Plastic Drainage Board To Set Up “return belt” Solution
Abstract: During the vacuum preloading of soft soil foundation, due to the complexity of muddy dispersion and deposition during the filling process, the friction between the drainage board and the bottom of the mud layer in the shallow foundation reinforcement area is too small, resulting in The soil is not entrained, causing a “return” phenomenon. Through the mechanical balance calculation and engineering practice, the vibration method, water injection method, pile shoe modification method, etc. are adopted to change the structure of the bottom plate of the drainage board or increase the load, so as to effectively solve the “return zone” of the plastic drainage board in the shallow foundation reinforcement area. The phenomenon of improving the quality and efficiency of plastic drainage board installation.
Keywords: shallow foundation reinforcement; plastic drainage board; belt back; solution
In the soft soil foundation treatment process of Tianjin Port Port Engineering, the construction process of “shallow drainage consolidation and deep vacuum preloading” is generally adopted. The specification stipulates that “vertical drainage system should adopt plastic drainage board”. Therefore, the application of plastic drainage boards in engineering is becoming more and more extensive, especially in the process of landfilling and landing, the sediment with a higher density is generally deposited at the bottom of the harbor basin, and the fluid mud with lower density is usually easy to stay. In the upper part of the harbor basin, this results in the construction of the plastic drainage board in the shallow foundation reinforcement area where the muddy surface is thinner, and the friction between the drainage board and the drainage board is small, resulting in frequent non-entrainment, causing the drainage board to return. The specification clearly stipulates that “the length of the retraction shall not exceed 500 mm when the device is set, and the number of rewinds shall not exceed 5% of the total number”. In the actual construction process, the length and number of the returning belts of the drainage board are far greater than the standard regulations. The drainage boards that do not meet the requirements of the specification need to be repaired, which not only has a high cost, low efficiency but also affects the construction period. In order to solve this problem, the technical personnel actively carry out technical research, through the mechanical balance calculation and engineering practice, take measures such as vibration method, water injection method, pile shoe modification method, etc., change the bottom plate structure of the drainage board or increase the load to increase The friction at the end of the plate makes the bottom mud smoothly clamp the drainage plate.
1. Project Overview
Tianjin Nangang Industrial Zone, the port channel and the port pool dredging combined with the landfilling and landing project (the land area of the west side of B03 Road), the total treatment area of the ground treatment area are 2,158,100 m2, divided into 91 communities, and the design uses “shallow layer”. The combination of pumping consolidation and deep in-line vacuum preloading process requires a plastic drainage board of 5.396 million meters in the shallow layer and 78.445 million meters of plastic drainage board in the deep layer.
2. Stress analysis of the end of the plastic drainage board
The plastic drainage board enters the soil together with the pile pipe, and the pile is formed into a hole. During the lifting process of the pile pipe, the soil around the hole wall collapses to clamp the drainage plate. In the process of lifting the pile pipe, the force analysis of the drainage board is shown in Figure 1.
Note: G1 is the gravity of the plastic drainage board; G2 is the gravity of the pile shoe; f is the vertical friction force of the soil acting on the plastic drainage board; N is the frictional resistance of the plastic drainage board when the pile tube is lifted.
Figure 1 The end of the plastic drainage board is stressed
According to the force diagram analysis, when G1+G2+f>N, the plastic drainage board can stay in the soil smoothly; when G1+G2+f<N, the “return” phenomenon of the plastic drainage board will not be avoided. Therefore, in order to reduce the retraction rate of the plastic drainage board, it is necessary to increase the force in the gravity direction of the plastic drainage board while minimizing the reverse force in the gravity direction of the plastic drainage board. Under normal circumstances, G1 and G2 do not change substantially. Therefore, increasing f and decreasing N will be the direct way to theoretically solve the phenomenon of plastic drainage board rewound.
3. Plastic drainage board back to the solution
3.1 Vibration method
In the process of setting up the plastic drainage board, after the pile pipe is inserted into the soil body, it is not appropriate to lift the pile rod quickly, and the vibrating hammer on the pile machine can be first opened to stir the pile tube in the soil. After vibrating several times, the pile tube is slowly lifted up, and the soil around the pile hole becomes loose due to the disturbance, which is easy to collapse, and the plastic drainage board can be well clamped. The method is mainly to fully contact the soil body and the drainage board to increase the vertical friction force of the soil body acting on the plastic drainage board, thereby satisfying the G1+G2+f>N, and reducing the effect of reducing the “return zone” of the plastic drainage board.
3.2 Water injection method
In the process of stacking the pile pipe, water is injected into the pile pipe, thereby additionally increasing the force of the gravity direction of the plastic drainage board, so that G3 (water weight in the pile pipe) + G1+G2+f>N solves the phenomenon of plastic belt drainage. The water injection device is mainly composed of a high pressure water pump and a high pressure resistant plastic water pipe. At the upper end of the pile pipe, one end of the plastic water pipe is welded at the inlet of the plastic drainage board, and the hose is connected to the groundwater pump. During the process of setting up the pile pipe, the operator opens the water pump and injects water into the pile pipe. The degree of water injection is controlled by the on-site operator according to the geological conditions. If there is thick silt or muddy soil layer in the soil, the water injection time can be increased appropriately, that is, the water injection is stopped after a moment of insertion into the design depth. When the water injection is stopped, the water in the pile pipe is not overflowed from the inlet, and then the pipe is raised. This not only increases the force of the gravity of the plastic drainage board but also discharges the sludge squeezed into the pile pipe and increases the plastic drainage board. Set the success rate.
3.3 Pile shoe modification method
3.3.1 Types and specifications of pile boots
The main purpose of the pile shoe is to increase the weight and friction of the pile shoe and meet G1+G2+f>N. The use of the pile shoe modification method to assist in the design of plastic drainage board is mainly for complex soil, especially clay soil, and the suitability of the pile shoe modification method is strong. According to the actual situation of the site, pile shoes of different types and specifications were produced, as shown in Table 1.
Table 1 Physical indicators of different types and specifications of pile boots
Note: The special stick-shaped rubber pile boots have a special shape and are characterized by two ends wide and thin in the middle; iron plate pile boots with arched handles, the arch body has a diameter of 0.5cm and a volume of 2.85cm3.
3.3.2 Pile shoe construction
(1) Construction of stick pile boots. First, pull the drainage board out from the bottom of the pile tube, wrap the stick-type pile boots, fix it with large staples, and tighten the other end of the drainage board so that the drainage board is wrapped around the stick pile boots. In the groove at the bottom of the pile tube, the board construction begins. During the construction process, the pile shoes are left in the soil, thereby increasing the vertical friction of the soil acting on the plastic drainage board.
The advantage of this process is that the waste steel head or the low-cost rubber rod can be used as the pile shoe, the engineering cost is slightly lower, and the process is simple; the disadvantage is that if the hard soil layer is encountered, the bending phenomenon is easy to occur, resulting in the bottom soil. The friction is reduced. The stick pile boots are shown in Figures 2 to 4.
Figure 2 stick pile boots
Figure 3 different types of stick pile boots
Figure 4 stick wire rope pile boots
(2) Construction of fancy pile boots. The ends of the steel bar are manually disrupted, and the friction with the lower soil is increased. The operation mode is similar to that of the stick pile shoe. The advantage is that the friction with the soil is increased, and the certain success rate is improved; the disadvantage is that the head of the steel bar is disturbed, a certain labor is required, and when the hard soil layer is encountered, the bending phenomenon is easy to occur, which affects the quality of the entrainment. Fancy pile boots are shown in Figures 5 and 6.
Figure 5 fancy pile boots
Figure 6 fancy wire rope pile boots
(3) Construction of iron plate pile shoes. The disposable steel sheet pile boots are in the upper part of the rectangular steel plate, and the spot welding handle shape steel strip. During the construction process, the drainage board is first pulled out from the bottom of the pile tube, passed through the pull-type steel strip and folded back, folded back to a length of 15 to 20 cm, and then fixed. The operator tightens the other end of the drainage board so that the steel plate is in close contact with the bottom of the pile shoe, and finally, the board is constructed. The process is to leave the steel sheet pile boots in the soil, which not only increases the self-weight of the pile shoes, but also increases the vertical friction force of the soil body acting on the plastic drainage board due to the connection of the drainage board and the steel plate, thereby effectively controlling the “return zone”. “phenomenon.
The advantage of the process is that when the pile pipe is set to the silt layer, the steel plate is tightly wrapped at the bottom end of the pile pipe, and the sludge is not easily squeezed into the pile pipe, thereby reducing the frictional resistance of the plastic pipe to the plastic pipe when the pile pipe is lifted; When it is set to a hard soil layer, the drainage plate is protected by the steel plate, and the phenomenon of broken plate is not easy to occur; the disadvantage is that one steel plate is required to set one drainage plate, the engineering cost is slightly higher, and the steel plate is not recyclable. Iron plate pile boots are shown in Figures 7 and 8.
Figure 7 iron plate pile boots
4. Application effect
Under normal circumstances, the soil in the lower part of the dredging area is not uniform due to factors such as the order of filling, soil deposition, and water flow direction. It is necessary to conduct trials and make a rational selection. Therefore, in this project, six communities were selected as test areas, and the drainage board installation test was carried out. Each district took different measures to summarize and analyze the installed drainage boards and set up the “return zone” situation. See Table 2.
It can be seen from Table 2 that the return rate reached 11.1% before the measures were taken, and the return length exceeded 500 mm, accounting for 23.7% of the total number of return belts. After taking measures, the return rate was 1.3% to 4.7%. The length of the retracting belt is not more than 500mm, and the quality of the drainage board is obviously improved. Since the quality of the drainage board after the installation meets the requirements of the specification, there is no need to make up, which improves the efficiency of the installation and reduces the construction cost.
Table 2 “Drawback” situation of the plastic drainage board
Note: The stick-type pile boots only tested the drainage board for the stick-type steel pile boots.
(1) By adopting different plastic drainage board construction methods such as vibration method, water injection method and pile shoe reforming method, the problem of “returning belt” in shallow foundation reinforcement is effectively solved, and the success rate of plastic drainage board is greatly improved.
(2) In the construction, if the vibration method and the water injection method are used, the trial can be carried out first. If the soil entrainment problem cannot be solved, the pile shoe modification method can be considered.
(3) Through field tests, the vibration method is applicable to silty soil or loose silt and sand. The water injection method is applicable to silt, cohesive soil and shell soil.
(4) When the stick-type pile boots hit the hard soil layer, the bending phenomenon is easy to occur, resulting in a decrease in the friction with the bottom soil. The pile boots are suitable for soft clay soil; the fancy pile boots encounter hard soil. When the layer is layered, it is prone to bending and affects the quality of the entrainment. This kind of pile shoe is suitable for sticky soil; the iron plate type pile shoe is equipped with plastic drainage board “return rate”, the bottom can be reduced to below 2%, suitable for all The soft soil layer and the hard soil layer will cause certain bending and deformation when used in the hard soil layer, but generally do not affect the entrainment effect, and the iron plate type pile shoe is slightly higher in cost.
(5) Using the above method, the quality and efficiency of the plastic drainage board in the shallow foundation reinforcement process is improved, which lays a good foundation for the vacuum preloading in the later stage, and also provides a strong guarantee for the foundation reinforcement effect.