Assessment of different methods for designing bored piles

Abstract

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.

Article in English.

Skirtingų metodų vertinimas projektuojant gręžtinius pamatus

Santrauka. Straipsnyje nagrinėjami įvairūs skaičiavimo metodai, skirti gręžtiniams pamatams projektuoti. Lietuvos elektrinės teritorijoje projektuojant kombinuoto ciklo dujų turbinos bloko pamatus, dėl ypatingų statinio reikalavimų polių nuosėdžiams bei laikomajai gebai buvo parinkti gręžtiniai poliai. Pagrindinis darbo tikslas – įvertinti gręžtinių polių nuosėdžių ir laikomosios gebos skačiavimus pagal statinio zondavimo duomenis bei analitinius sprendimus, taikant skirtingus skaičiavimo metodus. Polio nuosėdžiams ir laikomajai gebai vertinti buvo taikyti šie metodai bei standartai: Bowles, Schmertmann, de Ruiter ir Beringen, Bustamante (LCPC), EN 1997-2 ir NEN 6743 standartų metodika. Remiantis statinio ir dinaminio zondavimo bei laboratorinių tyrinių duomenimis, suminio nuosėdžio reikšmė buvo gauta lygi 16 mm, iš jų 2 mm sudaro nuo vertikaliosios apkrovos gniuždomo polio deformacija. Tokio tipo statinių pagrindo nuosėdis negali būti didesnis nei 2 % polio skersmens. Atlikti skaičiavimai parodė, kad 800 mm skersmens ir 27 m ilgio polio visiškai pakanka statinėms ir dinaminėms apkrovoms atlaikyti.

Reikšminiai žodžiai: statinis zondavimas, gręžtinių polių laikomoji galia, polių nuosėdžių skaičiavimai, dujų turbinos, gilieji pamatai

Keyword : design of bored piles, foundation for the gas and steam turbine, cone penetration test, pile bearing capacity, pile settlement analysis