Peat deposits are comprised of high organic content substances primarily derived from dead plant vegetation. Peat itself is not inert but undergoes continuous biological decomposition that causes progressive destruction of the peat fabric, reductions in fibre and organic contents and biogas generation. Depending on the degree of decomposition, the organic solids can exist as fresh (intact) fibres, slightly decomposed or ultimately completely decomposed (amorphous) material. From a geotechnical perspective, an understanding of the relationship between degree of decomposition and engineering properties, including the level of compressibility, is important in dealing with such problematic deposits. However a review of the literature indicates that such relationships have not been sufficiently investigated. Moreover, potential impacts of uncontrolled or unexpected decomposition in-situ are regularly discounted in geotechnical practice. This paper reviews decomposition effects in peat and potentially significant knock-on effects in terms of the material's physical properties and compressibility. Progressive reduction in solids volume and deterioration in the integrity of the organic structure due to on-going decomposition may cause significant additional settlement to occur over time. More decomposed peat generally undergoes lower primary consolidation and creep strains and is also less prone to future decomposition, compared with lesser decomposed peat.

Peat deposits are highly compressible, undergoing significant long-term settlement, particularly for fibrous peat.Since decomposition of organic matter can have a significant effect on compression behaviour, the ability to artificially stimulate and pre-decompose a bearing peat stratum prior to the main construction works may mitigate against increased compression rates reported to occur over the design life. This paper presents a feasibility trial of a technique that accelerates the decomposition process in peat prior to entering into full laboratory and field testing.The main decomposition limiting factors of pH and carbon:nitrogen (C:N) ratio are adjusted by adding sufficient amounts of basic and nitrogenous materials. The proposed technique can be tailored for specific peat deposits and can also be achieved in a more sustainable manner. In the trial on moderately decomposed fibrous Sphagnum peat, different peat blends were prepared using additives of peat pulverised fuel ash (PPFA) and urea. Optimum C:N ratio and pH for decomposition were achieved by adding ,21 kg PPFA and ,1 . 5 kg urea per cubic metre of wet peat. The availability of natural decomposers in the peat was also confirmed pre-and post-treatment using a plate count technique, with measured microorganism populations of the order of 10 5 colony-forming units (CFU)/g dry peat.

This paper reviews the mechanisms of decomposition and, in particular, its effect on the compressibility of fibrous peat. Decomposition refers to the breakdown of fresh plant tissue, with the detritus becoming progressively finer, until eventually all vestiges of the original fibrous structure are lost and the peat has an amorphous, almost granular form. From a geotechnical perspective, the partly decomposed plant matter has a major influence on peat behavior, with the peat properties closely related to the average state of decomposition. The destruction of the plant tissue causes a reduction in the material's water holding capacity and weakens the absorption complex which results in the reduction of both its water content and liquid limit values. The reduction in the organic content and the destruction of the inherent fibrous structure also causes an increase in bulk density. An evaluation of the compressibility characteristics of peats at different states of decomposition has been performed from a review of previous research reported in literature. The C /C c ratio of peat ranges between 0.035 and 0.1, with the lower value corresponding to amorphous peat. Hence, it is expected that the more decomposed the peat material, the lower its compressibility. This paper also presents a preliminary study on the potential for decomposition of peats maintained at a constant temperature of 30 C and under partially-saturated conditions over an extended period.

22Organic matter in peat deposits undergoes slow microbial decomposition, which causes permanent material change, including reduction in volume of solids, and is a significant factor in the development of creep settlements for such deposits. Microorganisms accountable for decomposition are readily available in peat, although generally less populous compared with other soils on account of the unfavourable environment. This paper presents a novel experimentallaboratory study that stimulates the growth of microorganisms in a fibrous peat and hence its decomposition rate.The carbon/nitrogen ratio and pH value of the peat were adjusted within optimum ranges for microbial activity to occur by blending sufficient amounts of pulverised fuel ash and urea with the wet peat. This process did not adversely affect indigenous microbial populations within the peat. Blended material was incubated aerobically and anaerobically at 30°C over periods of up to 126 days. Microbial populations increased by approximately 3500-and 1800-fold under aerobic and anaerobic incubation, respectively, which was an indicator of increased decomposition rate. Water content, specific gravity of solids, volatile organic content and fibre content were measured both before and after incubation treatment. Fibre content was found to be approximately inversely proportional to incubation period, with relatively greater reductions in fibre content occurring for blended material under aerobic conditions.