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On the treatment of common quality problems in the construction of wet rotary excavation method with static slurry wall protection

2019-03-08

Based on many years of on-site construction management experience, the paper briefly discusses the problems that should be paid attention to in the wet rotary excavation construction of static slurry wall protection.

 

Handling of common quality problems


1. Pile foundation displacement and treatment method


The displacement is caused by errors in pile position measurement and setting out, excavation at night, low visibility, noise caused by drilling bit encountering different types of underground soil and rock layers in the process of construction, and the soil layer around the pile hole caused by the components of construction machinery. In order to ensure the accurate position of pile foundation, the measurement method should be improved, the polar coordinate method of total station should be selected to measure, and theodolite should be avoided as far as possible; construction at night should be prevented as far as possible, even if graduation is not possible, the illumination at night should be strengthened; the driver's operation should be strictly controlled, and the soil cutting should be carried out at a constant speed to prevent the machinery from being too violent; Steel casing should be used as far as possible to ensure the stability of surrounding soil layer.


2. Hole collapse and treatment method


Because the rotary drilling adopts the slurry wall protection, there is no mud circulation, the wall protection is thin, and the slurry proportion is unstable, the pipe mouth adding the stabilizing fluid directly scours the soil layer, the groundwater level is high, the underground presents quicksand and silt, and the soil texture is uneven. During the drilling process, the disturbance of scattered soil around the retaining wall is formed, and the reinforcement cage is lowered because the crane lifts the steel If the reinforcement cage is not vertical and unstable in the process of lowering, it will bump into the hole wall to form the damage of mud skin or hole wall, and then cause the pouring process and the collapse of pile hole. If the thickness of the bottom sediment is too high, the waste hole can be directly formed if the sediment is in serious condition, which will cause accidents such as broken pile and waste pile, which will constitute the secondary construction and increase the construction cost. In order to ensure the construction quality and reduce the construction cost, we should strictly control the component ratio of the slurry (stable fluid) to ensure accurate measurement and error within the specified range. Correct the method of adding stabilizing fluid so that the nozzle is directly against the casing wall. Strictly control the driver's operation and try to keep the soil cutting down at a uniform speed to prevent the soil layer from being disturbed by the construction process. After the completion of pile hole, the standard operation shall be carried out in the process of hole cleaning, reinforcement cage placing and concrete pouring, so as to prevent the collapse of hole forming. The corresponding construction technology should be formulated according to different geological conditions, so as to ensure the smooth drilling and hole forming and prevent the occurrence of construction accidents.


3. Over diameter of rotary drilling hole and its control method


Because the drilling tools disturb the surface layer of the construction ground, it tends to liquefy and lose its original stable state. Part of the soil on the hole wall falls into the hole, the mud quality is unqualified, the mud proportion, viscosity or sand content rate can not meet the technical requirements, and the mud surface in the hole is lower than the underground water level, which makes the mud unable to effectively protect the wall, and the underground water in the hole enters into the hole, forming the surface soil subsidence and loose collapse Collapse. When rotary drilling, the bit and drill pipe form a certain angle, which makes the reverse diameter of the drill bit larger than the drill bit diameter, and the hole forming diameter is larger than the designed hole diameter. Then the quality problems such as pile foundation deviation and pile cap deviation are formed. In order to ensure the engineering quality and prevent the borehole from exceeding the diameter, the mud quality is strictly controlled in the construction process, the height of the slurry surface in the hole is insisted, and the pressure on the hole wall is increased. When the soft flow plastic and loose area with high groundwater level, the quality of slurry should be ensured first, so that all objectives can meet the construction requirements, and the slurry can be fully protected. In order to adhere to the mud surface in the hole, a cofferdam is made at the hole opening to connect with the mud in the hole. When the drilling tool is put forward, the mud in the cofferdam is added to the hole, so that the mud in the hole is always higher than the groundwater level. The higher the mud surface is, the greater the pressure on the hole wall is, and the more stable the hole wall is. Control the drilling speed of the drilling rig to prevent the phenomenon of mud bag bit. When drilling depth is controlled, the footage in soft fluid plastic formation should be less than that in normal formation to prevent or reduce the resistance caused by piston effect during drilling. Adhere to the smooth surface of the drill bit to prevent or reduce the drilling paste in the soft fluid plastic formation. When lifting the drill bit, some cohesive soil is often attached around the bit, which should be removed in time to reduce the adhesion between the bit and the surrounding clay to prevent the formation of paste drilling. In order to reduce the joint gap between drill bit and drill pipe, it is necessary to check the connection clamp of drill bit and drill pipe, repair the wear of drill bit in time and reduce the connection gap.


4. Reinforcement cage deformation and treatment method


Because the rotary drilling rig is basically used for the construction of deep pile foundation, the reinforcement cage is relatively slender, and the reinforcement cage is directly inserted into the rock at the bottom of the hole, and the reinforcement deformation occurs simply because of its large component and slender instability. In one case, in order to save the construction cost and time, some workers directly selected the auxiliary steel wire rope of the rotary drilling rig to hoist the reinforcement cage, which made the reinforcement cage and the drill arm collide and deform. During the construction, the reinforcement cage can be reinforced with binwood pole and steel wire rope, and the crane higher than 6 meters or the construction shall be used to lift the reinforcement cage vertically, so that the reinforcement cage can be installed vertically up and down, so as to prevent the deformation of the reinforcement cage caused by the collision between the reinforcement cage and the pile hole wall.


5. Protection of anchorage reinforcement at pile top


Because the wet rotary excavation method of static slurry wall protection is selected in the construction, the excavated soil is relatively thin and soft, the liquidity is large, and the distance between piles is small, so it is very simple to bury the cast piles nearby. In the later excavation and the process of rotary excavator transposition, the reinforcement at the top of pile is repeatedly bent, which damages the structure of reinforcement and even leads to the fracture of reinforcement, resulting in insufficient anchorage length of reinforcement and affecting the quality of the project. In the process of construction, in order to prevent this phenomenon, excavators and rotary excavators should be selected for construction, and the soil excavated by the rotary excavator should be timely sorted out to prevent the soil from burying the reinforcement. Determine the pile foundation construction sequence in advance, and try to make the rotary excavator backward and transpose construction.


6. Concrete pouring quality assurance method


As the depth of rotary excavation pile is generally large, the tremie method is generally used to pour underwater concrete, which is much more difficult than ordinary concrete pouring. In order to ensure the quality of underwater concrete, it is necessary to adopt measures from the following aspects: 1) strictly control the concrete mix ratio, ensure that the concrete bulk density is not less than 2100Kg / m3, and ensure the workability of concrete The bleeding rate of huningtu can ensure the adherence of concrete fluidity.

 

 



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