The mixture of cement and foundation soil is the main working material of cement-soil mixing pile. Whenever we mix cement into soft soil, a series of hydration and hydrolysis reactions between cement particles will occur when cement meets water in soil, forming cement hydrate and gel, which is the core principle of the process of strengthening soft soil foundation with cement-soil mixing pile. After that, small soil bodies or soil particles will agglomerate together to form a stable structural whole, and these hydrates will continue to harden themselves and eventually form a cement skeleton. At the same time, some of them will reunite with other clay particles with certain activity and exchange ions, and finally form stable water-insoluble crystalline compounds through hard coagulation reaction and carbonic acid reaction, thus forming a complete and stable cement-soil plus solid.

The concrete application of cement mixing pile technology in foundation treatment of high-rise buildings is illustrated by examples

Briefly describe the general situation of a large office building in Beijing. The plane layout of an I-shaped large office building in Beijing, the main direction is north-south, about 24m in the east, 9. 1m in the west, 9. 1m in the west, 40.4m in the west and 15m in width. The total height of the office building is 16 floors, with a total area of about 87655.

Field survey data. Before the concrete construction of foundation reinforcement project, according to the field investigation, the office building is located in the upper part of Quaternary alluvium, and its main lithology is mainly divided into silt, silt, fine sand and silty clay, which can be divided into 12 layer, and its maximum exposure depth is 51.4m. If cement mixing foundation reinforcement method is adopted, its pile height should be located in the second to ninth layers. At the same time, the buried depth of water level is 7. 1m, the bearing capacity of the second to eighth floors of foundation is about1140kpa, and the bearing capacity of the eighth to ninth floors of natural foundation can reach 160~250kPa. At the same time, the design replacement ratio m of cement-soil mixing pile is 19. The pile spacing is 1.2m, and the total number of piles n is 142 1. The bearing capacity of reinforced composite foundation is about 300kPa. For elevator shaft with large additional stress, the diameter of cement-soil mixing pile is 600mm to ensure local stress concentration and excessive settlement.

Quality inspection during construction. In the concrete implementation process of cement mixing pile project, strict quality supervision and testing are needed to ensure the smooth progress of the whole project, improve work efficiency and save construction funds. The specific supervision and inspection are as follows: 1) Do a good job in positioning the central pile position of cement mixing pile, so that the pile position difference is strictly controlled below 2cm; 2) Before construction, the whole system shall be strictly tested; 3) Pay attention to the quality of cement and the proportion of slurry preparation; 4) Jumping in rows and columns to prevent pile deformation; 5) Mark the laid piles to avoid mistakes; 6) After four working days, the personnel of relevant departments need to recheck the quality of cement piles.

Settlement observation and pile foundation detection of cement mixing pile. After the completion of the construction of pile foundation and superstructure in this project, we have tested and observed the settlement and foundation pile respectively, and the specific test results are as follows. 2.4. 1 After the completion of the pile foundation and superstructure construction in this project, we observed and detected the settlement deformation respectively. It is observed that the cumulative settlement does not change obviously with the increase of time, and its curve is relatively gentle, and all data indexes are within the control range. 2.4.2 The foundation reinforcement project of this office building was implemented on September 28th, 2009. During the construction, two pile drivers operate at the same time, with a total construction period of 20 days. Four working days after the completion of pile foundation construction, relevant professional inspectors from relevant departments carried out static load tests on 20 piles. The settlement of the three test piles increases with the increase of load, and basically changes linearly before the load reaches 200kPa, with obvious regularity. After that, with the increase of load, the settlement rate has a gradual increase trend, but the trend is relatively flat. The maximum settlement of other test piles is only 38.2mm, and the maximum settlement of two test piles is 22.4mm and 24.0mm respectively. Through the mathematical statistics analysis of 20 test piles, it is concluded that the characteristic value of bearing capacity of composite foundation is 307.5kPa, which meets the design requirements. In 20 groups of coring tests, the unconfined compressive strength of coring is 7. 1~ 15.2kPa. Finally, 500 piles are sampled, and the low-strain dynamic test results show that the integrity of the pile body is good, and the reinforcement effect of cement mixing pile is generally good. Through the description of a successful case of a large office building in Beijing, it is concluded that the cement-soil mixing pile composite foundation is completely suitable for the foundation construction of high-rise buildings. According to the test results of professionals from relevant state departments, in the process of foundation reinforcement of high-rise buildings, cement-soil mixing pile composite foundation has the functions of shortening construction period, reliable quality and reducing project capital cost. It is a standard, efficient and scientific foundation treatment technology for high-rise buildings. We should make full and rational use of this technology to promote urban development, social progress and national prosperity.

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