Application of Novak’s Method for Design of low frequency rotating equipment foundation block on driven cast In Situ RCC piles in weak soil-A Case Study considering Site Specific Modifications

By Sanjoy Chowdhury & Shalini Roy

Numbers of rotating machines having wide range of frequencies operate in a thermal power plant. These machines except the Turbo-Generator are mounted on reinforced concrete block foundations. Induced Draft (ID) Fan is a rotating machine which enables passage of flue from Electro static precipitator (ESP) to the Chimney and finallyto the atmosphere. This paper covers a case study for the dynamic analysis of ID Fan foundation on piles applying Novak, solution with some site specific judgements and design considerations. The site in the present case Is 2X300 MW Thermal Power Plant at Haidia, West Bengal, India and owned by Haldia Energy Limited (HEL). Geotechnical Investigation was performed in detail and soil characteristic was found to be weak (predominantly clayey/sandy silt with low ‘N. value). As such, 600 mm diameter RCC cast in situ driven friction pile was adopted for major equipment foundations and heavy structures. Simple methods of calculation considering the concept of participating soil mass & Its quantitative evaluation in case of pile foundations subjected to dynamic actions are yet to be well established. The problem Is further compounded in the present case since soil stratification alongthe embedded length of pile is not uniform. The pioneering work done by Novak in this field Is very popular among practising engineers despite of various limitations. Improvements to Novak, solution were done by eminent researchers subsequently to offset some of the limitations. This paper discusses a considered and judicious approach in simulating the soil stratification at site while applying Novak, solution for calculation of the pile stiffness and damping ratio in the dynamic analysis of the foundation. The ID Fan is under operation for the last ten months and performance of the equipment system with foundation had been reported to be satisfactory. This paper, by presenting the case study of ID Fan foundation, thus tries to establish that Novak’s method despite having limitations can be used when site specific soil parameters are accounted judiciously for the design of low frequency rothting machine foundation on driven cast in situ piles in a multilayered weak soil.

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