Assessment of Different Methods for Designing Bored Piles / Skirtingu Metodu Vertinimas Projektuojant Grežtinius Pamatus

This work concerns the design of bored piles on the site of Elektrenai Power Plant in Lithuania. Equipment for supporting bored pile foundation in the power plant consists of a gas turbine, a steam turbine and a generator. The foundation not only endures high loads but also sustains a strong dynamic impact due to the vibration of equipment in the power plant under working conditions. A solution to the pilling problem was adopted for the following reasons: i) the insufficient capacity of soil to support acute stresses; ii) high requirements of slab settlements and bearing capacity with regard to the main equipment used in the power plant. The objective of this work is to assess the methods used for estimating immediate settlement and carrying capacity of the pile considering cone penetration tests conducted in Elektrenai power plant. For settlement estimation, four methods, including Bowles (Bowles 1997) and Schmertmann methods (Schmertmann 1978) as well as the methods described in EN 1997-2 and NEN 6743 (NEN 6743:1991/A1:1997) were employed. A carrying capacity of the pile was evaluated with the help of direct methods that utilize data on the cone penetration test (CPT). The following direct methods such as the Schmertmann method (Schmertmann 1978), the de Ruiter and Beringen method (de Ruiter, Beringen 1979), the Bustamante method (LCPC) (Bustamante, Gianeeselli 1982) and the methods described in EN 1997-2 and NEN 6743 (NEN 6743:1991/A1:1997) were applied. Pilling foundation was evaluated performing immediate settlement analysis and included the examination of soil data received from cone and dynamic penetration tests, boreholes and laboratory tests. Soil properties were estimated taking into account investigation into the site of Elektrenai power plant and a soil exploration program developed according to Lithuanian standards. Pile settlement analysis showed that settlement value made 13.6 mm (pile toe settlement) and the settlement value of an elastic deformation of the pile from vertical compressive loads was 2.3 mm, for the most conservative situation. For such structure, foundation settlement should not exceed 16 mm. Elektrenai power plant has high reliability requirements, and therefore the pile having the diameter of 800 mm with a pile length of 27 m was adopted to endure overall loads.