In the construction of medium-thick pebble strata, fishing sand drills and rock drills (roller drills, barrel drills) are mostly used.

The characteristics of the sand fishing bit: First, there is a baffle plate at the bottom of the bit to prevent the slag from falling off, which ensures the effective drilling efficiency; Intensity perturbs the pebbles. When encountering dense and hard geological conditions with pebbles, use rock drills (roller drills, barrel drills) to disturb and destroy the stratum structure, and then use sand digging bits to remove the slag.

Precautions for construction operation

1. Safety in the hole

Pay attention to check the mud liquid in the casing and the surrounding terrain changes. If there is an abnormality around the casing, the drilling should be stopped, and the cause should be identified to prevent accidents. During the construction of the pebble layer, the drill bit should be lifted and lowered slowly to prevent the collapse of the hole caused by the formation of a partial vacuum due to the excessively fast lifting speed of the drill bit. Appropriate mud should be used to effectively form the retaining wall: the drilling mud should select the mud performance index parameters according to the formation conditions. The mud performance index parameters are too high or too low, which are not conducive to smooth hole formation. In practice, mud performance indicators mainly include density, viscosity, and sand content. Among the three mud performance indexes, the parameters of density can be obtained by theoretical calculation, and it is required that mud pressure ≥ active earth pressure + groundwater pressure. The calculation method of soil and water combined calculation and soil and water calculation can be used for calculation, and the calculation formula can refer to 3.4.2 of "Technical Regulations for Building Foundation Pit Support JGJ120-2012".

Sawdust, paper pulp and cottonseed chips can be added to the mud when stirring the mud. It is best to add the above substances to the mud that has been impacted into holes to form a high-viscosity mud. When using this high-viscosity mud, the water head height is 1.5m higher than the groundwater level. above. If there is no thick layer of silt and silt, the specific gravity of the mud should be higher, reaching more than 1.3/ml. During drilling, pay attention to observe whether there is looseness or collapse around the protective tube, and whether the protective tube is in its original position. Also pay attention to the situation of bubbles on the surface of the mud.

It is necessary to remember the depth of footage before each drill bit is lifted, and compare it with the depth after drilling again, so that the changes in the hole can be found in time, and the accident in the hole can be effectively prevented.

2. Operation safety

When using the machine-lock drill pipe to pressurize, pay attention to the pressure change of the drilling rig, and do not exceed the designed capacity of the hydraulic system. The lift rod must be fully unlocked to lift the drill. The driller often checks the wire rope and lifter of the drilling rig during construction, and if the wire rope is found to be worn beyond the instructions, it should be replaced in time.

3. Ensure the verticality of the mast

Pebble is a mechanically loose stratum, which has the characteristics of large pores, strong water permeability, low compressibility, and high shear strength. The arrangement of the pebble layer is irregular, the compressibility is low, the force on the gears during drilling is uneven, the whole machine vibrates violently, the drill pipe jumps and the drill pipe sways, which is very harmful to the drill pipe and the drilling tool. The operator needs to find out the distribution law of the stratum, and at the same time, according to the change of load and vibration and the deflection of the drill pipe, to determine the density of pebbles, the number and position of boulders.

In conclusion

Therefore, in the case of complex geology, the use of rotary drilling rigs for construction in thick pebble layers with uneven density requires gradually accumulating experience during the construction process. The operating points of the drilling rig are systematically analyzed from various aspects such as geological conditions, equipment factors, and operational factors, and targeted preventive measures are formulated to reduce the occurrence of quality and safety accidents and ensure the quality of pile foundation construction.