Adfreeze Forces on Lightly Loaded Pile Foundations of Solar PV Farms in Cold Regions

Abstract

Renewable energy generation through utility scale ground mounted solar photo-voltaic systems has gained steady popularity with increasing number of such facilities being constructed in various regions worldwide. Solar PV systems are very popular in the province of Ontario in Canada and strong growth in this sector is led by the popular initiatives of the Government of Ontario which offers extremely attractive rates for generation of renewable energy through Ontario Hydro’s popular Feed-In Tariff (FIT) Program. Many other countries offer incentives on such generation of renewable energy while many governments aim at increasing the percentages of renewable energy in their systems tremendously. Most ambitious plan has recently been launched by the state of Hawaii to deploy 100% of renewable energy in their grid by 2045. Solar PV systems are a cheap source of renewable energy as the energy released by the sun is harnessed as electricity by the solar photo-voltaic panels which is fed to the main transmission systems after raising its voltage. The costs of solar photo-voltaic panels meanwhile have also kept downward trends while the manufacture of various types of solar panels has multiplied rapidly. These renewable energy generation facilities are fully sustainable being completely recyclable on completion of their design/ contract period. Typical utility scale ground mounted solar PV facilities usually comprise of solar PV panels mounted on series of racking tables supported on foundations mostly comprising of partially embedded steel pipes. The governing loads for the foundations of these lightly loaded solar PV structures are usually frost loads in areas facing extremely cold winters. In fine grained soils like silty/ clayey soils, large adfreeze stresses develop due to penetrating frost deep into the soil resulting into uplift of foundation piles. Typical winter conditions in Ontario are harsh with extreme frost conditions in most areas which poses unique issues for design and construction of such foundations. Being a relatively newer technology, codes and standards for design and testing of such lightly loaded solar PV structures are still in the formulation stages. Frost heaving and its effects often create adverse conditions for these structures thereby affecting the production and continuous supply of renewable energy. Due to larger depths of frost penetration in extreme winter conditions, understanding the action of frost and related development of adfreeze stresses on these lightly loaded pile foundations is extremely important. Calculating reasonable frost depths and thereby the design loads is an important part of pile design for such facilities while the contractors tend to save on pile lengths to save on costs and compromising the structural design. Many such Solar PV facilities have experienced frost uplift of foundation piles either during the construction phase or during its lifetime. Since frost heave is more of a serviceability related issue, unfactored adfreeze loads without any factor of safety is a usual tendency by the EPC contractors. This paper investigates the frost depths and adfreeze stress related issues with the foundation piles of solar PV facilities hence the governing design forces on these piles and suggests appropriate frost related design stresses for the foundation piles. The authors have been heavily involved in design/ design reviews, pile selection/ design and pile load testing in the majority of the solar PV farms in Canada and US along with rehabilitation of piles affected by frost [1,2,3].