Abstract

To determine a robust geotechnical model from measurements of drilling parameters is one of the great desires of geotechnical investigations. Drilling parameters have the advantage of presenting very granular data (data-points every centimetre) much like the CPT. They allow for the identification of soil samples during the drilling process and can traverse all terrain types, from soft clays to hard rocks. However, interpreting each parameter, like penetration rate or thrust, in isolation can be challenging, as these parameters can vary greatly within each soil layer due to natural heterogeneity, the drill rig’s configuration and the noise introduced by the discontinuous interaction between soil and drill bit. Various authors propose using compound parameters such as specific energy, penetration resistance and alteration index alongside cautious filtering allows for a better interpretation, giving physical meaning to the measurements. These compound parameters have been developed from simple correlations, aiming to normalise parameters heavily influenced by drilling conditions or even aiming to evaluate the work or the energy spent in excavating the soil. Beyond that, many authors have devised algorithms to automate or standardize the interpretation of drilling logs by identifying homogenous zones or the probability that a given point belongs to a certain layer. This paper presents a comparison of such methodologies for identifying soil layers based on MWD profiles proposed in the literature. Assessment of the geotechnical structure may be made through different analytic and advanced statistical methods. MWD profiles from worksites throughout France will be used to compare and qualify these methods.

Abstract

Drilling boreholes for geotechnical purposes such as sampling and in situ tests is usually performed with the aid of drilling machines. In France, typically, the machine’s drilling parameters such as applied torque and thrust are monitored in realtime and registered so that information about the subsoil’s structure can be acquired. As these machines tend to use hydraulic systems, the oil pressures fed into each motor and actuator are the parameters commonly monitored. Recently, a new type of sensor and its application in geotechnical investigations through the measuring of drilling parameters was presented. This sensor is directly mounted on top of the drill string of a drilling machine and records the actual thrust, torque, and rotation speed transmitted to the drill string and bit. This bypasses the energy losses present in the hydraulic circuit between the machine’s pressure sensors and the hydraulic actuators. This mounting position allows a more accurate measurement of the forces and, furthermore, the effective rotation speed applied to the drill bit. The data is transmitted through a wireless Bluetooth connection enabling real time monitoring. Nevertheless, finding the right balance between a soil’s resistance, the drill rig’s power and the sensor’s optimal measuring range remains a topic for further development. This analysis of several worksites with different soils throughout France contributes to a better understanding of the last generation’s sensor’s precision and application range.

Abstract

The submersible drilling rig presented in this paper was designed and constructed with the aim to allow conducting soil or rock characterization in offshore investigations in shallow water projects when short target penetrations are required. The system is instrumented with sensing elements allowing the full control of the operation in real-time for three different purposes: (i) positioning, (ii) continuous drilling parameters (MWD, measuring while drilling) and (iii) visual inspection. The suitability in sandy and silty soils was reported by Romero et al. (2012). The system is outstanding as an interesting alternative to traditional methodologies for rock investigations in relative shallow water projects where the knowledge of the first metres of rock profile is critical (e.g., pipelines, submarine interconnections, dredging, foundations). Some benefits of the system are presented in this paper, especially in ground models composed of a rock layer underneath a soil deposit or by rocks with variable strength with depth.