Bored Pile Installation Method and Slurry Borehole Protection Mechanism in Karst Terrains
To resolve the problems of the borehole caving,deflection,and lost drill bit during the drilling process of the rotary excavation of bore piles in well-developed karst terrains,this paper presents a case history of bored pile installation for a high-rise residential building.Based on the analysis of the geological characteristics of the karst terrain at the site,the common in-stallation hazards occurred in the bored pile installation in the karst area,such as,borehole out of plumbness,drill bit jam,borehole caving at the soil-rock interface and in the filling material,could be mitigated throughout double casing method as well as slurry borehole protection.The results show that the main characteristics of the karst terrain at the project site are the pres-ence of vertically developed fractures,irregular filling,semi filled,and fully filled karst rock cavities and pinnacles.In rock cavities in the borehole during the drilling out of plumbness and the advancement reach 50％,and the linear dissolution rate of each borehole ranges from 45％to 78％.For the problem of drilling out of plumbness and drill bit jam,three control parame-ters,namely,drilling downward pressure,rotational speed,and drilling rate,are proposed for different soil layers and rock formations.For the problem of borehole caving at the soil rock interface,the double steel casing method was proposed,and a large cone angle,wear resistant spiral drill bit was used for advancing near the rock surface.For the problem of caving in the filling material,measures for grouting improvement and slurry borehole protection are proposed.After adding of 0.573kg spe-cial polymer slurry into the borehole can effectively improve the borehole wall stability and reduce hazards such as slurry lost,borehole caving,drill bit lost,and borehole necking.
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