strengthened in the process of steel bar planting. After the construction is completed, the pull-out test is carried out on each steel bar planting, so that it all meets the design requirements

in this way, the load of the upper main body is transferred to the new bearing platform through the expanded structural pile, and then transferred to the newly formed reinforced concrete bored pile by the new platform, so as to realize the transfer of the load. At the same time, the residual bearing capacity of the original repaired damaged pile is also participating in the work, so as to achieve the purpose of safety reserve

Figure 1 Schematic diagram of cast-in-place pile cap and column reinforcement

5. Acceptance of reinforced engineering piles

acceptance of reinforced engineering piles is mainly completed by using self balancing technology to detect the bearing capacity of a single pile, small strain detection technology to detect the integrity of the pile body, and pressure test of concrete test blocks to detect the strength of concrete in the pile body

the test basis of single pile bearing capacity is the Jiangsu Provincial aboveground standard "technical specification for self balancing test of pile bearing capacity" and JGJ Annex C "vertical compressive static load test of single pile". The self balancing pile test technology is a revolutionary new test technology in this field of Southeast University. It has no special requirements for site space, and is especially suitable for the adverse factors of the height of the basement of the project, without any surcharge and large equipment. The principle is as follows:

in the process of pile construction, a special loading equipment load box is welded with the main reinforcement of the pile reinforcement cage. After calculation, it is buried 2m away from the pile bottom, and the high-pressure oil pipe of the load box and the displacement rod are led to the ground together. After 15 days of concrete pouring, the high-pressure oil pump is used to fill the load box with oil according to the requirements of the loading specification, and the load box transmits the pressure to the pile body through the upper and lower thick steel plates, The friction of the upper pile body is balanced with the friction and end resistance of the lower pile body. Determine the bearing capacity of the pile according to the upper and lower Q-S, S-lgt, s-lgq curves. According to the specification, a total of 2 piles were tested, and the test results met the design requirements

the detection of the integrity of the concrete in the pile body is realized by using the low strain detection technology. Each pile adopts multi-point agitation, and a well consistent original waveform is collected as the analysis basis. The test results show that 28 class a piles and 10 class B piles meet the requirements and can be used as engineering piles

through the pressure test of concrete test blocks, with the increase of market demand for plastic products, the uniaxial compressive strength meets the design requirements

6. On the seventh floor, the order volume of injection molding machine in the fourth quarter fell by nearly 5 percentage points compared with the same period in 2012. Several problems needing attention in the design and construction of some towers

due to the late local pile foundation reinforcement at the basement floor, the foundation