New digital image analysis method could improve understanding of peat structure

Peat consists of plant remains that have decomposed over a long period in waterlogged environments. The material contains both larger fibres and smaller particles. How these are distributed affects, among other things, the peat’s strength, permeability and settlement characteristics. These properties are important to understand in construction and civil engineering projects on peatland. Despite this, there are currently no standardised methods for measuring fibre size and distribution in detail.

“Today, peat is described according to its degree of humification. However, what we need is a way of describing the peat’s engineering properties,” says David Rudebeck, geotechnical engineer at the Swedish Geotechnical Institute (SGI).

The method can distinguish between different types of peat

Peat is described using the von Post scale, which indicates its degree of humification, in other words, how decomposed the material is. The method involves squeezing peat by hand and assessing the material visually. It is simple to use but relies partly on subjective judgement.

“In fact, we would like to be able to describe peat in the same way as mineral soils. For those materials, sieve analysis is used. However, that approach works poorly because peat cannot be sieved. The fibres become entangled, rather like a pile of branches,” says David Rudebeck.

The researchers therefore chose a different approach. To separate the peat fibres, the peat was placed in water. The fibres were then placed on filter paper, dried and photographed at high resolution. Using image analysis, computer software was able to identify each individual fibre and calculate its size.

“The results show that the method produces relatively consistent results in repeated analyses. It clearly distinguishes between peat with different degrees of decomposition,” says David Rudebeck.

The peat from Mullsjö, which was less decomposed, generally contained larger fibres than the peat from Hyltena, where the plant material had broken down further. The results are in good agreement with previous observations, although such observations are usually not supported by measured data.

More peat types to be tested

The image analysis method is still at an early stage of development. The next step is to test a wider range of peat types and develop more advanced methods for analysing large, curved and branched fibres.

In the future, the method could make it easier to compare peat from different locations. This would improve ways to share knowledge and experience from research and construction projects on peat, both in Sweden and internationally.

The study was carried out by the Swedish Geotechnical Institute (SGI) in collaboration with RISE and MoRe Research. The project aimed to test and evaluate a method for quantifying the fibre size distribution in low- and moderately humified peat using digital photographic analysis.