Cement mixing pile technology is often used in foundation treatment, especially in soft soil foundation treatment, because of its advantages of economy, applicability, no pollution, no vibration and good reinforcement effect. High-quality cement mixing piles need suitable parameters, accurate construction technology, clear precautions and excellent quality inspection.
In 1960s, SwedishGeotechnicalInstitute and Japan Port Research Institute developed deep mixing method respectively, using lime and cement as curing agents, and using special mixing machinery to form mixing piles to strengthen soft soil foundation. Cement mixing pile technology is often used in foundation treatment, which has important practical value for the research, exploration and continuous improvement of cement mixing pile technology.
China began to study this technology on 1978. In 1980s, cement mixing pile technology was applied to soft soil foundation treatment, and it developed rapidly in 1990s. This paper discusses the parameter design, construction technology, quality inspection and matters needing attention of cement mixing pile technology in foundation treatment.
Parameter design of 1 cement mixing pile in foundation treatment
The composite foundation of cement mixing pile is mainly composed of pile body, soil between piles and cushion. Before the application of cement mixing pile technology, it is mainly necessary to determine the cement content, pile diameter, pile length, reinforcement range, cushion, pile bearing capacity and pile layout.
Cement dosage: the cement dosage is 15% of the reinforced soil weight. It is recommended to use ordinary portland cement with strength grade of 32.5 or above as curing agent for cement mixing pile.
Pile diameter: According to Technical Specification for Building Foundation Treatment (JGJ79-2002), the diameter of engineering cement mixing pile should be 500mm.
Pile length: Also according to regulations, the length of cement mixing pile should penetrate the soft soil layer with relatively high bearing capacity. The effective pile length of engineering cement mixing pile is not less than 9m, and the pile body must enter the fifth layer of silty fine sand, which shall not be less than 0.5m.
Reinforcement scope: According to regulations, cement mixing piles can only be laid in the foundation plane. The engineering foundation adopts reinforced concrete strip foundation, and the cement mixing piles are arranged within the width of the strip foundation.
Cushion: According to the regulations, the cement mixing pile composite foundation should be provided with a cushion between the foundation and the pile. The thickness of cushion is 300mm, and the material is medium-coarse sand.
Pile-soil bearing capacity: the bearing capacity of a single pile determined by the strength of pile material should be greater than or equal to the bearing capacity provided by the resistance of pile soil and pile tip soil. Generally, the bearing capacity of single pile should be greater than or equal to 80KN, and that of composite foundation should be greater than or equal to 150KN.
Pile layout: set small stakes as pile points as required.
2 cement mixing pile construction technology in foundation treatment
2. 1 Selection and leveling of construction site
Cement mixing pile technology is mainly suitable for treating normally consolidated silt and mucky soil, plain fill, mucky soil, peat soil, organic soil, cohesive soil and silt with standard bearing capacity not exceeding 120kpa.
2.2 Inspection of mixing machinery before construction
Cement mixing construction machinery should be inspected and accepted by the supervision engineer and project manager before all drilling rigs are started, and special attention should be paid to whether the cement mixing pile pipeline is blocked; Cement mixer construction machinery must maintain good stability; Check whether the computer recorder and printing equipment equipped with cement mixer are installed in sequence before construction, so as to avoid knowing and controlling the dosage of cement slurry and the uniformity of sprayed concrete at any time, resulting in unsatisfactory foundation quality.
2.3 Test pile
According to the actual situation of the construction site, in the field of soft foundation treatment, some soil properties are taken from the surface, middle and pile bottom for comparison. The materials with the worst soil quality are selected as construction materials, and 3-5 groups are generally selected for mix proportion test. In the mixture ratio test, cement and soil mixture are made of various soil properties and several components of cement, and then made into cylindrical specimens for indoor standard maintenance.
The selected cement can only be used after passing the inspection. It is best to use ordinary portland cement with P032.5 or above as the cement. Slag cement or pozzolanic cement is strictly prohibited. The water-cement ratio should be 0.45~0.50, and a small amount of gypsum powder and water reducing agent can be added. The dosage is 0.5%~ 1% of the cement dosage respectively to ensure the quality of mixing piles. After the construction comparison and cooperation is completed, the process test pile can adopt the mixing construction technology of two sprays and four stirs, that is, after the first positive circulation drilling reaches the design depth, the high-pressure grouting pump is turned on, then the cement slurry is lifted to 0.5m below the working datum in reverse circulation, and then the slurry is sprayed to the design depth after the second repeated stirring, and finally the drilling is lifted to the surface in reverse circulation. The purpose of re-mixing is to make the cement slurry and soil fully and evenly mixed. 2.4 pulping and stacking
Set the pile-making point with small stakes, level the drilling rig, and keep the verticality of drill pipe less than or equal to 1%. Start the mixing rig and control the penetration rate, which shall not be greater than1.2m/min; When crossing the clay layer, the drilling speed should not be greater than 0.8m/min. After drilling 50 meters, start the air compressor to inject compressed air to prevent the nozzle from clogging during drilling. At the same time, the load torque can be reduced by compressed air, so that drilling can be carried out smoothly. When pulping, enough slurry should be prepared at one time according to the needs of each pile to ensure the stable mixture ratio of each pile and sufficient slurry. The normal piling time of each pile is not less than 40 minutes, and the injection pressure is not less than 0.5mpa.
3 matters needing attention in cement mixing pile construction
(1) Assign a special person to be responsible for the construction of cement mixing pile, and monitor the whole process of cement mixing pile.
(2) The relevant person in charge mainly checks the cement dosage, whether there is slurry breakage in the grouting process of the cement mixer, and whether the mixing time of sprayed concrete and the frequency of re-mixing are normal.
(3) During the construction, the shotcreting time and stop time should be strictly controlled, and each pile should be operated continuously after drilling, and shotcreting should not be interrupted. It is forbidden to lift the drill pipe without guniting.
(4) If insufficient shotcrete is found during construction, the whole pile shall be re-stirred according to the requirements of the supervision engineer. The re-shotcreting quantity shall not be less than the design quantity.
(5) On-site construction site shall be equipped with construction recording personnel to record the date of construction pile, weather, shotcrete depth, grout stopping elevation, drilling rig speed, mud flow and re-mixing depth in detail.
4 quality inspection of cement mixing pile in foundation treatment
(1)n 10 The main purpose of portable penetration test within 3 days after the completion of construction is to check the uniformity of cement slurry distribution in the cement mixing pile. Generally, the depth of portable penetration test is not more than 4m, and the detection frequency is 1% of the total number of construction piles, and there are not less than 3 piles.
(2) The bearing capacity (static load) test of cement mixing pile after 28 days of construction can adopt composite foundation bearing capacity test and single pile bearing capacity test. The main purpose is to check whether the bearing capacity of the foundation is improved after the cement mixing pile is completed and whether the pile body meets the design and specification requirements. The inspection quantity is 0.5%~ 1.0% of the total number of construction piles. And each single project should not be less than 3.
(3) When there is doubt about the quality of the pile body after the portable static cone penetration test and static load test, the core of the pile body is cored with a core-pulling machine after 28 days of pile formation. The main purpose is to test the strength and integrity of the pile body, the uniformity of pile-soil mixing and the length of the pile body. To test the strength of pile, it is necessary to take core samples and send them to testing institutions for 28d and 90d unconfined compressive strength tests. The number of inspection shall be 0.5% of the total number of construction piles, and there shall be no less than 3 piles.
In the field quality inspection of cement mixing piles constructed by deep mixing method, in addition to light dynamic penetration test, core drilling and pile hanging load test according to the national standard JGJ790-2002 "Technical Specification for Building Foundation Treatment", the database of field strength and mixed strength in piles can also be established to improve the inspection method. For example, the invention patent: on-site detection method of deep mixing mixed soil based on mixing uniformity.
With the rapid development of highways in China, the expansion of first-class and second-class highways, and the continuous increase and improvement of infrastructure, the construction of village and township highways is imminent; With the continuous expansion and improvement of the urban traffic network, the urban road network gradually radiates to the surrounding areas of the city, which used to be cultivated land, fish ponds and easy-to-walk mud. The foundation treatment of these projects needs cement mixing pile technology to strengthen soil. Cement mixing piles have been widely used in soft soil foundation treatment of expressways, foundation pit support of high-rise buildings, isolation of contaminated sites and other projects because of the use of special machinery to spray cement powder or cement slurry into soft soil and force mixing to form composite foundation or water retaining structure. At present, this technology is also widely used in soft foundation treatment and reinforcement projects in China's railways, highways, municipal engineering, ports and docks, industrial and civil buildings and other industries. Cement mixing pile technology will play an increasingly important role in practical application and scientific research.
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