The inner barrel penetrated into the soil firstly, and the pressure is higher due to its smaller diameter. It open the central pilot hole to break the hard soil strata. The outer barrel following the guiding of the inner barrel cuts the outer soil strata, and collects the soil. It is two stage, stepped soil breaking process.
Double shell drilling buckets are used to stiff plastic clay and hard silt inter layers, cobble，gravel including boulder layer. It is also used in highly weathered rock, moderately weathered soft rock, and calcareous cemented hard inter layers. It just applied to 800mm large hole diameter, and deep piling foundation. It is needed in those construction site where holes are prone to deviation while hole position accuracy is strictly required.