Contents lists available at ScienceDirect International Journal of Rock Mechanics and Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms Estimating rock strength parameters using drilling data Sajjad Kalantari b, ⁎ , Hamid Hashemolhosseini a , Alireza Baghbanan b a Department of Civil Engineering, Isfahan University Of Technology, Iran b Department of Mining Engineering, Isfahan University Of Technology, Iran ARTICLE INFO Keywords: Estimating rock strength parameters Operational drilling data Analytical model ABSTRACT Estimating rock strength parameters using operational drilling data can be a fast and reliable method. In this case, several researchers have proposed different analytical models based on force or energy equilibrium methods. Most of them propose methods to estimate uniaxial compressive strength through the investigation of interaction between the bit and rock in drilling process. Although in the proposed models, operational drilling system, rock strength parameters, bit geometry and contact friction were considered, some of the important factors such as crushed zone and its mechanical properties, contact frictions between the bit and rock and friction between the rock and crushed zone need to be explicitly considered. In this research work, a theoretical model is developed based on limit equilibrium of forces and considering contact frictions, crushed zone and bit geometry in the rotary drilling process by a T-shaped drag bit. Based on the model, a method is used to estimate rock strength parameters form operational drilling data. The operational drilling parameters such as thrust force, torque, rate of penetration and speed of rotation were obtained by a developed portable drilling machine. The portable drilling machine is able to drill the rocks with different strength range coincident with measure and record the parameters. A set of drilling experiments were conducted on three different rocks ranged from weak, medium and hard strength. Obtained results based on proposed model for uniaxial compressive strength, co- hesion and internal friction angle of rock are well fitted to the results of the conventional standard tests. 1. Introduction Unconfined Compressive Strength (UCS) of intact rock has been considered as the main parameter in most rock mass classification systems and also as a high limit of the rock mass strength. Determining UCS using suggested standard methods is costly and time consuming mainly due to difficult sample preparation and destructive procedure. Therefore, many indirect methods such as scratch test, point load, Schmidt hammer and punch tests have been proposed for this purpose. Estimating the UCS of rock using drilling parameters is also an alter- native method. In this method, continuous measurement of drilling parameters and evaluating of rock strength during the drilling is pos- sible. Beside the advantage of continues measuring, it can be used as a quasi-nondestructive method in field. In large scale, this method can be easily facilitated due to simply movement and no need for sampling. In this case, the drilling methods as well as the bit are most important. Regarding extensive use of rotary drilling systems, it has attempted more attention compared with the other systems and most of the re- presented methods are accommodated in this drilling system. The drilling process is conducted through two stages of feeding and cutting. At the first stage, the bit indents into the intact rock by the thrust force. As a result of the indentation, a part of rock is crushed and pressed. The compressed crushed zone is formed in front of bit due to the rake angle, cutting face friction and spiral motion of the bit. The next stage of ro- tary drilling occurs due to the rotational motion of the bit. At this stage, the horizontal force is transferred to the rock along rotating bit but unlike the first stage, the force is transferred to the rock through an intermediary. The intermediary is the compressed crushed zone, which damps a part of the force as frictional one. The crushed zone is more important in investigating the interaction between the bit and rock as well as estimating the strength parameters of rock. This issue, mostly have not been considered in previous proposed models. According to the literature, previous proposed models which esti- mate rock strength using drilling technique or propose methods for this aim through the investigation of interaction between the bit and rock is categorized in two forces equilibrium-based and energy equilibrium- based models. The former category has been introduced by Evans 1 as “Indentation Model” which estimates the necessary force to coal plowing. It was also further developed by Nishimatsu 2 as “Shear Model” for estimating the necessary force to shear failure of rock which is based on Merchant's metal cutting Model. 3 There are other models in this category which are developed based on different types of bits such as Detournay et al. 4 and Gerbaud et al. 5 , Chiaia, 6 Wojtanowicz et al. 7 or Nakajima et al. 8 models for drag and Polly crystalline Diamond https://doi.org/10.1016/j.ijrmms.2018.02.013 Received 10 January 2017; Received in revised form 1 December 2017; Accepted 5 February 2018 ⁎ Corresponding author. International Journal of Rock Mechanics and Mining Sciences 104 (2018) 45–52 1365-1609/ © 2018 Elsevier Ltd. All rights reserved. T