Determining of rock strength parameters such as cohesion, internal friction angle and uniaxial compressive strength using suggested standard methods are costly and time consuming mainly due to difficult sample preparation and destructive procedure. Estimating rock strength parameters using operational drilling data can be a fast and reliable method. In this case, several researchers have proposed different experimental and analytical methods. The experimental methods are based on the measurement of the required thrust force during drilling under constant speed of rotation and rate of penetration as drilling resistance. The amount of the drilling resistance is correlated with the uniaxial compressive strength of a rock. The analytical methods have proposed models based on force or energy equilibrium methods. Most of the previus suggested models try to predict the uniaxial compressive strength based on interaction between a bit and rock especially in rotary drilling process. In order to increase the accuracy of drilling technique and also to investigate the effects of important parameters on the results, it is necessary to consider several factors of interaction between the bit and rock in the model. Therefore, attempts have been made to introduce deciding parameters of the drilling process, such as crushed zone, contact friction between the rock and crushed zone, contact friction between the bit and crushed zone and contact friction between the bit and rock in a new developed model based on limit equilibrium of forces approach. In this model beside the uniaxial compressive strength of rock, cohesion and internal friction angle of rock are also estimated. The new model provides an analytical relationship between the rock strength parameters, bit geometry and operational drilling parameters such as thrust force, torque, rate of penetration and speed of rotation. To estimate required parameters for developed analytical model a portable drilling machine was developed and used. The portable drilling machine allows for drilling tests on site as well as on rock samples. Rock types with different range of strength from weak and medium to hard were examined, and the obtained results were compared with the results of standard laboratory tests. The drilling experiments show that the results of the proposed model are in good agreement with the results of standard laboratory tests. In addition, unlike previous models, it is possible to estimate rock strength parameters only using a blunt bit, which the bit bluntness and its diameter does not affect the estimated results. Due to the small dimensions of the drilling, this technique can be considered as a non-destructive method
