# Evaluation of Methods Based on CPTu Testing for Prediction of the Bearing Capacity of CFA Piles
## 2 Materials and Methods
### Cone Penetration Testing and Pile Bearing Capacity Prediction  
The cone penetration test is based on the determination of the penetration resistance of soil against static pressing of the cone. It is one of the quick and reliable survey methods of soils and semi-rocks. For practical application, appropriate interpretation of observed data is crucial. Therefore, combination with other survey methods, such as boreholes, is suitable. Probing is performed without sampling of soil specimens and because of this, the method is less disturbing to the environment. The method is preferred mainly for fine-grained clayey, silty, and sandy soils [10]. In principle, a penetration cone is similar to a small driven \"pile\" when the cone tip resistance _q_s and shaft friction _f_s correspond to the pile tip bearing capacity and shaft bearing capacity, respectively. The piezoelectric penetration cone used in this study and the principle of pore pressure measurement are displayed in Figure 4.  
Evaluation of pile bearing capacity represents one of the most frequent applications of in situ test results. Utilization of CPT testing as a survey method arises the need of a proper connection between the test data and applicable quantities for designing structures. Because of the indirect nature of output data, a relation must be set up to convert the cone tip resistance or the shaft friction into components of pile bearing capacity. Pile bearing capacity analysis underwent a considerable development recently. Aside from the formulation of shaft friction based on soil strength parameters, alternative approaches have been adopted involving investigation of friction at the soil-pile interface. Additional variables that affect the pile bearing capacity were studied such as stress history, pile slenderness, soil sensitivity, plasticity of clayey soils, relative density of cohesionless  
Figure 4: Piezoelectric penetration cone: (**a**) The cone; (**b**) scheme of pore pressure measurement: \(u_{2}\): pore pressure; \(A_{a}\), \(A_{b}\), \(A_{c}\), \(A_{s}\): cross-section area [50]. Figure 3: Example of evaluation of loading curve for pile No. 5 [40]. soils, failure mechanism progress, pile installation method or compressibility of pile and soil. Similarly, the pile tip bearing capacity component primarily depends on the conditions within the influence zone, pile tip shape, installation process, strength and stiffness of the ground around the pile tip, loading rate, groundwater effect, etc. Several theories of bearing capacity and their modifications were proposed such as limit plasticity, elastoplasticity, theory of dilatancy strength or particle breaking theory. This led to derivation of more sophisticated calculation formulas [51]. Considering the above-mentioned similarity of a penetration cone and a pile, the bearing capacity components can be generally expressed as follows [52; 53]:  
\[Q_{p}=Q_{b}+Q_{s}, \tag{1}\]  
\[Q_{p}=A_{b}\times q_{b}+O_{p}\int_{0}^{h}f_{p}\;dz=A_{b}\times q_{b}+O_{p}\sum _{i}f_{pi}\;\times\;h_{i}, \tag{2}\]  
where \(Q_{p}\) is overall bearing capacity of a pile; \(Q_{b}\) is bearing capacity of a pile tip; \(Q_{s}\) is a pile shaft bearing capacity, \(q_{b}\) is average penetration resistance at the pile tip derived from interval depending on the method; \(f_{pi}\) is unit shaft friction in \(i\)-th layer; \(A_{b}\) is pile cross-section; \(d\) is diameter of a pile; \(O_{p}\) is circumference of a pile; and \(h_{i}\) is the thickness of an \(i\)-th soil layer. Pile bearing capacity calculation based on CPT data can be performed in two main ways: The first group is designated as direct methods and the second one as indirect or rational methods with division into another sub-groups. Despite the major contribution to this problem, several design methods have fundamental disadvantages [50; 54; 55]. Soil behavior is governed by the complex stress-strain state changes already occurring during manufacturing or installation and throughout the service life of a pile.