Changes of Thornthwaite’s Total Moisture Indices in Victoria from 1948-2007 and the effect on seasonal foundation movements

D. Lopes and N. Y. Osman

C.W. Thornthwaite first presented the Total Moisture Index which was later known as the Thornthwaite’s Moisture Index (TMI) and mapped these values for the whole of the USA in 1948. Thornthwaite defined the TMI as the first base for his climate classification. This index was found to be particularly useful in comparing changes in the near surface soil and atmospheric conditions and has been used in the USA since the 1940’s as an indicator of the supply of water in an area relative to the demand under prevailing climatic conditions (McCabe and Wollock, 1992). This paper presents the TMI values calculated over 3 x 20 year periods (1948-1967, 1968-1987 and 1988-2007) for the state of Victoria (Australia) which have been plotted on 3 TMI isopleth maps. These maps show that Victoria has experienced drying conditions in the past 60 years. A drying climate has a major engineering impact on clay foundation movements. Buildings with shallow footings founded in dry reactive clay are prone to differential heave from the watering of surrounding gardens or poor drainage. In the longer term, buildings built in clay profiles in drier climate, experience greater foundation movement due to the greater depth of soil drying and wetting. This paper discusses the possible “Characteristic Surface Movement” (ys) changes as defined in Australian Standard 2870–Residential slabs and footings construction (1996) due to the new climatic conditions indicated by the TMI changes.