Abstract: The unloading zones in the rock mass are frequently encountered in the deeply cut river valleys. The geological properties and limits of these unloading zones might determine the locations the proposed bridge abutments. The conventional exploration drift method has the disadvantage of time consuming, high cost and poor safety. However, a new exploration method, which utilizes a semiclosed single action double tube rock core bit to extract the undisturbed rock cores, can provide a rapid, cost effective and safe method to investigate the river banks. This method can efficiently analysis the structure plane, perform statistical analysis on rock core RQDs and provide qualitative and semi quantitative evaluation results for the unloading zones at both sides of the river banks. This paper presents the exploration results for the proposed Jiangdi River Bridge in Yongren to Dayao Highway Project. In this project, the semiclosed, single action and double tube rock core bit and undisturbed rock core preservation technique were first used in the evaluation of the unlading geologic zone.

Key words: Semi-closed Single Action Double Tube Rock Core Bit, Undisturbed Rock Cores, Unloading Zone, Rapid Explorations, River Bank Field Reconnaissance, Jinsha River Bridge