CFG short pile composite foundation technology is a brand-new foundation reinforcement theory based on the architectural geological conditions in northern China and aiming at the shortcomings existing in the construction of ordinary CFG piles (referred to as long piles).
199865438+February applied for an invention patent, and in August 2002, the State Patent Office granted the invention patent with the patent number of 98 125248.6. After years of construction practice, the popularization and application of CFG short pile composite foundation technology has made great progress and achieved good economic, technical and social benefits. It is summarized as follows.
1. Design and construction
Take the south area 1# and 2# buildings of the General Staff Headquarters in Beijing as an example.
1. 1 project overview
The South 1# and 2# buildings of the Beijing Retired Cadres Residential Area are located in Datun, outside Andingmen, Chaoyang District, east of anding road, between Dachuansi North Road and Xindian Road. The construction is invested by the General Staff Administration Department, and the construction drawing is designed by the Architectural Design Institute of the General Staff Administration Department.
The south 1# and 2# buildings have 24 floors above ground and 2 floors underground, with a building length of 37.2m, a width of 24m along the south edge and a width of 36. 1m along the north edge. For box foundation and shear wall structure, plus or minus 0 elevation is equivalent to the absolute elevation of 1# building 44. 12m and 2# building 44. According to the design requirements, the design value of foundation bearing capacity is 450kPa.
1.2 engineering geological and hydrogeological conditions
1.2. 1 stratum profile
According to the investigation reportNo. Beijing Survey and Design Institute 10 14, the stratum soil of the proposed site is divided into artificial accumulation layer, recently deposited layer and general Quaternary deposited layer.
Artificial accumulation layer: ① layer of silty clay fill and ① layer of house muck are distributed on the surface, with a thickness of1.4 ~ 3.2m. ..
New sedimentary layer: It is located under the artificial accumulation layer and distributed in the northwest and southeast direction of the proposed South 2# building in a strip shape, and consists of silty clay and clayey silt.
Quaternary sedimentary layer: this layer is located under artificial accumulation layer and recent sedimentary layer, and mainly consists of the following soil layers:
(1) below the elevation of 38.20~40.38m are clayey silt, silty clay ③ and sandy silt, clayey silt ③ 1, ③ Es=6.6MPa, ③ ES = 1+ 1.
(2) There are four layers of silty clay and clayey silt, four layers of sandy silt and clayey silt with medium and low compressibility, four layers of clay with medium and high compressibility and four layers of heavy silty clay below the elevation of 35.57 ~ 36.83m..
(3) The medium-fine sand layer is below the elevation of 32.22 ~ 32.97m.. The top of this layer is distributed with fine silty sand ⑤ 1 layer. The total thickness of sand layer is 3.40 ~ 3.90m.
(4) Below the elevation of 28.42~29.57m, there are 6 layers of silty clay and clayey silt, 6 layers of clay and heavy silty clay (1). There are two layers of low compressibility silt and fine sand, and clayey silt and sandy silt.
(5) Below the elevation of 20.60~2 1.90m, there are silty clay ⑦ layer and clay and heavy silty clay ⑦ layer, and there are low compressibility silty sand ⑦ 1 layer and clay silt and sandy silt ⑦ layer.
1.2.2 general situation of groundwater
See table 1 for the distribution of groundwater level in each layer. Only the corrosive medium C 1 in the first underground water in this site is weakly corrosive to the steel bars in reinforced concrete under the condition of alternating dry and wet conditions.
Groundwater layer distribution table 1
1.2.3 Relevant information on seismic design of buildings
According to the exploration results, the average shear wave velocity value υsm=240m/s within the depth range of overburden 15m of the proposed site. The soil type of the site is medium soft soil, the thickness of the site overburden is more than 80m, and the construction site is classified as Class III.
According to the seismic intensity zoning map of Beijing area, the basic seismic intensity of the proposed site is 8 degrees.
In the seismic design of Beijing area, only the influence of recent earthquakes is considered, and there is no possibility of liquefaction of foundation soil in this area.
1.3 foundation treatment scheme design
The design principle is 1.3. 1
After depth correction, the bearing capacity of composite foundation meeting the design requirements is not less than 450kPa. The weak substratum meets the strength requirements. The final settlement of building foundation is not more than 8cm, and the inclination value meets the specification requirements.
1.3.2 design basis
Geotechnical Engineering Investigation Report of Retired Cadres Residential Quarter in Beijing Area of General Staff Department (1# Building, 2# Building on the south side) by Beijing Survey and Design Institute, with project number 98 and technical transformation 0 14.
The Architectural Design Institute (JJ-2) of the General Staff Organs Administration Bureau showed the foundation plans (foundation treatment) of the residential buildings of Datun Nantui 1# and Datun Nantui 2#.
1.3.3 Selection of foundation treatment scheme
The investigation report puts forward four schemes:
(1) Natural foundation scheme with shallow soil replacement treatment. Excavate ④ layer, ④ 1 layer and ④2 layers of soft cohesive soil, and the bottom of the trench will be deepened to 32.20~32.70m, and the soil of the bearing layer will be fine sand, silty sand ⑤ 1 and medium fine sand ⑤, and then it will be compacted and backfilled by layers with low-grade plain concrete or graded sand to the proposed design.
(2) In view of the fact that the proposed high-rise residential buildings are all single buildings, the soil layers of each building are evenly distributed, and there are no favorable conditions such as podium connection, under the condition of meeting the allowable deformation value of the foundation of high-rise buildings, after comparing the foundation cost, it can be considered to deepen the bottom of the trough to an elevation of 34.00m, which is the bearing layer of fine silt ⑤ 1 and medium fine sand ⑤. ④ Silty clay and clayey silt, ④ clay and heavy silty clay have a thickness of1.20 ~1.50m, and the uniformity of the reserved soil layer is controlled in combination with the general drilling results at the bottom of the trench, and then it is backfilled with low-elevation plain concrete to the designed base elevation. The standard value (fka) of the foundation bearing capacity for this soil replacement treatment is 180kPa.
(3) Adopt bored pile scheme
The standard value qp of bearing capacity of bored pile tip and the standard value qs of soil friction around the pile are calculated according to the numerical values given in the survey report.
(4) Precast pile foundation scheme is adopted.
1) Choose φ400~900mm prestressed centrifugal concrete pipe pile, the pile tip must enter the bearing layer of fine sand not less than 0.50m, and the underlying layer is pebbles and pebbles. It is suggested that the standard value of vertical bearing capacity of single pile should be considered as 1400kN/ root.
2) In order to carry out the pile foundation construction smoothly, it is suggested to drill first and then drill, and the pilot hole test and diameter of pre-drilling shall be determined by the test pile. At the same time, before construction, the groundwater should be lowered below the elevation of 28.50 meters to prevent sand gushing and hole collapse during pre-drilling and piling.
13 times, of which the observation data of 12 times and 13 times are shown in Table 3.
Table 3
Technical and economic comparison and conclusion of 1.6
According to the comparison of market prices in Beijing:
Scheme (1), the total replacement cost (thickness of 3m) is 6.5438+0.4 million yuan, and the settlement is more than 7 cm;
Option 2, the replacement thickness is about 2m, plus the external expansion 2m, the cost is 6,543,800 yuan+3,000 yuan, and the settlement is more than 8cm; ;
Option 3: The unilateral cost of cast-in-place pile and reinforced concrete is 850 yuan, with a total cost of more than 2 million yuan;
Scheme ④: Precast piles, with a single cost of 1.020 yuan and a total cost of 2.4 million yuan;
Scheme ⑤, CFG long pile composite foundation, with a unilateral cost of 700 yuan, with a total cost of 6.5438+0.7 million yuan;
Scheme ⑥, CFG short pile composite foundation, with a single cost of 550 yuan, with a total cost of 6,543,800 yuan and a settlement of less than 8cm.
Conclusion: Through the above comparison, CFG short pile composite foundation can not only save investment but also meet the design requirements, which is the best scheme among the above comparison schemes.
2. Formulated the corresponding quality acceptance standard.
The construction quality of CFG short pile composite foundation technology strictly follows the Code for Acceptance of Construction Quality of Building Foundation Engineering (GB50202-2002). At the same time, according to the construction characteristics of CFG short piles, we uniformly stipulate the allowable deviation of short pile quality acceptance. Such as table: allowable deviation, inspection quantity and inspection method of CFG (or plain concrete) short piles.
Table 4
Study on reduction coefficient of bearing capacity of soil between piles in 3 CFG short pile composite foundation
There is generally no disturbance and compaction, and α is 1.0.
β is usually 0.75~ 1.0, which has a wide range and has a great influence on the design results. If the value is too conservative, it will cause great waste.
According to the calculation, the range of β value is 0.907≤β≤ 1.0, and the average value of β is 0.967. Therefore, it is suggested that the β value of the reduction coefficient of soil bearing capacity between piles should be 0.90≤β≤ 1.0. When β= 1.0 is taken, the error range between the calculated bearing capacity of composite foundation and the load test value is 0 ≤δ≤ 4. 13%.
It is suggested that the β value in the formula (1) should be 0.90≤β≤ 1.0. When there is no test data of pile design, its average value can also be 0.967 or a slightly conservative 0.95.
4 Characteristics and advantages of "short pile" compared with "long pile"
We call ordinary CFG piles "long piles" and short CFG piles "short piles".
After nearly two years of analysis, demonstration and research, we invented CFG short pile composite foundation technology. The technical feature of CFG short pile composite foundation technology is that the bearing layer of CFG pile tip is selected on the first sand layer or other hard soil layer in Beijing stratum rhythm, and its action mechanism can be regarded as end-bearing pile. The pile length of the foundation pit of 7~8 m in the second basement is generally 2.50~5.0m m. The hole-forming process is long spiral or manual Luoyang shovel dry method, the bottom of the hole is tamped with a heavy hammer, and CFG mixture or plain concrete is vibrated and compacted with a vibrating rod. Similarly, taking the 22 ~ 25th floor with the treatment area of 1000 m2 as an example, the foundation treatment cost is only 350,000 ~ 500,000 yuan, which can save 20%~60% compared with the "long pile scheme". Its remarkable characteristics are: saving money, good construction quality and meeting the requirements of design bearing capacity and final settlement.
Compared with the "long pile" composite foundation technology, the "short pile" technology has the following differences:
(1) design guiding ideology: "long pile" means "sparse pile theory", that is, sparse and long, making full use of the soil between piles to bear the upper vertical load; "Short pile" is "dense and short", which makes the pile bear more upper vertical load with large area replacement rate, and the pile and soil work together to meet the needs of the upper load. "Short pile" technology makes good use of the law of foundation settlement.
(2) Pile length: the length of "long pile" is more than 6 meters, and some even reach 16~23 meters, while the length of "short pile" is less than 6 meters.
(3) Replacement rate: the area replacement rate of "long piles" is generally not greater than 10%, and that of "short piles" is generally greater than 10%, and some are as high as 20%.
(4) Pile spacing: generally, the pile spacing is 3~6 times the pile diameter when "long piles" are arranged, and 1~3 times the pile diameter when "short piles" are arranged.
(5)CFG material: the strength grade of "long pile" CFG material is generally higher than C20, and there are many pumping agents; The strength grades of CFG materials used for "short piles" are generally C 10 and C 15, without pumping agent, and plain concrete materials made of coarse aggregate can also be used.
(6) length of protective piles: the length of protective piles for "long piles" is generally 0.5~0.7m, and that for "short piles" is generally 0.1~ 0.3m. ..
(7) Construction technology: the "long pile" adopts superfluid technology, that is, the hole is drilled with a long auger in the middle hole, and the CFG material is injected into the pile by a concrete pump, or the hole is drilled with a long auger, and the CFG material is injected into the pile by a vibrating immersed tube; "Short pile" is drilled with a long auger or a manual Luoyang shovel, the bottom of the hole is compacted, then CFG (or plain concrete) is poured, and the pile is rammed with a vibrator.
(8) Construction quality: It is difficult to guarantee the construction quality of "long piles", but it is extremely easy to guarantee the construction quality of "short piles" because of their short piles.
(9) Mechanical mechanism: "long piles" are mostly friction piles or end friction piles, and there is not necessarily a hard layer at the pile end; The "short pile" is mainly end bearing, and the pile end must have a hard layer, not a weak layer.
(10) Economy: the unit treatment area cost of "long pile" is higher, and the cost of "short pile" is lower. Taking the retired cadres' residential area 12 high-rise building in Beijing of the General Staff Department as an example, the lowest bid for cfg pile composite foundation treatment of each building in the "long pile scheme" is 750,000 yuan, and that in the "short pile scheme" is 450,000 yuan, which saves 3.6 million yuan in foundation treatment alone, which is equivalent to reducing the construction cost per square meter 10 yuan. Take Beiyuan Community as an example. The short pile scheme of a building is only 300 thousand, and the long pile scheme is 800 thousand. Only one cost of foundation treatment can save 500 thousand for one building, and there are 100 high-rise buildings in the whole region. This alone will save 50 million yuan.
Engineering example of 5 application
(1) Beijing Beiyuan Community 4-0 1#, 4-02#, 3-05#, 3-06#, 3- 12#, 5- 13# (both underground floors,
(2) Residential buildings 1, 2, 4, and 5 on the south side of the retired cadres' residential community in Beijing (2 floors underground, 24 floors above ground on the second floor of 1, 28 floors above ground on the fourth and fifth floors, shear wall structure);
(3) Building 5, Datun Staff Dormitory, Beijing No.1 Housing Management Company (2 floors underground, shear wall structure on the ground 16 floors);
(4) Buildings A and B of Beijing Jianguo Apartment (2 floors underground, 22-24 floors above ground, shear wall structure);
(5) Building 302, Sun Palace shaoyaoju Community, Chaoyang District, with 24 floors above ground and 3 floors underground;
(6) No.9 10 # residential building in the first phase of Shifoying Guangsha Times Home, Chaoyang District, in which Building No.9 has 28 floors above ground and 2 floors underground, and Building10 has 2 floors underground and 22 floors above ground;
(7) The first phase of Xinfadi Residential Quarter is the basement of Building No.3, with floors 10~ 13 above ground;
(8) Foundation treatment project of mineral wool sound-absorbing board project with an annual output of 25 million square meters on the east side of Xia 'an Road, Dachang Hui Autonomous County Industrial Park in Langfang;
(9) Beijing Haoting Garden 1# Residential Building (2 floors underground and 16 floors above ground).
6 benefit analysis
6. 1 Economic benefit analysis
Compared with "long pile", "short pile" has a remarkable effect in saving engineering investment.
There are 7 residential buildings with 2 floors underground and 24-26 floors above ground in Beijing Beiyuan Community, 4 residential buildings with 2 floors underground and 24-28 floors above ground in the south of Beijing retired cadres' residential area, and 2-24 floors underground and 22-24 floors above ground in Beijing Jianguo Apartment A and B, Beijing Haoting Garden 1# residential building. The original design is CFG pile, and the pile length is changed to plain concrete short pile, with 3.5. ..
Suppose Beijing builds 500 high-rise buildings with basements every year, and adopts "short pile" composite foundation technology. Calculated by saving 300,000 yuan per building, the annual project investment can be saved by 65.438+500 million yuan. Since 1998, the project investment should save no less than 654.38+0.5 billion yuan. It can be explained as the technical achievement of "short pile" composite foundation.
6.2 technical benefits: from the engineering practice effect, the aforementioned characteristics and construction points, the quality is easy to control, the qualified rate is 100%, and the construction period is short, which is shorter than other construction methods 1/4 ~ 1/2.
6.3 Social benefits: Compared with CFG piles, changing long piles into short piles saves the consumption of cement and sand, saves natural resources and saves a lot of energy; Fly ash, an industrial waste, is used as an additive of plain concrete, which solves the reuse problem of industrial waste.
6.4 Environmental benefits: During the construction of plain concrete short piles, there is no dust and vibration, which is suitable for the old city reconstruction. In particular, plain concrete can be purchased from commercial concrete suppliers in a unified way, which not only saves the cost, but also eliminates the environmental pollution caused by mixing in the construction site, which fully meets the requirements of green construction.
7. Publish or exchange academic papers
Discussion on the application of (1) CFG short pile composite foundation technology in foundation treatment of high-rise buildings (investigation project 1999 No.4)
(Application of CFG short pile composite foundation technology in foundation treatment of a residential area (No.3, Survey Engineering, 2000)
(3) Application of CFG short pile composite foundation with different diameters in a stepped high-rise residential building (200 1 supplement to exploration project).
(4) Study on reduction coefficient of bearing capacity of soil between piles of CFG short-pile composite foundation (survey engineering supplement in 2003)
(5) Application of CFG Short Pile Composite Foundation Technology in Foundation Treatment of High-rise Buildings (5th National Geotechnical Engineering Record Exchange Meeting), Ordnance Industry Press, September 2000.
(6) (6) Study on settlement law of CFG short pile composite foundation (Journal of changchun institute of technology (Natural Science Edition) 20 10003).
8. Conclusions and awards
To sum up, CFG short pile composite foundation technology is mature from theory to site construction practice, and it is the first technology in China, and has achieved remarkable economic, social and environmental benefits. We will conscientiously sum up and popularize it throughout North China, so that CFG short pile composite foundation technology will have a better application prospect.
"CFG (plain concrete) short pile composite foundation construction method" won the provincial construction method in 2008, and the construction method number is JSGF- 1 19-2008. And won the third prize of Nantong Science and Technology Progress Award and the first prize of Nantong Tongzhou District Science and Technology Progress Award. The project of "Foundation Reinforcement Project of Beijing Jianguo Apartment" completed by this technology won the first prize of excellent engineering survey of China Building Materials Engineering Construction (ministerial level) in 2002.
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