# Evaluation of Methods Based on CPTu Testing for Prediction of the Bearing Capacity of CFA Piles
## 1 Introduction  
Various types of piles can be utilized in civil engineering practice. The response of the pile to the load considerably varies depending on the installation method or technology, respectively. There are non-displacement piles on one side of the spectrum (e.g., bored piles or drilled shaft piles) and the full-displacement piles on the other side (e.g., closed-end pipe piles or precast reinforced concrete piles). Non-displacement piles are constructed by removing drilled-out soil and replacing the soil with concrete or another suitable material [1, 2, 3]. Full-displacement piles are driven in the ground using variety of materials, technical design, and utilization [4, 5, 6, 7, 8, 9]. Significant changes in porosity and overall stress state in the soil occur during the driving of the pile. The soil around the pile shaft is drifted mainly in the horizontal direction while the soil beneath the pile tip is compacted (densification of the soil). These factors lead to improving the ground stiffness during the installation of full-displacement piles in comparison with non-displacement piles, especially in the case of sandy soils [10]. Pile utility depends on the proper approximation of the pile-soil relation. Uncertainties in the understanding of such a relation consists of the variability of ground properties, measurement errors, and transformation of data during experimental works. Several uncertainties that cannot be described with sufficient reliability level lead to adopting the correlations between ground test outputs and required \"structural\" parameters of the rock environment [11, 12, 13]. Analysis of pile bearing capacity is an important task in the investigation of soil-structure interaction and the safe, economic, and effective design of the structure should be the output of the analysis. Despite the large database of case studies, practical application of pile design methods based on in situ testing requires proper care because of a large variety of ground conditions, pile types, and some shortcomings of the methods itself [14; 15; 16]. Statistical analyses with empirical approaches are widely used to avoid the above-mentioned uncertainties [17; 18; 19; 20]. Uncertainties in the pile bearing capacity evaluation based on the strength and deformation characteristics of soils, designated as indirect methods, lead to adopting the direct relation between in situ test results of penetration testing (cone tip resistance \(q_{b}\), shaft friction \(f_{s}\), pore pressure \(u_{1}\) and \(u_{2}\), or shear and pressure waves velocity) and the pile bearing capacity [21; 22]. The direct approach allows to avoid the two-step derivation of soil parameters and reduces the level of uncertainty that is increased by each step. Laboratory testing provides useful information in the case of clayey soils but in the case of sand or coarse-grained soils, it is difficult to obtain a non-disturbed soil specimen in the original stress state. Despite the improvement of numerical and analytical methods, the empirical approach is widely used for the design pile foundations. This approach is supported by the load tests of piles and outputs of in situ surveys, especially penetration and borehole testing [23; 24; 25; 26]. Interpretation and application of penetration test data requires the appropriate approach and evaluation method [27; 28]. The paper is dedicated to the direct design or prediction methods for the pile bearing capacity calculation based on the cone penetration test (CPT) results, namely the UniCone method, Laboratoire Central des Ponts et Chaussees method (LCFC), and the method involved in the Eurocode 7--2. Design methods were selected in accordance with the recent research and the possibility of application of the CPT method for the design of Continuous Flight Auger piles (CFA). These methods are developed on the basis of a large set of various pile types and ground conditions, e.g., soil types and properties.