bstract Determining soil strength parameters such as uniaxial compressive strength using suggested standard methods is costly and time consuming mainly due to difficult sample preparation and destructive procedure. Estimating soil 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 on the rocks and concretes. 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 rock and concrete. The analytical methods have proposed models based on force or energy equilibrium methods. Most of the models propose methods to estimate the uniaxial compressive strength through the investigation of interaction between the bit and sample especially in rotary drilling process. These models use the parameters of interaction between the bit and rock in drilling process to offer relationships between these parameters and rock strength. In order to increase the efficiency of this technique and also to investigate the roles of parameters affecting the estimated results, it is necessary to consider several factors of interaction between the bit and soil in the drilling process. Therefore, in this thesis attempts to introduce more parameters of the drilling process, such as crushed zone, contact friction between the soil and crushed zone, contact friction between the bit and crushed zone and contact friction between the bit and soil, a new analytical model is proposed based on limit equilibrium of forces in the drilling process. Based on the model, a method is used to estimate the soil strength parameters from the operational drilling data. Unlike direct shear test that is useful just for fine-grained soil, this method can be used also for sandy soils and can estimate uniaxial compressive strength of every soils without wasting time and cost. The new model provides an analytical relationship between the soil strength parameters, bit geometry and operational drilling parameters such as thrust force, torque, rate of penetration and speed of rotation. The model reveals the effects of these parameters on the estimated results. In addition, it is possible to estimate soil strength parameters only by a blunt bit. Using a is very simple bit like this kind that was used in this investigation can cause that there is a little time to research on soils. In addition, bit bluntness is very low and its efficiency is more. 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 soil samples. Due to the small dimensions of the drilling, this technique can be considered as a non-destructive method. The drilling experiments performed by this machine on the finely sanded clay, clay (with sand), clay silty with low plastic limit, poorly graded clay sand (large), show that the results of the proposed model are in good agreement with the results of standard laboratory tests. Using different ranges of humidity on soils which consist a variety of strength, causes that this investigation has been known as a general and expansive experimental and analytical model for soils, also not only can this model estimate strength parameters for soil very accurate, but also there is a way to expand it for all soils and all conditions. It is showed that this analytical model is very accurate and successful because of more than 96 percent conjunction between datum. Key words Unconfined Compressive Strength, non-destructive method, drilling, sand, clay