How to calculate the static load of composite foundation
There is no calculation formula, which can be determined by the following methods:
(1) in situ test method: it is a method to determine the bearing capacity through direct field test. It includes (static) load test, static cone penetration test, standard penetration test, pressuremeter test and so on
(2) theoretical formula method: it is a method to determine the bearing capacity according to the theoretical formula calculated by the shear strength index of soil
(3) standard table method: according to the indoor test index, field test index or field identification index, the bearing capacity can be obtained by consulting the table listed in the specification. Different specifications (including specifications of different departments, instries and regions) will not have the same bearing capacity, so it is necessary to pay attention to their respective service conditions
(4) local experience method: it is a method to determine the bearing capacity by analogy based on the regional experience. It is a macro auxiliary method 
extended data
advantages of composite foundation
(1) screw pile composite foundation forms suitable plane and space stiffness gradient in the foundation, so as to obtain high strength composite foundation
(2) the three-dimensional stress state of soil is formed in the screw pile composite foundation, which makes the strength of soil higher than the basic value of its own bearing capacity, so that the participation coefficient of soil is greater than 1, which is impossible for any other type of composite foundation (3) the optimized vertical stiffness of screw pile composite foundation forms three-layer foundation, which reces the settlement of composite foundation. In particular, it effectively solves the problem of uneven settlement of buildings or structures (4) the design of screw pile composite foundation can effectively rece the impact of seismic force on the structure, at the same time, even in the case of excessive horizontal displacement of buildings, it can still effectively transfer the vertical load, and because the liquefiable soil layer is eliminated after reinforcement, it can be widely used in seismic areas (5) screw pile composite foundation can be constructed by domestic machinery, so it has wide adaptability of equipment and technology (6) the screw pile composite foundation is suitable for the soil layer which needs artificial methods to improve its bearing capacity. For example: miscellaneous fill, macroporous soil, muddy soil, expansive soil, collapsible loess, loose silty sand, various continental and marine deposits or their interbedding (7) screw pile composite foundation can greatly improve the bearing capacity of foundation, improve the performance of soil between piles and rece settlement, so it can be widely used in the foundation treatment of high-rise buildings, airports, storage yards, subgrade engineering, bridge foundation, oil storage tanks and other projects (8) in the composite foundation of screw pile, the screw pile can be used as conventional building materials, so it has the characteristics of wide sources of materials and cheap materials (9) the test and acceptance method of screw pile composite foundation meets the requirements of national standardsFor each test, 0.5% of the total number of piles should be carried out, and the number of piles should not be less than 3
The test ofCFG pile requires the sampling test of the bearing capacity of single pile and composite foundation, and the static load test is generally used. Single pile is to test whether the quality and bearing capacity of single pile meet the design requirements, and composite foundation bearing capacity is to test whether the bearing capacity of integrated foundation meets the design requirements and upper load requirements
In general, two piles, three piles and four piles are used to test the bearing capacity of composite foundation together with the soil and cushion between piles. The main reason is that the requirements of load plate size are different. It is the most direct and accurate means to test the bearing capacity of composite foundation at present. Because the composite foundation is the entity directly bearing the upper load
extended data:
precautions for single pile static load test
(1) ring the transportation of test equipment and instruments, it should be ensured that they are not damaged to ensure the accuracy of field test data
(2) necessary safety measures should be taken to ensure the correct position of equipment and the safety of personnel and equipment(3) the installation and welding of reaction frame should be firm and reliable, and the reaction device that does not meet the requirements cannot be formally tested and loaded.
(4) it is strictly forbidden to overload the reaction steel beam in the test, so as to avoid the damage of personnel and instruments
(5) temporary tents or facilities should be set up on the test site to protect the instruments and equipment (6) high pressure oil pump and other equipment should be placed in accordance with the principle of proximity, convenience and safety(7) the power supply connected to the test site must meet the requirements of temporary erection of power lines. It is forbidden to pull the power supply and wires randomly to prevent leakage, electric shock and other accidents
composite foundation is a kind of artificial foundation, which is composed of two parts: matrix (natural foundation soil or improved natural foundation soil) and reinforcement body. Under the load, the matrix and reinforcement bear the load together. According to the load transfer mechanism of composite foundation, the composite foundation is divided into vertical reinforced composite foundation and horizontal reinforced composite foundation, and the vertical reinforced composite foundation is divided into three kinds: loose material pile composite foundation, flexible pile composite foundation and rigid pile composite foundation
the detection of foundation bearing capacity can be divided into natural foundation bearing capacity detection and composite foundation bearing capacity detection, but the detection methods and principles are similar. Plate static load test is usually used to test
the specific method is roughly as follows:
steel plates of a certain size (with certain stiffness, so generally multiple steel plates are stacked from top to bottom) are used and placed on the foundation soil to be tested. In case of composite foundation testing, a certain thickness of cushion layer is laid under the steel plate and a jack is placed on it, The load is erected on the jack, and the counterweight is stacked on it. Cement blocks or sand and gravel in bags can be used as counterweight. If the bearing capacity is large, anchor piles can also be used as reaction force. Then the jack is loaded step by step and the corresponding settlement is measured to guide the foundation soil settlement to meet the unstable conditions. The bearing capacity of the foundation can be calculated by dividing the measured load weight by the area of the steel plate.
2. The load plate area of single pile composite foundation load test is the treatment area of a pile; The load plate area of multi pile composite foundation load test is the treatment area of multi pile
3. If you know the calculation principle of the treatment area, you can calculate it directly according to the geometric proportion. If you don't want to know so much, you can remember the following fixed parameters: the equivalent diameter of the treatment area borne by a pile is 1.05 times of the pile spacing. When you calculate the area of this circle, it is the same as when the piles are arranged in an equilateral triangle, Treatment area of single pile; The calculation principle of square pile arrangement is the same as that of triangle pile arrangement, but the equivalent diameter is 1.13 times of pile spacing
4. For the static load test of multi pile composite foundation, the static load test of several piles can be calculated according to several times of the treatment area of single pile.
1. Load test of composite foundation
this test is used to determine the bearing capacity and deformation parameters of composite foundation within the main influence range of stress under pressure plate
the bearing plate used in composite foundation loading test should have enough stiffness. The bearing plate of single pile composite foundation load test can be round or square, and the area is the treatment area of one pile; The bearing plate of multi pile composite foundation load test can be square or rectangular, and its size is determined according to the treatment area borne by the actual number of piles. The center (or centroid) of the pile should be consistent with the center of the bearing plate and coincide with the load action point
The bottom elevation of bearing plate should be consistent with the design elevation of pile top. Coarse sand or medium sand cushion should be laid under the bottom of bearing plate, with the cushion thickness of 50-150 mm, and the larger value should be taken when the pile strength is high. The length and width of the test pit at the test level shall not be less than 3 times the size of the bearing plate. The fulcrum of the reference beam shall be set outside the test pit Before the test, measures should be taken to prevent the water content change or disturbance of the foundation soil in the test site, so as not to affect the test results The loading level can be divided into 8-12 levels. The maximum loading pressure shall not be less than 2 times of the design pressurethe settlement of bearing plate should be read and recorded before and after each level of load, and then every half an hour. When the settlement within one hour is less than 0.1 mm, the next level load can be added
the unloading stage can be half of the loading stage, and it can be carried out in the same amount. For each unloading stage, the rebound amount is read and recorded every half an hour. After unloading all the loads, the total rebound amount is read and recorded every three hours
When one of the following phenomena occurs, the test can be terminated (2) the accumulated settlement of bearing plate is more than 6% of its width or diameter(3) when the ultimate load is not reached, the maximum loading pressure is more than 2 times of the design pressure
(1) when the ultimate load on the pressure settlement curve can be determined and its value is not less than 2 times of the corresponding proportional limit, the proportional limit can be taken; When its value is less than 2 times of the corresponding proportional limit, half of the ultimate load can be taken (2) when the pressure settlement curve is smooth and gentle, it can be determined according to the relative deformation value:A. for sand gravel pile, vibroflotation pile composite foundation or dynamic compaction replacement Pier - when the foundation is mainly composed of cohesive soil, the pressure corresponding to S / D equal to 0.015 can be taken (s is the settlement of bearing plate in load test; s is the settlement of bearing plate in dynamic compaction test); B and D are the width and diameter of the bearing plate respectively. When the value is greater than 2m, it is calculated as 2m); When the foundation is mainly composed of silt or sand, the pressure corresponding to S / D equal to 0.01 can be taken
For the composite foundation of soil compaction pile, lime pile or column hammer pile, the pressure corresponding to S / D equal to 0.012 can be taken. For lime soil compaction pile composite foundation, the pressure corresponding to S / D equal to 0.008 can be taken For the composite foundation of cement fly ash gravel pile or rammed cement soil pile, or for the foundation mainly composed of pebble, round gravel and dense coarse and medium sand, the pressure corresponding to S / D equal to 0.008 can be taken; When the foundation is mainly composed of cohesive soil and silt, the pressure corresponding to S / D equal to 0.01 can be taken For the composite foundation of cement soil mixing pile or jet grouting pile, the pressure corresponding to S / D equal to 0.006 can be taken For the experienced area, the relative deformation value can also be determined according to the local experience. The characteristic value of bearing capacity determined by relative deformation value should not be more than half of the maximum load value The number of test points should not be less than 3. When the range is not more than 30% of the average value, the average value can be taken as the characteristic value of composite foundation bearing capacity Fujian Academy of Building Sciences. 2005. Foundation and foundation engineering construction and testing technology http://www.fjjky.com/kycg.htm
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In dynamic penetration test, the cone probe of a certain specification is driven into the soil by hammering function, and the change of soil layer is judged according to the impedance of driving into the soil