As an effective method to treat soft soil foundation, drainage consolidation method has been widely used in engineering. Drainage consolidation method can solve both settlement and stability problems. The settlement of the foundation is mostly or basically completed in the preloading period, and the building will not have adverse settlement and poor settlement during the service life, and the shear strength of the foundation soil will be accelerated, thus improving the bearing capacity and stability of the foundation.
Drainage consolidation method consists of drainage system and pressurization system. The drainage system mainly changes the original drainage boundary conditions of the foundation, increases the drainage channels of pore water and shortens the drainage channels. It consists of vertical drainage wells and horizontal drainage pads. As a vertical drainage channel, plastic drainage board composed of plastic core board and filter membrane jacket has been used more and more in engineering. Plastic drainage boards can be made in factories, which is convenient for transportation, especially suitable for areas lacking sand sources such as Sanmen, and can save investment at the same time. The pressurization system applies a load that plays a consolidation role, and the pore water in the soil oozes out to consolidate the soil due to the pressure difference.
Drainage system is a means, if there is no pressurization system, the water in the pores can't be discharged naturally without pressure difference, and the foundation can't be reinforced. If only the consolidation pressure is applied without shortening the drainage distance of the soil layer, the settlement required by the design cannot be completed as soon as possible during the preloading period, the strength of the soil cannot be improved in time, and the loading at all levels cannot be carried out smoothly. Therefore, when designing, drainage system and loading system always need to be considered together.
According to the analysis and calculation of settlement deformation, the soft soil layer is treated by plastic drainage board preloading method. It is required that after the site of each treatment area is leveled, a layer of geotextile and 0.8m thick gravel cushion shall be laid on the beach surface, and then plastic drainage boards shall be inserted. The plastic drainage board adopts SPB-IB standard drainage board with a width of 100mm and a thickness of 4mm, which is arranged in a quincunx shape with a spacing of 1 .5m. The depth of the drainage board should penetrate the bottom of the silt layer, as shown in figure1. See Table 2 for the plastic drainage boards required for each foundation treatment area.
Figure 1 Layout of Plastic Drainage Board
Table 2 Table of plastic drainage boards in foundation treatment areas
Area of foundation treatment area (ha) Number of plastic drainage boards (root) Site elevation of treatment area (m)
T3-2 6.50 33358 About 0~4.2
T 1-2 7.77 39876 approx.-1.0~2.0
T2-1-2 20.48105104 about-1.0~2.5
Total 34.75 178338
In order to adapt to the deformation of the foundation in the foundation treatment area and the direct backfill area, and prevent the foundation from cracking and sudden change of bearing capacity due to the great difference of the consolidation degree at the boundary line, the spacing of plastic drainage plates is changed from 1.5m to 2.0m within 20m near the direct backfill area in the treatment area, and the setting depth can also be appropriately reduced.
4 Determination of stowage diagram
According to the characteristics of the soil layer, the settlement and deformation of the soil layer under the surcharge preloading load are calculated, and then the time to reach a certain degree of consolidation and the increase of foundation strength after surcharge preloading are estimated, so as to evaluate the foundation conditions and put forward a reasonable backfill surcharge scheme and foundation treatment scheme.
Because the building area is large, it is assumed to be piled up in a large area. Backfill materials are dirt and stones dug from the mountains. In the calculation, the rock pile density r= 17.0kN/m3, the average height of backfill layer is 7.0m, the calculated thickness of soft soil layer (including silt layer and muddy soft clay) is 14.0m (south beach of the factory), and the consolidation coefficient is ch = cv = 2×10-3cm2/s.
4. 1 Final settlement calculation result
When the backfill height is 7.0m, the final deformation of foundation under preloading load is:
=1.50m.
Where: Sf-final vertical deformation;
E0i—— the void ratio corresponding to the dead weight pressure of point soil in the I layer;
E 1i—— the void ratio corresponding to the sum of the dead weight pressure and additional pressure of the point soil in the I layer;
Hi-I layer soil thickness;
X- empirical coefficient, 1. 1~ 1.4, when the load is large and the foundation soil is weak, take a larger value.
4.2 No treatment measures have been taken
According to Fellenius formula, the allowable first-class load on natural foundation P 1:
= 42.83 kPa
Among them, the undrained shear strength of natural foundation is determined by in-situ vane shear test results.
K- safety factor.
Under the load of P 1, when the degree of consolidation U=70%, the improved foundation strength is Cu 1:
Where: h- is the strength reduction factor considering shear creep;
Ut-the average degree of consolidation of foundation;
-Additional vertical pressure caused by preloading load;
Corresponding required time t:
= 12. 1 1 year
According to the above calculation, under the allowable load of P 1, the time required to reach 70% consolidation degree is as long as12.1/year, which can not meet the construction period requirements, and the shear strength of foundation does not increase much, only 37.6%.
The thickness of surcharge is 7.0m After preloading for 5 years, the consolidation degree and strength of foundation increase by 17.44kPa, so the shear strength of foundation only increases by about 7.44kPa.
Conclusion analysis:
After preloading for 5 years under 7.0m thick surcharge, the consolidation degree of soft soil layer is only 44.7%. No? Can meet the requirements of settlement and consolidation during construction;
Directly superpose 7.0m thick soil layer. Load P0 =119kpa > 42.83kpa (the first allowable load),? Will the foundation be cut? Destroy;
Judging from the above two points, Foundation treatment must be carried out. Considering the engineering characteristics, foundation conditions, construction period and other factors, the preloading drainage consolidation method is adopted for treatment. The drainage system adopts plastic drainage board.
4.3 Simplified calculation method of foundation with preloading drainage consolidation measures
1. Standard 2 adopts quasi-plastic drainage board (width b= 100mm, thickness d=4mm). The construction length of drainage board passing through soft clay layer is V, that is,14.0m. The converted diameter of plastic drainage board is:
= 66.2 mm.
The intransitive verb plastic drainage boards are arranged in an equilateral triangle, vii. Spacing l= 1.5m, viii. The equivalent circle diameter is:
=1.575m. 。
Nine. Some simplification has been made in the calculation. X. no xi. Considering the vertical drainage consolidation of soft soil layer, xii. Only inward radial drainage consolidation is considered.
Under the first-class allowable load P 1=42.83kPa (stacking thickness H 1=2.52m), the final settlement of foundation is S1= 0.65 m. When the degree of consolidation reaches 90%, the settlement is s 1'= 0.59m, and the foundation is improved.
=23.8
t 1=0.273yr=3.28mo
Repeat the above calculation, and the results are listed in Table 3.
Table 3 Calculation of Consolidation Settlement by Simplified Method
As can be seen from Table 3, the tertiary load is *** 12.46m thick, which is greater than the sum of 7.0m (calculated backfill thickness required by the factory elevation) and 1.50m (final settlement). According to the final backfill thickness of 8.50m, the shear strength Cu=29.64kPa.
Under the condition of inserting plastic drainage board, the total pile height is 8.50m, the settlement after 0.82 years is 1.50m, and the consolidation degree reaches 90%. The shear strength of foundation can be increased from 10.0kPa to 29.64kPa
4.4 Improved Shunsuke calculation method for takagi
The improved Takagi Shunsuke method is recommended in the code for foundation treatment. The form and arrangement of the plastic drainage board are described above. Considering the condition of graded constant velocity loading, the average consolidation degree of foundation corresponding to the total load at time t Ut:
Where:-loading rate of Class I load;
-Load rate of all levels of load;
—— The termination and start time of the first-class load (counting from zero).
See Table 4 for the calculation results of consolidation settlement of the improved Takagi Shunsuke method, and Table 5 for the corresponding safety factor of foundation bearing capacity.
Table 4 Calculation of Consolidation Settlement by Improved Takagi Shunsuke Method
Table 5 Safety margin of bearing capacity of load foundation during construction period
Although the minimum safety margin of foundation bearing capacity in the above table is small, and the lowest value is 1.0, considering the large backfill area, the foundation will not slide due to the mutual suppression of soil layers. This value is instantaneous. Because the plastic drainage board has good drainage effect, the pore water pressure dissipates quickly and the foundation soil consolidates, the corresponding bearing capacity will also be improved. Therefore, it is reasonable to think the foundation is safe.
4.5 Summary and comparison of two calculation methods
The simplified method is basically consistent with the calculation results used in the specification, as shown in Table 6.
Table 6 Comparison of calculation results between simplified method and recommended method in specification
Time (days) for the project to reach 90% consolidation degree, minimum safety factor and final settlement (expressed by load formula) (m)
Simplified Method 300 (Level 3) 1.2 Grading Instances 1.50
Recommended Method of Specification 370 (Grade 4) 1.0 Graded Isokinetic 1.74
4.6 stowage plan
According to the engineering geological conditions and different uses of the backfill site, the surcharge backfill can be divided into three categories: load-bearing backfill area, control backfill area and general backfill area. The load-bearing backfill area is used for backfilling the load-bearing foundation of buildings and structures, such as T 1-2 area. The controlled backfill area is used for backfilling the construction site area, the temporary construction area and the equipment and material storage yard, and T2- 1-2 and T3-2 which need foundation treatment belong to this area. The backfill area is generally the backfill of the standby site, mainly the area that does not need foundation treatment.
Rock is used to backfill the stacking load, and the packing grading requirements are good. The maximum block size of coarse stone is 500mm, which is not more than 2/3 of the laying thickness of each layer. The volume content of 500mm stone is not more than 20%, and it should be evenly distributed when backfilling. The compaction coefficient of T 1-2 bearing backfill area is 0.92 ~ 0.95, and that of T2- 1-2 and T3-2 controlling backfill area is 0.9 ~ 0.92.
In the foundation treatment area, the soft soil foundation is treated by plastic drainage board preloading method. The maximum pile height is 8.5m (including the backfill thickness of 7m and foundation settlement of 1.5m), and it is loaded in four stages, with loading thicknesses of 2.5m, 2.5m, 2.5m and 1m respectively. The loading process is shown in Figure 2, and the time difference between different levels of the construction loading plan can meet the requirements of earthwork excavation construction progress. The first-stage accumulation thickness includes 0.8m gravel filter cushion. For the convenience of construction, each layer can be backfilled in two layers, but the backfilling height of each layer should not be too high, about1~1.5m. Only when the first stage is completely filled can the second stage be filled. The backfill time difference between two levels in the same fixed area should be greater than 4~5 months. For areas with small backfill thickness, the height of the last backfill should be determined according to the natural elevation of the site and the required plant-level elevation, and the needs of backfill settlement and leveling design should be considered. However, the height of graded backfill load is still limited to 2.5m, and the graded interval should be met.
Figure 2 Loading Time History
It belongs to the range of 1.5m at the top of the load-bearing backfill area in T 1-2 area, and within the range of 1m at the periphery of the building, and there shall be no stones with a particle size exceeding 100mm; Within 5m away from the building, the thickness of each layer of coarse stone backfill layer is 0.75m, and stones with particle size exceeding 500mm should be removed.
5 Foundation treatment construction requirements
Backfill construction shall meet the following requirements:
The water in the fish and shrimp pond should be drained in advance to remove the silt and sundries on the surface. When filling on a stable hillside, the vegetation on the basement should be removed.
The filling construction shall be backfilled and compacted in nearly horizontal layers, and the upper layer can be filled only after the compaction coefficient and compaction range meet the design requirements.
According to the natural slope of the beach site, the site shall be leveled with each construction division as the unit, and the acceptance shall be made according to the grid of 10m× 10m. After leveling, the ground elevation difference in the reinforcement area should be within 20cm, and the leveling material should be gravel.
The silt content of the gravel cushion is less than 3%, the permeability coefficient is not less than 1× 10-2 cm/s, the vertical discharge flux of the plastic drain board is required to be more than 40× 10-6m2/s, and the filter membrane of the drain board is required to be made of medium-long or filament non-woven fabrics, with the permeability coefficient being more than 2×1.
Gravel cushion with a thickness of 0.8m shall be gravel or pebbles with good particle gradation and a particle size of ≤50mm.
Before backfilling, intermediate acceptance shall be made for the filling base and completed concealed works, and records shall be made. The compactness of the lower layer can be inspected before the upper layer can be constructed.
It is forbidden to backfill with unsuitable materials, including peat, wood, organic matter and perishable materials; Materials liable to spontaneous combustion; Clay and silt with liquid limit exceeding 80 and plasticity index exceeding 55; Materials with water content greater than the maximum allowable value of the material in the specification.
If the bad weather conditions affect the backfill quality, backfill should be stopped, and the temporary drainage system or ditch arranged by the construction unit should be removed after the work is completed.
Before backfilling and compaction, the construction unit should select representative plots and corresponding construction machinery according to previous engineering experience, and take the thickness, water content, rolling times and rolling speed of graded backfill layer determined by grading test as parameters to conduct tests to find out the reasonable parameters that best meet the design requirements.
6 On-site test monitoring
In order to ensure the expected effect of drainage consolidation method in soft foundation treatment, monitor the deformation and consolidation of foundation during construction, control the loading rate and process in time, prevent the shear failure and slip of foundation, and ensure the safety and quality of construction, it is necessary to monitor and analyze the deformation, consolidation and strength growth of foundation on the spot, so as to provide necessary foundation parameters for designing ground buildings in the future to meet the design requirements.
The main contents of field test monitoring include:
Basement settlement: the settlement plate is connected by double sleeves. According to the standard of fourth-class leveling? Quasi-measurement. About every 10000m2 1 settling plate.
Pore water pressure of foundation: Bored embedded steel string pore water pressure gauge is used to measure with frequency meter. About 1 group of pore water pressure measuring points are arranged every 20000m2. Each group is arranged along the depth. Monitor the dissipation of excess pore water pressure and the consolidation of foundation after plastic drainage board treatment.
Layered compression deformation of foundation: embedded layered settlement measuring tube and magnetic ring are adopted. Observe with electromagnetic sedimentation instrument. About every 30000m2, 1 layered settlement measuring holes are arranged. Understand the deformation and consolidation of each soil layer.
Lateral displacement monitoring of foundation: embedded inclinometer tube is adopted. Measure with an inclinometer. The lateral displacement measuring hole is arranged outside the treatment area. Monitor the influence of backfill stone on seawall.
On-site vane and soil-taking test: pre-embedded vane holes are used, and on-site vane cutting is carried out? Test instrument. Arrange 1 group of reserved holes for vane test every 30000m2. Shear at intervals with a depth of about 1.0m? Try it once.
Field load plate test: the load plate area is 2m2. About every 30000m2, arrange 1 field load plate test. Test the bearing capacity of foundation after backfilling.
According to the content of field test and monitoring, the corresponding control standards are put forward:
A) Control standard of Smax value of settlement observation point set on drainage gravel cushion b) Standard: [SMAX] ≤10 mm/d;
C) Control standard of lateral horizontal displacement rate Mmax of side piles set at the distance from pile toe 1m d) Accuracy: [mmax] ≤ 4 mm/d;
E) F) Control standard of excess pore water pressure coefficient Af value of pore water pressure gauges with different burial depths in foundation g) Standard: [AF] ≤ 0.6;
H) The control standards of other monitoring items i) Accurate: the values have not changed rapidly.
During the construction period, if one of the above control standards does not meet the requirements, the filling shall be stopped immediately.
7 Conclusions and recommendations
According to the monitoring experience of similar projects, the preloading of plastic drainage plate obviously improves the drainage consolidation process of soft soil layer and makes the whole project go smoothly. Under the load of backfill layer, the excess pore water pressure of foundation soil dissipates continuously, and the bearing capacity of foundation soil increases continuously. The influence depth of soft soil layer is mainly related to the overlying load and the depth of drainage plate treatment. According to the test results of layered settlement, pore water pressure and horizontal displacement, the main influence depth is within the treatment depth of drainage plate, and the influence on the stratum below drainage plate is small, and the physical and mechanical indexes of soil also change from silt to clay.
After drainage consolidation treatment, the main consolidation settlement of foundation soil in the site will be completed or nearly completed within two years of construction. However, due to the obvious secondary consolidation characteristics of soft soil foundation, secondary consolidation deformation will be long-term. It is suggested that all buildings on soft soil foundation should be provided with permanent settlement observation points according to regulations and observed regularly. According to the deformation of the building, the development law of the secondary consolidation deformation of the building can be calculated to prevent the building from being dangerous.
Based on the principle of safety and economy, Sanmen nuclear power plant adopts drainage consolidation method to treat foundation, which not only saves investment, but also meets the urgent requirements of construction period. The construction of plastic drainage board is convenient and fast, and it is relatively cheap in areas lacking sand sources. The construction technology is mature, the effect is remarkable and it is widely used. The drainage consolidation foundation treatment scheme of Sanmen Nuclear Power Plant has been fully affirmed by experts after many reviews. It is considered that the feasibility, economy and construction feasibility of the scheme should be highlighted: according to the engineering geological conditions and site use requirements, the zoning is basically reasonable; Reliable technology, economical and reasonable; Backfill requirements are feasible and will play a good role in large-scale excavation and backfilling.
refer to
Handbook of Foundation Treatment (2nd Edition), China Building Industry Press, August 2000;
Handbook of Engineering Geology (3rd Edition), China Building Industry Press,1February, 992;
Qian, Soil Mechanics, Hohai University Press, April1988;
Liu Songyu, Highway Foundation Treatment, Southeast University Press, 200 1 1.
Yin, Gong Xiaonan, Foundation Treatment Engineering Example, China Water Resources and Hydropower Press, July 2000.