In-situ digital profiling of soil to rock strength from drilling process monitoring of 200 m deep drillhole in loess ground

Abstract

Digital drilling process monitoring (DPM) technique is an in-situ technique that continuously records the digital data of drilling process parameters in real time. These parameters in real time series are associated with the drilling and operation process of a drilling machine being used to drill a hole in the soil and rock ground for exploration and/or production. This paper presents the results and analyses of the digital DPM data along a 200 m deep drillhole and presents the in-situ profile of ground soil to rock strength in the loess tableland of Weihe Plain, Xi'an, P.R. China. The digital data in real time series is recorded with DPM on site when an ordinary hydraulic rotary drilling machine is drilling the ground for geological exploration. The original DPM data in time series is divided into different sub-sets for differentiating the net drilling process and other drilling operations. The sub-set data of the net drilling process is used to construct the variation curve of the drill bit depth with the net drilling time. This curve is further used to identify the different zones with linear variations of constant gradients (i.e., constant drilling speeds) along the hole depth in association with the drilling force-power parameters. They are used to evaluate the in-situ strength profile of the ground soil and rock strata. Three ground strata of loess, gravelly silt and silty clay are encountered and evaluated along the 200 m drillhole. The in-situ strength profile in terms of constant drilling speed has 17 zones and 75 sub-zones, which shows the depth variation of the homogeneous soil and rock layers. The constant drilling speeds are further compared with laboratory test results of deformation moduli and bearing capacity of the soil and rock core samples, as well as the drilling speeds of other types of soils and rocks. The results and findings of this paper demonstrate that digital DPM technique and data in time series can upgrade the current drilling practice to become an in-situ testing tool of soil and rock strength profile in ground.