1 Preface

 

　　The riverside pump house of a power plant is designed to be constructed as a reinforced concrete caisson, with a clear height of 31.5 m and a plan dimension of 30.1 m × 40 m. Because it is located on the inner side of the river embankment and construction spans the flood season, and the soil layer to be sunk through is a clay layer, the hydraulic mechanical drainage sinking method is proposed. This method has simple equipment, fast construction progress and safe operation. It has been used in many caisson constructions in China and considerable construction experience has been accumulated.

 

 

　　The hydraulic mechanical method of caisson sinking mainly uses high-pressure water jet equipment to cut, scour and agitate the soil at the bottom of the caisson into slurry, and then uses a hydraulic mud suction machine to discharge the slurry out of the caisson, thereby causing the caisson to sink.

 

 

　　The main equipment for sinking caissons by the hydraulic mechanical method includes:

 

　　3.1 High-pressure water pump: mainly used for water supply;

 

　　3.2 Hydraulic soil-scouring machine: also called a water gun, mainly uses its jet to scour and cut the soil layer at the bottom of the caisson, and uses its jet to agitate the soil layer to turn it into slurry.

 

　　3.3 Hydraulic mud suction machine: mainly used to discharge the slurry formed by the soil-scouring machine out of the caisson, causing the caisson to sink.

 

　　3.4 Piping system: divided into high-pressure water inlet pipes and low-pressure mud discharge pipes.

 

 

　　4.1 Construction process

 

　　Cleaning inside the shaft → excavation of mud pit and water storage tank → installation of piping system → installation of hydraulic machinery inside the shaft → installation of high-pressure water pump → commissioning of hydraulic machinery → hydraulic soil scouring and mud discharge → monitoring of caisson position → cyclic construction of continued soil scouring, mud discharge and position monitoring → caisson in place → removal of mud at the caisson bottom

 

　　4.2 Operation method: according to the compartments divided by the caisson bottom beams, equip a corresponding number of hydraulic soil-scouring machines. The mechanical equipment layout is shown in Figure 1:

 

　　During construction, first use the hydraulic soil-scouring machine to scour and excavate a mud collection pit with a diameter of 2000-2500 mm below its nozzle, then use the hydraulic soil-scouring machine to open 2-4 water trenches leading to the mud collection pit in various directions, with a longitudinal slope of 3%-5%. After that, the pot bottom can be excavated outward. To prevent sudden sinking of the caisson from causing large deviations and to reduce disturbance and collapse of soil outside the shaft, a soil berm 500-1000 mm wide may be retained beside the cutting curb around the caisson, and after the pot bottom excavation is completed, the soil berm is gradually and evenly scoured and excavated.

 

　　Scouring and excavation should be carried out according to the compartments divided by the bottom beams, and simultaneous excavation of multiple compartments should be sought. When conditions for simultaneous excavation truly cannot be met, symmetrical and uniform excavation may be adopted, with the principle of ensuring uniform sinking of the caisson.

 

　　4.3 Construction preparation

 

　　4.3.1 Before operation, ensure that the water source is unobstructed, the mud-water discharge system is unobstructed, the power supply system is normal, and the hydraulic machinery equipment has been installed and commissioned normally before mud suction and discharge operations may proceed.

 

　　4.3.2 After the caisson is completed, organize the cleaning of debris inside the shaft, and chisel out the originally poured concrete cushion and transport it out of the shaft before hydraulic mechanical mud suction sinking can begin.

 

　　4.3.3 Before the caisson sinks to the wall-penetrating sleeve openings, the wall-penetrating sleeves shall be plugged.

 

　　4.4 Key points of construction operation

 

　　4.4.1 Install the hydraulic mechanical facilities as required: high-pressure water pump, low-pressure water intake pump, regulating water tank, hydraulic mud suction machine, water gun, high-pressure water inlet pipes and low-pressure mud discharge piping above and below the shaft, water gun operation platform, etc.

 

　　4.4.2 After the hydraulic mechanical facilities are installed, carry out load trial operation to ensure that the equipment can operate normally as required. Both the hydraulic mud suction machine and the water gun shall pass a water pressure test, with the test pressure being 1.5 times the working pressure.

 

　　4.4.3 To ensure the effect of hydraulic soil breaking and the normal operation of the mud suction machine, all kinds of debris and garbage at the bottom of the shaft must be cleaned up.

 

　　4.4.4 The sinking construction of the caisson by hydraulic soil discharge shall be operated according to the principles of "center first, edges later; layered symmetrical soil breaking; high first, low later; timely correction of deviation".

 

　　4.4.5 A soil berm about 0.5-1.0 m wide shall be retained beside the outer cutting curb of the caisson, so that the caisson squeezes soil and sinks at the outer cutting curb, thereby reducing disturbance to the surrounding soil. Only when the soil in the middle of the caisson has all been scoured away and the caisson still does not sink, or when correcting deviation, may the soil at the outer cutting curb be properly scoured away, but it is still strictly prohibited to use a water gun to dig and wash the soil outside the outer cutting curb tread.

 

　　4.4.6 When the caisson tilt is very small, the height difference of the soil surface in each shaft compartment should preferably be controlled within 1 m, so that the caisson maintains uniform vertical sinking.

 

　　4.4.7 The depth of the pot bottom in the middle of the caisson compartment should generally preferably be controlled within 1-2 m. If the pot bottom is too deep, sudden sinking is likely to occur, making the caisson sinking amount and tilt uncontrollable; at the same time, soil outside the shaft is also likely to flow plastically into the shaft, causing excessive settlement of the ground around the shaft.

 

　　4.4.8 When scouring mud with hydraulic machinery, a mud collection pit about 0.5 m deep should first be scoured out at the suction head of the mud suction machine (generally located in the middle of the shaft compartment), and then a water gun should be used to scour soil in all directions to continuously form funnel-shaped soil pits, so that the muddy water continuously collects into the mud collection pit and is sucked and discharged out of the shaft.

 

　　4.4.9 The suction head of the mud suction machine should be suspended with a rope; the suction head should not be buried in the floating mud in the mud collection pit, which would cause soil blocks to block the mesh cover of the suction head. To prevent silt and sand from accumulating in the mud collection pit, a water gun should often be used to scour and agitate at the suction head so that soil discharge remains unobstructed.

 

　　4.4.10 When the water gun is scouring mud, attention should be paid to controlling the amount of muddy water in the shaft compartment, and the water gun should directly scour the soil body. When there is excessive muddy water, the water output of the water gun may be adjusted or soil scouring may be suspended. The normal operation of the mud suction machine should be checked frequently.

 

　　4.4.11 When the high-pressure water pump is stopped, the control valve on the water gun and the check valve on the mud suction machine should both be closed to prevent water in the regulating water tank from flowing back into the shaft.

 

　　4.4.12 During construction, various construction monitoring tasks should be carried out promptly, with particular attention to settlement observation and on-site monitoring of ground structures and underground pipelines within the influence range of caisson sinking.

 

　　4.5 Common problems during construction and treatment measures

 

　　4.5.1 The hydraulic mud suction machine does not suck mud

 

　　Cause: The mud suction pipe is blocked or the throat pipe is blocked.

 

　　Treatment measure: Remove the mud and sand blocking the mud suction pipe, or use backwashing to remove the obstruction in the throat pipe.

 

　　4.5.2 Water flows back into the shaft through the mud suction pipe of the hydraulic mud suction machine

 

　　Cause: The throat pipe or nozzle is blocked.

 

　　Treatment measure: Clean the throat pipe or nozzle.

 

　　4.5.3 When the hydraulic mud suction machine is working normally, there is a lot of accumulated water in the shaft

 

　　Cause: The nozzle diameter of the hydraulic soil-scouring machine is too large.

 

　　Treatment measure: Replace the nozzle of the hydraulic soil-scouring machine to reduce the water volume during soil scouring.

 

 

　　Caisson sinking is a task with a complex process and considerable technical difficulty. It is necessary to first understand and analyze the site conditions, geological data, meteorological data and design conditions in detail before deciding which method to use for caisson sinking. This paper mainly introduces one of many caisson sinking methods, and also a relatively practical one, for reference by peers.